Article pubs.acs.org/est
Oxidative Degradation of Decabromodiphenyl Ether (BDE 209) by Potassium Permanganate: Reaction Pathways, Kinetics, and Mechanisms Assisted by Density Functional Theory Calculations Jiaqi Shi,† Ruijuan Qu,† Mingbao Feng, Xinghao Wang, Liansheng Wang, Shaogui Yang,* and Zunyao Wang* State Key Laboratory of Pollution Control and Resources Reuse, School of Environment, Nanjing University, No. 163 Xianlin Avenue, Qixia District, Nanjing, Jiangsu 210023, People’s Republic of China S Supporting Information *
ABSTRACT: This study found that decabromodiphenyl ether (BDE 209) could be oxidized effectively by potassium permanganate (KMnO4) in sulfuric acid medium. A total of 15 intermediate oxidative products were detected. The reaction pathways were proposed, which primarily included cleavage of the ether bond to form pentabromophenol. Direct oxidation on the benzene ring also played an important role because hydroxylated polybrominated diphenyl ethers (PBDEs) were produced during the oxidation process. The degradation occurred dramatically in the first few minutes and fitted pseudo-first-order kinetics. Increasing the water content decelerated the reaction rate, whereas increasing the temperature facilitated the reaction. In addition, density functional theory (DFT) was employed to determine the frontier molecular orbital (FMO) and frontier electron density (FED) of BDE 209 and the oxidative products. The theoretical calculation results confirmed the proposed reaction pathways.
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INTRODUCTION As an important class of brominated flame retardants, polybrominated diphenyl ethers (PBDEs) are extensively used, and large amounts have been released into the environment. PBDEs have been detected in soil, sediment, air, water, and organisms around the world, even in the Arctic and Antarctica.1−8 PBDEs have similar structures and properties to other environmentally persistent contaminants, such as polychlorinated dioxins, polychlorinated furans, and polychlorinated biphenyls; therefore, they have become an increasing concern.9−11 Among the PBDE congeners, 2,2′,3,3′,4,4′,5,5′,6,6′- decabromodiphenyl ether (BDE 209) constitutes approximately 75% of the worldwide use of PBDEs.12 The concentration of BDE 209 ranged from 0.4 to 7340 ng/g of dry weight in sediments in the Pearl River Delta and the adjacent South China Sea, accounting for 72.6−99.7% of the total PBDEs.13 The highest atmospheric concentrations of BDE 209 were 48 pg/m3 in Kyoto14 and 100−340 pg/m3 in Osaka.15 BDE 209 has also been detected in living organisms, including humans. The concentration of ∑PBDEs ranged from 1121 to 53 470 mg/kg of lipid weight (lw) in British Columbia grizzly bears, of which BDE 209 contributed up to 83%.16 The highest BDE 209 concentration observed among rubber workers in Sweden was 280 pmol/g of lw, and the median concentration was 37 pmol/g of lw.17 Because BDE 209 is toxic to microorganisms, animals, and humans,18−21 it has been banned in the European Union and parts of the United States.22 © 2015 American Chemical Society
Although PBDEs can be degraded by different methods, such as ultraviolet (UV) photolysis,23 reductive degradation,24,25 and biodegradation, 22,26,27 other treatment methods are still requered.28 Oxidation is commonly used to remove organic contaminants. Numerous studies have shown that common oxidants can be used to treat emerging micropollutants containing electron-rich moieties,29,30 but these studies primarily focused on water-soluble chemicals.31,32 Studies on the oxidation of PBDEs only focused on the reaction kinetics, and limited information is available on the oxidative products.25,31 PBDEs have high hydrophobicity and strong stability, resulting in difficulties in acquiring and analyzing the products. Mn(VII) has been demonstrated to be useful in treating certain antibiotics and endocrine-disrupting chemicals,33,34 but BDE 209 could not be effectively oxidized by potassium permanganate (KMnO4) alone. Hence, referencing to the Hummers method, which has been employed in the production of graphene oxide,35 we oxidized BDE 209 using KMnO4 in sulfuric acid medium in this study. The reaction pathways were then revealed, and kinetic studies were performed. Furthermore, density functional theory (DFT) was employed to determine the Received: Revised: Accepted: Published: 4209
October 20, 2014 February 28, 2015 March 9, 2015 March 9, 2015 DOI: 10.1021/es505111r Environ. Sci. Technol. 2015, 49, 4209−4217
Article
Environmental Science & Technology
presented. The standard deviations were consistently
