Biomimetic degradability of linear perfluorooctanesulfonate (L-PFOS): Degradation products and pathways

The reductive degradability and decomposition pathways of linear perfluorooctanesulfonate (L-PFOS) were investigated in a biomimetic system consisting of Ti(III)-citrate and Vitamin B₁₂. Biomimetic degradation of L-PFOS could well be described by a first-order exponential decay model. Accompanied by the release of fluoride ion, technical PFOS could not only be transformed to perfluorocarboxylates (PFCAs) and perfluoroalkylsulfonates (PFSAs) with perfluoroalkyl carbon chain length < C8 (thereafter referred as carbon-chain-shortened degradation products), but also be transformed to PFCAs with perfluoroalkyl carbon chain length ≥ C8 (thereafter referred as carbon-chain-lengthened degradation products). Perfluorohexanesulfonate and perfluorotetradecanoate were the most abundant carbon-chain-shortened and -lengthened degradation products of technical PFOS, respectively. Based on the various degradation products detected during biomimetic reduction of linear [1,2,3,4-¹³C₄]-PFOS, the degradation pathways of L-PFOS were proposed as follows: L-PFOS was first reduced to C₈F₁₇• radical by cleavage of C-S bond, and then transformed to PFOA through hydrolysis. However, the carbon-chain-shortened products were not generated through the sequential chain-shortening via C₈F₁₇• radicals and/or L-PFOS, while the carbon-chain-lengthened products were not formed via C₈F₁₇• radicals by stepwise addition of CF₂ moiety. In fact, C₈F₁₇• radical and/or L-PFOS were further reduced to form C_nF_2n+1• (n = 1, 2, 3, 4) radicals, and these radicals were chain-lengthened by stepwise addition of C₄F₈ moiety and eventually transformed to various degradation products via hydrolysis (PFCAs) or combination reaction with sulfonyl hydroxide (PFSAs). All carbon-chain-lengthened chemicals were first reported as the degradation products during the decomposition of L-PFOS, while carbon-chain-shortened compounds were first identified as the biomimetic reduction products of L-PFOS.