Comparative DFT and molecular dynamics investigation of PFAS adsorption on heteroatom-engineered metal-organic framework linker models.

Per- and polyfluoroalkyl substances (PFAS) represent persistent contaminants in aquatic environments, and developing effective absorbents for their removal is a major challenge. This study has been conducted for absorbent design using density functional theory (DFT) and molecular dynamics (MD) in order to understand the process of adsorption of the following PFAS species: PFBA-, PFBS-, PFOA-, and PFOS- on nitrogen-, oxygen-, and sulfur-containing linker models. For this, geometry optimizations were performed at ωB97XD/6-311++G(d, p) level under implicit aqueous solvation, and the adsorption energies were calculated using the counterpoise correction technique. The final results showed that four PFAS molecules exhibited appropriate counterpoise-corrected adsorption energies ranging from -0.700 to -20.047 kcal mol-1, with the strongest interaction between the PFOS- molecule and the sulfur-containing linker model (-20.047 kcal mol-1). As for the effect of sulfur substitution on the mean energy of adsorption for various PFAS species, it was negative, but the identity of the heteroatom affected PFAS adsorption behavior. Conceptual DFT descriptors based on frontier orbitals showed weak and statistically insignificant linear relations with adsorption energies, indicating that a single comprehensive electronic descriptor is not sufficient to explain the trend in adsorption energy across the data set. The analyses based on ALIE, RDG and NCI showed that the electronic activity depends on the ligands and involves both dispersive and attractive non-covalent interactions. Hirshfeld analysis revealed the predominance of ligand-H contacts, while other analyses (ELF and DOS) indicated that electron localization and redistribution depend on the heteroatoms. Finally, the 100 ns aqueous MD simulations provided evidence of continuous interaction between PFOS and ligands, accompanied by significant conformational and interfacial alterations, as well as sampling of low-free-energy states. The conclusions drawn from these results indicate that PFAS adsorption at the linker level arises from the combined effects of molecular structure, the electronic environment influenced by heteroatoms, and noncovalent interactions.