In situ sequestration of per- and polyfluoroalkyl substances in aquifer materials using polydiallyldimethyl ammonium chloride-stabilized powdered activated carbon

Abstract

The widespread use of aqueous film-forming foams (AFFF) for firefighting and fire fighter training activities has led to groundwater contamination with per- and polyfluoroalkyl substances (PFAS). PFAS can be effectively sorbed onto powdered activated carbon (PAC) and the aqueous cationic polymer, polydiallyldimethyl ammonium chloride (PDM). These sorbents form a stable suspension (S-PAC) that can be injected into the subsurface to create a permeable adsorptive barrier (PAB), providing the basis for field-scale in situ PFAS sequestration. A series of bench-scale one-dimensional column experiments were performed to assess the transport and sorption of PFAS in aquifer materials from two field sites with PFAS-contaminated groundwater. Experiments included testing the effect of pre-treatment with PDM and sequential injections of individual PFAS and mixtures. In all experiments, S-PAC enhanced PFAS sorption on site media from > 2-fold (e.g. perfluorohexanoic acid) to > 100-fold (e.g. perfluorooctane sulfonic acid) depending on headgroup, chain length, and influent concentration. Pretreatment of influent with PDM alone increased total PFAS sorption for compounds with sulfonic acid and sulfonamido headgroups by up to ∼2-fold relative to S-PAC treatment without PDM pretreatment. Results also demonstrated competition for sorption sites with long-chain PFAS displacing shorter chain length PFAS from the S-PAC, an effect that can potentially be addressed by expanding the treatment zone. S-PAC is a viable treatment for in situ sequestration of PFAS and upstream injection of PDM may enhance removal. Competitive displacement by more strongly sorbed PFAS should be a design consideration when implementing this technology in the field.