Comparison of the catalytic performances of different commercial cobalt oxides for peroxymonosulfate activation during dye degradation

The catalytic performances of different commercial cobalt oxides(CoxOy, i.e., Co2O3, Co3O4, and CoO) for peroxymonosulfate(PMS) activation in dye degradation were compared in this study. The CoO/PMS system exhibited the highest catalytic oxidation performances with more than 96% methylene blue(MB) removal at 90 min. The concentration of dissolved cobalt ions from CoO was 1.474 mg/L at neutral pH and 5.231 mg/L at acidic pH at 90 min which accounted for the high MB removal and CoO activated PMS homogeneously compared to the associated CoSO4/PMS. Co3O4 and Co2O3 exhibited similar catalytic performances and characteristics. The MB removal in Co3O4/PMS and Co2O3/PMS system were more than 88% at neutral pH and over 64% at acidic pH, respectively, in 90 min. The concentration of dissolved cobalt ions from Co3O4 and Co2O3 was less than 0.018 mg/L at neutral pH and 0.036 mg/L at acidic pH at 90 min. The MB removal achieved by Co3O4/PMS and Co2O3/PMS were approximately three times higher than that achieved by their associated CoSO4/PMS systems and the heterogeneous catalysis played a principal role in the processes. According to scavenger experiments, the contributions of the hydroxyl radicals in the MB degradation were negligible compared to sulfate radicals in the CoO/PMS system. Hydroxyl radicals contributed one-third or more and sulfate radicals were the dominant oxidizing species in the Co3O4/PMS and Co2O3/PMS systems. CoxOy could also effectively activate PMS to generate free radicals for degradation of other dyes. The dyedegradation processes in Co3O4/PMS, Co2O3/PMS and CoSO4/PMS systems followed pseudo-first-order kinetic model and dye degradation rates in different systems were also compared.