Oxidative Degradation Of Phenolic Contaminants By Permanganat:Products And Effects Of Manganese Intermediates In Situ Formed

As a mild oxidant,permanganate（KMnO₄）has been widely used due to its characteristics of ease of handling and non-halogenated property.Particularly,KMnO₄ is attractive because its manganese product（e.g.,MnO₂ and Mn（III））in situ formed may act as a catalyst or oxidant to improve the decay extent of contaminants.However,to date,little information is available on the mechanism involved in the oxidation of organic contaminants especially phenolics by KMnO₄.In addition,the mechanism involved in the enhanced degradation remained unclear yet.In this work,the transformation products of representative phenolic contaminants 4-chlorophenol（4-CP）,2,4-dichlorophenol（2,4-DCP）,2,6-dichlorophenol（2,6-DCP）,and tetrachlorobisphenol A（TClBPA）and triclosan（TCS）by KMnO₄.Due to the marked difference in product formation from TCS oxidation by KMnO₄ versus manganese intermediates,TCS was used as a probe to to explore the mechanism involved in the enhanced degradation of KMnO₄ by manganese intermediates through examining the products formation of TCS under various conditionsSimilar to the case of MnO₂,the oxidation of substituted phenolic contaminants（i.e.,4-chlorophenol,2,4-dichlorophenol,2,6-dichlorophenol,and tetrachlorobisphenol A）underg oxidative coupling pathway with the formation of polymeric products.However,this transformation pathway distinctly differed from that involved in KMnO₄ oxidation of TCS,where ether-bond cleavage product 2,4-dichlorophenol（2,4-DCP）was the major product.This finding suggested that TCS might be used as a potential probe to further confirm the role of MnO₂ in the oxidation of organic contaminants by KMnO₄.In other words,if MnO₂ acted as a mild oxidant,both the kinetics and the pathways of TCS oxidation by KMnO₄ would be appreciably affected in the presence MnO₂,resulting in the decrease of yields of 2,4-DCP（molar ratios of 2,4-DCP formed to TCS consumed）.Comparatively,if MnO₂ acts as a catalyst,the pathways of TCS oxidation by KMnO₄would not be affected with the identical yields as compared to that in the case of individual KMnO₄.It was found that MnO₂ formed in situ or prepared ex situ greatly accelerated the kinetics but negligibly affected the pathways of TCS oxidation（i.e.,the yields of 2,4-DCP）by KMnO₄,indicating the catalytic role of MnO₂ in accelerating KMnO₄ oxidation.Mn（III）as a one-electron transfer oxidant oxidized 4-CP,2,4-DCP,2,6-DCP,TClBPA,and TCS thorugh oxidative coupling pathway leading to the formation of polymeric products as major products and negligible generation of 2,4-DCP..In the presence of complexing ligands,Mn（III）in situ formed from KMnO₄ reduction could be stabilized to a certain extent to prevent its spontaneous disproportionation and thus the relatively long-lived Mn（III）as a potent oxidant could contribute to the oxidation of contaminants.We found that the presence of complexing ligands including pyrophosphate and nitrilotriacetate slightly affected the yields of DCP,although they greatly enhanced the oxidation kinetics of TCS by KMnO₄.This result could not be reasonably explained by taking the contribution of Mn（III）_L into account.Comparatively,the degradation of TCS by MnO₂ was also greatly enhanced in the presence of these ligands with negligible formation of 2,4-DCP,which could be rationalized by the contribution of Mn（III）-L.In addition,it was demonstrated that 2,4-DCP could not be generated from KMnO₄oxidation of unstable phenoxy radical intermediates and stable oxidation products formed from TCS by Mn（III）.These findings indicate that manganese intermediates(Mn（V）and Mn（VI）other than Mn（III）were likely involved in the KMnO₄/TCS/ligand systems responsible for the high yields of DCP product.Humic acid ubiquitous existing in aquatic environments as a reductant and a complexing ligand may affect the transformation of organics.The presence of humic acid appreciably accelerated the degradation of TCS by KMnO₄ with appreciable formation of 2,4-DCP under slightly acid condition.MnO₂ in situ formed from KMnO₄ reduction by humic acid as well as manganese intermediates（Mn（III）,Mn（V）,and Mn（VI））stabilized by humic acid was responsible for the accelerated kinetics and appreciable formation of 2,4-DCP.In contrast,under neutral and alkaline condition,the degradation of TCS was negligibly affected,which might be due to combied effect of the competitive consumption of KMnO₄ by humic acid and the pH-afected contribution of MnO₂ and manganese intermediates.Comparatively,the effect of humic acid on TClBPA oxidation by KMnO₄ was investigated,where reactions underwent via oxidative coupling.The reactions was enhanced under slightly acid solution similar to the case of TCS,while suppressed under neutral and alkaline conditions.Here,it was found that humic acid could also competitively react with the phenoxy radical of TClBPA and reversed it back to parent TClBPA,resulting the inhibitory effect of humic acid.A commonly used reductant bisulfite could be used to activate KMnO₄ to greatly enhance the oxidation of organic contaminates,where Mn（III）,Mn（V）and Mn（VI）were expected to be generated.Also,the extent of TCS decay in the KMnO₄/bisulfite system was found to be maximized at the[bisulfite]:[KMnO₄]ratio of 5:1-10:1,appreciably higher than the stoichiometric ratio of 2:1.Comparatively,arsenite showed negligible activating effect.Considering the instability of manganese intermediates as well as the complexing ability of bisulfite,these findings suggested that bisulfite acted as complexing agents other than a reductant.The yield of 2,4-DCP from TCS oxidation by KMnO₄ was²0-30%.Since that negligible 2,4-DCP was generated from TCS oxidation by KMnO₄,Mn（V）and Mn（VI）were also involved in the KMnO₄/bisulfite system to account for the enhanced degradation of TCS as well as the formation of 2,4-DCP.