Oxidation of Flame Retardant Tetrabromobisphenol A by Singlet Oxygen

• Published in: Environmental science & technology Vol. 42; no. 1; pp. 166 - 172
• Main Authors: Han, S.-K, Bilski, P, Karriker, B, Sik, R. H, Chignell, C. F
• Format: Journal Article
• Language: English
• Published: Washington, DC American Chemical Society 01.01.2008
• Subjects: Acetonitriles - chemistry; Applied sciences; Aqueous solutions; Environmental monitoring; Environmental Pollutants - chemistry; Environmental Pollutants - radiation effects; Environmental Processes; Exact sciences and technology; Flame Retardants - radiation effects; Oxidation; Oxidation-Reduction; Oxygen; Pollution; Polybrominated Biphenyls - chemistry; Polybrominated Biphenyls - radiation effects; Singlet Oxygen - chemistry; Spectrum analysis; Ultraviolet Rays; Ionization; Visible radiation; EPR spectrometry; Hydroxyl radicals; Flame retardant; Oxidation; Kinetics; Solar radiation; Spin trapping; Photosensitizer
• ISSN: 0013-936X; 1520-5851
• DOI: 10.1021/es071800d

Wide use of flame retardants can pose an environmental hazard, and it is of interest to investigate how they may degrade. We report here that 3,3′,5,5′-tetrabromobisphenol A (TBBPA) is subject to photosensitized oxidation involving singlet molecular oxygen (1O2). By using visible light and rose bengal or methylene blue as 1O2 photosensitizers, we have found that TBBPA is a 1O2 quencher. The quenching rate constant, k q, depends on TBBPA ionization (pK = 7.4). In acetonitrile, where TBBPA is undissociated, the k q value is 6.1 × 105 M−1 s−1 for a TBBPA monomer and decreases to 2.9 × 104 M−1 s−1 for TBBPA dimers and/or aggregates. TBBPA dissociates in aqueous solutions, and its k q value is 1.44 × 109 M−1 s−1 in alkaline solution, decreasing to 3.9 × 108 M−1 s−1 at pH 7.2. The strong 1O2 quenching by TBBPA anion initiates an efficient oxidation of TBBPA, which results in oxygen consumption in aqueous micellar (e.g., Triton X-100) solutions containing photosensitizer. This oxygen consumption is mediated by transient radical species, which we detected by using EPR spectroscopy. We observed two major radicals and one minor radical generated from TBBPA by reaction with 1O2 at pH 10. One was identified as the 2,6-dibromo-p-benzosemiquinone radical (a 2 H = 2.36 G, g = 2.0056). A second radical (a H = 2.10 G, g = 2.0055) could not be identified but was probably a 2,6-dibromo-p-benzosemiquinone radical containing an EPR-silent substituent at the 3-position. Spin trapping with 5,5-dimethyl-1-pyrroline N-oxide (DPMO) showed that other minor radicals (hydroxyl, carbon-centered) are also generated during the reaction of TBBPA with 1O2. The photosensitized production of radicals and oxygen consumption were completely inhibited by the azide anion, an efficient physical 1O2 quencher. Because TBBPA is a stable compound that at neutral pH does not absorb much of the atmosphere-filtered solar radiation, its photosensitized oxidation by 1O2 may be the key reaction initiating or mediating TBBPA degradation in the natural environment.