(71f) Probing Mechanisms of Electrochemical Sulfur Oxidation in Wastewater and Crude Oil

After hydrogen and carbon, sulfur is the next most abundant element in crude oil;¹ as the most prevalent contaminant, sulfur conversion plays a central role in designing oil refining processes. Considerable costs, energy, and chemical inputs are expended to remove sulfur, making processing costs and crude oil prices highly dependent on sulfur content.² Electrochemical sulfur oxidation overcomes obstacles facing conventional sulfur abatement in sour crude oil by effectively removing organosulfur compounds, avoiding intensive reaction conditions, and producing valuable products that can offset treatment costs.

Without accounting for recovered sulfur, electrochemical sulfur oxidation costs approximately 50-75% less than conventional hydrodesulfurization (Figure 2). Both inorganic sulfur (e.g., H₂S and elemental sulfur) and organosulfur compounds can be oxidized to sulfate, which can be selectively recovered as value-added sulfate products, such as ammonium sulfate fertilizer.¹⁰ In particular, a large portion of sulfur in sour crudes is comprised of bulky benzothiophenes, which are recalcitrant to most conventional treatments (resonance stabilization of 125 kJ/mol)² but may be degraded electrochemically.¹¹ Electrochemical approaches replace expensive, energy-intensive chemical inputs and reaction conditions with ambient electrons;¹² this substitution obviates expensive supply chains and enables sulfur removal in remote and decentralized settings, where many oil wells are located. Electrochemical oxidation of hydrogen sulfide has been observed in municipal wastewater, but leads to unpredictable mixtures of organic sulfur, sulfide, and sulfate.¹³ While the current electrochemical process could remove inorganic sulfides from sour crude oil, the oxidation mechanisms of organic sulfur compounds and product speciation are not well characterized. Passivation by deposition of elemental sulfur has been identified as one rate-limiting phenomenon,¹⁴ but has not been systematically compared to other transport and transformation steps. Thus, we address two fundamental knowledge gaps in this work: identifying the rate-limiting phenomena of the process and prioritizing electrolyte constituents that affect product speciation.

Mechanisms of electrochemical sulfur oxidation were probed through three complementary tasks: (i) bulk solution experiments to determine observed rate constants, (ii) electrode characterization to determine effects of sulfur oxidation on the anode surface and interior, and (iii) analysis of transient solution species to understand boundary layer reactions. Rate constants were calculated separately and compared in two-component solutions. Cyclic voltammetry and scanning electrochemical microscopy were used to identify boundary layer intermediates. Limiting steps of the sulfur oxidation have been identified, and parameters such as current density have been tuned to achieve ideal sulfate production. Result of the study will be utilized to develop an electrochemical device optimized for extracting sulfuric acid from effluents and producing ammonium sulfate.