Adsorption of fluoride by hydrous iron(III)–tin(IV) bimetal mixed oxide from the aqueous solutions

The synthetic iron(III)–tin(IV) mixed oxide (HITMO) has been characterized by the FTIR, XRD and SEM image analyses. The mixed oxide is found hydrated and amorphous with irregular surface morphology. The relevant parameters studied for fluoride removal by using HITMO are the effects of pH, contact time, and equilibrium. The fluoride adsorption capacity is nearly constant in the pH range 5.0–7.5. The pseudo-second order equation explains the kinetic data well and, the overall rate is multi-stage controlled. The Langmuir isotherm describes the equilibrium data well and gives high Langmuir capacity (∼10.50 mg g−1) value. The mean adsorption energy (9.05 kJ mol−1) computed from the Dubinin–Redushkevich isotherm suggests the ion exchange mechanism for fluoride adsorption. The evaluated change of enthalpy (ΔH0), entropy (ΔS0) and free energy (ΔG0) of adsorption values indicates the endothermic and non-spontaneous nature of the present reaction. Excepting bicarbonate, other common ions show no adverse effect on fluoride removal. The regeneration (∼75%) of fluoride-rich (10.0 mg g−1) material is possible by a solution of pH 13.0. Two gram HITMO per litre of the fluoride-doped (2.97 mg L−1) natural water reduces fluoride level below 1.5 mg L−1.