Effect of Organic Compounds and Copper on Chromium(VI) Reduction: Electrochemical Investigation of Electron Transfer Rates

Chromium contamination is redox-controlled, with Cr occurring in environmental settings as toxic and mobile Cr(VI) or less toxic Cr(III). The mechanisms and kinetics of Cr(VI) reduction and removal under variable environmental conditions are not fully understood. Here, we measure the charge transfer kinetics of Cr reduction under laboratory-controlled conditions using electrochemical techniques to provide mechanistic insights into the chemical reactions affecting the reduction of Cr(VI). By the addition of common anions, organic compounds, and copper, we determined the impact of each of these constituents on Cr(VI) reduction through voltammetric measurements using a gold rotating disk electrode. Copper acted as an electron shuttle and promoted faster Cr(VI) reduction rates. Sulfate and chloride as well as the organic citrate adsorbed to the electrode and slowed Cr(VI) reduction. The oxidation of certain organics (ascorbate and cysteine as well as oxalate to a lesser extent) was coupled to the reduction of Cr(VI), increasing the Cr(VI) reduction rate, while other organics (glutamate and citrate) were not electrochemically oxidized. Our work on a relatively simple Cr(VI) system illustrates how electrochemical measurements can be applied in an innovative way to provide a more direct measurement of the electron transfer kinetics of environmental and geochemical reactions.