Groundwater solutes influence the adsorption of short-chain perfluoroalkyl acids (PFAA) to colloidal activated carbon and impact performance for in situ groundwater remediation

Subsurface injection of colloidal activated carbon (CAC) is an in situ remediation strategy for perfluoroalkyl acids (PFAA), but the influence of groundwater solutes on longevity is uncertain, particularly for short-chain PFAA. We quantify the impact of inorganic anions, dissolved organic matter (DOM), and stabilizing polymer on PFAA adsorption to a commercial CAC. Surface characterization supported PFAA chain-length dependent adsorption results and mechanisms are provided. Inorganic anions decreased adsorption for short-chain PFAA (<7 perfluorinated carbons) due to competitive effects, while long-chain PFAA (≥ 7 perfluorinated carbons) were less impacted. DOM decreased adsorption of all PFAA in a chain-length dependent manner. High DOM concentrations (10mg/L, ~5mg OC/L) decreased PFOA adsorption by a factor of 2, PFPeA by one order of magnitude, and completely hindered PFBA adsorption. High MW DOM has less impact on short-chain PFAA than low MW DOM, possibly due to differences in the ability to access CAC micropores. Low DOM concentrations (1mg/L, ~0.5mg OC/L) did not impact adsorption. CMC (90kDa average MW) had negligible impact on PFAA adsorption likely due to minimal CAC surface coverage. Longevity modeling demonstrated that groundwater solutes limit the capacity for PFAA in a CAC barrier, particularly for short-chain PFAA. Environmental Implication Subsurface injection of colloidal activated carbon (CAC) is an emerging in situ approach to treat perfluoroalkyl acid (PFAA)-impacted groundwater. However, long-term effectiveness is uncertain for short-chain PFAA and for a range of water chemistry scenarios. The impact of inorganic ions, dissolved organic matter (DOM), and CAC delivery polymer on the adsorption of long- and short-chain PFAA were measured experimentally. Our results and modeling indicates that these water quality variables significantly lower retention and expected performance of short-chain PFAA in CAC barriers. Quantitative data will support more accurate longevity assessments and future barrier design.