Final Report: Anaerobic Microbial Reductive Debromination of Polybrominated Diphenyl Ethers

EPA Grant Number: R830251
Title: Anaerobic Microbial Reductive Debromination of Polybrominated Diphenyl Ethers
Investigators: Nies, Loring , Ahn, Mi Youn , Filley, Timothy
Institution: Purdue University
EPA Project Officer: Lasat, Mitch
Project Period: August 12, 2002 through August 13, 2004 (Extended to September 1, 2006)
Project Amount: $230,795
RFA: Futures Research in Natural Sciences (2001) RFA Text |  Recipients Lists
Research Category: Land and Waste Management , Ecological Indicators/Assessment/Restoration , Hazardous Waste/Remediation

Objective:

The overall goal of this project was to determine the extent to which decabromodiphenyl ether (BDE-209) will undergo reductive debromination in anaerobic sediments. Based on data regarding the environmental occurrence of polybrominated diphenyl ethers (PBDEs), as well as knowledge about the reductive dehalogenation of polychlorinated biphenyls (PCBs), polybrominated biphenyls (PBBs), and polychlorinated dibenzo-p-dioxins (PCDDs), we postulate that BDE-209 will undergo reductive debromination, producing less brominated and more bioavailable and toxic PBDE congeners. In order to obtain the most complete picture possible of the anaerobic fate of PBDEs, we will conduct sediment microcosm studies and parallel abiotic biomimetic dehalogenation studies on both the highly brominated BDE-209 and the lesser brominated 2,2',4,4'-tetrabromodiphenyl ether (BDE-47) and 2,2',4,4',5-pentabromodiphenyl ether (BDE-99). BDE-47 and BDE-99 are two PBDE congeners that are being detected in the biosphere in increasing concentrations.

Summary/Accomplishments (Outputs/Outcomes):

Two parallel experimental methods were used to study the fate of PBDEs under environmentally relevant reducing conditions. Three important congeners, BDE-209, BDE-99, and BDE-47, were used as starting material for experiments with both methods. BDE-209 is the single most widely produced and used PBDE congener by a wide margin, while BDE-99 and BDE-47 are the predominant congeners detected in the biosphere. Reductive debromination of BDE-209 could potentially produce BDE-99 and BDE-47 as well as other PBDE congeners. Using the co-solvent enhanced biomimetic system, the debromination rate and product distribution were monitored over the following times: 24 hours for BDE-209, 7 days for BDE-99, and 15 days for BDE-47. Parallel sediment microcosm incubations were monitored for total PBDE concentration and for the appearance of debromination products. In the case of BDE-209, two microcosm sets were constructed: one “new” set was assembled about the same time as the BDE-99 and BDE-47 microcosms, and a second “old” set was assembled more than 3 years earlier.

Sediment Microcosms

Only a very slight decrease in the mole fraction of BDE-209, with a concurrent increase in the three nonabromodiphenyl ethers, was measured in the “new” sediment microcosm set after 10 months. However, debromination was more extensive in the older BDE-209 microcosm set after 3.5 years of incubation. One replicate of the older microcosm incubations exhibited debromination that was substantially more extensive than in the other two. Therefore, data for replicates one and two are shown together and replicate three is shown separately. Significant debromination of BDE-209 was observed in all three replicates, along with the concurrent increase in a number of daughter congeners (Figure 1A). We observed a statistically significant increase in the mole fraction of each of the three nonabromodiphenyl ethers. This is in contrast to Gerecke, et al. (2005) who observed only para debromination to BDE-208 and meta debromination to BDE-207, but no ortho debromination to BDE-206. A view of only those congeners representing less than 10% mole fraction clearly illustrates the distinct difference of replicate three from the other two (Figure 1B). It should also be noted that nine new congeners that were not present in the starting parent material appeared. Furthermore, the total sum of all debromination products was greater than the starting fraction of non-BDE-209 PBDE congeners, showing that debromination of BDE-209 in sediment must have contributed to the distribution of lesser brominated congeners. The half-life of BDE-209 in the six live microcosms ranged from 6 to 50 years, with an average of just over 14 years.

Figure 1. Mole Fraction Distribution of BDE-209 After 3.5 Years Incubation in Anaerobic Sediment Microcosms.

Figure 1. Mole Fraction Distribution of BDE-209 After 3.5 Years Incubation in Anaerobic Sediment Microcosms.

Figure 1. Mole Fraction Distribution of BDE-209 After 3.5 Years Incubation in Anaerobic Sediment Microcosms. All congeners are shown in panel A, and only congeners less than 10% mole fraction are shown in panel B.

Anaerobic Transformation of BDE-99 in Anaerobic Sediment Microcosms

After 8 months, the BDE-99 mole fraction in the anaerobic sediment microcosms decreased by more than 3 percent and BDE-28 increased by approximately 2 percent, likely via a sequential two-step debromination through BDE-47 and BDE-49. The appearance of a new congener, BDE-66, accounted for about 1 percent of the debromination products. In addition, although it is a small amount, the mole fraction of BDE-47 doubled.

Anaerobic Transformation of BDE-47 Anaerobic Sediment Microcosms

A PBDE mole balance could not be closed for the BDE-47 microcosms, so the data were analyzed as the absolute mass of PBDE congener/mass sediments rather than a mole fraction distribution. After 8 months, there was a high degree of variability among the eight live microcosm incubations containing BDE-47. In several of the microcosms, there was a decrease in the absolute BDE-47 concentration of more than 30% without a consistent concurrent increase of daughter debromination products, except for a slight increase in the concentration of BDE-17. This would indicate that there is likely a BDE-47 transformation process occurring other than, or in addition to, reductive debromination. Since there are several reports of hydroxylated and methoxylated derivatives of tetra-BDEs occurring in biota, their appearance in sediment should be explored in the future.

Reductive Debromination of BDE-209 in a Biomimetic System

The BDE-209 parent starting material used in this experiment also contained small amounts of the three nonabromodiphenyl ethers, 2.0% BDE-206, 1.9% BDE-207, and 0.9% BDE-208, on a mole fraction basis (Figure 2A). The experiment was initiated by the addition of the titanium citrate reducing agent, and within 5 minutes, BDE-209 completely debrominated in the biomimetic system to more than a dozen daughter products, dominated by hexabromodiphenyl ether congeners. After 24 hours, the debromination reaction pathways had converged to primarily two pentabromodiphenyl ether congeners, BDE-99 and BDE-119 (2,3',4,4',6-,), and two tetrabromodiphenyl ethers, BDE-47 and BDE-66 (2,3',4,4'-,) (Figure 2B). It is noteworthy that all four of the major terminal congeners are double para substituted, indicating that in the biomimetic system, ortho and meta debromination regiospecificity dominates. Detailed examination of the debromination trajectory indicates that the vitamin B12- mediated reaction has broader specificity (more products) than the sediment debromination.

Figure 2. Reductive Debromination of BDE-209 in a Co-Solvent Enhanced Biomimetic System. (A) Starting material, and (B) reactive at 24 hours.

Figure 2. Reductive Debromination of BDE-209 in a Co-Solvent Enhanced Biomimetic System. (A) Starting material, and (B) reactive at 24 hours.

Figure 2. Reductive Debromination of BDE-209 in a Co-Solvent Enhanced Biomimetic System. (A) Starting material, and (B) reactive at 24 hours.

Reductive Debromination of BDE-99 in a Biomimetic System

The parent BDE-99 material was greater than 97% pure (mole fraction), but contained small amounts of the hexabromodiphenyl ethers BDE-153 and BDE-154, which are also found in the commercial Penta-product. After 7 days, the BDE-99 was almost completely debrominated. The daughter products were primarily three tetrabrominated congeners, BDE-47, BDE-66, and BDE-49 (2,2',4,5'-), which converged by debromination to primarily two tribrominated congeners, BDE-28 (2,4,4'-) and BDE-17 (2,2',4-), and a very small amount of BDE-37 (3,4,4'-). In the first few hours of the experiment, before the appearance of any tribromodiphenyl ethers, BDE-47, BDE-66, and BDE-49 appeared in a mole ratio of about 5:4:1, respectively. Subsequently, BDE-17, BDE-28, and BDE-37 appeared in a mole ratio of about 3:2:0.15, respectively. Of the six tetra- and tri-BDE product congeners appearing from BDE-99, four were double para-substituted and accounted for more than 79% of the total.

Reductive Debromination of BDE-47 in a Biomimetic System

The BDE-47 starting material used in the biomimetic experiment was greater than 99% pure. After 15 days, more than 80% of the initial BDE-47 remained, with about 12% being transformed to BDE-28 and slightly less than 5% to BDE-17. Since BDE-28 is the dominant product of vitamin B12- mediated BDE-47 debromination, we can conclude that a significant portion of the BDE-17 appearing in the BDE-99 experiment goes through the BDE-49 intermediate. The results indicate that BDE-47 has the lowest reactivity for debromination when compared to BDE-99 and BDE-209. The BDE-209 debromination half-life was 18 seconds, compared to a half-life of about 19.9 hours for BDE-99, and almost 58 days for BDE-47 in the biomimetic system. Clearly, the PBDE debromination rate was proportional to the number of bromine substitutions, proving our first hypothesis. These results demonstrate that BDE-47 is the dominant first debromination product of BDE-99, but BDE-47 is not the only product, nor is it the terminal product.

Conclusions

In sediment microcosms after approximately 3.5 years of anaerobic incubation at room temperature (22°C), the mole fraction of BDE-209 decreased by more than 12%, with a corresponding increase in nona-, octa-, hepta-, and hexa-PBDEs. The calculated laboratory half-life of BDE-209 was well over a decade, and was in good agreement with the predicted value obtained from the biomimetic experiment. Nine new PBDE congeners appeared in sediment microcosms as products of reductive debromination, but the terminal debromination products of BDE-209 are as yet unknown. We can conclude with certainty that BDE-209 can undergo sequential anaerobic reductive dehalogenation to less brominated congeners. It is important to determine whether this transformation is a significant source of lesser brominated PBDEs to the environment. The finding that BDE-99 also undergoes slow debromination to tetra- and tribromodiphenyl ether congeners in sediment strongly indicates that anaerobic reductive debromination of BDE-209 could be responsible for some portion of the flux of lower molecular weight PBDEs into the biosphere. The biomimetic experiments clearly demonstrate that BDE-209, BDE-99, and to some extent BDE-47 are highly amenable to reductive debromination transformation. However, in aquatic environments there are competing influences of hydrophobicity and reactivity, and therefore, significant retardation in the rate of debromination.


Journal Articles on this Report : 3 Displayed | Download in RIS Format

Other project views: All 6 publications 3 publications in selected types All 3 journal articles
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Journal Article Ahn M-Y, Filley TR, Jafvert CT, Nies L, Hua I, Bezares-Cruz J. Photodegradation of decabromodiphenyl ether adsorbed onto clay minerals, metal oxides, and sediment. Environmental Science & Technology 2006;40(1):215-220. R830251 (Final)
R830398 (Final)
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  • Journal Article Ahn M-Y, Filley TR, Jafvert CT, Nies L, Hua I. Birnessite mediated debromination of decabromodiphenyl ether. Chemosphere 2006;64(11):1801-1807. R830251 (Final)
    R830398 (Final)
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  • Journal Article Tokarz III JA, Ahn M-Y, Leng J , Filley TR, Nies L. Reductive debromination of polybrominated diphenyl ethers in anaerobic sediment and a biomimetic system. Environmental Science & Technology 2008;42(4):1157-1164. R830251 (Final)
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  • Supplemental Keywords:

    environmental fate, brominated flame retardants, decabromodiphenyl ether, biomimetic, environmental chemistry,, RFA, Scientific Discipline, Waste, Water, Ecosystem Protection/Environmental Exposure & Risk, Contaminated Sediments, Environmental Chemistry, Restoration, Environmental Monitoring, Ecology and Ecosystems, Aquatic Ecosystem Restoration, Exp. Research/future, Futures, reducitve debromination, reductive debromination, biodiversity, contaminated sediment, flame retardants, PBDE, conservation, contaminant uptake, ecological pollutants, exploratory research, remediation, environmental rehabilitation, environmental stress, ecotoxicology, anaerobic biotransformation

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    Original Abstract
  • 2003 Progress Report
  • 2004
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  • 2006