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Peroxymonosulfate (PMS) decomposition,hydroxyl radical (·OH) generation,and acetaminophen (ACT) degradation by the Co/PMS system using homogeneous (dissolved cobalt) and heterogeneous (suspended Co3O4) cobalt were assessed.For the homogeneous process,＞99％ PMS decomposition was observed and 10 mmol/L of ·OH generation was produced using 5 mmol/L of PMS and different dissolved cobalt concentrations after 30 min.A dissolved cobalt concentration of 0.2 mmol/L was used to achieve ＞99％ ACT degradation using the homogeneous process.For the heterogeneous process,60％ PMS decomposition and negligible ·OH generation were observed for 5 mmol/L of the initial PMS concentration using 0.1 and 0.2 g/L of Co3O4.Degradation of ACT greater than 80％ was achieved for all experimental runs using 5 mmol/L of the initial PMS concentration independently of the initial Co3O4 load used.For the heterogeneous process,the best experimental conditions for ACT degradation were found to be 3 mmol/L of PMS and 0.2 g/L of Co3O4,for which ＞99％ ACT degradation was achieved after 10 min.Because negligible ·OH was produced by the Co3O4/PMS process,a second-order kinetic model was proposed for sulfur-based free radical production to allow fair comparison between homogeneous and heterogeneous processes.Using the kinetic data and the reaction by-products identified,a mechanistic pathway for ACT degradation is suggested.