Article pubs.acs.org/est
Oxidation of Flame Retardant Tetrabromobisphenol A by Aqueous Permanganate: Reaction Kinetics, Brominated Products, and Pathways Su-Yan Pang,† Jin Jiang,‡,* Yuan Gao,‡ Yang Zhou,‡ Xiaoliu Huangfu,‡ Yongze Liu,‡ and Jun Ma*,‡ †
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, China ‡ State Key Laboratory of Urban Water Resource and Environment, School of Municipal and Environmental Engineering, Harbin Institute of Technology, Harbin 150090, China S Supporting Information *
ABSTRACT: In this work, the most widely used brominated flame retardant tetrabromobisphenol A (TBrBPA) was shown to exhibit appreciable reactivity toward potassium permanganate [Mn(VII)] in water over a wide pH range of 5−10 with the maxima of second-order rate constants (kMn(VII) = 15−700 M−1 s−1) at pH near its pKa values (7.5/8.5). A novel precursor ion scan (PIS) approach using negative electrospray ionization-triple quadrupole mass spectrometry (ESIQqQMS) was adopted and further optimized for fast selective detection of brominated oxidation products of TBrBPA by Mn(VII). By setting PIS of m/z 79 and 81, two major products (i.e., 4-(2-hydroxyisopropyl)-2,6dibromophenol and 4-isopropylene-2,6-dibromophenol) and five minor ones (including 2,6-dibromophenol, 2,6-dibromo-1,4-benzoquinone, and three dimers) were detected and suggested with chemical structures from their product ion spectra and bromine isotope patterns. Reaction pathways mainly involving the initial one-electron oxidation of TBrBPA and subsequent release and further reactions of 2,6dibromo-4-isopropylphenol carbocation intermediate were proposed. The effectiveness of Mn(VII) for treatment of TBrBPA in real waters was confirmed. It is important to better understand the reactivity and toxicity of primary brominated products before Mn(VII) can be applied for treatment of TBrBPA-contaminated wastewater and source water.
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INTRODUCTION Tetrabromobisphenol A (TBrBPA) is primarily and widely used as brominated flame retardants (BFRs) in the manufacture of printed circuit boards for information technology, communication, and other electronic equipment.1 The widespread use of TBrBPA leads to its ubiquitous occurrence in the environments.2,3 For instance, TBrBPA concentrations in the effluents of municipal wastewater treatment plants in Europe were reported to be as high as 85 ng/L, and some industrial landfill leachates in Japan contained TBrBPA up to 620 ng/L.4,5 Since TBrBPA is structurally similar to steroid hormones, it has been reported that TBrBPA can act as an endocrine disruptor.6−8 It is therefore necessary to evaluate the efficiency of currently used water treatment processes and/or develop new effective techniques to remove TBrBPA from the contaminated water. TBrBPA can undergo oxidative transformation in the presence of naturally occurring manganese oxides, contributing to the natural attenuation of this important anthropogenic contaminant.9 This also suggests that the process such as manganese sand filtration might be potentially used for treatment of TBrBPA-containing water or remediation of © 2013 American Chemical Society
TBrBPA-polluted environmental matrices. The naturally occurring laccase enzyme has been shown to be effective to transform TBrBPA, suggesting a novel enzymatic method for the control of TBrBPA contamination.10 In addition, several novel advanced oxidation processes have been proposed and tested in recent years.11−13 Nevertheless, to date little information is known on the transformation of TBrBPA in the presence of common oxidants/disinfectants (e.g., ozone (O3), chlorine dioxide (ClO2), free chlorine (HOCl), and permanganate [Mn(VII)]) during water and wastewater treatment. Numerous studies have shown that these selective oxidants are effective in treating emerging micropollutants containing electron-rich moieties (ERM).14,15 Among them, Mn(VII) shows an attractive characteristic of comparative stability, ease of handling, and relatively low cost, as well as non-halogenated property. It has already widely used by water utilities to control dissolved Received: Revised: Accepted: Published: 615
September 14, 2013 November 8, 2013 December 2, 2013 December 2, 2013 dx.doi.org/10.1021/es4041094 | Environ. Sci. Technol. 2014, 48, 615−623
Environmental Science & Technology
Article
manganese, taste/odor/color, and biological growth.16−19 In particular, Mn(VII) has been recently demonstrated to be fairly effective in treating a few antibiotics and several endocrine disrupting chemicals (EDCs). For instance, Hu et al.16,20,21 reported that Mn(VII) could readily oxidize ciprofloxacin, lincomycin, trimethoprim, and carbamazepine by electrophilic attack at the tertiary aromatic amine, secondary aliphatic amine, thioether, and olefin groups. Reaction kinetics for these pharmaceuticals followed a generalized second-order rate law with apparent rate constants (kMn(VII)) at pH 7 and 25 °C of 0.61−300 M−1 s−1, respectively. Our studies showed that Mn(VII) could rapidly oxidize the biocide triclosan (TCS) and natural/synthetic EDCs including estrone (E1), 17β-estradiol (E2), estriol (E3), and 17α-ethinylestradiol (EE2), as well as 4n-nonylphenol (4-n-NP) and bisphenol A (BPA) with kMn(VII) at pH 7 and 25 °C of 35.8−122.8 M−1 s−1.14,22,23 Product analysis by high performance liquid chromatography/electrospray ionization-triple quadrupole mass spectrometry (HPLC/ ESI−QqQMS or HPLC/MS−MS) showed the initial attack of Mn(VII) at the hydroxyl groups in the aromatic ring of these EDCs, leading to a series of quinone-like and ring-opening products.14 In this work, the reactions of the important anthropogenic contaminant TBrBPA with aqueous Mn(VII) were studied. First, the reaction kinetics were examined in synthetic buffered waters over a wide pH range of 5−10. Then, oxidation products were identified by a novel and powerful precursor ion scan (PIS) approach using ESI−QqQMS and reaction pathways were tentatively proposed. Further, the effectiveness and oxidation dynamics of Mn(VII) for treatment of TBrBPA in source water and wastewater effluent were examined. The PIS approach, as a unique mode of the QqQMS, has been recently developed for the differentiation of halogenated disinfection byproducts in finished drinking water and tap water from tens of thousands of halogen-free organics in pools.24,25 This method has also been extended to the selective detection of polar halogen-containing contaminants in groundwater, surface water and wastewater.24 Because halogen-containing compounds may generate fragmented halogen ions such as Cl− and Br− in the collision chamber of the QqQMS, by setting PIS of m/z 79 or 81 for instance, almost all polar brominecontaining compounds can be rapidly detected in a sample. When bromine-containing precursor ions were selectively picked out by PIS of m/z 79 or 81, they were then analyzed by product ion scans to obtain structural information. It is of note that the PIS can only detect polar halogen-containing compounds that are ionizable in negative ESI.24,25 Fortunately, the oxidation by Mn(VII) always leads to the formation of products rich in carboxyl, carbonyl, and alcohol groups rather than the mineralization of target organics to inorganic products.16,20 So, it is anticipated that the negative ESI− QqQMS PIS approach can be used for the fast selective detection of oxidation products of TBrBPA with Mn(VII).
water and standardized spectrophotometrically. Since TBrBPA is sparsely soluble in water with the solubility at 25 °C of 7.15 μM and logKow of 4.50,26 its stock solutions were prepared in HPLC-grade acetonitrile. Working solutions (0.1−3.0 μM) were obtained by dilution and contained acetonitrile of
