ABSTRACT

The photolysis of tetrabromobisphenol A (TBBPA) in aqueous solution under  simulated solar light irradiation was studied under different conditions to find out mechanisms and pathways that control the transformation of TBBPA during photoreaction. Particular attention was paid to the identification of intermediates and elucidation of photolysis mechanism of TBBPA by UPLC, LC/MS, FT-ICR-MS, NMR, ESR and stable isotope techniques (13C and 18 24 O). The results showed that the photolysis of TBBPA could occur under simulated solar light irradiation in both aerated 26 and deaerated conditions. A magnetic isotope effect (MIE)-hydrolysis transformation was proposed as the predominant pathway for TBBPA photolysis in both cases. 2,6-Dibromophenol and two isopropylphenol derivatives were identified as photo-oxidation products of TBBPA by singlet oxygen. Reductive debromination products tribromobisphenol A and dibromobisphenol A were also observed. This is the first report of a photolysis pathway involving the formation of  hydroxyl-tribromobisphenol A.

INTRODUCTION

Tetrabromobisphenol A (TBBPA) is extensively used in plastics, textiles and electronics as a commercial brominated flame retardant to improve fire safety1-3 . Release of TBBPA to environment occurs during manufacturing, recycling and disposal of various textiles and solid waste materials4 . For instance, TBBPA concentrations were up to 540 ng/L in dumps dialyzates and up to 620 ng/L in industrial landfill leachates as reported in Japan5,6 . TBBPA was found in human tissues 7 , maternal and cord serum, breast milk in women8 , and serum from exposed and general population9,10 . Researchers suggest that TBBPA is a potentially persistent, bioaccumulative and toxic (PBT) compound1 . It has been identified as an environmental endocrine disruptor and reported to display low but multiple hormonal activities11 .

Due to the extensive use, bioaccumulation, toxicity and potential hazard of TBBPA, it is urgent to investigate its transformation in water and develop effectivedecontamination methods. Both biotic and abiotic methods have been developed to remove TBBPA from aquatic environment12-19 . Most reactions under aerobic conditions were performed in the presence of catalysts, aiming to develop more effective methods with high debromination and TOC removal efficiency. Photolysis is one of the main abiotic degradation pathways of TBBPA and may affect its fate and ecological risk in the natural environment. The degradation of TBBPA was reported to involve different photochemical processes20-28 . TBBPA could be degraded by UV irradiation20,21, photocatalytic oxidation (TiO2 22,23, Ag/Bi5Nb3O15 24, BiOBr25,titanomagnetite26, [Mn(VII)]27) and photosensitized oxidation28,29 . In these photoreactions, photogenerated electrons and reactive oxygen species (ROS) were responsible for its degradation. Debromination and beta scission were proposed as two major pathways26 .However, the mechanism of direct photochemical transformation inependent of the function of ROS was overlooked.  

In this work, phototransformation of TBBPA in water under both aerobic and anaerobic conditions was investigated. Kinetic isotope effects (KIE) as a powerful tool was used to determine the reaction mechanism. Together with 18 O isotope tracing technique, we proposed TBBPA photolysis pathway that is different from previous studies. It was our aims to investigate (1) the possible photolysis pathways of TBBPA in water; (2) the validity of stable isotope techniques and products analysis combined for the degradation pathway differentiation; (3) the active species responsible for the photolysis of TBBPA. This study should be helpful for better understanding the photochemical behavior of aromatic bromide in aquatic environments.

MATERIALS AND METHODS

Materials and Chemicals. Tetrabromobisphenol A (purity 99%) was obtained from the Dr. Ehrenstofer Germany. H2 18O (18 76 O > 97%) was purchased from Shanghai Research Institute of Chemical Industry (SRICI). Reagents NaCl, Na2SO4, Na2CO3, Fe(ClO4)—6H2O, HClO4, NaOH，rose bengal, NaN3，furfuryl alcohol, fulvic acid were purchased from the China National Medicines Co. (China), HPLC-grade methanol was obtained from Shanghai Xingke Biochemical Co. (China). All experimental solutions were prepared by dissolving reagents directly in ultra-pure water. All stock solutions were refrigerated after preparation.

Photochemical Experiments.

Photochemical experiments were conducted in a photochemical reaction chamber XPA-7 (350W-Xe, Xujiang, China, λ > 290 nm) to simulate solar light. Irradiation of aqueous solutions (50 mL) was carried out in quartz glass reaction cells, which were positioned at a distance of 5.5 cm to the lamp center. The light intensity impinging on the solutions was 20 mW/cm2 as measured with a 88 radiometer (CEL-NP2000, Beijing Aulight Co. China). The initial TBBPA concentrations were 10-4 M. The initial pHs of the aqueous solutions were fixed at 8.0 by adjusting the solutions with NaOH and HClO4, and was not controlled during the course of the reaction. At given reaction time intervals, samples were taken out and analyzed by ultra performance liquid chromatography (UPLC) (Figure S1). Each  experiment was conducted at least twice with relative errors less than 5%. The deaerated experiment was done under ultrapure N2 atmosphere.

  Sample pretreatment and Instrument Analysis Methods. The concentration of TBBPA in the reaction process was measured by UPLC (Waters, ACQUITY UPLC H-Class, USA) fitted with a C18 column (2.1 × 50 mm, 1.7 µm). The mobile phase composition was methanol - 0.2% acetic acid (70:30, V/V) at a flow rate of 0.2 mL/min. Samples were analyzed with photodiode array (PDA) detector at a wavelength of 210 nm. Solid phase extraction (SPE) was optimized as the primary extraction and cleaning procedure for all water samples for liquid chromatography/mass spectrometry (LC/MS) and carbon isotope analysis. After preconditioned by 10 mL methanol and 10 mL purified water, 50 mL of sample was loaded onto the C18 cartridge at a rate of 5 mL/min, and the cartridges were subsequently eluted with 9 mL methanol. The elution was then concentrated to ca. 1 mL by rotary evaporation. The LC/MS methods for analyzing photolysis products of TBBPA was performed using a LCQ Fleet (Thermo Fisher Scientific, USA) equipped with a Waters SunFireTM-C18 column (4.6 mm × 108 250 mm, 5 µm). Electron spray ionization (ESI) was performed with a spray voltage set at 5000 V, sheath gas flow rate and aux gas flow rate and capillary temperature were set at 30 arb, 10 arb and 300℃, respectively. The 18 O isotopic tracer experiment was conducted in H2O , and the sample was analyzed by LC/MS. The isotopic proportion of parent peak or fragment ion in the observed MS matches excellently the simulated 113 spectra using the Chemdraw software. The electron spin resonance (ESR) technique was used to detect radicals on a Bruker (ESP 300E) spectrometer equipped with a 355 nm laser. The ESR settings were modulation amplitude 1.94 G and microwave 116 frequency 9.75 GHz.