281101 Integrated Electrokinetic and Microbial Fuel Cell Technologies for Enhanced Transport and Bioremediation of Hexavalent Chromium in Groundwater

Wednesday, October 31, 2012: 4:35 PM
408 (Convention Center )
Ryan Thacher, Lewis Hsu and Massoud Pirbazari, Sonny Astani Department of Civil and Environmental Engineering, University of Southern California, Los Angeles, CA

Groundwater contamination threatens potable water supplies around the world, and this fact has encouraged the progression of novel research for groundwater treatment. Hexavalent chromium (CrVI) is an EPA priority contaminant due to its high toxicity, and its prevalence in groundwater around the world as a result of improper disposal practices from a variety of industries. CrVI is highly mobile in water systems due to its solubility, making treatment by conventional methods difficult and costly. 

Electrokinetic remediation is a technique that can be used for transporting ionic contaminants, including CrVI (as HCrO4-) by the application of an electric potential across a contaminated aquifer. Electrokinetic remediation is often limited by a pH change due to electrolysis reactions. These reactions create OH- at the cathode and H+ at the anode, which migrate towards the opposite electrodes by electrokinetic phenomena. At the point of intersection of these fronts a significant drop in electrical conductivity occurs, causing transport to slow or stop completely. Additionally, the dynamic pH change affects precipitation and dissolution reactions, which govern the solution chemistry and availability of the contaminant for transport. Many enhancement techniques have been investigated to mitigate this problem, however they can add significant economic and environmental cost.

This study investigates the potential to enhance electrokinetic transport of CrVI in groundwater while also promoting its reduction to CrIII by integrating an electrokinetic system with microbial fuel cell (MFC) technology. Recent studies have shown MFCs to be effective in CrVI reduction, as CrVI can act as an electron acceptor when introduced to the MFC cathode compartment. This reaction is catalyzed by the presence of biofilm on the cathode, which facilitates a complex electron transport system from the cathode to CrVI in solution. In this context, Shewanella putrefaciens MR-1 bacteria have been documented to be effective biocatalysts for this application, and different carbon sources including lactate, acetate, formate, and pyruvate have demonstrated to be effective electron donors in the anode compartment.

In this study, the integration of these technologies is evaluated using a soil column under an applied electric potential, and an MFC with anode and cathode reservoirs with a 300 mL capacity. The electric potential applied across the soil column is varied in different experiments from 0.5 – 2.0 V/cm in an effort to optimize transport by counteracting the advective pull of CrVI towards the cathode by water flow. The soil column is operated with a continuous flow of simulated groundwater spiked with CrVI at a rate comparable to that of groundwater in sandy soils. The flow direction is from the anode towards the cathode, while CrVI is transported by electrokinetic phenomena in the opposite direction. During operation, the soil column effluent adjacent to the anode reservoir is pumped out into the cathode of the MFC. Recirculating solution to and from this location in the column will mitigate the dramatic drop in pH, which occurs in the absence of enhancement strategies from H+ generation at the anode. Effluent from the cathode reservoir of the soil column is sampled and analyzed for CrVI by ion-exchange chromatography to determine the extent of treatment.

The optimization of the electrokinetic and MFC integration will be discussed in this presentation. Key points of discussion will be CrVI reduction rates and mechanisms at the biocathode, cathode effluent CrVI concentrations, effects of solution circulation to enhance CrVI transport, and energy production by the MFC.

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