Enhanced removal of per- and poly-fluoroalkyl substances (PFAS) by modified clay: Adsorption behavior, simulation, and regeneration

A modified clay adsorbent was developed by coating poly(diallyldimethylammonium) chloride (PDADMAC) to improve its performance in removing per- and poly-fluoroalkyl substances (PFAS) from water. The adsorption equilibrium was well described by the Freundlich isotherm, with K f values of 2.29 ± 0.80 (mg/g)(L/mg)¹⁄ⁿ for perfluorobutanoic acid (PFBA) and 38.21 ± 6.73 (mg/g)(L/mg)¹⁄ⁿ for perfluorooctanoic acid (PFOA), while the kinetic behavior fit both pseudo-first-order and pseudo-second-order models, with all R² values exceeding 0.95. Adsorption performance decreased with increasing pH, particularly for short-chain PFAS. Compared to NaCl, the presence of natural organic matter had a more pronounced inhibitory effect on PFAS uptake. The PDADMAC-modified clay demonstrated excellent reusability, retaining >98% of its initial efficiency after five regeneration cycles using simple NaCl brine. This effective brine-based regeneration outperformed commercial adsorbents such as ion exchange resins and activated carbon. Quantum mechanical calculations and molecular dynamics simulations revealed that adsorption is mainly driven by noncovalent interactions, with strong electrostatic attraction (–42.73 kcal/mol for PFBA and –43.26 kcal/mol for PFOA) complemented by van der Waals forces (–10.03 kcal/mol for PFBA and –15.80 kcal/mol for PFOA). These negative interaction energies confirm the thermodynamic favourability of adsorption. These results demonstrate that PDADMAC modification substantially improves performance of clay-based adsorbents and offers a practical, low-cost, and scalable strategy for PFAS remediation in water treatment applications. • A polymer-modified clay adsorbent was developed for PFAS removal from water. • PFAS uptake was influenced by pH and natural organic matter. • Effective regeneration was achieved using 1% NaCl solution. • QM analysis revealed noncovalent interactions as key adsorption mechanism. • Molecular simulation aligned well with experimental adsorption behavior.