Potential use of Electrochemical Redox for In Situ Remediation of Benzene

Permeable, reactive electrochemical redox barriers can be engineered for in situ remediation of groundwater by enhancing oxidation conditions at a permeable anode and/or reducing conditions at a permeable cathode. However, it is necessary to assess the potential for transformation of target contaminants by electrochemical redox. This paper evaluates the effect of current density and presence of chloride ions in groundwater on electrochemical degradation of benzene in solution. Electrochemical reactors, composed of an anode and a cathode separated by a Nafion® membrane, were used to evaluate the effect of three current densities (1, 9 and 18 mA/L) on the transformation of 40 mg/L benzene solution. Transformation is demonstrated at different rates depending on the current density. Under the lowest current density (1.8 mA/L), 56% of benzene was transformed in the first 48 hours and more than 96% was transformed after 216 hours. A much faster rate is noted under 9 mA/L, where 87% of benzene was transformed after 48 hours. The higher current density (18.2 mA/L) produced results similar to the 9 mA/L. The results show that most of the benzene transformation occurred in the first four hours, where both the 9 and 18.2 mA/L densities resulted in benzene concentration less than 2 mg/L after 72 hours. Transformation of by products, including chlorobenzene compounds, continued to occur and final concentrations levels of less than 3 mg/L were reached after 300 hours. The effect of electrolysis on anolyte pH and redox potential was evaluated. Under 9 mA/L, the redox potential increased significantly at the anolyte in the first 8 hours to about 900 mV. After that, the redox potential continued to increase, but at a lower rate until it reached 1380 mV at the end of processing. Similar behavior is noted for the anolyte pH, which decreased significantly in the first 8 hours to less than 2.5 and continued to decrease until it reached pH value of 1.86 at the end of testing.