Implications for modeling anion exchange treatment of perfluoroalkyl substances in drinking water and related natural...

To better understand natural organic matter (NOM) impacts on per- and polyfluoroalkyl substances (PFAS) removal in drinking water, this study investigated PFAS breakthrough from pilot-scale fixed-bed anion exchange columns fed a stable water quality influent with and without addition of reconstituted Ohio River water NOM at four empty bed contact times (EBCTs; 0.5-, 1.0-, 1.5-, and 2.5- min). PFAS elution order was mostly consistent with previously reported batch-derived selectivity with respect to chloride (K_x/Cl), with deviations in bed volumes to early breakthrough (BV10) attributed to molecular size-exclusion kinetic limitations. Earlier breakthrough was observed with NOM compared to without NOM, and NOM impacts on BV10 were greater for larger, later-eluting PFAS. Subsequently, an ion exchange column model (IEX-CM; https://github.com/USEPA/Water_Treatment_Models) was applied to evaluate correlations for estimating film mass transfer coefficient (k_L) and to estimate PFAS intraparticle diffusion coefficient (D_s) and K_x/Cl by fitting PFAS effluent data to IEX-CM simulated concentrations, resulting in most PFAS breakthrough curves being well-described by IEX-CM simulations. Some deviations from the IEX-CM were noted, including unexplained chromatographic effects on perfluorohexanoic acid (PFHxA), and most surprisingly, apparent non-adsorption removal of hexafluoropropylene oxide dimer acid (HFPO-DA or GenX chemicals), which was enhanced by increasing EBCT and absence of NOM.