Redefining the competitive paradigm: influence of co-occurring ions on PFAS adsorption from polluted water onto granular activated carbon and anion exchange resin.

The effects of four inorganic ions: magnesium chloride (MgCl2), potassium nitrate (KNO3), sodium phosphate (Na2PO4), sodium bicarbonate (NaHCO3), and three organic ions: sodium dodecyl sulfate (SDS), ethylenediaminetetraacetic acid (EDTA), and humic acid (HA) on the adsorption behaviour of five per- and polyfluoroalkyl substances (PFAS) with varying chain lengths (C5-C11) were systematically investigated using granular activated carbon (GAC) and anion exchange resin (AER). Pseudo-second order (PSO) kinetic modelling showed adsorption capacities (qₑ) ranged from 1.11 to 6.89 mg/g and 4.67 to 8.62 mg/g for GAC and AER, respectively, across all experimental conditions and controls. AER consistently maintained high PFAS removal across diverse water matrices, whereas GAC performance was influenced by PFAS molecular structure and background ion composition. Competitive adsorption among PFAS followed a chain length-dependent trend on GAC; however, distinct selectivity patterns were observed for both adsorbents, highlighting material-specific adsorption mechanisms. Slight pH variations from background water are unlikely to destabilize PFAS adsorption, indicating stable performance of adsorption-based treatment under realistic conditions. Notably, this work reveals a previously unreported ion-mediated enhancement in the adsorption of short- and medium-chain PFAS (perfluoropentanoic acid, PFPeA; perfluorononanoic acid, PFNA) on GAC, attributed to cation-bridging and strengthened electrostatic interactions. In contrast, the longest-chain PFAS (perfluoroundecanoic acid, PFUnDA) exhibited a reversed response, with suppressed adsorption in the presence of ions. Moreover, desorption experiments revealed that co-occurring ions can enhance the retention of highly mobile, shorter-chain PFAS (PFPeA) on adsorbents, limiting PFAS remobilization, thereby improving the long-term reliability of adsorption-based water treatment systems. Overall, these findings provide new mechanistic insight into PFAS-adsorbent interactions under realistic water matrix conditions and offer practical guidance for adsorbent selection and treatment optimization especially for shorter-chains that are more problematic.