Remediation of toxic and mobile Cr(VI) to less toxic Cr(III) can be influenced by redox and adsorption reactions in the presence of organic and inorganic constituents. Here, we provide mechanistic insights into the chemical reactions affecting the reduction of Cr(VI) in the presence of organic compounds and copper using electrochemical techniques which enable measuring charge transfer kinetics of Cr reduction under laboratory controlled conditions. By adding different electrolytes, organic compounds, and copper, we determined the impact of each of these constituents on Cr(VI) reduction using voltametric measurements through a gold rotating disk electrode and measuring the resulting current. Electrolytes, such as sulfate and chloride, adsorbed to the electrode, and slowed Cr(VI) reduction. Cr(VI) reduction had kinetic barriers preventing significant reduction at more oxidizing conditions. Copper acted as an electron shuttle and promoted Cr(VI) reduction at more positive voltages. The oxidation of ascorbate and cysteine (as well as oxalate to a lesser extent) was coupled to the reduction of Cr(VI), enhancing Cr(VI) reduction rate. Glutamate and citrate were not electrochemically oxidized, but the negatively charged citrate ion adsorbed to the electrode, slowing Cr(VI) reduction. Electrochemical measurements provide a more direct measurement of the thermodynamics and kinetics of Cr(VI) reduction than other aqueous chemistry tools used in environmental chemistry and geochemistry. Similar electrochemical analyses may be useful for improved mechanistic understanding of how environmental conditions impact the redox behavior and transport of other redox-active toxic contaminants.

Electrochemical investigation of chromium(VI) reduction: effect of organic compounds and copper

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