Oxidation Performance And Intrinstic Mechanism Of Manganese Oxides Activated Peroxymonosulfate-based Processes

In recent years,advanced oxidation processes（AOPs）based on peroxymonosulfate（PMS）have attracted increasing interests for water decontaminant.PMS,in the form of Oxone,have the advantages of convenient storage and transportation as well as great solubility in water.Generally,PMS is not effective for most of organic contaminants.Various activation methods are required to improve the decontaminantion capacity.Thereinto,manganese oxides are recognized as promising PMS catalysts.In the past,many crystalline manganese oxides were synthesized and induced PMS decomposition to produce hydroxyl（^·OH）and sulfate(SO₄^·-)radicals with strong oxidative activity towards organic pollutants degradation.However,PMS activation by permanganate（KMnO₄）and amorphous manganese dioxide（MnO₂）has not been fully studied,and the intrinsic mechanisms are unclear yet.Moreover,if abundant natural manganese-containing minerals（NMMs）on earth are effective for PMS activation,it will be more cost-effective for pollutants removal from surface/ground water and urban runoff.In this work,the feasibility of PMS activation by manganese oxides（i.e.,KMnO₄,amorphous MnO₂,and NMMs）was studied.The generation of reactive oxidative species was identified,and underlying mechanisms were deeply revealed.The removal efficiencies and transformation pathways of organic contaminants in these three oxidation systems were explored.It was proposed that the combined use of KMnO₄ and PMS caused a synergetic effect on refractory benzoic acids pollutants degradation firstly.By means of radical quenching tests and electron spin resonance（ESR）analysis,^·OH and SO₄^·-were confirmed to be the primary oxidation species for organics degradation.A tentative mechanism of direct PMS activation by KMnO₄ was demonstrated to involve substitution of oxo atoms in KMnO₄ by peroxo groups,one-electron reduction,and intramolecular disproportionation processes.The ubiquitous bicarbonate（HCO₃^-）and chloride（Cl^-）ions obviously inhibited the degradation of 4-CBA in KMnO₄/PMS system for their competitively consumption of reactive radicals.In constract,the addition of metal ions greatly facilitated the 4-CBA oxidation efficiency in KMnO₄/PMS system.This result could be attributed to the promotion on KMnO₄ and PMS interaction through cation bridging function.In the reaction of KMnO₄,it is generally reduced to amorphous MnO₂.The degradation of bisphenol A（BPA）by PMS was significantly accelerated in the presence of amorphous MnO₂.By the means of quenching experiments,ESR tests,and Raman spectra analysis,it has been verified that amorphous MnO₂ greatly enhanced the oxidative capacity of PMS via a nonradical mechanism.PMS complexed with the active sites on amorphous MnO₂ surface through inner-sphere interaction producing PMS-MnO₂ reactive complexes.In terms of the relatively mild oxidative capacity of generated complexes,the amorphous MnO₂/PMS system exhibited obvious substrate-selective reactivities.It was highly reactive towards phenolic compounds especially with electron-donating moieties,while carbamazepine,4-CBA,nitrobenzene and atrazine were hardly degraded（<10%）by amorphous MnO₂/PMS oxidation.The presence of HCO₃^-,Cl^-,and humic acid（HA）had little influence on the degradation of BPA,while metal ions showed positive effect for BPA removal at a certain degree.By identifying the oxidative products,the transformation of BPA in amorphous MnO₂/PMS system mainly involved one-electron oxidation,radical coupling,βbond cleavage,and hydroxylation processes.Further experiments found that bisphenol compounds（i.e.,BPA,bisphenol F（BPF）,bisphenol AF（BPAF）,and bisphenol S（BPS））could be effectively removed in cases of PMS activation by different sources of natural manganese-containing minerals（NMMs）.The degradation of four bisphenols was well-fitted with pseudo-first order kinetics,and the observed rate constants(k_obs)followed the order of BPF(0.1210 min^-1)>BPA(0.1055 min^-1)>BPAF(0.0152 min^-1)>BPS(0.0011min^-1)under the similar experimental conditions.By means of the characterization on NMMs,the primary active component was less-crystalline birnessite-type MnO₂.Analogous to amorphous MnO₂/PMS system,PMS was activated by NMMs via a nonradical mechanism,where surface reactive complexes were produced for organic compounds oxidation.The common inorganic anions（i.e.,HCO₃^-and Cl^-）and HA showed negligible influence on the degradation of BPAF by NMMs/PMS system.As a common transition metal ions in waters,trace Cu（II）（10μM）greatly enhanced the oxidation decontaminantion efficiency of GNMM/PMS system(0.4926 min^-1).The generation of Cu（I）has been proved by analyzing the mechanism of Cu（II）/PMS,which reduced Mn（IV）to Mn（III）on the suface of NMMs.Except for surface reactive complexes,the increase of Mn（III）content was conducive to produce^·OH and SO₄^·-in PMS activation,resulting in the great acceleration on BPAF degradation process.By analyzing the transformation products,the oxidation pathways of BPAF in natural manganese-containing minerals/PMS system mainly involved one-electron oxidation,βbond cleavage,hydroxylation,aromatic ring-opening,and decarboxylation processes.