Evolution of Singlet Oxygen by Activating Peroxydisulfate and Peroxymonosulfate: A Review

Advanced oxidation processes (AOPs) based on peroxydisulfate (PDS) or peroxymonosulfate (PMS) activation have attracted much research attention in the last decade for the degradation of recalcitrant organic contaminants. Sulfate (SO(4)(•−)) and hydroxyl ((•)OH) radicals are most frequently generated from catalytic PDS/PMS decomposition by thermal, base, irradiation, transition metals and carbon materials. In addition, increasingly more recent studies have reported the involvement of singlet oxygen ((1)O(2)) during PDS/PMS-based AOPs. Typically, (1)O(2) can be produced either along with SO(4)(•−) and (•)OH or discovered as the dominant reactive oxygen species (ROSs) for pollutants degradation. This paper reviews recent advances in (1)O(2) generation during PDS/PMS activation. First, it introduces the basic chemistry of (1)O(2), its oxidation properties and detection methodologies. Furthermore, it elaborates different activation strategies/techniques, including homogeneous and heterogeneous systems, and discusses the possible reaction mechanisms to give an overview of the principle of (1)O(2) production by activating PDS/PMS. Moreover, although (1)O(2) has shown promising features such as high degradation selectivity and anti-interference capability, its production pathways and mechanisms remain controversial in the present literatures. Therefore, this study identifies the research gaps and proposes future perspectives in the aspects of novel catalysts and related mechanisms.