Structure-selectivity relationship of anion exchange resins with different quaternary amine functional groups for highly selective removal of PFAS from chromium-plating wastewater

Anion exchange resin (AER) adsorption is an effective technology for the removal of per- and polyfluoroalkyl substances (PFAS) from wastewater. However, existing AERs with tributylamine functional groups have poor adsorption selectivity for perfluorinated carboxylic acids (PFCAs) and 6:2 fluorotelomer sulfonate (6:2 FTS), and the structure-selectivity relationship is still unclear. In this study, several novel gel AERs with long-chain amine groups were prepared. It was found that their adsorption selectivity for 6:2 FTS was 3.3-5.1 times that of commercial AERs, and the adsorption amount for 6:2 FTS in chromium-plating wastewater was 2.1 times that of the commercial PFA694E. On this basis, we synthesized 16 AERs with different quaternary amine functional groups, and explored the structure-selectivity relationship through the selectivity coefficients and adsorption energies of different AERs for seven typical PFAS. The order of adsorption selectivity of AERs with different quaternary amine groups for PFAS was AER-(12-1-1) > AER-(8-1-1) > AER-(4-4-4) > AER-(4-1-1) approximate to AER-(2-2-2) > AER-(1-1-1), where the three numbers are the carbon-chain lengths of the three alkyl groups attached to the nitrogen atom of the quaternary amine group. Density functional theory (DFT) calculations confirmed the enhanced adsorption selectivity and contribution of both non-electrostatic and electrostatic interactions by the long-chain amine groups, and a quantitative relationship between theoretical calculations and experimental results was established. These results could provide guidance for the development of efficient adsorbents for PFAS removal from wastewater.