Surface-confined singlet oxygen enables coupled roxarsone degradation and arsenate immobilization over Co-doped LaOCl.

Organoarsenical contaminants such as roxarsone (ROX) pose a persistent challenge in water treatment because oxidative degradation of the parent compound inevitably generates inorganic arsenic (iAs), whereas adsorption-based processes immobilize iAs without removing the organoarsenical precursor. As a result, arsenic is redistributed among species rather than eliminated, and its overall fate during transformation and immobilization remains poorly quantified. Here, we develop a Co-doped LaOCl catalyst (Co0.5LOC) that enables coupled oxidative transformation and arsenic immobilization under peroxymonosulfate (PMS) activation via spatially integrated Co and La sites. Critically, a complete time-resolved arsenic mass balance is achieved throughout the reaction, with total arsenic recovery maintained at 100 ± 2%, thereby directly linking ROX degradation, iAs formation, and final immobilization. The catalyst exhibits a pseudo-first-order ROX degradation rate of 0.2935 min-1, 2.2-fold higher than pristine LaOCl, with >96% removal across pH 5-9 and >81% activity retention after five cycles. Mechanistic analysis identifies surface-confined singlet oxygen (1O2) as the dominant reactive species, providing strong resistance to chloride ions and natural organic matter. A synergy index of 6.72 quantifies non-additive oxidation-adsorption coupling. Total organic carbon monitoring reveals that structural transformation substantially exceeds mineralization, with approximately 26% of the initial carbon remaining at 120 min; LC-HRMS tentatively identifies four aromatic intermediates in the residual fraction, consistent with the limited ring-opening capability of surface-confined 1O2. Overall, this work establishes a unified strategy for organoarsenic remediation by integrating oxidative destruction with quantitative arsenic sequestration, enabling comprehensive contaminant fate accountability in advanced water treatment systems.