Enhanced adsorption of diclofenac and naproxen toward rice husk biochars via decoration of UiO-66 and UiO-66-NH₂: Roles of π-π interaction and hydrogen bonding

Pharmaceutical residues such as naproxen (NPX) and diclofenac (DCF) are frequently detected in aquatic environments and remain challenging to remove using conventional treatment processes. In this work, rice husk biochar (RHB) was modified with UiO-66 and UiO-66-NH₂ to enhance adsorption performance and to clarify the roles of specific noncovalent interactions. Metal-organic frameworks (MOFs) decoration substantially increased surface area from 130.5 m2/g for RHB to 1177.2 m2/g for UiO-66@RHB and 926.5 m2/g for UiO-66-NH₂@RHB, indicating successful formation of highly porous composites. Under pH 7.0 and 25 °C, UiO-66-NH₂@RHB achieved much higher equilibrium capacities than RHB, reaching 23.9 mg/g for NPX and 23.8 mg/g for DCF at 5 mg/L. Isotherm analysis was better described by the Freundlich model, while the maximum adsorption capacities estimated from the Langmuir model were 92.7 mg/g for NPX and 67.7 mg/g for DCF on UiO-66-NH₂@RHB. Thermodynamic results indicated spontaneous and endothermic adsorption for the MOF-decorated composites. Spectroscopic evidence supported adsorption dominated by pore filling, π-π interactions, and hydrogen bonding, with the amino functionality strengthening binding. UiO-66-NH₂@RHB retained reusability over five solvent-based regeneration cycles, demonstrating its suitability for sustained operation in pharmaceutical adsorption processes.