Hydrophobic interaction is the dominant mechanism of zwitterionic PFAS adsorption to carbon-based sorptive materials in water and soil

Abstract

The diversity, persistence, bioaccumulation potential and mobility of per- or polyfluoroalkyl substances (PFASs) make these contaminants particularly formidable when determining their environmental fate and behaviour. While most work has been on anionic PFASs, recent findings indicated that PFASs found in highly contaminated soils are more diverse in charge and functional groups than previously realised, and cationic and zwitterionic PFASs make up a substantial proportion of PFASs found at these sites. This complexity presents challenges with respect to their remediation, which has mostly targeted anionic PFASs. Remediation using sorbents, for example, may need mixed modes for PFAS removal from solution and soil. Graphene-based materials (GBMs) with a range of functionalities and two commercially available activated carbon-based sorbents were used to remediate selected zwitterionic and anionic PFASs with similar carbon chain lengths from contaminated solution and soil. As for anionic PFASs, the activated carbon-based sorbents showed greater sorption of PFASs compared to the GBMs from both solution and soil. Among the GBMs, graphene showed greater sorption of anionic and zwitterionic PFASs due to its greater hydrophobic interactions. The sorption of PFASs was weaker for GBMs that were less hydrophobic due to the presence of functional groups. Thus, hydrophobic interactions appeared to be the most important mechanisms of sorption for zwitterionic PFASs. While PFAS sorption by activated carbon-based materials was the same in both solution and soil, the efficiency of all GBMs was less in soil. Adding a combination of the GBMs to contaminated soil did not improve PFAS immobilisation in soil likely due to hydrophobic interactions being more important than other mechanisms (i.e., electrostatic) for immobilisation of zwitterionic and anionic PFASs. However, mixed modes of remediation should not be dismissed as a holistic approach to contaminated site remediation.