Sulfate radical-based advanced oxidation processes (SR-AOPs) employing heterogeneous catalysts to generate sulfate radical (SO4 −) from peroxymonosulfate (PMS) and persulfate (PS) have been extensively employed for organic contaminant removal in water. This article aims to provide a state–of–the–art review on the recent development in heterogeneous catalysts including single metal, mixed metal, and nonmetal carbon catalysts for organic contaminants removal, with particular focus on PMS activation. The hybrid heterogeneous catalyst/PMS systems integrated with other advanced oxidation technologies is also discussed. Several strategies for the identification of principal reactive radicals in SO4 −–oxidation systems are evaluated, namely (i) use of chemical probe or spin trapping agent coupled with analytical tools, and (ii) competitive kinetic approach using selective radical scavengers. The main challenges and mitigation strategies pertinent to the SR-AOPs are identified, which include (i) possible formation of oxyanions and disinfection byproducts, and (ii) dealing with sulfate produced and residual PMS. Potential future applications and research direction of SR-AOPs are proposed. These include (i) novel reactor design for heterogeneous catalytic system based on batch or continuous flow (e.g. completely mixed or plug flow) reactor configuration with catalyst recovery, and (ii) catalytic ceramic membrane incorporating SR-AOPs.

Generation of sulfate radical through heterogeneous catalysis for organic contaminants removal: Current development, challenges and prospects

Reports that promote persulfate-based advanced oxidation process (AOP) as a viable alternative to hydrogen peroxide-based processes have been rapidly accumulating in recent water treatment literature. Various strategies to activate peroxide bonds in persulfate precursors have been proposed and the capacity to degrade a wide range of organic pollutants has been demonstrated. Compared to traditional AOPs in which hydroxyl radical serves as the main oxidant, persulfate-based AOPs have been claimed to involve different in situ generated oxidants such as sulfate radical and singlet oxygen as well as nonradical oxidation pathways. However, there exist controversial observations and interpretations around some of these claims, challenging robust scientific progress of this technology toward practical use. This Critical Review comparatively examines the activation mechanisms of peroxymonosulfate and peroxydisulfate and the formation pathways of oxidizing species. Properties of the main oxidizing species are scrutinized and the role of singlet oxygen is debated. In addition, the impacts of water parameters and constituents such as pH, background organic matter, halide, phosphate, and carbonate on persulfate-driven chemistry are discussed. The opportunity for niche applications is also presented, emphasizing the need for parallel efforts to remove currently prevalent knowledge roadblocks.

/pdf/persulfate-based-advanced-oxidation-critical-assessment-of-ru3om4hbrg.pdf

Persulfate-Based Advanced Oxidation: Critical Assessment of Opportunities and Roadblocks.

Chemistry of persulfates in water and wastewater treatment: A review

Reactive species in advanced oxidation processes: Formation, identification and reaction mechanism

Metal-free activation of superoxides provides an efficient and environmentally benign strategy for heterogeneous catalytic oxidation. In this study, nanocarbons with varying carbon-conjugation structures and functional groups were investigated for peroxymonosulfate (PMS) activation. It was discovered that radical and nonradical oxidations could occur on different carbocatalysts depending on the carbon structure. Radical oxidation occurs exclusively on MWCNTs and CMK-3, similar to a metal oxide, MnO2. Both radical and nonradical oxidations are very pronounced in nanodiamond (AND-900)/PMS whilst nonradical oxidation is dominated in reduced graphene oxide (rGO-900)/PMS. Density functional theory (DFT) calculations were employed to explore the PMS adsorption and O O bond activation on the different carbon configurations for an in-depth probe of the activation mechanism. The intact sp2-conjugated π system in MWCNTs and electron-rich ketonic groups (as Lewis basic sites) in CMK-3 can stimulate PMS dissociation to generate SO4 − and OH, similar to metal-based catalysts. However, the defective edges at the boundary of carbon network are able to facilitate the organic degradation without generation of the reactive radicals, which is well supported by both experiments and the DFT calculation. The emerging nonradical oxidation induced by the carbocatalysis is superior to the radical oxidation on most metal oxides for effective degradation of various organics. The influences of solution pH, various anions (H2PO42−, HCO3− and Cl−) and background organic matters (humic acid) on the nonradical oxidation were further evaluated. The nonradical oxidation on carbocatalysts can be utilized as a green and effective oxidation strategy for aqueous environmental remediation and nonaqueous phase oxidation.

Occurrence of radical and nonradical pathways from carbocatalysts for aqueous and nonaqueous catalytic oxidation

E.s.r. spectra are reported for the radicals obtained by the oxidation of a variety of organic compounds by the titanium(III)–persulphate redox system. Evidence is obtained that the sulphate radical anion is formed throughout the pH range employed (1–10·5) and that it can react with organic compounds in at least three ways: by abstracting a hydrogen atom from saturated carbon, by adding to unsaturated or aromatic carbon, and by removing one electron from carboxylate anions and from certain neutral molecules. It displays a marked degree of selectivity in its addition reactions and it behaves differently from the hydroxyl radical in several respects.

P. M. Storey

Papers

Electron spin resonance studies. Part XXV. Reactions of the sulphate radical anion with organic compounds