Multispectroscopic Insights into the Role of Extracellular Polymeric Substances in PFOS Adsorption and Retention onto Sewage Sludge

Per- and polyfluoroalkyl substances (PFASs) in sewage sludge have attracted increasing attention due to their persistence, bioaccumulation potential, and environmental risks. However, the molecular mechanisms by which extracellular polymeric substances (EPSs) regulate PFAS adsorption onto sludge remain poorly understood. Herein, a multi-spectroscopic strategy combined with adsorption kinetics was employed to elucidate the interaction mechanisms between sludge EPS and perfluorooctane sulfonate (PFOS), a representative long-chain sulfonate PFAS, and to quantify the contribution of EPS to sludge PFOS adsorption. UV–vis spectroscopy revealed that lower PFOS concentrations initially induced electrostatic interactions with amide-related groups in EPS, followed by hydrophobic interactions with aromatic structures at higher concentrations. Raman spectroscopy coupled with principal component analysis identified protein-associated amide III bands and hydrophobic C–H structures as key responsive sites during EPS–PFOS association. Three-dimensional excitation–emission matrix fluorescence spectroscopy further demonstrated that protein-like and humic-like components within EPS jointly participated in PFOS binding. X-ray photoelectron spectroscopy and Fourier transform infrared spectroscopy analyses indicated that hydrophobic carbon structures, protonated amine groups, and protein secondary structures underwent significant rearrangement after PFOS exposure, supporting the involvement of both hydrophobic partitioning and electrostatic interactions. Furthermore, adsorption kinetic experiments suggested that EPS removal reduced sludge PFOS adsorption capacity, with the equilibrium distribution coefficient (Log Kd) decreasing from 3.580 to 3.004. Collectively, this study reveals that EPS mediates sludge PFOS adsorption through competitive and synergistic interactions among diverse organic components, providing multi-spectroscopic and molecular-level insights into PFAS fate and risk control during sludge management.