Carboxyl-Rich Carbon Triggers Radical-Dominated Oxidation in the Cr(VI)/S(IV) System via Ligand-Exchange and Interfacial Electron Transfer

The reaction between hexavalent chromium (Cr(VI)) and sulfite (S(IV)) holds promise for water decontamination but intrinsically favors a non-radical pathway, leading to toxic Cr(V) accumulation and inefficient radical yields. Herein, we constructed a heterogeneous carbon/Cr(VI)/S(IV) advanced oxidation system using carboxyl-rich carbon materials (CP-T and MWCNTs-COOH) to steer the reaction toward a radical-dominated pathway. Combined experimental and theoretical analyses unravel a surface-mediated ligand-exchange activation mechanism. Surface carboxyl groups function as specific anchoring sites, undergoing ligand exchange with Cr(VI) species to form a reactive Carbon-COO-Cr(VI) intermediate. Concurrently, the highly conductive carbon framework serves as an efficient electron conduit, facilitating rapid interfacial electron transfer from S(IV) to the anchored Cr centers. A dual-descriptor kinetic model was established, quantitatively demonstrating that the catalytic turnover is governed by a symmetric co-limitation regime; the intrinsic catalytic activity of the carboxyl sites is non-linearly amplified by the enhanced electron transport capability of the carbon network. This synergistic interplay simultaneously achieves complete Cr(VI)-to-Cr(III) detoxification and efficient sulfate radical (SO 4 •− ) generation. Furthermore, density functional theory (DFT) calculations elucidated the SO 4 •− -initiated degradation pathways for diclofenac (DCF). Operating efficiently in authentic water matrices under near-neutral conditions, this work provides a scalable and sustainable strategy for the synergistic remediation of complex metal-organic co-contamination.