Per- and polyfluoroalkyl substances (PFAS) removal by microalgae-derived biochar: Valorising from spent biomass after lipid extraction

Per- and polyfluoroalkyl substances (PFAS) are persistent environmental contaminants of concern due to their toxicity and resistance to conventional water treatment. This study investigates the removal of perfluorooctanoic acid (PFOA) and perfluorooctanesulfonic acid (PFOS) – two most prominent PFAS compounds, using biochar derived from microalgae spent biomass. Biochars derived from C. vulgaris and Scenedesmus sp. biomass were evaluated for PFAS adsorption. Compared to granular activated carbon (GAC), biochars showed faster kinetics, reaching equilibrium within 2 h. Scenedesmus sp. biochar exhibited higher adsorption capacities (30.1 µg/g PFOA; 43.1 µg/g PFOS) and, at 0.25 g/L of biochar, both biochars completely removed PFAS in single-solute systems. Under natural organic matter competition, Scenedesmus sp. biochar maintained >80% PFOA and 87% PFOS removal at 1 g/L of biochar. Adsorption was dominated by hydrophobic interactions with secondary cation bridging. While capacities were lower than GAC, microalgal biomass represents a sustainable feedstock to produce biochars for PFAS adsorption. The results demonstrate that the developed biochar exhibits rapid uptake of PFAS, highlighting its potential as an effective and sustainable material for water treatment applications. ● Microalgae-derived biochar shows faster adsorption kinetics than activated carbon ● PFAS adsorption onto microalgae-derived biochar is species dependent ● Background organics show limited effect on PFAS adsorption to microalgae-derived biochar ● Hydrophobic interactions dominate PFAS adsorption, followed by cation bridging