No abstract

This study investigates structure−reactivity relationships within branched per-and polyfluoroalkyl substances (PFASs) undergoing cobalt-catalyzed reductive defluorination reactions. Experimental results and theoretical calculations reveal correlations among the extent of PFAS defluorination, the local C−F bonding environment, and calculated bond dissociation energies (BDEs). In general, BDEs increase in the following order: tertiary C−F bonds < secondary C−F bonds < primary C−F bonds. A tertiary C−F bond adjacent to three fluorinated carbons (or two fluorinated carbons and one carboxyl group) has a relatively low BDE that permits an initial defluorination to occur. Both a biogenic cobalt−corrin complex (B 12 ) and an artificial cobalt−porphyrin complex (Co-PP) are found to catalytically defluorinate multiple C−F bonds in selected PFASs. In general, Co-PP exhibits an initial rate of defluorination that is higher than that of B 12 . Neither complex induced significant defluorination in linear perfluorooctanoic acid (PFOA; no tertiary C−F bond) or a perfluoroalkyl ether carboxylic acid (tertiary C−F BDEs too high). These results open new lines of research, including (1) designing branched PFASs and cobalt complexes that promote complete defluorination of PFASs in natural and engineered systems and (2) evaluating potential impacts of branched PFASs in biological systems where B 12 is present.

The ubiquitous occurrence of organophosphate flame retardants (OPFRs) in aquatic and soil environments poses significant risks to human health and ecosystems. Here, we report on the hydrolysis of six OPFRs and three structural analogues in the absence and presence of metal (hydr)oxide minerals. Eight of the target compounds showed marked degradation in alkaline solutions (pH 9-12) with half-lives ranging from 0.02-170 days. Kinetics follow a second-order rate law with apparent rate constants for base-catalyzed hydrolysis (k) ranging from 0.69-42 000 M d. Although hydrolysis in homogeneous solution at circumneutral pH is exceedingly slow (t > 2 years, except for tris(2,2,2-trichloroethy) phosphate), rapid degradation is observed in the presence of metal (hydr)oxide minerals, with half-lives reduced to <10 days for most of the target OPFRs in mineral suspensions (15 m/L mineral surface area loading). LC-qToF-MS analysis of transformation products confirmed ester hydrolysis as the active degradation pathway. Values of k for individual OPFRs are highly variable and correlate with acid dissociation constants (pK) of the corresponding alcohol leaving groups. In contrast, kinetic parameters for mineral-catalyzed reactions are much less sensitive to OPFR structure, indicating that other factors like mineral-OPFR interactions are rate controlling. Given the documented recalcitrance of OPFRs to biodegradation and photodegradation, these results suggest that mineral-catalyzed hydrolysis may be a major fate-controlling sink in natural environments.

Despite benefits to the firefighting industry, the release of per-and polyfluoroalkyl substances (PFASs) from aqueous film-forming foam (AFFF) into aquatic systems poses significant risks to human health and other organisms. While anion-exchange technologies have proven to be effective for removing perfluoroalkyl acids (PFAAs) from water, their effectiveness for removing the diverse PFAS structures discovered in AFFF remains unknown. Here, we report on the adsorption of 75 PFASs, including 63 polyfluorinated substances, in a diluted AFFF mixture using 14 commercially available ion-exchange (IX)/nonionic resins and granular activated carbon (GAC). Results showed that anion-exchange resins (AERs) exhibited significant adsorption of PFASs compared to cation-exchange resins (CERs), nonionic resins (NIRs), and GAC regardless of the PFAS's predicted charge. Isotherm data showed that macroporous AERs have a higher PFAS adsorption capacity compared to gel-type AERs. Cross-correlation comparison of PFAS/Cl − selectivity coefficients (K ex ) for each PFAS−AER combination showed that the hydrophobicity of the AER functional group, and polymer matrix played a dominant role in determining resin affinity for PFASs. PFAS structural characteristics also significantly affected adsorption, with increasing chain length and a net negative charge increasing the extent of adsorption. Results from this study provide guidelines for the selection of resins to adsorb a wider range of PFASs and meaningful insights for the development of quantitative models for IX treatment of AFFF-impacted water.

No abstract