(762d) Transport and Remediation of Chromium (VI) In Groundwater Using An Integrated Electrokinetic and Nanoscale Particle Technology

         Hexavalent
chromium [Cr(VI)] is one of the EPA's target contaminants due its prevalence in
natural waters in concentrations above levels regarded as safe. When present in
soils of low hydraulic conductivity such as clay, conventional treatment
technologies become ineffective due to the difficulty and energy consumption in
pumping through tightly packed soil. An emerging novel technology,
electrokinetic remediation, uses electric potential to transport ionic
contaminants through saturated soils and groundwater, eliminating problematic
issues associated with state-of-the-art remediation technologies. Strategic
placement of electrodes surrounding a contaminated zone with low voltage
application facilitates transport of ions through aquifers.

         Hexavalent
chromium in water is found as the anions chromate, dichromate, or chromic acid,
and has been shown to migrate by electrokinetic phenomena towards the anode. At
the anode, acidic conditions generated by electrolysis of water and reductive
electrochemical conditions promote Cr(VI) reduction to Cr(III), which is a more
stable oxidation state with lower solubility and negligible toxicity.
Nonetheless, to enhance the remediation process, it is proposed to inject
zero-valent iron nanoparticles (nZVI) directly into the subsurface. nZVI have
tremendous potential in remediation work due to their high surface area to
volume ratio, high reaction rate, and the ability to travel through soil pore
space. Furthermore, as compared to macroscale iron particles, nZVI are less
expensive and have the added flexibility of being able to inject them into the
subsurface at multiple locations and at great depth.

         A
concern regarding the use of nZVI in natural systems is their tendency to
agglomerate and form larger particles, or adsorb to soil particles. This
reduces their reactivity and mobility in the subsurface, rendering them
potentially inactive. Many conditions affect the degree of agglomeration, such
as pH, the presence of multi- or mono-valent cations, and natural organic
matter. The likelihood of agglomeration can be determined by the zeta-potential
of individual nanoparticles. This is a measure of electrokinetic potential that
exists across the interface of all solids and liquids, and as the absolute
value of zeta-potential increases, the suspension becomes more stable.

         This
study explores the use of nZVI in electrokinetic systems for the treatment of
Cr(VI) in clay soils. Zeta-potential analysis was performed to determine the
stability of surface-stabilized nZVI under experimental conditions. Results
indicated a significant negative surface charge on the nZVI, which is increased
by the presence of both Cr(VI), and natural organic matter found in all soils
and groundwater. The clay soil was also shown to be highly stable under neutral
to basic conditions due to a negative surface charge, thereby repelling nZVI in
solution.

         Integration
of nZVI with an electrokinetic soil-column apparatus was conducted to simulate
the effectiveness of these two technologies in a natural system. A 20cm column
was loaded with a clay soil spiked with Cr(VI), and electric potential was
continuously supplied to electrodes housed in reservoirs on either end of the
soil column. The reduction and transport of Cr(VI) was investigated with two
different methods of nZVI application; as a permeable reactive barrier (PRB)
and direct injection. Use of nZVI as PRB effectively reduced Cr(VI) on the
cathode-side of the barrier, but had no impact on Cr(VI) on the anode side.
Direct injection of nZVI into the electrokinetic cell near the cathode
increased reduction rates within the first 72 hours of treatment.

         It
is found that nZVI can remain stable and dispersed upon injection into a
simulated groundwater environment, and move through interstitial space under an
electric field to enhance remediation of groundwater and soils contaminated
with Cr(VI).