Aluminium-modified kaolinite for the adsorption of long and short-chain PFAS: Batch and fixed-bed systems with mechanistic insights

Clay-based adsorbents have attracted attention for remediating per- and polyfluoroalkyl substances (PFAS) owing to their abundance, feasibility, and scalability. However, the negative surface charge of kaolinite limits its affinity for anionic PFAS. In this study, aluminium-modified kaolinite (Al³⁺Kao) was developed to enhance PFAS adsorption by introducing hydroxylated Al-bearing sites and shifting the surface charge. SEM–EDX, BET, XRD, FTIR, ζ-potential, and XPS confirmed Al incorporation, increased Al₂O₃ content from 10.26% to 40.96%, and an increase in the isoelectric point from pH 3.5 for raw kaolinite (RKao) to 7.3 for Al³⁺Kao. Batch adsorption capacities increased after Al³⁺ modification from 4251 to 5687 ng g⁻¹ for perfluorooctanoic acid (PFOA), 2493–2869 ng g⁻¹ for perfluorobutanoic acid (PFBA), 4055–4897 ng g⁻¹ for perfluorobutane sulfonic acid (PFBS), and 2395–4108 ng g⁻¹ for 6:2 fluorotelomer sulfonic acid (6:2FTSA). In fixed-bed columns, Al³⁺Kao delayed breakthrough and extended operating performance for all PFAS. For PFOA, the breakthrough increased from 3.0 to 7.9 bed volumes, the operating limit increased from 7.1 to 39.9 bed volumes, and total removal efficiency improved from 37% to 75%. Regeneration showed reusability over five cycles, with removal efficiencies declining from 43% to 37% for PFBA, 70–65% for PFBS, 82–55% for PFOA, and 66–51% for 6:2FTSA. Adsorption depended on PFAS molecular properties, pH, coexisting ions, and dissolved organic matter. Mechanistic evidence indicated electrostatic attraction at Al-rich hydroxylated sites and perfluoroalkyl-chain interactions, demonstrating the potential of Al³⁺Kao as a low-cost adsorbent for PFAS remediation.