Temperature effect on the zeta potential and fluoride adsorption at the α-Al2O3/aqueous solution interface

The effect of temperature and pH on the zeta potential of α-Al2O3 and adsorption of fluoride ions at the α-Al2O3/aqueous solution interface has been investigated through electrophoretic mobility measurements and adsorption studies, to delineate mechanisms involved in the removal of fluoride ions from water using alumina as adsorbent. When the temperature increases from 10 to 40 °C, the pH of the point of zero charge (pHpzc) shifts to smaller values, indicating proton desorption from the alumina surface. The pHpzc increases linearly with 1/T, which allowed estimation of the standard enthalpy change for the surface-deprotonation process. Fluoride ion adsorption follows a Langmuir-type adsorption isotherm and is affected by the electric charge at the α-Al2O3/aqueous solution interface and the surface density of hydroxyl groups. Such adsorption occurs through an exchange between fluoride ions and surface-hydroxyl groups and it depends on temperature, pH, and initial fluoride ion concentration. At 25 and 40 °C, maximum fluoride adsorption density takes place between pH 5 and 6. Increasing the temperature from 25 to 40 °C lowers the adsorption density of fluoride. Fluoride ion adsorb at the α-Al2O3/aqueous solution interface following the Langmuir isotherm. Adsorption is high and maxima at pH 5 due to specific interaction of fluoride on AlOH+2 and AlOH surface sites and is low at pH 9 due to electrostatic repulsion between fluoride ions and the negative surface of alumina.