Poly-/Perfluoroalkyl Substance (PFAS) Immobilization Using a Novel Adsorptive Organoclay Compared with Colloidal-Activated Carbon in Soil under Dynamic Redox Conditions and the Response of Microbial Communities

Poly-/perfluoroalkyl substance (PFAS) contamination is a significant global environmental concern and requires effective remediation solutions. Although it is often assumed that PFAS sorption is not redox sensitive, redox fluctuations can significantly modify the biogeochemical environment, influencing PFAS behavior indirectly. This study evaluated the effectiveness of soil amendments, a novel adsorptive organoclay (AOC) and a colloidal-activated carbon (CAC) on PFAS immobilization under dynamic redox conditions. PFAS-contaminated soil was treated with AOC and CAC. We created a slurry, set systematically five predefined redox windows (from -100 to +500 mV), and collected samples at each redox window. PFAS concentrations were measured in the dissolved phase (<0.45 μm), and phospholipid fatty acid (PLFA) was analyzed in the solid phase (>0.45 μm). The contaminated soil contained 13 PFAS compounds. CAC treatment drastically reduced the dissolved PFAS concentrations across all redox windows compared to the control and AOC treatment. Perfluorooctanoic acid (PFOA), perfluorooctanesulfonate (PFOS), perfluorononanoic acid (PFNA), and 6:2 fluorotelomer sulfonamide propyl betaine (6:2 FTAB) dominated in the dissolved phase of control and AOC treatment. The dissolved phase of CAC mainly contained shorter-chain perfluoropentanoic acid (PFPeA) and perfluorobutanoic acid (PFBA). The specific surface area (SSA) along with total carbon (TC) content of amendments are key factors in reducing PFAS concentration in the dissolved phase. Relatively similar PLFA profiles indicated no adverse effects on soil microbial community from either amendment compared to the control. This study highlights CAC as an effective amendment for PFAS immobilization even under changing environmental redox conditions.