Pore size effects upon adsorption of PFAS in covalent organic frameworks: molecular dynamics study

Abstract

Per- and polyfluoroalkyl substances (PFAS) are a large class of industrial chemicals whose diversity, spread, and environmental/health impacts have recently become a major concern for environmental and health policy makers. This concern is further exacerbated by their pervasiveness and chemical resilience, which complicates their removal from watersheds and other contaminated environments. Due to the chemical stability of the carbon–fluoride bonds, they are difficult to degrade. Instead, an alternative presents itself in the form of adsorption, concentration, and then removal of PFAS from contaminated sites. Both metal organic frameworks (MOFs) and covalent organic frameworks (COFs) have recently come under significant investigation as possible adsorption media which could be adapted for the removal of PFAS from contaminated sites. To gain greater insight into the adsorption capabilities of COFs for the removal of PFAS from waterways, we have studied the adsorption of PFAS molecules in COFs of differing pores sizes using molecular dynamics simulations. We examine the absorption of aqueous PFBA, PFOA, and PFOS into Covalent Triazine-Based Frameworks (CTF) of different pore sizes. This mechanistic adsorption data shows that a goldilocks zone occurs in pores with diameters of around 8 Å where the PFAS thread through the pores smoothly. Kinetic factors from diffusion into these nanopores favors the adsorption of short chain PFAS even though larger PFAS are thermodynamically favored. Each pore tends to initially adsorb only one PFAS, occupying the mouth of the pore, until the local COF surface is saturated and then multiple occupancy per pore can occur. Discussion on the impacts of PFAS concentration and interaction with the pores will inform design principles for enhanced selectivity and capacity for PFAS adsorbent material.