Influence of coexisting anions on the one-step electrochemical reduction and precipitation removal of Cr(VI): Implications for advanced wastewater treatment.

Electroreduction of Cr(VI) coupled with in-situ precipitation of Cr(III) on the cathode is a promising method for removing Cr(VI) from wastewaters. However, the influence of coexisting anions in wastewaters on the electrochemical removal process remains unclear. This study investigated the impact of common inorganic anions, including nitrate (NO), chloride (Cl), phosphate (PO) and sulfate (SO), on the electrochemical removal processes of Cr(VI). The results indicated that HCrO was directly electrochemically reduced to Cr, and the OH generated through electro-mediated water reduction could complex with Cr, thereby transforming Cr into chromium hydroxide (Cr(OH)) coated at cathode. Coexisting anions would partially penetrate the alkaline Cr(III) complexes, inhibiting the formation of Cr(OH) passivation layer and promoting the electroreduction of Cr(VI), whose penetration ability followed the order of SO > PO > Cl > NO. Both the inhibitory effect on Cr(III) precipitation and promoting effect on Cr(VI) reduction were intensified with increasing concentrations of these anions in the range of 1-100 mmol L. Accordingly, after electrolysis of 10 mg L Cr(VI) at an initial pH of 3.0 and -0.2 V (vs. Ag/AgCl), the highest electrochemical reduction ratio of Cr(VI) (99.9%) was achieved in the presence of 100 mmol L SO, while the total Cr removal ratio was minimal (3.3%). In contrast, the presence of NO at 1 mmol L resulted in a nearly lowest reduction ratio of Cr(VI) (92.9%), with the maximum total Cr removal ratio (92.8%). These findings provide new insights into the electrochemical removal mechanisms of Cr(VI) in complex solution environments.