(Invited) Electrochemical Selenium Remediation: Direct Reduction Pathways and System Optimization

Abstract

Selenium (Se) is a naturally occurring metalloid in the earth's crust that is released through weathering processes. Anthropogenic activities have significantly accelerated geogenic release, increasing Se concentrations in aquatic environments far beyond the ppb levels required for biologic function. Meanwhile, existing biological and physicochemical Se remediation processes are energy-, resource-, and cost-intensive, driving new Se separation processes to consistently meet the WHO and EPA guidelines. Se direct electrochemical reduction (SeDER) is a chemical-free and thermodynamically favorable approach for aquatic Se separation, but evaluating the feasibility of SeDER in application requires a comprehensive understanding of reduction pathways and system-level performance in complex water matrices. This study focuses on the thermodynamic and kinetic performance of SeDER and competing ion behavior in the aquatic environment. 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 C 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. Eventually, we evaluate a series of metal- and carbon-based electrode materials to ensure enhanced Faradaic efficiency, consistent Se removal, and lower material costs. The results from this study will inform energy-efficient and cost-effective electrochemical approaches for Se remediation in various water and wastewater matrices.