Defluoridation from aqueous solution by lanthanum hydroxide

This research was undertaken to evaluate the feasibility of lanthanum hydroxide for fluoride removal from aqueous solutions. A batch sorption experiments were conducted to study the influence of various factors such as pH, presence of competing anions, contact time, initial fluoride concentration and temperature on the sorption of fluoride on lanthanum hydroxide. The optimum fluoride removal was observed in the pH(eq)≤7.5. The presence of competing anions showed no adverse effect on fluoride removal. The equilibrium data reasonably fitted the Langmuir isotherm model, and the maximum monolayer sorption capacity was found to be 242.2 mg/g at pH(eq)≤7.5 and 24.8 mg/g at pH(eq)>10.0. The pseudo-second-order kinetic model described well the kinetic data, and resulted in the activation energy of 53.4-68.8 kJ/mol. It was suggested that the overall rate of fluoride sorption is likely to be controlled by the chemical process. Thermodynamic parameters such as ΔG°, ΔH° and ΔS° indicated that the nature of fluoride sorption is spontaneous and endothermic. The used lanthanum hydroxide could be regenerated by washing with NaOH solution. Results from this study demonstrate the potential usability of lanthanum hydroxide as a good fluoride selective sorbent.