Fluoride adsorption studies of montmorillonite clay

Batch adsorption studies were conducted to determine the effects of contact time and temperature on fluoride removal by montmorillonite clay at neutral pH. The adsorption of fluoride was studied at four different temperatures, viz. 30, 40, 50 and 60°C. The kinetics of adsorption as well as adsorption isotherms at different temperatures were studied. Adsorption obeyed both Langmuir and Freundlich isotherm models. The percentage of fluoride removed increased with time and reached an optimum level at 50 min. The material with particle size of 75 microns registered maximum percentage of fluoride adsorption, compared to the other particle sizes. Thermodynamic studies revealed that the adsorption of fluoride by montmorillonite is an endothermic process, showing increase in sorption at higher temperature. The negative values of ΔG° indicate the spontaneity of the sorption process. Adsorption takes place on the surface as well as through intraparticle diffusion pattern of the adsorbent material. SEM studies revealed the morphological characteristics of the untreated sorbents as well as the changes in the treated sorbents. X-ray diffraction studies also confirmed the deposition of fluoride on the surface of the clay material. FTIR studies showed the involvement of hydroxyl group present on the surface in the adsorption interaction.