Linear free energy relationship based estimates for the congener specific relative reductive defluorination rates of perfluorinated alkyl compounds

Abstract

Linear free energy relationships (LFERs) were developed to estimate the congener specific relative rates of reductive defluorination for a suite of perfluorinated compound (PFC) classes. The LFERs were based on the semiempirically calculated lowest unoccupied molecular orbital energy (E_LUMO) using gas and aqueous phase computations with the PM6 and RM1 methods. PFC classes in the modeling effort included the C₁ through C₈ perfluoroalkyl sulfonates (PFSAs), carboxylates (PFCAs), sulfonyl fluorides (PFSFs), sulfonamides and their derivatives (SAs), and the perfluorotelomer alcohols (PFTAls), olefins (PFTOls), and acids (PFTAcs). Gas and aqueous phase calculations using the PM6 method predict that branched PFSA, PFCA, and PFSF congeners will have more rapid reductive defluorination kinetics than their linear counterparts. The RM1 method predicts that only PFSFs will display intra-homologue dependent branching effects. For the PFSAs and PFSFs, both the PM6 and RM1 methods predict no significant difference in mean rates of reductive defluorination between the homologue groups. For the PFCAs, the PM6 method suggests no significant difference in inter-homologue mean rates of reductive defluorination, whereas the RM1 method predicts a significant increase with a lengthening perfluoroalkyl chain. All approaches used suggest that the intrahomologue variability in reduction rates increases with increasing chain length for PFSAs, PFCAs, and PFSFs, implying that the larger homologue groups in these classes will see a more rapid linearization of the congener profiles under reducing conditions than their lower homologue counterparts. Chain length has a negligible effect on the estimated rates of SA reductive defluorination, but a significant role for the fluorotelomer derivatives. Ratios of rates between the C₈:C₁ straight chain telomeric congeners are expected to range up to 200-fold depending on the computational combination. The kinetics for reductively defluorinating PFC starting materials will likely be 2 to 3 orders of magnitude more rapid than for most of the partially defluorinated degradation products. Significant quantities of partially defluorinated PFCs are thus expected to be observed under steady state conditions during reductive treatment processes, leading to a potentially significant reservoir of these compounds residing in reducing environmental and biological systems.

Keywords:

• Reduction
• perfluorinated compounds
• defluorination
• congener specific
• perfluoroalkyl sulfonates
• perfluoroalkyl carboxylates
• perfluoroalkyl sulfonyl fluorides
• perfluoroalkyl sulfonamides
• perfluoroalkyl telomers

Acknowledgment

S.R. thanks the Natural Sciences and Engineering Research Council (NSERC) of Canada for financial support.