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The presence of triclosan (TCS) in water resources has drawn significant attention due to its endocrine disruption potential. Sulfate radicals (SRs), generated particularly by the metal-mediated activation of peroxymonosulfate (PMS), have been proposed to effectively decompose TCS and many other pharmaceuticals and personal care products. In spite of its significance for understanding the efficiency of SR generation and catalytic/non-catalytic nature of the oxidation reaction, metal speciation has not been adequately highlighted in previous studies. This study investigated the detailed changes in metal speciation in cobalt/PMS and iron/PMS systems and correlated it with TCS decomposition under different pH conditions. A rapid oxidation of Co 2þ to Co 3þ and Fe 2þ to Fe 3þ generally corresponded with pseudo-steady state decomposition kinetics of TCS after its initial fast decomposition. The presence of potential threshold concentrations of metals to effectively activate PMS was found. A strong catalytic activity was observed for Co/PMS system at pH 3, where most of Co was present in the form of dissolved Co 2þ . The pH impacts were different for Co/PMS and Fe/ PMS. TCS oxidation was fast at pH 5 for Co and pH 3 for Fe. However, long-term mineralization of TCS seemed less dependent on pH.