PNNL-13501 - Pacific Northwest National Laboratory

Electrochemical Enhancement of Permeable Treatment Barriers
Johanes H. Sukamto, Wesley E. Lawrence, Vilayanur V. Viswanathan, Jonathan S. Fruchter
Study Control Number: PN00038/1445
In Situ Redox Manipulation (ISRM) is an innovative technology for treating chromate-contaminated groundwater plumes
at the Hanford Site. Currently, the technology requires injection of expensive reducing agents. This project evaluated the
feasibility of using ex situ electrochemical synthesis of dithionite to reduce both the chemical cost and waste volume
associated with the current process. The successful demonstration of this technology may lead to further development of
a process for in situ generation of dithionite.
Project Description
A procedure for the electrochemical synthesis of
dithionite (S2O4 -2 ) was developed through modification of
a single-pass process developed by Oloman et al. (1999).
The effects of reactant concentration, reactant flow rate,
and current density on product concentration were
investigated. This process was used to generate dithionite
from a simulant of spent in situ redox manipulation
solution. The stability of the generated dithionite in air
and under nitrogen was investigated. In addition, a cost
analysis was performed to estimate large-scale production
cost using this method. This project advanced our ability
to electrochemically synthesize dithionite from low-cost
feedstock such as spent in situ redox manipulation
solution.
Introduction
In situ redox manipulation creates a permeable treatment
zone in the subsurface by reducing ferric iron in the
aquifier to ferrous iron. Conventional groundwater wells
are placed at the treatment site. Sodium dithionite
solution is injected into the wells for 10 to 20 hours. The
goal of the in situ redox manipulation method is to create
a permeable treatment zone in the subsurface to remediate
redox-sensitive contaminants. Redox-sensitive
contaminants in the plume are immobilized or destroyed
as they migrate through the manipulated zone.
The chemical solution of sodium dithionite is injected into
the wells and when the solution reaches the groundwater
it reacts with iron in the soil to form a large barrier.
When the groundwater flows through the barrier, the
targeted contaminants are destroyed or immobilized.
Activation of a permeable treatment barrier requires the
use of reducing agents. Currently, the reducing agents are
purchased and shipped to the treatment site and the spent
reducing agent solutions disposed of as waste. Using
electrochemical synthesis of dithionite, it is possible to
generate reducing agents on-site and possibly recycle
(reuse) the spent solution. This approach could reduce
chemical costs and minimize waste generation. In this
study, electrochemical preparation of dithionite was
studied. The effect of residence time; feed concentration;
and residence time on dithionite concentration,
conversion, and coulombic efficiency was determined. A
cost estimate was completed to determine production cost
for dithionite using this method.
Results and Accomplishments
Dithionite Generation
Dithionite was generated electrochemically using a
microflow cell (Electrocell AB). Sodium hydroxide of
varying concentration was used as the anolyte. The
catholyte consisted of a 0.2 M solution of sodium sulfite
in either deionized water or in a mixture of sulfurous acid
and sodium hydroxide. The anode was platinized
titanium, while the cathode consisted of a nickel foam
electrode with 80 pores per inch and a surface area of
32 cm 2 /geometric cm 2 . The flow pattern within each
chamber consisted of flow past the electrode at varying
rates, such that the residence time inside the cell ranged
from 1 to 10 seconds. The cathode and anode
compartments were separated by Nafion 450 membrane.
The membrane was pretreated by immersion in boiling
deionized water for 40 minutes, followed by overnight
immersion in deionized water. The exit solution pH was
controlled at 6.0 by addition of 0.5 M and 1.5 M sulfuric
acid or 1 M sulfurous acid. The catholyte was
continuously purged with nitrogen during the process.
Catholyte samples were taken downstream of the cathode
compartment exit and analyzed for dithionite
concentration by titrating against 0.5 M K3Fe(CN)6 using
methylene blue as an indicator. Calibration was done by
titrating a known volume of 0.2 M sodium dithionite
against 0.5 M K3Fe(CN)6 under a nitrogen atmosphere.
The current ranged from 0.13 to 0.51 A, which
Earth System Science 213