Brominated Biphenyls Prime Extensive Microbial Reductive Dehalogenation of Aroclor 1260 in Housatonic River Sediment

Abstract

The upper Housatonic River and Woods Pond (Lenox, Mass.), a shallow impoundment on the river, are contaminated with polychlorinated biphenyls (PCBs), the residue of partially dechlorinated Aroclor 1260. Certain PCB congeners have the ability to activate or "prime" anaerobic microorganisms in Woods Pond sediment to reductively dehalogenate the Aroclor 1260 residue. We proposed that brominated biphenyls might have the same effect and tested the priming activities of 14 mono-, di-, and tribrominated biphenyls (350 µM) in anaerobic microcosms of sediment from Woods Pond. All of the brominated biphenyls were completely dehalogenated to biphenyl, and 13 of them primed PCB dechlorination. Measured in terms of chlorine removal and decrease in the proportion of hexa- through nonachlorobiphenyls, the microbial PCB dechlorination primed by several brominated biphenyls was nearly twice as effective as that primed by chlorinated biphenyls. Congeners containing a meta bromine primed Dechlorination Process N (flanked meta dechlorination), and congeners containing an unflanked para bromine primed Dechlorination Process P (flanked para dechlorination). Two ortho-substituted congeners, 2-bromobiphenyl and 2,6-dibromobiphenyl (2-BB and 26-BB), also primed Process N dechlorination. The most effective primers were 26-BB, 245-BB, 25-3-BB, and 25-4-BB. The microbial dechlorination primed by 26-BB converted ~75% of the hexa- through nonachlorobiphenyls to tri- and tetrachlorobiphenyls in 100 days and removed ~75% of the PCBs that are most persistent in humans. These results represent a major step toward identifying an effective method for accelerating PCB dechlorination in situ. The challenge now is to identify naturally occurring compounds that are safe and effective primers.

FIG. 1

Structure of halogenated biphenyls, showing the numbering scheme (A) and the dechlorination of a major heptachlorobiphenyl by Process N, Processes N plus LP, and Process P (B). Each chlorinated or brominated biphenyl congener can have from 1 to 10 halogens (chlorines or bromines) at the positions shown in panel A. The halogens at positions 2 and 6 on either ring are designated ortho, those at carbons 3 and 5 are designated meta, and those at carbon 4 are designated para. Panel B shows one of the major components of Aroclor 1260 and the terminal dechlorination products produced from this congener by Process N, Processes N plus LP, and Process P. The most extensive chlorine removal results from sequential dechlorination by Processes N and LP (27).

FIG. 2

Time course of dechlorination of Aroclor 1260 primed by 26-BB (450 μM) in Woods Pond sediment. (A) Change in the total hexa- through nonachlorobiphenyls (Hexa-Nona-CBs) as a function of time. Symbols: □, live controls; ▪, 26-BB-primed samples. (B) Change in PCB homolog distribution in 26-BB-primed samples as a function of time. Symbols: ⧫, nonachlorobiphenyls; ▪, octachlorobiphenyls; ▴, heptachlorobiphenyls; •, hexachlorobiphenyls; ▵, pentachlorobiphenyls; ○, tetrachlorobiphenyls; □, trichlorobiphenyls. The data shown are averages for triplicate samples ± standard deviations (shown as error bars); if no bar is evident, the deviation was smaller than the size of the symbol.

FIG. 3

PCB congener distribution of the Aroclor 1260 residue in Woods Pond sediment after 100 days in live controls (A) and in samples primed with 26-BB (450 μM) (B). The PCB congener designations indicate the positions of the chlorine atoms on each phenyl ring, and the hyphen represents separation of the rings. The data shown are averages for triplicate samples.

FIG. 4

Absolute differences in the PCB congener distributions of the Aroclor 1260 residues (controls minus experimental samples) as a result of priming with 26-BB (450 μM) (A) and priming with 4-4-BB (350 μM) (B). The PCB congener designations indicate the positions of the chlorine atoms on each phenyl ring, and the hyphen represents separation of the rings. The data shown are averages for duplicate samples primed with 4-4-BB (obtained at 120 days) and for triplicate samples primed with 26-BB (obtained at 100 days). The scale is the same for both panels.

FIG. 5

Effect of the concentration of 26-BB used as primer on the time course and extent of dechlorination. The samples contained 26-BB at the following concentrations: 0 μM (live control) (□), 50 μM (▪), 100 μM (▵), 200 μM (▴), 500 μM (○), and 1,000 μM (•). No malate was added. The data shown are means for duplicate samples. Hexa- through Nona-CBs, hexa-through nonachlorobiphenyls.