Adsorption mechanisms of short-chain and ultrashort-chain PFAS on anion exchange resins and activated carbon

Abstract

Short-chain and ultrashort-chain per-/polyfluoroalkyl substances (PFAS) have become ubiquitous in aquatic environments worldwide, and their concentrations are rising. Studies have shown adsorption on activated carbon (AC) and anion exchange resins (AERs) as efficient removal techniques for long-chain PFAS (C ≥ 8). However, limited data are available on the adsorption of short-chain PFAS (C ≤ 4), especially ultrashort-chain PFAS. In this study, isotherm experiments were conducted to elucidate the possible adsorption mechanisms of widely detected short-chain perfluorobutanesulfonic acid (PFBS) and perfluorobutanoic acid (PFBA), and ultrashort-chain perfluoropropionic acid (PFPrA) on AC and AERs. Various factors, such as adsorbate concentration and characteristics, adsorbent properties, and the water matrix, influenced the adsorption of the target compounds. At concentrations > 1 mg L⁻¹, strong base AER (A900) displayed the highest adsorption affinity among the four adsorbents investigated. An average 20 times decrease in the adsorption of three PFAS in the presence of competing CaCl₂ salt affirmed the importance of ionic interactions. In contrast, both ionic interactions and hydrophobic interactions were equally important at concentrations < 1 mg L⁻¹ for adsorption on AER and AC. The higher dipole moment of PFBS could be responsible for its higher adsorption on AERs compared to PFPrA and PFBA, while PFBS's greater adsorption on AC could be attributed to hydrophobic partitioning, which was supported by the calculated Langmuir and Freundlich model parameters. The isotherm data also suggested adsorption through additional mechanisms(s), which could include negative charge-assisted hydrogen bonds between PFBA and AC functional groups. Among the three short-chain PFAS, PFPrA exhibited the least adsorption and maximum desorption irrespective of the adsorbent type and adsorbate concentrations. Overall, our results suggest that AERs and ACs can be used to remove short-chain PFBA and PFBS through electrostatic and non-electrostatic interactions. This implies that an adsorption treatment train consisting of a series of stages, each targeting different interaction mechanisms, is needed to remove a wide range of PFAS.