Kinetic and isotherm study for the adsorption of per- and polyfluoroalkyl substances (PFAS) on activated carbon in the low ng/L range

Activated carbon adsorption is a widely used technology for the removal of per- and polyfluoroalkyl substances (PFAS). However, the rapid breakthrough of PFAS in activated carbon filters poses a challenge to meet the very low allowable PFAS concentrations in drinking water, leading to high operational costs. In this study, we conducted batch isotherm and kinetic adsorption experiments using nine different types of PFAS molecules at concentrations typically found in water sources used for drinking water production (0.1-100 ng/L). The isotherm experiments at these low concentrations reveal that the maximum adsorption capacity of several PFAS is much lower than reported in literature. The estimated isotherms were included in a dynamic model that includes mass transport based on surface diffusion. This model effectively describes the experimental kinetic data, and the obtained surface diffusion coefficients indicate a very slow PFAS surface mobility. Additionally, our findings indicate that PFAS surface mobility decreases in scenarios with more available adsorption sites. Notably, mesoporous activated carbon, with its higher adsorption capacity, exhibits lower PFAS surface mobility than microporous carbon with lower PFAS adsorption capacity. Moreover, for both carbons, we observed a decrease in PFAS surface mobility at higher carbon loadings when the surface is less saturated with PFAS. Our findings suggest potential inherent limitations in activated carbon technology for PFAS removal under environmentally relevant conditions, as we observed lower adsorption capacities than previously reported at higher concentrations, and a decrease in PFAS surface mobility with more available adsorption sites.