Transformation and defluorination by nNiFe-activated carbon nanocomposites: PFAS structure and matrix effects

Abstract

The ability of nNiFe-activated carbon (AC) to transform a variety of per- and poly-fluoroalkyl substances (PFAS) in single solute, synthetic mixtures, and water types impacted by aqueous film forming foam (AFFFs) was evaluated in different matrices for 1-d, 3-d or 5-d at 50 °C and/or 60 °C. Perfluoroalkyl sulfonates (PFSAs) defluorinated the most and shorter chain perfluoroalkyl acids (PFAAs) transforming less and more slowly. Except for GenX (least transformed), low levels of perfluoroalkyl carboxylates (PFCAs) and some polyfluoroalkyl sulfonates/carboxylates were generated from all PFAS tested. C-S cleavage was evident for both PFSAs and fluorotelomer sulfonates (FTSs). Also, FTS transformation generated some fluorotelomer carboxylic acids and PFCAs. Temperature effects (50 °C versus 60 °C) were similar on transformation and defluorination except for PFOS (x2 higher at 50 °C). Transformation in single solute and PFAA mixtures in deionized water were similar but decreased in 30 mg/L NaCl or 10 mM bicarbonate buffer (pH ~ 7.5). For AFFF-impacted waters, which included a high salt fire-training (FT) pit water and Cape Cod groundwater sample, precursor presence complicated comparisons between different PFAS structures. However, for PFSAs, which are unlikely to be generated from precursor transformation by nNiFe-AC, transformation in both AFFF-impacted waters were lower than observed in deionized water. Shorter chain PFCAs and some saturated fluorotelomer carboxylates were the major products of precursor transformation in FT pit water. Exploration in the component-specific role in nNiFe-AC nanocomposites showed that AC indirectly enhances reactivity by preventing nanoparticle aggregation and increasing proximity to reactive surfaces through sorption.