Environ. Sci. Technol. 2005, 39, 3128-3133
Hydrolysis of Environmental Contaminants as an Experimental Tool for Indication of Their Persistency SARA RAHM,* NICHOLAS GREEN, JESSICA NORRGRAN, AND ÅKE BERGMAN Department of Environmental Chemistry, Stockholm University, SE-10691 Stockholm, Sweden
To predict the persistency of a chemical in the environment, the chemical’s physical-chemical properties and its reactivity in the environment need to be known or at least estimated. The partitioning of a chemical can be described on the basis of its water solubility, its octanol/water partitioning coefficient, and its vapor pressure. The mechanisms by which a chemical can be transformed may be categorized as being hydrolysis, oxidation, reduction, and photolysis. This study establishes a method for estimating the relative susceptibility of some potential environmental pollutants to undergo hydrolysis reactions. The method used the second-order rate constant for the reaction with sodium methoxide in methanol/N,Ndimethylformamide (DMF) as an indicator of relative susceptibility toward hydrolysis. The decabromodiphenyl ether is rapidly hydrolyzed, that is, undergoes nucleophilic aromatic substitution, while the rate of reaction of less brominated diphenyl ethers decreased by roughly a factor of 10 for each decrease in the level of bromination. Hexachlorobenzene was found to have a similar rate to a nonabromodiphenyl ether. 2,2-Bis(4-chlorophenyl)-1,1,1trichloroethane (DDT) was transformed to 2,2-bis(4chlorophenyl)-1,1-dichloroethene (DDE) immediately under these conditions, while DDE showed no apparent reaction. The results show that chemicals that can undergo elimination reactions are rapidly transformed, as are perhalogenated chemicals that can undergo substitution reactions. These chemicals are not likely to persist in the environment, while those that did not show any observable reactivity under similar hydrolytic conditions may persist for a very long time.
Introduction Understanding and predicting the persistence and lipophilicity of existing and new organic chemicals in the environment is fundamental to assessing their potential hazard to humans and wildlife. The lipophilicity, and a range of other chemico-physical constants, are well-known for most chemicals commercially produced. Several of these constants can also be modeled, such as log Kow, pKa, vapor pressures, aqueous solubility, log Koa, log Ksc, and Henry’s law constant (1-6). Persistence, on the other hand, and its relation to degradation is rarely well defined. Furthermore, the degra* Corresponding author phone: +46-8-163675; fax: +46-8-163979; e-mail:
[email protected]. 3128
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dation of organic chemicals in the various parts of the environment is highly complex, and currently available methods for determining or predicting degradation are generally regarded as being inadequate. This is especially so for compounds of relatively high persistence, many of which are also highly hydrophobic and hence not suited to standard degradation protocols such as photolysis in water (7) or ready biodegradability tests (8). The official European Union risk assessments (RA) of a number of brominated flame retardants (BFRs) have recently been completed or are nearing completion, and in many cases the outcome of the assessment hinges on the degradation potential of the compound. The persistence of the pentaBDE and octa-BDE (bromodiphenyl ether) products were, for example, crucial to the agreed banning of their manufacture and use within the EU (9). The potential for decaBDE to form polybrominated dibenzofurans (PBDFs) and diphenyl ethers of lesser bromination was an important consideration in assessing the risks associated with its use and release to the environment (10). Conversely, the relative ease of degradation of tetrabromobisphenol A (TBBPA) facilitated its anticipated approval in the current draft RA document (11). The data on degradation were not clear-cut in any of these cases, and more reliable assessments of degradation potential would have enabled such pivotal decisions to be made with greater confidence. It is very difficult to assess the environmental degradation potential of a chemical since it is impossible to replicate the huge variety of environmental conditions into a representative laboratory test scheme. However, an understanding of the chemistry of organic pollutants would provide some insight into their potential for degradation in the environment. Studies on the photochemistry of a few PBDEs, TBBPA, and some other relatively persistent organic compounds have recently been presented (12-14), but understanding of other forms of transformation is often lacking. The purpose of the present study was to broaden the understanding of chemical hydrolytic reactivity of some organic pollutants to determine their relative stability. Rates of hydrolysis, including elimination and substitution reactions, influence the persistency of the chemicals. This work is specifically aimed to develop a method to describe rates of hydrolysis under experimental conditions. Reaction with nucleophiles (including water) is an important mechanism of transformation of chemicals in the environment (15, 16), and this study focused on providing a general method for assessing susceptibility of compounds to this mode of attack. The outcome was intended to be a measure of the inherent susceptibility of the chemical to such a mode of reaction, rather than to attempt to replicate any one of the myriad sets of conditions a chemical might encounter in the environment. The nucleophile selected for use in this study was sodium methoxide in a MeOH/N,Ndimethyl formamide (DMF) solvent mixture; the reactivity of the conditions was varied by the solvent composition. The study focused on selected BFRs as a test set of contaminants, but hexachlorobenzene (HCB) was used for method development work since it was known to be a sufficiently reactive substance when reacted with, for example, sodium methanethiolate and sodium methoxide (17, 18). HCB, 2,2-bis(4-chlorophenyl)-1,1,1-trichloroethane (4,4′DDT), and related compounds were also included since they are well-studied environmental contaminants of similar nature to the compounds of our test set. These compounds could therefore act as a reference for interpreting the results of the study. 10.1021/es0484698 CCC: $30.25
2005 American Chemical Society Published on Web 04/02/2005
Materials and Methods Chemicals. Pentabromophenol (PBP) was purchased from Acros Organics (New Jersey), and tetrabromobisphenol A diallyl ether was from Aldrich Chemical Co., Inc. (Milwaukee, WI). Tetrabromobisphenol A bis(2,3-dibromopropyl ether) (FR-720) was from (Beer Sheva, Israel), 2,2-bis(4-chlorophenyl)-1,1,1-trichloroethane (4,4′-DDT) was from Sigma (St. Louis, MO), decabromodiphenyl ether (BDE-209) was from Fluka Chemie AG (Buchs, Switzerland), sodium was from Kebo (Stockholm, Sweden), and hexachlorobenzene (HCB) was from Sigma-Aldrich Chemie GmbH (Steinheim, Germany). 2,2-Bis(4-chlorophenyl)-1,1-dichloroethene (4,4′DDE) was prepared in house. 2,4,4′-Tribromodiphenyl ether (BDE-28), 2,2′,4,4′-tetrabromodiphenyl ether (BDE-47), 2,2′,4,4′,6-pentabromodiphenyl ether (BDE-100), 2,2′,4,4′,5,5′hexabromodiphenyl ether (BDE-153), 2,2′,3,4,4′,5′,6-heptabromodiphenyl ether (BDE-183), 2,2′,3,4,4′,5,5′,6-octabromodiphenyl ether (BDE-203), 2,2′,3,3′,4,4′,5,5′,6-nonabromodiphenyl ether (BDE-206), and 6-OH-2,2′4,4′-tetrabromdiphenyl ether (6-OH-BDE-47) were synthesized as described elsewhere (19-23). Dichloromethane (DCM), toluene, tetrahydrofuran (THF), acetic acid (analytical grade), and water were purchased from Scharlau (Barcelona, Spain). Sodium sulfate, silica gel (size 0.063-0.200 mm), acetonitrile, hexane, and acetone were from Merck (Darmstadt, Germany). N,N-Dimethylformamide (DMF) from Fluka/Riedel de Haen, was distilled weekly and stored over a molecular sieve (4 Å) under nitrogen. Methanol, from BDH (Poole, England), was freshly distilled before use. Instruments. High-performance liquid chromatography (HPLC) was performed on either a Waters (Milford, MA) chromatograph equipped with a UV detector (Waters 486) or a Shimadzu (Kyoto, Japan) chromatograph equipped with a UV detector (Shimadzu). A C18 column (Ace 5 C18, 250 × 4.6 mm, 5 µm particles) from Advanced Chromatography Technologies (Aberdeen, Scotland) was used in both instruments. The mobile phase was acetonitrile/water (8:2 or 92:8) with a flow rate of 1.5 mL min-1, and the detection wavelength was set at 235 nm. A vortex mixer from Scientific Industries (New York) and an ultrasound bath from Branson (Soest, Taiwan) were used. All equipment (glassware, stirrer, and septum) used in the distillation of solvents and in all reactions was cleaned with tetrahydrofuran (THF) and acetone before use to prevent contamination. The reaction flasks were further cleaned with distilled DMF before use. Method for Hydrolysis. A stock solution of sodium methoxide (the reagent) in MeOH (0.163 mol/L) was produced freshly every fortnight by reacting sodium metal with freshly distilled MeOH. The stock solution was stored under nitrogen atmosphere and the container was sealed. A stock solution of each test compound was prepared in freshly distilled DMF (24) with the aid of an ultrasound bath. The solution of the test compound was diluted to a concentration of 0.163 mM, and 1 mL (0.163 µmol) of the solution was transferred to the reaction flask. Reactions carried out at room temperature were performed in round-bottomed flasks equipped with magnetic stirrer bar and a seal through which the sodium methoxide solution was added to initiate the reaction. The flask was suspended in an unheated oil bath to the top level of the reaction solution. The temperature of the oil bath was monitored throughout the reaction and it was generally observed to rise by up to 1 °C. The reactions were performed under an atmosphere of dry nitrogen and the reaction flask was covered with aluminum foil to eliminate potential interference from photolysis reactions. The test compound (0.163 µmol) in DMF was diluted to 10 mL with distilled DMF and allowed to stabilize before a zero time sample (0.5 mL) was withdrawn. Stock methoxide solution (0.5 mL) was added to the flask to initiate the reaction.
Aliquots (0.5 mL) of the reaction mixture were withdrawn at appropriate time intervals by syringe through the sealed arm of the flask and quenched immediately by transferral to a test tube containing a 0.10 M solution of acetic acid in water (0.5 mL). The test tube was stirred on a vortex and then diluted with acetonitrile. The quenched aliquot solution (100 µL) was thereafter injected onto the HPLC column. Seven aliquots (inclusive of the zero time sample) were taken from each reaction. Disappearance of the starting material was monitored quantitatively, and the appearance and disappearance of the primary products was observed qualitatively. The natural logarithm of the ratio of the HPLC area of the zero time sample (corrected to allow for dilution by the reagent solution) to that of the starting material in each aliquot was plotted against time (seconds), ln (A0/At) ) kt. A0 is the initial area of the test compound, and At is the area of the test compound after time t. The slope of the resulting straight line provided the pseudo-first-order rate constant for the reaction. The corresponding second-order rate constant was calculated from this and the concentration of the reagent. The test compound was not considered to undergo hydrolysis if the test compound had not reacted after 1 h at 60 °C in a solution of 0.5% methanol (and 99.5% DMF) when the reagent was added in an excess of 1000-fold relative to the test compound. For reactions at elevated temperature, the apparatus was equipped with a water-cooled condenser column and the oil bath was maintained at the desired temperature. In all other respects the conditions were as described for experiments at room temperature. Reduced temperatures were maintained by an ice bath. The MeOH/DMF ratio and the amount of reagent used were independently varied by using a reagent solution of appropriate concentration. The initial volume of reaction was always made up to 10 mL by addition of the appropriate amount of DMF. The final reaction mixture was quenched with acetic acid (6.5 mL, 0.1 M) and diluted with water (80 mL) before it was extracted with dichloromethane (DCM) (3 × 20 mL). The organic phase was dried on a sodium sulfate column and evaporated. Toluene was used as an azeotrope for removal of DMF from the collection flask. The reaction mixture was then dissolved in DCM and cleaned on a small silica column (0.4 g), with DCM (10 mL) as mobile phase. The solvent of the cleaned mixture was evaporated and the residue was dissolved in hexane prior to GC-MS characterization.
Results The reaction rate of HCB with sodium methoxide in MeOH/ DMF was investigated through a series of experiments performed (i) over a range of temperatures, (ii) by using MeOH/DMF ratios between 0.05% and 6% methanol in the solution, and (iii) by applying 10-1000 molar excess of the reagent (sodium methoxide). The second-order rate constant for each reaction carried out with HCB in 5:95 MeOH/DMF is presented in Table 1, and the natural logarithm of this rate constant is plotted against reciprocal temperature (K-1) in Figure 1. The concentration of HCB, as presented in Table 1, was higher (32.5 µM) than the concentrations of HCB as presented in Table 2. The reaction rates were investigated at different temperatures as shown in Table 1. For the reaction with HCB, the activation energy was calculated to be 65 kJ mol-1. The second-order rate constant was independent of the concentration of reagent and independent of the ratio of substrate to reagent, in accordance with second-order kinetics. The second-order rate constant for reaction at 293 K decreased exponentially with increasing proportion of MeOH in the solvent mixture. This is shown in Figure 2, where the logarithm of the second-order rate constant for VOL. 39, NO. 9, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 1. Summary of the Second-Order Rate Constants of HCB at Different Temperatures when Sodium Methoxide Is Added in 100-fold Excess k′ (×10-3 s-1)
T (K)
k2 (L mol-1‚s-1)
11.6 10.3 5.6 6.4 3.1 2.1 0.42 0.5
333 333 323 323 313 313 293 293
3.66 3.25 1.77 2.02 0.98 0.66 0.13 0.16
a k′ is the measured rate reaction constant, T is the temperature in kelvins, and k2 is the second-order rate reaction constant.
FIGURE 1. Relationship between temperature and the natural logarithm of the second-order rate constant for reaction between hexachlorobenzene and sodium methoxide in methanol/DMF (5:95). each reaction performed at 293 ((2) K is plotted against the proportion of MeOH as a percentage of the total solvent mixture. Table 2 lists all compounds used in this study, the temperatures at which reactions were performed, the solvent composition as percent MeOH, the measured second-order rate constant in each case, and the calculated half-lives. Where reactions were performed at elevated or suppressed temperatures, the rate constant at 293 K is listed, as calculated from the Arrhenius expression. For test chemicals that reacted too slowly in 5% MeOH/DMF, a rough extrapolation of the second-order rate constant to 293 K and 5% MeOH was made by applying the relationship between ln k and solvent composition established for HCB (Figure 2). These values are listed in Table 2.
Discussion Choice of Reaction Conditions. The choice of sodium methoxide as the nucleophile for this study was made bearing in mind four dominant factors that affect reagent reactivity: nucleophilic strength, hard-soft acid-base interactions, steric effects, and solvent effects. No single reagent can provide a representative assessment of the possible chemistry (including microbiochemistry) encountered in the environment, but we considered that sodium methoxide would provide the most useful information about a pollutant’s reactivity. Since the study focused on relatively persistent compounds, a strong nucleophile was required. Earlier work using sodium methanethiolate (a stronger nucleophile than the methoxide) had showed promise (25), but the thiolate presented problems with solubility that the methoxide did not. The methoxide ion is a hard nucleophile, being small, charged, and nonpolarizable. Pearson (26) has shown that relative rates of reaction vary for harder/softer reactants dependent on the hardness/softness of the nucleophile. A ranking of the susceptibility of a series of pollutants to nucleophilic attack by a hard reagent may be different than the ranking derived from attack by a soft nucleophile. For 3130
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example, elimination reactions are particularly favored by hard nucleophiles since the nucleophile directs its attack toward a hydrogen substituent of the target molecule, with protons being the ultimate hard acid. Conversely, an unsaturated carbon center is more polarizable (softer) and may be preferentially attacked by a softer nucleophile than, say, a saturated carbon. For this study a hard nucleophile was chosen, but this alone may not provide sufficient information about the inherent chemical reactivity of a chemical to predict environmental persistence. Since there are many nucleophiles active in the environment, with varying degrees of softness/hardness, it may be necessary to use a series of reagents from soft to hard to best determine the reactivity (hydrolytic sensitivity) of chemicals. The methoxide ion is also small and, consequently, the measured reactivity of the test set of pollutants toward nucleophilic attack would not be masked by any inability of access of the nucleophile. This is arguably the most pertinent information for environmental nucleophilic reactions, where the predominant nucleophile is water. Nevertheless, the response to bulkier nucleophiles would also be helpful to understand, especially if polysulfides or humic substances are thought to be significant environmental nucleophiles for hydrophobic compounds. Finally, sodium methoxide is readily soluble in many solvents, or solvent systems, that are equally compatible with the pollutants of our study set and most organic chemicals in general. However, a high energy of solvation for the nucleophile can obscure inherent properties of the substrate in determining the reaction rate. DMF is a polar, aprotic solvent with a high dielectric constant ( ) 36.7) but with a poor capacity for hydrogen bonding (24). It is a good solvent for cations but a poor one for anions. As such, the methoxide ion in DMF is freely available for reaction. In MeOH, on the contrary, the methoxide ion is highly solvated. By using a mixture of these two solvents, most organic chemicals can be dissolved, and by varying the proportion of the solvents, the reactivity of the conditions can be modified. In the reaction with HCB, a decrease in MeOH content of 1% resulted in roughly a doubling of the reaction rate. By projection, the reaction in pure MeOH would have to be conducted at 1000 °C to give the same rate as 5% MeOH/ DMF at 20 °C. Such control over the reaction conditions by varying the solvent composition could enable the testing of a wide variety of chemicals. Modifying the temperature of reaction also allows a degree of control (see Figure 1), but we were restricted to operating between 0 and 60 °C, to ensure the solubility of the reagent at one end and that we did not alter the solvent composition through refluxing of the azeotropic mix at the other. Modest control of the observed pseudo-first-order rate of reaction could be achieved by varying the concentration of the reagent. In the work with HCB, the reagent concentration was varied between 10 and 1000 times that of the substrate. The rate of reaction increased in direct relation to the concentration of reagent, confirming the independence of the second-order rate constant from the concentration of reagent. Results and Their Relation to Organic Chemistry. As reported in Table 2, 4,4′-DDT was found to react immediately, even under the mildest conditions used, while 4,4′-DDE showed no reaction. TBBPA bis(2,3-dibromopropyl ether) also reacted immediately, while the diallyl ether analogue showed no reaction. These results are not unexpected: they demonstrate the inherent facility of 4,4′-DDT and TBBPA bis(2,3-dibromopropyl ether) to undergo dehydrohalogenation and mirror the observed environmental fate for 4,4′DDT. In each case the unsaturated alkene substituent in the corresponding compound (4,4′-DDE and TBBPA diallyl ether) was resistant to the nucleophile.
TABLE 2. Sodium Methoxide Reaction Conditions, Second-Order Rate Constants, and Calculated Half-lives substancea HCB BDE-209 BDE-209 BDE-206 BDE-203 BDE-183 BDE-183 BDE-183 BDE-183 BDE-153 BDE-153 BDE-153 BDE-153 BDE-100 BDE-100 BDE-100 BDE-47 BDE-28 TBBPA-bis(2,3-dibromopropyl ether) DDT DDE TBBPA-diallyl ether 6-OH-BDE-47 PBP
temp (K) MeOH/DMF (% MeOH) second-order k (L‚mol-1‚s-1) half-lives, T1/2 (h) kb (L‚mol-1‚s-1) 293 293 273 293 293 333 323 313 293 333 323 313 293 333 323 293 333 333 273 273 293 333 333 333
5 5 5 5 5 5 5 5 5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 5 5 5 0.5 0.5 0.5
0.092c 0.83c 0.17c 0.080c 0.030c 0.27 0.096 0.040 0.0045d 0.34 0.13 0.070 0.011d 0.12 0.044 0.0013d c c >1c >1c c c c
0.26 0.028 0.30 0.77
0.092 0.83 0.88 0.080 0.030
5.3
0.0045
22.3
0.0005
188 >240 >240 240 >240 >240
0.00006 >5 >5
a An abbreviated numbering system for PBDE congeners is used, analogous to that described for polychlorinated biphenyl congeners (33). Second-order rate reaction constant extrapolated to 293 K and 5% MeOH. c Average value from 2-6 experiments under the same conditions. d Arrhenius equation has been used to estimate a value at 293 K. b
FIGURE 2. Relationship between solvent composition and the natural logarithm of the second-order rate constant for reaction between hexachlorobenzene and sodium methoxide in methanol/DMF at 293 K. The proportion of methanol in the solvent mix is indicated by the x-axis. The two phenolic compounds in the test set (pentabromophenol and 6-OH-2,2′4,4′-tetrabromdiphenyl ether) did not react either. This is to be expected since the acidity of the phenolic protons is easily such that the methoxide ion will deprotonate them. The resulting negative charge on the ring eliminates the possibility of a nucleophile attacking. The reactivity of the PBDEs was correlated to the degree of bromination in the compounds. Rates were measurable for penta- to decabrominated diphenyl ether congeners, although different temperatures and different solvent ratios were required to cover the range of rates encountered. The tetra- and tri-BDE did not react even under the harshest conditions employed. To compare the results obtained at different temperatures, the Arrhenius expression can be used to extrapolate results to a common standard; in this case 293 K was used. If experiments were carried out at several solvent compositions, then an analogous expression could be formed for each compound to extrapolate back to the standard of 5% MeOH used here. However, this was only carried out for HCB, and in the absence of an alternative, the extrapolations for BDE-153 and BDE-100 were made by use of the expression derived for HCB. The results (listed in the final column of Table 2) are therefore only rough but are useful for comparison. Aromatic compounds are more normally associated
with electrophilic substitution, but in the presence of strongly electron-withdrawing effects, nucleophilic attack is feasible. In the sequence from deca- through to penta-BDE, the rate constant decreases by roughly a factor of 10 for each bromine removed. Bromine substituents are electron-withdrawing while bromide is a good leaving group, making the perbrominated compound highly suitable for nucleophilic aromatic substitution (NAS). The decrease in reactivity for lesserbrominated congeners reflects the decreased electronwithdrawing effect of fewer bromine substituents. The other perhalogenated compound in the test set, HCB, also had a high rate constant, although lower than that of deca-BDE. This may reflect the observation that chlorine is a poorer leaving group than bromine. GC-MS analysis showed several HO-P (n-1) BDEs to be the primary products of reaction for the PBDEs, indicating that substitution was not restricted to any particular position in the ring (data not shown). This is in keeping with an observation of Bergman (17) that NAS reactions of PCBs with more than three chlorine substituents in one of the rings could result in both meta and para substitution. Products from the reaction of BDE-209, in particular, included compounds with mass spectra consistent with di- and trimethoxylated PBDEs and others consistent with brominated anisole structures, indicating cleavage of the diphenyl ether bridge. The present study is not focused on identification of substitution or elimination products from the reactions performed. However, it would be of interest to also identify products formed in the reactions, but that is beyond the scope of this work. The observed chemistry can be summed up as showing that (i) deprotonation of phenols precludes any subsequent substitution, (ii) elimination reactions are generally facile, and (iii) NAS is only encountered in compounds that have a high degree of electron-withdrawing from the aromatic ring, which are consistent with standards in chemical understanding. Relation to Environmental Degradation. This study was not looking to develop an empirical formula between the experimental test results and environmental degradation rate VOL. 39, NO. 9, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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but rather to use the understanding of the general chemistry of the compounds to give indications of their relative reactivity in the environment. Ranking of such chemicals by their reactivity toward various modes of attack may provide a basis for predicting their relative rate of degradation in the environment. Understanding the susceptibility of chemicals toward nucleophilic attack can help predict environmental degradation through this mechanism. Many substitution reactions occur in the environment, the most obvious being hydrolysis, reaction with water. Water is a very weak nucleophile and, although abundant, may not be the most important. Additionally, when reaction with water is catalyzed (at mineral surfaces, by enzymes, by metal ions, etc., in addition to acid or base catalysis), the rate becomes greatly increased. Thus direct measurement of reaction of a compound with water [as in the Organisation for Economic Cooperation and Development (OECD) standard protocol (27)] may not provide a full indication of the potential for the compound to undergo hydrolyses or other substitution or elimination reactions. It is well-known that DDT is transformed to DDE through dehydrohalogenation relatively quickly in the environment, while DDE is extremely persistent. The calculated environmental half-lives of DDT and DDE have been proposed to be 12 years and more than 120 years, respectively, at pH 5 (28). It is well-known that DDT is only present at higher concentrations relative to DDE in the environment if recently discharged; results analogous to this are observed when the inherent chemistry of DDT is studied. The extensive knowledge of DDT/DDE makes them relevant benchmarks useful for comparing the other chemicals tested for hydrolysis. The relative rates of hydrolysis can thus play a role in the indication of contaminant persistency. Although there are no published studies on the environmental degradation of TBBPA bis(2,3-dibromopropyl ether), from the results of our study we might confidently predict to observe the elimination product TBBPA bis(bromopropenyl ether) in sediments, for example, or possibly reduced versions of it (in the way that DDD is commonly observed). Nothing is known of the environmental degradation of TBBPA-diallyl ether either, but from our findings, we might predict this to be highly resistant to hydrolysis and other forms of nucleophilic attack. As discussed above, one or several bromine atoms in BDE209 are rapidly substituted for a methoxyl group, indicating a relatively lower persistency of this PBDE congener compared to lower brominated congeners, in particular tri- and tetra-BDEs (cf. Table 2) that showed no observed reactivity toward methoxide. The lower brominated PBDEs are thus more DDE-like while the higher are more like HCB in their hydrolytic stability. Tracing substitution products of PBDEs in the environment would be more difficult since, as the nucleophile is incorporated into the product structure, its nature must be known. Nevertheless, there are now many reports of hydroxylated and also methoxylated PBDEs being found in environmental matrices (29-32), some of which may derive from anthropogenic PBDEs. Further Developments. In this study a single hard, strong, small nucleophile has been applied to a test set of chemicals that was specifically intended to contain relatively persistent compounds. The strength of the reaction conditions can be effectively altered by use of elevated or suppressed temperatures and/or by changing the proportion of MeOH to DMF. The latter, in particular, has the potential to create conditions of much lower reactivity so that less persistent compounds might also be investigated (including DDT). Both size and hardness of a nucleophile are important parameters for reactivity. Previous results with sodium methanethiolate, a softer nucleophile, reacted with PBDE congeners showed similar results to the present study (25), but it is desirable 3132
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to incorporate more bulky and also softer nucleophiles into a battery of reactions that would provide a fuller assessment of inherent chemical reactivity. For an investigation of this sort to be feasible, in terms of time and costs, the tests would need to be simplified.
Acknowledgments This study was financially supported by the Swedish Foundation for Strategic Environmental Research (MISTRA), the Swedish Research Council for Environment, Agricultural Sciences and Spatial Planning (FORMAS), and CEFIC (Contract NMLRI-LUND-MISTRA0201).
Literature Cited (1) Braekevelt, E.; Tittlemier, S. A.; Tomy, G. T. Direct Measurement of Octanol-Water Partition Coefficients of Some Environmentally Relevant Brominated Diphenyl Ether Congeners. Chemosphere 2003, 51, 563-567. (2) Ba¨rring, H.; Bucheli, T. D.; Broman, D.; Gustafsson, O ¨ . SootWater Distribution Coefficients for Polychlorinated Dibenzop-Dioxins, Polychlorinated Dibenzofurans and Polybrominated Diphenyl ethers Determined With the Soot Cosolvency-Column Method. Chemosphere 2002, 49, 515-523. (3) Harner, T.; Shoeib, M. Measurements of Octanol-Air Partition Coefficients (KOA) for Polybrominated Diphenyl Ethers (PBDEs): Predicting Partitioning in the Environment. J. Chem. Eng. Data 2002, 47, 228-232. (4) Palm, A.; Cousins, I. T.; Mackay, D.; Tysklind, M.; Metcalfe, C.; Alaee, M. Assessing the Environmental Fate of Chemicals of Emerging Concern: a Case Study of the Polybrominated Diphenyl Ethers. Environ. Pollut. 2002, 117, 195-213. (5) Tittlemier, S. A.; Halldorson, T.; Stern, G. A.; Tomy, G. T. Vapor Pressures, Aqueous Solubilities, and Henry’s Law Constants of Some Brominated Flame Retardants. Environ. Toxicol. Chem. 2002, 21, 1804-1810. (6) Li, X.; Shibata, E.; Nakamura, T. Theoretical Calculation of Thermodynamic Properties of Polybrominated Dibenzo-pDioxins. J. Chem. Eng. Data 2003, 48, 727-735. (7) OECD. Phototransformation of Chemicals in WatersDirect and Indirect Photolysis; Draft New Guideline; Organisation for Economic Co-operation and Development: Paris, 2000. (8) OECD. Guidelines for testing of chemicals: 301. Ready biodegradability; Organisation for Economic Co-operation and Development: Paris, 1992. (9) Cox, P.; Efthymiou, P. Directive 2003/11/EC of the European Parliament and of the Council of February 6 2003 Amending for the 24th Time Council Directive 76/669/EEC Relating to Restrictions on the Marketeing and Use of Certain Dangerous Substances and Preparations (Pentabromodiphenyl Ether, Octabromodiphenyl Ether). Off. J. Eur. Union 2003, OJ L 42, 4546. (10) European Commission. European Union Risk Assessment Report: Bis(pentabromophenyl) ether; EUR 20402 EN; Office for Official Publications of the European Communities: Luxembourg, 2002; Vol. 17. (11) European Commission. Draft Risk Assessment of 2,2′,6,6′Tetrabromo-4,4′-isopropylidene diphenol (Tetrabromobisphenol A); Environmental, R402 0311 ENV; Rapporteur: U.K., 2003. (12) Palm, W.-U.; Sossinka, W.; Ruck, W.; Zetzsch, C. Abiotic Degradation of Polybrominated Diphenyl Ethers: 1. Kinetics and Quantum Yields of the Photolysis in Tetrahydrofuran, Environ. Toxicol. Chem. 2004. (13) Eriksson, J.; Green, N.; Marsh, G.; Bergman, Å. Photochemical Decomposition of Fifteen Polybrominated Diphenyl Ether Congeners in Methanol/Water. Environ. Sci. Technol. 2004, 38, 3119-3125. (14) Eriksson, J.; Rahm, S.; Green, N.; Bergman, Å.; Jakobsson, E. Photochemical Transformations of Tetrabromobisphenol A and Related Phenols in Water. Chemosphere 2004, 54, 117-126. (15) Larson, R. A.; Weber, E. J. Reaction Mechanisms in Environmental Organic Chemistry; Lewis Publishers: Chelsea, MI, 1994. (16) Schwarzenbach, R. P.; Gschwend, P. M.; Imboden, D. M. Environmental Organic Chemistry; John Wiley & Sons: New York, 1993. (17) Bergman, Å. Studies on some persistent environmental contaminants. Ph.D. Thesis, Department of Organic Chemistry, University of Stockholm, Sweden, 1980.
(18) Jansson, B.; Bergman, Å. Sulfur-Containing Derivatives of Hexachlorobenzene (HCB)sMetabolites in the Rat. Chemosphere 1978, 257-268. (19) Eriksson, J.; Marsh, G.; Bergman, Å. Photo decomposition of brominated diphenyl ethers in methanol/water. The Second International Workshop on Brominated Flame Retardants, 1-12001; BFR 2001; p 203. (20) Eriksson, J.; Eriksson, L.; Marsh, G.; Bergman, Å. A one step synthesis of all nona-brominated diphenyl ethers. Organohalogen Compd. 2003, 61, 191. (21) Marsh, G.; Hu, J.; Jakobsson, E.; Rahm, S.; Bergman, Å. Synthesis and Characterization of 32 Polybrominated Diphenyl Ethers. Environ. Sci. Technol. 1999, 33, 3033-3037. (22) Marsh, G.; Stenutz, R.; Bergman, Å. Synthesis of Hydroxylated and Methoxylated Polybrominated Diphenyl EtherssNatural Products and Potential Polybrominated Diphenyl Ether Metabolites. Eur. J. Org. Chem. 2003, 2566-2576. (23) O ¨ rn, U.; Eriksson, L.; Jakobsson, E.; Bergman, Å. Synthesis and Characterization of Polybrominated Diphenyl EtherssUnlabeled and Radiolabeled Tetra-, Penta- and Hexa-Bromodiphenyl Ethers. Acta Chem. Scand. 1996, 50, 802-807. (24) Furniss, B. S.; Hannaford, A. J.; Smith, P. W. G.; Tatchell, A. R. Vogel’s Textbook of Practical Organic Chemistry; Longman Scientific & Technical: Essex, U.K., 1989. (25) Rahm, S.; Jakobsson, E. Reactivity of brominated diphenyl ethers vs methanethiolate. Second International Workshop on Brominated Flame Retardants; BFR 2001; 2001, p 227. (26) Pearson, R. G. Hard and Soft Acids and Bases. J. Am. Chem. Soc. 1963, 85, 3533-3539. (27) OECD. Guideline for testing of chemicalssHydrolysis as a function of pH; Report number 111; Organisation for Economic Co-operation and Development: Paris, 1981.
(28) Wolfe, N. L.; Zepp, R. G.; Paris, D. F.; Baughman, G. L.; Hollis, R. C. Metoxychlor and DDT Degradation in Water: Rates and Products. Environ. Sci. Technol. 1977, 11, 1077-1081. (29) Haglund, P. S.; Zook, D. R.; Buser, H. R.; Hu, J. Identification and Quantification of Polybrominated Diphenyl Ethers and Methoxy-Polybrominated Diphenyl Ethers in Baltic Biota. Environ. Sci. Technol. 1997, 31, 3281-3287. (30) Letcher, R. J.; D’Sa, I.; Valters, K.; Marsh, G.; Hongxia, L.; Bennett, E.; Alaee, M. Polybrominated diphenyl ethers and hydroxylated and methoxylated analogues in Detroit river fish. Organohalogen Compd. 2003, 61, 29. (31) Marsh, G.; Athanasiadou, M.; Bergman, Å.; Asplund, L. Identification of Hydroxylated and Methoxylated Polybrominated Diphenyl Ethers in Baltic Sea Salmon (Salmo salar) Blood. Environ. Sci. Technol. 2004, 38, 10-18. (32) Asplund, L.; Athanasiadou, M.; Sjo¨din, A.; Bergman, Å.; Bo¨rjeson, H. Organohalogen Substances in Muscle, Egg and Blood From Healthy Baltic Salmon (Salmo salar) and Baltic Salmon That Produced Offspring With the M74 Syndrome. Ambio 1999, 28, 67-76. (33) Ballschmiter, K.; Mennel, A.; Buyten, J. Long Chain AlkylPolysiloxanes As Non-Polar Stationary Phases in Capillary Gas Chromatography. Fresenius J. Anal. Chem. 1993, 346, 396-402.
Received for review September 28, 2004. Revised manuscript received January 19, 2005. Accepted February 24, 2005. ES0484698
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