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Oxidation of Bromophenols and Formation of Brominated Polymeric Products of Concern during Water Treatment with Potassium Permanganate Jin Jiang, Yuan Gao, Su-yan Pang, Qiang Wang, Xiaoliu Huangfu, Yongze Liu, and Jun Ma Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/es5008577 • Publication Date (Web): 19 Aug 2014 Downloaded from http://pubs.acs.org on August 24, 2014
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Oxidation of Bromophenols and Formation of
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Brominated Polymeric Products of Concern during
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Water Treatment with Potassium Permanganate
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Jin Jiang,‡, * Yuan Gao,‡ Su–Yan Pang,† Qiang Wang,† Xiaoliu Huangfu,‡ Yongze Liu,‡
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and Jun Ma‡, *
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‡
State Key Laboratory of Urban Water Resource and Environment, School of Municipal and
7 Environmental Engineering, Harbin Institute of Technology, Harbin 150090, China 8
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Key laboratory of Green Chemical Engineering and Technology of College of Heilongjiang Province,
9 College of Chemical and Environmental Engineering, Harbin University of Science and Technology, 10 Harbin 150040, China 11 12
*
Corresponding authors: Dr. Jin Jiang and Prof. Jun Ma
13 Tel: 86–451–86283010; fax: 86–451–86283010; Email:
[email protected] (J.J.),
[email protected] 14 (J.M.). 15 16 Abstract 17 The extensive use of bromophenols (BrPs) in industrial products leads to their occurrence in freshwater 18 environments. This study explored the oxidation kinetics of several BrPs (i.e., 2-BrP, 3-BrP, 4-BrP, 2,4ACS Paragon Plus Environment
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19 diBrP, and 2,6-diBrP) and potential formation of brominated polymeric products of concern during 20 water treatment with potassium permanganate [Mn(VII)]. These BrPs exhibited appreciable reactivity 21 toward Mn(VII) with the maxima of second-order rate constants (kMn(VII)) at pH near their pKa values, 22 producing bell-shaped pH-rate profiles. The unusual pH-dependency of kMn(VII) was reasonably 23 explained by a tentative reaction model, where the formation of an intermediate between Mn(VII) and 24 dissociated BrP was likely involved. A novel and powerful precursor ion scan (PIS) approach was used 25 for selective detection of brominated oxidation products by liquid chromatography/electrospray 26 ionization-triple quadrupole mass spectrometry. Results showed that brominated dimeric products such 27 as hydroxylated polybrominated diphenyl ethers (OH-PBDEs) and hydroxylated polybrominated 28 biphenyls (OH-PBBs) were readily produced. For instance, 2’-OH-BDE-68, one of the most naturally 29 abundant OH-PBDEs, could be formed at a relatively high yield possibly via the coupling between 30 bromophenoxyl radicals generated from the one-electron oxidation of 2,4–diBrP by Mn(VII). Given the 31 altered or enhanced toxicological effects of these brominated polymeric products compared to the BrP 32 precursors, it is important to better understand their reactivity and fate before Mn(VII) is applied by 33 water utilities for the oxidative treatment of BrP-containing waters. 34 35 Introduction 36 Bromophenols (BrPs) are widely used in many industrial products as polymer intermediates, flame 37 retardant intermediates, and wood preservatives and thus are often released into freshwater 38 environments.1, 2 For instance, the concentrations of 2,4-diBrP and 2,6-diBrP were reported to be as high 39 as 40 and 3 μg/L in Indian river waters primarily impacted by the discharge of municipal wastewater 40 effluent.3 BrPs are of high toxicity and have possible harmful effects on human health and aquatic 41 ecology.4-6 Also, BrPs can cause odorous problems in drinking water, and the odor threshold 42 concentrations of some BrPs are in the ng/L range, even comparable to the earthy/musty odorous 43 geosmin and methyl isoborneol.7 Therefore, investigation of the transformation/treatment of BrPs in 44 natural/engineered processes is of considerable importance. ACS Paragon Plus Environment
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Recently, Lin et al.8 reported that BrPs could undergo oxidative transformation in the presence of
46 naturally occurring manganese oxides. Eriksson et al.9 and Liu et al.10 demonstrated that BrPs could also 47 undergo facile photo-degradation under simulated sunlight. These processes could contribute to the 48 natural abiotic attenuation of BrPs. Meanwhile, product analysis in these studies showed that BrPs could 49 be converted to polymeric products such as hydroxylated polybrominated diphenyl ethers (OH-PBDEs) 50 and hydroxylated polybrominated biphenyls (OH-PBBs). For example, one of the most naturally 51 abundant OH-PBDEs in the environmental matrices, 2’-hydroxy-2,3’,4,5’-tetrabromodiphenyl ether (2’52 OH-BDE-68), was formed as a dimeric product from 2,4-diBrP.8,
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Consequently, an alternative
53 formation pathway of OH-PBDEs from BrPs other than from most identified precursor PBDEs has been 54 suggested.8, 10 Compared to their precursors, brominated polymeric products such as OH-PBDEs have 55 altered or enhanced toxicological effects and thus their presence/persistence in the environmental 56 matrices has caused great concerns.11-14 57
Potassium permanganate (KMnO4 or Mn(VII)), which is already widely used by water utilities for the
58 control of dissolved manganese(II), taste/odor/color, and biological growth,15-17 has been demonstrated 59 to be fairly effective for the treatment of some phenolic contaminants.18-24 In particular, product analysis 60 has suggested that the transformation pathways of phenolic contaminants by Mn(VII) are likely 61 substrate-dependent. For instance, our recent work showed that one of the most widely used brominated 62 flame retardants tetrabromobisphenol A (TBrBPA) could be readily transformed by Mn(VII) to several 63 dimeric products.24 In contrast, Wu et al. reported that the oxidation of the widely used phenolic biocide 64 triclosan (TCS) by Mn(VII) did not produce dimers but generated 2,4-diCP (i.e., the ether bond cleavage 65 product of TCS), which was further converted to a dimeric product instead.19 However, the reactions of 66 Mn(VII) with another important class of phenolic contaminants BrPs have not been characterized so far, 67 and it is unknown whether brominated polymeric products of concern such as OH-PBDEs can be 68 formed. 69
Recently, a novel and powerful precursor ion scan (PIS) approach using high pressure liquid
70 chromatography and electrospray ionization-triple quadrupole mass spectrometry (HPLC/ESI-QqQMS) ACS Paragon Plus Environment
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71 has been developed, which can allow fast and selective detection of polar (electrospray-ionizable) 72 bromine-containing compounds in the aquatic environmental samples compared to traditional full scan 73 approach.25, 26 Because polar bromine-containing compounds can produce fragment bromide ions in the 74 collision chamber of the QqQMS at negative ESI, by performing PIS of m/z 79 or 81, almost all of them 75 are selectively picked out from the complex background matrices.25-29 The working principle for the PIS 76 approach has been well described in literature25-29 and is briefly presented in the Supporting Information 77 (SI; Text S1). Given the large number of bromine atoms in the polymeric products of BrPs if they are 78 formed when oxidized by Mn(VII), the HPLC/ESI–QqQMS PIS approach is well suited to their 79 detection. 80
The objective of this study was to evaluate the oxidative transformation of several BrPs (i.e., 2-BrP,
81 3-BrP, 4-BrP, 2,4-diBrP, and 2,6-diBrP) during water treatment with Mn(VII). First, the reaction 82 kinetics were determined in synthetic waters over a wide pH range of 5~10. Then, the potential 83 formation of brominated polymeric products in the reactions of Mn(VII) with selected BrPs (i.e., 4-BrP, 84 2,4-diBrP, and 2,6-diBrP) were explored by the HPLC/ESI–QqQMS PIS approach and the formation 85 pathways were tentatively proposed. Finally, the oxidative treatment of selected BrPs by Mn(VII) were 86 examined in natural waters. 87 88 Experimental Section 89 Material. 2-BrP, 3-BrP, 4-BrP, 2,4-diBrP, 2,6-diBrP, 2’-HO-BDE-68, 4,4’-dihydroxybiphenyl, 490 phenoxyphenol, and 2,2-azino-bis(3-ethylbenzothiazoline)-6-sulfonic acid diammonium (ABTS) were 91 obtained from commercial sources (Aldrich-Sigma, AccuStandard, Fisher, and Sinopharm) with the 92 purities of 98%, 98%, 99%, 95%, 99%, 98.6%, 97%, 99%, and 98%, respectively. Acetonitrile of HPLC 93 grade was purchased from Merck. Deionized water (>17.8 MΩ·cm) was obtained by passage of distilled 94 water through a Millipore Milli-Q water purification system. Other chemicals were of analytical grade or 95 better and used without further purification. Stock solutions of Mn(VII) were prepared by dissolving the
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96 crystals of KMnO4 in deionized water and standardized spectrophotometrically by determining the 97 absorbance at 525 nm (ε = 2500 M-1 cm-1). 98
Reaction Kinetics in Synthetic Waters. Kinetic reactions were initiated by adding excess Mn(VII) of
99 75-200 μM into pH-buffered solutions containing each BrP at a desired concentration of 5 μM (i.e., the 100 molar ratio of [Mn(VII)]:[BrP] = 15-40). Samples were periodically collected and quenched with 101 ascorbic acid (similar results were also obtained with hydroxylamine as the quenching agent) before 102 analyzed with HPLC and UV detection (see SI Text S2 for the details). The following buffers (10 mM) 103 were used: sodium acetate for pH 5-6 and sodium borate for pH 7-10. Negligible shifts from the initial 104 pH values (≤0.1 units) measured at the end of kinetic runs were observed even in the cases at 105 circumneutral pH, where the buffering capacity was relatively low. Unlike phosphate, acetate and borate 106 buffers were demonstrated to have no influence on Mn(VII) reactions in previous studies,21-24 and their 107 negligible effects were also confirmed in this study (SI Figure S1). By using stopped-flow technique 108 (Applied Photophysics, SX20), it was demonstrated that ascorbic acid in excess (0.1‰; w/w) could 109 quickly quench Mn(VII) (
