FeCl3-Activated Digested Sludge-Derived Biochar for PFOS and PFOA Removal in Real Municipal Wastewater: Mechanistic Insights and Reusability

Per- and polyfluoroalkyl substances (PFAS), including perfluorooctanesulfonic acid (PFOS) and perfluorooctanoic acid (PFOA), are persistent, bioaccumulative and difficult to remove from wastewater using conventional treatment processes. Developing effective adsorbents that perform under complex wastewater conditions while supporting sustainable waste valorization is therefore critical. In this study, FeCl 3 -activated biochar was synthesized from municipal anaerobic digester sludge and evaluated for PFOS and PFOA removal from real municipal wastewater pre-UV secondary effluent. By converting sludge generated at the same wastewater treatment plant into an adsorbent for treating its effluent, this work demonstrates a closed-loop strategy for simultaneous PFAS mitigation and resource recovery. FeCl 3 -A-BC exhibited adsorption capacities of 54.9 mg/g for PFOS and 12.5 mg/g for PFOA, with high removal efficiency under real wastewater conditions. This performance was associated with its high surface area, hierarchical porosity, and Fe-/O-containing surface sites, while kinetic and isotherm modeling suggested surface-mediated monolayer adsorption. The material demonstrated good PFOA reusability over repeated regeneration cycles, while maintaining low leaching (<5.3%). The treatment reduced COD and TOC by up to 62% and 53%, respectively, demonstrating concurrent improvement in effluent quality and highlighting the potential of FeCl 3 -A-BC as an effective adsorbent within circular economy frameworks.