Article pubs.acs.org/est
Understanding the Role of Manganese Dioxide in the Oxidation of Phenolic Compounds by Aqueous Permanganate Jin Jiang,*,‡ Yuan Gao,‡ Su-Yan Pang,† Xue-Ting Lu,† Yang Zhou,‡ Jun Ma,*,‡ and Qiang Wang† ‡
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 † Key Laboratory of Green Chemical Engineering and Technology of College of Heilongjiang Province, College of Chemical and Environmental Engineering, Harbin University of Science and Technology, Harbin 150040, People’s Republic of China S Supporting Information *
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.
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INTRODUCTION Phenolic compounds including chlorophenols (CPs) and bromophenols (BrPs) as well as some emerging endocrine disrupting chemicals and personal care products such as estrogens, tetrabromobisphenol A (TBrBPA), and triclosan (TCS) have been frequently detected in environmental samples (e.g., soils, sediments, surface waters, and wastewaters).1−5 The oxidative treatment of these phenolic contaminants of concern in various environmental matrices has been extensively studied over the past decades.6−10 Many studies have shown that the common oxidant potassium permanganate (KMnO4; Mn(VII)) is a promising technology for in situ decontamination of soil as well as for advanced treatment of water/wastewater due to the characteristics of good stability, ease of handling, and effectiveness over a wide pH range.11−19 On the reaction kinetics between Mn(VII) and phenolic compounds, one interesting finding is the promoting effect of manganese dioxide (MnO2) formed in situ or prepared ex situ at slightly acidic pH (e.g., pH 5) but negligible effect at neutral/ alkaline pH.14−19 This finding is generally consistent with the fact that MnO2 can oxidize many phenolic compounds (e.g., phenol, TCS, and CPs) with much higher reaction rates at acidic pH vs at neutral/alkaline pH.20−25 So, it was reasonably © XXXX American Chemical Society
concluded that MnO2 acted as a mild co-oxidant to enhance the oxidation kinetics of phenolic compounds by Mn(VII) in a very recent study.19 However, in our previous study, it was found that MnO2 could enhance Mn(VII) oxidation of TCS and 2,4diCP but also refractory 4-nitrophenol (4-NP), which otherwise showed negligible reactivity toward MnO2 and Mn(VII) individually.14 This finding indicated a rather catalytic effect of MnO2. One possible explanation for the discrepancy is a substance-dependent role of MnO2 (i.e., the role of MnO2 changing from a catalyst for refractory phenolic compounds such as NPs to a mild oxidant for those that are susceptible to oxidation such as TCS and CPs). It has been described extensively in the literature that transformation of phenolic compounds by MnO2 always proceeds via oxidative coupling pathway, where the initial formation of phenoxyl radicals through one-electron transfer and subsequent coupling between these radicals are involved.20−25 Comparatively, several studies have demonstrated Received: September 30, 2014 Revised: November 20, 2014 Accepted: December 1, 2014
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dx.doi.org/10.1021/es504796h | Environ. Sci. Technol. XXXX, XXX, XXX−XXX
Environmental Science & Technology
Article
Figure 1. . The HPLC/ESI−QqQMS PIS chromatograms at m/z 35 of the samples separately containing TCS treated by MnO2-I (a), TCS treated by Mn(VII) (b), and 2,4-diCP treated by Mn(VII) (c). The inserts showed the corresponding molecular ion mass spectra of chromatographic peaks. Experimental conditions: [TCS] = [2,4-diCP] = 20 μM, [MnO2-I] = [Mn(VII)] = 60 μM, pH = 5, and reaction time of 20 min.
(i.e., TCS) in the literature. Wu et al. reported that 2,4-diCP was a major product of TCS (i.e., ∼60% of TCS lost) oxidized by Mn(VII),13 while Zhang and Huang found that a negligible amount of 2,4-diCP (i.e.,
