Non-radical reactions in persulfate-based homogeneous degradation processes: A review

Recently there is vigorous growth of reports on persulfate (PS)-based advanced oxidation processes (AOPs), inclining to development of newfangled catalysts and treatment of a wide range of organic pollutants. PS-related non-radical oxidation pathways are lately proposed, whereas there still exist debatable observations and explanation on these issues. While non-radical reactions in heterogeneous systems have been well commented, this review focuses on the latest discovery of non-radical oxidation in homogeneous persulfate (especially peroxymonosulfate (PMS)) systems. The application of direct oxidation process is firstly summarized and the roles of PMS distal oxygen in attack on organic pollutants like antibiotics are highlighted. Besides, the effects of exogenic and endogenic halogen on the reactivity and mechanisms of HOX in PS activation are discussed. Then the formation and reaction pathways of singlet oxygen (¹O₂) triggered by phenol/carbonyl compounds and nucleophiles are elaborated. Recent discovery on formation of high valent metal species by PS activation and their reactivity is reviewed lastly. By sorting out a large number of cutting-edge research reports, this review conducts a detailed and critical analysis and overview on the homogeneous PS non-radical mechanisms, and points out the controversies and challenges so as to better understand the reaction mechanism and further improve their treatment performance.