Efficient removal of short- and long-chain perfluoroalkyl carboxylate acids from surface water matrices using a quaternary ammonium functionalized adsorbent derived from waste Karanja shells†

Per- and polyfluoroalkyl substances (PFASs) have garnered significant global attention due to their widespread use in consumer products, persistence in environmental media, potential toxicity, and high bioaccumulation. Regulatory and voluntary phase-out initiatives targeting long-chain PFASs have led to increased usage of short-chain PFASs, which are now frequently detected in aquatic environments. The shorter chain length increases their mobility and persistence in aquatic environments, posing a significant risk of toxicity as well as complicating remediation efforts. This study investigates the efficacy of an activated carbon (AC) synthesized from waste Karanja shells for the removal of both short- and long-chain PFASs, specifically perfluoropropanoic acid (PFPrA) (C3), perfluorobutanoic acid (PFBA) (C4), perfluoropentanoic acid (PFPeA) (C5), and perfluorooctanoic acid (PFOA) (C8). The surface of the adsorbent was impregnated with a quaternary ammonium group using cetyltrimethylammonium bromide (CTAB) to improve its electrostatic interaction effects. The adsorbent demonstrated high removal efficiencies, achieving ∼90% removal of PFPrA and PFBA, and ∼95% for PFPeA and PFOA at an initial concentration of 100 μg L⁻¹, comparable to the performance of commercially available ion-exchange resins tested under the same conditions in the present study. The adsorption performance of the AC was significantly influenced by the PFAS chain length, with longer-chain PFOA exhibiting higher selectivity and adsorption capacities as compared to the short-chain PFASs; however, conversely the adsorbent exhibited faster kinetics for the short-chain PFASs as reflected in the relative values of the respective rate constants of each PFAS studied. In continuous flow packed-bed columns, the adsorbent's selectivity followed the order: PFPrA (C3) < PFBS (C4) < PFPeA (C5) < PFOA (C8). The presence of the background divalent cations and anions of the river water matrix significantly reduced the breakthrough times of PFPrA (C3) and PFBA (C4) from 930 to 230 BV and 1260 to 550 BV, respectively. A solution containing 50% methanol with 1% NH₄OH exhibited a higher value for PFAS desorption among all the tested specs of regenerants. These findings suggest that the waste-derived surface functionalized adsorbent offers a cost-effective and environmentally sustainable option for short-chain PFAS remediation in contaminated water sources, with the added benefit of utilizing the repurposed waste material.