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Photocatalytic debromination of preloaded decabromodiphenyl ether on the TiO2 surface in aqueous system
Chemosphere 89 (2012) 420–425
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Photocatalytic debromination of preloaded decabromodiphenyl ether on the TiO2 surface in aqueous system Chunyan Sun a,b, Jincai Zhao b, Hongwei Ji b, Wanhong Ma b, Chuncheng Chen b,⇑ a b
Department of Chemistry, Shaoxing University, Zhejiang Shaoxing 312000, China Beijing National Laboratory for Molecular Sciences, Key Laboratory of Photochemistry, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China
h i g h l i g h t s " A preloaded method was investigated to photocatalytic debromination of PBDEs in water. " The organic-like micro-environment on the surface of TiO2 prevented the effect of water. " It provide a potential remediation technology in water not only for PBDEs but also for other hydrophobic contaminants.
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Article history: Received 3 February 2012 Received in revised form 10 April 2012 Accepted 19 May 2012 Available online 12 June 2012 Keywords: Decabromodiphenyl ether TiO2 Preload Aqueous system Photocatalytic degradation
a b s t r a c t There have been serious concerns about polybromodiphenyl ethers (PBDEs) in the environment because of their global distribution and bioaccumulation. Owing to strong hydrophobicity of PBDEs, the regular photocatalytic system, in which the substrate is solvated in the bulk solution, is not applicable to the removal of the PBDEs in water. In this work, the photocatalytic reduction degradation of decabromodiphenyl ether (BDE209), the most-used PBDEs, was investigated in aqueous system, by pre-adsorbing it on the surface of photocatalyst. It was found that the preloaded BDE209 underwent efficient reductive debromination in aqueous system under irradiation with wavelength larger than 360 nm in the presence of electron donors such as methanol. Our experiments further show that such a preloaded system exhibits different characteristics from that in the organic solution. The meta-debrominated intermediate is predominant in the present system, while the ortho-debrominated one is the main nona-BDE products in the organic solution. In addition, different from other photocatalytic system, the pH has little effect on the photocatalytic reaction. We propose that these differences are originated from the formation of overlayer of hydrophobic BDE209 to limit the motion of BDE209 and the access of water and H+/OH to the TiO2 surface. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction Recently, polybrominated diphenyl ethers (PBDEs) arouse high environmental concern due to their global distribution and bioaccumulation (Wolkers et al., 2004; Mai et al., 2005; WyrzykowskaCeradini et al., 2011). PBDEs as brominated flame retardants have been widely used in the consumer electronics, furniture, and building materials (Alaee et al., 2003; Du et al., 2010). Owning to their increasing usage, and their high hydrophobicity, persistence and bioaccumulation, PBDEs have been detected in sediments, marine organisms, food samples, and human mother’s milk, even in the animals of Arctic regions (de Wit, 2002; Routti et al., 2009). Exposure to PBDEs, the balance of the thyroid of human is able to be ⇑ Corresponding author. Present address: Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China. Tel./fax: +86 10 8261 6495. E-mail address: [email protected] (C. Chen). 0045-6535/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.chemosphere.2012.05.076
affected, which will cause neurotoxicity (Kierkegaard et al., 1999; Meerts et al., 2001; Behnisch et al., 2003). Deca-BDE has been classified as a possible human carcinogen by the US Environmental Protection Agency (EPA) (Agency for Toxic Substances, 2004). Research on PBDEs has mainly focused on the distribution and transformation of PBDEs in the environment. PBDEs can be photolytically debrominated on the surface of clay minerals, metal oxides, silica gel, sand, soil and sediment (Ahn et al., 2006; Li et al., 2010), and in solution of toluene and hexane (Bezares-ruz et al., 2004). The biotic reductive debromination has also been reported by anaerobic bacteria (He et al., 2006), Juvenile rainbow trout and common carp (Stapleton et al., 2006). In contrast to the abundant studies on the environmental action of PBDEs, the development of potential methods for PBDEs removal in contaminated environmental system remains relatively undeveloped. Hitherto, only a few studies have been reported to the degradation of PBDEs. Gerecke reported that anaerobic bacteria could effective degrade
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BDE209 (Gerecke et al., 2005). Keum studied the reductive debromination of BDE209 by zerovalent iron powder (Keum and Li, 2005). Within 40 days, 92% of BDE209 has been converted into lower bromo congeners. Jiang’s group (Li et al., 2007) reported that resin-bound zerovalent iron nanoparticles exhibited a more rapid debromination activity. BDE209 could be converted into lower bromo congeners within 8 h. Photocatalysis is an effective method for degradation of environmental contaminants (Agrios and Pichat, 2005; Robertson et al., 2005; Fujishima et al., 2008; Gupta et al., 2012). Photocatalytic degradation of PBDEs is less favorable under conventional oxidation conditions, due to the recalcitrancy of PBDEs to oxidation. In our previous studies (Sun et al., 2009), it was found that PBDEs could undergo rapid debromination by TiO2-mediated photocatalytic reduction method in organic solutions such as CH3CN and CH3OH. It was also observed that presence of H2O markedly suppressed the degradation of BDE209 by TiO2 in organic solution. The reason is BDE209 owns strong hydrophobic, but the surface of TiO2 is quite hydrophilic. The surface of TiO2 would be preferentially adsorbed by water, which would block the interaction between the TiO2 and BDE209, and further inhibit the electron transformation from conduction band to BDE209 molecular. That places some impediments on the treatment of BDE209, since water is ubiquitous in environment. In addition, water is a desirable greener solvent for treatment of organic pollutants than the organic ones, because of its obvious advantage. In this study, we report a method of photocatalytic debromination of decabromodiphenyl ether in aqueous system. Preloading BDE209 on the surface of TiO2 before it is introduced into the water solution, which makes the hydrophobic BDE209 to be photocatalytically degradation in water. The degradation intermediates and influencing factors on the degradation rate have been further studied and compared with pre-solvated organic system. This study may provide helpful information for the photocatalytic removal and remediation of not only PBDEs but also other highly hydrophobic pollutants such as polycyclic aromatic hydrocarbons in water media.
2. Materials and methods 2.1. Materials Decabromophenyl ether (BDE209) was purchased from Aldrich Chemical Company (Milwaukee, Wisconsin, USA). It was a solid white powder with 98% purity and used without further purification. BDE206, BDE207 and BDE208 were purchased from Cambridge Isotope Laboratories (CIL, Andover, MA). Titanium dioxide particle (TiO2) (P25, ca. 80% anatase, 20% rutile; Brunauer– Emmett–Teller specific surface area, ca. 50 m2 g1) was kindly supplied by the Degussa Company (Clausthal-Zellerfeld, Germany). Na2SO4, Tetrahydrofuran (THF), methanol and toluene were guaranteed reagent (Chemical Co., Beijing). HCl or NaOH were analytical reagents (Chemical Co., Beijing). They were used without further purification. Deionized and doubly distilled water was used in this study.
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coverage (mol g1) was calculated. By the same method, BDE209/ SiO2 and BDE209/Al2O3 were also prepared. BDE209/TiO2 was added to aqueous solution in a 15 mL Pyrex glass vessel. The pH was adjusted with HCl or NaOH if required. Prior to irradiation, the suspension was magnetically stirred in the dark for 10 min to disperse. For the experiment under the anoxic conditions, the Pyrex vessel was sealed and purged with argon gas for 30 min to remove O2 and protected under argon atmosphere during the irradiation. A PLS-SXE300 Xe lamp (Beijing Trusttech Co. Ltd.) was used as the light source. To eliminate the direct photolysis of BDE209 (Fig. S1), a cutoff filter was employed to remove completely any radiation at wavelengths below 360 nm. At given time intervals, the whole suspensions were centrifuged, and the solution was extracted by toluene (1 mL 3). The deposit was washed by toluene (1 mL 3). The extract and eluent were combined. After they were dried by Na2SO4, the final volume was brought to 4.0 mL for HPLC analysis. The extraction efficiency was about 92%. BDE209 in samples was quantified with a SHIMADZU HPLC system (LC-20AT pump and UV/VIS SPD-20A detector) with a DIKMA Platisil ODS C-18 column (250 4.6 mm, 5 lm film thickness). The mobile phase was 2 vol.% water in acetonitrile at 1 mL min1 (Hua et al., 2003) and the detector wavelength was set at 240 nm. The quantification was done with a calibration cure with a BDE-209 standard (Fig. S2). 3. Results and discussion 3.1. Degradation kinetics As seen from Fig. 1, when BDE209 was preloaded on the surface of TiO2 (BDE209/TiO2), the degradation of BDE209 occurred in aqueous system in presence of small amount methanol, and almost 30% of BDE209 disappeared after 4 h of irradiation (curve a). In control experiment, if TiO2 was replaced by SiO2 or Al2O3 with addition of same amount of CH3OH, no degradation of BDE209 was observed under otherwise identical conditions (curve c, d), suggesting that the degradation was a photocatalytic reaction, rather than a surface-mediated photolysis or direct photolysis. No degradation of BDE209 was observed in the BDE209/TiO2 system, when water was used as the sole solvent (curve b). As discussed in the previous paper (Sun et al., 2009), the main reductant for the photocatalytic reductive of PBDEs in TiO2
2.2. Methods Preparation of preloaded BDE209 on TiO2 (BDE209/TiO2) was carried out by adding the TiO2 particles into the THF solution of BDE209 and then the dispersion was magnetically stirred at room temperature until THF was volatilized completely. During this process, BDE209 was adsorbed on the surface of TiO2 particles. According to the weight of BDE209 and TiO2 added, the surface
Fig. 1. Temporal course of photodegradation of BDE209 in aqueous system. Reaction conditions: 10 mg BDE209/TiO2, BDE209/Al2O3, or BDE209/SiO2 (5.5 106 mol g1), wavelength >360 nm, anaerobic condition. (a) BDE209/TiO2, 0.5 mL CH3OH, 9.5 mL H2O; (b) BDE209/TiO2, 10 mL H2O; (c) BDE209/SiO2, 0.5 mL CH3OH, 9.5 mL H2O; (d) BDE209/Al2O3, 0.5 mL CH3OH, 9.5 mL H2O.
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Scheme 1. (a) The proposed photocatalytic pathway for degradation of PBDEs by TiO2; (b) preloaded hydrophobic BDE209 on the surface of TiO2 in the aqueous suspension with methanol as hole scavenger.
dispersions should be the conduction electron, therefore, the accumulation of electron in conduction band of TiO2 and the interaction between the substrate and BDE209 should play essential roles in the reaction (Scheme 1a). First of all, the accumulation of conduction band electron of TiO2 demands the presence of electron donor to scavenge the VB hole for preventing the charge recombination. Evidently, this is the reason that the methanol has to be added. However, even the electron is accumulated in the conduction band, the reductive reaction cannot always occur smoothly. In the photocatalytic reduction of PBDEs in the organic solution, the PBDEs are solvated in the bulk organic phase. The presence of the water layer on the surface of TiO2 (Pichat, 2010) would hinder the interaction between PBDEs and the surface, and hence block the electron transformation channel between them. Under the present experimental conditions, the surface of TiO2 was first physically covered with the hydrophobic BDE209, which prevent the surface from interaction with water even in the aqueous system. It means the hydrophobicity of BDE209 avoids the formation of depressive water barrier. On the other hand, the organic methanol molecule is able to go through the BDE209 layer to the surface of TiO2 for reaction with the photogenerated hole, because methanol molecule partially solubility with BDE209. In other word, the preloading of the hydrophobic substrate creates an organic-like micro-environment (Enriquez and Pichat, 2001) in surface layer of the catalyst in the water suspension (Scheme 1b), which makes the reaction just as in the organic solution. We believe that such concept should be applicable for the photocatalytic reduction or oxidation of other hydrophobic substrate in water.
electron can be changed greatly (Michael et al., 1983; Sun et al., 2009). As a result, the pH values usually have some influence on photocatalytic reactions. However, in the aqueous system, the hydrophobicity of preloaded BDE209 prevents the H+ or OH from approaching the surface of TiO2, that is, by avoiding the formation of depressive water barrier, the BDE209 cover weakens the effect of pH on the photocatalytic debromination reaction.
3.2. Effect of pH value
3.3. Effect of concentration of CH3OH and surface coverage of BDE209
The pH values of the reaction system usually have great effect on the photocatalytic degradation rates of organic pollutants in TiO2 systems (Zhao et al., 1998). In our previous studies, for example, we found that the photocatalytic degradation of BDE209 by TiO2 in organic solution is enhanced in the presence of bases, but greatly depressed by addition of acids. To investigate the effect of pH values on the photocatalytic debromination of PBDEs in preloaded aqueous system, the reactions were further carried out in different pH values. However, the experiments with pH values over 2–12 showed that the effect of pH on the debromination rates was insignificant (Fig. 2). In systems where the substrates were solvable in the bulk solution, the acid–base properties of the solution might influence the adsorption and desorption of the reactant and intermediates on the surface of TiO2 particles (Zhao et al., 1998). More importantly, upon the adsorption of H+ or OH on the surface of TiO2, the reductive ability of the conduction band
Methanol played important role in the reaction. It not only acted as the VB hole scavenger but also as hydrogen atom donor in the process of debromination. Accordingly, it was found that the rate of debromination was accelerated with the ratio of CH3OH increasing (Fig. 3). It is possible that the presence of too much methanol might solvate partially the BDE209, and destroys the BDE209 layer. Our experiments indicated that, up to the methanol concentration of 80%, the water cannot reach the surface to block the interaction of substrate with TiO2. The effect of surface coverage on the photocatalytic debromination of BDE209 in aqueous system was investigated by loading different amount of BDE209. As shown in Fig. 4, the relative photodebromination conversion rate decreased with the surface coverage in the tested range from 5.5 106 to 2.1 105 mol g1 (Fig. 4a). However, the absolute conversion amount is not changed very much with the increase of surface coverage (Fig. 4b). It
Fig. 2. Effect of pH on the photocatalytic debromination of BDE209 in aqueous system. Reaction conditions: 10 mg BDE209/TiO2 (5.5 106 mol g1); 0.5 mL menthol, 9.5 mL H2O; wavelength >360 nm; anaerobic condition.
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para-debrominated intermediates BDE208 are the most minor nona-BDEs, probably because the number of the para-Br is only one half of the ortho-Br or meta-Br in BDE209 molecule. However, the two other mono-debrominated intermediates BDE206 and BDE207 exhibited completely opposite relative yields in the two systems. The predominant nona-BDE in aqueous system is BDE207, while in organic solution BDE206 is the major monodebrominated intermediate (Sun et al., 2009) (Fig. 5b), indicating the different debromination mechanism in these two photocatalytic systems. For the reductive cleavage of C–X bond, two reaction pathways, stepwise and concerted mechanism (Eqs. 1 and 2), have been proposed (Isse et al., 2001). In the stepwise mechanism, the radical anion, RX , was first formed, which further cleaves to R and X Eq. (1). In the concerted mechanism, electron transfer and C–X bond cleavage occur in a coherent one-step manner Eq. (2) Fig. 3. The effect of different ratio CH3OH on the photocatalytic debromination of BDE209 in aqueous system. Reaction conditions: 10 mg BDE209/TiO2 (5.5 106 mol g1); wavelength >360 nm; anaerobic condition.
indicates that the loading amount of BDE209 (5.5 106 mol g1) is enough to form a complete overlayer on the surface, so the absolute rate of photocatalytic debromination is concentration -independent. The increase in the amount of BDE209/TiO2 has little effect on the photodegradation rates in the range of 5–15 mg (Fig. S2), However, the degradation rate was significantly decreased when 30 mg of BDE209/TiO2 was used, probably because of the increase in the concentration of BDE209. 3.4. Debromination products The reaction intermediates were analyzed by HPLC technique and identified by the chemical standards. It was observed that BDE209 was transformed to its lower bromo congeners in a stepwise way in aqueous system (Fig. 5a). Before the photoreaction, only the chromatogramic peak of BDE209 was detected. After 2 h of irradiation, BDE209 decreased, and two new peaks appeared. By the chemical standards, they are ascribed to nona-BDEs, specifically, ortho- and meta-debrominated intermediates of BDE209, respectively. After irradiation of 4 h, Octa-BDEs were formation. In the reported debromination systems for BDE209, the relative yield of isomer of the mono-debrominated intermediates changed drastically. (Bezares-ruz et al., 2004; Gerecke et al., 2005; Stapleton et al., 2006; Li et al., 2007). It suggests that the debromination mechanism of BDE209 is condition-dependent. It was found that, in both the organic and the present aqueous systems, the
RAX þ e ! RAX ! R þ X
ð1Þ
RAX þ e ! ½R X e ! R þ X
ð2Þ
The calculated bond dissociation energies (BDEs) of C–Br in the BDE209 anion show that ortho C–Br (190.4 kJ mol1) has a BDE of 6 kJ mol1 lower than those of meta (196.3 kJ mol1) and para C– Br (196.4 kJ mol1) (Sun et al., 2009), indicating that ortho C–Br bond is the weakest. The relative yield of the mono-debrominated intermediates during photocatalytic debromination of BDE209 in organic solution is consistent with the BDE calculation, suggesting that the debromination might involve the BDE209 anion, that is, the cleavage of the C–Br occur according to a stepwise mechanism in the bulk solution. However, in aqueous system, BDE209 has been restricted on the surface of TiO2 due to its unsolvability in water. It can present significant effect on the reductive debromination process (Maurino et al., 2005), which may probably suppress the ortho-C–X cleavage. Because of the steric effect, it is more difficult for the ortho-Br than for the meta-Br to interact with the surface sites of TiO2 Eq. (3). Upon the formation of conduction band electron, the surface Ti can trap it Eq. (4). Based on the difference in adsorption, two possible interpretations for the enhancement of cleavage of the meta-C–Br bond can be tentatively made. Stronger interaction may decrease BDE of the meta-C–Br to a value lower than the ortho one, which can make the splitting of the meta-C– Br easier that the ortho one. Alternatively, the interaction might change the breaking of C–Br from the stepwise mechanism to concerted one. This is likely because the trapped electron (TiIII<) can polarize C–Br bond, and the surface Ti site of TiO2 is strong Lewis acid to accept the leaving bromine.
Fig. 4. Change on the photocatalytic debromination rate of BDE209 along with loading amount of BDE209 in aqueous system. (a) Relative photodebromination conversion rate; (b) absolute conversion amount. Reaction conditions: 10 mg BDE209/TiO2; 0.5 mL menthol, 9.5 mL H2O; wavelength >360 nm; anaerobic condition.
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Fig. 5. (a) HPLC chromatograms of debromination products of BDE209 in aqueous system at different irradiation time. Reaction conditions: 10 mg BDE209/TiO2 (5.5 106 mol g1); 0.5 mL menthol, 9.5 mL H2O; wavelength >360 nm; anaerobic condition. (b) The different mono-debromination intermediates during the photocatalytic debromination of BDE209 in organic and aqueous system.
h i VI VI m RABr þ Ti
h
VI
i
III
i
m RABr Ti < þ
m RABr Ti <
ecb
h
ð3Þ
III
! m RABr Ti <
i
ð4Þ
h i III ! m R Br Ti < VI
! m R þ Br Ti <
Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.chemosphere. 2012.05.076. References
ð5Þ
4. Conclusions Photocatalytic reductive debromination of preloaded decabromodiphenyl ether in aqueous system has been investigated. The formation of hydrophobic BDE209 layer on TiO2 surface creates an organic-like micro-environment (Enriquez and Pichat, 2001) in surface layer of the catalyst in the water suspension, which prevents the surface from interaction with water and H+/OH in water, and makes the reaction just as in the organic solution. The organic methanol molecule is able to go through the BDE209 layer to the surface for reaction with the photo-generated hole. However, the H+/OH in water cannot penetrate into the surface, so the pH of solution has little effect on the debromination rate. We also observed that the predominant nona-BDE in aqueous system is BDE207, while in organic solution BDE206 is the major monodebrominated intermediate. Such a difference is interpreted by the steric effect and restriction effect of the surface BDE209 layer. The central concept of the study is that the preloaded model makes photocatalytic reaction of the unsolvable substrate possible to occur in aqueous systems. The abstraction by organic solvent and reloading process can be used, and the solvents can be recycled in the preadsorption step to achieve in a cost effective manner in a practical treatment process. It should be applicable for the photocatalytic reduction or oxidation of other hydrophobic substrate in water.
Acknowledgements The authors appreciate the generous financial support of this work by the National Science Foundation of China (Nos. 21107073, 20920102034 and 21137004) and the Natural Science Foundation of Zhejiang province (No. Y5110347).
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Chemosphere journal homepage: www.elsevier.com/locate/chemosphere
Photocatalytic debromination of preloaded decabromodiphenyl ether on the TiO2 surface in aqueous system Chunyan Sun a,b, Jincai Zhao b, Hongwei Ji b, Wanhong Ma b, Chuncheng Chen b,⇑ a b
Department of Chemistry, Shaoxing University, Zhejiang Shaoxing 312000, China Beijing National Laboratory for Molecular Sciences, Key Laboratory of Photochemistry, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China
h i g h l i g h t s " A preloaded method was investigated to photocatalytic debromination of PBDEs in water. " The organic-like micro-environment on the surface of TiO2 prevented the effect of water. " It provide a potential remediation technology in water not only for PBDEs but also for other hydrophobic contaminants.
a r t i c l e
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Article history: Received 3 February 2012 Received in revised form 10 April 2012 Accepted 19 May 2012 Available online 12 June 2012 Keywords: Decabromodiphenyl ether TiO2 Preload Aqueous system Photocatalytic degradation
a b s t r a c t There have been serious concerns about polybromodiphenyl ethers (PBDEs) in the environment because of their global distribution and bioaccumulation. Owing to strong hydrophobicity of PBDEs, the regular photocatalytic system, in which the substrate is solvated in the bulk solution, is not applicable to the removal of the PBDEs in water. In this work, the photocatalytic reduction degradation of decabromodiphenyl ether (BDE209), the most-used PBDEs, was investigated in aqueous system, by pre-adsorbing it on the surface of photocatalyst. It was found that the preloaded BDE209 underwent efficient reductive debromination in aqueous system under irradiation with wavelength larger than 360 nm in the presence of electron donors such as methanol. Our experiments further show that such a preloaded system exhibits different characteristics from that in the organic solution. The meta-debrominated intermediate is predominant in the present system, while the ortho-debrominated one is the main nona-BDE products in the organic solution. In addition, different from other photocatalytic system, the pH has little effect on the photocatalytic reaction. We propose that these differences are originated from the formation of overlayer of hydrophobic BDE209 to limit the motion of BDE209 and the access of water and H+/OH to the TiO2 surface. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction Recently, polybrominated diphenyl ethers (PBDEs) arouse high environmental concern due to their global distribution and bioaccumulation (Wolkers et al., 2004; Mai et al., 2005; WyrzykowskaCeradini et al., 2011). PBDEs as brominated flame retardants have been widely used in the consumer electronics, furniture, and building materials (Alaee et al., 2003; Du et al., 2010). Owning to their increasing usage, and their high hydrophobicity, persistence and bioaccumulation, PBDEs have been detected in sediments, marine organisms, food samples, and human mother’s milk, even in the animals of Arctic regions (de Wit, 2002; Routti et al., 2009). Exposure to PBDEs, the balance of the thyroid of human is able to be ⇑ Corresponding author. Present address: Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China. Tel./fax: +86 10 8261 6495. E-mail address: [email protected] (C. Chen). 0045-6535/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.chemosphere.2012.05.076
affected, which will cause neurotoxicity (Kierkegaard et al., 1999; Meerts et al., 2001; Behnisch et al., 2003). Deca-BDE has been classified as a possible human carcinogen by the US Environmental Protection Agency (EPA) (Agency for Toxic Substances, 2004). Research on PBDEs has mainly focused on the distribution and transformation of PBDEs in the environment. PBDEs can be photolytically debrominated on the surface of clay minerals, metal oxides, silica gel, sand, soil and sediment (Ahn et al., 2006; Li et al., 2010), and in solution of toluene and hexane (Bezares-ruz et al., 2004). The biotic reductive debromination has also been reported by anaerobic bacteria (He et al., 2006), Juvenile rainbow trout and common carp (Stapleton et al., 2006). In contrast to the abundant studies on the environmental action of PBDEs, the development of potential methods for PBDEs removal in contaminated environmental system remains relatively undeveloped. Hitherto, only a few studies have been reported to the degradation of PBDEs. Gerecke reported that anaerobic bacteria could effective degrade
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BDE209 (Gerecke et al., 2005). Keum studied the reductive debromination of BDE209 by zerovalent iron powder (Keum and Li, 2005). Within 40 days, 92% of BDE209 has been converted into lower bromo congeners. Jiang’s group (Li et al., 2007) reported that resin-bound zerovalent iron nanoparticles exhibited a more rapid debromination activity. BDE209 could be converted into lower bromo congeners within 8 h. Photocatalysis is an effective method for degradation of environmental contaminants (Agrios and Pichat, 2005; Robertson et al., 2005; Fujishima et al., 2008; Gupta et al., 2012). Photocatalytic degradation of PBDEs is less favorable under conventional oxidation conditions, due to the recalcitrancy of PBDEs to oxidation. In our previous studies (Sun et al., 2009), it was found that PBDEs could undergo rapid debromination by TiO2-mediated photocatalytic reduction method in organic solutions such as CH3CN and CH3OH. It was also observed that presence of H2O markedly suppressed the degradation of BDE209 by TiO2 in organic solution. The reason is BDE209 owns strong hydrophobic, but the surface of TiO2 is quite hydrophilic. The surface of TiO2 would be preferentially adsorbed by water, which would block the interaction between the TiO2 and BDE209, and further inhibit the electron transformation from conduction band to BDE209 molecular. That places some impediments on the treatment of BDE209, since water is ubiquitous in environment. In addition, water is a desirable greener solvent for treatment of organic pollutants than the organic ones, because of its obvious advantage. In this study, we report a method of photocatalytic debromination of decabromodiphenyl ether in aqueous system. Preloading BDE209 on the surface of TiO2 before it is introduced into the water solution, which makes the hydrophobic BDE209 to be photocatalytically degradation in water. The degradation intermediates and influencing factors on the degradation rate have been further studied and compared with pre-solvated organic system. This study may provide helpful information for the photocatalytic removal and remediation of not only PBDEs but also other highly hydrophobic pollutants such as polycyclic aromatic hydrocarbons in water media.
2. Materials and methods 2.1. Materials Decabromophenyl ether (BDE209) was purchased from Aldrich Chemical Company (Milwaukee, Wisconsin, USA). It was a solid white powder with 98% purity and used without further purification. BDE206, BDE207 and BDE208 were purchased from Cambridge Isotope Laboratories (CIL, Andover, MA). Titanium dioxide particle (TiO2) (P25, ca. 80% anatase, 20% rutile; Brunauer– Emmett–Teller specific surface area, ca. 50 m2 g1) was kindly supplied by the Degussa Company (Clausthal-Zellerfeld, Germany). Na2SO4, Tetrahydrofuran (THF), methanol and toluene were guaranteed reagent (Chemical Co., Beijing). HCl or NaOH were analytical reagents (Chemical Co., Beijing). They were used without further purification. Deionized and doubly distilled water was used in this study.
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coverage (mol g1) was calculated. By the same method, BDE209/ SiO2 and BDE209/Al2O3 were also prepared. BDE209/TiO2 was added to aqueous solution in a 15 mL Pyrex glass vessel. The pH was adjusted with HCl or NaOH if required. Prior to irradiation, the suspension was magnetically stirred in the dark for 10 min to disperse. For the experiment under the anoxic conditions, the Pyrex vessel was sealed and purged with argon gas for 30 min to remove O2 and protected under argon atmosphere during the irradiation. A PLS-SXE300 Xe lamp (Beijing Trusttech Co. Ltd.) was used as the light source. To eliminate the direct photolysis of BDE209 (Fig. S1), a cutoff filter was employed to remove completely any radiation at wavelengths below 360 nm. At given time intervals, the whole suspensions were centrifuged, and the solution was extracted by toluene (1 mL 3). The deposit was washed by toluene (1 mL 3). The extract and eluent were combined. After they were dried by Na2SO4, the final volume was brought to 4.0 mL for HPLC analysis. The extraction efficiency was about 92%. BDE209 in samples was quantified with a SHIMADZU HPLC system (LC-20AT pump and UV/VIS SPD-20A detector) with a DIKMA Platisil ODS C-18 column (250 4.6 mm, 5 lm film thickness). The mobile phase was 2 vol.% water in acetonitrile at 1 mL min1 (Hua et al., 2003) and the detector wavelength was set at 240 nm. The quantification was done with a calibration cure with a BDE-209 standard (Fig. S2). 3. Results and discussion 3.1. Degradation kinetics As seen from Fig. 1, when BDE209 was preloaded on the surface of TiO2 (BDE209/TiO2), the degradation of BDE209 occurred in aqueous system in presence of small amount methanol, and almost 30% of BDE209 disappeared after 4 h of irradiation (curve a). In control experiment, if TiO2 was replaced by SiO2 or Al2O3 with addition of same amount of CH3OH, no degradation of BDE209 was observed under otherwise identical conditions (curve c, d), suggesting that the degradation was a photocatalytic reaction, rather than a surface-mediated photolysis or direct photolysis. No degradation of BDE209 was observed in the BDE209/TiO2 system, when water was used as the sole solvent (curve b). As discussed in the previous paper (Sun et al., 2009), the main reductant for the photocatalytic reductive of PBDEs in TiO2
2.2. Methods Preparation of preloaded BDE209 on TiO2 (BDE209/TiO2) was carried out by adding the TiO2 particles into the THF solution of BDE209 and then the dispersion was magnetically stirred at room temperature until THF was volatilized completely. During this process, BDE209 was adsorbed on the surface of TiO2 particles. According to the weight of BDE209 and TiO2 added, the surface
Fig. 1. Temporal course of photodegradation of BDE209 in aqueous system. Reaction conditions: 10 mg BDE209/TiO2, BDE209/Al2O3, or BDE209/SiO2 (5.5 106 mol g1), wavelength >360 nm, anaerobic condition. (a) BDE209/TiO2, 0.5 mL CH3OH, 9.5 mL H2O; (b) BDE209/TiO2, 10 mL H2O; (c) BDE209/SiO2, 0.5 mL CH3OH, 9.5 mL H2O; (d) BDE209/Al2O3, 0.5 mL CH3OH, 9.5 mL H2O.
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Scheme 1. (a) The proposed photocatalytic pathway for degradation of PBDEs by TiO2; (b) preloaded hydrophobic BDE209 on the surface of TiO2 in the aqueous suspension with methanol as hole scavenger.
dispersions should be the conduction electron, therefore, the accumulation of electron in conduction band of TiO2 and the interaction between the substrate and BDE209 should play essential roles in the reaction (Scheme 1a). First of all, the accumulation of conduction band electron of TiO2 demands the presence of electron donor to scavenge the VB hole for preventing the charge recombination. Evidently, this is the reason that the methanol has to be added. However, even the electron is accumulated in the conduction band, the reductive reaction cannot always occur smoothly. In the photocatalytic reduction of PBDEs in the organic solution, the PBDEs are solvated in the bulk organic phase. The presence of the water layer on the surface of TiO2 (Pichat, 2010) would hinder the interaction between PBDEs and the surface, and hence block the electron transformation channel between them. Under the present experimental conditions, the surface of TiO2 was first physically covered with the hydrophobic BDE209, which prevent the surface from interaction with water even in the aqueous system. It means the hydrophobicity of BDE209 avoids the formation of depressive water barrier. On the other hand, the organic methanol molecule is able to go through the BDE209 layer to the surface of TiO2 for reaction with the photogenerated hole, because methanol molecule partially solubility with BDE209. In other word, the preloading of the hydrophobic substrate creates an organic-like micro-environment (Enriquez and Pichat, 2001) in surface layer of the catalyst in the water suspension (Scheme 1b), which makes the reaction just as in the organic solution. We believe that such concept should be applicable for the photocatalytic reduction or oxidation of other hydrophobic substrate in water.
electron can be changed greatly (Michael et al., 1983; Sun et al., 2009). As a result, the pH values usually have some influence on photocatalytic reactions. However, in the aqueous system, the hydrophobicity of preloaded BDE209 prevents the H+ or OH from approaching the surface of TiO2, that is, by avoiding the formation of depressive water barrier, the BDE209 cover weakens the effect of pH on the photocatalytic debromination reaction.
3.2. Effect of pH value
3.3. Effect of concentration of CH3OH and surface coverage of BDE209
The pH values of the reaction system usually have great effect on the photocatalytic degradation rates of organic pollutants in TiO2 systems (Zhao et al., 1998). In our previous studies, for example, we found that the photocatalytic degradation of BDE209 by TiO2 in organic solution is enhanced in the presence of bases, but greatly depressed by addition of acids. To investigate the effect of pH values on the photocatalytic debromination of PBDEs in preloaded aqueous system, the reactions were further carried out in different pH values. However, the experiments with pH values over 2–12 showed that the effect of pH on the debromination rates was insignificant (Fig. 2). In systems where the substrates were solvable in the bulk solution, the acid–base properties of the solution might influence the adsorption and desorption of the reactant and intermediates on the surface of TiO2 particles (Zhao et al., 1998). More importantly, upon the adsorption of H+ or OH on the surface of TiO2, the reductive ability of the conduction band
Methanol played important role in the reaction. It not only acted as the VB hole scavenger but also as hydrogen atom donor in the process of debromination. Accordingly, it was found that the rate of debromination was accelerated with the ratio of CH3OH increasing (Fig. 3). It is possible that the presence of too much methanol might solvate partially the BDE209, and destroys the BDE209 layer. Our experiments indicated that, up to the methanol concentration of 80%, the water cannot reach the surface to block the interaction of substrate with TiO2. The effect of surface coverage on the photocatalytic debromination of BDE209 in aqueous system was investigated by loading different amount of BDE209. As shown in Fig. 4, the relative photodebromination conversion rate decreased with the surface coverage in the tested range from 5.5 106 to 2.1 105 mol g1 (Fig. 4a). However, the absolute conversion amount is not changed very much with the increase of surface coverage (Fig. 4b). It
Fig. 2. Effect of pH on the photocatalytic debromination of BDE209 in aqueous system. Reaction conditions: 10 mg BDE209/TiO2 (5.5 106 mol g1); 0.5 mL menthol, 9.5 mL H2O; wavelength >360 nm; anaerobic condition.
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para-debrominated intermediates BDE208 are the most minor nona-BDEs, probably because the number of the para-Br is only one half of the ortho-Br or meta-Br in BDE209 molecule. However, the two other mono-debrominated intermediates BDE206 and BDE207 exhibited completely opposite relative yields in the two systems. The predominant nona-BDE in aqueous system is BDE207, while in organic solution BDE206 is the major monodebrominated intermediate (Sun et al., 2009) (Fig. 5b), indicating the different debromination mechanism in these two photocatalytic systems. For the reductive cleavage of C–X bond, two reaction pathways, stepwise and concerted mechanism (Eqs. 1 and 2), have been proposed (Isse et al., 2001). In the stepwise mechanism, the radical anion, RX , was first formed, which further cleaves to R and X Eq. (1). In the concerted mechanism, electron transfer and C–X bond cleavage occur in a coherent one-step manner Eq. (2) Fig. 3. The effect of different ratio CH3OH on the photocatalytic debromination of BDE209 in aqueous system. Reaction conditions: 10 mg BDE209/TiO2 (5.5 106 mol g1); wavelength >360 nm; anaerobic condition.
indicates that the loading amount of BDE209 (5.5 106 mol g1) is enough to form a complete overlayer on the surface, so the absolute rate of photocatalytic debromination is concentration -independent. The increase in the amount of BDE209/TiO2 has little effect on the photodegradation rates in the range of 5–15 mg (Fig. S2), However, the degradation rate was significantly decreased when 30 mg of BDE209/TiO2 was used, probably because of the increase in the concentration of BDE209. 3.4. Debromination products The reaction intermediates were analyzed by HPLC technique and identified by the chemical standards. It was observed that BDE209 was transformed to its lower bromo congeners in a stepwise way in aqueous system (Fig. 5a). Before the photoreaction, only the chromatogramic peak of BDE209 was detected. After 2 h of irradiation, BDE209 decreased, and two new peaks appeared. By the chemical standards, they are ascribed to nona-BDEs, specifically, ortho- and meta-debrominated intermediates of BDE209, respectively. After irradiation of 4 h, Octa-BDEs were formation. In the reported debromination systems for BDE209, the relative yield of isomer of the mono-debrominated intermediates changed drastically. (Bezares-ruz et al., 2004; Gerecke et al., 2005; Stapleton et al., 2006; Li et al., 2007). It suggests that the debromination mechanism of BDE209 is condition-dependent. It was found that, in both the organic and the present aqueous systems, the
RAX þ e ! RAX ! R þ X
ð1Þ
RAX þ e ! ½R X e ! R þ X
ð2Þ
The calculated bond dissociation energies (BDEs) of C–Br in the BDE209 anion show that ortho C–Br (190.4 kJ mol1) has a BDE of 6 kJ mol1 lower than those of meta (196.3 kJ mol1) and para C– Br (196.4 kJ mol1) (Sun et al., 2009), indicating that ortho C–Br bond is the weakest. The relative yield of the mono-debrominated intermediates during photocatalytic debromination of BDE209 in organic solution is consistent with the BDE calculation, suggesting that the debromination might involve the BDE209 anion, that is, the cleavage of the C–Br occur according to a stepwise mechanism in the bulk solution. However, in aqueous system, BDE209 has been restricted on the surface of TiO2 due to its unsolvability in water. It can present significant effect on the reductive debromination process (Maurino et al., 2005), which may probably suppress the ortho-C–X cleavage. Because of the steric effect, it is more difficult for the ortho-Br than for the meta-Br to interact with the surface sites of TiO2 Eq. (3). Upon the formation of conduction band electron, the surface Ti can trap it Eq. (4). Based on the difference in adsorption, two possible interpretations for the enhancement of cleavage of the meta-C–Br bond can be tentatively made. Stronger interaction may decrease BDE of the meta-C–Br to a value lower than the ortho one, which can make the splitting of the meta-C– Br easier that the ortho one. Alternatively, the interaction might change the breaking of C–Br from the stepwise mechanism to concerted one. This is likely because the trapped electron (TiIII<) can polarize C–Br bond, and the surface Ti site of TiO2 is strong Lewis acid to accept the leaving bromine.
Fig. 4. Change on the photocatalytic debromination rate of BDE209 along with loading amount of BDE209 in aqueous system. (a) Relative photodebromination conversion rate; (b) absolute conversion amount. Reaction conditions: 10 mg BDE209/TiO2; 0.5 mL menthol, 9.5 mL H2O; wavelength >360 nm; anaerobic condition.
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Fig. 5. (a) HPLC chromatograms of debromination products of BDE209 in aqueous system at different irradiation time. Reaction conditions: 10 mg BDE209/TiO2 (5.5 106 mol g1); 0.5 mL menthol, 9.5 mL H2O; wavelength >360 nm; anaerobic condition. (b) The different mono-debromination intermediates during the photocatalytic debromination of BDE209 in organic and aqueous system.
h i VI VI m RABr þ Ti
h
VI
i
III
i
m RABr Ti < þ
m RABr Ti <
ecb
h
ð3Þ
III
! m RABr Ti <
i
ð4Þ
h i III ! m R Br Ti < VI
! m R þ Br Ti <
Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.chemosphere. 2012.05.076. References
ð5Þ
4. Conclusions Photocatalytic reductive debromination of preloaded decabromodiphenyl ether in aqueous system has been investigated. The formation of hydrophobic BDE209 layer on TiO2 surface creates an organic-like micro-environment (Enriquez and Pichat, 2001) in surface layer of the catalyst in the water suspension, which prevents the surface from interaction with water and H+/OH in water, and makes the reaction just as in the organic solution. The organic methanol molecule is able to go through the BDE209 layer to the surface for reaction with the photo-generated hole. However, the H+/OH in water cannot penetrate into the surface, so the pH of solution has little effect on the debromination rate. We also observed that the predominant nona-BDE in aqueous system is BDE207, while in organic solution BDE206 is the major monodebrominated intermediate. Such a difference is interpreted by the steric effect and restriction effect of the surface BDE209 layer. The central concept of the study is that the preloaded model makes photocatalytic reaction of the unsolvable substrate possible to occur in aqueous systems. The abstraction by organic solvent and reloading process can be used, and the solvents can be recycled in the preadsorption step to achieve in a cost effective manner in a practical treatment process. It should be applicable for the photocatalytic reduction or oxidation of other hydrophobic substrate in water.
Acknowledgements The authors appreciate the generous financial support of this work by the National Science Foundation of China (Nos. 21107073, 20920102034 and 21137004) and the Natural Science Foundation of Zhejiang province (No. Y5110347).
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