title: Understanding the role of manganese dioxide in the oxidation of phenolic compounds by aqueous permanganate.
abstract: Recent studies have shown that manganese dioxide (MnO2) can significantly accelerate the oxidation kinetics of phenolic compounds such as triclosan and chlorophenols by potassium permanganate (Mn(VII)) in slightly acidic solutions. However, the role of MnO2 (i.e., as an oxidant vs catalyst) is still unclear. In this work, it was demonstrated that Mn(VII) oxidized triclosan (i.e., trichloro-2-phenoxyphenol) and its analogue 2-phenoxyphenol, mainly generating ether bond cleavage products (i.e., 2,4-dichlorophenol and phenol, respectively), while MnO2 reacted with them producing appreciable dimers as well as hydroxylated and quinone-like products. Using these two phenoxyphenols as mechanistic probes, it was interestingly found that MnO2 formed in situ or prepared ex situ greatly accelerated the kinetics but negligibly affected the pathways of their oxidation by Mn(VII) at acidic pH 5. The yields (R) of indicative products 2,4-dichlorophenol and phenol from their respective probes (i.e., molar ratios of product formed to probe lost) under various experimental conditions were quantified. Comparable R values were obtained during the treatment by Mn(VII) in the absence vs presence of MnO2. Meanwhile, it was confirmed that MnO2 could accelerate the kinetics of Mn(VII) oxidation of refractory nitrophenols (i.e., 2-nitrophenol and 4-nitrophenol), which otherwise showed negligible reactivity toward Mn(VII) and MnO2 individually, and the effect of MnO2 was strongly dependent upon its concentration as well as solution pH. These results clearly rule out the role of MnO2 as a mild co-oxidant and suggest a potential catalytic effect on Mn(VII) oxidation of phenolic compounds regardless of their susceptibility to oxidation by MnO2.
authors.affiliation: State Key Laboratory of Urban Water Resource and Environment, School of Municipal and Environmental Engineering, Harbin Institute of Technology , Harbin 150090, People's Republic of China.
authors.forename: Jin
authors.initials: J
authors.lastname: Jiang
authors.forename: Yuan
authors.initials: Y
authors.lastname: Gao
authors.forename: Su-Yan
authors.initials: SY
authors.lastname: Pang
authors.forename: Xue-Ting
authors.initials: XT
authors.lastname: Lu
authors.forename: Yang
authors.initials: Y
authors.lastname: Zhou
authors.forename: Jun
authors.initials: J
authors.lastname: Ma
authors.forename: Qiang
authors.initials: Q
authors.lastname: Wang
mesh_terms: D002384:Catalysis; D002733:Chlorophenols; D007700:Kinetics; D017895:Manganese Compounds / Q000737:chemistry*; D009596:Nitrophenols; D009930:Organic Chemicals; D016877:Oxidants / Q000737:chemistry; D010084:Oxidation-Reduction; D010087:Oxides / Q000737:chemistry*; D010636:Phenols / Q000737:chemistry*; D011196:Potassium Permanganate / Q000737:chemistry*; D014260:Triclosan / Q000737:chemistry*; D014867:Water; D014874:Water Pollutants, Chemical / Q000737:chemistry*
chemical_list: D002733:Chlorophenols; D017895:Manganese Compounds; D009596:Nitrophenols; D009930:Organic Chemicals; D016877:Oxidants; D010087:Oxides; D010636:Phenols; D014874:Water Pollutants, Chemical; D011196:Potassium Permanganate; D014867:Water; C048856:permanganic acid; D014260:Triclosan; C045573:2-nitrophenol; C004762:2,4-dichlorophenol; C016552:manganese dioxide; C024836:4-nitrophenol