Removal of Emerging Contaminants from Wastewater via Adsorption in H-MFI Zeolite: A Hybrid QM/MM Study

Abstract

Abstract Nanoporous adsorbents such as zeolites offer a promising route for the selective removal of contaminants of emerging concern (CECs) from wastewater streams, where conventional technologies such as activated carbon often lack selectivity at sub-ppm concentrations. Here, we employ a first-principles-based hybrid quantum mechanical/molecular mechanical (QM/MM) framework to investigate the adsorption of representative CECs, aspirin, bisphenol A, and clofibric acid, at Brønsted acid sites of H-MFI zeolite under both gas- and aqueous-phase conditions. Gas-phase QM/MM calculations at the ωB97X-D/def2-TZVP level of theory reveal strong adsorption of all three CECs via hydrogen bonding to the acidic proton, and the resulting adsorption enthalpies are in excellent agreement with periodic QM/QM calculations. To explicitly quantify solvent effects, adsorption in condensed water was examined using an explicit solvation scheme for zeolites (eSZS), combined with grand canonical Monte Carlo (GCMC) sampling of water in the μVT ensemble. Inclusion of condensed water substantially alters the adsorption thermodynamics and leads to pronounced selectivity among the CECs. While adsorption (ΔGadsgas→liquid) of aspirin and clofibric acid remains exergonic in condensed water at 298 K (−0.57 ± 0.01 eV and −1.07 ± 0.07 eV, respectively), adsorption of bisphenol A becomes endergonic (1.19 ± 0.03 eV) due to a large solvent-induced free-energy penalty. This behavior originates from the formation of a significantly larger solvent cavity by bisphenol A owing to its larger molecular size. By combining the computed condensed water-phase adsorption free energies with Henry’s law under dilute aqueous conditions, threshold CEC concentrations (where ΔGads,CEC = 0 eV) were determined. Compared to aspirin (39.76 ppm), clofibric acid exhibited an extremely low threshold concentration of only 3.5 × 10–7 ppm, well below typical concentrations reported in wastewater, highlighting the potential of H-MFI for the selective removal of clofibric acid at environmentally relevant concentrations.