Competitive adsorption of per- and polyfluoroalkyl substances (PFAS) on a gel-type anion-exchange resin: Experiments...

Ion-exchange resins (IXR) have been used for a range of water treatment applications, and recently, their ability to remove per- and polyfluoroalkyl substances (PFAS) from water has gained increasing attention. Since multiple PFAS are typically present in contaminated water, it is important to understand how competitive effects will impact the performance and longevity of IXR. The aim of this study was to investigate the competitive adsorption behavior of six PFAS, perfluorononanoic acid (PFNA), perfluorooctane sulfonic acid (PFOS), perfluorooctanoic acid (PFOA), perfluorohexane sulfonic acid (PFHxS), perfluorohexanoic acid (PFHxA), and perfluorobutanoic acid (PFBA), on a representative gel-type IXR (Amberlite࣪ PSR2 Plus). Single solute batch experiments were conducted to determine baseline adsorption behavior, followed by a series of bi-solute experiments to examine competitive behavior. In single-solute solutions, carboxylates and shorter-chain length compounds (e.g., PFHxA) exhibited greater maximum adsorption capacities compared to sulfonates and longer-chain length compounds (e.g., PFOS). In bi-solute experiments, however, PFOS exhibited preferential adsorption, resulting in significant reductions in adsorption of the competing PFAS. To describe the competitive adsorption of PFAS on gel-type IXR, a mathematical model was developed that incorporates Langmuir-based competitive adsorption and a pore blocking factor, m. The model, which accounts for adsorption on both the exterior and interior domains of IXR, was able to accurately reproduce the observed bi-solute adsorption behavior, where m quantifies the effect of PFOS adsorption on interior site accessibility for short-chain PFAS. Experimental results demonstrate that PFOS can dramatically suppress the adsorption of competing PFAS, attributed to pore blocking that limits access to the interior of gel-type IXR.