A review of state per‐ and polyfluoroalkyl substances (PFAS) guidelines indicates that four long‐chain PFAS (perfluorooctanesulfonic acid [PFOS] and perfluorooctanoic acid [PFOA] followed by perfluorohexanesulfonic acid [PFHxS] and perfluorononanoic acid [PFNA]) are the most frequently regulated PFAS compounds. Analysis of 17 field‐scale studies of colloidal activated carbon (CAC) injection at PFAS sites indicates that in situ CAC injection has been generally successful for both short‐ and long‐chain PFAS in the short‐term (0.3–6 years), even in the presence of low levels of organic co‐contaminants. Freundlich isotherms were determined under competitive sorption conditions using a groundwater sample from an aqueous film‐forming foam (AFFF)‐impacted site. The median concentrations for these PFAS of interest at 96 AFFF‐impacted sites were used to estimate influent concentrations for a CAC longevity model sensitivity analysis. CAC longevity estimates were shown to be insensitive to a wide range of potential cleanup criteria based on modeled conditions. PFOS had the greatest longevity even though PFOS is present at higher concentrations than the other species because the CAC sorption affinity for PFOS is considerably higher than PFOA and PFHxS. Longevity estimates were directly proportional to the CAC fraction in soil and the Freundlich Kf, and were inversely proportional to the influent concentration and average groundwater velocity.
Groundwater contamination by per-and polyfluoroalkyl substances (PFAS) is a global problem, and current treatment methods are expensive and/or inefficient. Attenuation of PFAS plume by subsurface Colloidal Activated Carbon (CAC) sorptive barrier is a potential interim alternative. However, competitive adsorption can impact the longevity of CAC barriers. Competitive adsorption is predicted using models such as ideal adsorbed solution theory (IAST) and competitive Langmuir model (CLM). This study tested the existing competitive adsorption models and developed a new model, Modified CLM (MCLM), to predict the competitive adsorption equilibria of PFAS on CAC. The findings indicate that neither IAST nor CLM accurately predicts the competitive adsorption equilibria, but MCLM performs best and suggests a relationship between PFAS competitive adsorption and their molecular weights. A 1D transport model with CLM was also developed which was able to simulate the chromatographic effect and the gradual breakthrough curve of long-chained PFAS observed in the literature.
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