Structure-performance in zwitterionic and anionic biosurfactant-assisted foam fractionation of PFAS: Interfacial and molecular mechanisms.

Foam fractionation has emerged as a promising technology for remediating per- and polyfluoroalkyl substances (PFAS) from aquatic environments. However, its sustainable application is critically hindered by the reliance on toxic synthetic co-surfactants and the intrinsic hydrodynamic trade-off between pollutant removal and enrichment. Here, we investigate how the molecular structure and charge characteristics of zwitterionic and anionic biosurfactants regulate PFAS capture and foam-fractionation performance. The study compares zwitterionic lauramidopropyl betaine (LAB) and lauryldimethylbetaine (BS-12) with anionic rhamnolipids (RLs) and sophorolipids (SLs). Interfacial thermodynamics and mechanistic characterization using FTIR, zeta-potential measurements, and molecular docking indicate that zwitterionic biosurfactants form cohesive interfacial assemblies with PFAS, whereas bulky anionic biosurfactants experience severe steric and electrostatic constraints. The stronger hydrogen-bonding capacity and polarizability of the sulfonate group also contribute to the more favorable interfacial association of perfluorooctane sulfonate (PFOS) than perfluorooctanoic acid (PFOA). Systematic variation of biosurfactant dosage, pH, ionic strength, and gas velocity reveals how interfacial adsorption and foam drainage jointly determine the balance between PFAS removal and enrichment. Under the optimized conditions, the LAB-assisted system achieved a maximum PFOS removal efficiency of 97.4% and an enrichment ratio of 10.4-13.2. The performance of the LAB system was further evaluated in deionized water, tap water, river water, and simulated wastewater, and supplementary experiments were conducted for the representative short-chain PFAS. Overall, this study links biosurfactant structure, interfacial interaction, foam hydrodynamics, and PFAS separation performance, providing a mechanistic basis for designing biosurfactant-assisted foam-fractionation processes for PFAS-contaminated wastewater and concentrated process streams.