Cobalt Single‐Atom Catalysts for Ultrafast Sulfamethoxazole Degradation: Unveiling the Chloride‐Ion‐Enhanced Formation of Co(IV)=O

ABSTRACT Chloride ions, widely regarded as radical scavengers, can paradoxically enhance pollutant degradation in a Co–N ₄ single‐atom catalyst (SAC) activated peroxymonosulfate (PMS) system. In the presence of Cl ⁻ , sulfamethoxazole (SMX) oxidation is dramatically accelerated via an HOCl‐mediated non‐radical route that promotes high‐valent Co(IV) = O generation, increasing the observed rate constant from 0.736 to 2.53 min ^{− 1} and enabling rapid, selective SMX removal with minimal matrix interference. In situ surface‐enhanced Raman spectroscopy and density functional theory calculations analyses confirm this Cl ⁻ ‐enhanced pathway, with Co(IV) = O emerging as the predominant oxidant. We further translate this mechanism into a flexible polytetrafluoroethylene (PTFE)‐supported Co–N ₄ membrane reactor, which sustains high SMX removal under continuous flow with minimal hydraulic loss. Toxicity assays show significantly reduced ecotoxicity of degradation byproducts and confirm the treated effluent's biocompatibility. These findings establish a chloride‐augmented oxidation strategy that transforms a common wastewater constituent into a co‐promoter, demonstrating a robust, selective, and scalable platform for advanced water purification.