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

Abstract

The catalytic performances of different commercial cobalt oxides(Co _x O _y , 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. Co _x O _y 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.