Photochemical mass balance of 2,3,7,8-TeCDD in aqueous solution under UV light shows formation of chlorinated dihydroxybiphenyls, phenoxyphenols, and dechlorination products.

Polychlorinated dibenzo-p-dioxins (PCDDs) are a class of halogenated diaryl compounds that are environmentally important because of their high toxicity and bioaccumulatory properties. There is an incomplete understanding of their photochemistry because the majority of photoproducts, as indicated by incomplete mass balances, have not been identified. We studied the photochemical transformation of 2,3,7,8-tetrachlorodibenzo-p-dioxin (2,3,7,8-TeCDD) in aqueous solution using 302 nm light. Our results allow for the first photochemical mass balance (92-99%) for this compound and confirm the operation of a novel photochemical pathway, which gives rise to 2,2'-dihydroxy-4,4',5,5'-tetrachlorobiphenyl (4,4',5,5'-TeCDHBP) at >50% conversion from the starting material. Rearrangement from 2,3,7,8-TeCDD to 4,4',5,5'-TeCDHBP takes place following preferential homolytic C-O bond cleavage via a proposed mechanism analogous to the parent dibenzo-p-dioxin system. Photochemical conversion from the starting material to dechlorinated PCDDs or chlorinated phenoxyphenols are minor pathways, although the exact contribution of the strictly dechlorination pathways remains uncertain because of the complexity of the system. The results suggest that the photochemical conversion of PCDDs to chlorinated dihydroxybiphenyls, which are also polychlorinated biphenyl (PCB) metabolites, may be an important part of their environmental fate.