Impact of coexisting nitrate and sulfate ions on per- and polyfluoroalkyl substances removal by anion exchange resins

The impact of coexisting anions on the removal of per- and polyfluoroalkyl substances (PFAS) by three strong base anion exchange resins (AER) was investigated. Two short-chain (perfluorobutanoic acid (PFBA); perfluorobutane sulfonic acid (PFBS)) and two long-chain (perfluorooctanoic acid (PFOA); perflourooctane sulfonic acid (PFOS)) PFAS were selected as model compounds representing varying chain lengths and functional groups. Batch sorption experiments were conducted using single PFAS and multi-solute (four PFAS) matrix in the presence and absence of nitrate and sulfate ions. Divalent (sulfate) anions inhibited the sorption of PFAS by nearly 2-times compared to monovalent (nitrate) anions in the single solute system. However, the trend was inconsistent in the multi-solute matrix due to inter-competition between PFAS. The sorption capacities of the resins for PFAS were greatly suppressed by the presence of coexisting anions, with FerrIX A33E (arsenic-specific resin) showing nearly 2-times more impact than A592E (PFAS-specific resin) and A520E (nitrate-specific resin). The observed trends were related to competitive sorption to the exchange sites of the resins by the inorganic anions and their relative affinity to the resins investigated. The impact of inorganic anions on PFAS removal by the AER followed the trend: PFBA > PFBS ≥ PFOA > PFOS, with PFBA sorption suppressed by 4X-7X in both the single-solute and multi-solute systems. Higher inhibitions were observed for carboxylates compared to sulfonates, likely due to the higher electronegativity of the sulfonate functional group and/or hydrophobicity of perfluoroalkyl sulfonates allowing for their enhanced sorption on AER.