Synthesis, characterization, kinetic and thermodynamic investigation of silica nanoparticles and their application in mefenamic acid removal from aqueous solution

Three types of silica nanoparticles (SiO2-ZnO(CTAB), SiO2-(CTAB), and SiO2-ZnO) were prepared by sol-gel method as adsorbent for mefenamic acid drug. The morphology of ZnO and silica nanoparticles were investigated using scanning and transmission electron microscopy. The results showed that the CTAB cationic surfactant and ZnO nanoparticles improved the efficiency for removing mefenamic acid from aqueous solution. The adsorption of mefenamic acid onto SiO2-ZnO(CTAB) nanoparticles was characterized by infrared spectroscopy and the kinetic and thermodynamic adsorption mechanisms were investigated. Pseudo-first and pseudo-second-order kinetic adsorption models were applied to the adsorption of mefenamic acid onto SiO2-ZnO(CTAB) nanoparticles. The results indicated that the experimental data fitted the pseudo-second-order model better than pseudo-first-order model. Also, the results showed that Langmuir isotherm model described efficiently the adsorption process rather than Freundlich isotherm model. The activation energy value of (74.73 kJ/mole) indicated that a chemisorption process is predominant in the adsorption mechanism due to cationic and anionic electrostatic attraction. Thermodynamic parameters of standard enthalpy, standard entropy and standard Gibbs free energy were determined at different temperatures and indicated an exothermic and spontaneous process in nature.