Copper catalysts for radical and nonradical persulfate based advanced oxidation processes: Certainties and uncertainties

[Display omitted] •Reactive species in Cu-catalyzed persulfate activation are summarized.•Cu-catalyzed radical/nonradical persulfate activation pathways are elaborated.•Uncertainties on role of Cu-catalysts in persulfate activation are explicated.•Controversies over persulfate activation mechanism are discussed. Persulfate-based Advanced oxidation processes (AOPs) have gained a lot of attentions in organic decontamination in the past decades. Copper (Cu)/persulfate is an interesting coupling system due to its versatile activation pathways with formation of multiple radical and nonradical species. Thus, in the review, the generation mechanisms of various reactive species in Cu/persulfate system are analyzed and summarized. Firstly, the general activation mechanisms of Peroxydisulfate (PDS) and Peroxymonosulfate (PMS) catalyzed by Cu are summarized, and formation processes of four types of free radicals (SO4−, OH, SO5− and O2−) and three types of non-radicals (Cu(III), surface activated PDS/PMS and 1O2) in the system are compared. Moreover, the role of copper ions and copper oxides in generation of these reactive species is discussed in details. Finally, the concluding remarks and perspectives for future research on Cu/persulfate AOPs are proposed. The review provides a comprehensive and deep insight into the formation and reaction mechanisms of radical and nonradical species, points out current controversies over reaction mechanisms involved in the Cu-persulfate-based AOPs, and proposes a guideline for future research and engineering application of persulfate.