Sulfur recovery from spent caustic streams: enabling selective transformation via tandem electrochemical-chemical...

Spent caustic streams (SCS), rich in sulfides (predominantly HS^-), pose severe environmental risks and represent an underutilized sulfur resource, while existing treatment technologies suffer from high energy consumption, harsh operating conditions, or poor selectivity towards valuable sulfur products. To address the critical challenges of an unclear sulfide oxidation mechanism and an uncontrollable product distribution that hinder practical application, this study developed a tandem electrochemical-chemical strategy for selective sulfur recovery from SCS. A series of transition metal sulfide catalysts on nickel foam (TMS/NF) were synthesized via a two-step hydrothermal method. The optimized Cu_xS-0.2/NF electrode achieved 95% sulfide removal efficiency with up to 87% selectivity towards soluble polysulfides (S_n^2-) in a simulated SCS solution. A combined analytical approach using ion chromatography (IC) and UV-vis spectroscopy revealed that the Cu_xS-0.2/NF electrode catalyzes the oxidation of HS^- to S_n^2- via zero-order kinetics, ensuring stable performance under complex matrices. The superior activity is attributed to the synergistic effect between abundant Cu⁺ active sites (for HS^- adsorption and activation) and Cu²⁺ sites (for optimizing electron transfer). The process exhibits a low cell voltage of 0.81 ± 0.1 V, thus achieving a notably low energy consumption of 0.62 kWh·kg^-1 S, which confirms its techno-economic viability. Furthermore, high-purity elemental sulfur (α-S₈) was successfully recovered from real SCS through CO₂-induced protonation of S_n^2-, validating the practical feasibility of this resource-oriented treatment. This work elucidates the oxidation mechanism and kinetics of sulfide in alkaline media, providing a scalable, energy-efficient solution for sulfur resource recovery from SCS and advancing the development of green desulfurization technologies for industrial applications.