Transition metal catalyzed sulfite auto-oxidation systems for oxidative decontamination in waters: A state-of-the-art minireview

Sulfate radical (SO4−) is believed to be one of the most highly reactive oxidants, as superior as hydroxyl radical (HO), for various organic/inorganic contaminants removal in the field of pollution control chemistry. In the recent decade, sulfate radical-based advanced oxidation processes (SR-AOPs) have been developed quickly primarily due to the selective oxidation and high oxidative potential and therefore hold great promises. Although peroxydisulfate (PDS) and peroxymonosulfate (PMS) have been extensively utilized in various SR-AOPs, new attempts have been made to replace PDS/PMS with sulfite for the purpose of SO4− generation at lower cost. Indeed, some significant progresses have been achieved in driving SO4− generation from transient metal catalyzed sulfite auto-oxidation systems to oxidize contaminants. The background, basic mechanisms, and application of the transition metal catalyzed sulfite auto-oxidation systems in contaminants detoxification and microorganism inactivation are reviewed in this work. Meanwhile, we hereby also want to point out several important unresolved issues for future investigation. (1) How to realize quick reactions at near neutral pH? (2) How to achieve high rate of mineralization as equally as or at least close to the apparent complete elimination of substrates? (3) What are the relative contributions of various oxysulfur radicals to the transformation of contaminants. (4) Is it possible to control the extent of substrates oxidation so as to get target transformed products with desired properties? If so, SR-AOPs can be upgraded as product-oriented AOPs (PO-AOPs). This state-of-art minireview aims to discuss abovementioned issues and presents some recent progresses in this field.