Understanding the adsorption trilemma: Achieving load, level, and selectivity for the removal of pharmaceuticals with zeolites

Abstract

The persistence and ecotoxic effects of pharmaceuticals in aquatic environments demand their removal from wastewater treatment plant effluents. Adsorbents for this purpose must combine high uptake at low concentration levels and a high adsorption load (capacity) with strong selectivity over competing wastewater constituents. Yet, a systematic strategy to address this trilemma remains elusive. Here, we present such an approach using commercial zeolites, which are promising candidates for selective removal owing to the strong influence of molecular shape–topology relationships on interaction energies. The removal of carbamazepine (CBZ) and diclofenac (DCF) with maximum capacities and affinities of 59.71 mg/g and 3.37 L/mg, as well as 104.28 mg/g and 0.21 L/mg, respectively, could be maintained for CBZ adsorption in a secondary-treated municipal wastewater effluent (WWTP effluent), while the adsorption of DCF was significantly reduced. Thermodynamic interpretation of the experimental isotherm parameters showed an excellent quantitative agreement with free energy perturbation simulations, yielding consistent free adsorption energies for CBZ across all zeolites. This confirms that for structurally rigid pollutants, mostly consisting of annulated ring structures, the shape-topology fit is the dominant and predictive factor for adsorption, providing a clear design principle for the targeted selection of zeolites. In contrast, the deviations observed for the DCF molecule with a more flexible structure, consisting of two rings that can more freely arrange with respect to each other, highlight that for non-rigid, more complex pollutants, dynamic conformational effects and guest-guest interactions can become critically important. This study reveals the zeolite topology as a determinant of selective pharmaceutical uptake from wastewater and paves the way for contaminant-specific selection.