Alternate Reductants with VB12 to Transform C8 and C6 Perfluoroalkyl Sulfonates: Limitations and Insights into Isomer-Specific Transformation Rates, Products and Pathways

摘要

Previous studies evaluating Vitamin B12 (VB12) with Ti(m)-citrate for potential use in in situ remediation of perfluorooctanesulfonate (PFOS) found that linear (L)-PFOS was unaltered. We explored if alternate reductants could overcome this limitation with a primary focus on nanoscale zerovalent zinc (nZn°). Transformation over time with VB12-nZn° was quantified at 22, 70, and 90 ℃ for PFOS, at 70 ℃ for perfluorohexanesulfonate (PFHxS), and VB12-nFe~0 and VB12-Pd~0Fe~0 at 70 ℃ for PFOS. Only branched (br-) isomers were transformed generating F~- (no SO_4~(2-)) and polyfluoroalkyl intermediates/products. The absence of L-PFOS transformation by VB12 appears to be due to the inability of L-perfluoroalkyl sulfonates to complex with VB12 and not an activation energy issue that can be overcome by stronger reductants/catalysts. At 90 ℃, 95% of br-PFOS isomers were transformed within 5 days. Isomer-specific removal rates were positively correlated to the br-CF_3's proximity to the terminal CF_3. Br-PFHxS transformation was approximately two times slower with less defluorination than br-PFOS. C8/C7 and C6/C5 polyfluorinated sulfonates from br-PFOS and br-PFHxS, respectively, were identified as both intermediates and apparent dead-end products. Pathways included 4 F replaced by 2 H and a C=C bond, and serial F replacement by H with up to 12 F atoms removed from br-PFOS.