Performance of the HSDM to predict competitive uptake of PFAS, NOM and inorganic anions by suspended ion exchange processes

Abstract

Per- and polyfluoroalkyl substances (PFAS) are potential human carcinogens that have been ubiquitously detected in drinking water sources. Ion exchange (IX) resins offer promising potential for the treatment of water sources impacted by high natural organic matter (NOM) and PFAS concentrations. Several kinetic models such as the pseudo-first-order (PFO), pseudo-second-order (PSO) and intraparticle diffusion (ID) have been investigated to examine the PFAS uptake kinetics on IX resins. However, the kinetic parameters are strongly impacted by the sorption test conditions, especially the sorbate-to-sorbent ratio, which were highly variable in past studies, resulting in several discrepancies when comparing published data. This study examined the use of the homogenous surface diffusion model (HSDM) coupled with the equivalent background concentration (EBC) model as an approach to describe competitive uptakes of regulated long- and short-chain PFAS, NOM, and inorganic ions (such as sulphate and nitrate) with IX resins. Kinetic studies confirmed surface diffusion as the rate-limiting step in NOM-rich waters (Biot number >30), while the multicomponent equilibrium model demonstrated that the initial NOM concentration of approximately 0.35–0.95 μmol L⁻¹ (C₀ = 5 mg C per L) competed with PFAS for active exchange sites. More importantly, the HSDM (R² > 0.98)/EBC (R² > 0.95) models provided an adequate fit to describe PFAS removal in a recycled wastewater.