Per- and polyfluoroalkyl substances (PFAS) and alternatives: structure, characterization, and degradation

Abstract

Since their synthesis and commercialization over seven decades ago, per- and polyfluoroalkyl substances (PFAS) have notoriously become known as worldwide contaminants due to their toxicity, persistence, and bioaccumulation. Thus, it is paramount to accurately identify these compounds in environmental samples for effective remediation, as well as implement alternatives which are less recalcitrant and toxic. The resistance of PFAS degradation is a result of their chemical and thermal stability, which may be a result of their unique helical conformations. To understand if PFAS robustness can be attributed to their structures, the relationship between helicity and fundamental molecular properties of a class of four-carbon polyfluoroalkanes is explored. Additionally, helicity is not significantly impacted by polar head groups but is broken with radicals located towards the center of the molecule, which may have implications in facilitating degradation. The functional diversity within the PFAS family also creates unique challenges in identification and remediation. 19F NMR spectroscopy has recently become a suitable technique to characterize PFAS in concentrated samples. Thus, this work first develops a computational protocol to accurately predict chemical shifts (within 2-4 ppm) specifically for perfluoroalkanes. This method is further applied to environmentally relevant short chain PFAS and shows that implicit solvation may be necessary for accurate predictions. Trimethylsiloxane (TriSil) surfactants are a possible fluorine-free replacement to PFAS as fire suppressants. To evaluate their environmental impact and support including TriSil in foam formulations, potential degradation products of a truncated TriSil in high temperature and aqueous (hydrolysis, reduction, and oxidation) conditions are determined. All degradation products are small organics and PDMS-like products, which are relatively safer than those of PFAS. The work presented herein uses density functional theory methods to study the structure of PFAS, its connection to properties, characterization by 19F NMR, and additionally identify prominent degradation pathways and products of TriSil.