Critical role of pore size on perfluorooctanoic acid adsorption behaviors in carbonaceous sorbents

Per- and polyfluoroalkyl substances (PFAS) are an emergent threat to the environment due to their toxic, carcinogenic, and environmentally persistent nature. Commonly, these harmful micropollutants are removed from contaminated water sources through adsorption by porous sorbents such as activated carbon. While studies suggest a relationship between sorbent pore size and their PFAS remediation performance, the underlying mechanisms-particularly those related to sorbate morphology-have not been elucidated through direct experimental observations. This work investigates how pore size in carbonaceous sorbents impacts the morphology of adsorbed perfluorooctanoic acid (PFOA) aggregates and their sorption behavior, using microporous and mesoporous carbons as models. Contrast-matching small-angle neutron scattering (CM-SANS) is used to determine the structure of adsorbed PFOA molecules, supported by molecular dynamics simulations and physisorption experiments. Our findings reveal that the larger pore sizes in mesoporous sorbents enable the formation of PFOA assemblies during adsorption, which is hindered in microporous sorbents. Collectively, this work provides direct insights into the adsorption and assembly mechanisms of PFAS molecules within confined pores, offering important insights for the rational design of effective remediation systems.