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Electrochemically enhanced reduction of hexavalent chromium in contaminated clay: Kinetics, energy consumption, and application of pulse current

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Abstract

Electrochemically enhanced reduction of Cr(VI) in clay medium is a technique based on inputting extra energy into the clay to drive the favorable redox reaction. In this study, the reducing reagent Fe(II) was transported into Cr(VI) spiked kaolinite clay by direct current to investigate the dependency of reaction rate on energy consumption. A modified electrophoresis cell with platinum wires as working electrodes was used to run experiments. Results showed that the reduction rate of Cr(VI) was significantly increased by application of current with the pseudo-first-order rate constant kpse from 0.002min-1 at current density of 0mA/cm2 to 0.016min-1 at current density of 0.6mA/cm2, and the corresponding reduction efficiency after 60min experimental time was increased from 8.5% to 57.5%. Mass transport process of Fe(II) in clay pore fluid was determined as the rate controlling step. With the increasing rate of Cr(VI) reduction, both of the productive and non-productive energy consumption increased, from 0.53 and 0.15mWh at current density of 0.1mA/cm2 to 18.9 and 12.6mWh at current density of 0.6mA/cm2, respectively. The non-productive energy consumption was caused by the formation of [(Cr,Fe)(OH)3] precipitates. XRD analysis suggested that the [(Cr,Fe)(OH)3] formed at the clay surface and grew into the pore fluid. SEM-EDX results indicated that the overall Fe(III):Cr(III) ratio of the precipitates was approximately 1.26:1. Application of pulse current decreased the non-productive energy consumption by decreasing the polarization potential drop of each cycle. This effect was more significant in lower pulse frequency due to the better restoration of equilibrium state of clay medium during relaxation period.