Molecular Simulations of Nanoporous Materials for PFAS Adsorption

Abstract

Per- and polyfluoroalkyl substances (PFASs), a broad class of synthetic organic micropollutants, have emerged as a threat due to their chronic toxicity in humans, bioaccumulation potential, and resistance to degradation. Developing efficient and affordable removal processes is a complex but necessary engineering challenge, motivated by established scientific consensus and stricter global regulations. Recently, adsorption onto all-silica zeolite beta has shown promising results for PFAS filtration from water, their primary medium for accumulation. However, molecular-scale insights into sorption under industrially relevant conditions are lacking, leaving unanswered questions and limiting opportunities for process optimization. This work conducts molecular simulations to characterize the behaviour of PFASs in both the aqueous and adsorbed phases in the presence of solvated inorganic ions. Using a broad set of tools from computational statistical mechanics, their concerted interactions are studied across multiple domains: molecular dynamics simulations in bulk water, Monte Carlo sampling in the adsorbed phase, and adaptive biasing methods to characterize the water-zeolite interface.

The intra-diffusion coefficients in water, ranging from 5.4(5) to 9.2(11) × 10⁻⁶ cm² s⁻¹ depending on PFAS moiety and salinity, agree with experimental findings within a constant systematic error. The mobility of PFASs is weakly affected by ionic content, with microviscous effects playing a central role for shorter molecules. Adsorption sites are identified inside the zeolite beta framework and at the zeolite-water interface. Computing host-guest interaction energies reveals that adsorption is driven by both enthalpic effects (favourable van der Waals interactions) and entropic effects (hydrophobic interactions between framework and adsorbate). Free energy profiles at the water-zeolite interface suggest that inorganic ions stabilize PFASs into the interfacial adsorption sites, increasing the barrier for desorption by 10.1 to 27.1 kJ mol⁻¹ for long- and short-chain PFASs at 0.1 M CaCl₂. However, the ultra-short chain trifluoroacetic acid is not affected by the presence of inorganic ions. Additionally, higher salinities hinder adsorption instead, likely due to increased attraction between PFAS anions and solvated inorganic cations.