Defluoridation of drinking water by calcined MgAl-CO3 layered double hydroxides

This work used calcined MgAl-CO3 layered double hydroxides (CLDH) to remove fluoride with the reconstruction of their original layered structure in the presence of appropriate anions. The influences of pH of aqueous solution, dosage of CLDH and initial fluoride concentration on the distribution coefficient of fluoride with CLDH were investigated. The maximum distribution coefficient takes place at pH 6.0, 2.0 g/1.8 L CLDH dose, and 50 mg F−/L. It was found from the equilibrium investigation that the defluoridation capacity decreases with increased temperature, which shows that the uptake process is exothermic in nature. The data of fluoride removal fitted well into the linearly transformed Langmuir. Thermodynamic parameters including ΔG0, ΔH0 and ΔS0 were calculated. The values of ΔG0 at 30, 40, 50 and 60°C were −17.18, −17.55, −17.60 and −17.76 kJ/mol, indicating the spontaneous nature of defluoridation. The negative value of ΔH0 (−11.67 kJ/mol) confirms the exothermic nature of defluoridation. The positive value of ΔS0 (18.40 J/mol/K) suggests the increased randomness at the solid/solution interface for the uptake of fluoride onto CLDH. The mechanism of defluoridation was confirmed by powder X-ray diffraction and TG-MS.