Regulating activation pathway of Cu/persulfate through the incorporation of unreducible metal oxides: Pivotal role of surface oxygen vacancies

Abstract

In this paper, we surprisingly found that the incorporation of unreducible metal oxides MxOy (M = Mg, Zn, Ca, Ba, Al) onto CuO hybrid magnetic nano ferric oxide (Cu@Fe3O4) may alter the reaction pathway in persulfate activation, and increase the reaction rate constant. The activation of peroxymonosulfate (PMS) by Cu@Fe3O4 led to a classic sulfate radical based oxidation process (SR-AOP) with an acetaminophen (ACE) degradation rate constant of 0.004 min−1, while 1O2-dominated nonradical oxidation process was disclosed in CuM@Fe3O4 with wildly fluctuated reaction rate constants from 0.003 to 0.242 min−1. Mechanism studies indicated that singlet oxygen (1O2) derived from the direct oxidation of superoxide anions radicals (O2−) or the recombination of O2− was the main reactive oxygen species (ROS) in CuM@Fe3O4/PMS system. A series of characterization experiments (pHpzc tests, XPS, H2-TPR, et al.) and DFT calculation disclosed that the addition of an unreducible metal M yielded many positive effects: (1) the formation of surface oxygen vacancies (OV) raised the zero point charge (pHpzc) of CuM@Fe3O4, thus enhanced the adsorption and activation of PMS; (2) promoting the generation of a new Cu species (Cu3+) on the surface of CuM@Fe3O4, which then participated in the generation of 1O2. The different reducibility of Cu3+ led to differences in the catalytic properties of CuM@Fe3O4. In addition, the effects of various water matrix species and the results of reusability experiment, mineralization experiment, and ecotoxicity test exhibited that CuM@Fe3O4/PMS system possessed excellent practical application value.