Kinetics of reduction of nitrobenzene and carbon tetrachloride at an iron-oxide coated gold electrode

The rates of reduction of carbon tetrachloride (CT) and nitrobenzene (NB) by iron-oxide coated gold electrodes were studied to gain insight into the processes that control reduction of groundwater contaminants by zerovalent metal permeable reactive barriers. Fe(III)-oxide films were deposited on gold electrodes with a small fraction of the Fe(III) electrochemically reduced to Fe(II) to investigate the role of Fe(II) in the reduction of the CT and NB. Mass transport to the surface of the oxide film was controlled through use of a well-defined flow-through system similar to a wall-jet electrode. The factors affecting the overall reduction rate were investigated by varying the Fe(II) content in the iron-oxide, controlling mass transport of the electroactive species to the oxide surface, and varying the thickness of the oxide film. The rates of reduction of CT and NB were found to be independent of Fe(II) content in the iron-oxide and were only slightly dependent on the rate of transport to the surface of the oxide under a few sets of reaction conditions. Conversely, the rates of reduction were greatly dependent on the thickness of the oxide film, with the reduction rate decreasing as the oxide thickness increased. Evidence suggests that the location of the reduction reaction for CT and NB is at the gold surface and supports a barrier model for the system studied, in which the oxide film physically impedes direct contact of the electroactive species and the gold electrode, increases the diffusion path length, and creates adsorption sites.