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).