Mechanochemical Synthesis of Lignin-Based Adsorbents for PFAS Removal from Water

This study presents a straightforward, green synthesis of lignin-based adsorbents for the removal of short-and long-chained per-and polyfluoroalkyl substances (PFAS). Lignin was functionalized via solvent-free reactive ball-milling with glycidyltrimethylammonium chloride (GTMAC) and 1,2-epoxytetradecane. This reaction was performed by epoxide ring-opening chemistry, leading to positively charged lignin platforms bearing alkyl chains. Subsequent wet-chemical crosslinking utilizing hexamethylene diisocyanate (HDI) and 1,6-hexanedithiol resulted in the water-insoluble scaffolds as PFAS adsorbers. Single-solute adsorption screening at high concentrations (500 mg L-1) identified the HDI-crosslinked cationic lignin (L-N+-HDI) as the most effective adsorbent, achieving maximum adsorption capacities (ɋmax) of 590 mg g-1 for perfluorooctanoic acid (PFOA) and 200 mg g-1 for perfluorobutanoic acid (PFBA), surpassing most of the commercially available granular activated carbons reported in the literature. Mechanistic investigations revealed that the positive charge density, rather than hydrophobic modification, predominantly drives PFAS adsorption. At environmentally relevant low concentrations, L-N+-HDI exhibited linear isotherm behavior consistent with Henry's Law, effectively acting as an "infinite reservoir" of active binding sites. These findings demonstrate that mechanochemically engineered biopolymers adsorbents represent a highly efficient and sustainable alternative to conventional fossil-fuel-derived ion-exchange resins for water remediation applications.