Abstract
The key role of trivalent manganese (Mn(III)) species in promoting sulfate radical-based advanced oxidation processes (SR-AOPs) has recently attracted increasing attention. This review provides a comprehensive summary of Mn(III) (oxyhydr)oxide-based catalysts used to activate peroxymonosulfate (PMS) and peroxydisulfate (PDS) in water. The crystal structures of different Mn(III) (oxyhydr)oxides (such as α-Mn2O3, γ-MnOOH, and Mn3O4) are first introduced. Then the impact of the catalyst structure and composition on the activation mechanisms are discussed, as well as the effects of solution pH and inorganic ions. In the Mn(III) (oxyhydr)oxide activated SR-AOPs systems, the activation mechanisms of PMS and PDS are different. For example, both radical (such as sulfate and hydroxyl radical) and non-radical (singlet oxygen) were generated by Mn(III) (oxyhydr)oxide activated PMS. In comparison, the activation of PDS by α-Mn2O3 and γ-MnOOH preferred to form the singlet oxygen and catalyst surface activated complex to remove the organic pollutants. Finally, research gaps are discussed to suggest future directions in context of applying radical-based advanced oxidation in wastewater treatment processes.
Highlights
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Conventional water treatment technologies, such as filtration [4,5], precipitation [6,7], coagulation–flocculation [8,9,10], and biological treatment [11,12] exhibited a minimal effect on the removal of recalcitrant pollutants
Based on various reaction conditions, Advanced oxidation processes (AOPs) can be classified into different categories, including Fenton reaction [14], Fenton-like reaction [15,16], photochemical oxidation [17,18], ultrasonic oxidation [19,20], electrochemical oxidation [21,22], ozone oxidation [23,24], and sulfate radical-based AOPs (SR-AOPs) [25,26,27]
Summary
Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. The activation of PMS/PDS by different transition metal ions (i.e., Co(II), Ru(III), Fe(II), Fe(III), Ag(I), Mn(II), Ni(I), and V(III)) for organic pollutant degradation has been reported [32]. Among the transition metal oxides, the manganese oxides have been widely developed in PMS/PDS activation for recalcitrant pollutant degradation due to their excellent properties, such as various Mn valences, ubiquitous existence, cost-efficiency, and low toxicity [40]. No attempt has been made to provide a comprehensive review on Mn(III) (oxyhydr)oxides activated PMS/PDS for recalcitrant pollutants removal. The commonly reported Mn(III) (oxyhydr)oxides munity because of their promising technological applications, such as in catalysis, water include manganese(III) oxide (α-Mn2 O3 ), groutite (α-MnOOH), feitknechtite (β-MnOOH), treatment, and ion exchange. Mn ions occupy the tetrahedral and octahedral sites, respectively (Figure 1c)
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