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Catalytic and electrocatalytic hydrogenolysis of brominated diphenyl ethers
Chemosphere 58 (2005) 961–967 www.elsevier.com/locate/chemosphere
Catalytic and electrocatalytic hydrogenolysis of brominated diphenyl ethers Pascale M.L. Bonin a, Patrick Edwards a, Dorin Bejan a, Chun Chi Lo a, Nigel J. Bunce a,*, Alexandre D. Konstantinov b a b
Department of Chemistry, University of Guelph, Guelph, Ont., Canada N1G 2W1 Wellington Laboratories, 345 Southgate Drive, Guelph, Ont., Canada N1G 3M5
Received 18 December 2003; received in revised form 8 September 2004; accepted 29 September 2004
Abstract Polybrominated diphenyl ethers (PBDEs) are ubiquitous environmental contaminants due to their use as additive flame-retardants. Conventional catalytic hydrogenolysis in methanol solution and electrocatalytic hydrogenolysis in aqueous methanol were examined as methods for debrominating mono- and di-bromodiphenyl ethers, as well as a commercial penta-PBDE mixture, in each case using palladium on alumina as the catalyst. Electrocatalytic hydrogenolysis employed a divided flow-through batch cell, with reticulated vitreous carbon cathodes and IrO2/Ti dimensionally stable anodes. Both methods gave efficient sequential debromination, with essentially complete removal of bromine from the PBDEs, but the electrocatalytic method was limited by the poor solubility of PBDEs in aqueous methanol. 2004 Published by Elsevier Ltd. Keywords: Hydrogenolysis; Electrochemical debromination; Flame-retardants; Polybrominated diphenyl ethers
1. Introduction Polybrominated diphenyl ethers (PBDEs) are part of the family of brominated flame-retardants, which are widely used in textiles, paints, plastics, and electronic equipment (IPCS, 1994). Commercial PBDEs, which are prepared by bromination of diphenyl ether, are mixtures of congeners; the commercial mixtures presently in use are nominally penta-, octa-, and deca-BDEs
* Corresponding author. Tel.: +1 519 824 4120x53962; fax: +1 519 766 1499. E-mail address: [email protected] (N.J. Bunce).
0045-6535/$ - see front matter 2004 Published by Elsevier Ltd. doi:10.1016/j.chemosphere.2004.09.099
(Wenning, 2002; Darnerud, 2003). The penta-BDE mixture, for example, typically comprises 37% of 2,2 0 ,4, 4 0 -tetra-BDE (congener 47) and 35% of 2,2 0 ,4,4 0 ,5-penta-BDE (congener 99), which are the most prevalent congeners found in the environment (Lindstrom et al., 1999), along with smaller amounts of congeners 100, 153, 154, and 85 (cited in Chen and Bunce, 2003)—the numbering of PBDE congeners is analogous to that used for polychlorinated biphenyls (ATSDR, 2003). The consumption of brominated flame-retardants is high; in 1999, 67 000 metric tons of PBDE mixtures were produced worldwide (McDonald, 2002). PBDEs are persistent hydrophobic contaminants that bioaccumulate in organisms and biomagnify in the food chain (Palm et al., 2002). Their concentrations
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in sediments, fish, aquatic birds, marine mammals, and other wildlife (Wenning, 2002), and in human breast milk, e.g., in Sweden (Meironyte et al., 1999; Lind et al., 2003) and North America (Ryan and Patry, 2000; Schecter et al., 2003), have been increasing in parallel with their production trends. Although environmental concentrations of PBDEs have so far been much lower than those of organochlorines such as PCBs, a worrisome trend is that concentrations of PBDEs in fish from the Columbia River, British Columbia, were predicted to surpass those of PCBs in the year 2003 (Rayne et al., 2003). Concern exists over possible health effects of PBDEs, including endocrine disruption, neurodevelopment deficits and cancer (McDonald, 2002; Chen, 2003). As a result, the European Union has implemented a ban on the future use of penta-BDE (European Chemicals Bureau, 1999a,b, 2000). Risk assessment for octa- and deca-BDE commercial mixtures are ongoing and regulations concerning their future use are pending (ECB, 1999a,b). One North American manufacturer has already announced a voluntary phase-out of the pentaand octa- commercial PBDE mixtures (Tullo, 2003; Hogue, 2003). Hydrogenolysis offers a possible method to degrade PBDEs by replacing the bromine atoms with hydrogen atoms, leaving diphenyl ether. Although this compound is potentially bioaccumulable and toxic to fish, it is biodegradable (Schmidt et al., 1992), suggesting that hydrogenolysis might be useful as a front-end technology for dehalogenating the PBDE molecules prior to biodegradation (Rodgers et al., 1999). In this study we have examined conventional and electrocatalytic hydrogenolysis (ECH) as methods for the debromination of PBDEs. ECH (Tsyganok et al., 1998) involves the electrochemical reduction of a protic solvent to produce adsorbed ‘‘nascent’’ hydrogen atoms that react chemically with an adsorbed organic substrate (Chapuzet et al., 1998). The innate reactivity of nascent hydrogen atoms means that ECH generally proceeds under much milder conditions than conventional hydrogenation (Miller and Christensen, 1978). For example, glucose has been electrocatalytically hydrogenated to sorbitol at 60 C and atmospheric pressure, compared with 80–140 C and 20–140 atm for the conventional reaction (Pintauro and Bontha, 1991), and even 4-phenoxyphenol has been catalytically electroreduced to the corresponding cyclohexanol (Dabo et al., 1999). ECH has also been applied to the hydrogenolysis of aryl C– Cl bonds (Dabo et al., 2000; Stock and Bunce, 2002), which are stronger than the C–Br bonds present in PBDEs. A further advantage of ECH is that the cathodic polarization of the catalyst appears to offer protection against the adsorption of catalyst poisons, thereby extending the catalystÕs working lifetime (Laborde et al., 1990; Robin et al., 1990).
2. Experimental section 2.1. Chemicals Phenyl ether, 4-bromodiphenyl ether (4-MBDE, congener #3), 1-naphthol, and 5% palladium on alumina were obtained from Aldrich (Oakville, Ontario). 4,4 0 Dibromodiphenyl ether (4,4 0 -DBDE, congener #15) was obtained from Alfa Aesar (Wardhill, Massachussetts). Other PBDE congeners (Table 1) were available from a previous study (Chen et al., 2001), with the exception of 2-bromodiphenyl ether (2-MBDE, congener #1) and 3-bromodiphenyl ether (3-MBDE, congener #2) which were prepared by the method of Marsh et al. (1999). The identities of the prepared compounds, which were single spots by thin layer chromatography, were confirmed by 1H NMR at 400 MHz, CD2Cl2 as solvent; low resolution GC–MS indicated >99% purity, based on peak intensities; HPLC showed only a single peak. A commercial penta-BDE mixture (Great Lakes DE-71) was provided by Great Lakes Chemical Co. (West Lafayette, Indiana). Solvents used were HPLC grade methanol (Fisher Scientific, Toronto, Ontario), HPLC grade benzene (Aldrich, Oakville, Ontario) and water purified with a Millipore Milli-Q Reagent Water System. All other chemicals used were of reagent grade. 2.2. Analytical procedures The samples were analyzed by HPLC, using a Waters model 600 pump (flow rate 1.0 ml min1), a Rheodyne injector containing a 20 ll sample loop, and a Waters model 486 tunable absorbance detector operated at 220 nm (except for congeners #153 and #154, which were monitored at 210 nm). The detector output was processed with Waters Millennium version 3.2 software. Separation was achieved on a Waters Spherisorb ODS1 (5 lm · 4.6 mm · 25 cm) column. The mobile phase was 90:10 v/v methanol:water, which was degassed before use. Peaks were identified by coinjection of hydrogenolyzed mixtures and authentic congeners. Quantitation was achieved using standard curves prepared with 1naphthol as internal standard. 1-Naphthol was chosen for this purpose because it did not undergo hydrogenolysis under our experimental conditions, unlike those reported for other naphthalene derivatives by Jobin and Lessard (2003), and had HPLC retention time well removed from any reactants or products. Some samples were also analyzed by API-MS in the positive atmospheric pressure chemical ionization (APCI) mode, using a Waters single quadrupole ZQLC/MS 2000 system equipped with an APCI probe and a Z-spray source. The mobile phase was delivered by a Waters 600 gradient pump system with a Waters 600 controller (Waters, Milford, MA/Micromass, Manchester, UK). A Compaq Workstation with Masslynx
P.M.L. Bonin et al. / Chemosphere 58 (2005) 961–967
perature 400 C) and as cone gas (160 l min1). The source temperature was set to 120 C to prevent sample recondensation inside the ion source. Due to the high flow rate requirement of APCI-MS, methanol mixture was used as carrier phase and was pumped continuously into the probe at a rate of 0.3 ml min1. Sample was analyzed by the ‘‘flow infusion method’’ without prior chromatographic separation. The samples were dissolved in a 1:1 methanol:benzene (v/v) and were injected into the probe at a rate of 20 ll min1, using an onboard syringe pump and a Hamilton 500 ll syringe. Under APCI positive condition, the BDE congeners can be ionized by the benzene molecular ions through the ‘‘Charge exchange mechanism’’ (Waters Connection University, 2001):
Table 1 PBE congeners used in this study BDE No.
Structure
1 2 3
2-MBDE 3-MBDE 4-MBDE
15
4,4 0 -DBDE
17 28
2,2 0 ,4-Tri-BDE 2,4,4 0 -Tri-BDE
47 66
2,2 0 ,4,4 0 -Tetra-BDE 2,3 0 ,4,4 0 -Tetra-BDE
85 99 100
2,2 0 ,3,4,4 0 -Penta-BDE 2,2 0 ,4,4 0 ,5-Penta-BDE 2,2 0 ,4,4 0 ,6-Penta-BDE 0
0
þ C6 Hþ þ C6 H6 6 þM ! M
0
2,2 ,4,4 ,5,5 -Hexa-BDE 2,2 0 ,4,4 0 ,5,6 0 -Hexa-BDE
153 154
963
The scan speed was 1 s scan1 with 0.1 s interscan delay. Total ion current (TIC) spectra were obtained by integrating the first 40 scans in each acquisition. The MS was scanned from m/z 160 to 650. The resulting spectra allowed identification of the BDE congeners. In experiments involving the commercial penta-mixture, groups of peaks corresponding to diphenyl ethers containing zero, one, two, three, four, five, and six bromines were observed (Fig. 1), as determined by the bromine isotope patterns of the molecular ion clusters.
v.4.0 software was used for instrumental control and data acquisition. The analyses were carried out with the corona discharge needle maintained at approximately 15 lA. The probe was positioned perpendicular to the sampling orifice. Nitrogen (delivered by a Parker Balston model 75-72 nitrogen generator) was used as desolvation gas (flow rate 410 l min1, desolvation tem-
563.70 565.70
100 485.78
483.80
487.78
561.71
%
567.71 325.99 .
405.87 .
163.13
324.00 327.99
198.30
481.80 489.77 . 401.88
407.86 559.72
1 167.15
643.65 . 641.64 645.63
569.70
214.20 185.20
229.29 231.31
0 160
180
200
200
240
271.35
315.43 .
260
280
300
639.66
329.04
287.36
320
341.96
340
369.22 381.22
360
380
400
408.94 422.90 453.30
420
440
479.77
460
490.81 515.81
480
500
520
570.76
531.84
540
603.67
560
580
600
637.68
m/z 620
640
Fig. 1. Positive ion APCI mass spectrum of a commercial penta-mixture showing groups of peaks corresponding to diphenyl ethers containing zero, one, two, three, four, five, and six bromines (m/z 170.21 to 643.52).
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2.3. Catalytic hydrogenolysis Catalytic hydrogenolysis was carried out in 10 ml round-bottom flasks, using stirred solutions of BDE congeners and 1-naphthol (internal standard) in methanol, and amounts of catalyst powder representing 15– 40% by mass of the congeners. Solutions were protected from the light by covering the flask with aluminum foil. The flask was sealed by a rubber septum through which three syringe needles were inserted, one to bubble hydrogen gas into the solution, the others to allow excess gas to escape. Samples were taken at convenient times using a microsyringe and analyzed directly by HPLC or LC– MS. Agitation and gas flow were stopped during sampling to avoid uptake of catalyst into the microsyringe. 2.4. Electrocatalytic hydrogenolysis ECH was carried out under galvanostatic conditions using a PAR Model 363 potentiostat/galvanostat and a flow-through batch cell with a glass tube attached to the side to allow recirculation of the cathodic solution (Fig. 2). This cell was designed by Patrick Dube´ (University of Sherbrooke) and fabricated by Yves Savoret (University of Guelph). The anode and cathode compartments were separated with a Dupont Nafion 424 cation exchange membrane (CEM). The cathode was a circular piece of Duocel reticulated vitreous carbon (RVC) (100 ppi, 6 mm thick and 35 mm in diameter) with a copper wire lead enclosed in glass for stability. The anode was a rectangular piece of ELTECH IrO2/Ti dimensionally stable anode (DSA) (55 mm · 13 mm · 1.5 mm) with a stainless steel wire lead. The anolyte (5 ml) was 0.02 M aq. sulphuric acid, and the catholyte was the BDE in methanol–water (80:20 or
70:30), containing 0.02 M sodium sulphate as supporting electrolyte. A stir bar was placed in the cathode compartment; upon stirring, the catholyte solution and catalyst powder were forced through the RVC electrode, resulting in the impregnation of catalyst powder in the cathode. When the charge was removed and stirring was stopped, the catalyst powder returned to solution. Samples of the catholyte solution were collected for HPLC or LC–MS analyses at appropriate times.
3. Results and discussion The following experiments describe the first preliminary work on the (electro)catalytic hydrogenolysis of PBDEs. PBDEs are considered to be persistent organic pollutants (POPs), and already the penta-BDE commercial mixture is scheduled for phase out in Europe (European Chemicals Bureau, 2000), and the chief US manufacturer has agreed to a voluntary phase out of both the penta- and the octa-BDE commercial mixtures (Hogue, 2003). The eventual objective of our research is the remediation of the waste or unwanted PBDE residues that will eventually require destruction. In this work, however, convenient (mM) concentrations of PBDEs were employed, and no attempt was made to optimize the ratios of PBDE to catalyst to practical levels. 3.1. Catalytic hydrogenolysis A 1.25 mM solution of 4-MBDE (1.25 mg) in methanol (4 ml) was hydrogenolyzed in the presence of 0.5 mg of catalyst; after 90 min, 97% of the 4-MBDE was converted to diphenyl ether (Fig. 3). With 0.2 mg of catalyst,
1.4
Concentration (mM)
1.2 1 0.8 0.6 0.4 0.2 0 0
20
40
60
80
100
Time (min)
Fig. 2. Schematic of cell used for the electrocatalytic hydrogenation of BDEs.
Fig. 3. Catalytic hydrogenation of 4-MBDE. Concentration versus time for 4-MBDE (empty circle) and phenyl ether (empty square).
P.M.L. Bonin et al. / Chemosphere 58 (2005) 961–967
2 1.5 1 0.5 0 0
20
40
60
80
Time (min)
Fig. 6. Catalytic hydrogenation of a commercial penta-mixture. Concentration versus time for the three monobrominated compounds: 2-MBDE (empty square), 3-MBDE (empty triangle) and 4-MBDE (empty circle).
experiments were analyzed by LC–MS in positive ion mode, to show overall congener group patterns, and by HPLC to obtain congener-specific information. Sequential debromination was observed, and after 80 min, the reaction mixture comprised diphenyl ether (49%), MBDE isomers (38%), and DBDE isomers (12%). The monobrominated compounds were 4MBDE > 2-MBDE > 3-MBDE, the preponderance of o- and p-isomers being consistent with the initial composition of the penta-BDE mixture (principally congeners 47 and 99) (Fig. 6). Faster debromination was observed when more catalyst was used.
0 -0.05 -0.1 -0.15 Ln (C/C0)
2.5 Concentration (mM)
the reaction was slower (66% conversion after the same time). 4,4 0 -DBDE underwent sequential debromination to 4-MBDE and diphenyl ether: with 17.5 mg of catalyst, 98% of a 3.32 mM solution of 4,4 0 -DBDE (109 mg) in methanol (100 ml) had degraded after 45 min, affording 0.18 mM of 4-MBDE and 2.85 mM of diphenyl ether. A mixture of o-, m-, and p-MBDE was hydrogenated to determine their relative rates of debromination. The solution contained 0.95 mM 2-MBDE, 1.34 mM 3MBDE, and 1.15 mM 4-MBDE (total mass 3.43 mg) in methanol (4 ml) with 1.4 mg of catalyst. The reaction rates were 3-MBDE > 4-MBDE > 2-MBDE, with apparent first order rate constants 0.012, 0.008, and 0.002 min1 (Fig. 4). Catalytic hydrogenolysis of a 10 mM solution of commercial penta-BDE (33.8 mg) in methanol (6 ml) was carried out using 5.1 mg of catalyst (Fig. 5). These
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-0.2 -0.25 -0.3 -0.35 -0.4 -0.45 -0.5 0
10
20
30
40
Time (min)
Fig. 4. Catalytic hydrogenation of a mixture of the three MBDEs. Natural logarithm of the ratio of the concentration to the initial concentration versus time for 2-MBDE (empty square), 3-MBDE (empty triangle) and 4-MBDE (empty circle).
Fig. 5. Catalytic hydrogenation of a commercial penta-mixture. Mixture composition (%) versus time for phenyl ether and for the mono- to the hexa-brominated compounds. Initial mixture composition (white pillars), after 40 min (textured bars) and after 80 min (filled bars).
3.2. Electrocatalytic hydrogenolysis The limited solubility of PBDE congeners in aq. MeOH required a high proportion of methanol in the solvent mixture: 80:20 v/v MeOH:H2O for experiments with 4-MBDE and 70:30 for 4,4 0 -DBDE. This necessitated a trade-off; hydrogenolysis was slower when there was less water in the mixture, because of a slower rate of forming nascent hydrogen atoms. A problem was encountered in that bubbles of hydrogen gas formed an isolating layer around the electrode; this led to an increased ohmic drop, and hence to current overload. The hydrogen gas bubbles were dispersed by physically tapping the electrode. At a current of 0.06 A, a 44 mM solution of 4-MBDE (1 g) in methanol (100 ml) was hydrogenolyzed in the presence of 150 mg of catalyst. After 90 min, 48% of the 4-MBDE was converted to diphenyl ether. ECH of a 2.4 mM solution of 4,4 0 -DBDE (70.8 mg in 100 ml methanol, with 10.3 mg catalyst) at a current of 0.065 A gave 75% consumption of the reactant after 90 min, with the formation of 0.95 mM of 4-MBDE and 0.83 mM of diphenyl ether. Unfortunately, the poor solubility of PBDEs in a partly aqueous solution made it impractical to carry out electrocatalytic studies with the
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commercial penta-BDE mixture. Fortunately, in the case of hydrogenolysis of bromides, the C–Br bond is sufficiently weak that the conventional method involving H2/Pd proceeds satisfactorily at room temperature and pressure. In conclusion, catalytic hydrogenolysis has been shown to offer a convenient method for the debromination of polybrominated diphenyl ethers—and probably other brominated flame-retardants as well. Further work will be required to determine optimum substrate: catalyst ratios and to establish the longevity of the catalyst.
Acknowledgment Financial support was provided by the Natural Sciences and Engineering Research Council of Canada through a Postdoctoral Fellowship to PMLB, a summer scholarship to PE and a research grant to NJB.
References Agency for Toxic Substances and Disease Registry (ATSDR), 2003. (accessed November 28, 2003). Chapuzet, J.M., Lasia, A., Lessard, J., 1998. Electrocatalytic hydrogenation of organic compounds. In: Lipkowski, J., Ross, P.N. (Eds.), Frontiers of Electrochemistry. VCH Publishers Inc., New York, pp. 155–196. Chen, G., 2003. Interactive mechanism of halogenated aromatic hydrocarbons on Ah receptor signal transduction pathway. PhD Thesis, University of Guelph (Chapter 4). Chen, G., Bunce, N.J., 2003. Polybrominated diphenyl ethers as Ah receptor agonists and antagonists. Toxicol. Sci. 76, 310– 320. Chen, G., Konstantinov, A.D., Chittim, B.G., Joyce, E.M., Bols, N.C., Bunce, N.J., 2001. Synthesis of polybrominated diphenyl ethers and their capacity to induce CYP1A by the Ah receptor mediated pathway. Environ. Sci. Technol. 35, 3749–3756. Dabo, P., Cyr, A., Laplante, F., Jean, F., Menard, H., Lessard, J., 2000. Electrocatalytic dehydrochlorination of pentachlorophenol to phenol or cyclohexanol. Environ. Sci. Technol. 34, 1265–1268. Dabo, P., Cyr, A., Lessard, J., Brossard, L., Menard, H., 1999. Electrocatalytic hydrogenation of 4-phenoxyphenol on active powders highly dispersed in a reticulated vitreous carbon electrode. Can. J. Chem. 77, 1225–1229. Darnerud, P.O., 2003. Toxic effects of brominated flame retardants in man and in wildlife. Environ. Int. 29, 841–853. European Chemicals Bureau (ECB), 1999a. Draft risk assessment of diphenyl ether, octabromo derivative (octabromodiphenyl ether). Final Report of August.
tive (decabromodiphenyl ether). Final Report of August.
P.M.L. Bonin et al. / Chemosphere 58 (2005) 961–967 Schecter, A., Pavuk, M., Papke, O., Ryan, J.J., Birnbaum, L., Rosen, R., 2003. Polybrominated diphenyl ethers (PBDEs) in US mothersÕ milk. Environ. Health Perspect. 111, 1723– 1729. Schmidt, S., Wittich, R.-M., Erdmann, D., Wilkes, H., Francke, W., Fortnagel, P., 1992. Biodegradation of diphenyl ether and its monohalogenated derivative by Sphingomonas sp. strain SS3. Appl. Environ. Microbiol. 58, 2744–2750. Stock, N.L., Bunce, N.J., 2002. Electrocatalytic dechlorination of atrazine. Can. J. Chem. 80, 200–206. Tsyganok, A.I., Yamanaka, I., Otsuka, K., 1998. Pd-loaded carbon felt as the cathode for selective dechlorination of 2,4-
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dichlorophenoxyacetic acid in aqueous solution. J. Electrochem. Soc. 145, 3844–3850. Tullo, A., 2003. Great Lakes to phase out flame retardants. Chem. Eng. News (Nov 10), 13. Waters Connection University, 2001. Section 2: Atmospheric pressure ionization fundamental, Alliance LC/MS system: ZQ operator training, vol. 1. Waters Corporation, Milford, MA. Wenning, R.J., 2002. Uncertainties and data needs in risk assessment of three commercial polybrominated diphenyl ethers: probabilistic exposure analysis and comparison with European Commission results. Chemosphere 46, 779–796.
Catalytic and electrocatalytic hydrogenolysis of brominated diphenyl ethers Pascale M.L. Bonin a, Patrick Edwards a, Dorin Bejan a, Chun Chi Lo a, Nigel J. Bunce a,*, Alexandre D. Konstantinov b a b
Department of Chemistry, University of Guelph, Guelph, Ont., Canada N1G 2W1 Wellington Laboratories, 345 Southgate Drive, Guelph, Ont., Canada N1G 3M5
Received 18 December 2003; received in revised form 8 September 2004; accepted 29 September 2004
Abstract Polybrominated diphenyl ethers (PBDEs) are ubiquitous environmental contaminants due to their use as additive flame-retardants. Conventional catalytic hydrogenolysis in methanol solution and electrocatalytic hydrogenolysis in aqueous methanol were examined as methods for debrominating mono- and di-bromodiphenyl ethers, as well as a commercial penta-PBDE mixture, in each case using palladium on alumina as the catalyst. Electrocatalytic hydrogenolysis employed a divided flow-through batch cell, with reticulated vitreous carbon cathodes and IrO2/Ti dimensionally stable anodes. Both methods gave efficient sequential debromination, with essentially complete removal of bromine from the PBDEs, but the electrocatalytic method was limited by the poor solubility of PBDEs in aqueous methanol. 2004 Published by Elsevier Ltd. Keywords: Hydrogenolysis; Electrochemical debromination; Flame-retardants; Polybrominated diphenyl ethers
1. Introduction Polybrominated diphenyl ethers (PBDEs) are part of the family of brominated flame-retardants, which are widely used in textiles, paints, plastics, and electronic equipment (IPCS, 1994). Commercial PBDEs, which are prepared by bromination of diphenyl ether, are mixtures of congeners; the commercial mixtures presently in use are nominally penta-, octa-, and deca-BDEs
* Corresponding author. Tel.: +1 519 824 4120x53962; fax: +1 519 766 1499. E-mail address: [email protected] (N.J. Bunce).
0045-6535/$ - see front matter 2004 Published by Elsevier Ltd. doi:10.1016/j.chemosphere.2004.09.099
(Wenning, 2002; Darnerud, 2003). The penta-BDE mixture, for example, typically comprises 37% of 2,2 0 ,4, 4 0 -tetra-BDE (congener 47) and 35% of 2,2 0 ,4,4 0 ,5-penta-BDE (congener 99), which are the most prevalent congeners found in the environment (Lindstrom et al., 1999), along with smaller amounts of congeners 100, 153, 154, and 85 (cited in Chen and Bunce, 2003)—the numbering of PBDE congeners is analogous to that used for polychlorinated biphenyls (ATSDR, 2003). The consumption of brominated flame-retardants is high; in 1999, 67 000 metric tons of PBDE mixtures were produced worldwide (McDonald, 2002). PBDEs are persistent hydrophobic contaminants that bioaccumulate in organisms and biomagnify in the food chain (Palm et al., 2002). Their concentrations
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in sediments, fish, aquatic birds, marine mammals, and other wildlife (Wenning, 2002), and in human breast milk, e.g., in Sweden (Meironyte et al., 1999; Lind et al., 2003) and North America (Ryan and Patry, 2000; Schecter et al., 2003), have been increasing in parallel with their production trends. Although environmental concentrations of PBDEs have so far been much lower than those of organochlorines such as PCBs, a worrisome trend is that concentrations of PBDEs in fish from the Columbia River, British Columbia, were predicted to surpass those of PCBs in the year 2003 (Rayne et al., 2003). Concern exists over possible health effects of PBDEs, including endocrine disruption, neurodevelopment deficits and cancer (McDonald, 2002; Chen, 2003). As a result, the European Union has implemented a ban on the future use of penta-BDE (European Chemicals Bureau, 1999a,b, 2000). Risk assessment for octa- and deca-BDE commercial mixtures are ongoing and regulations concerning their future use are pending (ECB, 1999a,b). One North American manufacturer has already announced a voluntary phase-out of the pentaand octa- commercial PBDE mixtures (Tullo, 2003; Hogue, 2003). Hydrogenolysis offers a possible method to degrade PBDEs by replacing the bromine atoms with hydrogen atoms, leaving diphenyl ether. Although this compound is potentially bioaccumulable and toxic to fish, it is biodegradable (Schmidt et al., 1992), suggesting that hydrogenolysis might be useful as a front-end technology for dehalogenating the PBDE molecules prior to biodegradation (Rodgers et al., 1999). In this study we have examined conventional and electrocatalytic hydrogenolysis (ECH) as methods for the debromination of PBDEs. ECH (Tsyganok et al., 1998) involves the electrochemical reduction of a protic solvent to produce adsorbed ‘‘nascent’’ hydrogen atoms that react chemically with an adsorbed organic substrate (Chapuzet et al., 1998). The innate reactivity of nascent hydrogen atoms means that ECH generally proceeds under much milder conditions than conventional hydrogenation (Miller and Christensen, 1978). For example, glucose has been electrocatalytically hydrogenated to sorbitol at 60 C and atmospheric pressure, compared with 80–140 C and 20–140 atm for the conventional reaction (Pintauro and Bontha, 1991), and even 4-phenoxyphenol has been catalytically electroreduced to the corresponding cyclohexanol (Dabo et al., 1999). ECH has also been applied to the hydrogenolysis of aryl C– Cl bonds (Dabo et al., 2000; Stock and Bunce, 2002), which are stronger than the C–Br bonds present in PBDEs. A further advantage of ECH is that the cathodic polarization of the catalyst appears to offer protection against the adsorption of catalyst poisons, thereby extending the catalystÕs working lifetime (Laborde et al., 1990; Robin et al., 1990).
2. Experimental section 2.1. Chemicals Phenyl ether, 4-bromodiphenyl ether (4-MBDE, congener #3), 1-naphthol, and 5% palladium on alumina were obtained from Aldrich (Oakville, Ontario). 4,4 0 Dibromodiphenyl ether (4,4 0 -DBDE, congener #15) was obtained from Alfa Aesar (Wardhill, Massachussetts). Other PBDE congeners (Table 1) were available from a previous study (Chen et al., 2001), with the exception of 2-bromodiphenyl ether (2-MBDE, congener #1) and 3-bromodiphenyl ether (3-MBDE, congener #2) which were prepared by the method of Marsh et al. (1999). The identities of the prepared compounds, which were single spots by thin layer chromatography, were confirmed by 1H NMR at 400 MHz, CD2Cl2 as solvent; low resolution GC–MS indicated >99% purity, based on peak intensities; HPLC showed only a single peak. A commercial penta-BDE mixture (Great Lakes DE-71) was provided by Great Lakes Chemical Co. (West Lafayette, Indiana). Solvents used were HPLC grade methanol (Fisher Scientific, Toronto, Ontario), HPLC grade benzene (Aldrich, Oakville, Ontario) and water purified with a Millipore Milli-Q Reagent Water System. All other chemicals used were of reagent grade. 2.2. Analytical procedures The samples were analyzed by HPLC, using a Waters model 600 pump (flow rate 1.0 ml min1), a Rheodyne injector containing a 20 ll sample loop, and a Waters model 486 tunable absorbance detector operated at 220 nm (except for congeners #153 and #154, which were monitored at 210 nm). The detector output was processed with Waters Millennium version 3.2 software. Separation was achieved on a Waters Spherisorb ODS1 (5 lm · 4.6 mm · 25 cm) column. The mobile phase was 90:10 v/v methanol:water, which was degassed before use. Peaks were identified by coinjection of hydrogenolyzed mixtures and authentic congeners. Quantitation was achieved using standard curves prepared with 1naphthol as internal standard. 1-Naphthol was chosen for this purpose because it did not undergo hydrogenolysis under our experimental conditions, unlike those reported for other naphthalene derivatives by Jobin and Lessard (2003), and had HPLC retention time well removed from any reactants or products. Some samples were also analyzed by API-MS in the positive atmospheric pressure chemical ionization (APCI) mode, using a Waters single quadrupole ZQLC/MS 2000 system equipped with an APCI probe and a Z-spray source. The mobile phase was delivered by a Waters 600 gradient pump system with a Waters 600 controller (Waters, Milford, MA/Micromass, Manchester, UK). A Compaq Workstation with Masslynx
P.M.L. Bonin et al. / Chemosphere 58 (2005) 961–967
perature 400 C) and as cone gas (160 l min1). The source temperature was set to 120 C to prevent sample recondensation inside the ion source. Due to the high flow rate requirement of APCI-MS, methanol mixture was used as carrier phase and was pumped continuously into the probe at a rate of 0.3 ml min1. Sample was analyzed by the ‘‘flow infusion method’’ without prior chromatographic separation. The samples were dissolved in a 1:1 methanol:benzene (v/v) and were injected into the probe at a rate of 20 ll min1, using an onboard syringe pump and a Hamilton 500 ll syringe. Under APCI positive condition, the BDE congeners can be ionized by the benzene molecular ions through the ‘‘Charge exchange mechanism’’ (Waters Connection University, 2001):
Table 1 PBE congeners used in this study BDE No.
Structure
1 2 3
2-MBDE 3-MBDE 4-MBDE
15
4,4 0 -DBDE
17 28
2,2 0 ,4-Tri-BDE 2,4,4 0 -Tri-BDE
47 66
2,2 0 ,4,4 0 -Tetra-BDE 2,3 0 ,4,4 0 -Tetra-BDE
85 99 100
2,2 0 ,3,4,4 0 -Penta-BDE 2,2 0 ,4,4 0 ,5-Penta-BDE 2,2 0 ,4,4 0 ,6-Penta-BDE 0
0
þ C6 Hþ þ C6 H6 6 þM ! M
0
2,2 ,4,4 ,5,5 -Hexa-BDE 2,2 0 ,4,4 0 ,5,6 0 -Hexa-BDE
153 154
963
The scan speed was 1 s scan1 with 0.1 s interscan delay. Total ion current (TIC) spectra were obtained by integrating the first 40 scans in each acquisition. The MS was scanned from m/z 160 to 650. The resulting spectra allowed identification of the BDE congeners. In experiments involving the commercial penta-mixture, groups of peaks corresponding to diphenyl ethers containing zero, one, two, three, four, five, and six bromines were observed (Fig. 1), as determined by the bromine isotope patterns of the molecular ion clusters.
v.4.0 software was used for instrumental control and data acquisition. The analyses were carried out with the corona discharge needle maintained at approximately 15 lA. The probe was positioned perpendicular to the sampling orifice. Nitrogen (delivered by a Parker Balston model 75-72 nitrogen generator) was used as desolvation gas (flow rate 410 l min1, desolvation tem-
563.70 565.70
100 485.78
483.80
487.78
561.71
%
567.71 325.99 .
405.87 .
163.13
324.00 327.99
198.30
481.80 489.77 . 401.88
407.86 559.72
1 167.15
643.65 . 641.64 645.63
569.70
214.20 185.20
229.29 231.31
0 160
180
200
200
240
271.35
315.43 .
260
280
300
639.66
329.04
287.36
320
341.96
340
369.22 381.22
360
380
400
408.94 422.90 453.30
420
440
479.77
460
490.81 515.81
480
500
520
570.76
531.84
540
603.67
560
580
600
637.68
m/z 620
640
Fig. 1. Positive ion APCI mass spectrum of a commercial penta-mixture showing groups of peaks corresponding to diphenyl ethers containing zero, one, two, three, four, five, and six bromines (m/z 170.21 to 643.52).
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2.3. Catalytic hydrogenolysis Catalytic hydrogenolysis was carried out in 10 ml round-bottom flasks, using stirred solutions of BDE congeners and 1-naphthol (internal standard) in methanol, and amounts of catalyst powder representing 15– 40% by mass of the congeners. Solutions were protected from the light by covering the flask with aluminum foil. The flask was sealed by a rubber septum through which three syringe needles were inserted, one to bubble hydrogen gas into the solution, the others to allow excess gas to escape. Samples were taken at convenient times using a microsyringe and analyzed directly by HPLC or LC– MS. Agitation and gas flow were stopped during sampling to avoid uptake of catalyst into the microsyringe. 2.4. Electrocatalytic hydrogenolysis ECH was carried out under galvanostatic conditions using a PAR Model 363 potentiostat/galvanostat and a flow-through batch cell with a glass tube attached to the side to allow recirculation of the cathodic solution (Fig. 2). This cell was designed by Patrick Dube´ (University of Sherbrooke) and fabricated by Yves Savoret (University of Guelph). The anode and cathode compartments were separated with a Dupont Nafion 424 cation exchange membrane (CEM). The cathode was a circular piece of Duocel reticulated vitreous carbon (RVC) (100 ppi, 6 mm thick and 35 mm in diameter) with a copper wire lead enclosed in glass for stability. The anode was a rectangular piece of ELTECH IrO2/Ti dimensionally stable anode (DSA) (55 mm · 13 mm · 1.5 mm) with a stainless steel wire lead. The anolyte (5 ml) was 0.02 M aq. sulphuric acid, and the catholyte was the BDE in methanol–water (80:20 or
70:30), containing 0.02 M sodium sulphate as supporting electrolyte. A stir bar was placed in the cathode compartment; upon stirring, the catholyte solution and catalyst powder were forced through the RVC electrode, resulting in the impregnation of catalyst powder in the cathode. When the charge was removed and stirring was stopped, the catalyst powder returned to solution. Samples of the catholyte solution were collected for HPLC or LC–MS analyses at appropriate times.
3. Results and discussion The following experiments describe the first preliminary work on the (electro)catalytic hydrogenolysis of PBDEs. PBDEs are considered to be persistent organic pollutants (POPs), and already the penta-BDE commercial mixture is scheduled for phase out in Europe (European Chemicals Bureau, 2000), and the chief US manufacturer has agreed to a voluntary phase out of both the penta- and the octa-BDE commercial mixtures (Hogue, 2003). The eventual objective of our research is the remediation of the waste or unwanted PBDE residues that will eventually require destruction. In this work, however, convenient (mM) concentrations of PBDEs were employed, and no attempt was made to optimize the ratios of PBDE to catalyst to practical levels. 3.1. Catalytic hydrogenolysis A 1.25 mM solution of 4-MBDE (1.25 mg) in methanol (4 ml) was hydrogenolyzed in the presence of 0.5 mg of catalyst; after 90 min, 97% of the 4-MBDE was converted to diphenyl ether (Fig. 3). With 0.2 mg of catalyst,
1.4
Concentration (mM)
1.2 1 0.8 0.6 0.4 0.2 0 0
20
40
60
80
100
Time (min)
Fig. 2. Schematic of cell used for the electrocatalytic hydrogenation of BDEs.
Fig. 3. Catalytic hydrogenation of 4-MBDE. Concentration versus time for 4-MBDE (empty circle) and phenyl ether (empty square).
P.M.L. Bonin et al. / Chemosphere 58 (2005) 961–967
2 1.5 1 0.5 0 0
20
40
60
80
Time (min)
Fig. 6. Catalytic hydrogenation of a commercial penta-mixture. Concentration versus time for the three monobrominated compounds: 2-MBDE (empty square), 3-MBDE (empty triangle) and 4-MBDE (empty circle).
experiments were analyzed by LC–MS in positive ion mode, to show overall congener group patterns, and by HPLC to obtain congener-specific information. Sequential debromination was observed, and after 80 min, the reaction mixture comprised diphenyl ether (49%), MBDE isomers (38%), and DBDE isomers (12%). The monobrominated compounds were 4MBDE > 2-MBDE > 3-MBDE, the preponderance of o- and p-isomers being consistent with the initial composition of the penta-BDE mixture (principally congeners 47 and 99) (Fig. 6). Faster debromination was observed when more catalyst was used.
0 -0.05 -0.1 -0.15 Ln (C/C0)
2.5 Concentration (mM)
the reaction was slower (66% conversion after the same time). 4,4 0 -DBDE underwent sequential debromination to 4-MBDE and diphenyl ether: with 17.5 mg of catalyst, 98% of a 3.32 mM solution of 4,4 0 -DBDE (109 mg) in methanol (100 ml) had degraded after 45 min, affording 0.18 mM of 4-MBDE and 2.85 mM of diphenyl ether. A mixture of o-, m-, and p-MBDE was hydrogenated to determine their relative rates of debromination. The solution contained 0.95 mM 2-MBDE, 1.34 mM 3MBDE, and 1.15 mM 4-MBDE (total mass 3.43 mg) in methanol (4 ml) with 1.4 mg of catalyst. The reaction rates were 3-MBDE > 4-MBDE > 2-MBDE, with apparent first order rate constants 0.012, 0.008, and 0.002 min1 (Fig. 4). Catalytic hydrogenolysis of a 10 mM solution of commercial penta-BDE (33.8 mg) in methanol (6 ml) was carried out using 5.1 mg of catalyst (Fig. 5). These
965
-0.2 -0.25 -0.3 -0.35 -0.4 -0.45 -0.5 0
10
20
30
40
Time (min)
Fig. 4. Catalytic hydrogenation of a mixture of the three MBDEs. Natural logarithm of the ratio of the concentration to the initial concentration versus time for 2-MBDE (empty square), 3-MBDE (empty triangle) and 4-MBDE (empty circle).
Fig. 5. Catalytic hydrogenation of a commercial penta-mixture. Mixture composition (%) versus time for phenyl ether and for the mono- to the hexa-brominated compounds. Initial mixture composition (white pillars), after 40 min (textured bars) and after 80 min (filled bars).
3.2. Electrocatalytic hydrogenolysis The limited solubility of PBDE congeners in aq. MeOH required a high proportion of methanol in the solvent mixture: 80:20 v/v MeOH:H2O for experiments with 4-MBDE and 70:30 for 4,4 0 -DBDE. This necessitated a trade-off; hydrogenolysis was slower when there was less water in the mixture, because of a slower rate of forming nascent hydrogen atoms. A problem was encountered in that bubbles of hydrogen gas formed an isolating layer around the electrode; this led to an increased ohmic drop, and hence to current overload. The hydrogen gas bubbles were dispersed by physically tapping the electrode. At a current of 0.06 A, a 44 mM solution of 4-MBDE (1 g) in methanol (100 ml) was hydrogenolyzed in the presence of 150 mg of catalyst. After 90 min, 48% of the 4-MBDE was converted to diphenyl ether. ECH of a 2.4 mM solution of 4,4 0 -DBDE (70.8 mg in 100 ml methanol, with 10.3 mg catalyst) at a current of 0.065 A gave 75% consumption of the reactant after 90 min, with the formation of 0.95 mM of 4-MBDE and 0.83 mM of diphenyl ether. Unfortunately, the poor solubility of PBDEs in a partly aqueous solution made it impractical to carry out electrocatalytic studies with the
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commercial penta-BDE mixture. Fortunately, in the case of hydrogenolysis of bromides, the C–Br bond is sufficiently weak that the conventional method involving H2/Pd proceeds satisfactorily at room temperature and pressure. In conclusion, catalytic hydrogenolysis has been shown to offer a convenient method for the debromination of polybrominated diphenyl ethers—and probably other brominated flame-retardants as well. Further work will be required to determine optimum substrate: catalyst ratios and to establish the longevity of the catalyst.
Acknowledgment Financial support was provided by the Natural Sciences and Engineering Research Council of Canada through a Postdoctoral Fellowship to PMLB, a summer scholarship to PE and a research grant to NJB.
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