Sulfate-form anion exchange resins in mixed beds for the removal of NOM and PFAS from drinking water

Anion exchange resins (AERs) are increasingly applied for the simultaneous removal of natural organic matter (NOM) and per - and polyfluoroalkyl substances (PFAS) from surface and groundwater; however, conventional chloride-form AERs rapidly lose capacity due to their high affinity for sulfate (SO₄ 2− ), which dominates anion competition in natural waters. To address this limitation, AERs were converted to the sulfate form by pre-saturating exchange sites with SO₄ 2− , thereby modulating ion-exchange equilibria and selectivity toward target contaminants. This study presents the first long-term pilot-scale evaluation of sulfate-form AERs for combined NOM and PFAS removal and introduces a mixed-bed configuration combining polyacrylic (PA) and polystyrene (PS) resins to exploit complementary sorption affinity. Results demonstrate that both counter-ion form and polymer backbone strongly influence competitive adsorption behavior. Sulfate-form resins consistently outperformed their chloride-form counterparts, with PS-SO₄ 2− exhibiting enhanced PFAS and NOM uptake through combined hydrophobic and electrostatic interactions, while PA-SO₄ 2− preferentially removed high-molecular-weight NOM fractions. The mixed-bed configuration achieved 54% dissolved organic carbon (DOC) removal and 80% total PFAS removal over 150,000 bed volumes, corresponding to 1.4-fold and 3.5-fold improvements in DOC and PFAS removal, respectively, compared to single-bed and PFAS-specific resins. LC-OCD, principal component analysis, and FTIR analyses indicate reduced competition at exchange sites and cooperative adsorption behavior. These findings demonstrate that sulfate-form and mixed-bed AERs enable enhanced selectivity and sustained performance under multicomponent conditions, providing a mechanistically informed approach for advanced ion-exchange process design.