Environ. Sci. Technol. 2009, 43, 157–162
TiO2-Mediated Photocatalytic Debromination of Decabromodiphenyl Ether: Kinetics and Intermediates CHUNYAN SUN, DAN ZHAO, CHUNCHENG CHEN, WANHONG MA, AND JINCAI ZHAO* Beijing National Laboratory for Molecular Sciences, Key Laboratory of Photochemistry, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China
Received July 12, 2008. Revised manuscript received October 26, 2008. Accepted October 27, 2008.
In this work, we report the rapid photoreductive debromination of decabromodiphenyl ether (BDE209) by TiO2. The degradation of BDE209 is a stepwise process, and the bromines at the ortho positions are much more susceptible than those at the para positions. The photocatalytic degradation kinetics of BDE209 was further investigated under different reaction conditions (various solvents, in the presence of H2O, acids, or bases). A possible photoreductive debromination pathway was proposed on the basis of the identified reaction intermediates and density functional theory (DFT). This study provides a potential application of photocatalysis in removal of PBDE contamination.
Experimental Section
Introduction Polybrominated diphenyl ethers (PBDEs) are commonly used as brominated flame retardants (1), arousing high environmental concern because of their global distribution and bioaccumulation. They have been detected in sediments, marine organisms, food samples, and human mothers’ milk, even in Arctic animals (2-5). It was reported that PBDEs influence liver enzyme activity and act as endocrine disruptors, inducing immunotoxicity and influencing neurological development at a key period of brain growth (6, 7). Therefore, it is critical to understand the environmental transformation of PBDEs and to explore a feasible way for eliminating PBDE contamination. PBDEs are strongly resistant to oxidative degradation. Nonetheless, they have been reported to undergo reductive debromination to lower bromo congeners (8-16). For example, photolytic debromination of PBDEs has been observed on the surface of clay minerals, metal oxides, silica gel, sand, soil, and sediment (9, 12) and in solutions of toluene, hexane, or methanol/water (12, 14, 15). The biotic reductive debromination has also been observed in anaerobic bacteria (16, 19), Juvenile rainbow trout, and common carp (10). Besides the studies on the environmental transformation of PBDEs, the development of potential methods for PBDE removal in contaminated environmental systems is another urgent and significant issue. However, only a few studies were devoted to the degradation of PBDEs (16-20). Li and Keum studied the reductive debromination of PBDEs by zerovalent iron powder (17): 92% of BDE209 was converted * Corresponding author fax: 86-10-8261-6495; e-mail: jczhao@ iccas.ac.cn. 10.1021/es801929a CCC: $40.75
Published on Web 12/12/2008
into lower bromo congeners within 40 days. Jiang et al. (18) reported that resin-bound zerovalent iron nanoparticles exhibited much higher activity. BDE209 was converted to less brominated products within 8 h. The hydrothermal treatment was also introduced as a potential method for removal of PBDEs (20). TiO2 photocatalysis has been extensively studied for its environmental applications, especially in catalytic degradation of a variety of organic contaminants (21-24). Much research on the potential applications of TiO2 focused on the hole-initiated oxidative degradation of organic pollutants. However, because of the recalcitrancy of PBDEs to oxidation, the photocatalytic degradation of PBDEs is less favorable under conventional oxidation conditions, and no photocatalytic degradation of PBDEs has been investigated to our best knowledge. Considering that PBDEs can undergo reductive debromination, we realize that the conduction band (CB) electrons generated during the photocatalytic reaction might effectively reduce PBDEs to their lower bromo congeners under appropriate conditions, which makes the photocatalytic degradation of PBDEs possible. In this paper, for the first time the photocatalytic degradation of PBDEs was investigated. BDE209 as the major product of PBDEs (25) was selected as a target PBDE. The photocatalytic degradation kinetics of BDE209 was examined under different reaction conditions (various solvents, in the presence of H2O, acids, or bases). The reaction intermediates were identified, and product distributions were interpreted by DFT caculation. The photocatalytic mechanism was also proposed on the basis of experimental results and density functional theory. It could be a promising method in the removal of PBDEs.
2009 American Chemical Society
Materials. BDE209 was purchased from Aldrich Chemical Co. (Milwaukee, WI). BDE203, BDE204, and a standard solution of PBDEs (EO5103) were purchased from Cambridge Isotope Laboratories (CIL, Andover, MA). 13C-labeled surrogate standard solutions of PBDEs were obtained from Wellington Laboratories (Canada). TiO2 (P25, ca. 80% anatase, 20% rutile; surface area, ca. 50 m2 g-1) was supplied by the Degussa Company (Germany). Tetrahydrofuran, acetonitrile, isopropyl alcohol, N,N-dimethylformamide, hexane, toluene, tetrahydrofuran, acetone, and dimethyl sulfoxide were analytical reagents, and methanol was a guaranteed reagent (Chemical Co., Beijing). Formic acid and acetic acid were analytical reagents purchased from Acros Organics. Trifluoroacetic acid was chemically pure (Chemical Co., Beijing). Pyridine was HPLC grade (Dikma, ON, Canada) and triethylamine was analytical reagent (Chemical Co., Beijing). They were used without further purification. Deionized and doubly distilled water was used throughout the study. Experimental Setup. BDE209 stock solution (2 × 10-3 mol/L) in tetrahydrofuran was diluted with a certain solvent (typically acetonitrile) to 2 × 10-5 mol/L. The BDE209/TiO2 dispersions were prepared by adding TiO2 particulates (1 g/L) to a 40 mL solution of BDE209 in a Pyrex vessel. A 0.33 mol/L amount of isopropyl alcohol was added to the dispersions as the electron donor. The suspensions were stirred for 30 min and ultrasonically mixed 1 min in the dark before irradiation. Reaction suspensions were magnetically stirred during the irradiation. For experiments under anoxic conditions, the Pyrex vessel was purged with argon for 30 min to remove O2 and protected under an argon atmosphere during the irradiation. A PLS-SXE300 Xe lamp (Beijing Trusttech Co. Ltd.) was used as the light source. To eliminate VOL. 43, NO. 1, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 1. Temporal curves of the photodegradation of BDE209 in 40 mL acetonitrile solution or dispersion containing 0.33 mol/ L isopropyl alcohol) under different conditions. BDE209: 2.0 × 10-5 mol/L; UV: under irradiation of wavelength >360 nm; Ar: purged with argon; air: air-saturated; TiO2: 1 g /L. the direct photolysis of BDE209, a cutoff filter was used to cut the irradiation below 360 nm. To investigate the effect of H2O and acids/bases on the reaction kinetics, a given amount of H2O or acid/base was added to the dispersions under otherwise identical conditions. The effect of solvent was explored by replacing acetonitrile with hexane, toluene, tetrahydrofuran, acetone, dimethyl sulfoxide, methanol and N,N-dimethylformamide. At given time intervals, 3 mL aliquots were centrifuged and filtered through a Millipore filter (pore size 0.20 µm) to remove the TiO2 particles. The filtrates were used for HPLC and GC/HRMS analysis. HPLC Analysis. BDE209 in samples was quantified with a Shimadzu HPLC system (LC-20AT pump and UV/vis SPD20A detector) with a Dikma Platisil ODS C-18 column (250 × 4.6 mm, 5 µm film thickness). The mobile phase was 2% water in acetonitrile at 1 mL/min (26), and the detector wavelength was set at 240 nm. The quantification was done with a calibration cure with a BDE-209 standard. Product Analysis. Trace GC Ultra coupled with high resolution mass spectrometers (Thermofisher, MA, U.S.A.) (GC/HRMS) was used for products analysis. The HRMS was operated at 45 eV ionization energy, and the resolution was >10 000. The extracts were transformed to hexane from acetonitrile solution, and the internal standards (13C-BDE47, 13 C-BDE99, and 13C-BDE153) were added to all samples before analysis. Splitless 1 µL injection was performed manually at 280 °C. A DB-5 capillary column (30 m × 250 µm i.d. × 0.1 µm film thickness) was used. Helium was the carrier gas at a constant flow rate of 1.0 mL/min. The oven temperature was kept at 100 °C for 2 min, increased at 15 °C/min to 230 °C, increased at 5 °C/min to 270 °C, and finally increased at 10 °C/min to 330 °C for 8 min. Quantification was carried out with the internal calibration standard method.
Results and Discussion Degradation Kinetics. Figure 1 shows the degradation kinetics of BDE209 in acetonitrile under different conditions. The onset absorption of BDE209 in acetonitrile was at 340 nm. To eliminate its direct photolysis (8, 15), the irradiation below 360 nm was filtered completely. BDE209 in acetonitrile solution exhibited little degradation under the UV irradiation even in the presence of isopropyl alcohol (curve a). The reaction system containing TiO2 which was kept in the dark under anoxic conditions showed no disappearance of BDE209 (curve b). However, very rapid degradation of BDE209 occurred in the anoxic BDE209/TiO2 dispersions under UV irradiation, and more than 90% of BDE209 disappeared after 7.5 min of irradiation (curve d). The kinetics was fitted by pseudo-first-order process, giving a rate constant of 0.33 ( 0.02 min-1 (t1/2 ) 2.1 min). The 158
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decay rate was significantly faster than those in the zerovalent-iron systems (17, 18), in which the complete removal of BDE209 took 40 days and 8 h, respectively. The degradation of BDE209 was barely observed in the air-saturated TiO2 suspension under UV irradiation (curve c), which was the typical condition for photocatalytic oxidative degradation of organic compounds. This indicated that BDE209 was resistant to the oxidation by valence band (VB) holes or reactive oxygen species such as hydroxyl radical. When the TiO2 was replaced by SiO2 or Al2O3 nanoparticles (Figure S1, Supporting Information), no degradation of BDE209 was observed under identical conditions, indicating that the degradation was initiated by TiO2-based photocatalytic reaction. During the photocatalytic reaction, electrons are excited from VB into CB of TiO2 under UV irradiation which results in the generation of a VB hole and a CB electron (22). The VB hole is scavenged by isopropyl alcohol, and the electron is accumulated in CB of TiO2. Under photocatalytic conditions, the possible reductive species that are responsible for the reduction of DBE209 are CB electrons or the organic radicals which are generated from the one-electron oxidation of electron donors (isopropyl alcohol) or the solvents. CB electrons are usually trapped in the form of Ti(III) on the TiO2 nanoparticle surface. In a control experiment, the Arpurged TiO2/acetonitrile dispersion containing isopropyl alcohol, but in the absence of BDE209, was first irradiated by UV light (λ > 360 nm). During this process, TiO2 particles turned blue gradually, and electrons were accumulated in the CB of TiO2, owing to removal of photogenerated holes by isopropyl alcohol. After turning off the UV irradiation, an Ar-purged BDE209 (the final concentration was 2 × 10-5 mol/ L) was injected into the system in the dark. In this case, there should be no organic radicals present in the dispersion because they will disappear immediately upon turning off the irradiation. The blue color of the TiO2 dispersion disappeared, and the HPLC analysis showed that about 40% of BDE209 was degraded. This indicates that the stored conduction band electrons can indeed lead to the reductive degradation of BDE209 in the absence of organic radicals. The organic radicals are also known to be powerful reducing agents and are capable of reducing halogenated organics and nitro organics at near diffusion-controlled rates (27-29). However, whether and how the in situ organic radicals, as an additional channel, participate in the reduction degradation of BDE209 was not clear at the present stage. Product Identification. It is known that bioaccumulation and toxicity of PBDEs depend on the number of bromine atoms and their substituted position. For example, pentaBDEs were reported to be the most toxic among PBDE congeners. In our study, the degradation products were separated, identified, and quantified by GC-HRMS with chemical standards (Figure 2). The first step was the loss of one bromine atom, forming nona-BDEs. After 3 min of irradiation, all three nona-BDEs were observed, identified as DBE208, -207, and -206, respectively, according to their wellestablished GC elution order. There were five octa-BDEs observed after 7.5 min of irradiation. BDE-203 and BDE-204 were identified by the chemical standards. The peak with a retention time slightly longer than that of BDE203 was BDE196, and the peak with a retention time longer than that of BDE204 was BDE197, according to the relative retention times and the orders of GC elution obtained from the literature (9, 10, 14, 30, 31). Three hepta-BDEs were detected after 3 h of irradiation. BDE-183 and BDE-190 were identified with the chemical standards. The peak, whose retention time was slightly shorter than that of BDE183, could be assigned to BDE184 based on the retention time reported in the literature (30). Three hexa-BDEs peaks were identified to be BDE-138, BDE-153, and BDE-154, respectively, with the
FIGURE 2. GC-HRMS chromatograms of photodegradation of BDE209 in 40 mL TiO2/acetonitrile dispersion under anaerobic conditions after irradiation for (a) 3 min and (b) 24 h. BDE209, 2.0 × 10-5 mol/L; TiO2, 1.0 g/L; isopropyl alcohol, 0.33 mol/L.
FIGURE 3. Changes in homologue distribution at different irradiation time in photocatalytic degradation of BDE209 in 40 mL of anaerobic TiO2/acetonitrile dispersion. BDE209, 2.0 × 10-5 mol/L; TiO2, 1.0 g/L; isopropyl alcohol, 0.33 mol/L. “8, 9 Br” is the sum of nona-BDEs and octa-BDEs. The abundance of nona-BDEs and octa-BDEs was calculated from the amount of BDE-209 and hepta-BDEs because of the lack of related chemical standards. The concentrations of other congeners with no chemical standards were estimated by assuming that they had a similar response factor to their isomers with available standards. chemical standards. Three penta-BDEs were identified with chemical standards, and they were BDE-85, BDE-99, and BDE-100, respectively. BDE-47 was identified as tetra-BDE, which was one of the most commonly detected in the environment. Stepwise Debromination Processes. It was observed that BDE209 was rapidly transformed to its lower bromo congeners in a stepwise way under the present photocatalytic conditions (Figure 3 and Figure S2, Supporting Information). Before the photoreaction, only BDE209 was detected (Figure S2a). After 3 min of irradiation, BDE209 disappeared rapidly with nona-BDEs (major), and octa-BDEs (minor) appeared concomitantly (Figure S2b). Irradiation for 7.5 min resulted in more than 90% of BDE209 decomposed, and octa-BDEs became the dominant products (Figure S2c). Hepta-BDEs
were formed after 30 min of irradiation. The hexa-BDEs were observed at 3 h (Figure S2d), and penta-BDEs were formed at 9 h. The tetra-BDEs appeared after 24 h of irradiation (Figure S2e). The changes in homologue distribution with reaction time were shown in Figure 3. Before irradiation for 180 min, the predominant degradation products were nona-BDEs and octa-BDEs. The accumulation of nona- and octa-BDEs in the degradation system indicates that DBE209 debrominated easily to nona- and octa-BDEs, but the 8-Br and 9-Br PBDEs were difficult to further debrominate. After irradiation for 9 h, the hepta- and hexa-BDEs were the dominant intermediates, and the hexa-BDEs were observed as major products after 24 h irradiation. Debromination Pathways. The intermediate identification shows that the bromines at ortho or meta positions are more susceptible to debromination than those at the para position. In all the identified debrominated intermediates except nona-BDEs, the bromines at the para positions were preserved (Figure 4). In accordance with our study, Keum and Li (17) also observed that the reductive debromination of four PBDEs (2,4,4′-BDE28, 2,2′,4,4′-BDE47, 2,3′,4,4′-BDE66, and 2,2′,4,4′,6-BDE100) by zerovalent-iron resulted in the preferential accumulation of para-brominated products (4,4′BDE15, 2,4,4′-BDE28, 3,4,4′-BDE37, and 2,2′,4,4′-BDE47, respectively), suggesting that para-debromination is difficult under reducing conditions. Among all intermediates, the products of the first debromination step (nona-BDEs) provide direct information on the debromination pathway of BDE209. All the three nonaBDE intermediates were identified during the photocatalytic reductive reaction. They were BDE206 BDE207, and BDE208, corresponding to ortho-, meta-, and para-debrominated intermediates of BDE209, respectively. Their relative peak areas were in the order BDE206 . BDE207 > BDE208 (1: 0.25:0.16) at irradiation times of 3 and 7.5 min (upon further irradiation for 180 min, the nona-BDE peaks disappeared), indicating that the ortho debromination was much easier VOL. 43, NO. 1, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 4. Proposed photocatalytic debromination pathways of BDE209. Only identified congeners are shown. The names in bold indicate congeners with major peaks in GC-HRMS. These congeners were identified by GC-HRMS with responding standards, except those in parentheses which were assigned according to the literature. than those from other positions, and that the debromination from the para position was the most unfavorable pathway. Previous studies showed that the initial step of BDE209 debromination varied greatly with reducing systems. Direct photolysis of BDE209 in methanol/water (8:2) solution (15) or on the surface of sand and sediment (12) exhibited similar GC-MS patterns to our study, i.e. BDE206 was the most abundant intermediate and BDE208 was difficult to produce while microbial degradation of BDE209 in anaerobic sewage sludge produced only BDE207 and BDE208 (0.136 nmol and 0.15 nmol, respectively, after incubation of 138 days in the presence of 10 nmol BDE209), and no ortho-debrominated product (BDE206) was observed (16). The biotic debromination of BDE209 also mainly yielded BDE208 (10). However, the debromination was not position-selective in other systems. Jiang et al. (18) reported that resin-bound nanoiron reduction systems did not show much difference in the relative peak areas of the three nona-BDEs. In direct photolysis of BDE209 by sunlight in hexane, the relative peak 160
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areas of the three nona-BDEs were also similar (14). These results indicated that the debromination of BDE209 went through different pathways in varied systems. In the photocatalytic reduction, debromination of PBDEs may be initiated through two possible pathways: (1) direct electron transfer from the conduction band of TiO2; (2) addition of a hydrogen atom to an aromatic carbon. Following both initial steps is the elimination of bromine. The DFT population analysis on the DBE209 molecule showed that there was little correlation between the debromination rates and the atomic charges on the aromatic carbon (Table 1). The calculation results indicated that the hydrogenolysis pathway was not the predominate step to initiate the debromination reaction, since the attack of the hydrogen atom on the aromatic carbon would greatly depend on the local electronic characteristics of the target carbon. Accordingly, the most likely initial step was the one-electron reduction of BDE209 in our system. It was expected that the follow-up elimination process of bromine would strongly
TABLE 1. Position Dependence of Relative Amounts of Debromination of DBE209 (RGC-HRMS) and the Calculated Energy and Population Analysis by DFT Methodsa
ortho meta para
RGC-HRMSb
BDEc (kJ/mol)
MACBrd
MACCe
1 0.25 0.16
190.4 196.3 196.4
0.141 0.097 0.083
-0.244 -0.288 -0.239
a Hybrid density functional theory (DFT) was employed to obtain optimized geometries, electronic energies, and Mulliken population analysis of the BDE209 molecule and its anionic radical, and debrominated radicals at the (U)B3LYP/6-31G level of theory using Gaussian 03 package (32). b Estimated from relative abundance of the peaks in GC-HRMS and normalized by using the ortho-debrominated product as a unit. c C-Br bond dissociation energy (BDE) of DBE209 anionic radical. d Mulliken atomic charges on bromine (MACBr) of the DBE209 anionic radical. e Mulliken atomic charges on carbon (MACC) of the DBE209 molecule.
TABLE 2. Adsorption Amount and Photocatalytic Degradation Rates of BDE209 on TiO2 under Various Solvent Conditionsa
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18
solvents
adsorption amount (%)
degradation rate (%)d
toluene hexane acetone THF DMSO methanol DMF acetonitrile acetonitrile + H2O (0.1%) acetonitrile + H2O (0.2%) acetonitrile + H2O (0.5%) acetonitrile + H2O (1%) acetonitrile + formic acidb acetonitrile + acetic acidb acetonitrile + TFAb acetonitrile + pyridine c acetonitrile + triethylaminec acetonitrile+ DMFc
0 0 0 0 0.3 27.0 0.4 15.4 14.2 7.5 2.1 0.5 0 0.1 1.3 1.5 1.2 0.2
0 0 0 0 0 43.8 85.9 63.6 43.6 5.7 4.3 0 0 0 0 85.8 87.2 95.6
a 40 mL of TiO2 dispersion under anaerobic conditions. BDE209, 2.0 × 10-5 mol/L; TiO2, 1.0 g/L; isopropyl alcohol, 0.33 mol/L. b Concentrations of acids: 4.6 × 10-3 mol/L, c Concentrations of bases: 4.9 × 10-3 mol/L. d Removed ratios after 3 min of irradiation.
acetonitrile (Table 2, entries 6 and 8), despite the adsorption amount of BDE209 (27.0%) in methanol being larger than that (15.4%) in acetonitrile. The N,N-dimethylformamide (DMF) system exhibited higher activity for the degradation of BDE209 than the acetonitrile system, although the adsorption of BDE209 was only 0.4% (Table 2, entry 7). It was also found that when a small amount of water was added to the dispersions, the degradation of BDE209 was remarkably suppressed (Table 2, entries 8-12; Figure S4, Supporting Information): 1% water in the acetonitrile solution could stop the degradation. A similar effect of water on the adsorption of BDE209 was also observed. The addition of acids such as formic acid, acetic acid, and trifluoroacetic acid (TFA) greatly depressed the degradation of BDE209). On the contrary, all the investigated bases (pyridine, triethylamine, and DMF) were found to accelerate the degradation to some extent (Table 2, entries 13-18; Figure S5, Supporting Information). It is evident that the photocatalytic degradation of BDE209 greatly depends on the electronic structure and properties of TiO2 surface/solvent. We found that in hexane, toluene, THF, acetone, DMSO, and acetonitrile-H2O, where the adsorption of BDE209 was weak, degradation scarcely occurred. In general, the photocatalytic reaction primarily occurs on the surface of TiO2. The adsorption affects the accessibility of the surface reducing species to the BDE209 and the reduction kinetics. However, adsorption is not the only factor that controls the photocatalytic degradation of BDE209. For example, although the adsorption amounts of BDE209 were in the order of DMF < acetonitrile < methanol, the degradation rates were in the reverse order (Table 2). Although addition of both acids and bases significantly suppressed the adsorption, the degradation was accelerated in the presence of bases but was greatly depressed by acids. It is known that the Fermi level (flat band potential) of TiO2 particles shifts upon the adsorption of acids or bases on the surface of TiO2 (33). Generally, the adsorption of bases (such as 4-tert-butylpyridine) on the surface of TiO2 shifts the Fermi level toward a negative potential (34); on the contrary, the adsorption of acids tends to positively shift the conduction band edge. Accordingly, the addition of bases/acids can enhance/suppress the reduction of BDE209. It should also be noted that, because of the complexity of photocatalytic systems, many mechanistic details including the effect of solvents on the energy structure of TiO2 and on the formation and transfer of the reducing species were not understood thoroughly and need to be addressed further.
Acknowledgments depend on the strength of the C-Br bond in the anionic radical (BDE209•-). The calculated bond dissociation energy of C-Br in the radical showed that the most abundent orthodebrominated product (BDE206) corresponds to the weakest ortho C-Br bond, (the bond dissociation energy was 6 kJ/ mol lower than that of meta and para C-Br (Table 1)) implying that the cleavage of the C-Br with the release of Br- from the PBDE anionic radical is the rate-determining step of the whole debromination pathway. We also found that a strong correlation of position-selectivity of debromination with atomic charges (Table 1) and atomic spin densities (data not shown) on the bromines in the anionic radical. Effects of Solvent Properties on the Photocatalytic Degradation of BDE209. The reactions were further carried out in eight other solvents (Table 2 and Figure S3, Supporting Information). The degradation of BDE209 and its adsorption on TiO2 was not observed in hexane, toluene, tetrahydrofuran (THF), acetone, and dimethyl sulfoxide (DMSO) within the present experimental scale (Table 2, entries 1-5). In methanol, the degradation of BDE209 was slower than that in
Generous financial support by 973 project (2007CB613306), by the National Science Foundation of China (Nos. 20537010, 20677062, and 20877076), and the Chinese Academy of Sciences is gratefully acknowledged. The authors also thank Profs. G. B. Jiang and Q. H. Zhang, and Drs. Y. W. Wang and P. Wang. (Research Center for Eco-Environmental Sciences, CAS) for their kind GS-HRMS analysis and helpful discussion.
Supporting Information Available The effects of the degradation rates of BDE209 under different conditions (on SiO2 or Al2O3, various solvents, and in the presence of H2O, acids, or bases). The GC-HRMS of degradation products of BDE209 at different irradiation times and the details of ref 32. This material is available free of charge via the Internet at http://pubs.acs.org.
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