Enhanced adsorption and visible light photocatalytic removal of 5-Fluorouracil residuals using environmental NiO/geopolymer nanocomposite: Steric, energetic, and oxidation studies

Synthetic kaolinite-based geopolymer structure was impeded with NiO nanoparticles (NiO/GP) for promising decontamination of 5-Fluorouracil drug residuals (5-Fu) either by adsorption or oxidation. The adsorption of 5-Fu by NiO/GP follows the kinetic assumption of the Pseudo-First order model (R2 > 0.99). The equilibrium behaviors of 5-Fu uptake reactions were illustrated by the classic Langmuir and advanced Monolayer model of one energy site (R2 > 0.99). The steric parameters including both the active sites density (Nm) and 5-Fu saturation capacity (Qsat) increase regularly with the test temperature up to 40 oC (Nm = 323 mg/g and Qsat =329.7 mg/g) reflecting the exothermic properties of the reaction. The number of the captured 5-Fu molecules per site (n > 1) suggested vertical uptake of 1-2 molecules by a multimolecular mechanism. The estimated values of adsorption energy (14.6 -16.55 kJ/mol) and Gaussian energy (1.3-3.18 kJ/mol) reflect the physisorption of 5-Fu by NiO/GP. The energetic parameters of internal energy, free enthalpy, and entropy demonstrate spontaneous, feasible, and endothermic uptake reactions. As photocatalyst, the NiO/GP (0.1 g/L) caused complete oxidation for 5-Fu residuals (50 mg/L) after 20 min while the complete mineralization was achieved after 100 min under a visible light source. Based on the detected intermediate compounds, the 5-Fu oxidation pathway over NiO/GP occurred by hydroxylation and fluorine release processes considering the hydroxyl radicals as the main oxidizing species.