Microbially mediated transport and temporal community responses of 6:2 fluorotelomer sulfonate under variably saturated conditions.

Understanding of the transport and retention of perfluoroalkyl substances (PFAS) precursors in the unsaturated zone remains limited, particularly regarding the coupled effects of hydrological variability and microbial processes under variably saturated conditions. This study investigated the transport behavior of 6:2 fluorotelomer sulfonate (6:2 FTS), a representative PFAS precursor, through variably saturated columns packed with sand or soil. Quantification of retention was combined with solid-phase characterization and microbial community sequencing. An instrumented apparatus was used to collect in-situ samples and to continuously monitor water saturation (Sw) during the experiments. The results demonstrated that 6:2 FTS transport was co-regulated by porous media type, Sw, and microbial colonization. Retention was stronger in agricultural soil than in quartz sand and decreased with depth as increasing Sw reduced air-water interfacial area. Extracellular polymeric substances (EPS) introduced a rate-limited retention domain that operated alongside rapid air-water interfacial adsorption, which enhanced overall retention and produced nonideal transport behavior, including dual-plateau breakthrough curves. Greater EPS abundance and spatial heterogeneity in soil accounted for stronger and more prolonged retention despite higher Sw. Microbial community structure was shaped primarily by porous media type, followed by 6:2 FTS exposure. Co-occurrence network and biomarker analyses further identified media-specific and 6:2 FTS-tolerant genera. Overall, this study provides mechanistic evidence that microbial processes can substantially modulate hydrologically controlled PFAS transport in the vadose zone. Incorporating such coupled biophysical processes is therefore essential for accurate PFAS fate prediction and risk assessment in subsurface environments.