Effect of long-chain PFAS on the migration of short-chain PFAS: Adsorption mechanism and molecular-level analysis

The short-chain perfluoroalkyl and polyfluoroalkyl substances (PFAS) are produced and used in large quantities as substitutes for long-chain PFAS. Short-chain PFAS have strong migration capabilities due to their good water solubility and weak adsorption ability, posing increased risks to global groundwater safety. This study investigated the migration characteristics of short-chain PFAS in saturated porous media, their migration characteristics when coexisting with long-chain PFAS, and competitive effects. As shown in transport experiments, perfluorobutane sulfonate (PFBS) had stronger migration than perfluorobutyric acid (PFBA). Additionally, short-chain PFBA and PFBS showed accelerated migration in the presence of long-chain perfluorooctanoic acid (PFOA). Furthermore, molecular adsorption configurations were established, and adsorption energies were calculated. The absolute value of binding energy order was PFBS (|–3.4 eV|) < PFBA (|–3.91 eV|) < PFOA (|–4.02 eV|), indicating that PFBS had weaker adsorption on SiO 2 compared to PFBA and PFOA. Additionally, when PFOA was present, the binding energies of PFBA and PFBS were − 0.11 and − 0.46 eV, respectively. The difference between R values for PFBA/PFBS and tracer decreased in the PFBA/PFBS–PFOA system (0.152/0.015), compared with PFBA/PFBS pollution (0.232/0.166). The absolute values of binding energies of PFBA/PFBS also decreased, showing that the calculation results aligned with the experimental results. Overall, short-chain PFAS had faster migration rates and were prone to causing groundwater pollution, as inferred from both experiments and theoretical calculations. The results of this study are scientifically significant for better assessing and managing environmental risks of PFAS in soil. Environmental implication Previous studies have focused on adsorption and molecular dynamics simulation of PFAS in soil, microorganisms, and minerals. However, the molecular mechanism of long-chain PFAS on short-chain PFAS has not been addressed in binary pollution. The migration characteristics of short-chain PFAS and long-chain-short-chain PFAS binary pollution were quantitatively studied through breakthrough curve analysis. The adsorption binding energy and charge transfer of PFAS in natural quartz sand were calculated based on density functional theory, revealing molecular adsorption configurations and adsorption mechanisms.