Concurrent interactions between PFAS and bacteria during 6:2 fluorotelomer sulfonate transport in saturated and unsaturated porous media.

The role of microbial communities in regulating the transport and retention of per- and polyfluoroalkyl substances (PFAS) in the subsurface environment, particularly under water-unsaturated conditions, remains poorly understood. Here, miscible-displacement experiments were conducted with saturated and unsaturated sand-packed columns to investigate the influence of biofilms on the transport of 6:2 fluorotelomer sulfonate (6:2 FTS), considering biofilm age, bacterial species, and flow rate. The underlying mechanisms were further elucidated through characterization of extracellular polymeric substances (EPS), zeta potential, and cell surface hydrophobicity, while microbial community responses to 6:2 FTS exposure were also evaluated. Compared with clean sand, biofilm-coated sand enhanced 6:2 FTS retention under both saturated and unsaturated conditions. Under saturated conditions, retention differences among bacterial species were primarily governed by EPS content and protein-to-polysaccharide ratios. In contrast, species-level effects became less pronounced under unsaturated conditions, indicating a shift from species dependent controls to coupled biofilm interface processes. Quantitative analysis of these specific experiments showed that biofilms-associated processes contributed 42% of the additional apparent retention observed upon transition from saturated to unsaturated conditions, compared with 58% attributed to air-water-interfacial adsorption. These results demonstrate that biofilm-associated processes constitute an important and previously overlooked retention domain contributing substantially to the enhanced retention of 6:2 FTS under unsaturated conditions. Biofilm maturation further increased retention through EPS accumulation and associated changes in surface physicochemical properties, including surface potential, roughness, and hydrophobicity. Moreover, 6:2 FTS exposure altered bacterial community composition, suggesting reciprocal interactions between PFAS transport and microbial ecology. These findings provide new mechanistic insights into PFAS fate in the subsurface and improve the basis for predicting PFAS transport and designing remediation strategies for PFAS-impacted sites.