Sustainable redox processes induced by peroxymonosulfate and metal doping on amorphous manganese dioxide for nonradical degradation of water contaminants

Manganese oxides have been applied in advanced oxidation processes (AOPs), however, underlying oxidation regimes are still debatable. In this work, we synthesized various crystalline manganese oxides and single or dual metal-doped amorphous MnO2 (M-AMO, M = Fe, Co, Ni, and Cu) for organic oxidation with peroxymonosulfate (PMS). M-AMO at 1 mol% exhibited much higher activities than crystalline manganese oxides and Cu-AMO displayed the rate constant at 3.5 times as high as that of AMO. Different from conventional radical oxidation, nonradical degradation occurred via two pathways. Cu-doping resulted in relatively higher-crystallized structure, more oxygen vacancies, and a higher ratio of Mn4+/Mn3+ for a faster redox cycle between MnⅣ(s) and MnⅢ(s) to boost PMS activation and direct/indirect (Cu-AMO–PMS complex-based) phenol oxidation. This study contributes to a new insight to the structure-catalysis relationship in manganese-catalyzed PMS oxidation. The outcomes will direct the rational synthesis of reaction-oriented catalysts for nonradical AOPs and novel remediation technologies.