Direct Electrochemical Pathways for Selenium Reduction in Aqueous Solutions
Shiqiang Zou, Meagan S. Mauter
Abstract
Direct electrochemical reduction provides a novel strategy for selenium removal from complex wastewaters. While electrochemical Se(IV) reduction is thermodynamically favorable, anion structure reorganization hinders process kinetics and the phase of reduced Se(0) determines process performance. This study evaluates the thermodynamic and kinetic performance of Se(IV) removal via direct electrochemical reduction (SeDER) and proposes moderate heating to promote efficient and continuous process operation. We find that SeDER is a robust process that can handle 0.001–10 mM Se(IV) in a weakly acidic solution (pH 4–7). Se(IV) can be electrochemically removed 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). The four-electron pathway is a surface-limited process below 70 °C and terminates when the cathode is fully covered with the insulative amorphous 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. In a simple batch process design, we observe Se(IV) removal rates of up to 89 mg h–1 m–2 of electrode surface area, up to 10% Faradaic efficiency, and up to 95% removal, although we observe moderate trade-offs between these metrics depending on the electron pathway and the initial concentration of Se(IV). Our results suggest value in future work to enhance Faradaic efficiency via better reactor and electrode design, investigate parasitic reactions among competing ions, and select cost-effective electrodes for an economically competitive SeDER process.