Anaerobic transformation of polybrominated biphenyls with the goal of identifying unknown hexabromobiphenyls in Baltic cod liver

Polybrominated biphenyls (PBBs) have been introduced as flame retardants in 1970. Despite decreasing application rates since the mid-1970s, PBB residues are still reported in the environment. Furthermore, environmental PBB residues often do not match the PBB pattern in technical products. To get insights into the structures of environmentally-relevant PBBs, the congener patterns of technical hexabromobiphenyl (THBB), octabromobiphenyl (TOBB), synthesized PBB 209 were compared to PBB residues in a cod liver sample from the Baltic Sea. The most relevant PBB congeners in Baltic cod liver were not present in the technical products and therefore most likely metabolites. For this reason, TOBB and HPLC-fractions obtained from this technical product were incubated with super-reduced cyanocobalamine. Reductive debromination was found to be the predominant transformation process. Bromine substituents in ortho-positions proved to be more recalcitrant, and several of the unknown PBBs were tri- and tetra-ortho substituted congeners. Furthermore, the key-PBBs determined in Baltic cod liver were formed during this process. The most important hexabromobiphenyl in Baltic cod liver was identified as PBB 155 by parallel synthesis. PBB 155 which was not detected in the technical PBB product analyzed was suggested as an indicator PBB congener suited to decide whether PBB residues originate from the previous use of THBB (low relative abundance of PBB 155) or TOBB/TDBB (high relative abundance of PBB 155). The latter scenario was found to be valid for the Baltic cod liver sample. Thus, PBBs in the Baltic Sea appeared to originate from partially-weathered residues of PBB 209.