paper 66 - ACS: Division of Environmental Chemistry

Preprints of Extended Abstracts Vol. 42 No. 2
ELECTROCHEMICAL METHODS FOR WASTEWATER
AND POTABLE WATER TREATMENT
Organized by
J.M. Farrell
Symposia Papers Presented Before the Division of Environmental Chemistry
American Chemical Society
Boston, MA August 18-22, 2002
INVESTIGATION OF ARSENATE REDUCTION IN IRON MEDIA FILTERS
Nikos Melitas 1 , Martha Conklin 2 and James Farrell 1
1 Department of Chemical and Environmental Engineering
2 Department of Hydrology and Water Resources
University of Arizona, Tucson, AZ 85721
Introduction
Zerovalent iron filings have been proposed as a filter medium for removing As(V) and
As(III) compounds from potable water 1-4 . Although column studies have not observed
changes in the arsenic oxidation state 1,2 , reduction of As(V) and oxidation of As(III) has
been observed in batch experiments conducted in sealed vials containing iron filings 3 .
The issue of As(V) reduction and As(III) oxidation in iron filter media is important since
As(III) exhibits different binding characteristics than As(V) 5-8 . This research investigated
the electrochemical reactions that occur on the surface of zerovalent iron in solutions
containing As(V) and As(III).
Materials and Methods
All experiments were performed in sealed glass reactors in 3 mM CaSO4 background
electrolyte solutions. The effect of the As(V) concentration on the rate of iron corrosion
was assessed by Tafel analysis using a single 10 cm long iron wire placed in a 0.75 L
glass reactor. Chronoamperometry experiments were performed at potentials of –885
and –1385 mV/SHE to assess the effect of potential and As(V) concentration on
arsenate reduction. Chronopotentiometry experiments were performed to assess the
effect of As(V) concentration, current density, and pH on arsenate reduction.
Experiments were performed at pH values of 2, 6.5 and 11 and at cathodic current
densities of 0.5, 1, 5, 10, 20, 30 and 50 µA.
Results and Discussion
Figure 1 shows Tafel diagrams for an iron wire exposed to solutions containing As(V) at
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Preprints of Extended Abstracts Vol. 42 No. 2
concentrations ranging from 0 µg/L to 20,000 µg/L. The lower cathodic currents at each
potential in the four arsenate solutions indicate a decrease in the exchange current for
water reduction. The observed decrease in the exchange current for water reduction
can be attributed to the blocking of cathodic sites by arsenic compounds chemisorbed
to the iron surface. For potentials lower than -475 mV, the lower anodic currents in the
As(V) solutions indicate that the anodic exchange current for iron oxidation was
diminished in the presence of arsenic. The decrease in the anodic exchange current
can be attributed to the blocking of anodic sites by adsorbed arsenic compounds. The
change in the anodic slopes at a potential of approximately –475 mV may be attributed
to oxidation of iron complexed with arsenic.
Figure 2a shows chronoamperometry profiles for an iron wire cathode at an applied
potential –885 mV. For solutions with final concentrations of 10 and 100 mg/L, As(V)
was added to each solution at the indicated concentration at 5.5 minutes elapsed. In
the solution with a final concentration of 1000 mg/L, the As(V) was added in three
increments at 5.5, 5.8 and 6.1 minutes elapsed. Only when the solution concentration
reached 1000 mg/L was a current pulse for arsenic reduction observed. This current
pulse was for reduction of arsenate chemisorbed to the iron surface. Changes in
solution concentrations of As(III) confirmed that there was no reduction of solution
phase As(V) in any experiments conducted at neutral and alkaline pH values at a
potential of –885 mV. However, at a pH value of 2, chronopotentiometry experiments
showed that arsenate reduction occurred over a wide potential range.
2.5
1.5
0.5
-0.5
-1.5
0 µg/L
β c = 0.007mV -1
100 µg/L
-750 -650 -550 -450 -350
E (mV SHE)
5000 µg/L
β a = 0.016mV -1
10000 µg/L
20000 µg/L
Figure 1. Tafel scans for an iron wire electrode in anaerobic solutions containing As(V)
at the indicated concentration at a pH value of 6.5. The electrode was equilibrated with
each solution for 1 day prior to performing each scan.
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Preprints of Extended Abstracts Vol. 42 No. 2
250
200
150
E=-885 mV
100 mg/L
5 6 7 8 9 10
Elapsed Time (min)
1,000 mg/L
0 mg/L
10 mg/L
Figure 2. Chronoamperometry profiles for an iron wire electrode in anaerobic 3 mM
CaSO4 electrolyte solutions at a potential of -885 mV and a pH value of 6.5.
References
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