Adsorption characteristics of legacy and emerging per-and polyfluoroalkyl acids(PFAAs)on anion exchange resin
Per-and polyfluoroalkyl acids(PFAAs)exhibit remarkable surfactant properties,contributing to their widespread use in diverse commercial and industrial applications,and subsequent contamination of water environments.Conventional drinking water treatment methods,such as coagulation and advanced oxidation processes have encountered challenges in effectively eliminating these persistent contaminants,necessitating the exploration of alternative approaches.While anion exchange resins(AERs)have proven effective in removing long-chain PFAAs,their efficacy in addressing alternative forms of these legacy PFAAs,such as short-chain C4 PFAAs,and per-and polyfluoroalkyl ether carboxylic acids and sulfonic acids(PFECAs and PFESAs),remains uncertain.Trifluoroacetic acid(TFA),trifluoromethane sulfonic acid(TFMS),perfluorobutanoic acid(PFBA),and perfluorobutane sulfonic acid(PFBS)were chosen as representative short-chain perfluorocarboxylic acids(PFCAs)and perfluorosulfonic acids(PFSAs).Additionally,sodium p-perfluorous nonenoxybenzenesulfonate(OBS)and perfluoro-2-propoxypropanoic acid(GenX)were selected to represent emerging PFECAs and PFESAs.To provide a basis for comparison,the sorption of two legacy PFAAs,perfluorooctanoic acid(PFOA)and perfluorooctane sulfonic acid(PFOS),were also examined.This study investigates the impacts of PFAA structural features(carbon chain length and terminal functional group),resin properties(composition and structure),and solution chemistry factors(pH,co-occurring contaminants,inorganic ions,and salinity)on the removal of both legacy and emerging PFAAs by AERs.Results demonstrate that the adsorption rate of PFAAs on the macroporous resin(A860)surpasses that on gel(G)resin,with observed desorption of TFA on the macroporous resin.However,the G resin,characterized by a hydrophobic polystyrene(PS)composition,exhibits a higher adsorption capacity and removal rate for PFAAs compared to macroporous(MP)resins with a polyacrylic(PA)composition(adsorption capacity:G＞MP;PS＞PA).The carbon chain length and terminal functional groups of PFAAs significantly influence their adsorption and removal rate.Specifically,the adsorption and removal rates of PFSAs were higher than those of PFCAs with the same carbon chain length.The adsorption rate was higher for short-chain analogues than long-chain analogues with the same terminal functional groups.Competitive adsorption allows long-chain PFAAs to replace adsorbed short-chain PFAAs.Notably,the adsorption of PFAAs to AERs remains largely unaffected by pH levels ranging from 3.5 to 9.5.Elevated concentrations of humic acid(HA)decrease the removal efficiency of PFAAs,while lower HA concentrations could promote the sorption of hydrophobic PFOS and OBS onto the resin.The adsorbed amounts of PFAAs on the resin decrease with increasing concentrations of competing inorganic anions.Electrostatic interactions and anion exchange emerge as the principal driving forces for the adsorption of short-chain PFAAs onto resins.The adsorption of short-chain PFAAs,particularly PFCAs,is susceptible to the influence of inorganic ions and salinity.Additionally,short-chain PFAAs(such as TFA)could be removed by resins through pore filling.In the adsorption of long-chain PFAAs,electrostatic and hydrophobic effects appear to be of primary significance.Competing inorganic anions minimally affect the adsorption of OBS onto the resin.However,short-chain PFAAs(C＜4)and PFECAs(such as GenX)are strongly influenced by inorganic ions,resulting in low removal efficiencies by commonly used AERs,posing a significant challenge for future water treatment.Simultaneously,it is recommended that the adsorption treatment time not exceed 24 hours to prevent the desorption of short-chain PFAAs.
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