Quantifying and Characterizing Sulfide Oxidation to Inform Operation of Electrochemical Sulfur Recovery from Wastewater

Aqueous sulfur management is critical for polishing anaerobic wastewater effluent and manufacturing fertilizers. Although recovery processes that convert sulfur in wastewater effluent to commodity chemicals have been proposed, chemical mechanisms of aqueous sulfur transformation are not well characterized. Quantifying and characterizing aqueous sulfur transformation can enhance the understanding of fundamental reaction steps and subsequently guide process design. In this study, we investigated the rate-limiting steps, reaction barriers, and performance of aqueous electrochemical sulfide oxidation from wastewater under direct oxidation at the anode surface and indirect oxidation with oxygen evolution. Indirect oxidation outpaced direct oxidation in terms of sulfide removal efficiency (1.4× higher for indirect) and sulfate production efficiency (3× higher for indirect). We identified thiosulfate oxidation as the rate-limiting step from electrochemical experiments with single-component sulfur solutions and confirmed elemental sulfur deposition on the electrode as the major barrier for mass transfer via scanning electrochemical microscopy. These findings translated into demonstrated trade-offs between energy demand and performance in synthetic and real anaerobic effluents while informing different operating schemes. Additionally, we achieved an inorganic sulfur mass balance of >95% using combined ion and liquid chromatography methods and advanced the use of scanning electrochemical microscopy to characterize physicochemical water treatment techniques (especially emerging electrochemical processes). Insights from this study help prioritize directions for future process-level optimization and enable the treatment of various sulfide-containing wastewaters (e.g., anaerobic effluents, produced waters). In addition to fertilizer production and sulfur management, this work also has relevance for water reuse because it enhances energy-saving anaerobic processes and a circular sulfur economy.