(125c) Direct Electrochemical Reduction to Separate Selenium from Industrially Impacted Water

Aquatic selenium (Se) pollution is closely associated with anthropogenic activities, including mining, thermoelectric power generation, and high-tech fabrication and manufacturing industries. This anthropogenic Se is primarily released as Se(IV) and Se(VI) oxyanions, and needs to be effectively separated to prevent bioaccumulation and ecosystem impacts. Se direct electrochemical reduction (SeDER) is an effective and thermodynamically favorable approach to separate Se from the aqueous solution, but evaluating the feasibility of SeDER in application requires a comprehensive understanding of reduction pathways and system performance in complex water matrices. This study focuses on the thermodynamic and kinetic performance of SeDER and competing ion behavior in industrially impacted water. Our results indicate that anion structure reorganization hinders process kinetics in electrochemical Se(VI) reduction. However, Se(IV) can be electrochemically separated from the aqueous phase through either a four- or six-electron pathway, with the former generating Se(0) directly attached to the electrode surface and the latter producing Se(-II) that is subsequently converted to Se(0). We demonstrate that raising the solution temperature to 80 ℃ deposits Se(0) in a conductive crystalline form and enables continuous reduction on the electrode surface. We further investigate cathodic and anodic competing ion behavior in both electrochemical pathways. The results suggest that sulfate promotes electrochemical Se(IV) removal efficiency by 11-23%, but nitrate hinders Se(IV) removal (2-11% decrease) by occupying cathodic reaction sites. The anodic competing ions, especially chloride, decrease Se(IV) separation efficiency by generating strong oxidants and disrupting Se(IV) reduction pathways. We also find that four-electron Se(IV) reduction outperforms its six-electron counterpart when treating simulated industrial water matrix (containing 7 g/L Cl and other competing ions), with a highest removal efficiency of 96.9% and a threshold deposition capacity of 3.5 g Se/m2 in a 7-day semi-continuous operation. The results from this study will inform energy-efficient and cost-effective electrochemical approaches to separate Se from industrially impacted water and help address the emerging aquatic Se pollution.