Generation and engineering applications of sulfate radicals in environmental remediation

Sulfate radical (SO₄^•-)-based advanced oxidation processes (AOPs) have become promising alternatives in environmental remediation due to the higher redox potential (2.6-3.1 V) and longer half-life period (30-40 μs) of sulfate radicals compared with many other radicals such as hydroxyl radicals (•OH). The generation and mechanisms of SO₄^•- and the applications of SO₄^•--AOPs have been examined extensively, while those using sulfite as activation precursor and their comparisons among various activation precursors have rarely reviewed comprehensively. In this article, the latest progresses in SO₄^•--AOPs were comprehensively reviewed and commented on. First of all, the generation of SO₄^•- was summarized via the two activation methods using various oxidant precursors, and the generation mechanisms were also presented, which provides a reference for guiding researchers to better select two precursors. Secondly, the reaction mechanisms of SO₄^•- were reviewed for organic pollutant degradation, and the reactivity was systematically compared between SO₄^•- and •OH. Thirdly, methods for SO₄^•- detection were reviewed which include quantitative and qualitative ones, over which current controversies were discussed. Fourthly, the applications of SO₄^•--AOPs in various environmental remediation were summarized, and the advantages, challenges, and prospects were also commented. At last, future research needs for SO₄^•--AOPs were also proposed consequently. This review could lead to better understanding and applications of SO₄^•--AOPs in environmental remediations.