Arsenic Coagulation with Iron, Aluminum, Titanium, and Zirconium ...

Arsenic Water Technology Partnership
Arsenic Coagulation with Iron, Aluminum, Titanium,
and Zirconium Salts

Arsenic Coagulation with Iron, Aluminum, Titanium,
and Zirconium Salts
Prepared by:
Divagar Lakshmanan, Dennis A. Clifford, and Gautam Samanta
Department of Civil and Environmental Engineering
University of Houston
Houston, TX 77204-4791
Jointly Sponsored by:
Awwa Research Foundation
6666 West Quincy Avenue, Denver CO 80235-3098
and
U.S. Department of Energy
Washington, D.C. 20585-1290
Published by:
WERC, a Consortium for
Environmental Education and
Technology Development at
New Mexico State University
Awwa Research Foundation

DISCLAIMER
This study was jointly funded by the Awwa Research Foundation (AwwaRF) and the U.S.
Department of Energy (DOE) under Grant No. DE-FG02-03ER63619 through the Arsenic Water
Technology Partnership. The comments and views detailed herein may not necessarily reflect
the views of the Awwa Research Foundation, its officers, directors, affiliates or agents, or the
views of the U.S. Federal Government and the Arsenic Water Technology Partnership. The
mention of trade names for commercial products does not represent or imply the approval or
endorsement of AwwaRF or DOE. This report is presented solely for informational purposes.
Copyright 2008
By Awwa Research Foundation
and Arsenic Water Technology Partnership
All Rights Reserved
Printed in the U.S.A.

CONTENTS
LIST OF TABLES ......................................................................................................................... ix
LIST OF FIGURES ....................................................................................................................... xi
FOREWORD .............................................................................................................................. xvii
ACKNOWLEDGMENTS ........................................................................................................... xix
EXECUTIVE SUMMARY ......................................................................................................... xxi
CHAPTER 1 INTRODUCTION .................................................................................................... 1
Arsenic Occurrence, Health Effects and Regulations .......................................................1
Iron Coagulation-Filtration and Adsorption Processes .....................................................1
Background of the Study ..................................................................................................2
Significance of In-situ Formed Hydroxides in Arsenic Removal ........................... 2
Alum as Coagulant for Arsenic Removal ............................................................... 2
Zirconium Oxide/Hydroxide Adsorbent Media for Arsenic Removal ................... 3
Basis of the Project ................................................................................................. 3
Objectives .........................................................................................................................4
Anticipated Practical Benefits of the Project ....................................................................4
CHAPTER 2 MATERIALS AND METHODS ............................................................................. 7
Reagents and Stocks .........................................................................................................7
Preparation of NSFI-53 Challenge Water .........................................................................7
Preparation of Coagulant Stock ........................................................................................7
Coagulation-Filtration Procedure ......................................................................................8
Preservation and Speciation ..............................................................................................9
Instrumentation ...............................................................................................................10
Total Arsenic and Arsenic and Arsenic(III) Analysis Using FI- HG-AAS ....................10
Freundlich Isotherm ........................................................................................................11
CHAPTER 3 As(III) VS. As(V) REMOVAL .............................................................................. 13
Introduction .....................................................................................................................13
Arsenic Removal Study Procedure .................................................................................14
Arsenic Removal Using Ferric (III) Chloride ....................................................... 14
Arsenic Removal Using Alum .............................................................................. 15
Arsenic Removal Using Zirconium (IV) Chloride ............................................... 16
Arsenic Removal Using Titanium (IV) Chloride.................................................. 17
Arsenic Removal Using Titanium (III) Chloride .................................................. 18
Arsenic Removal Using Titanium (IV) Oxychloride ............................................ 19
Arsenic Removal Using Zirconium (IV) Oxychloride ......................................... 19
Arsenic Adsorption Isotherms ........................................................................................20
Comparison of Adsorption Capacities ............................................................................24
v

Arsenic Adsorption Comparison on a Mass Basis ................................................ 24
Arsenic Adsorption Comparison on a Molar Basis .............................................. 25
Oxidation Study of Arsenic(III) in the Presence of Titanium(III) chloride ....................28
TiCl 3 Oxidation Study Results .............................................................................. 28
Proposed Mechanism for the Oxidation of Arsenic(III) in Presence of
Titanium(III) ................................................................................................... 29
Summary and Conclusions .............................................................................................30
CHAPTER 4 EFFECT OF COMPETING IONS ON ARSENIC (III)/(V) ADSORPTION........ 31
Introduction .....................................................................................................................31
Effect of Silica in NSFI Challenge Water Containing Phosphate ..................................32
Effect of Silica in NSFI Water with Phosphate Using Ferric (III) Chloride as
Coagulant ........................................................................................................ 32
Effect of Silica in NSFI Water with Phosphate Using Alum as Coagulant .......... 34
Effect of Silica in NSFI Water with Phosphate Using Zirconium (IV)
Chloride as Coagulant ..................................................................................... 34
Effect of Silica in NSFI Water with Phosphate Using Titanium (IV)
Chloride as Coagulant ..................................................................................... 36
Effect of Silica in NSFI Water with Phosphate Using Titanium (III)
Chloride as Coagulant ..................................................................................... 37
Effect of Phosphate in NSFI Challenge Water Containing Silica ..................................38
Effect of Phosphate in NSFI Water with Silica Using Ferric (III) Chloride as
Coagulant ........................................................................................................ 39
Effect of Phosphate in NSFI Water with Silica Using Alum as Coagulant .......... 40
Effect of Phosphate in NSFI Water with Silica Using Zirconium (IV)
Chloride as Coagulant ..................................................................................... 41
Effect of Phosphate in NSFI Water with Silica Using Titanium (IV)
Chloride as Coagulant ..................................................................................... 42
Effect of Phosphate in NSFI Water with Silica Using Titanium (III)
Chloride as Coagulant ..................................................................................... 43
Effect of Vanadate in NSFI Challenge Water Containing Silica and Phosphate ...........45
Effect of Vanadate in NSFI Water with Silica and Phosphate Using Ferric
(III) Chloride as Coagulant ............................................................................. 45
Effect of Vanadate in NSFI Water with Silica and Phosphate Using Alum as
Coagulant ........................................................................................................ 46
Effect of Vanadate in NSFI Water with Silica and Phosphate Using
Zirconium (IV) Chloride as Coagulant ........................................................... 47
Effect of Vanadate in NSFI Water with Silica and Phosphate Using
Titanium (IV) Chloride as Coagulant ............................................................. 48
Effect of Vanadate in NSFI Water with Silica and Phosphate Using
Titanium (III) Chloride as Coagulant ............................................................. 49
Individual Effects of Silica, Phosphate and Vanadate with Ferric (III) Chloride as
Coagulant in NSFI Challenge Water Without Competing Ions................................50
Effect of Silica in the Absence of Phosphate and Vanadate ................................. 51
Effect of Phosphate in the Absence of Silica and Vanadate ................................. 52
Effect of Vanadate in the Absence of Silica and Phosphate ................................. 53
vi

Summarizing the Effects of Competing Ions on the Arsenic Adsorption Capacity
in NSFI Challenge Water ..........................................................................................55
Individual Effects of Competing Ions on the Arsenic Adsorption Capacity of
Ferric (III) Hydroxide ...............................................................................................61
Conclusions .....................................................................................................................63
CHAPTER 5 TOXICITY CHARACTERISTIC STUDIES ......................................................... 65
Introduction .....................................................................................................................65
Preliminary Toxicity Characteristic Evaluation ..............................................................65
Experimental Study Procedure ............................................................................. 66
Toxicity Characteristic Study Results .............................................................................67
Experimental Evaluation of the Adsorption of High Concentrations of Arsenic
onto Metal Hydroxides .............................................................................................67
Conclusions .....................................................................................................................68
CHAPTER 6 ARSENIC REMOVAL WITH FeCl 3 COAGULATION: COMPARISON OF
EXPERIMENTAL RESULTS WITH MINEQL+ 4.50 CHEMICAL EQUILIBRIUM
MODELING PROGRAM PREDICTIONS ........................................................................... 69
Introduction .....................................................................................................................69
Mineql+ Program Modeling Procedure ..........................................................................69
As(III) and As(V) Adsorption Isotherms Based on Mineql+ Modeling .........................72
Comparison of Model-predicted vs Experimentally Observed Arsenic Adsorption
onto Fe(OH) 3 (s) ........................................................................................................76
Conclusions .....................................................................................................................79
CHAPTER 7 SUMMARY AND CONCLUSIONS ..................................................................... 81
SUMMARY ....................................................................................................................81
Conclusions .....................................................................................................................81
Recommendations ...........................................................................................................85
REFERENCES ............................................................................................................................. 87
ABBREVIATIONS ...................................................................................................................... 91
vii

viii

TABLES
Table 2.1 Composition of NSFI 53 challenge water........................................................................8
Table 2.2 Composition of reagents in preparation of coagulant stock for coagulation study ........ 8
Table 2.3 Experimental conditions for the determination of As(III) and As(Tot) by FI-HG-
AAS.......................................................................................................................... 10
Table 3.1 Equilibrium constants for some important arsenic containing acids ............................ 13
Table 3.2 Comparison of chemical costs for treating one million gallons of NSFI-53 water
to reduce As(V)/As(III) from 50 to < 10 μg/L ......................................................... 27
Table 4.1 Composition of species of some important acids present in pH 6.5-8.5 ...................... 32
Table 5.1 Percent dry solids in the liquid wastes after 3-hr settling ............................................. 67
Table 5.2 Arsenic concentrations in the TCLP- or WET-defined extract (liquid waste after
filtration) .................................................................................................................. 68
Table 5.3 Comparison of expected and measured arsenic adsorption capacities of metal
hydroxides for all coagulants ................................................................................... 68
Table 6.1 Composition of NSFI-53 challenge water .................................................................... 70
Table 6.2 Mineql+ titration parameters ........................................................................................ 70
Table 6.3 As(V) adsorption capacity of Fe(OH) 3 based on Mineql+ program ............................. 71
ix

FIGURES
Figure 2.1 Coagulation-filtration procedure ................................................................................... 9
Figure 3.1 Mechanism of arsenate ligand exchange on the surface of metal oxyhydroxides ...... 14
Figure 3.2 Removal efficiency of As(V) and As(III) in NSFI challenge water as a function of
ferric chloride dose and pH ................................................................................................... 15
Figure 3.3 Removal efficiency of As(V) and As(III) in NSFI challenge water as a function of
alum dose and pH .................................................................................................................. 16
Figure 3.4 Removal efficiency of As(V) and As(III) in NSFI challenge water as a function of
zirconium(IV) chloride dose and pH .................................................................................... 16
Figure 3.5 Removal efficiency of As(V) and As(III) in NSFI challenge water as a function of
titanium (IV) chloride dose and pH ...................................................................................... 17
Figure 3.6 Removal efficiency of As(V) and As(III) in NSFI challenge water as a function of
titanium (III) chloride dose and pH ...................................................................................... 18
Figure 3.7 Removal/Conversion efficiency of As(III) in NSFI challenge water as a function of
titanium(III) chloride dose and pH ....................................................................................... 19
Figure 3.8 Removal efficiency of As(V) and As(III) in NSFI challenge water as a function of
titanium(IV) oxychloride dose and pH ................................................................................. 20
Figure 3.9 Removal efficiency of As(V) and As(III) in NSFI challenge water as a function of
zirconium(IV) oxychloride coagulant dose and pH .............................................................. 20
Figure 3.10 Comparison of As(V) adsorption isotherms for the in-situ formed oxyhydroxides of
Fe(III), Al(III), Zr(IV), Ti(IV), and Ti(III) at pH 6.5 ........................................................... 21
Figure 3.11 Comparison of As(V) adsorption isotherms for the in-situ formed oxyhydroxides of
Fe(III), Al(III), Zr(IV), Ti(IV), and Ti(III) at pH 7.5 ........................................................... 21
Figure 3.12 Comparison of As(V) adsorption isotherms for the in-situ formed oxyhydroxides of
Fe(III), Al(III), Zr(IV), Ti(IV), and Ti(III) at pH 8.5 ........................................................... 22
Figure 3.13 Comparison of As(III) adsorption isotherms for the in-situ formed oxyhydroxides of
Fe(III), Al(III), Zr(IV), Ti(IV), and Ti(III) at pH 6.5 ........................................................... 22
Figure 3.14 Comparison of As(III) adsorption isotherms for the in-situ formed oxyhydroxides of
Fe(III), Al(III), Zr(IV), Ti(IV), and Ti(III) at pH 7.5 ........................................................... 23
xi

Figure 3.15 Comparison of As(III) adsorption isotherms for the in-situ formed oxyhydroxides of
Fe(III), Al(III), Zr(IV), Ti(IV), and Ti(III) at pH 8.5 ........................................................... 23
Figure 3.16 Comparison of mass As(V) adsorbed per mass of Fe(III), Al(III), Ti(III), Ti(IV), or
Zr(IV) coagulant for an equilibrium concentration of 10 μg/L as a function of pH ............. 24
Figure 3.17 Comparison of mass As(III) adsorbed per mass of Fe(III), Al(III), Ti(III), Ti(IV), or
Zr(IV) coagulant for an equilibrium concentration of 10 μg/L as a function of pH ............. 25
Figure 3.18 Comparison of moles As(V) adsorbed per mol of Fe(III), Al(III), Ti(III), Ti(IV), or
Zr(IV) coagulant for an equilibrium concentration of 10 μg/L as a function of pH ............. 26
Figure 3.19 Comparison of moles As(III) adsorbed per mol of Fe(III), Al(III), Ti(III), Ti(IV), or
Zr(IV) coagulant for an equilibrium concentration of 10 μg/L as a function of pH ............. 26
Figure 3.20 As(III) and As(Tot) removed or converted during Ti(III) coagulation as a function of
flocculation and settling times .............................................................................................. 29
Figure 4.1 Effect of silica on arsenic(V) adsorption at different pHs with in-situ formed Fe(III)
hydroxide in NSFI water with phosphate ............................................................................. 33
Figure 4.2 Effect of silica on arsenic(III) adsorption at different pHs with in-situ formed Fe(III)
hydroxide in NSFI water with phosphate ............................................................................. 33
Figure 4.3 Effect of silica on arsenic(V) adsorption at different pHs with in-situ formed Al(III)
hydroxide in NSFI water with phosphate ............................................................................. 34
Figure 4.4 Effect of silica on arsenic(V) adsorption at different pHs with in-situ formed Zr(IV)
hydroxide in NSFI water with phosphate ............................................................................. 35
Figure 4.5 Effect of silica on arsenic(III) adsorption at different pHs with in-situ formed Zr(IV)
hydroxide in NSFI water with phosphate ............................................................................. 35
Figure 4.6 Effect of silica on arsenic(V) adsorption at different pHs with in-situ formed Ti(IV)
hydroxide in NSFI water with phosphate ............................................................................. 36
Figure 4.7 Effect of silica on arsenic(III) adsorption at different pHs with in-situ formed Ti(IV)
hydroxide in NSFI water with phosphate ............................................................................. 37
Figure 4.8 Effect of silica on arsenic(V) adsorption at different pHs with in-situ formed Ti(III)
hydroxide in NSFI water with phosphate ............................................................................. 37
Figure 4.9 Effect of silica on arsenic(III) adsorption at different pHs with in-situ formed Ti(III)
hydroxide in NSFI water with phosphate ............................................................................. 38
xii

Figure 4.10 Effect of phosphate on arsenic(V) adsorption at different pHs with in-situ formed
Fe(III) hydroxides in NSFI water with silica ........................................................................ 39
Figure 4.11 Effect of phosphate on arsenic(III) adsorption at different pHs with in-situ formed
Fe(III) hydroxides in NSFI water with silica ........................................................................ 40
Figure 4.12 Effect of phosphate on arsenic(V) adsorption at different pHs with in-situ formed
Al(III) hydroxides in NSFI water with silica ........................................................................ 40
Figure 4.13 Effect of phosphate on arsenic(V) adsorption at different pHs with in-situ formed
Zr(IV) hydroxide in NSFI water with silica ......................................................................... 41
Figure 4.14 Effect of phosphate on arsenic(III) adsorption at different pHs with in-situ formed
Zr(IV) hydroxides in NSFI water with silica ........................................................................ 41
Figure 4.15 Effect of phosphate on arsenic(V) adsorption at different pHs with in-situ formed
Ti(IV) hydroxides in NSFI water with silica ........................................................................ 42
Figure 4.16 Effect of phosphate on arsenic(III) adsorption at different pHs with in-situ formed
Ti(IV) hydroxides in NSFI water with silica ........................................................................ 43
Figure 4.17 Effect of phosphate on arsenic(V) adsorption at different pHs with in-situ formed
Ti(III) hydroxides in NSFI water with silica ........................................................................ 44
Figure 4.18 Effect of phosphate on arsenic(III) adsorption at different pHs with in-situ formed
Ti(III) hydroxides in NSFI water with silica ........................................................................ 44
Figure 4.19 Effect of vanadate on arsenic(V) adsorption at different pHs with in-situ formed
Fe(III) hydroxides in NSFI water with silica and phosphate ................................................ 45
Figure 4.20 Effect of vanadate on arsenic(III) adsorption at different pHs with in-situ formed
Fe(III) hydroxides in NSFI water with silica and phosphate ................................................ 46
Figure 4.21 Effect of vanadate on arsenic(V) adsorption at different pHs with in-situ formed
Al(III) hydroxides in NSFI water with silica and phosphate ................................................ 46
Figure 4.22 Effect of vanadate on arsenic(V) adsorption at different pHs with in-situ formed
Zr(IV) hydroxides in NSFI water with silica and phosphate ................................................ 47
Figure 4.23 Effect of vanadate on arsenic(III) adsorption at different pHs with in-situ formed
Zr(IV) hydroxides in NSFI water with silica and phosphate ................................................ 47
Figure 4.24 Effect of vanadate on arsenic(V) adsorption at different pHs with in-situ formed
Ti(IV) hydroxides in NSFI water with silica and phosphate ................................................ 48
xiii

Figure 4.25 Effect of vanadate on arsenic(III) adsorption at different pHs with in-situ formed
Ti(IV) hydroxides in NSFI water with silica and phosphate ................................................ 49
Figure 4.26 Effect of vanadate on arsenic(V) adsorption at different pHs with in-situ formed
Ti(III) hydroxides in NSFI water with silica and phosphate ................................................ 50
Figure 4.27 Effect of vanadate on arsenic(III) adsorption at different pHs with in-situ formed
Ti(III) hydroxides in NSFI water with silica and phosphate ................................................ 50
Figure 4.28 Effect of silica on arsenic(V) adsorption at different pHs with in-situ formed Fe(III)
hydroxides in the absence of phosphate and vanadate .......................................................... 51
Figure 4.29 Effect of silica on arsenic(III) adsorption at different pHs with in-situ formed Fe(III)
hydroxides in the absence of phosphate and vanadate .......................................................... 52
Figure 4.30 Effect of phosphate on arsenic(V) adsorption at different pHs with in-situ formed
Fe(III) hydroxides in the absence of silica and vanadate ...................................................... 53
Figure 4.31 Effect of phosphate on arsenic(III) adsorption at different pHs with in-situ formed
Fe(III) hydroxides in the absence of silica and vanadate ...................................................... 53
Figure 4.32 Effect of vanadate on arsenic(V) adsorption at different pHs with in-situ formed
Fe(III) hydroxides in the absence of silica and phosphate .................................................... 54
Figure 4.33 Effect of vanadate on arsenic(III) adsorption at different pHs with in-situ formed
Fe(III) hydroxides in the absence of silica and phosphate .................................................... 55
Figure 4.34 Comparison of arsenic(V) adsorption capacities of in-situ formed Fe(III) hydroxides
for an equilibrium concentration of 10 μg/L As(V) in the pH range 6.5-8.5 ....................... 56
Figure 4.35 Comparison of arsenic(III) adsorption capacities of in-situ formed Fe(III) hydroxides
for an equilibrium concentration of 10 μg/L As(III) in the pH range 6.5-8.5 ...................... 56
Figure 4.36 Comparison of arsenic(V) adsorption capacities of in-situ formed Al(III) hydroxides
for an equilibrium concentration of 10 μg/L As(V) in the pH range 6.5-8.5 ....................... 57
Figure 4.37 Comparison of arsenic(V) adsorption capacities of in-situ formed Zr(IV) hydroxides
for an equilibrium concentration of 10 μg/L As(V) in the pH range 6.5-8.5 ....................... 58
Figure 4.38 Comparison of arsenic(III) adsorption capacities of in-situ formed Zr(IV) hydroxides
for an equilibrium concentration of 10 μg/L As(III) in the pH range 6.5-8.5 ...................... 58
Figure 4.39 Comparison of arsenic(V) adsorption capacities of in-situ formed Ti(IV) hydroxides
for an equilibrium concentration of 10 μg/L As(V) in the pH range 6.5-8.5 ....................... 59
xiv

Figure 4.40 Comparison of arsenic(III) adsorption capacities of in-situ formed Ti(IV) hydroxides
for an equilibrium concentration of 10 μg/L As(III) in the pH range 6.5-8.5 ...................... 60
Figure 4.41 Comparison of arsenic(V) adsorption capacities of in-situ formed Ti(III) hydroxides
for an equilibrium concentration of 10 μg/L As(V) in the pH range 6.5-8.5 ....................... 60
Figure 4.42 Comparison of arsenic(III) adsorption capacities of in-situ formed Ti(III) hydroxides
for an equilibrium concentration of 10 μg/L As(III) in the pH range 6.5-8.5 ...................... 61
Figure 4.43 Comparison of arsenic adsorption capacities of in-situ-formed Fe(III) hydroxides for
an equilibrium concentration of 10 μg/L As(V) in the pH range 6.5-8.5 ............................. 62
Figure 4.44 Comparison of arsenic adsorption capacities of in-situ formed Fe(III) hydroxides for
an equilibrium concentration of 10 μg/L As(III) in the pH range 6.5-8.5 ............................ 62
Figure 5.1 Preliminary determination of percent solids ................................................................ 66
Figure 6.1 Modeling of As(V) adsorption isotherm onto Fe(OH) 3 (s) at pH 7.5 .......................... 72
Figure 6.2 Mineql+-predicted effect of silica, phosphate, and vanadate on As(V) adsorption on
the in-situ-formed Fe(III) hydroxide at pH 6.5 ..................................................................... 73
Figure 6.3 Mineql+-predicted effect of silica, phosphate, and vanadate on As(V) adsorption on
the in-situ-formed Fe(III) hydroxide at pH 7.5 ..................................................................... 73
Figure 6.4 Mineql+-predicted effect of silica, phosphate, and vanadate on As(V) adsorption on
the in-situ-formed Fe(III) hydroxide at pH 8.5 ..................................................................... 74
Figure 6.5 Mineql+-predicted effect of silica, phosphate, and vanadate on As(III) adsorption on
the in-situ-formed Fe(III) hydroxide at pH 6.5 ..................................................................... 74
Figure 6.6 Mineql+-predicted effect of silica, phosphate, and vanadate on As(III) adsorption on
the in-situ-formed Fe(III) hydroxide at pH 7.5 ..................................................................... 75
Figure 6.7 Mineql+-predicted effect of silica, phosphate, and vanadate on As(III) adsorption on
the in-situ-formed Fe(III) hydroxide at pH 8.5 ..................................................................... 75
Figure 6.8 Comparison of As(V) adsorption capacities of Fe(OH) 3 for an equilibrium
concentration of 10 μg/L at pH 6.5 ....................................................................................... 76
Figure 6.9 Comparison of As(V) adsorption capacities of Fe(OH) 3 for an equilibrium
concentration of 10 μg/L at pH 7.5 ....................................................................................... 77
Figure 6.10 Comparison of As(V) adsorption capacities of Fe(OH) 3 for an equilibrium
concentration of 10 μg/L at pH 8.5 ....................................................................................... 77
xv

Figure 6.11 Comparison of As(III) adsorption capacities of Fe(OH) 3 for an equilibrium
concentration of 10 μg/L at pH 6.5 ....................................................................................... 78
Figure 6.12 Comparison of As(III) adsorption capacities of Fe(OH) 3 for an equilibrium
concentration of 10 μg/L at pH 7.5 ....................................................................................... 78
Figure 6.13 Comparison of As(III) adsorption capacities of Fe(OH) 3 for an equilibrium
concentration of 10 μg/L at pH 8.5 ....................................................................................... 79
xvi

FOREWORD
The Awwa Research Foundation is a nonprofit corporation that is dedicated to the
implementation of a research effort to help utilities respond to regulatory requirements and
traditional high-priority concerns of the industry. The research agenda is developed through a
process of consultation with subscribers and drinking water professionals. Under the umbrella of
a strategic Research Plan, the Research Advisory Council prioritizes the suggested projects based
upon current and future needs, applicability, and past work; the recommendations are forwarded
to the Board of Trustees for final selection. The foundation also sponsors research projects
through the unsolicited proposal process; the Collaborative Research, Research Applications, and
Tailored Collaboration programs; and various joint research efforts with organizations such as
the U.S. Environmental Protection Agency, the U.S. Bureau of Reclamation, and the U.S.
Department of Energy.
This publication is a result of one of these sponsored studies, and it is hoped that its
findings will be applied in communities throughout the world. The following report serves not
only as a means of communicating the results of the water industry’s centralized research
program but also as a tool to enlist the further support of the nonmember utilities and individuals.
Projects are managed closely from their inception to the final report by the foundation’s
staff and large cadre of volunteers who willingly contribute their time and expertise. The
foundation serves a planning and management function and awards contracts to other institutions
such as water utilities, universities, and engineering firms. The funding for this research effort
comes primarily from the Subscription Program, through which water utilities subscribe to the
research program and make an annual payment proportionate to the volume of water they deliver
and consultants and manufacturers subscribe based on their annual billings. The program offers
a cost-effective and fair method for funding research in the public interest.
A broad spectrum of water supply issues is addressed by the foundation’s research
agenda: resources, treatment and operations, distribution and storage, water quality and analysis,
toxicology, economics, and management. The ultimate purpose of the coordinated effort is to
assist water suppliers to provide the highest possible quality of water economically and reliably.
The true benefits are realized when the results are implemented at the utility level. The
foundation’s trustees are pleased to offer this publication as a contribution towards that end.
David Ragner
Chair, Board of Trustees
Awwa Research Foundation
Robert C. Renner
Executive Director
Awwa Research Foundation
xvii

xviii

ACKNOWLEDGMENTS
This report is the product of a collaborative effort between the members of the Arsenic
Water Technology Partnership and was made possible by funds from Congress and the drinking
water community. A special thanks to U.S. Senator Pete Domenici for his support and assistance
in helping to bring low-cost, energy efficient solutions to remove arsenic from drinking water.
The authors kindly thank the following persons for their cooperation and participation in
this project:
1. Albert Ilges, AwwaRF Arsenic Program Project Manager, for his helpful and responsive
attitude and his technical and administrative guidance throughout the course of the
project.
2. Project Advisory Committee members Dr. Abbas Ghassemi and Dr. Malcolm Siegel for
reviewing the reports and for their helpful suggestions and observations throughout the
course of the project.
3. Dr. Issam Najm, President of Water Quality and Treatment Solutions, Inc. for his help
with proposal preparation and for his technical advice on field applications of coagulation
filtration for arsenic removal.
The authors are grateful for the funding provided by the Awwa Research Foundation and the inkind
matching funding provided by the University of Houston, and Water Quality and Treatment
Solutions, Inc.
xix

EXECUTIVE SUMMARY
Coagulation-Filtration (C-F) with Fe salts is an effective and widely used technology
for treating drinking water to remove arsenic. For small systems, however, adsorption onto
granular ferric hydroxide (GFH) or granular ferric oxide (GFO) is currently the process of
choice. In a comparison of C-F vs. adsorption, the arsenic loading, mg of As/g Fe, on the
coagulant was higher than on the solid media, which is unexpected in view of the fact that the
coagulant is in equilibrium with the effluent arsenic concentration (

4. Study the effect of competing ions (silica, phosphate, and vanadate) in NSFI-53 challenge
water on As(V) and As(III) removal during coagulation with ferric, aluminum, titanium,
and zirconium salts,
5. Quantify the individual effects of silica, phosphate and vanadate during coagulation with
FeCl 3 ,
6. Establish the toxicity characteristics of the sludges produced during coagulation with
ferric, aluminum, titanium, and zirconium salts, and compare the results with the
regulatory limits of the TCLP and WET tests, and
7. Determine if the adsorption of As(V) and As(III) onto Fe(OH) 3 could be accurately
modeled using the Mineql+ water chemistry equilibrium model, and compare the model
results with experimental results.
APPROACH
Extensive jar testing of the effectiveness of the aluminum, titanium, and zirconium
coagulants for arsenic removal in comparison with FeCl 3 was conducted. The effects of pH, and
competing ions such as silicate, phosphate, and vanadate on As(III) and As(V) removal were
determined. An existing surface complexation model (Mineql+) was applied to quantitatively
predict the influence of these parameters on arsenic adsorption. Finally, recommendations are
made as to the optimum coagulation-filtration processes suitable for representative small
community ground waters contaminated with arsenic. An outline of the comprehensive,
systematic study of arsenic III/V removal is given below:
1. NSFI challenge water (NSFI Standard-53) spiked with 50 µg/L of As(III) or As(V) was
used as the background water for the isotherm tests. The coagulant stocks were prepared
from the following salts: zirconium(IV) chloride (ZrCl 4 ), zirconium(IV) oxychloride
(ZrOCl 2 ), titanium(IV) chloride (TiCl 4 ,), titanium(IV) oxychloride (TiOCl 2 ), titanium(III)
chloride (TiCl 3 ), alum, and ferric chloride (FeCl 3 ).
2. Coagulation-filtration experiments were performed using jar tests. The predetermined
coagulant dose and the base (NaOH) required to reach the desired equilibrium pH (6.5,
7.5, or 8.5) were added at the start of the experiment. The jar test procedure consisted of
1 min rapid mix at 100 rpm followed by 20 min of slow mixing at 20 rpm. After 1-hr of
settling, the supernatant was filtered through a 0.2-μm filter, and the filtrate was then
preserved for arsenic analysis.
3. Experiments that assessed the effect of the competing ions in the challenge water were
conducted using all five coagulant metals (Fe(III), Al(III), Ti(IV), Ti(III), and Zr(IV)) at
pH 6.5, 7.5, and 8.5, spiked with two levels of silica, phosphate, and vanadate.
4. Experiments that assessed the individual effect of the competing ions were also
conducted using challenge water with Fe(III) as coagulant at three pHs spiked with two
levels of silica, phosphate, and vanadate in the absence of competing ions.
5. Toxicity characteristics of the sludge produced with Fe(III), Al(III), Ti(IV), Ti(III), and
Zr(IV) coagulant were studied according to TCLP and WET procedures and checked if
the toxicity was within regulatory limits.
6. The adsorption of arsenic (As(V) and As(III)) onto Fe(OH) 3 (s) was modeled using the
Mineql+ water chemistry equilibrium model, and the model results were compared with
coagulation experiment results.
xxii

CONCLUSIONS
The comparative study of the removal of arsenic in challenge water with the innovative
coagulants (TiCl 4 , TiCl 3 , TiOCl 2 , ZrCl 4 , and ZrOCl 2 ) compared to commonly used coagulants
(FeCl 3 , alum) resulted in the following conclusions:
1. The percent removal of As(V) was highly pH dependent in the NSFI challenge water,
and the removal increased with decreasing pH for all coagulants tested: FeCl 3 , alum,
TiCl 4 , TiCl 3 , TiOCl 2 , ZrCl 4 , and ZrOCl 2 . In particular, the adsorption capacity of As(V)
with zirconium salts decreased significantly with increasing pH.
2. The percent removal efficiency of As(III) was independent of pH for FeCl 3 , TiCl 4 , ZrCl 4 ,
and ZrOCl 2 , and it decreased with increasing pH for TiCl 3 and increased with increasing
pH for TiOCl 2 . The removal of As(III) by alum was insignificant.
3. At all doses, the removal efficiency of As(V) was significantly greater than As(III) at all
pHs with all seven coagulants tested: FeCl 3 , alum, TiCl 4 , TiCl 3 , TiOCl 2 , ZrCl 4 , and
ZrOCl 2 .
4. When comparing arsenic adsorption isotherms for all the coagulants, the highest As(V)
loadings on a coagulant on a mass basis (mg As(V)/g metal) were observed with FeCl 3 ,
which performed better than aluminum, titanium and zirconium salts at pHs of 6.5 and
7.5. However, at pH 8.5, As(V) loadings on FeCl 3 were approximately the same as TiCl 3
at equilibrium As(V) ≤ 10 μg/L.
5. When comparing adsorption isotherms, the highest As(V) loading on any coagulant on a
molar basis or a mass basis was observed for ferric chloride at all three pHs, and the
As(V) loading on iron was significantly greater than aluminum.
6. The ranking of As(V) adsorption capacities of the iron, aluminum, titanium, and
zirconium salts for an equilibrium concentration of 10 μg/L was as follows:
• pH 6.5: FeCl 3 > Alum > ZrOCl 2 ≈ ZrCl 4 > TiCl 4 > TiCl 3 > TiOCl 2
• pH 7.5: FeCl 3 >> TiCl 3 > TiCl 4 > Alum ≈ ZrOCl 2 > ZrCl 4 > TiOCl 2
• pH 8.5: FeCl 3 ≈ TiCl 3 > TiCl 4 > Alum > ZrCl 4 ≈ TiOCl 2 > ZrOCl 2
7. FeCl 3 was a far better coagulant than alum for As(V) removal on a mass basis and on a
molar basis.
8. When comparing arsenic adsorption isotherms for all the coagulants, the highest As(III)
loading on a coagulant (mg As(III)/g metal) was observed with titanium(III) chloride,
which performed better than ferric, titanium(IV), and zirconium salts at pHs of 6.5 and
7.5. However Ti(III) had similar adsorption capacity to that of Fe(III) and Ti(IV)
coagulants at pH 8.5. Alum did not have any adsorption capacity for As(III). TiCl 4
exhibited similar removal efficiency to that of FeCl 3 , and TiOCl 2 offered similar removal
xxiii

efficiency to FeCl 3 at pH 8.5. Zirconium salts did not have good adsorption capacity for
As(III). Thus, it appears that on an mg metal/L basis, TiCl 3 could be a better coagulant for
As(III) removal in coagulation-filtration processes.
9. On a mass basis, the ranking of As(III) adsorption capacities of iron, aluminum, titanium
and zirconium salts for an equilibrium concentration of 10 μg/L was as follows:
• pH 6.5: TiCl 3 > FeCl 3 ≈ TiCl 4 > TiOCl 2 >> ZrCl 4 ≈ ZrOCl 2 >> Alum
• pH 7.5: TiCl 3 > FeCl 3 ≈ TiCl 4 > TiOCl 2 >> ZrCl 4 ≈ ZrOCl 2 >> Alum
• pH 8.5: TiCl 3 ≈ FeCl 3 ≈ TiCl 4 ≈ TiOCl 2 >> ZrCl 4 ≈ ZrOCl 2 >> Alum
10. The highest As(III) removal/loading on the coagulant on a molar basis was observed for
titanium(III) chloride at pH 6.5 and 7.5, while at pH 8.5, ferric chloride had the highest
molar adsorption capacity. Note: As(III) was oxidized to As(V) by Ti(III).
11. When comparing chemical costs for FeCl 3 , alum, TiCl 4 , ZrOCl 2 , and TiOCl 2 coagulation
to remove As(V) or As(III), the most economical was FeCl 3 . Of the common coagulants,
alum was found to be 4-8 times more expensive than ferric chloride for As(V) removal.
The chemical cost of ferric chloride coagulation was calculated to be more than 5 to 20
times higher for As(III) treatment compared with As(V).
12. There was experimental evidence that the high removal efficiency of As(III) by TiCl 3 and
the unusual As(III) behavior of increasing removal with decreasing pH was due to
oxidation of As(III) to As(V) by H 2 O 2 , which based on the literature, formed from Ti(III)
hydrolysis in the NSFI challenge water that contained some dissolved oxygen. In spite of
its partial oxidation, the experimentally observed removal of As(V) oxidized from As(III)
was far less than the removal of a similar starting concentration of As(V), because (a) the
floc was already formed when it contacted As(V), and (b) the As(III) oxidation continued
for many hours during which the Ti(OH) 3 (s) formed had settled and was not in contact
with the As(V) formed.
The studies on effect of competing ions (silica, phosphate and vanadate) on arsenic
adsorption in NSFI challenge water with FeCl 3 , alum, TiCl 4 , TiCl 3 , and ZrCl 4 as coagulants
resulted in the following conclusions:
13. In the NSFI challenge water with phosphate, silica significantly reduced the adsorption of
arsenic, presumably by competing for adsorption sites. The As(V) and As(III) removal
efficiencies in the absence of silica were higher than in the presence of silica for all
coagulants tested (FeCl 3 , TiCl 4 , TiCl 3 , and ZrCl 4 ). With alum as coagulant, silica
significantly affected As(V) adsorption at pH 6.5-7.5, while it had no significant effect at
pH 8.5. In the NSFI challenge water without silica, the adsorption of As(III) increased
with increasing pH for TiCl 4 , FeCl 3 and ZrCl 4 , whereas pH did not significantly affect
As(III) adsorption on these coagulants in the standard challenge water with silica present.
The detrimental effect of silica on As(III) removal increased with increasing pH for all
coagulants except TiCl 3 .
xxiv

14. In the NSFI challenge water with silica, phosphate was found to reduce the adsorption of
As(V) significantly at pH 6.5 and 7.5, whereas it had a lesser effect at pH 8.5 with the
coagulants tested (FeCl 3 , TiCl 3 , TiCl 4 , and ZrCl 4 ). However, with alum as coagulant,
phosphate did not affect As(V) adsorption. In contrast to As(V) adsorption, the presence
of phosphate did not significantly affect the adsorption of As(III) with the coagulants
tested (FeCl 3 , TiCl 3 , TiCl 4 , and ZrCl 4 ).
15. In the NSFI challenge water with silica and phosphate, vanadate did not significantly
affect the adsorption of As(V) with the coagulants tested (FeCl 3 , alum, TiCl 3 , TiCl 4 , and
ZrCl 4 ). Similarly. in the NSFI challenge water with silica and phosphate, vanadate did
not significantly affect the adsorption of As(III) with the coagulants tested (FeCl 3 , TiCl 3 ,
TiCl 4 , and ZrCl 4 )
The studies on the individual effects of competing ions (silica, phosphate and vanadate)
on arsenic adsorption with FeCl 3 as coagulant resulted in the following conclusions:
16. In the absence of other competing ions, it was found that silica, phosphate, and vanadate
exhibited significant competitive effects on the adsorption of As(V) and As(III).
17. In the absence of phosphate and vanadate, silica exhibited a significant effect on the
adsorption of As(V) and As(III) and the effect increased with increasing pH.
18. In the absence of silica and vanadate, phosphate exhibited a significant effect on the
adsorption of As(V) at all pHs and a minor effect on the adsorption of As(III) at pH 7.5
and 8.5.
19. In the absence of silica and phosphate, vanadate exhibited a significant effect on the
adsorption of As(V) and had a minor effect on the adsorption of As(III) at pH 7.5 and 8.5
during FeCl 3 coagulation.
20. Based on the FeCl 3 experimental results with and without multiple competing
contaminants, the following inferences were made:
• The presence of silica significantly reduced the magnitude of the phosphate effect
on As(V) adsorption at pH 7.5 and 8.5.
• The presence of silica reduced the effect of phosphate in the case of As(III)
adsorption at all pHs.
• The combined presence of silica and phosphate reduced the effect of vanadate at
all pHs in the case of As(V) and As(III) removal.
The toxicity characteristics of the sludges produced during coagulation with FeCl 3 , alum,
TiCl 3 , TiCl 4 , and ZrCl 4 led to the following conclusions:
21. The adsorbent loading of As(V) was independent of initial arsenic concentration with the
coagulants tested (FeCl 3 , alum, TiCl 3 , TiCl 4 , and ZrCl 4 ).
22. Following coagulation and 3-hr settling, with the five coagulants tested (FeCl 3 , alum,
xxv

TiCl 3 , TiCl 4 , and ZrCl 4 ), the liquid wastes obtained were found to contain less than 0.5%
dry solids, and according to the TCLP regulations, did not require extraction. The liquid
filtrate, which is considered to be the extract in such cases, easily passed the TCLP and
WET test regulatory limits of 5 mg/L.
The attempt to model As(V)/As(III) adsorption during coagulation with FeCl 3 using the
chemical equilibrium modeling program, Mineql+, led to the following conclusions:
23. The unmodified Mineql+ chemical equilibrium modeling program could not simulate
As(III) or As(V) adsorption onto Fe(OH) 3 (s) during FeCl 3 coagulation.
24. The model predicts significantly less adsorption onto Fe(OH) 3 (s) compared with
experimental results, and the model also does not take into account the effect of silica,
phosphate and vanadate on As(V)/As(III) adsorption in most cases.
RECOMMENDATIONS
The main purpose of this project was to determine the technical and economic feasibility
of using aluminum, zirconium and titanium salts in comparison with ferric salt as coagulants for
arsenic removal in coagulation-filtration processes. Based on adsorption and economic
comparisons, this work showed that FeCl 3 was clearly superior to the other coagulants tested.
Although Ti(III) had the highest removal efficiency for As(III), its chemical cost was not
available and is expected to be higher considering the cost of Ti(IV) salts. A chemical cost
comparison of commonly used coagulants showed that alum did not remove As(III) and was 4-6
times more expensive than FeCl 3 for As(V) removal. Taking the detailed conclusions above into
consideration, the following recommendations are made for application of coagulation for
arsenic removal in small systems:
1. FeCl 3 should be considered as the preferred coagulant for As(V) and As(III) removal at
all pHs and background water compositions.
2. Alum could be considered as a coagulant for As(V) removal at pH ≤6.5 where its As(V)
capacity is closer to that of FeCl 3 .
3. As(III) should be pre-oxidized to As(V) for cost-effective treatment with alum and FeCl 3
coagulants.
4. Zirconium and/or titanium could be considered as alternative coagulants to alum, if their
prices drop significantly.
xxvi

CHAPTER 1
INTRODUCTION
ARSENIC OCCURRENCE, HEALTH EFFECTS AND REGULATIONS
Inorganic arsenic is considered as a human carcinogen with multiple sites of attack.
Epidemiological studies have demonstrated the higher risks of skin, bladder, lung, liver and
kidney cancer along with other non-cancerous health effects that result from continued
consumption of elevated levels of arsenic in drinking water (Chen et al. 1988, Guha Mazumder
et al. 1998, Ferreccio et al. 2000). Due to the elevated health risk, based on the analysis of the
United States Environmental Protection Agency (USEPA) and two independent reports by the
National Research Council (NRC), USEPA has reduced the maximum contamination level
(MCL) of arsenic in drinking water from 50 to 10 µg/L (USEPA 2001).
Arsenic is a widely distributed element to which humans are exposed through ingestion
of water and food, and by inhalation. People in many countries around the world are exposed to
elevated levels of arsenic in their drinking water. The International Agency for Research on
Cancer as well as the United States Environmental Protection Agency (USEPA) has designated
arsenic as a Group A "known" human carcinogen. There are numerous reports in the literature,
based on past and ongoing experience in various countries in Asia and South America
concerning the high risk of skin, bladder, lung, liver, and kidney cancer along with other noncancerous
ailments that result from continued consumption of elevated levels of arsenic in
drinking water. Indeed, arsenic is the only major demonstrated human carcinogen where the
principle route of human exposure is through drinking water. The most devastating incidence of
arsenic poisoning has been reported in Bangladesh, and West Bengal-India (Chowdhury et al.
2000, Chowdhury et al. 2000a, Das et al. 1995). Reportedly, more than 6 million people in West
Bengal, India, and more than 70 million people in Bangladesh are drinking ground water
containing elevated levels (50 µg/L and above) of arsenic (Chakraborti et al. 2002). In
Bangladesh out of an estimated 6-11 million shallow tube wells, approximately 27% are
contaminated with arsenic above 50 µg/L (Kinniburgh and Kosmus 2002). Here in the USA,
according to the US Geological Survey (USGS) and the USEPA, high concentrations of arsenic
are widespread in Western, Midwestern and Northwestern United States
(http://co.water.usgs.gov/trace/arsenic). The EPA has reported that more than 5.5% of the total
water supply systems in USA contain arsenic at a level greater than the present limit (10 µg/L).
Higher levels of arsenic tend to be found more often in groundwater than in surface water
sources. About 4,100 of the nation’s 54,000 Community Water Supplies and 1,100 of the 20,000
Non-Transient Non-Community Water Supplies exceed the current 10 µg/L limit. According to
the Natural Resources Defense Council (NRDC 2000), over 34 million Americans drink water
that increases the risk of arsenic-related cancer.
IRON COAGULATION-FILTRATION AND ADSORPTION PROCESSES
The most frequently used technology for removing arsenic from drinking water is
precipitation/coprecipitation followed by some form of settling and/or filtration. The process is
commonly referred to as coagulation-filtration. This technology is capable of treating a wide
range of influent concentrations to the revised MCL for arsenic. According to USEPA more than
52% of the identified applications of arsenic treatment technologies for water are based on
1

coagulation. Many studies have been done to examine the efficiency of arsenic removal using
coagulation with ferric and aluminum salts (Hering et al. 1996, Gulledge et al. 1973, Edwards
1994). Coagulant type and dosage, pH, composition of the water, and contaminant type have
significant effects on removal efficiency. It is well accepted that removal of arsenate [As(V)] is
much better than arsenite [As(III)] (Clifford et al. 1990) and that silica interferes with arsenic
removal at higher pH (Tong 1997). For better removal efficiency, pre-oxidation is necessary for
As(III) removal.
Small-scale systems and point-of-entry (POE) systems often use adsorption or ionexchange
(IX) in packed-bed processes for arsenic removal, especially when arsenic is the only
contaminant to be removed. As with iron and aluminum coagulation, numerous factors including
arsenic oxidation state (III or V), pH, competing anions, media particle size, and empty bed
contact time (EBCT) significantly affect arsenic removal by adsorption and IX. However, the
major factors limiting the use of adsorption and IX processes include the high cost of the media,
the complexity of regeneration, backwash and spent-regenerant disposal and spent media
disposal. Due to high arsenic concentrations in the backwash waters and spent regenerant, direct
discharge to a sanitary sewer is usually not acceptable. Therefore, the spent regenerants may
need to be further treated by a precipitation/coagulation process to produce a sludge that can be
thickened and dried prior to disposal. On the other hand, the arsenic-laden sludge produced by
coagulation processes does not require an additional precipitation step, and can be directly dried
and land filled in hazardous or non-hazardous land fills depending on arsenic concentration.
BACKGROUND OF THE STUDY
Significance of In-situ Formed Hydroxides in Arsenic Removal
Previous studies conducted by researchers at the University of Houston (Clifford et al.
1997, Ghurye et al. 2004) reported that Coagulation-Microfiltration with FeCl 3 was effective and
economical compared with adsorption and ion exchange. It was observed that pH and ferric dose
were the most important variables controlling arsenic removal. Experiments were also performed
that compared hydrolyzed in-situ Fe(OH) 3 (s) with preformed ferric hydroxide. Based on arsenic
uptake by the iron oxide/hydroxide surfaces, preformed Fe(OH) 3 (s) was not as effective as ferric
hydroxide formed-in-place (hydrolyzed in situ) by adding FeCl 3 to water and mixing it well. The
reason for the higher arsenic capacity on the formed-in-place coagulant is that the arsenate
anions are sorbed by surface complexation onto the short-chain polymers (oligomers) of
Fe x (OH) y z+ as they are forming into Fe(OH) 3 (s) floc particles, which can be filtered. Preformed
Fe(OH) 3 and granular ferric oxide/hydroxide (GFO/GFH) media simply do not have the
available surface area in comparison to the oligomers and polymers of Fe(OH) 3 (s) that are
formed during Fe 3+ hydrolysis in coagulation processes. This leads to much higher arsenic
loading on the coagulant and a smaller volume of Fe(OH) 3 (s) solids to be wasted in coagulation
compared with adsorption onto GFH/GFO. When the column performance of GFH is compared
with coagulated Fe(OH) 3 (s), an even greater difference in As(V) loading was observed.
Alum as Coagulant for Arsenic Removal
Alum is most widely used coagulant for water treatment in the USA, and it has been
reported to be equally as effective as ferric iron for arsenic adsorption on a molar basis.
2

However, this reported equality of iron and alum is questionable, and has not been shown in
direct coagulant comparisons. Alum, like ferric chloride, is an effective coagulant at pH < 6.5,
because it carries a strong cationic charge within this pH region. At pH > 6.5, alum is only
weakly cationic, and becomes much less effective for adsorbing anions. Sorg and Logsdon
(1978) demonstrated that arsenic removal with alum coagulation is most effective at pH 5 to 7.
Edwards (1994) reported that ferric and alum coagulation are equally effective on a molar basis
and at significant coagulant dosages, As(V) removal was similar for both alum and ferric
coagulants at pH 7.6 or lower. Ahmed and Rahaman (2000) also reported effective removal with
alum coagulation in the pH range of 7.2-7.5. McNeil & Edwards (1997) reported that aluminum
and iron flocs have equal adsorption capacity, but the aluminum hydroxide flocs with sorbed
arsenic can pass through 0.45-μm filters and decrease apparent arsenic removal by alum
coagulation. If microfiltration pore size influences arsenic removal, then arsenic removal by
alum coagulation followed by 0.2-μm membrane filtration as used in this research should
produce improved arsenic removal. As will be seen, this was not the case.
Zirconium Oxide/Hydroxide Adsorbent Media for Arsenic Removal
Zirconium-based adsorbents for arsenic removal have appeared in the literature and in the
market place. The tests have verified that zirconium-based dry powder adsorbent media are
effective for arsenic removal from drinking water and have significant advantages over the
highest quality alumina. Zirconium based adsorbents have been reported to have high capacity
for arsenic (Manna et al, 1999; Suzuki et al, 2000; Zhu et al, 2001; Clarke et al, 2002; Daus et al,
2004). Similarly hydrated titanium oxide/hydroxide powders and nanocrystalline TiO 2 have also
been shown to have high adsorption capacity for As(III) and As(V) in the pH 6.5 – 8.6 range
(Bissen et al, 2001; Lee et al, 2002; Dutta et al, 2004; Bang et al, 2005; Ferguson et al, 2005;
Meng et al, 2005; Pena et al, 2005). Thus, in addition to iron-based media, zirconium and
titanium based media are now being used for arsenic removal.
Basis of the Project
It has been shown that hydrolyzed in situ Fe(III) coagulants have a significant arseniccapacity
advantage over preformed coagulants and granular media such as GFH and GFO. It has
also been shown that zirconium oxide/hydroxide and titanium oxide/hydroxide powders and fine
crystals are good-excellent adsorbents for arsenic. Because of the higher arsenic capacity of
coagulants and the growing popularity of zirconium and titanium oxyhydroxides for arsenic
removal a comparative study of iron vs. titanium and zirconium coagulants seemed warranted.
The higher-arsenic-loading advantage of in-situ formed iron hydroxides during coagulation
compared with sorption onto iron hydrous oxide/hydroxides was also expected to occur with
alternative metal coagulants such as zirconium and titanium. Alum was also studied because of
(a) its wide used as a coagulant for water treatment, and (b) its reportedly equal effectiveness for
As(V) removal on a molar basis compared with iron-based coagulant. The suggested equal
effectiveness of alum and iron coagulants for arsenic removal has been called into question by
recent comparisons of iron- and aluminum-based sorbents (Wu, 2001) that demonstrate that ironbased
sorbents have far greater As(V) capacity compared with activated alumina. Thus it is
logical to do a comparative study of the soluble aluminum, zirconium and titanium salts as
coagulants in comparison with iron salt for As(III) and As(V) removal. As is the case with
3

Fe(OH) 3 (s), the arsenic-adsorption capacity of Al, Zr and Ti oxides/hydroxides will depend on
the presence of hydroxyl groups. These groups are highly reactive, and the loss of hydroxyl
groups leads to a loss of surface complexation sites such that the media becomes less efficient.
Hydrolyzed-in-place (coagulated) Al, Zr and Ti hydroxides will have a greater number of
available hydroxyl groups compared with the crystalline or powder compounds. So, the
adsorption capacity of crystalline or powder materials will be lower than the freshly prepared Al,
Zr and Ti amorphous oxides/hydroxides. Additional disadvantages of granular and crystalline
media such as poor kinetics of adsorption and the dependence of adsorption capacity on crystal
structure can be overcome in the coagulation process.
Although zirconium and titanium may be considered rare and expensive elements by
professionals in the water utility industry, such is not the case as titanium and zirconium are the
10th and the 20th most-abundant chemical in the earth’s crust, respectively. In fact, titanium is
more common than chlorine, phosphorus, manganese, sulfur, and chromium. Zirconium is nearly
as abundant as fluorine and nitrogen (Masterson et al. 1973). As already pointed out, zirconiumand
titanium-based media are already being marketed for arsenic removal from drinking water in
small systems and POU/POE systems. With regard to coagulation, zirconium and titanium
chemicals are available as water hydrolysable salts including TiCl 4 , TiCl 3 , TiOCl 2 , ZrCl 4 , and
ZrOCl 2 which could be used for studies. Aqueous solutions of these chemicals are also available
commercially.
OBJECTIVES
The objective of this work was to determine the technical and economic feasibility of
using aluminum, zirconium and titanium salts in comparison with ferric salt as coagulants for
arsenic removal in coagulation filtration processes. To attain the main objective, the following
specific tasks were carried out:
1. Study the removal efficiency of As(V) and As(III) in NSFI-53 challenge water with
aluminum, titanium and zirconium coagulants and compare the removal efficiency with
FeCl 3 .
2. Study the effect of pH on As(III) and As(V) removal with each of the coagulants.
3. Study the effect of competing ions such as silicate, phosphate, and vanadate on As (III)
and As(V) removal in NSFI challenge water.
4. Study the individual effect of competing ions such as silicate, phosphate, and vanadate on
As(III) and As(V) removal.
5. Test the toxicity characteristics of the sludge produced with each coagulant metal and
check with TCLP and WET sludge test regulatory limits.
6. Apply readily available surface complexation models to quantitatively predict the arsenic
adsorption and compare the model results with experimental results.
ANTICIPATED PRACTICAL BENEFITS OF THE PROJECT
Alum and ferric chloride are the most commonly used coagulants in drinking water
treatment. Alum is the coagulant preferred by many water utility managers and operators because
of industry experience with alum, as well as its effectiveness, availability, purity, and cost. The
controversies surrounding the reportedly equal-arsenic-removal effectiveness of alum and iron
4

coagulants on a molar basis will be resolved. The water supply community will benefit from the
results of this innovative coagulant research by having access to potentially more effective
zirconium and titanium coagulants for arsenic removal. Although these metals are not well
known in water treatment, they are 10 th and 20 th in occurrence in the earth’s crust, i.e., very
common and potentially low cost, and non-toxic by nature. Prior to carrying out this research it
was expected that zirconium and/or titanium coagulants would have the following advantages
over ferric coagulants: (As will be seen, most of these advantages could not be demonstrated.)
(1) Higher As(III) and As(V) loadings on the Zr(OH) 4 (s), Ti(OH) 3 (s), Ti(OH) 4 (s)
precipitates compared with Fe(OH) 3 (s). This means less solid waste to send to landfills.
(2) Lower costs for coagulation and filtration resulting from lower doses of coagulants.
(3) Less sensitivity to pH and interference by competing ions including: hydroxide,
silicate, vanadate, and phosphate.
(4) Potentially simpler process design by the elimination of the As(III) oxidation step for
coagulants that adsorb As(III) as well as or better than As(V). The claim of better
As(III) adsorption has been made for titanium oxide/hydroxide solid media.
(5) Less sensitivity to reducing (low E H ) conditions in the landfills that ultimately receive
the arsenic-contaminated sludge. Fe(OH) 3 (s) is susceptible to reduction to Fe(II) and
As(V) and dissolution of the precipitate with the subsequent release of arsenic. It is
anticipated that Zr(IV), Ti(IV) and Ti(III) hydroxides will not be easily reduced, and
even if they are, the resulting hydroxides will remain insoluble and prevent the release
of adsorbed arsenic III and V. It is also expected that these innovative hydroxides will
fare better in passing the California WET procedure for arsenic-contaminated sludge.
(6) Faster-settling sludge in the case of denser Zr(OH) 4 (s) in the event settling is used as a
pretreatment for filtration.
(7) Less-staining potential of white as-opposed-to the red-brown sludge that is produced by
ferric coagulation.
5

REAGENTS AND STOCKS
CHAPTER 2
MATERIALS AND METHODS
All reagents used were of analytical reagent grade. Primary standards of 100 mg As/L of
each species were prepared from arsenic trioxide (As 2 O 3 ) for As(III) and sodium arsenate for
As(V) (Na 2 HAsO 4 ) both from Sigma Chemical Co, Mo. The As(V) and As(III) stock solutions
(100 mg/L) were prepared and stored in bottles. The stock solutions were then used for spiking
the arsenic into the challenge water. Working standard solutions were prepared daily with proper
dilution.
To prepare challenge water, the following salts were used NaNO 3 , NaHCO 3 ,
Na 2 HPO 4·H 2 O, NaF, Na 2 SiO 3·9H 2 O, MgSO 4·7H 2 O, CaCl 2·2H 2 O, and NaVO 3 . All the chemicals
were purchased from Sigma Chemical Co or EM Science. Concentrated stocks were prepared
from these salts and used for the preparation of challenge water on the day of experiment.
The NSFI challenge water and the coagulants were prepared on the day of arsenic
analysis. Citric/citrate buffer solution was prepared using 2 M citric acid and pH was adjusted to
5.0 using NaOH. A 4-mg/mL solution of L-cysteine (Sigma Chemical Co, Mo.) in HCl solution
was used to reduce As(V) to As(III). Sodium tetrahydroborate (EM Science, Germany) solutions
were prepared fresh daily, and were supplemented with sodium hydroxide.
PREPARATION OF NSFI-53 CHALLENGE WATER
For all coagulation studies, NSFI-53 challenge water (hereafter referred to as challenge
water) whose composition is given in Table 2.1 was prepared and used as the background water
for the isotherm tests This water contains realistic concentrations of background contaminants
such as silica, sulfate, phosphate, fluoride, and hardness, which are known to affect the arsenic
capacity of adsorbents. Stability of the challenge water was not an issue because it was prepared
on the day of experiment, and studies conducted at University of Houston showed that the
challenge water was stable (did not precipitate calcium) for at least a week (Tripp, 2000;
Swaminathan, 2005). The pH of the synthetic groundwater was adjusted by using dilute HCl or
NaOH solution. The required amount of As(III) or As(V) was spiked to the challenge water for
the coagulation studies.
PREPARATION OF COAGULANT STOCK
The coagulants used in this study were reagent-grade anhydrous ferric chloride (FeCl 3 ),
alum (Al 2 (SO 4 ) 3·18H 2 O), anhydrous zirconium (IV) chloride (ZrCl 4 ), titanium (IV) chloride
solution (TiCl 4 ), titanium (III) chloride solution (TiCl 3 ), titanium (IV) oxychloride (TiOCl 2 ), and
zirconium (IV) oxychloride (ZrOCl 2 ). Table 2.2 lists the approximate amount of salts used for
the preparation of 1000 mg/L of the concentrated stock. Stock solutions were prepared for a
concentration of 1000 mg/L with DI water except in the case of TiCl 4 . A solution of 1000 mg/L
titanium (IV) chloride was prepared by adding TiCl 4 solution to 1-L solution of 0.25 N
hydrochloric acid solution to avoid the precipitation of meta-titanic acid in the stock solution.
The stock solutions were prepared on the day of the experiment and the exact concentration of
the coagulant metals in the stock were then analyzed using ICP or AAS.
7

Table 2.1
Composition of NSFI 53 challenge water
Cation MW meq/L mg/L Anion MW meq/L mg/L
Ca 2+ 40.1 2.00 40.1
-
HCO 3 61.0 3.00 183.
Mg 2+ 24.3 1.04 12.6
2-
SO 4 96.1 1.04 50.0
Na + 23.0 3.86 88.9 Cl - 35.5 2.00 71.0
NO 3 -N 14.0 0.143 2.00
F - 19.0 0.053 1.00
PO 4 -P 31.0 0.001 0.04
SiO 3 -SiO 2 60.1 0.67 20.0
As(III)/(V) 74.9 0.05
Σ = 6.90 142 Σ = 6.90 327
Table 2.2
Composition of reagents in preparation of coagulant stock for coagulation study
Reagents
Target concentration in
stock
Amount of reagent added
Solid FeCl 3 1000 mg/L of Fe 3+ 2.9 g/L in DIW
Solid Alum 1000 mg/L of Al 3+ 12.34 g/L in DIW
TiCl 4 , Soln.* 1000 mg/L of Ti 4+ 2.3 ml/L in 0.25N HCl
Solid ZrCl 4 1000 mg/L of Zr 4+ 2.555 g/L in DIW
TiCl 3 , Soln.* 1000 mg/L of Ti 3+ 26.83 ml/L in DIW
TiOCl 2 , Soln.* 1000 mg/L of Ti 4+ 2.3 ml/L in DIW
Solid
ZrOCl 2 .8H 2 O
1000 mg/L of Zr 4+ 3.54 g/L in DIW
* Note: The Ti concentrations of the original solutions were verified by flame AAS analysis.
COAGULATION-FILTRATION PROCEDURE
NSFI 53 challenge water spiked with 50 µg/L of As(III) or As(V) was prepared, and the
pH of the challenge water was adjusted to the desired pH in the 6.5 to 8.5 range by using dilute
HCl and NaOH solutions. Since the coagulants used were highly acidic, they reduced the pH of
the challenge water after dosing, thus it was necessary to bring the challenge water to the initial
pH after addition of coagulant. Pre-titrations of the challenge water dosed with varying amounts
of coagulant were performed to determine the amount of sodium hydroxide (NaOH) solution
required to achieve the desired equilibrium pHs at the different dosages.
The coagulation study was conducted using batch-scale jar tests. A six-position flat-blade
stirring apparatus manufactured by Phipps and Bird, USA was used. 2-L square glass jars were
8

used for each experiment. Before each experiment, 1-L of challenge water adjusted to a specific
pH (6.5, 7.5, or 8.5) was added to each jar. The coagulant dose used for As(V) and As(III)
removal was in the range of 0 to 10 mg/L of the metal. With the stirring apparatus preset at the
rapid mix speed of 100 rpm (G = 100 s -1 ), the stirring apparatus was started and a predetermined
amount of coagulant was added to each jar. Immediately after addition of coagulant, the
predetermined amount of NaOH solution was added to restore the challenge water to the initial
pH. After rapid mixing for 1 min, the jars were stirred slowly at 20 rpm (G = 14 s -1 ) for 20 min,
and then the precipitate was allowed to settle for 1 hour. During slow mixing, the pH of
challenge water was checked and adjusted to the initial pH by adding more NaOH, if necessary.
The final pH was measured after the settling period, and was typically within ± 0.15 units of the
initial pH. Figure 2.1 shows the steps involved in the coagulation-filtration process.
PRESERVATION AND SPECIATION
After settling, a supernatant water sample was collected using a 30-mL polyethylene
syringe, and then the water sample was filtered through a 0.2-μm pore size filter. Samples (30-
mL) were collected in 50-mL centrifuge tubes and acidified with 30 μL of concentrated nitric
acid to measure total arsenic (final conc. 0.1%, pH < 2). All samples were analyzed within a
week. For arsenic(III) measurement, 10-mL filtered sample was collected in a 15-mL centrifuge
tube containing 0.432 mL of 2 M acetic acid and 0.134 mL of 0.1 M EDTA as required by the
recently developed field speciation method for inorganic arsenic speciation (Clifford et al. 2004).
Rapid mix at 100 rpm for 1-min then
flocculate at 20 rpm for 20-min
Settle for 1 hr
Jar test apparatus
Analyze
sample by
FI-HG-AAS
Preserve with
HNO 3 for total
Arsenic analysis
Preserve with
EDTA-Acetic acid for
Arsenic(III) analysis
Filter supernatant
(0.2 μm filter)
Figure 2.1 Coagulation-filtration procedure
9

Table 2.3
Experimental conditions for the determination of As(III) and As(Tot) by FI-HG-AAS
Parameters Perkin-Elmer (Zeeman 5000)
Lamp
8 W EDL
Wavelength
193.7 nm
Slit
0.7 nm(low)
Sample volume 500 µL
HCl concentration
0.02 M
Citric/citrate buffer (pH 5.0) 2.0 M
HCl/citrate buffer flow rate 8 ml/min
NaBH 4 concentration (for As(Tot)) 0.4% in 0.2% NaOH
NaBH 4 concentration [for As(III)] 0.2% in 0.05% NaOH
NaBH 4 flow rate
5 ml/min
Carrier gas
Argon
Carrier gas flow rate
60-70 ml/min
Quartz cell temp
~900°C, Electrically heated
INSTRUMENTATION
The arsenic analyses were carried out by flow injection hydride-generation atomic
absorption spectroscopy (FI-HG-AAS) using a Perkin-Elmer (Model Zeeman 5000) atomic
absorption spectrometer (AAS) equipped with an electrodeless discharge lamp (EDL) operated at
8W from an external power supply. The AAS was coupled with a Perkin-Elmer Flow Injection
(FIAS-100) unit for hydride generation for the determinations of As(III) and As(Tot). Detail
descriptions of the instrumental conditions are given in Table 2.3. The samples for As(Tot) and
speciated As(III) were analyzed by FI-HG-AAS.
Total iron, aluminum, and titanium analyses were performed by flame atomic absorbance
spectrometry (Flame AAS). Samples were analyzed by direct aspiration into an air-acetylene
flame in the case of iron analysis and into a nitrous oxide-acetylene flame in the case of titanium
and aluminum analyses. Zirconium analysis was performed using an Inductively Coupled
Plasma-Mass Spectrometer (ICP-MS). The stock solutions were diluted and acidified before
analysis, and the calibration standards were prepared in the same matrix as that of the sample.
TOTAL ARSENIC AND ARSENIC AND ARSENIC(III) ANALYSIS USING FI- HG-AAS
To determine total arsenic (As(Tot) = As(III) + As(V)), samples were treated with L-
cysteine in 2 M HCl. The samples were kept for 15 minutes at room temperature for the
reduction of As(V) to As(III) and then diluted to such a volume as to maintain the concentrations
of L-cysteine, and acid to 4 mg/mL, and 0.02 M, respectively. The arsenic concentration was
measured by FI-HG-AAS against arsenic standards prepared as samples. Five standards were
prepared in the range of 0 to 6 μg/L arsenic, and the standards were periodically checked during
analysis. Samples were diluted to the range of 1 to 6 μg/L arsenic. The arsenic concentration of
the sample was then obtained from the calibration curve, derived by analysis of the absorbance
10

of the five different arsenic standards. Reduced arsenic sample was injected by means of a rotary
valve fitted with a 500µL sample loop into the stream of 0.02 M HCl solution which was flowing
at 8 mL/min. The injected sample together with carrier solution, met subsequently with a
continuous stream of 0.4% sodium tetrahydroborate in 0.2% NaOH flowing at 5 mL/min. After
mixing with sodium tetrahydroborate, the generated hydride (AsH 3 ) subsequently entered into
the gas-liquid separator. Inside this apparatus a continuous flow of argon carrier gas (70-80
mL/min) carried the hydride to the quartz tube fitted on an electrically heated heater at 900 ºC.
Arsenic adsorption was measured at 193.7 nm. At least triplicate measurements were
made for each sample and standard. For the determination of As(III) in the presence of As(V),
the samples were not treated with L-cysteine and the carrier HCl solution was replaced by 2 M
citric/citrate buffer of pH 5.0. Arsine was generated using 0.2% sodium tetrahydroborate in
0.05% NaOH. Under this condition only As(III) generates AsH 3 and As(V) concentration is not
measured. The As(V) was then calculated from the difference of As(Total) and As(III).
FREUNDLICH ISOTHERM
Equilibrium isotherms are constant temperature plots of the mass of contaminant
adsorbed per unit mass of adsorbent versus the concentration of the contaminant. In order to use
isotherms to estimate the mass adsorbed, equilibrium must be reached between the sorbent and
the sorbate. Although the Freundlich isotherm equation has a theoretical basis and can be derived
assuming a Boltzmann distribution of adsorption site energies and multi-layer adsorption, it is
used here only as an empirical curve fitting technique. The Freundlich equation is shown in
Equation 2.1.
q e
KC e
1/ n
= (2.1)
where q e = Mass of arsenic adsorbed per mass of the adsorbent, μg/mg
K = Freundlich constant indicative of adsorption capacity of adsorbent, L/mg
C e = Equilibrium concentration of arsenic in the liquid phase, μg/L
1/n = Freundlich exponent, a constant
Plotting the Freundlich isotherms for various pH and competing-ion conditions allows a
quick comparison of the effectiveness of arsenic removal under the various pH and competing
ion conditions.
11

CHAPTER 3
As(III) VS. As(V) REMOVAL
INTRODUCTION
It has been found in several studies of arsenic occurrence that only, inorganic arsenite
(As(III)), and arsenate (As(V)) are important in ground water supplies (Andrae 1977, Irgolic
1982, etc). As(V) is the thermodynamically stable species and As(III) is the metastable species.
In the natural pH range of 6 to 9, As(III) is mostly present as H 3 AsO 3 , while As(V) exists either
as H 2 AsO 4 - or HAsO 4 2- . Table 3.1 presents the equilibrium constants (pK a ’s) of arsenic and
arsenious acids. As(V), the oxidized form of arsenic is more easily removed from water by ion
exchange, adsorption, and membrane processes compared with As(III), which, in general,
requires pre-oxidation.
Various mechanisms explain contaminant removal in the coagulation process. The major
mechanisms are as follows: (1) compression of the double layer, (2) adsorption to produce
charge neutralization, (3) enmeshment in precipitates, (4) adsorption to permit interparticle
bridging, (5) surface precipitation, (6) ligand exchange-surface complexation, and (7) hydrogen
bonding.
In coagulation treatment, trace inorganic contaminants, are removed by the sorption onto
the surfaces of freshly formed metal hydroxides/oxyhydroxides (M x (OH) y + ), which are formed
upon metal coagulant addition and hydrolysis reaction. Stumm (1992) reported anion removal by
metal oxyhydroxide surfaces by the process of ligand exchange (Figure 3.1). During ligand
exchange, the ligand H 2 AsO 4 - replaces two hydroxides and forms a bidendate surface complex
on the metal oxyhydroxide surface. This reaction is facilitated by low pH and excess hydrogen
ions, which consume the hydroxides released. Silicate, phosphate, and vanadate, three common
competing anions that could compete for adsorption sites are shown in the figure. The surface
complex formed between the metal and the arsenate anion is very strong and not easily reversed
except at high pH when the reaction is reversed due to the presence of hydroxide ions, which are
ligands highly preferred by the central-atom metals.
Table 3.1
Equilibrium constants for arsenic and arsenious acids
Concentration of species
Arsenic pK a values Species
(μmol/L)
pH 6.5 pH 7.5 pH 8.5
Arsenic acid,
C T = 0.67 μM (50 μg/L)
Arsenous acid,
C T = 0.67 μM (50 μg/L)
Source: Schecher 1998.
pK a1 = 2.26
pK a2 = 6.79
pK a3 = 11.29
pK a1 = 9.23
pK a2 = 12.1
H 2 AsO 4
-
HAsO 4
2-
0.5 0.15 0.02
0.17 0.52 0.65
H 3 AsO 3 0.67 0.66 0.57
H 2 AsO 3
-
0.001 0.01 0.09
13

Figure 3.1 Mechanism of arsenate ligand exchange on the surface of metal oxyhydroxides
In addition to producing competing hydroxide ions, an increase in pH decreases the
fraction of positively charged adsorption sites on the metal hydroxide surface resulting in lesser
adsorption of negatively charged As(V) species. With decrease in pH, the concentration of
monovalent As(V) (H 2 AsO 4 - ) increases significantly from 0.02 to 0.5 μΜ from pH 8.5 to 6.5
(Table 3.1) which would result in greater removals of As(V) due to the process of ligand
exchange at low pH. Also, as can be seen in Table 3.1, with increasing pH, the concentration of
monovalent As(III) (H 2 AsO 3 - ) increases which would result in greater removals of As(III) due to
the process of ligand exchange at high pH. This trend of increasing As(III) removal with
increasing pH is offset by the increasing competition from hydroxide ions at the higher pH.
Iron coagulation followed by flocculation and filtration (Cheng et.al. 1994) or iron
coagulation-filtration without flocculation (Ghurye et.al. 2004) have been shown to be effective
technologies for removing arsenic from drinking water. The arsenic removal efficiency depends
on the pH, As(III/V) speciation, and coagulant dose. Among the coagulants, ferric chloride and
alum are the one most commonly used due to widespread use, availability, low cost, and high
capacity for arsenic. As mentioned, the purpose of the research was to test the effectiveness of
Al(III), Ti(IV), Ti(III), and Zr(IV) salts for coagulation-filtration in comparison with Fe(III) as
the benchmark coagulant for As(V) and As(III) removal.
ARSENIC REMOVAL STUDY PROCEDURE
Challenge water spiked with 50 µg/L of As(III) or As(V) was used in the coagulation
experiments, which were performed at pH 6.5, 7.5, and 8.5 with the following coagulants: FeCl 3 ,
alum, TiCl 3 , TiCl 4 , TiOCl 2 , ZrCl 4 , and ZrOCl 2 .
Arsenic Removal Using Ferric (III) Chloride
The removal efficiencies of As(V) and As(III) in NSFI challenge water using ferric
chloride as a function of coagulant dose and pH are shown in Figure 3.2, which indicates that
14

emoval efficiency of As(V) was pH dependent, whereas As(III) removal efficiency was
independent of pH. With increase in pH, the removal efficiency of As(V) decreased. The
percentage removals of As(V) with 1 mg/L Fe(III) dosage at pHs 6.5, 7.5, and 8.5 were 90, 76,
and 46%, respectively. At 2 mg/L Fe(III) dosage, approximately 26% of As(III) was removed
irrespective of pH. It can be observed from Figure 3.2 that the removal efficiency of As(V) was
much higher than As(III) at all pH values.
Arsenic Removal Using Alum
The removal efficiencies of As(V) and As(III) in NSFI challenge water using alum as a
function of coagulant dose and pH are shown in Figure 3.3, which indicates that removal
efficiency of As(V) was pH dependent, and no significant As(III) removal was observed at any
pH. With increase in pH, the removal efficiency of As(V) decreased. The percentage removals of
As(V) with 1 mg/L Al(III) dosage at pHs 6.5, 7.5, and 8.5 were 85, 49, and 20%, respectively,
which were less than the removal for the same Fe(III) dose. The removal efficiency of As(V)
during Al(III) coagulation was more influenced by pH, and the removals decreased significantly
with increase in pH. When alum was used as coagulant, no significant As(III) removal was
observed at any dose or pH. The removal efficiencies of As(V) and As(III) with alum were lower
than with ferric chloride.
% removal of As(V) & As(III)
100
80
60
40
20
0
pH 6.5-As(V)
pH 7.5-As(V)
pH 8.5-As(V)
pH 6.5-As(III)
pH 7.5-As(III)
pH 8.5-As(III)
0 2 4 6 8 10 12
Dose of FeCl 3 as Fe(III) (mg/L)
Figure 3.2 Removal efficiency of As(V) and As(III) in NSFI challenge water as a function of
ferric chloride dose and pH
15

% removal of As(V) & As(III)
100
75
50
25
0
pH 6.5-As(V)
pH 7.5-As(V)
pH 8.5-As(V)
pH 6.5-As(III)
pH 7.5-As(III)
pH 8.5-As(III)
0 2 4 6 8 10
Dose of Alum as Al(III) (mg/L)
Figure 3.3 Removal efficiency of As(V) and As(III) in NSFI challenge water as a function of
alum dose and pH
% removal of As(V) & As(III)
100
80
60
40
20
0
pH 6.5-As(V)
pH 7.5-As(V)
pH 8.5-As(V)
pH 6.5-As(III)
pH 7.5-As(III)
pH 8.5-As(III)
0 2 4 6 8 10 12 14
Dose of ZrCl 4 as Zr(IV) (mg/L)
Figure 3.4 Removal efficiency of As(V) and As(III) in NSFI challenge water as a function of
zirconium(IV) chloride dose and pH
Arsenic Removal Using Zirconium (IV) Chloride
The removal efficiencies of As(V) and As(III) in NSFI challenge water using
zirconium(IV) chloride as a function of coagulant dose and pH are shown in Figure 3.4, which
demonstrates that As(V) removal was highly pH dependent, whereas As(III) removal was
independent of pH. The removal efficiency of As(V) during Zr(IV) coagulation was more
influenced by pH than was Fe(III) coagulation. With increase in pH, the removal efficiency of
As(V) decreased. The percentage removal of As(V) with 1 mg/L Zr(IV) dosage at pHs 6.5, 7.5,
and 8.5 were 62, 28, and 8%, respectively, which were significantly less than the removal for the
16

same Fe(III) dose. With 2 mg/L Zr(IV) dosage, approximately 8% of As(III) was removed
irrespective of pH, which was significantly less than the removal for the same Fe(III) dose. So
the removal efficiencies of As(V) and As(III) with zirconium(IV) chloride were lower than with
ferric chloride. It can be observed from Figure 3.4 that the removal efficiency of As(V) was
much higher than As(III) at all pH values.
Arsenic Removal Using Titanium (IV) Chloride
The removal efficiencies of As(V) and As(III) in NSFI challenge water as a function of
coagulant dose and pH titanium (IV) chloride are shown in Figure 3.5, which demonstrates that
As(V) removal was highly pH dependent, whereas As(III) removal was independent of pH. With
increase in pH, the removal efficiency of As(V) decreased. The percent removals of As(V) with
1 mg/L Ti(IV) dosage at pHs 6.5, 7.5, and 8.5 were 54, 40, and 26%, respectively, which were
significantly less than the removal for the same Fe(III) dose. With 2 mg/L Ti(IV) dosage,
approximately 26% of As(III) was removed irrespective of pH, which was the same with Fe(III).
So the removal efficiencies of As(V) in the presence of titanium (IV) chloride were lower than in
the presence of ferric chloride, but the removal efficiency of As(III) was similar to that of ferric
chloride. It can be observed from Figure 3.5 that the removal efficiency of As(V) was much
higher than As(III) at all pH values.
% removal of As(V) & As(III)
100
80
60
40
20
0
pH 6.5-As(V)
pH 7.5-As(V)
pH 8.5-As(V)
pH 6.5-As(III)
pH 7.5-As(III)
pH 8.5-As(III)
0 2 4 6 8 10 12 14 16
Dose of TiCl 4 as Ti(IV) (mg/L)
Figure 3.5 Removal efficiency of As(V) and As(III) in NSFI challenge water as a function of
titanium (IV) chloride dose and pH
17

Arsenic Removal Using Titanium (III) Chloride
The removal efficiencies of As(V) and As(III) in NSFI challenge water as a function of
coagulant dose and pH using titanium (III) chloride are shown in Figure 3.6, which demonstrates
that both As(V) and As(III) removal were highly pH dependent. As was observed with the other
coagulants, the removal efficiency of As(V) decreased with increasing pH. The percent removals
of As(V) with 1 mg/L Ti(III) dosage at pHs 6.5, 7.5, and 8.5 were 49, 37, and 30%, respectively,
which were significantly less than the removals for the same Fe(III) dose.
The removal efficiency of As(III) by Ti(III) increased with decreasing pH specifically for
As(III) removal. The removal efficiencies were 42, 32, and 27% at pH 6.5, 7.5 and 8.5,
respectively for a dose of 2 mg/L Ti(III). Thus the removal efficiency with titanium (III) chloride
was higher than ferric chloride, which had a removal efficiency of 26% and was pH independent
for all dosages.
An interesting observation made during the titanium (III) chloride coagulation tests was
that, based on speciation of the sample after coagulation As(III) appeared to be oxidized to
As(V). The percent change or removal efficiency of As(III) species was found to be much higher
than the removal efficiency of As(Total), which is shown in Figure 3.7. With a Ti(III) dose of 2
mg/L, the arsenic removal efficiencies calculated as total arsenic were 42, 32, and 27% at pH
6.5, 7.5, and 8.5, respectively, whereas the As(III) that was removed or converted was
approximately 70% in all cases. This was possible presumably As(III) was efficiently oxidized
by Ti(III) to As(V), whereas the As(V) produced was not as efficiently removed by adsorption
onto the hydrolyzed Ti(III) oxyhydroxide. Further oxidation studies were carried out to elucidate
the mechanism of As(III) oxidation during Ti(III) coagulation, and the results are discussed later
in this chapter.
In summary, the removal efficiency of As(V) during titanium(III) chloride coagulation
was lower than during ferric chloride coagulation while the removal efficiency of As(III) during
titanium(III) chloride coagulation was higher than that of ferric chloride.
% removal of As(V) & As(III)
100
75
50
25
0
pH 6.5-As(V)
pH 7.5-As(V)
pH 8.5-As(V)
pH 6.5-As(III)
pH 7.5-As(III)
pH 8.5-As(III)
0 3 6 9 12
Dose of TiCl 3 as Ti(III) (mg/L)
Figure 3.6 Removal efficiency of As(V) and As(III) in NSFI challenge water as a function of
titanium (III) chloride dose and pH
18

100
As(III) change/removal in %
80
60
40
20
0
pH 6.5, As(III) removed+oxidized
pH 7.5, As(III) removed+oxidized
pH 8.5, As(III) removed+oxidized
pH 6.5, As(Total) removed
pH 7.5, As(Total) removed
pH 8.5, As(Total) removed
0 2 4 6 8 10 12
Dose of TiCl 3 as Ti(III) (mg/L)
Figure 3.7 Removal/Conversion efficiency of As(III) in NSFI challenge water as a function
of titanium(III) chloride dose and pH
Arsenic Removal Using Titanium (IV) Oxychloride
The removal efficiencies of As(V) and As(III) in NSFI challenge water as a function of
titanium (IV) oxychloride coagulant dose and pH are shown in Figure 3.8, which demonstrates
that both As(V) and As(III) removal were pH dependent. The percent removals of As(V) with 2
mg/L Ti(IV) dosage at pHs 6.5, 7.5, and 8.5 were 54, 37, and 29%, respectively, which were
significantly less than the removals for the same Fe(III) dose. The removal efficiency of As(III)
was also pH dependent; it increased with increasing pH. The removal efficiencies of As(III) were
16, 20, and 26% at pH 6.5, 7.5 and 8.5, respectively for a dose of 2 mg/L Ti(IV), which shows
that the removal efficiency was less than that observed for ferric chloride at pH 6.5 and 7.5 while
it was the same at pH 8.5. So the removal efficiencies of As(V) by titanium (IV) oxychloride
coagulation were lower than those for ferric chloride coagulation, while the removal efficiencies
of As(III) were less than or equal to those of ferric chloride. It can be observed from Figure 3.8
that the removal efficiency of As(V) was higher than As(III) at all pH values.
Arsenic Removal Using Zirconium (IV) Oxychloride
The removal efficiencies of As(V) and As(III) in NSFI challenge water as a function of
zirconium (IV) oxychloride dose and pH are shown in Figure 3.9, which demonstrates that As(V)
removal was highly pH dependent whereas As(III) removal was independent of pH. The removal
efficiency of As(V) by zirconium (IV) oxychloride was more influenced by pH than was As(V)
removal by ferric chloride. The percent removals of As(V) with 2 mg/L Zr(IV) dosage at pHs
6.5, 7.5, and 8.5 were 94, 59, and 23%, respectively, which were significantly less than the
removals for the same Fe(III) doses. With 2 mg/L Zr(IV) dosage, approximately 8% of As(III)
was removed irrespective of pH. So the removal efficiencies of As(V) and As(III) with
zirconium(IV) oxychloride were lower than with ferric chloride. It can be observed from Figure
3.9 that the removal efficiency of As(V) was much higher than As(III) at all pH values.
19

% removal of As(V) & As(III)
100
80
60
40
20
0
pH 6.5-As(V)
pH 7.5-As(V)
pH 8.5-As(V)
pH 8.5-As(III)
pH 7.5-As(III)
pH 6.5-As(III)
0 2 4 6 8 10 12
Dose of TiOCl 2 as Ti(IV) (mg/L)
Figure 3.8 Removal efficiency of As(V) and As(III) in NSFI challenge water as a function of
titanium(IV) oxychloride dose and pH
% removal of As(V) & As(III)
100
80
60
40
20
0
pH 6.5-As(V)
pH 7.5-As(V)
pH 8.5-As(V)
pH 6.5-As(III)
pH 7.5-As(III)
pH 8.5-As(III)
0 2 4 6 8 10 12
Dose of ZrOCl 2 as Zr(IV) (mg/L)
Figure 3.9 Removal efficiency of As(V) and As(III) in NSFI challenge water as a function of
zirconium(IV) oxychloride coagulant dose and pH
ARSENIC ADSORPTION ISOTHERMS
Graphs showing percent arsenic removal as a function of coagulant dose and pH are of
practical importance, but it is the equilibrium adsorption isotherm that is generally used to
compare the effectiveness of coagulants. The adsorption isotherms of As(V) on the in-situ
formed hydroxides of Fe(III), Al(III), Zr(IV), Ti(III), Ti(IV) at pH 6.5, 7.5, and 8.5 are shown in
Figures 3.10, 3.11, and 3.12, respectively. The most favorable As(V) adsorption isotherm was
observed with ferric chloride, which performed better than other coagulants except at pH 8.5
20

where it was similar to that of TiCl 3 . At pH 8.5, the adsorption of As(V) on the oxyhydroxides of
TiCl 3 were greater or equal to that of ferric hydroxide at equilibrium As(V) concentration less
than 10 μg/L. Alum performed less efficiently in comparison with FeCl 3 for As(V) sorption at all
pHs. Alum performed better than zirconium and titanium coagulants at pH 6.5, while it was
similar to or poorer at pH 7.5 and 8.5.
qe (μg As(V)/mg of metal)
90
75
60
45
30
15
0
FeCl3
ZrOCl2
ZrCl4
TiCl4
Alum
TiCl3
TiOCl2
0 5 10 15 20 25 30
C e (μg As(V)/L)
Figure 3.10 Comparison of As(V) adsorption isotherms for the in-situ formed
oxyhydroxides of Fe(III), Al(III), Zr(IV), Ti(IV), and Ti(III) at pH 6.5
qe (μg As(V)/mg of metal)
50
40
30
20
10
0
FeCl3
TiCl3
TiCl4
Alum
ZrOCl2
ZrCl4
TiOCl2
0 5 10 15 20 25 30 35 40
C e (μg As(V)/L)
Figure 3.11 Comparison of As(V) adsorption isotherms for the in-situ formed
oxyhydroxides of Fe(III), Al(III), Zr(IV), Ti(IV), and Ti(III) at pH 7.5
21

q e (μg As(V)/mg of metal)
30
25
20
15
10
5
0
FeCl3
TiCl3
TiCl4
Alum
TiOCl2
ZrOCl2
ZrCl4
0 5 10 15 20 25 30 35 40 45 50
C e (μg As(V)/L)
Figure 3.12 Comparison of As(V) adsorption isotherms for the in-situ formed
oxyhydroxides of Fe(III), Al(III), Zr(IV), Ti(IV), and Ti(III) at pH 8.5
The As(III) adsorption isotherms for the in-situ formed oxyhydroxides of Fe(III), Al(III),
Zr(IV), Ti(IV), Ti(III), at pH 6.5, 7.5, and 8.5 are shown in Figures 3.13, 3.14, and 3.15,
respectively. The highest As(III) loading was observed with the oxyhydroxide of TiCl 3 , which
performed better than other coagulants at pHs 6.5 and 7.5. But at pH 8.5, the highest As(III)
loading was observed with oxyhydroxides of TiCl 3 , FeCl 3 , TiCl 4 , and TiOCl 2 which had similar
adsorption isotherms. The oxyhydroxide of TiCl 4 exhibited similar loading to that of FeCl 3 at all
pHs, but the oxyhydroxide of TiOCl 2 exhibited similar loading only at pH of 8.5. Alum had
virtually no measurable capacity for As(III) at any pH.
qe (μg As(III)/mg of metal)
12
9
6
3
0
TiCl3
FeCl3
TiCl4
TiOCl2
ZrCl4
ZrOCl2
Alum
0 10 20 30 40 50 60
C e (μg As(III)/L)
Figure 3.13 Comparison of As(III) adsorption isotherms for the in-situ formed
oxyhydroxides of Fe(III), Al(III), Zr(IV), Ti(IV), and Ti(III) at pH 6.5
22

qe (μg As(III)/mg of metal)
10
8
6
4
2
0
v
TiCl3
FeCl3
TiCl4
TiOCl2
ZrCl4
ZrOCl2
Alum
0 10 20 30 40 50 60
C e (μg As(III)/L)
Figure 3.14 Comparison of As(III) adsorption isotherms for the in-situ formed
oxyhydroxides of Fe(III), Al(III), Zr(IV), Ti(IV), and Ti(III) at pH 7.5
qe (μg As(III)/mg of metal)
10
8
6
4
2
0
TiCl3
FeCl3
TiOCl2
TiCl4
ZrCl4
ZrOCl2
Alum
0 10 20 30 40 50 60
C e (μg As(III)/L)
Figure 3.15 Comparison of As(III) adsorption isotherms for the in-situ formed
oxyhydroxides of Fe(III), Al(III), Zr(IV), Ti(IV), and Ti(III) at pH 8.5
23

COMPARISON OF ADSORPTION CAPACITIES
The best-fit isotherm equations (power curves) were used to calculate the adsorption
capacities of the metal hydroxides for an equilibrium concentration of 10 μg /L As(V) or 10 μg/L
As(III).
Arsenic Adsorption Comparison on a Mass Basis
The comparisons of As(V) and As(III) adsorption capacities of aluminum, titanium and
zirconium salts with ferric chloride on a mass basis for an equilibrium concentration of 10 μg/L
in NSFI challenge water are shown in Figures 3.16 and 3.17. As was observed in the isotherm
comparisons, As(V) capacity was typically 3 to 20 times greater than As(III) depending on the
coagulant and pH (Al and Zr coagulants were not considered due to negligible As(III)
adsorption). Also, for all coagulants, As(V) adsorption capacity decreased significantly as
equilibrium pH increased from 6.5 to 8.5 as observed in Figure 3.16. Ferric chloride was found
to have the highest adsorption capacity at pH 6.5 and 7.5, while at pH 8.5 TiCl 3 had similar
adsorption capacity. Alum was found to have As(V) adsorption capacities higher than the
titanium- and zirconium-based coagulants at pH 6.5, while at pH 7.5 and 8.5 the adsorption
capacities were similar or less. The order of As(V) adsorption capacities of different coagulants
at three different pHs based on an equilibrium concentration of 10 μg/L was as follows:
pH 6.5: FeCl 3 > Alum > ZrOCl 2 ≈ ZrCl 4 > TiCl 4 > TiCl 3 > TiOCl 2
pH 7.5: FeCl 3 >> TiCl 3 > TiCl 4 > Alum ≈ ZrOCl 2 > ZrCl 4 > TiOCl 2
pH 8.5: FeCl 3 ≈ TiCl 3 > TiCl 4 > Alum > ZrCl 4 ≈ TiOCl 2 > ZrOCl 2
70
qe (μg As(V)/mg of metal)
60
50
40
30
20
10
0
FeCl3; 59.6
TiCl3; 20.0
TiCl4; 25.1
Alum; 40.9
ZrCl4; 28.1
ZrOCl2; 30.1
TiOCl2; 9.3
FeCl3, 37.3
TiCl3; 16.1
TiCl4; 13.9
Alum; 13
ZrCl4; 9.89
ZrOCl2; 11.2
TiOCl2; 5.94
FeCl3; 14.3
TiCl3; 14.0
TiCl4; 10.7
Alum; 7.4
pH 6.5 pH 7.5 pH 8.5
Figure 3.16 Comparison of mass As(V) adsorbed per mass of Fe(III), Al(III), Ti(III),
Ti(IV), or Zr(IV) coagulant for an equilibrium concentration of 10 μg/L as a function of
pH
ZrCl4; 5.02
ZrOCl2; 4.14
TiOCl2; 4.78
24

7
qe (μg As(III)/mg of metal)
6
5
4
3
2
1
0
TiCl3; 6.13
FeCl3; 3.00
TiCl4; 2.74
TiOCl2; 1.44
ZrCl4; 0.29
ZrOCl2; 0.03
Alum; 0.00
TiCl3; 4.00
FeCl3; 2.96
TiCl4; 2.86
TiOCl2; 2.00
ZrCl4; 0.76
ZrOCl2; 0.10
Alum; 0.00
TiCl3; 3.00
FeCl3; 3.12
TiCl4; 2.74
TiOCl2; 2.71
ZrCl4; 0.51
pH 6.5 pH 7.5 pH 8.5
ZrOCl2; 0.04
Alum; 0.00
Figure 3.17 Comparison of mass As(III) adsorbed per mass of Fe(III), Al(III), Ti(III),
Ti(IV), or Zr(IV) coagulant for an equilibrium concentration of 10 μg/L as a function of
pH
A similar comparison of As(III) adsorption (Figure 3.17) showed that the As(III)
adsorption capacities (a) were independent of pH for FeCl 3 , alum, TiCl 4 , ZrCl 4 , and ZrOCl 2 , (b)
decreased with increasing pH for TiCl 3 and (c) increased with increasing pH for TiOCl 2 . The
As(III) adsorption capacity with TiCl 3 was high compared with other coagulants at pHs 6.5 and
7.5, while it was same as FeCl 3 and Ti(IV) salts at pH 8.5. The As(III) adsorption capacity of
TiCl 4 was the same as that of FeCl 3 at all three pHs. The As(III) adsorption capacities of
zirconium salts were significantly less than titanium and ferric salts, while alum did not have any
measurable As(III) adsorption capacity. Thus, it appears that based on a mg metal/L basis and
ignoring costs, Ti(III) could be the coagulant of choice for As(III) removal. The order of As(III)
adsorption capacities of different coagulants at three different pHs based on an equilibrium
concentration of 10 μg/L was as follows:
pH 6.5: TiCl 3 > FeCl 3 ≈ TiCl 4 > TiOCl 2 >> ZrCl 4 ≈ ZrOCl 2 >> Alum
pH 7.5: TiCl 3 > FeCl 3 ≈ TiCl 4 > TiOCl 2 >> ZrCl 4 ≈ ZrOCl 2 >> Alum
pH 8.5: TiCl 3 ≈ FeCl 3 ≈ TiCl 4 ≈ TiOCl 2 >> ZrCl 4 ≈ ZrOCl 2 >> Alum
Arsenic Adsorption Comparison on a Molar Basis
The comparisons of As(V) and As(III) adsorption capacities of aluminum, titanium and
zirconium salts with ferric chloride on a molar basis for an equilibrium concentration of 10 μg/L
in NSFI challenge water are shown in Figures 3.18 and 3.19. Similar to what was observed in the
mass comparison, the highest As(V) loading on a molar basis (mmol As(V)/mol coagulant metal)
was observed for ferric chloride at all three pHs. The highest As(III) loading on the coagulant on
a molar basis was observed for titanium(III) chloride at pH 6.5 and 7.5, while at pH 8.5, ferric
25

chloride had the highest molar adsorption capacity. Although aluminum was reported to have
equal adsorption capacity (mmol As(V)/mol metal) to that of iron(III) (Edwards, 1994; McNeil
& Edwards, 1997), our study found that ferric chloride had significantly higher adsorption
capacity than alum at all three pHs. Finally, on a molar basis, alum was found to have As(V)
adsorption capacities less than or equal to titanium and zirconium based coagulants.
50
qe (mmol As(V)/mol metal)
40
30
20
10
0
FeCl3; 46.7
ZrCl4; 34.2
ZrOCl2; 36.6
TiCl3; 12.8
TiCl4; 16.1
TiOCl2; 6.0
Alum; 14.7
FeCl3; 27.8
ZrCl4; 12.0
ZrOCl2; 13.6
TiCl3; 10.3
TiCl4; 8.9
TiOCl2; 3.8
Alum; 4.7
FeCl3; 10.7
ZrCl4; 6.1
ZrOCl2; 5.0
TiCl3; 8.9
pH 6.5 pH 7.5 pH 8.5
TiCl4; 6.8
TiOCl2; 3.1
Figure 3.18 Comparison of moles As(V) adsorbed per mol of Fe(III), Al(III), Ti(III), Ti(IV),
or Zr(IV) coagulant for an equilibrium concentration of 10 μg/L as a function of pH
Alum; 2.6
4
q e (mmol As(III)/mol metal)
3
2
1
0
TiCl 3 ; 3.91
FeCl 3 ; 2.24
TiCl 4 ; 1.75
TiOCl 2 ; 0.92
ZrCl 4 ; 0.35
ZrOCl 2 ; 0.04
Alum; 0.00
TiCl 3 ; 2.55
FeCl 3 ; 2.21
TiCl4; 1.83
TiOCl 2 ; 1.28
ZrCl 4 ; 0.93
ZrOCl 2 ; 0.12
Alum; 0.00
TiCl 3 ; 1.92
FeCl 3 ; 2.33
TiCl 4 ; 1.75
TiOCl 2 ; 1.73
pH 6.5 pH 7.5 pH 8.5
Figure 3.19 Comparison of moles As(III) adsorbed per mol of Fe(III), Al(III), Ti(III),
Ti(IV), or Zr(IV) coagulant for an equilibrium concentration of 10 μg/L as a function of
pH
ZrCl 4 ; 0.62
ZrOCl 2 ; 0.04
Alum; 0.00
26

COMPARISON OF COAGULANT COSTS
The determination of the economic feasibility of using these salts as coagulants for
arsenic removal was studied by comparing the costs of the industrial grade coagulants: FeCl 3
(Thatcher Company, Chemical Market Reporter), alum (Thatcher Company, General Chemicals),
ZrOCl 2 (MEI), TiOCl 2 (Millennium Chemicals), and TiCl 4 (Millennium Chemicals).
Unfortunately the chemical costs of industrial-grade TiCl 3 , and ZrCl 4 were not available, and so
for these chemicals, cost comparison could not be made. The coagulant cost comparisons were
based on reducing As(V) or As(III) from 0.05 to 0.01 mg/L in the NSFI challenge water (Table
3.2). The calculations indicated that compared with alum, ZrOCl 2 , TiCl 4 and TiOCl 2 , ferric
chloride was the most cost effective coagulant ($/million gallons water treated) at pH 6.5-8.5 for
both As(V) and As(III) (Table 3.2). When comparing the common coagulants alum and ferric
chloride for As(V) removal, chemical costs for alum were found to be 4-8 times greater than
ferric chloride depending on the pH.
Table 3.2
Comparison of chemical costs for treating one million gallons of NSFI-53 water to reduce
As(V)/As(III) from 50 to < 10 μg/L
Coagulant
Cost for treating As(V)
from 0.05 to 0.01 mg/L
($/Mgal)
Cost for treating As(III)
from 0.05 to 0.01 mg/L
($/Mgal)
FeCl 3 2.46 51.2
Alum 10.5 > 120,000*
pH 6.5 TiCl 4 40.0 366
TiOCl 2 109 709
ZrOCl 2 116 120,000
FeCl 3 4.12 52.0
Alum 32.9 > 120,000*
pH 7.5 TiCl 4 72.1 351
TiOCl 2 172 510
ZrOCl 2 311 36,500
FeCl 3 10.7 49.3
Alum 58.4 > 120,000*
pH 8.5 TiCl 4 93.9 367
TiOCl 2 213 377
ZrOCl 2 843 99,400
* There was no measurable adsorption of As(III) onto alum, thus precise cost estimates
could not be made.
27

When comparing the chemical costs of FeCl 3 coagulation to remove As(III) vs As(V)
from the NSFI challenge water, As(III) removal was found to be 5-20 times more costly than
As(V) removal depending on the pH. Thus, As(III) should be oxidized to As(V) for costeffective
treatment. Although Ti(III) was found to have the highest capacity for As(III) a
comparison of treatment cost could not be made with ferric chloride because of the unavailability
of costs for industrial-grade Ti(III) coagulant. However, due to the very high coagulant costs for
As(V) and As(III) removal using TiCl 4 and TiOCl 2 , it is unlikely that TiCl 3 would be cost
competitive with FeCl 3 for As(III) removal.
OXIDATION STUDY OF ARSENIC(III) IN THE PRESENCE OF TITANIUM(III)
CHLORIDE
During the removal of As(III) using TiCl 3 , it was observed that the μg/L As(III)
remaining was far less than that of the As(Tot) remaining, which suggested that in the presence
of Ti(III), As(III) was oxidized to As(V). To verify and characterize the Ti(III)-facilitated
oxidation of As(III) to As(V) jar tests were conducted at pH 7.5 for a dose of 2 mg/L Ti(III).
Flocculation (0, 5, 10, 20, and 30 min) and settling (0, 1, and 2 hr) times were varied in order to
monitor the As(III) to As(V) oxidation rate.
TiCl 3 Oxidation Study Results
The results of the study on oxidation of arsenic(III) with TiCl 3 are shown in Figure 3.20,
which demonstrates that the oxidation of As(III) to As(V) by Ti(III) proceeded slowly, i.e., it
continued for several hours during which the conversion increased with time. From Figure 3.20 it
can be seen that, for a flocculation time of 20 minutes, the As(Tot) removals were 36, 37, and
39% for 0, 1, and 2 hours of settling, i.e., As(Tot) removal was approximately the same
regardless of the settling time. However, for this same 20-min flocculation time, As(III) removed
or converted to As(V) was 71, 64, and 51% after 0, 1, and 2 hours of settling, i.e., coagulated
samples that were settled for two hours showed greater As(III) oxidation than coagulated
samples settled for one hour which in turn showed greater oxidation than coagulated samples that
were not settled. Thus it was concluded that even though the As(III) continued to get oxidized to
As(V) during the settling stage, arsenic removal was restricted to the time period during
coagulation. The reason is that during settling and after the floc had settled, As(III) oxidation
continued, but the new As(V) that was formed was not adsorbed onto the Ti(OH) 3 (s) because of
severe mass transfer limitations between As(V) in the bulk solution and Ti(OH) 3 (s) on the
bottom of the beaker. Another factor to consider is that, based on prior experience with
preformed metal oxyhydroxide flocs, the already-formed floc could not adsorb as much As(V)
(or As(III)) as was adsorbed by in-situ formed floc during rapid mixing.
Flocculation time in the range of 0-30 minutes did have an effect on arsenic removal in
the samples that were not settled. During slow mixing of the floc, additional As(III) was oxidized
to As(V) and adsorbed onto the floc particles as they were slowly flocculated.
28

As removal/conversion (%)
100
80
60
40
20
0
As(III) removal/conversion after Settling=2hr
As(III) removal/conversion after Settling=1hr
As(III) removal/conversion after Settling=0hr
Total As removal after Settling=2hr
Total As removal after Settling=1hr
Total As removal after Settling=0hr
0 5 10 15 20 25 30 35 40
Flocculation Time (min)
Figure 3.20 As(III) and As(Tot) removed or converted during Ti(III) coagulation as a
function of flocculation and settling times
Proposed Mechanism for the Oxidation of Arsenic(III) in Presence of Titanium(III)
The oxidation of hydrolyzed titanium(III) by means of molecular oxygen consumes more
oxygen than is necessary to convert Ti(III) to Ti(IV), and the extra oxygen requirement has been
attributed to the formation of H 2 O 2 during Ti(III) oxidation in water (Remy, 1956).
TiCl 3 + H 2 O Ti(OH) 3 + 3HCl (3.1)
Ti(OH) 3 + ½O 2 + H 2 O Ti(OH) 4 + ½ H 2 O 2 (3.2)
If the hydrogen peroxide formed is used up as it is formed, it is found that for each atom
of Ti(III) oxidized, ½ molecule of oxygen is taken up, and ½ molecule of H 2 O 2 is formed.
Hydrogen peroxide thus formed can promote the oxidation of As(III) to As(V), but studies have
shown that peroxide oxidation of As(III) is very slow at pH < 9.0 (Pettine et al, 1999). However,
the rate has been shown to be faster in the presence of metals, particularly Fe(II) and copper, in
the near-neutral pH range (Pettine et al, 2000; Hug et al, 2003; Bissen et al, 2003; Voegeline et
al, 2003). If titanium (III) behaves in a similar catalytic manner to Fe(II), one might expect an
enhanced rate of As(III) oxidation by peroxide, as was observed in this work.
Thus, the results from these tests show that the oxidation of As(III) by Ti(III) is a slow
step, i.e., it continues for several hours during which the As(III) conversion to As(V) increases
with time, but As(V) is not removed by the floc, because the floc is not in contact with the
supernatant solution. Furthermore pre-formed floc has been shown to adsorb less arsenic than
floc formed in-situ during rapid mixing.
29

SUMMARY AND CONCLUSIONS
The following commercially available, hydrolysable Fe, Al, Zr and Ti salts were studied:
FeCl 3 , alum, TiCl 4 , TiCl 3 , TiOCl 2 , ZrCl 4 , and ZrOCl 2 . Variable-coagulant-dose jar tests were
conducted at pH 6.5, 7.5, and 8.5 using the standard NSFI challenge water spiked with 50 μg/L
As(III) or As(V).
The percent removal of As(V) was highly pH dependent and the removal increased with
decreasing pH for all coagulants tested. In particular, the adsorption capacity of As(V) with
zirconium salts decreased significantly with increasing pH. When comparing arsenic adsorption
isotherms for all the coagulants, the highest As(V) loadings on a coagulant on a mass basis (mg
As(V)/g metal) were observed with FeCl 3 , which performed better than aluminum, titanium, and
zirconium salts at pHs of 6.5 and 7.5. However, at pH 8.5, As(V) loadings on ferric chloride
were approximately the same as TiCl 3 at equilibrium As(V) ≤ 10 μg/L. When comparing
adsorption isotherms, the highest As(V) loading on any coagulant on a molar basis or a mass
basis was observed for ferric chloride at all three pHs, and the As(V) loading on iron was
significantly greater than aluminum. Regardless of the basis of comparison, ferric chloride was a
far better coagulant than alum for As(V) removal.
The percent removal efficiency of As(III) was independent of pH for FeCl 3 , TiCl 4 , ZrCl 4 ,
and ZrOCl 2 , and it decreased with increasing pH for TiCl 3 and increased with increasing pH for
TiOCl 2 . The removal of As(III) by alum was insignificant. When comparing arsenic adsorption
isotherms for all the coagulants, the highest As(III) loading on a coagulant (mg As(III)/g metal)
was observed with TiCl 3 , which performed better than ferric, titanium(IV), and zirconium salts at
pHs of 6.5 and 7.5. However Ti(III) had similar adsorption capacity to that of Fe(III) and Ti(IV)
coagulants at pH 8.5. Alum did not have any adsorption capacity for As(III). The highest As(III)
loading on the coagulant on a molar basis was observed for titanium(III) chloride at pH 6.5 and
7.5, while at pH 8.5, ferric chloride had the highest molar adsorption capacity. At all doses, the
removal efficiency of As(V) was significantly greater than As(III) at all pHs with all seven
coagulants tested: FeCl 3 , alum, TiCl 4 , TiCl 3 , TiOCl 2 , ZrCl 4 , and ZrOCl 2 .
When comparing chemical costs for FeCl 3 , alum, ZrOCl 2, TiCl 4 , or TiOCl 2 coagulation to
remove As(V) or As(III) the most economical was FeCl 3 . Of the common coagulants, alum was
found to be 4-8 times more expensive than ferric chloride. The chemical cost of ferric chloride
coagulation was calculated to be more than 5 to 20 times higher for As(III) treatment compared
with As(V).
There was experimental evidence that the high removal efficiency of As(III) by TiCl 3 and
the unusual As(III) behavior of increasing removal with decreasing pH was due to oxidation of
As(III) to As(V) by H 2 O 2 , which based on the literature, was formed from Ti(III) hydrolysis in
the challenge water, which contained some dissolved oxygen. In spite of its partial oxidation, the
experimentally observed removal of As(V) oxidized from As(III) was far less than the removal
of a similar starting concentration of As(V), because (a) the floc was already formed when it
contacted As(V), and (b) the As(III) oxidation continued for many hours during which the
Ti(OH) 3 formed had settled and was not in contact with As(V) formed.
30

CHAPTER 4
EFFECT OF COMPETING IONS ON ARSENIC (III)/(V) ADSORPTION
INTRODUCTION
The adsorption of arsenic occurs by the process of ligand exchange as discussed in
Chapter 3 (Figure 3.1). However the adsorption is influenced by the presence of competing
anions in source water such as bicarbonate, sulfate, silica, phosphate, and vanadate. These anions
are expected to sorb onto the metal hydroxides and competition between these substances for
adsorption sites will significantly interfere with As removal. This competition has the potential to
reduce the overall effectiveness of arsenic removal from source water.
The NSFI challenge water under study contains silica and phosphate, which will compete
with arsenates for adsorption sites on the oxyhydroxide surfaces because they form strong
surface complexes with iron, aluminum and similar metal oxides (Hingston 1981). Vanadate is
also expected to affect the adsorption of arsenic and reduce the adsorption capacity of arsenic
onto the coagulant hydroxides. Table 4.1 gives the equilibrium constants (pKa’s) for arsenic
containing acids, silicic acid, phosphoric acid and vanadic acid.
Soluble silica in water exists as silicic acid (H 4 SiO 4 ) which dissociates to H 3 SiO - 4 (pK a1 =
9.84). So at pH 6.5-8.0, silica exists almost entirely as uncharged H 4 SiO 4 , but at pH 8.5-9.0, the
-
fraction of silica existing as anionic silicate, H 3 SiO 4 is significant. Both neutral H 4 SiO 4 and
anionic H 3 SiO - 4 can act as ligands and form surface complexes, but the anions are stronger
ligands. Although the neutral species dominates at pH 6.5-8.5, the concentration of silica is so
high (712 μM) that the monovalent silicate anion concentration (H 3 SiO - 4 ) is very high in
comparison to the arsenic species at pHs 7.5 and 8.5 and is nearly equal at pH 6.5 (Table 4.1).
Because of the high concentration of silicate anion (H 3 SiO - 4 ), compared with the concentration
of arsenic, silica is expected to compete significantly with arsenic for adsorption sites at pH 6.5-
8.5.
Phosphoric acid has similar pK a values to those of arsenic acid, and phosphate exists as
monovalent and divalent anions (H 2 PO - 4 and as HPO 2- 4 ) at pH 6.5-8.5, which are similar in
chemical behavior to H 2 AsO - 4 and HAsO 2- -
4 . Both H 2 PO 4 and HPO 2- 4 can have ligand exchange
reactions with the hydroxides formed and will compete for adsorption sites with the arsenates.
Since the concentration of phosphorous (1.29 μM) is higher than that of arsenic (0.67 μM),
phosphate was expected to compete strongly with arsenic for adsorption sites.
Vanadate in water is actually a mixture of mono and oligovanadates, the composition of
which depends on the pH, the overall vanadium concentration, and the ionic strength. Vanadic
acid has similar pK a values to those of arsenic and phosphoric acids, and exists as H 2 VO - 4 and
2- -
2-
HVO 4 anions at pH 6.5-8.5 which are similar to H 2 AsO 4 and HAsO 4 anions. Since the
concentration of vanadium (0.98 μΜ) is higher than that of arsenic (0.67 μM), and since
vanadate is chemically similar to phosphate, it was expected that vanadate would compete for
adsorption sites with arsenic.
Experiments were conducted using NSFI challenge water at three pHs spiked with two
levels of silica, phosphate, and vanadate in order to assess the combined effects of the competing
ions under study. The competing ions were also studied separately; i.e., when silicate was
present, phosphate and vanadate were absent so as to be able to isolate the influence of each
contaminant.
31

Table 4.1
Composition of species of some important acids present in pH 6.5-8.5
Concentration of species (μmol/L) in
Acid
NSFI challenge water
pKa pH 6.5 pH 7.5 pH 8.5
-
pK
Arsenic acid (0.67 μmol/L * 1 =2.22 H 2 AsO 4 0.50 0.15 0.02
2-
pK 2 =6.98 HAsO 4 0.17 0.52 0.65
50 μg/L as As
pK 3 =11.5
-
pK
Phosphoric acid (1.29 μmol/L)* 1 =2.16 H 2 PO 4 1.07 0.43 0.06
2-
pK 2 =7.2 HPO 4 0.22 0.86 1.23
40 μg/L as P
pK 3 =12.4
Arsenous acid (0.67 μmol/L)* pK 1 =9.23 H 3 AsO 3 0.67 0.66 0.57
50 μg/L as As pK 2 =12.1
-
H 2 AsO 3 0.001 0.01 0.09
Silicic acid (333 μmol/L)*
20 mg/L as SiO 2
Vanadic acid (0.98 μmol/L)**
50 μg/L as V
*Source: Schecher 1998.
** Source: Sigel 1995.
pK 1 =9.84 Si(OH) 4 712 709 681
pK 2 =13.2
-
H 3 SiO 4 0.33 3.24 31
pK 1 =7.91 H 3 VO 3 0.94 0.71 0.20
pK 2 =13.4
-
H 2 VO 3 0.04 0.27 0.78
The effects of silicate, phosphate and vanadate in NSFI challenge water were studied
with five coagulants: FeCl 3 , alum, TiCl 3 , TiCl 4 and ZrCl 4 . The coagulants TiOCl 2 and ZrOCl 2
were dropped from further studies because they did not appear to offer any advantage over TiCl 4
and ZrCl 4 , respectively. The effect of competing ions on As(III) adsorption using alum as
coagulant was also not carried out since it was found that aluminum does not have any
significant As(III) adsorption.
EFFECT OF SILICA IN NSFI CHALLENGE WATER CONTAINING PHOSPHATE
Coagulation experiments were first performed using the standard challenge water with
silica (SiO 2 = 20 mg/L) and without silica (SiO 2 = 0 mg/L) in the presence of phosphate. The
objective was to study the effect of silica in the challenge water on arsenic removal in the pH
range of 6.5 to 8.5 using all five coagulants tested: FeCl 3 , alum, ZrCl 4 , TiCl 4 , and TiCl 3 .
Effect of Silica in NSFI Water with Phosphate Using Ferric (III) Chloride as Coagulant
Based on jar tests in the presence and absence of silica, it was found that silica does have
a significant impact on the adsorption of arsenic. The removal percentages of As(V) at 1 mg/L
Fe(III) dosage at pHs 6.5, 7.5, and 8.5 were 90, 76, and 46%, respectively, in the presence of
silica. In the absence of silica, the removal percentages at the same pHs were 94, 85, and 76%,
respectively for the same doses, which shows that removal efficiency was higher in the absence
of silica .This can be observed in Figure 4.1 through the more favorable adsorption isotherms in
the absence of silica at all three pH’s. It was somewhat surprising that silica had such a
significant effect at pH 6.5, where 99.95% of the silica is present as non-ionic H 4 SiO 4 .
32

q e (μg As(V)/mg Fe(III))
140
120
100
80
60
40
20
0
pH6.5, SiO2=0mg/L
pH7.5, SiO2=0mg/L
pH6.5, SiO2=20mg/L
pH8.5, SiO2=0mg/L
pH7.5, SiO2=20mg/L
pH8.5, SiO2=20mg/L
0 10 20 30 40 50
C e (μg As(V)/L)
Figure 4.1 Effect of silica on arsenic(V) adsorption at different pHs with in-situ formed
Fe(III) hydroxide in NSFI water with phosphate
qe (μg As(III)/mg of Fe(III))
25
20
15
10
5
0
pH 8.5, SiO2=0mg/L
pH 7.5, SiO2=0mg/L
pH 6.5, SiO2=0mg/L
pH 6.5, SiO2=20mg/L
pH 7.5, SiO2=20mg/L
pH 8.5, SiO2=20mg/L
0 5 10 15 20 25 30 35 40 45
C e (μg As(III)/L)
Figure 4.2 Effect of silica on arsenic(III) adsorption at different pHs with in-situ formed
Fe(III) hydroxide in NSFI water with phosphate
The As(III) adsorption isotherms shown in Figure 4.2 illustrate the strong negative
influence of silica on As(III) adsorption onto Fe(III) hydroxide in the 6.5-8.5 pH range. The
adsorption behavior of As(III) was similar to that observed for the As(V) in the absence of silica
where the removal efficiency was higher than with silica present. In the absence of silica, the
adsorption of As(III) was found to be pH dependent, and the As(III) removal efficiency
increased with increasing pH, which was not observed in the presence of silica were the removal
was the same regardless of pH. At 2 mg/L Fe(III) dosage, approximately 26% of As(III) was
removed irrespective of pH in the presence of silica. But in the absence of silica, higher removal
33

efficiencies of 41, 53, and 71% were obtained at pHs 6.5, 7.5, and 8.5, respectively. Thus, the
As(III) removal efficiency was highly pH dependent in the absence of silica and the removal
efficiencies increased with increasing pH.
Effect of Silica in NSFI Water with Phosphate Using Alum as Coagulant
Based on jar tests in the presence and absence of silica, it was found that silica does have
a effect on the adsorption of As(V) at low pH during coagulation with alum. The removal
percentages of As(V) at 1 mg/L Al(III) dosage at pHs 6.5, 7.5, and 8.5 were 85, 49, and 20%,
respectively, in the presence of silica. In the absence of silica, the removal percentages at the
same pHs were 95, 73, and 12%, respectively for the same doses, which shows that removal
efficiency was higher in the absence of silica at pH 6.5 and 7.5. However there was no effect of
silica on As(V) adsorption at pH 8.5, but in fact the adsorption decreased in the absence of silica
at pH 8.5. This can be observed in Figure 4.3 through the more favorable adsorption isotherms in
the absence of silica at pH 6.5 and 7.5. It was somewhat surprising that silica had so litter effect
at pH 8.5, where silica is present as ionic species compared to low pH where silica is non-ionic.
Effect of Silica in NSFI Water with Phosphate Using Zirconium (IV) Chloride as
Coagulant
Based on jar tests in the presence and absence of silica, it was found that silica does have
a significant impact on the adsorption of arsenic during coagulation with ZrCl 4 . The removal
percentages of As(V) at 1 mg/L Zr(IV) dosage at pHs 6.5, 7.5, and 8.5 were 62, 28, and 8%,
respectively, in the presence of silica, while in the absence of silica, the removals were 83, 56,
and 30%, respectively for the same dose and pHs. Thus, removal efficiency was higher in the
absence of silica. The adsorption isotherms in Figure 4.4 clearly indicate the greater As(V)
adsorption in the absence of silica at all three pH values.
q e (μg As(V)/mg Al(III))
120
100
80
60
40
20
0
pH 6.5, SiO2=0mg/L
pH 6.5, SiO2=20mg/L
pH 7.5, SiO2=0mg/L
pH 7.5, SiO2=20mg/L
pH 8.5, SiO2=20mg/L
pH 8.5, SiO2=0mg/L
0 10 20 30 40 50
C e (μg As(V)/L)
Figure 4.3 Effect of silica on arsenic(V) adsorption at different pHs with in-situ formed
Al(III) hydroxide in NSFI water with phosphate
34

q e (μg As(V)/mg Zr(IV))
50
40
30
20
10
0
pH 6.5, SiO2=0mg/L
pH 6.5, SiO2=20mg/L
pH 7.5, SiO2=0mg/L
pH7.5, SiO2=20mg/L
pH 8.5, SiO2=0mg/L
pH8.5, SiO2=20mg/L
0 10 20 30 40 50
C e (μg As(V)/L)
Figure 4.4 Effect of silica on arsenic(V) adsorption at different pHs with in-situ formed
Zr(IV) hydroxide in NSFI water with phosphate
q e (μg As(III)/mg Zr(IV))
7
6
5
4
3
2
1
0
pH8.5, SiO2=0mg/L
pH7.5, SiO2=0mg/L
pH6.5, SiO2=0mg/L
pH6.5, SiO2=20mg/L
pH7.5, SiO2=20mg/L
pH8.5, SiO2=20mg/L
0 10 20 30 40 50 60
C e (μg As(III)/L)
Figure 4.5 Effect of silica on arsenic(III) adsorption at different pHs with in-situ formed
Zr(IV) hydroxide in NSFI water with phosphate
The As(III) adsorption isotherms shown in Figure 4.5 illustrate the negative influence of
silica on arsenic adsorption onto Zr(IV) hydroxide at the three pHs tested: 6.5, 7.5, and 8.5. The
As(III) adsorption behavior was similar to that observed for the adsorption of As(V) where the
removal efficiency was higher in the absence of silica. In the absence of silica, the adsorption of
As(III) was found to be pH dependent, and the removal efficiency of As(III) increased with
increasing pH, which was not observed in the presence of silica were the removal was the same
regardless of pH. At 2 mg/L Zr(IV) dosage, approximately 8% of As(III) was removed
irrespective of pH in the presence of silica. But in the absence of silica, removal efficiencies of
35

16, 19, and 24% were obtained at pHs 6.5, 7.5, and 8.5, respectively. Thus, the As(III) removal
efficiency was highly pH dependent in the absence of silica and the removal efficiencies
increased with increasing pH.
Effect of Silica in NSFI Water with Phosphate Using Titanium (IV) Chloride as Coagulant
Based on jar tests in the presence and absence of silica, it was found that silica does have
a significant impact on the adsorption of arsenic during coagulation with TiCl 4 . The As(V)
removals at 1 mg/L Ti(IV) dosage at pHs 6.5, 7.5, and 8.5 were 54, 40, and 26%, respectively, in
the presence of silica (20 mg/L as SiO 2 ), while in the absence of silica, the removals were 73, 57,
and 42%, respectively for the same dose and pHs. Clearly, the As(V) removal efficiency was
higher in the absence of silica as shown by the isotherms in Figure 4.6.
In the absence of silica, the adsorption of As(III) was pH dependent, and its removal
efficiency increased with increasing pH, which was not observed in the presence of silica were
the removal was the same regardless of pH. For example, at 2 mg/L Ti(IV) dosage, As(III)
removal in the presence of silica was 26%, irrespective of pH. But, in the absence of silica,
removal efficiencies of 49, 51, and 53% were obtained at pHs 6.5, 7.5, and 8.5, respectively. The
As(III) adsorption isotherms shown in Figure 4.7 illustrate the negative influence of silica on
arsenic adsorption onto Ti(IV) hydroxide at all three pHs.
qe (μg As(V)/mg Ti(IV))
60
50
40
30
20
10
0
pH6.5, SiO2=0mg/L
pH7.5, SiO2=0mg/L
pH6.5, SiO2=20mg/L
pH8.5, SiO2=0mg/L
pH7.5, SiO2=20mg/L
pH8.5, SiO2=20mg/L
0 5 10 15 20 25 30 35 40
C e (μg As(V)/L)
Figure 4.6 Effect of silica on arsenic(V) adsorption at different pHs with in-situ formed
Ti(IV) hydroxide in NSFI water with phosphate
36

q e (μg As(III)/mg Ti(IV))
14
12
10
8
6
4
2
0
pH8.5, SiO2=0mg/L
pH7.5, SiO2=0mg/L
pH6.5, SiO2=0mg/L
pH6.5, SiO2=20mg/L
pH7.5, SiO2=20mg/L
pH8.5, SiO2=20mg/L
0 5 10 15 20 25 30 35 40
C e (μg As(III)/L)
Figure 4.7 Effect of silica on arsenic(III) adsorption at different pHs with in-situ formed
Ti(IV) hydroxide in NSFI water with phosphate
Effect of Silica in NSFI Water with Phosphate Using Titanium (III) Chloride as Coagulant
Based on jar tests in the presence and absence of silica, it was found that silica does have
a significant impact on the adsorption of arsenic during coagulation with TiCl 3 . The removals of
As(V) at 1 mg/L Ti(III) dosage at pHs 6.5, 7.5, and 8.5 were 49, 37, and 30%, respectively, in
the presence of silica, while in the absence of silica, higher removals of 77, 58, and 48%,
respectively, were observed for the same dose and pHs. Thus, As(V) removal by TiCl 3 was
higher in the absence of silica. This can be observed in Figure 4.8 through the more favorable
adsorption isotherms in the absence of silica.
qe (μg As(V)/mg Ti(III))
45
40
35
30
25
20
15
10
5
0
pH6.5, SiO2=0mg/L
pH7.5, SiO2=0mg/L
pH6.5, SiO2=20mg/L
pH8.5, SiO2=0mg/L
pH7.5, SiO2=20mg/L
pH8.5, SiO2=20mg/L
0 5 10 15 20 25 30 35 40 45
C e (μg As(V)/L)
Figure 4.8 Effect of silica on arsenic(V) adsorption at different pHs with in-situ formed
Ti(III) hydroxide in NSFI water with phosphate
37

qe (μg As(III)/mg Ti(III))
25
20
15
10
5
0
pH6.5, SiO2=0mg/L
pH7.5, SiO2=0mg/L
pH8.5, SiO2=0mg/L
pH6.5, SiO2=20mg/L
pH7.5, SiO2=20mg/L
pH8.5, SiO2=20mg/L
0 5 10 15 20 25 30 35 40 45
C e (μg As(III)/L)
Figure 4.9 Effect of silica on arsenic(III) adsorption at different pHs with in-situ formed
Ti(III) hydroxide in NSFI water with phosphate
Although As(III) adsorption was lower, its behavior with silica absent was similar to that
observed for the adsorption of As(V). In the presence and absence of silica, the adsorption of
As(III) increased with decreasing pH. For example, in the presence of silica and with 2 mg/L
Ti(III) dosage, As(III) removals of 42, 32, and 27% were obtained at pHs 6.5, 7.5, and 8.5,
respectively. In the absence of silica, As(III) removals of 76, 61, and 51% were obtained at the
same dose and pHs. As was observed earlier, the As(III) concentration remaining after Ti(III)
coagulation was far less than that of the total arsenic. This was due to oxidation of As(III) to
As(V) in the presence of Ti(III) during coagulation, as was described in Chapter 3. Thus, the
As(III) removal efficiency was pH dependent both in the presence and absence of silica and the
As(III) removal was higher in the absence of silica. The As(III) adsorption isotherms shown in
Figure 4.9 illustrate the negative influence of silica on arsenic adsorption onto Ti(III) hydroxide
at all three pHs.
Thus, in general it was found that silica does reduce the adsorption of arsenic
(As(V)/As(III)) and the effect increases with increasing pH with the following coagulants tested:
FeCl 3 , ZrCl 4 , TiCl 4 , and TiCl 3 . In the case of alum as coagulant, silica did have a significant
effect on As(V) adsorption at pH 6.5-7.5, while it had little if any effect at pH 8.5.
EFFECT OF PHOSPHATE IN NSFI CHALLENGE WATER CONTAINING SILICA
Coagulation experiments were first performed using challenge water with phosphate
(PO 4 -P = 40 μg/L) and without phosphate (PO 4 -P = 0 μg/L) in the presence of silica. The overall
objective was to study the effect of phosphate in the NSFI challenge water containing silica on
arsenic removal in the pH range of 6.5 to 8.5 using five coagulants: FeCl 3 , alum, ZrCl 4 , TiCl 4 ,
and TiCl 3 .
38

Effect of Phosphate in NSFI Water with Silica Using Ferric (III) Chloride as Coagulant
For FeCl 3 coagulation, the presence of phosphate in NSFI water with silica had a
significant effect at pH 6.5-7.5 on the adsorption of As(V) and no significant effect on the
adsorption of As(III). The removals of As(V) in the presence of phosphate at 1 mg/L Fe(III)
dosage at pHs 6.5, 7.5, and 8.5 were 90, 76, and 46%, respectively. In the absence of phosphate,
the removals at the same pHs were 98, 88, and 51%, respectively for the same dose, which
shows that the decrease in removal efficiency was significant in the presence of phosphate at pHs
6.5 and 7.5 and was less significant at pH 8.5. This can be observed in Figure 4.10 through the
more favorable adsorption isotherms in the absence of phosphate at pHs of 6.5 and 7.5 as
compared with the influence of phosphate on the adsorption isotherms at pH 8.5.
The presence or absence of phosphate in the NSFI water with silica had no significant
effect on As(III) adsorption, which was found to be pH independent using FeCl 3 as coagulant. At
2 mg/L Fe(III) dosage, approximately 26% of As(III) was removed irrespective of pH in the
presence of phosphate. In the absence of phosphate, approximately 29% of As(III) was removed
irrespective of pH. The As(III) adsorption isotherms shown in Figure 4.11 illustrate no
significant influence of phosphate or pH on As(III) adsorption onto Fe(III) hydroxide at the three
pHs tested: 6.5, 7.5, and 8.5.
qe (μg As(V)/mg Fe(III))
180
160
140
120
100
80
60
40
20
0
pH6.5, PO4-P=0ug/L
pH7.5, PO4-P=0ug/L
pH6.5, PO4-P=40ug/L
pH7.5, PO4-P=40ug/L
pH8.5, PO4-P=0ug/L
pH8.5, PO4-P=40ug/L
0 5 10 15 20 25 30
C e (μg As(V)/L)
Figure 4.10 Effect of phosphate on arsenic(V) adsorption at different pHs with in-situ
formed Fe(III) hydroxides in NSFI water with silica
39

q e (μ g As(III)/mg of Fe(III))
9
8
7
6
5
4
3
2
1
0
pH8.5, PO4-P=0ug/L
pH7.5, PO4-P=0ug/L
pH6.5, PO4-P=0ug/L
pH8.5, PO4-P=40ug/L
pH7.5, PO4-P=40ug/L
pH6.5, PO4-P=40ug/L
0 10 20 30 40 50
C e (μg As(III)/L)
Figure 4.11 Effect of phosphate on arsenic(III) adsorption at different pHs with in-situ
formed Fe(III) hydroxides in NSFI water with silica
Effect of Phosphate in NSFI Water with Silica Using Alum as Coagulant
When silica was present in the NSFI water, phosphate had no significant effect on the
adsorption of As(V) during coagulation with alum. The removals of As(V) in the presence of
phosphate at 1 mg/L Al(III) dosage at pHs 6.5, 7.5, and 8.5 were 85, 49, and 20%, respectively.
In the absence of phosphate, the removals at the same pHs were 88, 53, and 19%, respectively
for the same dose. This can be observed in Figure 4.12 through similar adsorption isotherms in
the presence and absence of phosphate at all pH 6.5, 7.5, and 8.5.
qe (μg As(V)/mg Al(III))
90
75
60
45
30
15
0
pH 6.5, PO4-P=40ug/L
pH 6.5, PO4-P=0ug/L
pH 7.5, PO4-P=40ug/L
pH 7.5, PO4-P=0ug/L
pH 8.5, PO4-P=40ug/L
pH 8.5, PO4-P=0ug/L
0 10 20 30 40 50
C e (μg As(V)/L)
Figure 4.12 Effect of phosphate on arsenic(V) adsorption at different pHs with in-situ
formed Al(III) hydroxides in NSFI water with silica
40

Effect of Phosphate in NSFI Water with Silica Using Zirconium (IV) Chloride as
Coagulant
When silica was present in the NSFI water, phosphate had a significant effect at pH 6.5-
7.5 on the adsorption of As(V) and no significant effect on the adsorption of As(III) during
coagulation with ZrCl 4 . The removals of As(V) in the presence of phosphate at 1 mg/L Zr(IV)
dosage at pHs 6.5, 7.5, and 8.5 were 62, 28, and 8%, respectively. In the absence of phosphate,
the removals at the same pHs were 79, 41, and 16%, respectively for the same dose, which
shows that the decrease in removal efficiency was significant in the presence of phosphate at pHs
6.5 and 7.5 and was less significant at pH 8.5. This can be observed in Figure 4.13 through the
qe (μg As(V)/mg Zr(IV))
50
45
40
35
30
25
20
15
10
5
0
pH6.5, PO4-P=0ug/L
pH6.5, PO4-P=40ug/L
pH7.5, PO4-P=0ug/L
pH7.5, PO4-P=40ug/L
pH8.5, PO4-P=0ug/L
pH8.5, PO4-P=40ug/L
0 10 20 30 40 50
C e (μg As(V)/L)
Figure 4.13 Effect of phosphate on arsenic(V) adsorption at different pHs with in-situ
formed Zr(IV) hydroxide in NSFI water with silica
qe (μg As(III)/mg Zr(IV))
9
8
7
6
5
4
3
2
1
0
pH8.5, PO4-P=0ug/L
pH7.5, PO4-P=0ug/L
pH6.5, PO4-P=0ug/L
pH8.5, PO4-P=40ug/L
pH7.5, PO4-P=40ug/L
pH6.5, PO4-P=40ug/L
0 10 20 30 40 50 60
C e (μg As(III)/L)
Figure 4.14 Effect of phosphate on arsenic(III) adsorption at different pHs with in-situ
formed Zr(IV) hydroxides in NSFI water with silica
41

more favorable adsorption isotherms in the absence of phosphate at pHs of 6.5 and 7.5 as
compared with the influence of phosphate on the adsorption isotherms at pH 8.5.
When silica was present in the NSFI water, phosphate did not significantly effect As(III)
adsorption, which was independent of pH using ZrCl 4 coagulant. At 2 mg/L Zr(IV) dosage,
approximately 8% of As(III) was removed irrespective of pH in the presence of phosphate. In the
absence of phosphate, approximately 7% of As(III) was removed irrespective of pH in the
presence of phosphate. The As(III) adsorption isotherms in Figure 4.14 demonstrate very little
As(III) adsorption and no significant effect of phosphate or pH during Zr(IV) coagulation at the
three pHs tested: 6.5, 7.5, and 8.5.
Effect of Phosphate in NSFI Water with Silica Using Titanium (IV) Chloride as Coagulant
When silica was present in the NSFI water, phosphate had a significant effect at pH 6.5-
7.5 on the adsorption of As(V) and no significant effect on the adsorption of As(III) during
coagulation with TiCl 4 . The removals of As(V) at 1 mg/L Ti(IV) dosage at pHs 6.5, 7.5, and 8.5
were 54, 40, and 26%, respectively, in the presence of phosphate. In the absence of phosphate,
the removals at the same pHs were 65, 47, and 30%, respectively for the same dose, which
shows that the decrease in removal efficiency was significant in the presence of phosphate at pHs
6.5 and 7.5 and was less significant at pH 8.5. This can be observed in Figure 4.15 through the
more favorable adsorption isotherms in the absence of phosphate at pHs of 6.5 and 7.5 as
compared with the lesser influence of phosphate on the adsorption isotherms at pH 8.5.
The presence or absence of phosphate in the NSFI water with silica had little effect on
As(III) adsorption, which was found to be pH independent using TiCl 4 as coagulant. At 2 mg/L
Ti(IV) dosage, approximately 26% of As(III) was removed irrespective of pH in the presence of
phosphate. In the absence of phosphate, removal efficiencies of 31, 30, and 26% were obtained
at pHs 6.5, 7.5, and 8.5, respectively. The As(III) adsorption isotherms shown in Figure 4.16
illustrate little if any influence of phosphate or pH on As(III) adsorption onto Ti (IV) hydroxide
in the 6.5-8.5 pH range.
qe (μg As(V)/mg Ti(IV))
35
30
25
20
15
10
5
0
pH6.5, PO4-P=0ug/L
pH6.5, PO4-P=40ug/L
pH7.5, PO4-P=0ug/L
pH7.5, PO4-P=40ug/L
pH8.5, PO4-P=0ug/L
pH8.5, PO4-P=40ug/L
0 5 10 15 20 25 30 35 40 45
C e (μg As(V)/L)
Figure 4.15 Effect of phosphate on arsenic(V) adsorption at different pHs with in-situ
formed Ti(IV) hydroxides in NSFI water with silica
42

qe (μg As(III)/mg Ti(IV))
8
7
6
5
4
3
2
1
0
pH6.5, PO4-P=0ug/L
pH7.5, PO4-P=0ug/L
pH8.5, PO4-P=0ug/L
pH6.5, PO4-P=40ug/L
pH7.5, PO4-P=40ug/L
pH8.5, PO4-P=40ug/L
0 5 10 15 20 25 30 35 40
C e (μg As(III)/L)
Figure 4.16 Effect of phosphate on arsenic(III) adsorption at different pHs with in-situ
formed Ti(IV) hydroxides in NSFI water with silica
Effect of Phosphate in NSFI Water with Silica Using Titanium (III) Chloride as Coagulant
When silica was present in the NSFI water, phosphate had a significant effect at low pH
on the adsorption of As(V) and no significant influence on the adsorption of As(III) during
coagulation with TiCl 3 . The removals of As(V) in the presence of phosphate at 1 mg/L Ti(III)
dosage at pHs 6.5, 7.5, and 8.5 were 49, 37, and 30%, respectively. In the absence of phosphate,
the removals at the same pHs were 66, 51, and 36%, respectively for the same dose, which
shows that the decrease in removal efficiency was significant in the presence of phosphate at pHs
6.5 and 7.5 and was less significant at pH 8.5. This can be observed in Figure 4.17 through the
more favorable adsorption isotherms in the absence of phosphate at pHs of 6.5 and 7.5 as
compared with the influence of phosphate on the adsorption isotherms at pH 8.5.
When silica was present in the NSFI water, phosphate had no significant effect on
As(III) adsorption using TiCl 3 coagulant. In the presence or absence of phosphate, the adsorption
of As(III) was found to be pH dependent, and the removal efficiency of As(III) increased with
decreasing pH. At 2 mg/L Ti(III) dosage, As(III) removal efficiencies of 42, 32, and 27% were
obtained at pHs 6.5, 7.5, and 8.5, respectively in the presence of phosphate, while in the absence
of phosphate, removal efficiencies of 41, 33, and 25% were obtained at the same pHs and dose,
respectively. Thus, the As(III) removal efficiency was the same and was pH dependent both in
the presence and absence of phosphate in NSFI water with silica. As was observed earlier, the
As(III) concentration remaining after Ti(III) coagulation was far less than that of the total
arsenic. This is due to oxidation of As(III) to As(V) in the presence of Ti(III) during coagulation,
which was described earlier. The As(III) adsorption isotherms shown in Figure 4.18 illustrate no
significant influence of phosphate on As(III) adsorption onto Ti(III) hydroxide at the three pHs
tested: 6.5, 7.5, and 8.5.
43

qe (μg As(V)/mg Ti(III))
40
35
30
25
20
15
10
5
0
pH6.5, PO4-P=0ug/L
pH6.5, PO4-P=40ug/L
pH7.5, PO4-P=0ug/L
pH7.5, PO4-P=40ug/L
pH8.5, PO4-P=0ug/L
pH8.5, PO4-P=40ug/L
0 5 10 15 20 25 30 35 40 45
C e (μg As(V)/L)
Figure 4.17 Effect of phosphate on arsenic(V) adsorption at different pHs with in-situ
formed Ti(III) hydroxides in NSFI water with silica
12
q e (μg As(III)/mg Ti(III))
10
8
6
4
2
0
pH 6.5, PO4-P=0ug/L
pH 6.5, PO4-P=40ug/L
pH 7.5, PO4-P=0ug/L
pH 7.5, PO4-P=40ug/L
pH 8.5, PO4-P=0ug/L
pH 8.5, PO4-P=40ug/L
0 5 10 15 20 25 30 35 40 45
C e (μg As(III)/L)
Figure 4.18 Effect of phosphate on arsenic(III) adsorption at different pHs with in-situ
formed Ti(III) hydroxides in NSFI water with silica
Thus, in NSFI challenge water containing silica, it was found that phosphate significantly
lowers the adsorption of As(V) at pH 6.5-7.5 but does not influence the adsorption of arsenic(III)
in the 6.5-8.5 pH range with any of the coagulants tested: FeCl 3 , ZrCl 4 , TiCl 4 and TiCl 3 .
However with alum as coagulant, their was no significant effect of phosphate on As(V)
adsorption at any pH tested: 6.5, 7.5, and 8.5.
44

EFFECT OF VANADATE IN NSFI CHALLENGE WATER CONTAINING SILICA
AND PHOSPHATE
Coagulation experiments were performed using the standard NSFI challenge water
without vanadate (VO 3 -V = 0 μg/L). Then, the standard challenge water containing silica and
phosphate was spiked with vanadate (VO 3 -V = 50 μg/L) and tested again. The objective was to
study the effect of vanadate in the NSFI challenge water on arsenic removal in the pH range of
6.5 to 8.5 using all five coagulants: FeCl 3 , alum, ZrCl 4 , TiCl 4 , and TiCl 3 .
Effect of Vanadate in NSFI Water with Silica and Phosphate Using Ferric (III) Chloride as
Coagulant
Based on jar tests in the presence and absence of vanadate, it was found that vanadate
does not significantly influence the adsorption of As(V) or As(III) in the standard NSFI
challenge water containing silica and phosphate during coagulation with FeCl 3 . The removals of
As(V) at 1 mg/L Fe(III) dosage at pHs 6.5, 7.5, and 8.5 were 90, 76, and 46%, respectively, in
the absence of vanadate. In the presence of vanadate, the removals at the same pHs were 89, 74,
and 37%, respectively for the same dose. The As(V) adsorption isotherms shown in Figure 4.19
illustrate no significant influence of vanadate on As(V) adsorption onto Fe(III) hydroxide at the
three pHs tested: 6.5, 7.5, and 8.5.
The presence or absence of vanadate in the NSFI water with silica and phosphate had no
significant effect on As(III) adsorption and was found to be pH independent using FeCl 3 as
coagulant. At 2 mg/L Fe(III) dosage, approximately 25% of As(III) was removed irrespective of
pH in the presence of vanadate. In the absence of vanadate, approximately 26% of As(III) was
removed irrespective of pH. Thus, the As(III) removal efficiency was not pH dependent, nor was
it significantly affected by the presence of vanadate in NSFI water containing silica and
phosphate. The As(III) adsorption isotherms shown in Figure 4.20 illustrate no significant
influence of vanadate or pH on As(III) adsorption onto Fe(III) hydroxide at the three pHs tested:
6.5, 7.5, and 8.5.
q e (μg As(V)/mg Fe(III))
80
70
60
50
40
30
20
10
0
pH6.5, VO3-V=0ug/L
pH6.5, VO3-V=50ug/L
pH7.5, VO3-V=0ug/L
pH7.5, VO3-V=50ug/L
pH8.5, VO3-V=0ug/L
pH8.5, VO3-V=50ug/L
0 5 10 15 20 25 30 35
C e (μg As(V)/L)
Figure 4.19 Effect of vanadate on arsenic(V) adsorption at different pHs with in-situ
formed Fe(III) hydroxides in NSFI water with silica and phosphate
45

q e (μg As(III)/mg Fe(III))
8
7
6
5
4
3
2
1
0
pH8.5, VO3-V=0ug/L
pH7.5, VO3-V=0ug/L
pH6.5, VO3-V=0ug/L
pH8.5, VO3-V=50ug/L
pH7.5, VO3-V=50ug/L
pH6.5, VO3-V=50ug/L
0 5 10 15 20 25 30 35 40 45
Figure 4.20 Effect of vanadate on arsenic(III) adsorption at different pHs with in-situ
formed Fe(III) hydroxides in NSFI water with silica and phosphate
Effect of Vanadate in NSFI Water with Silica and Phosphate Using Alum as Coagulant
Based on jar tests in the presence and absence of vanadate, it was found that vanadate
does not significantly influence the adsorption of As(V) in the standard NSFI challenge water
containing silica and phosphate during coagulation with alum. The removals of As(V) at 1 mg/L
Al(III) dosage at pHs 6.5, 7.5, and 8.5 were 85, 49, and 20%, respectively, in the absence of
vanadate. In the presence of vanadate, the removals at the same pHs were 82, 54, and 15%,
respectively for the same dose. Thus there was not a significant decrease in removal efficiency in
the presence of vanadate in NSFI water containing silica and phosphate. The As(V) adsorption
isotherms shown in Figure 4.21 illustrate no significant influence of vanadate on As(V)
adsorption onto Al(III) hydroxide at the three pHs tested: 6.5, 7.5, and 8.5.
q e (μg As(V)/mg Al(III))
70
60
50
40
30
20
10
0
pH 6.5, VO3-V=0ug/L
pH 6.5, VO3-V=50ug/L
pH 7.5, VO3-V=0ug/L
pH 7.5, VO3-V=50ug/L
pH 8.5, VO3-V=0ug/L
pH 8.5, VO3-V=50ug/L
0 10 20 30 40 50
C e (μg As(V)/L)
Figure 4.21 Effect of vanadate on arsenic(V) adsorption at different pHs with in-situ
formed Al(III) hydroxides in NSFI water with silica and phosphate
46

Effect of Vanadate in NSFI Water with Silica and Phosphate Using Zirconium (IV)
Chloride as Coagulant
Based on ZrCl 4 coagulation jar tests in the presence and absence of vanadate, it was
found that vanadate does not significantly influence the adsorption of As(V) or As(III) in NSFI
challenge water containing silica and phosphate. The removals of As(V) at 1 mg/L Zr(IV)
dosage at pHs 6.5, 7.5, and 8.5 were 62, 28, and 8%, respectively, in the absence of vanadate. In
the presence of vanadate, the removals at the same pHs were 59, 28, and 8%, respectively for the
same dose. Thus there was no significant effect of vanadate in NSFI water containing silica and
phosphate. The As(V) adsorption isotherms shown in Figure 4.22 illustrate insignificant effect of
vanadate on As(V) adsorption onto Zr(IV) hydroxide at the three pHs tested: 6.5, 7.5, and 8.5.
qe (μg As(V)/mg Zr(IV))
40
35
30
25
20
15
10
5
0
pH6.5, VO3-V=0ug/L
pH6.5, VO3-V=50ug/L
pH7.5, VO3-V=0ug/L
pH7.5, VO3-V=50ug/L
pH8.5, VO3-V=0ug/L
pH8.5, VO3-V=50ug/L
0 10 20 30 40 50
C e (μg As(V)/L)
Figure 4.22 Effect of vanadate on arsenic(V) adsorption at different pHs with in-situ
formed Zr(IV) hydroxides in NSFI water with silica and phosphate
q e (μg As(III)/mg Zr(IV))
7
6
5
4
3
2
1
0
pH6.5, VO3-V=0ug/L
pH7.5, VO3-V=0ug/L
pH8.5, VO3-V=0ug/L
pH6.5, VO3-V=50ug/L
pH7.5, VO3-V=50ug/L
pH8.5, VO3-V=50ug/L
0 10 20 30 40 50 60
C e (μg As(III)/L)
Figure 4.23 Effect of vanadate on arsenic(III) adsorption at different pHs with in-situ
formed Zr(IV) hydroxides in NSFI water with silica and phosphate
47

The presence or absence of vanadate in the NSFI water with silica and phosphate had no
significant effect on As(III) adsorption and was pH independent using ZrCl 4 as coagulant. At 2
mg/L Zr(IV) dosage, approximately 6% of As(III) was removed irrespective of pH in the
presence of vanadate. In the absence of vanadate, approximately 8% of As(III) was removed
irrespective of pH. Thus, the As(III) removal efficiency was not pH dependent, nor was it
significantly affected by the presence of vanadate in NSFI water containing silica and phosphate.
The As(III) adsorption isotherms shown in Figure 4.23 illustrate no significant effect of vanadate
or pH on As(III) adsorption onto Zr(IV) hydroxide at the three pHs tested: 6.5, 7.5, and 8.5.
Effect of Vanadate in NSFI Water with Silica and Phosphate Using Titanium (IV) Chloride
as Coagulant
Based on jar tests in the presence and absence of vanadate, it was found that vanadate
does not significantly affect the adsorption of As(V) or As(III) in challenge water containing
silica and phosphate during coagulation with TiCl 4 . The removals of As(V) at 1 mg/L Ti(IV)
dosage at pHs 6.5, 7.5, and 8.5 were 54, 40, and 26%, respectively, in the absence of vanadate. In
the presence of vanadate, the removals at the same pHs were 54, 39, and 25%, respectively for
the same dose. Thus, there was no effect of vanadate in NSFI water containing silica and
phosphate. The As(V) adsorption isotherms shown in Figure 4.24 illustrate insignificant effect of
vanadate on As(V) adsorption onto Ti(IV) hydroxide at the three pHs tested: 6.5, 7.5, and 8.5.
When coagulating with TiCl 4 , the presence or absence of vanadate in the NSFI water
with silica and phosphate did not significantly affect As(III) adsorption, which was independent
of pH. At 2 mg/L Ti(IV) dosage, approximately 26% of As(III) was removed irrespective of pH
in the presence of vanadate, while in the absence of vanadate, approximately 26% of As(III) was
removed, again, irrespective of pH. The As(III) adsorption isotherms shown in Figure 4.25
illustrate no significant influence of vanadate or pH on As(III) adsorption onto Ti(IV) hydroxide
at the three pHs tested: 6.5, 7.5, and 8.5.
q e (μg As(V)/mg Ti(IV))
30
25
20
15
10
5
0
pH6.5, VO3-V=0ug/L
pH6.5, VO3-V=50ug/L
pH7.5, VO3-V=0ug/L
pH7.5, VO3-V=50ug/L
pH8.5, VO3-V=0ug/L
pH8.5, VO3-V=50ug/L
0 5 10 15 20 25 30 35 40
C e (μg As(V)/L)
Figure 4.24 Effect of vanadate on arsenic(V) adsorption at different pHs with in-situ
formed Ti(IV) hydroxides in NSFI water with silica and phosphate
48

qe (μg As(III)/mg Ti(IV))
7
6
5
4
3
2
1
0
pH6.5, VO3-V=0ug/L
pH7.5, VO3-V=0ug/L
pH8.5, VO3-V=0ug/L
pH6.5, VO3-V=50ug/L
pH7.5, VO3-V=50ug/L
pH8.5, VO3-V=50ug/L
0 5 10 15 20 25 30 35 40
C e (μg As(III)/L)
Figure 4.25 Effect of vanadate on arsenic(III) adsorption at different pHs with in-situ
formed Ti(IV) hydroxides in NSFI water with silica and phosphate
Effect of Vanadate in NSFI Water with Silica and Phosphate Using Titanium (III) Chloride
as Coagulant
Based on TiCl 3 coagulation jar tests in the presence and absence of vanadate, it was
found that vanadate does not significantly influence the adsorption of As(V) or As(III) in NSFI
challenge water containing silica and phosphate. The removals of As(V) at 2 mg/L Ti(III) dosage
at pHs 6.5, 7.5, and 8.5 were 49, 37, and 30%, respectively in the absence of vanadate, while in
the presence of vanadate, the removals were 47, 35, and 28%, respectively for the same dose and
pHs. Although As(V) adsorption decreased with increasing pH, the As(V) adsorption isotherms
shown in Figure 4.26 illustrate no significant influence of vanadate on As(V) adsorption onto
Ti(III) hydroxide at the three pHs tested: 6.5, 7.5, and 8.5.
Based on TiCl 3 coagulation tests, the presence or absence of vanadate in the NSFI water
with silica and phosphate was found to have no significant effect on As(III) adsorption. In the
presence and absence of vanadate, the adsorption of As(III) was found to be pH dependent, and
the removal of As(III) increased with decreasing pH. At 2 mg/L Ti(III) dosage, As(III) removals
of 42, 32, and 27% were obtained at pHs 6.5, 7.5, and 8.5, respectively in the absence of
vanadate, while in the presence of vanadate, removal efficiencies of 42, 30, and 25% were
obtained at the same pHs and dose, respectively. Thus, the As(III) removal was the same and pH
dependent both in the presence and absence of vanadate in NSFI water containing silica and
phosphate. The As(III) adsorption isotherms shown in Figure 4.27 illustrate no significant effect
of vanadate on As(III) adsorption onto Ti(III) hydroxide at the three pHs tested: 6.5, 7.5, and 8.5.
Thus, for the five coagulants tested (FeCl 3 , alum, ZrCl 4 , TiCl 4 , and TiCl 3 ) in NSFI
challenge water containing silica and phosphate, it was found that vanadate does not affect the
adsorption of arsenic(V) in the 6.5-8.5 pH range. Similarly, vanadate did not affect the
adsorption of arsenic(III) in NSFI challenge water containing silica and phosphate in the same
pH range with the four coagulants tested (FeCl 3 , , ZrCl 4 , TiCl 4 , and TiCl 3 ).
49

qe (μg As(V)/mg Ti(III))
30
25
20
15
10
5
0
pH6.5, VO3-V=0ug/L
pH6.5, VO3-V=50ug/L
pH7.5, VO3-V=0ug/L
pH7.5, VO3-V=50ug/L
pH8.5, VO3-V=0ug/L
pH8.5, VO3-V=50ug/L
0 5 10 15 20 25 30 35 40 45
C e (μg As(V)/L)
Figure 4.26 Effect of vanadate on arsenic(V) adsorption at different pHs with in-situ
formed Ti(III) hydroxides in NSFI water with silica and phosphate
qe (μg As(III)/mg Ti(III))
11
10
9
8
7
6
5
4
3
2
1
0
pH6.5, VO3-V=0ug/L
pH6.5, VO3-V=50ug/L
pH7.5, VO3-V=0ug/L
pH7.5, VO3-V=50ug/L
pH8.5, VO3-V=0ug/L
pH8.5, VO3-V=50ug/L
0 5 10 15 20 25 30 35 40 45
C e (μg As(III)/L)
Figure 4.27 Effect of vanadate on arsenic(III) adsorption at different pHs with in-situ
formed Ti(III) hydroxides in NSFI water with silica and phosphate
INDIVIDUAL EFFECTS OF SILICA, PHOSPHATE AND VANADATE WITH FERRIC
(III) CHLORIDE AS COAGULANT IN NSFI CHALLENGE WATER WITHOUT
COMPETING IONS
Although not included of the original research plan, limited coagulation experiments
were performed with ferric chloride to determine the effects of silicate, phosphate, and vanadate
ions in the absence of one another. By contrast, the previously reported effects of silica,
phosphate, or vanadate had been determined in the presence of other competing ions, e.g., the
50

effect of silica (0 and 20 mg/L SiO 2 ) had been determined using NSFI challenge water
containing phosphate, and the effect of phosphate (0 and 40 μg P/L) had been determined in the
challenge water containing silica. So, to study the individual effect of silica on arsenic adsorption
in the absence of competing ions, experiments were performed using NSFI water with silica
(SiO2 = 20 mg/L) and without silica (SiO2 = 0 mg/L), in the absence of phosphate and vanadate
in the pH range of 6.5 to 8.5 with ferric chloride as coagulant. Similarly, the effect of phosphate
on arsenic removal in the absence of silica and vanadate, and the effect of vanadate in the
absence of silica and phosphate were studied in the same pH range with FeCl 3 .
Effect of Silica in the Absence of Phosphate and Vanadate
Based on FeCl 3 coagulant tests using challenge water without phosphate, it was found
that silica interferes with the adsorption of As(V) and As(III) and the effect increases with
increasing pH. For example, the removals of As(V) in the presence of silica at 0.5 mg/L Fe(III)
dosage at pHs 6.5, 7.5, and 8.5 were 92, 77, and 30%, respectively. In the absence of silica, the
corresponding removals were 96, 93, and 82% at the same Fe(III) dose. Thus, silica significantly
interfered with As(V) adsorption at pHs 7.5 and 8.5; while somewhat less silica interference was
observed at pH 6.5. This can be observed in Figure 4.28 through the more favorable adsorption
isotherms in the absence of silica at pHs of 7.5 and 8.5 as compared with the influence of silica
on the adsorption isotherms at pH 6.5.
Silica also interferes with the adsorption of As(III) onto Fe (III) hydroxide in the absence
of phosphate and vanadate, and the effect of silica increases with increasing pH. In the absence
of silica, the adsorption of As(III) was found to be pH dependent, and the removal efficiency of
As(III) increased with increasing pH, which was not observed in the presence of silica where the
removal was independent of pH. At 2 mg/L Fe(III) dosage, approximately 29% of As(III) was
removed irrespective of pH in the presence of silica. In the absence of silica, removal efficiencies
of 48, 64, and 78% were obtained at pHs 6.5, 7.5, and 8.5, respectively for the same dose. The
As(III) adsorption isotherms shown in Figure 4.29 illustrate the significant effect of silica on
As(III) adsorption onto Fe(III) hydroxide at all pHs and the effect increasing with increase in pH.
q e (μg As(V)/mg Fe(III))
180
160
140
120
100
80
60
40
20
0
pH 6.5, SiO2=0mg/L,P=0ug/L,V=0ug/L
pH 6.5, SiO2=20mg/L,P=0ug/L,V=0ug/L
pH 7.5, SiO2=0mg/L,P=0ug/L,V=0ug/L
pH 8.5, SiO2=0mg/L,P=0ug/L,V=0ug/L
pH 7.5, SiO2=20mg/L,P=0ug/L,V=0ug/L
pH 8.5, SiO2=20mg/L,P=0ug/L,V=0ug/L
0 5 10 15 20 25 30
C e (μg As(V)/L)
Figure 4.28 Effect of silica on arsenic(V) adsorption at different pHs with in-situ formed
Fe(III) hydroxides in the absence of phosphate and vanadate
51

q e (μg As(III)/mg Fe(III))
25
20
15
10
5
0
pH8.5, SiO2=0mg/L,P=0ug/L,V=0ug/L
pH7.5, SiO2=0mg/L,P=0ug/L,V=0ug/L
pH6.5, SiO2=0mg/L,P=0ug/L,V=0ug/L
pH8.5, SiO2=20mg/L,P=0ug/L,V=0ug/L
pH7.5, SiO2=20mg/L,P=0ug/L,V=0ug/L
pH6.5, SiO2=20mg/L,P=0ug/L,V=0ug/L
0 10 20 30 40 50
C e (μg As(III)/L)
Figure 4.29 Effect of silica on arsenic(III) adsorption at different pHs with in-situ formed
Fe(III) hydroxides in the absence of phosphate and vanadate
Effect of Phosphate in the Absence of Silica and Vanadate
In the absence of silica and vanadate, phosphate significantly interfered As(V) adsorption
and had and a minor effect on the adsorption of As(III) during coagulation with FeCl 3 . The
removals of As(V) in the presence of phosphate at 0.5 mg/L Fe(III) dosage at pHs 6.5, 7.5, and
8.5 were 80, 71, and 57%, respectively. In the absence of phosphate, the corresponding removals
were 96, 93, and 82%, which shows that the decrease in removal efficiency was significant in the
presence of phosphate. This can be observed in Figure 4.30 through the more favorable
adsorption isotherms in the absence of phosphate at all three pHs tested: 6.5, 7.5, and 8.5.
Phosphate had a minor effect on the adsorption of As(III) onto Fe(III) hydroxide in the
absence of silica and vanadate at pHs of 7.5 and 8.5. In the presence and absence of phosphate,
the adsorption of As(III) was found to be pH dependent, and the removal of As(III) increased
with increasing pH. The removals of As(III) in the presence of phosphate at 2 mg/L Fe(III)
dosage at pHs 6.5, 7.5, and 8.5 were 41, 53, and 71%, respectively. In the absence of phosphate,
the removals at the same pHs were 48, 64, and 78%, respectively for the same dose, which
shows a minor decrease in removal in the presence of phosphate. Thus, As(III) removal was pH
dependent and was affected by the presence of phosphate. This can be observed in Figure 4.31
through the more favorable adsorption isotherms in the absence of phosphate at pHs of 7.5 and
8.5 as compared with the lesser influence of phosphate on the adsorption isotherms at pH 6.5.
52

q e (μg As(V)/mg Fe(III))
200
160
120
80
40
0
pH6.5, SiO2=0mg/L,P=0ug/L,V=0ug/L
pH7.5, SiO2=0mg/L,P=0ug/L,V=0ug/L
pH8.5, SiO2=0mg/L,P=0ug/L,V=0ug/L
pH6.5, SiO2=0mg/L,P=50ug/L,V=0ug/L
pH7.5, SiO2=0mg/L,P=50ug/L,V=0ug/L
pH8.5, SiO2=0mg/L,P=50ug/L,V=0ug/L
0 5 10 15 20 25 30 35 40 45
C e (μg As(V)/L)
Figure 4.30 Effect of phosphate on arsenic(V) adsorption at different pHs with in-situ
formed Fe(III) hydroxides in the absence of silica and vanadate
q e (μg As(III)/mg Fe(III))
25
20
15
10
5
0
pH8.5, SiO2=0mg/L,P=0ug/L,V=0ug/L
pH8.5, SiO2=0mg/L,P=40ug/L,V=0ug/L
pH7.5, SiO2=0mg/L,P=0ug/L,V=0ug/L
pH7.5, SiO2=0mg/L,P=40ug/L,V=0ug/L
pH6.5, SiO2=0mg/L,P=0ug/L,V=0ug/L
pH6.5, SiO2=0mg/L,P=40ug/L,V=0ug/L
0 5 10 15 20 25 30 35
C e (μg As(III)/L)
Figure 4.31 Effect of phosphate on arsenic(III) adsorption at different pHs with in-situ
formed Fe(III) hydroxides in the absence of silica and vanadate
Effect of Vanadate in the Absence of Silica and Phosphate
In the absence of silica and phosphate, vanadate exhibited a significant effect on the
adsorption of As(V) and minor effect on the adsorption of As(III) during coagulation with FeCl 3 .
This vanadate effect was not observed in the presence of silica and phosphate. The removals of
As(V) in the presence of vanadate at 0.5 mg/L Fe(III) dosage at pHs 6.5, 7.5, and 8.5 were 91,
86, and 66%, respectively. In the absence of vanadate, the corresponding removals were 96, 93,
53

and 82%. This can be observed in Figure 4.32 through the more favorable adsorption isotherms
in the absence of vanadate at the three pHs tested: 6.5, 7.5, and 8.5.
Vanadate had a minor effect on the adsorption of As(III) onto Fe (III) hydroxide in the
absence of silica and phosphate at pHs of 7.5 and 8.5. In the presence and absence of vanadate,
the adsorption of As(III) was found to be pH dependent, and the removal efficiency of As(III)
increased with increasing pH. The removals of As(III) in the presence of vanadate at 2 mg/L
Fe(III) dosage at pHs 6.5, 7.5, and 8.5 were 46, 58, and 72%, respectively. In the absence of
vanadate, the corresponding removals were 48, 64, and 78%, which shows a minor decrease in
removal efficiency in the presence of vanadate at pHs of 7.5 and 8.5. Thus, the As(III) removal
efficiency was pH dependent and was affected by the presence of vanadate. This can be observed
in Figure 4.33 through the more favorable adsorption isotherms in the absence of vanadate at
pHs of 7.5 and 8.5 as compared with the lesser influence of vanadate on the adsorption isotherms
at pH 6.5.
q e (μg As(V)/mg Fe(III))
180
150
120
90
60
30
pH6.5, SiO2=0mg/L,P=0ug/L,V=0ug/L
pH7.5, SiO2=0mg/L,P=0ug/L,V=0ug/L
pH6.5, SiO2=0mg/L,P=0ug/L,V=50ug/L
pH7.5, SiO2=0mg/L,P=0ug/L,V=50ug/L
pH8.5, SiO2=0mg/L,P=0ug/L,V=0ug/L
pH8.5, SiO2=0mg/L,P=0ug/L,V=50ug/L
0
0 5 10 15 20 25 30
C e (μg As(V)/L)
Figure 4.32 Effect of vanadate on arsenic(V) adsorption at different pHs with in-situ
formed Fe(III) hydroxides in the absence of silica and phosphate
54

qe (μg As(III)/mg Fe(III))
25
20
15
10
5
0
pH8.5, SiO2=0mg/L,P=0ug/L,V=0ug/L
pH8.5, SiO2=0mg/L,P=0ug/L,V=50ug/L
pH7.5, SiO2=0mg/L,P=0ug/L,V=0ug/L
pH7.5, SiO2=0mg/L,P=0ug/L,V=50ug/L
pH6.5, SiO2=0mg/L,P=0ug/L,V=0ug/L
pH6.5, SiO2=0mg/L,P=0ug/L,V=50ug/L
0 5 10 15 20 25 30 35
C e (μg As(III)/L)
Figure 4.33 Effect of vanadate on arsenic(III) adsorption at different pHs with in-situ
formed Fe(III) hydroxides in the absence of silica and phosphate
SUMMARIZING THE EFFECTS OF COMPETING IONS ON THE ARSENIC
ADSORPTION CAPACITY IN NSFI CHALLENGE WATER
Figure 4.34 compares the As(V) adsorption capacities of ferric (III) hydroxides for an
equilibrium concentration of 10 μg/L As(V) in the pH range 6.5-8.5 in the presence and absence
of silica, phosphate, and vanadate in NSFI challenge water. Based on the experiments, the
presence of silica significantly reduced the adsorption capacity of As(V) onto Fe(OH) 3 .
Phosphate also significantly lowered the adsorption of As(V) at pHs of 6.5 and 7.5 but had a
lesser effect at pH 8.5. Vanadate did not significantly affect the adsorption of As(V) onto
Fe(OH) 3 in the presence of silica and phosphate.
Figure 4.35 compares the As(III) adsorption capacities of ferric (III) hydroxides for an
equilibrium concentration of 10 μg/L As(III) in the pH range 6.5-8.5 in the presence and absence
of silica, phosphate, and vanadate in NSFI challenge water. The presence of silica significantly
reduced the adsorption capacity of As(III) and the effect increased with increasing pH. However,
there was no significant competition of phosphate in the adsorption of As(III) onto Fe(OH) 3 in
the presence of silica. Vanadate also did not significantly affect the adsorption of As(III) onto
Fe(OH) 3 in the presence of silica and phosphate.
Figure 4.36 compares the As(V) adsorption capacities of aluminum (III) hydroxides for
an equilibrium concentration of 10 μg/L As(V) in the pH range 6.5-8.5 in the presence and
absence of silica, phosphate, and vanadate in NSFI challenge water. Based on the experiments,
the presence of silica significantly reduced the adsorption capacity of As(V) at pH 6.5 and 7.5,
while at pH 8.5 there was no significant effect of silica on As(V) adsorption. Phosphate did not
affect As(V) adsorption onto Al(OH) 3 in the presence of silica at any pH. Vanadate also did not
significantly affect the adsorption of As(V) onto Al(OH) 3 in the presence of silica and phosphate
at pH 6.5-8.5.
55

q e (mg As(V)/g Fe(III))
160
140
120
100
80
60
40
20
0
NSFI*
NSFI without silica
NSFI without phosphate
NSFI with vanadate
pH 6.5 pH 7.5 pH 8.5
NSFI* 62.6 37.3 14.3
NSFI without silica 83.2 63.8 43.1
NSFI without phosphate 151 67.9 16.6
NSFI with vanadate 61.3 32.8 11.9
*NSFI-53 challenge water contains 20 mg/L SiO 2 , 40 μg/L PO 4 -P, and 0 μg/L VO 3 -V.
Figure 4.34 Comparison of arsenic(V) adsorption capacities of in-situ formed Fe(III)
hydroxides for an equilibrium concentration of 10 μg/L As(V) in the pH range 6.5-8.5
qe (mg As(III)/g Fe(III))
16
14
12
10
8
6
4
2
0
NSFI*
NSFI without silica
NSFI without phosphate
NSFI with vanadate
pH 6.5 pH 7.5 pH 8.5
NSFI* 3 2.96 3.12
NSFI without silica 4.95 7.23 13.4
NSFI without phosphate 2.2 2.52 2.76
NSFI with vanadate 2.53 2.52 2.65
*NSFI-53 challenge water contains 20 mg/L SiO 2 , 40 μg/L PO 4 -P, and 0 μg/L VO 3 -V.
Figure 4.35 Comparison of arsenic(III) adsorption capacities of in-situ formed Fe(III)
hydroxides for an equilibrium concentration of 10 μg/L As(III) in the pH range 6.5-8.5
56

qe (mg As(V)/mg Al(III))
70
60
50
40
30
20
10
0
NSFI*
NSFI without silica
NSFI without phosphate
NSFI with vanadate
pH 6.5 pH 7.5 pH 8.5
NSFI* 40.90 13.05 7.36
NSFI without silica 60.98 30.56 8.39
NSFI without phosphate 43.67 14.80 8.03
NSFI with vanadate 43.26 14.48 7.36
*NSFI-53 challenge water contains 20 mg/L SiO 2 , 40 μg/L PO 4 -P, and 0 μg/L VO 3 -V.
Figure 4.36 Comparison of arsenic(V) adsorption capacities of in-situ formed Al(III)
hydroxides for an equilibrium concentration of 10 μg/L As(V) in the pH range 6.5-8.5
Figure 4.37 compares the As(V) adsorption capacities of zirconium (IV) hydroxides for
an equilibrium concentration of 10 μg/L As(V) in the pH range 6.5-8.5 in the presence and
absence of silica, phosphate, and vanadate in NSFI challenge water. Silica significantly lowered
the adsorption capacity of As(V) onto Zr(OH) 4 . Phosphate also significantly lowered the
adsorption of As(V) at pHs of 6.5 and 7.5 but had a lesser effect at pH 8.5. Vanadate did not
significantly affect the adsorption of As(V) onto Zr(OH) 4 in the presence of silica and phosphate.
Figure 4.38 compares the As(III) adsorption capacities of zirconium (IV) hydroxides for
an equilibrium concentration of 10 μg/L As(III) in the pH range 6.5-8.5 in the presence and
absence of silica, phosphate, and vanadate in NSFI challenge water. It can be observed that the
As(III) capacities are extremely low compared with As(V) adsorption capacities for Zr(IV). The
presence of silica significantly reduced the adsorption capacity of As(III) onto Zr(OH) 4 and the
effect increased with increasing pH. Even though Figure 4.38 shows that adsorption capacity in
the absence of phosphate is higher than in the presence of phosphate, it is not true as adsorption
capacities were calculated with the Freundlich isotherm equation. But it can be seen from Figure
4.14, the isotherms were similar regardless of the presence or absence of phosphate in the
equilibrium concentration range of 35 to 50 μg/L, which was the experimental range. So there
was no significant competition of phosphate in the adsorption of As(III) onto Zr(OH) 4 in the
presence of silica. Vanadate also did not significantly affect the adsorption of As(III) onto
Zr(OH) 4 in the presence of silica and phosphate.
Figure 4.39 compares the As(V) adsorption capacities of titanium (IV) hydroxides for an
equilibrium concentration of 10 μg/L As(V) in the pH range 6.5-8.5 in the presence and absence
of silica, phosphate, and vanadate in NSFI challenge water. Silica significantly lowers the
adsorption capacity of As(V) onto Ti(OH) 4 . Phosphate also significantly lowered the adsorption
57

of As(V) at pHs of 6.5 and 7.5, but had a lesser effect at pH 8.5. Vanadate did not significantly
affect the adsorption of As(V) onto Ti(OH) 4 in the presence of silica and phosphate.
q e (mg As(V)/g Zr(IV))
50
45
40
35
30
25
20
15
10
5
0
NSFI*
NSFI without silica
NSFI without phosphate
NSFI with vanadate
pH 6.5 pH 7.5 pH 8.5
NSFI* 28.1 9.89 5.02
NSFI without silica 46.8 23.9 11.8
NSFI without phosphate 41.7 13.6 5.28
NSFI with vanadate 24.2 9.84 3.95
*NSFI-53 challenge water contains 20 mg/L SiO 2 , 40 μg/L PO 4 -P, and 0 μg/L VO 3 -V.
Figure 4.37 Comparison of arsenic(V) adsorption capacities of in-situ formed Zr(IV)
hydroxides for an equilibrium concentration of 10 μg/L As(V) in the pH range 6.5-8.5
q e (mg As(III)/g Zr(IV))
3
2
1
NSFI*
NSFI without silica
NSFI without phosphate
NSFI with vanadate
0
pH 6.5 pH 7.5 pH 8.5
NSFI* 0.29 0.76 0.51
NSFI without silica 0.72 1.11 2.75
NSFI without phosphate 1.12 1.07 1.81
NSFI with vanadate 0.84 0.85 0.71
*NSFI-53 challenge water contains 20 mg/L SiO 2 , 40 μg/L PO 4 -P, and 0 μg/L VO 3 -V.
Figure 4.38 Comparison of arsenic(III) adsorption capacities of in-situ formed Zr(IV)
hydroxides for an equilibrium concentration of 10 μg/L As(III) in the pH range 6.5-8.5
58

q e (mg As(V)/g Ti(IV))
45
40
35
30
25
20
15
10
5
0
NSFI*
NSFI without silica
NSFI without phosphate
NSFI with vanadate
pH 6.5 pH 7.5 pH 8.5
NSFI* 25.1 13.9 10.7
NSFI without silica 40.9 24.1 15.9
NSFI without phosphate 30.1 19.2 13.3
NSFI with vanadate 22 13.9 9.75
*NSFI-53 challenge water contains 20 mg/L SiO 2 , 40 μg/L PO 4 -P, and 0 μg/L VO 3 -V.
Figure 4.39 Comparison of arsenic(V) adsorption capacities of in-situ formed Ti(IV)
hydroxides for an equilibrium concentration of 10 μg/L As(V) in the pH range 6.5-8.5
Figure 4.40 compares the As(III) adsorption capacities of titanium (IV) hydroxides for an
equilibrium concentration of 10 μg/L As(III) in the pH range 6.5-8.5 in the presence and absence
of silica, phosphate, and vanadate in NSFI challenge water. The presence of silica dramatically
reduced the adsorption capacity of As(III) onto Ti(OH) 4 at all three pH values. However there
was no significant competition of phosphate in the adsorption of As(III) onto Ti(OH) 4 in the
presence of silica. Vanadate also did not significantly affect the adsorption of As(III) onto
Ti(OH) 4 in the presence of silica and phosphate.
Figure 4.41 compares the As(V) adsorption capacities of titanium (III) hydroxides for an
equilibrium concentration of 10 μg/L As(V) in the pH range 6.5-8.5 in the presence and absence
of silica, phosphate, and vanadate in NSFI challenge water. Silica and phosphate significantly
lower the adsorption capacity of As(V) onto Ti(OH) 3 at pH 6.5 and 7.5, but have a lesser
interference at pH 8.5. Vanadate did not significantly affect the adsorption of As(V) onto
Ti(OH) 3 in the presence of silica and phosphate.
Figure 4.42 compares the As(III) adsorption capacities of titanium (III) hydroxides for an
equilibrium concentration of 10 μg/L As(III) in the pH 6.5-8.5 range in the presence and absence
of silica, phosphate, and vanadate in NSFI challenge water. The presence of silica significantly
reduced the adsorption capacity of As(III) onto Ti(OH) 3 at all pHs. Phosphate interference was
less that that of silica in the adsorption of As(III) onto Ti(OH) 3 . Phosphate and Vanadate did not
significantly affect the adsorption of As(III) onto Ti(OH) 3 in the NSFI challenge water.
59

q e (mg As(III)/g Ti(IV))
12
10
8
6
4
2
NSFI*
NSFI without silica
NSFI without phosphate
NSFI with vanadate
0
pH 6.5 pH 7.5 pH 8.5
NSFI* 2.74 2.86 2.74
NSFI without silica 9.96 9.25 8.28
NSFI without phosphate 2.97 2.95 2.65
NSFI with vanadate 3.04 2.77 2.4
*NSFI-53 challenge water contains 20 mg/L SiO 2 , 40 μg/L PO 4 -P, and 0 μg/L VO 3 -V.
Figure 4.40 Comparison of arsenic(III) adsorption capacities of in-situ formed Ti(IV)
hydroxides for an equilibrium concentration of 10 μg/L As(III) in the pH range 6.5-8.5
qe (mg As(V)/g Ti(III))
40
35
30
25
20
15
10
5
0
NSFI*
NSFI without silica
NSFI without phosphate
NSFI with vanadate
pH 6.5 pH 7.5 pH 8.5
NSFI* 20 16.2 14.2
NSFI without silica 37 25.4 19.7
NSFI without phosphate 24.2 20.9 15.1
NSFI with vanadate 18.3 14.7 12.9
*NSFI-53 challenge water contains 20 mg/L SiO 2 , 40 μg/L PO 4 -P, and 0 μg/L VO 3 -V.
Figure 4.41 Comparison of arsenic(V) adsorption capacities of in-situ formed Ti(III)
hydroxides for an equilibrium concentration of 10 μg/L As(V) in the pH range 6.5-8.5
60

qe (mg As(III)/g Ti(III))
20
15
10
5
0
NSFI*
NSFI without silica
NSFI without phosphate
NSFI with vanadate
pH 6.5 pH 7.5 pH 8.5
NSFI* 6.13 4 2.36
NSFI without silica 17.5 10.7 7.93
NSFI without phosphate 6.45 4.2 2.4
NSFI with vanadate 5.63 4.06 2.76
*NSFI-53 challenge water contains 20 mg/L SiO 2 , 40 μg/L PO 4 -P, and 0 μg/L VO 3 -V.
Figure 4.42 Comparison of arsenic(III) adsorption capacities of in-situ formed Ti(III)
hydroxides for an equilibrium concentration of 10 μg/L As(III) in the pH range 6.5-8.5
INDIVIDUAL EFFECTS OF COMPETING IONS ON THE ARSENIC ADSORPTION
CAPACITY OF FERRIC (III) HYDROXIDE
Figure 4.43 compares the individual effects of silica, phosphate and vanadate on the
As(V) adsorption capacities of ferric (III) hydroxides for an equilibrium concentration of 10
μg/L As(V) in the pH range 6.5-8.5. In the absence of phosphate and vanadate, silica
significantly reduced the adsorption capacity of As(V) onto Fe(OH) 3 and the effect increased
significantly with increasing pH. Phosphate significantly lowered the adsorption of As(V) at all
pHs in the absence of silica and vanadate. Vanadate also significantly affected the adsorption of
As(V) in the absence of silica and phosphate at all pHs during coagulation with FeCl 3 .
Figure 4.44 compares the individual effects of silica, phosphate and vanadate on the
As(III) adsorption capacities of ferric (III) hydroxides for an equilibrium concentration of 10
μg/L As(III) in the pH range 6.5-8.5. In the absence of phosphate and vanadate, silica
significantly reduced the adsorption capacity of As(III) onto Fe(OH) 3 and the effect increased
with increasing pH. Phosphate and vanadate had a minor effect on As(III) adsorption onto
Fe(OH) 3 , and the effect increased with increasing pH..
61

q e (mg As(V)/g Fe(III))
200
180
160
140
120
100
80
60
40
20
0
NSFI with no competing ion*
SiO2=20mg/L
PO4-P=40ug/L
VO3-V=50ug/L
pH 6.5 pH 7.5 pH 8.5
NSFI with no competing ion* 178 152 88.5
SiO2=20mg/L 151 67.9 16.6
PO4-P=40ug/L 83.2 63.8 43
VO3-V=50ug/L 112 102 53.8
*NSFI with no competing ion contains 0 mg/L SiO 2 , 0 μg/L PO 4 -P, and 0 μg/L VO 3 -V.
Figure 4.43 Comparison of arsenic adsorption capacities of in-situ-formed Fe(III)
hydroxides for an equilibrium concentration of 10 μg/L As(V) in the pH range 6.5-8.5
q e (mg As(III)/g Fe(III))
20
18
16
14
12
10
8
6
4
2
0
NSFI with no competing ion*
Si=20mg/L
PO4-P=40ug/L
VO3-V=50ug/L
pH 6.5 pH 7.5 pH 8.5
NSFI with no competing ion* 5.03 9.22 17.3
Si=20mg/L 2.2 2.52 2.76
PO4-P=40ug/L 4.95 7.23 13.4
VO3-V=50ug/L 4.1 7.9 14.4
*NSFI with no competing ion contains 0 mg/L SiO 2 , 0 μg/L PO 4 -P, and 0 μg/L VO 3 -V.
Figure 4.44 Comparison of arsenic adsorption capacities of in-situ formed Fe(III)
hydroxides for an equilibrium concentration of 10 μg/L As(III) in the pH range 6.5-8.5
62

CONCLUSIONS
In the NSFI challenge water with phosphate, silica significantly reduced the adsorption of
arsenic by competing for adsorption sites. So the As(V) and As(III) removal efficiencies in the
absence of silica were higher than in the presence of silica with the four coagulants tested: FeCl 3 ,
TiCl 4 , TiCl 3 , and ZrCl 4 . With alum as coagulant, silica did affect As(V) adsorption at pH 6.5 and
7.5, while at pH 8.5, it had no effect. The effect of silica at 20 mg/L increased with increasing pH
in the case of As(III) removal with all coagulants (FeCl 3 , TiCl 4 , and ZrCl 4 ) except TiCl 3 . In the
NSFI challenge water with silica, phosphate at 40 μg P/L was found to reduce the adsorption of
As(V) significantly at pH 6.5 and 7.5, whereas it had a lesser effect at pH 8.5 with all four
coagulants tested: FeCl 3 , TiCl 4 , TiCl 3 , and ZrCl 4 . With alum as coagulant, phosphate did not
have a significant effect on As(V) adsorption in the 6.5-8.5 pH range. In contrast to its effect on
As(V) adsorption, the presence of phosphate did not have a significant effect on the adsorption of
As(III) for all four of the coagulants tested (FeCl 3 , TiCl 4 , TiCl 3 , and ZrCl 4 ). In the NSFI
challenge water with silica and phosphate, vanadate at 50 μg V/L did not significantly affect the
adsorption of As(V) or As(III) with all the coagulants tested including FeCl 3 , alum, TiCl 4 , TiCl 3 ,
and ZrCl 4 .
In the absence of other competing ions with ferric chloride as coagulant, it was found that
silica, phosphate, and vanadate exhibited significant competitive effects on the adsorption of
As(V) and As(III). In the absence of phosphate and vanadate, silica exhibited a significant effect
on the adsorption of As(V) and As(III) and the effect increased with increasing pH. In the
absence of silica and vanadate, phosphate exhibited a significant effect on the adsorption of
As(V) at all pHs and a minor effect on the adsorption of As(III) at pH 7.5 and 8.5 during
coagulation with FeCl 3 . Similarly, in the absence of silica and phosphate, vanadate exhibited a
significant effect on the adsorption of As(V) and a minor effect on the adsorption of As(III) at
pH 7.5 and 8.5 during coagulation with FeCl 3 .
So, as expected, the competing ions under study did affect the adsorption of arsenic in
most of the cases. Silica did compete with As(V) and As(III) for adsorption sites and affected the
adsorption capacity regardless of the presence of competing ions. Phosphate did compete with
As(V) and As(III) adsorption in the absence of competing ions. However, the phosphate effect
was suppressed in the presence of silica: the phosphate interference on As(V) adsorption
decreased with increasing pH and had no effect on As(III) adsorption. The effect of vanadate on
arsenic adsorption was observed in the absence of competing ions while no effect was observed
in the presence of competing ions which suggests that the presence of silica and phosphate
suppressed the effect of vanadate at all pHs in the case of As(V) and As(III) removal.
63

CHAPTER 5
TOXICITY CHARACTERISTIC STUDIES
INTRODUCTION
The Toxicity Characteristic (TC) is used to determine whether a solid waste is classified
as a hazardous waste due to its toxicity. The TC of waste is established by determining the levels
of the contaminant in the TC extract of the waste. Under USEPA regulations, the toxicity
characteristic is assessed by the Toxicity Characteristic Leaching Procedure (TCLP), which
measures the leachability of toxic contaminants from the wastes utilizing a buffered acetic acid
solution as an extraction fluid. In addition to using the TCLP to determine hazardous waste
status, California employs a method known as the Waste Extraction Test (WET), which employs
buffered citric acid as a solvent and tends to leach more metals than the TCLP test. Both the tests
have a common preliminary-evaluation-of-solids procedure, which is followed by a leaching
procedure which differentiates them if needed. The TCLP and WET test regulatory limit for
arsenic is 5.0 mg/L.
PRELIMINARY TOXICITY CHARACTERISTIC EVALUATION
A preliminary TC evaluation is performed on a waste and includes: (1) determination of
the percent solids; (2) determination of whether the waste contains insignificant solids and is,
therefore, its own extract after filtration; (3) determination of whether the solid portion of the
waste requires particle size reduction; and (4) determination of which of two extraction fluids are
to be used for the extraction of the waste.
Figure 5.1 provides a flow chart which delineates preliminary determination of percent
solids. The first step is to take the waste and pass it through a 0.6 to 0.8 μm filter and determine
the percent dry solids, and, depending on the dry solids, determine if further studies are needed
as follows:
i) For liquid wastes (i.e., those containing less than 0.5% dry solid material), the waste,
after filtration through a 0.6 to 0.8 μm glass fiber filter is defined as the TCLP extract.
ii) For wastes containing greater than or equal to 0.5% solids, the liquid, if any, is
separated from the solid phase and stored for later analysis; the particle size of the
solid phase is reduced, if necessary. The solid phase is extracted with an amount of
extraction fluid equal to 20 times the weight of the solid phase. The composition of
the extraction fluid employed is a function of the alkalinity of the solid phase of the
waste. Following extraction, the liquid extract is separated from the solid phase by
filtration through a 0.6 to 0.8 μm glass fiber filter.
iii) If compatible (i.e., multiple phases will not form on combination), the initial liquid
phase of the waste is added to the liquid extract, and these are analyzed together. If
incompatible, the liquids are analyzed separately and the results are mathematically
combined to yield a volume-weighted average concentration.
65

Sample of
waste
Solids are < 0.5%.
Discard solids.
Filtrate = TCLP
extract
< 0.5% Determine % ≥ 0.5%
dry solids in
the waste
Leachate
studies
needed
100%
Examine solids.
Leachate studies
needed
Figure 5.1 Preliminary determination of percent solids
Experimental Study Procedure
The preliminary-determination-of-percent-solids studies were conducted at pH 7.5 with
an initial concentration of 500 μg/L As(V) in the case of ferric chloride and 250 μg/L As(V) with
all other coagulants (Alum, TiCl 3 , TiCl 4 , and ZrCl 4 ). The higher (500 or 250 μg/L As(V)) initial
arsenic concentrations were used because the small amount of coagulant needed in the case of
initial concentration of 50 μg/L As(V) would not yield enough sludge for further testing. The
coagulant doses required to achieve equilibrium arsenic concentrations ≤10 μg/L in the challenge
water were calculated based on the isotherms obtained previously for the four coagulants tested:
FeCl 3 , Alum, ZrCl 4 , TiCl 4 , and TiCl 3 . The jar tests were then conducted as before with the
calculated amount of metal being dosed. After rapid mixing and flocculation, the waste was
allowed to settle in an Imhoff cone for 3 hours. The volume of settled sludge was noted, and the
initial weight of the waste was calculated assuming a density of 1.0 g/mL. The waste was
vacuum filtered through a 0.7-μm glass fiber filter. The residue on the filter was defined as the
solid phase of the waste, and the liquid phase was defined as the filtrate. The filter with the solid
phase was then removed from the filtration apparatus and dried at 110 ° C until two successive
weighings yielded the same value within ± 1%. The dry weight of the solid was then calculated,
which was used to calculate the percent dry solids using Equation 5.1.
Weight of dry waste
Percent dry solids = *100
(5.1)
Initial weight of waste
66

According to the Toxicity Characteristic Leaching procedure (TCLP) test method 1311
(USEPA SW 846) and California Waste Extraction Test (WET) procedure (California Code of
Regulations, Title 22, Division 4.5, Chapter 11, Article 5), liquid wastes containing less than
0.5% dry solids do not require extraction. The liquid waste, after filtration, is defined as the
extract. The filtered extract was analyzed and the resulting concentration compared directly to
the appropriate regulatory concentration.
TOXICITY CHARACTERISTIC STUDY RESULTS
As explained earlier (TCLP and WET procedures), the percent dry solids in the 3-hr
settled sludges were calculated for each of the coagulants tested and the values are given in Table
5.1.
It can be observed that the percent dry solids results were less than 0.5% for all
coagulants tested. So, the liquid wastes (containing less than 0.5% dry solids) did not require
extraction. The liquid waste, after filtration, is defined as the extract or filtrate. It was expected
that the arsenic concentration in the filtrate would be less than or equal to 10 μg/L, since it is in
equilibrium with the effluent concentration. As expected the measured arsenic concentration as
reported in Table 5.2 were ≤ 10 μg/L, which easily passed the TCLP and WET test regulatory
limit of 5 mg As/L (5,000 μg As/L).
EXPERIMENTAL EVALUATION OF THE ADSORPTION OF HIGH
CONCENTRATIONS OF ARSENIC ONTO METAL HYDROXIDES
The initial arsenic concentrations in these extraction studies were 500 or 250 μg/L As(V),
which were much higher than the initial concentration of arsenic (50 μg/L) used in previous jar
tests. Thus, it was necessary to determine if the higher initial arsenic concentrations would
influence the equilibrium adsorption capacity of arsenic on the metal hydroxides. The highinitial-concentration
arsenic loadings were determined by dissolving the metal hydroxide
precipitates and measuring the arsenic released. These arsenic loadings were compared with the
calculated loadings based on previously determined arsenic isotherms beginning with challenge
water containing 50 μg /L As(V). Specifically, the high-initial-arsenic-concentration loading of
As(V) onto precipitated solids was determined by dissolving the dry solids obtained in the
leachate studies into 1:1 nitric acid. The acid solution was then analyzed for arsenic
concentration, which was then converted to an As(V) loading onto the known amount of
precipitated metal hydroxides. The comparison of the adsorption capacities based on the
calculated and experimental work is given in Table 5.3, where it can be seen that the loadings are
almost the same. Therefore, it was concluded that the As(V) adsorption capacity was the same,
regardless of the initial concentration of arsenic when the final As(V) concentration was ≤ 10
μg/L.
Table 5.1
Percent dry solids in the liquid wastes after 3-hr settling
Coagulant FeCl 3 Alum ZrCl 4 TiCl 4 TiCl 3
% dry solids 0.27 ± 0.02% 0.31 ± 0.04% 0.4 ± 0.05% 0.22 ± 0.02% 0.21 ± 0.02%
67

Table 5.2
Arsenic concentrations in the TCLP- or WET-defined extract (liquid waste after filtration)
Coagulant FeCl 3 Alum ZrCl 4 TiCl 4 TiCl 3
Arsenic
concentration in
extract (μg/L)
3.39 ± 0.24 13.45 ± 0.68 3.02 ± 1.23 4.09 ± 0.5 11.2 ± 0.89
Table 5.3
Comparison of expected and measured arsenic adsorption capacities of metal hydroxides
for all coagulants at 10 μg /L As(V) equilibrium concentration.
Coagulant
Theoretical adsorption
capacity (μg As/mg
metal)
Experimental
adsorption capacity
based on filtrate (μg
As/mg metal)
Experimental
adsorption capacity
based on arsenic
adsorbed on solid (μg
As/mg metal)
Initial
concentration
of As(V)
FeCl 3 Alum ZrCl 4 TiCl 4 TiCl 3
50 μg/L 37.3 13 10 14 16.2
250 or 500
μg/L
250 or 500
μg/L
37.3 ±
0.02
35.6 ±
0.65
10.81 ±
0.03
12.51 ±
1.2
10.2 ±
0.05
8.79 ±
0.7
14.1 ±
0.03
13.3 ±
0.52
16.4 ±
0.06
15.1 ±
0.84
CONCLUSIONS
Following coagulation and 3-hr settling, with all five coagulants tested (FeCl 3 , Alum,
TiCl 3 , TiCl 4 , and ZrCl 4 ), the liquid wastes obtained were found to contain less than 0.5% dry
solids, and therefore did not require extraction. The liquid filtrate, which is the extract in such
cases, easily passed the TCLP and WET test regulatory limit of 5.0 mg/L. It was also found that
the adsorption capacity of As(V) remained the same regardless of initial arsenic concentration
with all four of the coagulants tested. Thus the adsorption capacity of arsenic at a final arsenic
concentration of 10 μg/L was found to be independent of initial arsenic concentration in the
range of 50-500 μg/L As(V).
68

CHAPTER 6
ARSENIC REMOVAL WITH FeCl 3 COAGULATION: COMPARISON OF
EXPERIMENTAL RESULTS WITH MINEQL+ 4.50 CHEMICAL
EQUILIBRIUM MODELING PROGRAM PREDICTIONS
INTRODUCTION
The scope of work for this project included a requirement to model the effects of arsenic
concentration and background ion concentrations on the adsorption of As(III) and As(V) onto the
coagulants tested using commercially available software. The objective of the modeling research
described in this chapter was to determine if the adsorption of arsenic (As(V) and As(III)) onto
Fe(OH) 3 could be accurately modeled using the Mineql+ Chemical Equilibrium Modeling
Program. To determine the accuracy of the Mineql+ predictions, modeling results were
compared with experimental results. Fe(III) was the only coagulant modeled because the
unmodified Mineql+ program is not capable of modeling adsorption onto zirconium or titanium
oxyhydroxides.
MINEQL+ is a widely used chemical equilibrium modeling program capable of
calculating aqueous-, solid-, and gas-phase concentrations of chemical species derived from
input components. An extensive thermodynamic database is included in the model. MINEQL+ is
a powerful and easy-to-use chemical equilibrium modeling system that can be used to perform
calculations in low temperature (0-50 o C) and low-to-moderate ionic strength (

Table 6.1
Composition of NSFI-53 challenge water
Ion Concentration (mg/L) MW Concentration (M)
Ca 2+ 40.1 40.1 1.000E-3
Mg 2+ 12.6 24.3 5.185E-4
Na + 88.87 23.0 3.864E-3
-
HCO 3 183 61.0 3.000E-3
2-
SO 4 50 96.1 5.203E-4
Cl - 71 35.5 2.000E-3
NO 3 -N 2.0 14.0 1.429E-4
F - 1.0 19.0 5.263E-5
PO 4 -P 0.04 31.0 1.290E-6
SiO 3 -SiO 2 20 60.1 3.328E-4
As(III)/(V) 0.05 74.9 6.676E-7
Table 6.2
Mineql+ titration parameters
Parameter Start End Points
Total conc. of Fe 3+ 8.952E-6 M 8.952E-5 M 10
Total conc. of Cl - 2.852E-3 M 3.094E-3 M 10
Total conc. of Na + 3.891E-3 M 4.133E-3 M 10
concentration of Cl - in the NSFI challenge water plus the amount of Cl - added with Fe 3+ , which
would be three times the Fe 3+ concentration. In the “pH wizard”, the calculation type was set to
“pH is supplied by the user” and the “pH value was set at pH 7.5”. Although the coagulation jar
tests were done in the atmosphere, the coagulated solutions were not actually in equilibrium with
the atmosphere. Therefore, the calculation type was set to “Closed to the atmosphere” and the
“Total CO 3 ” concentration was then set in the “CO 2 wizard”. The “Ionic Strength Corrections”
was set to “Calculated” and “2-layer FeOH” was selected in the “Adsorption model section” in
the “Run Time Manager”. The calculation was then run.
From within the “Output Manager”, “Report on Ion balance sample QA/QC” was opened
from the “Special Reports Output Type”. The ion balance for this system showed a value of
8.272E-4 which corresponds to the amount of acid (HCl) anion that was not accounted for in the
system at fixed pH 7.5. So, going back to calculation wizard, the required concentration of Cl -
was added to the initial concentration. The pH calculation type was changed to “pH calculated by
Mineql+” and the “Base pH calculation on Electroneutrality” was selected in the “pH wizard”.
After the initial conditions were set, the addition of FeCl 3 was started. The “Multirun Option”
was selected and the type of calculation was set to “Titration” and the variables Fe 3+ , Cl - and Na +
were selected. It was necessary that a 3:1 ratio of Cl - to Fe 3+ be maintained, and also that that we
would have to add an equivalent amount of Na + for every Cl - added to simulate NaOH
neutralization of HCl resulting from the hydrolysis Fe 3+ to produce Fe(OH) 3 (s). A summary of
the titration parameters that were selected are shown in Table 6.2.
70

The problem was then rerun, and from the output, the pH of the system was checked for
each addition of Fe 3+ , and was found to be around “pH 7.5 ± 0.01” in the “Alkalinity Summary
Special Reports”. The ion balance was also checked from the “Report on Ion balance sample
QA/QC” and was found to “pass” the ion balance. From the “Output Manager”, the “Component
Groups” was selected and the “Data Object” was set to “AsO 4 3- ”. The “Obs × Variables” was set
to “Total adsorbed AsO 4 3- ”, which gave the concentration of AsO 4 3- that was adsorbed for each
addition of Fe 3+ . The values are tabulated with the corresponding concentration of Fe 3+ that was
added in Table 6.3. The adsorption capacity then was calculated as follows.
Total adsorbed AsO
3 −
Adsorption capacity ( μ g As / mg Fe)
=
4
(6.1)
Concentrat ion of Fe
3 +
Table 6.3 gives the As(V) adsorption capacity of Fe(OH) 3 (s) calculated at pH 7.5 with
NSFI-53 challenge water from the concentration of arsenic adsorbed for every addition of Fe 3+ in
the range of 0.5-5.0 mg/L. The As(V) adsorption isotherm, i.e., the As(V) adsorption capacity
(μg As(V)/mg Fe(III)) vs. the final concentration of arsenic in challenge water was plotted and is
shown in Figure 6.1. A representative As(V) adsorption capacity was calculated for 10 μg/L
As(V) in the challenge water. The adsorption capacity thus calculated was then compared with
the experimental result.
Titration
#
Table 6.3
As(V) adsorption capacity of Fe(OH) 3 based on Mineql+ program
Arsenic Arsenic Concentration
Concentration Concentration
of Fe 3+ (M) of Fe 3+ adsorbed adsorbed of Arsenic in
(mg/L)
(M) (μg/L) water, C e (μg/L)
Adsorption
capacity, Q e (μg
As(V)/mg Fe)
Initial 0 0 0 0 50.00 0
1 8.95E-6 0.5 2.63E-7 19.7 30.3 39.41
2 1.79E-5 1 4.74E-7 35.5 14.5 35.52
3 2.69E-5 1.5 6.00E-7 44.94 5.06 29.97
4 3.58E-5 2 6.45E-7 48.31 1.69 24.16
5 4.48E-5 2.5 6.58E-7 49.28 0.72 19.72
6 5.37E-5 3 6.62E-7 49.58 0.42 16.53
7 6.27E-5 3.5 6.64E-7 49.73 0.27 14.21
8 7.16E-5 4 6.65E-7 49.81 0.19 12.46
9 8.06E-5 4.5 6.66E-7 49.88 0.12 11.09
10 8.95E-5 5 6.66E-7 49.88 0.12 9.98
71

q e (μg As(V)/mg of Fe(III))
50
45
40
35
30
25
20
15
10
5
0
NSFI-53 water
Mineql+ Modeling Results
0 5 10 15 20 25 30 35
C e (μg As(V)/L)
Figure 6.1 Modeling of As(V) adsorption isotherm onto Fe(OH) 3 (s) at pH 7.5
Similarly the Mineql+ modeling program was used to calculate the adsorption capacity of
As(V) or As(III) for the following compositions of challenge water at pH 6.5, 7.5, and 8.5.
• NSFI-53 challenge water
• NSFI-53 without phosphate (i.e., P = 0 μg/L, Si = 20 mg/L)
• NSFI-53 with vanadate (i.e., VO 3 -V = 50 μg/L, Si = 20 mg/L, PO 4 -P = 40 μg/L)
• NSFI-53 with vanadate and without silica or phosphate (i.e., V = 50 μg/L, Si = 0 mg/L
and P = 0 μg/L )
• NSFI-53 without silica (i.e., Si = 0 mg/L, PO 4 -P = 40 μg/L)
• NSFI-53 without silica or phosphate (i.e., Si = 0 mg/L and PO 4 -P = 0 μg/L)
The Fe 3+ concentration ranges used in the calculations were 8.952e-6 M (0.5 mg/L) to
8.952e-5 (5 mg/L) in the case of As(V) adsorption and 1.791e-5 M (1 mg/L) to 1.791e-4 (10
mg/L) in the case of As(III) adsorption.
As(III) AND As(V) ADSORPTION ISOTHERMS BASED ON MINEQL+ MODELING
The arsenic adsorbed for each dose of FeCl 3 was calculated by Mineql+ for the 18 pH,
silica, phosphate, and vanadate conditions described above for As(V) and As(III). The resulting
theoretical isotherms are compared in Figures 6.2-6.7.
Figure 6.2 shows the As(V) adsorption isotherms onto Fe(III) hydroxide at pH 6.5 for all
the six cases modeled. Only two different isotherms resulted from the six cases modeled. It can
be seen that the adsorption isotherms for the three cases which had phosphate (lower curves)
were different from other cases which did not have phosphate (upper curves). So presence of
phosphate significantly reduced the adsorption capacity at pH 6.5. But the presence of neither
silica nor vanadate had any effect on As(V) adsorption capacity according to the Mineql+
program. Figure 6.3 shows the As(V) adsorption isotherms onto Fe(III) hydroxide at pH 7.5 for
all the six cases modeled. It can be seen that the adsorption isotherms are similar in all cases. So
72

presence of silica, phosphate, and vanadate did not have any effect on the adsorption capacity at
pH 7.5 according to the Mineql + program. Figure 6.4 shows the As(V) adsorption isotherms
onto Fe(III) hydroxide at pH 8.5 for all the six cases modeled. It can be seen that the adsorption
isotherms are similar in all cases. So presence of silica, phosphate, and vanadate did not have any
effect on the adsorption capacity at pH 8.5 according to the Mineql + program.
q e (μg As(V)/mg Fe(III))
70
60
50
40
30
20
10
0
NSFI-53 with no silica
or phosphate
NSFI-53 without phosphate
NSFI-53 with
vanadate
NSFI-53
NSFI-53 with vanadate and
without silica or phosphate
NSFI-53 without silica
Mineql+ Modeling Results
0 5 10 15 20 25 30
C e (μg As(V)/L)
Figure 6.2 Mineql+-predicted effect of silica, phosphate, and vanadate on As(V) adsorption
on the in-situ-formed Fe(III) hydroxide at pH 6.5
q e (μg As(V)/mg Fe(III))
50
40
30
20
10
0
NSFI-53 with
vanadate
NSFI-53
without
phosphate
NSFI-53 with no
silica or phosphate
NSFI-53 without silica
NSFI-53
NSFI-53 with vanadate
and without silica or
phosphate
Mineql+ Modeling Results
0 5 10 15 20 25 30 35
C e (μg As(V)/L)
Figure 6.3 Mineql+-predicted effect of silica, phosphate, and vanadate on As(V) adsorption
on the in-situ-formed Fe(III) hydroxide at pH 7.5
73

q e (μg As(V)/mg Fe(III))
35
28
21
14
7
0
NSFI-53 with
vanadate
NSFI-53 with
vanadate and without
silica or phosphate
NSFI-53 without
phosphate
NSFI-53 with no
silica or phosphate
NSFI-53 without silica
NSFI-53
Mineql+ Modeling Results
0 5 10 15 20 25 30 35 40
C e (μg As(V)/L)
Figure 6.4 Mineql+-predicted effect of silica, phosphate, and vanadate on As(V) adsorption
on the in-situ-formed Fe(III) hydroxide at pH 8.5
q e (μg As(III)/mg Fe(III))
18
16
14
12
10
8
6
4
2
0
NSFI with no silica
or phosphate
NSFI-53 with vanadate
and without silica or
phosphate
NSFI-53 with
vanadate
NSFI-53 without
phosphate
NSFI-53 without
silica
NSFI-53
Mineql+ Modeling Results
0 5 10 15 20 25 30 35 40
C e (μg As(III)/L)
Figure 6.5 Mineql+-predicted effect of silica, phosphate, and vanadate on As(III)
adsorption on the in-situ-formed Fe(III) hydroxide at pH 6.5
Figure 6.5 shows the As(III) adsorption isotherms onto Fe(III) hydroxide at pH 6.5 for all
the six cases modeled. Only two different isotherms resulted from the six cases modeled. It can
be seen that the adsorption isotherms for the three cases which had phosphate (lower curves) are
different from other cases which did not have phosphate (upper curves). So, the presence of
phosphate reduced the adsorption capacity at pH 6.5. But the presence of neither silica nor
vanadate had any effect on the adsorption capacity according to the Mineql + program. Figure
6.6 shows the As(III) adsorption isotherms onto Fe(III) hydroxide at pH 7.5 for all the six cases
74

modeled. It can be seen that the adsorption isotherms are similar in all cases. So neither the
presence of silica, phosphate, nor vanadate had any effect on the adsorption capacity at pH 7.5
according to the Mineql + program. Figure 6.7 shows the As(III) adsorption isotherms onto
Fe(III) hydroxide at pH 8.5 for all the six cases modeled. It can be seen that the adsorption
isotherms are similar in all cases. So, neither the presence of silica, phosphate, nor vanadate had
any effect on the adsorption capacity at pH 8.5 according to the Mineql + program.
q e (μg As(III)/mg Fe(III))
25
20
15
10
5
0
NSFI-53 with no silica
or phosphate
NSFI-53 without
phosphate
NSFI-53 with
vanadate
NSFI-53 without silica
NSFI-53
NSFI-53 with vanadate and
without silica or phosphate
Mineql+ Modeling Results
0 5 10 15 20 25 30 35
C e (μg As(III)/L)
Figure 6.6 Mineql+-predicted effect of silica, phosphate, and vanadate on As(III)
adsorption on the in-situ-formed Fe(III) hydroxide at pH 7.5
q e (μg As(III)/mg Fe(III))
20
15
10
5
0
NSFI-53 with vanadate
and without silica or
phosphate
NSFI-53 with
vanadate
NSFI-53 with no
silica or phosphate
NSFI-53 without silica
NSFI-53 without
phosphate
NSFI-53
Mineql+ Modeling Results
0 5 10 15 20 25 30 35
C e (μg As(III)/L)
Figure 6.7 Mineql+-predicted effect of silica, phosphate, and vanadate on As(III)
adsorption on the in-situ-formed Fe(III) hydroxide at pH 8.5
75

COMPARISON OF MODEL-PREDICTED VS EXPERIMENTALLY OBSERVED
ARSENIC ADSORPTION ONTO Fe(OH) 3 (S)
The adsorption isotherms previously plotted were used to calculate the adsorption
capacity for an equilibrium concentration of 10 μg/L As(V) or As(III) as explained earlier. The
results were then compared (Figures 6.8-6.13) with the experimental results obtained during
coagulation experiments.
Figures 6.8, 6.9, and 6.10 compare the As(V) adsorption capacities of Fe(OH) 3 by model
and experiment at pH 6.5, 7.5, and 8.5, respectively. For different compositions of challenge
water, the modeling results differed significantly from the experimental results, which was
expected in light of the adsorption isotherms generated from the model (Figs. 6.2, 6.3, and 6.4).
The Mineql+ program did not show any effect of silica, phosphate, or vanadate except in the
presence of phosphate at pH 6.5. But from experiment results, the presence of silica, phosphate,
and vanadate had effect on the adsorption of As(V), which was not predicted by the unmodified
Mineql+ 4.50 model.
q e (mg As(V)/g Fe(III))
200
160
120
80
40
Model
Experiment
0
NSFI-53
NSFI-53
without
phosphate
NSFI-53 with
vanadate
NSFI-53 with
vanadate &
without silica
or phosphate
NSFI-53
without silica
NSFI-53 with
no silica or
phosphate
Figure 6.8 Comparison of As(V) adsorption capacities of Fe(OH) 3 for an equilibrium
concentration of 10 μg/L at pH 6.5
76

q e (mg As(V)/g Fe(III))
160
120
80
40
Model
Experiment
0
NSFI-53
NSFI-53
without
phosphate
NSFI-53 with
vanadate
NSFI-53 with
vanadate &
without silica
or phosphate
NSFI-53
without silica
NSFI-53 with
no silica or
phosphate
Figure 6.9 Comparison of As(V) adsorption capacities of Fe(OH) 3 for an equilibrium
concentration of 10 μg/L at pH 7.5
100
q e (mg As(V)/g Fe(III))
80
60
40
20
Model
Experiment
0
NSFI-53
NSFI-53
without
phosphate
NSFI-53 with
vanadate
NSFI-53 with
vanadate &
without silica
or phosphate
NSFI-53
without silica
NSFI-53 with
no silica or
phosphate
Figure 6.10 Comparison of As(V) adsorption capacities of Fe(OH) 3 for an equilibrium
concentration of 10 μg/L at pH 8.5
A similar comparison between model-predicted and experimentally observed As(III)
adsorption was made and the results are shown in Figures 6.11-6.13. Figures 6.11, 6.12, and 6.13
have compared the As(III) adsorption capacities of Fe(OH) 3 by model and experiment at pH 6.5,
7.5, and 8.5, respectively. For different compositions of challenge water, the modeling results
differed significantly from the experimental results, which was expected in light of the
77

adsorption isotherms generated from the model (Figs. 6.5, 6.6, and 6.7). The Mineql+ program
did not show any effect of silica, phosphate, or vanadate except in the presence of phosphate at
pH 6.5. But from experiments, the presence of silica, phosphate, and vanadate did significantly
affect the adsorption of As(III) which was not predicted by the Mineql+ model.
q e (mg As(III)/g Fe(III))
7
6
5
4
3
2
1
Model
Experiment
0
NSFI-53
NSFI-53
without
phosphate
NSFI-53 with
vanadate
NSFI-53 with
vanadate &
without silica
or phosphate
NSFI-53
without silica
NSFI-53 with
no silica or
phosphate
Figure 6.11 Comparison of As(III) adsorption capacities of Fe(OH) 3 for an equilibrium
concentration of 10 μg/L at pH 6.5
10
q e (mg As(III)/g Fe(III))
8
6
4
2
Model
Experiment
0
NSFI-53
NSFI-53
without
phosphate
NSFI-53 with
vanadate
NSFI-53 with
vanadate &
without silica
or phosphate
NSFI-53
without silica
NSFI-53 with
no silica or
phosphate
Figure 6.12 Comparison of As(III) adsorption capacities of Fe(OH) 3 for an equilibrium
concentration of 10 μg/L at pH 7.5
78

q e (mg As(III)/g Fe(III))
20
16
12
8
4
0
NSFI-53
NSFI-53
without
phosphate
NSFI-53 with
vanadate
NSFI-53 with
vanadate &
without silica
or phosphate
Model
Experiment
NSFI-53
without silica
NSFI-53 with
no silica or
phosphate
Figure 6.13 Comparison of As(III) adsorption capacities of Fe(OH) 3 for an equilibrium
concentration of 10 μg/L at pH 8.5
CONCLUSIONS
The Mineql+ (version 4.50) program used for modeling the adsorption of arsenic onto
Fe(OH) 3 during coagulation gave results significantly different from the experimental results.
The reasons for the variation could be due to the following.
1. Co-precipitation of arsenic onto iron solids is not included.
2. According to the model, there is no adsorption of silica onto Fe(OH) 3 , whereas silica
adsorption does occur experimentally.
3. According to the model, there is no adsorption of vanadate onto Fe(OH) 3 , whereas
vanadate adsorption does occur experimentally.
4. According to the model, phosphate is precipitated at pH greater than 6.5 and so does not
get adsorbed onto Fe(OH) 3 . So phosphate does not affect the adsorption of arsenic at pH
7.5 and 8.5 according to model, whereas phosphate adsorption does occur at pH 7.5 and
8.5 experimentally.
Thus, the unmodified Mineql+ model (version 4.50), although widely used for chemical
equilibrium modeling, cannot be used to accurately model arsenic adsorption onto Fe(OH) 3
during coagulation experiments.
79

CHAPTER 7
SUMMARY AND CONCLUSIONS
SUMMARY
The objectives of the proposed work were to determine the technical and economic
feasibility of using aluminum, zirconium and titanium salts in comparison with ferric salts as
coagulants for arsenic removal in coagulation-filtration processes. The project objectives were
attained during an experimental and modeling study comprising seven phases:
1. study the removal efficiencies of As(V) and As(III) during coagulation with alum, TiCl 4 ,
TiCl 3 , TiOCl 2 , ZrCl 4 , ZrOCl 2 salts in comparison with FeCl 3 ,
2. establish the effect of pH on As(V) and As(III) removals using these seven coagulants,
3. compare the As(III)/As(V) adsorption on a molar, mass and cost basis for these seven
coagulants and recommend the best coagulant,
4. study the effect of competing ions (silica, phosphate and vanadate) in NSFI-53 challenge
water on As(V) and As(III) removal during coagulation with FeCl 3 , alum, TiCl 4 , TiCl 3 ,
and ZrCl 4 ,
5. quantify the individual effects of silica, phosphate and vanadate during coagulation with
FeCl 3 ,
6. establish the toxicity characteristics of the sludges produced during coagulation with
FeCl 3 , alum, TiCl 4 , TiCl 3 , and ZrCl 4 , and compare the results with the regulatory limits of
the TCLP and WET tests, and
7. determine if the adsorption of As(V) and As(III) onto Fe(OH) 3 could be accurately
modeled using the MINEQL+ (version 4.50) water chemistry equilibrium model and
compare the model results with experimental results.
CONCLUSIONS
The study of the removal of arsenic in NSFI-53 challenge water with the innovative
coagulants (TiCl 4 , TiCl 3 , TiOCl 2 , ZrCl 4 , and ZrOCl 2 ) compared to commonly used coagulants
(FeCl 3 and alum) resulted in the following conclusions:
1. The percent removal of As(V) was highly pH dependent in the NSFI-53 challenge water,
and the removal increased with decreasing pH for all coagulants tested: FeCl 3 , alum,
TiCl 4 , TiCl 3 , TiOCl 2 , ZrCl 4 , and ZrOCl 2 . In particular, the adsorption capacity of As(V)
with zirconium salts decreased significantly with increasing pH.
2. The percent removal efficiency of As(III) was independent of pH for FeCl 3 , TiCl 4 , ZrCl 4 ,
and ZrOCl 2 , and it decreased with increasing pH for TiCl 3 and increased with increasing
pH for TiOCl 2 . The removal of As(III) by alum was insignificant..
3. At all doses, the removal efficiency of As(V) was significantly greater than As(III) at pH
6.5, 7.5, and 8.5 with all seven coagulants tested: FeCl 3 , alum, TiCl 4 , TiCl 3 , TiOCl 2 ,
ZrCl 4 , and ZrOCl 2 .
81

4. When comparing arsenic adsorption isotherms for all the coagulants, the highest As(V)
loadings on a coagulant on a mass basis (mg As(V)/g metal) were observed with FeCl 3 ,
which performed better than aluminum, titanium and zirconium salts at pHs of 6.5 and
7.5. However, at pH 8.5, As(V) loadings on FeCl 3 were approximately the same as TiCl 3
at equilibrium As(V) ≤ 10 μg/L.
5. When comparing adsorption isotherms, the highest As(V) loading on any coagulant on a
molar basis or a mass basis was observed for ferric chloride at all three pHs, and the
As(V) loading on iron was significantly greater than aluminum.
6. On a mass basis, the comparison of As(V) adsorption capacities of titanium and
zirconium salts with FeCl 3 for an equilibrium concentration of 10 μg/L was as follows:
• pH 6.5: FeCl 3 > Alum > ZrOCl 2 ≈ ZrCl 4 > TiCl 4 > TiCl 3 > TiOCl 2
• pH 7.5: FeCl 3 >> TiCl 3 > TiCl 4 > Alum ≈ ZrOCl 2 > ZrCl 4 > TiOCl 2
• pH 8.5: FeCl 3 ≈ TiCl 3 > TiCl 4 > Alum > ZrCl 4 ≈ TiOCl 2 > ZrOCl 2
7. Regardless of the basis of comparison (mass or molar), FeCl 3 was a far better coagulant
than alum for As(V) removal.
8. When comparing arsenic adsorption isotherms for all the coagulants, the highest As(III)
loading on a coagulant (mg As(III)/g metal) was observed with titanium(III) chloride,
which performed better than ferric, titanium(IV), and zirconium salts at pHs of 6.5 and
7.5. However Ti(III) had similar adsorption capacity to that of Fe(III) and Ti(IV)
coagulants at pH 8.5. Alum did not have any adsorption capacity for As(III). TiCl 4
exhibited similar removal efficiency to that of FeCl 3 , and TiOCl 2 offered similar removal
efficiency to FeCl 3 at pH 8.5. Zirconium salts did not have good adsorption capacity for
As(III). Thus, it appears that on an mg metal/L basis, TiCl 3 could be a better coagulant for
As(III) removal in coagulation-filtration processes.
9. On a mass basis, the comparison of As(III) adsorption capacities of titanium and
zirconium salts with ferric chloride for an equilibrium concentration of 10 μg/L was as
follows:
• pH 6.5: TiCl 3 > TiCl 4 ≈ FeCl 3 > TiOCl 2 >> ZrCl 4 ≈ ZrOCl 2 >> Alum
• pH 7.5: TiCl 3 > TiCl 4 ≈ FeCl 3 > TiOCl 2 >> ZrCl 4 ≈ ZrOCl 2 >> Alum
• pH 8.5: TiCl 3 ≈ TiCl 4 ≈ FeCl 3 ≈ TiOCl 2 >> ZrCl 4 ≈ ZrOCl 2 >> Alum
10. On a molar basis, the highest As(III) loading on a coagulant was observed for
titanium(III) chloride at pH 6.5 and 7.5, while at pH 8.5, ferric chloride and had the
highest molar adsorption capacity.
11. When comparing chemical costs for FeCl 3 , alum, TiCl 4 , ZrOCl 2 , and TiOCl 2 coagulation
to remove As(V) or As(III) the most economical was FeCl 3 . Of the common coagulants,
alum was found to be 4-8 times more expensive than ferric chloride for As(V). The
82

chemical cost of ferric chloride coagulation was calculated to be more than 5 to 20 times
higher for As(III) treatment compared with As(V).
12. There was experimental evidence that the high removal efficiency of As(III) by TiCl 3 and
the unusual As(III) behavior of increasing removal with decreasing pH was due to
oxidation of As(III) to As(V) by H 2 O 2 , which based on the literature, formed from Ti(III)
hydrolysis in the NSFI challenge water, which contained some dissolved oxygen. In spite
of its partial oxidation, the experimentally observed removal of As(V) oxidized from
As(III) was far less than the removal of a similar starting concentration of As(V), because
(a) the floc was already formed when it contacted As(V), and (b) the As(III) oxidation
continued for many hours during which the Ti(OH) 3 formed had settled and was not in
contact with As(V) formed.
The studies on effect of competing ions (silica, phosphate and vanadate) on arsenic
adsorption in NSFI-53 challenge water with FeCl 3 , alum, TiCl 4 , TiCl 3 , and ZrCl 4 as coagulants
resulted in the following conclusions:
13. In the NSFI-53 challenge water with phosphate, silica significantly reduced the
adsorption of arsenic presumably by competing for adsorption sites. The As(V) and
As(III) removal efficiencies in the absence of silica were higher than in the presence of
silica for all coagulants tested (FeCl 3 , TiCl 4 , TiCl 3 , and ZrCl 4 ). With alum as coagulant,
silica significantly affected As(V) adsorption at pH 6.5-7.5, while it had no significant
effect at pH 8.5. In the NSFI-53 challenge water without silica, the adsorption of As(III)
increased with increasing pH for TiCl 4 , FeCl 3 and ZrCl 4 , whereas pH did not significantly
affect As(III) adsorption on these coagulants in the standard challenge water with silica
present. The detrimental effect of silica on As(III) removal increased with increasing pH
for all coagulants except TiCl 3 .
14. In the NSFI-53 challenge water with silica, phosphate was found to reduce the adsorption
of As(V) significantly at pH 6.5 and 7.5, whereas it had a lesser effect at pH 8.5 with all
coagulants tested (FeCl 3 , TiCl 3 , TiCl 4 , and ZrCl 4 ). However with alum as coagulant,
phosphate did not affect As(V) adsorption. In contrast to As(V) adsorption, the presence
of phosphate did not significantly affect the adsorption of As(III) with the coagulants
tested (FeCl 3 , TiCl 3 , TiCl 4 , and ZrCl 4 ).
15. In the NSFI-53 challenge water with silica and phosphate, vanadate did not significantly
affect the adsorption of As(V) with all coagulants tested (FeCl 3 , alum, TiCl 3 , TiCl 4 , and
ZrCl 4 ). Similarly, in the NSFI-53 challenge water with silica and phosphate, vanadate
did not significantly affect the adsorption of As(III) with all coagulants tested (FeCl 3 ,
TiCl 3 , TiCl 4 , and ZrCl 4 )
The studies on individual effect of competing ions (silica, phosphate and vanadate) on
arsenic adsorption with FeCl 3 as coagulant resulted in the following conclusions:
16. In the absence of other competing ions, it was found that silica, phosphate, and
vanadate exhibited significant competitive effects on the adsorption of As(V) and As(III).
83

17. In the absence of phosphate and vanadate, silica significantly decreased the adsorption
of As(V) and As(III), and the effect increased with increasing pH.
18. In the absence of silica and vanadate, phosphate significantly decreased the adsorption
of As(V) at all pHs, and a had minor effect on the adsorption of As(III) at pH 7.5 and 8.5.
19. In the absence of silica and phosphate, vanadate significantly decreased the adsorption
of As(V), and had a minor effect on the adsorption of As(III) at pH 7.5 and 8.5 during
FeCl 3 coagulation.
20. Based on the FeCl 3 experimental results with and without multiple competing
contaminants the following inferences were made:
• The presence of silica significantly reduced the magnitude of the phosphate effect
on As(V) adsorption at pH 7.5 and 8.5.
• The presence of silica reduced the effect of phosphate in the case of As(III)
adsorption at all pHs.
• The combined presence of silica and phosphate reduced the effect of vanadate at
all pHs in the case of As(V) and As(III) removal.
The toxicity characteristics of the sludges produced during coagulation with FeCl 3 , alum,
TiCl 3 , TiCl 4 , and ZrCl 4 led to the following conclusions:
21. The adsorption capacity of As(V) was independent of initial arsenic concentration with
all the coagulants tested (FeCl 3 , alum, TiCl 3 , TiCl 4 , and ZrCl 4 ).
22. Following coagulation and 3-hr settling, with all four coagulants tested (FeCl 3 , alum,
TiCl 3 , TiCl 4 , and ZrCl 4 ), the liquid wastes obtained were found to contain less than 0.5%
dry solids, and according to the TCLP regulations, did not require extraction. The liquid
filtrate, which is considered to be the extract in such cases, easily passed the TCLP and
WET test regulatory limit of 5 mg/L arsenic.
The attempt to model As(V)/As(III) adsorption during coagulation with FeCl 3 using
Mineql+ (version 4.50) program led to the following conclusions:
23. The unmodified Mineql+ chemical equilibrium modeling program could not simulate
As(III) or As(V) adsorption onto Fe(OH) 3 during FeCl 3 coagulation.
24. The model predicts significantly less adsorption onto Fe(OH) 3 compared with
experimental results, and the model also does not take into account the effect of silica,
phosphate and vanadate on As(V)/As(III) adsorption in most cases.
84

RECOMMENDATIONS
The main purpose of this project was to determine the technical and economic feasibility
of using aluminum, zirconium and titanium salts in comparison with ferric salt as coagulants for
arsenic removal in coagulation-filtration processes. Based on adsorption and economic
comparisons, this work showed that FeCl 3 was clearly superior to the other coagulants tested.
Although Ti(III) had the highest removal efficiency for As(III), its chemical cost was not
available and is expected to be higher considering the cost of Ti(IV) salts. A chemical cost
comparison of commonly used coagulants showed that alum was 4-6 times more expensive than
FeCl 3 for As(V) removal. Taking the detailed conclusions above into consideration, the
following recommendations are made for application of coagulation for arsenic removal in
drinking water treatment systems:
1. FeCl 3 should be considered as the preferred coagulant for As(V) and As(III) removal at
all pHs and background water compositions.
2. Alum could be considered as a coagulant for As(V) removal at pH ≤6.5 where its As(V)
capacity is closer to that of FeCl 3 .
3. As(III) should be pre-oxidized to As(V) for cost-effective treatment with alum and FeCl 3
coagulants.
4. Zirconium and/or titanium could be considered as alternative coagulants to alum, if their
prices drop significantly.
85

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89

ABBREVIATIONS
AAS
As
AWWA
AwwaRF
Atomic Absorption Spectrometer
Arsenic
American Water Works Association
Awwa Research Foundation
C e
Equilibrium concentration (μg As/L)
Co.
company
°C degree Celcius
DIW
$/Mgal
ed.
EDL
FI-HG-AAS
g
g/mL
hr
ICP-MS
IX
Jour
Kg/L
L
M
MCL
min
mg/g
mg/L
mL
mM
mmol/mol
μg/L
deionized water
dollars per million gallons
edition
Electrodeless discharge lamp
Flow Injection Hydride Generation Atomic Absorption Spectrometer
gram
gram per milliliter
hour
Inductively Coupled Plasma-Mass Spectrometer
Ion Exchange
Journal
Kilograms/liter
Liter
Molar
Maximum contaminant level
minute
milligram/gram
milligram/liter
milliliter
millimolar
millimole/mole
microgram/liter
91

μg/mg
μL
μm
nm
NSFI
pK a
POU/POE
PO 4 -P
ppb
ppm
microgram/milligram
microliter
micrometer
nanometer
National Sanitation Foundation International
Negative logarithm of an ionization/ equilibrium constant
Point-of-use/Point-of-entry
Phosphate as Phosphorous
parts per billion
parts per million
q e
Adsorption capacity (μg As/mg metal)
rpm
(s)
sec
TCLP
USEPA
USGS
VO 3 -V
WET
Revolutions per minute
solid
second
Toxicity Characteristic Leaching Procedure
U.S. Environmental Protection Agency
U.S. Geological Survey
Vanadate as Vanadium
Waste Extraction Test
92