Elucidating Mechanisms of Aqueous Electrochemical Sulfur Oxidation

Anaerobic wastewater treatment is a promising technique to facilitate environmental sustainability due to energy recovery, lower energy consumption, and lower biosolid production compared to aerobic processes. Sulfide, produced from sulfate reduction by sulfur-reducing bacteria in anaerobic processes, is toxic, odorous, and corrosive. Meanwhile, sulfur species play critical roles in chemical manufacturing and food production, and can be recovered into valuable products (e.g., sulfuric acid and ammonium sulfate) from wastewater. Electrochemical methods have been applied to sulfur removal and recovery because of its advantages in versatility, controllability, cost-effectiveness, and potential to be integrated into wastewater treatment train. However, more detailed studies are still required to address recovery selectivity, product purity, and end-product applications before electrochemical methods are fully utilized for sulfur removal and recovery.

In this study, we probed the kinetics and thermodynamics of fundamental reaction mechanisms of electrochemical sulfur oxidation. Chronopotentiometric measurements were conducted on an electrochemical membrane reactor to determine the rate-limiting steps of sulfur oxidation for different inorganic sulfur species. Cyclic voltammetry was performed to evaluate the thermodynamic favorability of sulfur oxidation reactions in bulk solution. Scanning electrochemical microscopy was used to assist the investigations of intermediate products in the diffusion layer and passivation by deposition of insoluble sulfur species.

To better study sulfur speciation, we combined several analytical methods (i.e., high-performance liquid chromatography, ion chromatography, and gas chromatography) to improve the resolution of the inorganic sulfur mass balance. Results of this mechanistic study will provide molecular-level foundation for designing electrochemical sulfur recovery processes that extract sulfuric acid and produces ammonium sulfate from anaerobic effluent. Overall, this work will progress anaerobic treatment towards a full potable reuse treatment train, facilitate the shift from pollutant removal to resource recovery, and thus enhance environmental sustainability.