Tailoring adsorption properties of Kekulene nanoring via functionalization for pharmaceutical pollutant removal

Pharmaceutical pollutants such as Diclofenac and Naproxen are emerging environmental contaminants due to their persistence and potential biological hazards. In this study, pristine and functionalized Kekulene nanorings (KNRs) were theoretically explored as novel adsorbents using density functional theory (DFT). Various functional groups (-COOH, -NO₂, -NO, -N, -O, -S) were introduced to modulate the interaction with drug molecules. Adsorption energy (E_a) calculations confirmed spontaneous physisorption across all systems, with values ranging from -2.473 to -0.441 eV. Notably, the KNR-NO₂-N system in aqueous phase exhibited the lowest E_a (-0.441 eV) and shortest recovery time (τ = 2.864 × 10^-5 s), making it the most promising candidate for rapid desorption and recyclability. Non-covalent interaction (NCI) analysis revealed that van der Waals forces and weak electrostatic interactions dominate the adsorption mechanism. Natural Bond Orbital (NBO) analysis of the oxygen atom (O6) confirmed variable charge transfer behaviour, reflecting the influence of surface functionalization. HOMO-LUMO analysis showed frontier orbital localization patterns that shifted upon functionalization, especially in Naproxen complexes, indicating enhanced electronic interactions. Compared to benchmark materials such as PTX@rGO and FPV@GN, KNR-based adsorbents demonstrated competitive or superior tunability and desorption potential. These results suggest that functionalized KNRs-particularly KNR-NO₂-N-are promising candidates for efficient, reversible pharmaceutical pollutant capture in both gas and aqueous environments.