Iron oxide nanoparticle preparation and its use for the removal of fluoride from aqueous solution: application of isotherm, kinetic, and thermodynamics

Fluoride in drinking water above permissible levels is responsible for human and skeletal fluorosis. In this study, iron oxide nanoparticles (Fe 3 O 4 -NPs) were synthesized through an electrochemical method and used as an adsorbent for fluoride removal from aqueous solutions. The adsorption results indicated that Fe 3 O 4 -NPs could quickly capture fluoride in the initial 20 min, and the kinetic data could be well described by the pseudo-second-order model with a rate constant k 2 of 0.100–0.187 g/mg.min. The adsorption isotherm data fitted better to the Langmuir model (R 2 = 0.969) relative to Freundlich (R 2 = 0.851), Temkin (R 2 = 0.743), and Dubinin–Radushkevich (R 2 = 0.904) models, indicating that monolayer adsorption occurred on the Fe 3 O 4 -NPs, and there was no interaction between adsorbed fluoride. Based on thermodynamics, the negative enthalpy (ΔH° = –0.011 kJ/mol) indicated that the adsorption process was exothermic, and the negative entropy (ΔS° = –0.5 kJ/mol.K) suggested that the degree of freedom at solid solution level declined during the adsorption process. In addition, the influences of important parameters including pH, adsorbent dosage, initial fluoride concentration, contact time, and temperature were investigated, and the optimal adsorption conditions were determined as pH 6, Fe 3 O 4 -NPs 0.02 g/L, initial fluoride 25 mg/L, contact time 45 min, and temperature 25°C.