Anthropogenic selenium (Se) released in industrial and agricultural wastewaters presents toxicity challenges for local ecosystems. Se direct electrochemical reduction (SeDER) is an effective and thermodynamically favorable approach for Se(IV) removal, 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 cathodic and anodic competing ion behavior in simulated flue-gas desulfurization wastewater. Our results indicate that 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). 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 SeDER performance 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 flue-gas desulfurization wastewater (with 7 g/L Cl), with the 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 manage industrial wastewaters and help engineers and policymakers to retrofit current treatment trains to meet more stringent Se regulations.

Managing aquatic selenium pollution through direct electrochemical reduction

ACS Fall 2021

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