Adsorption of fluoride onto crystalline titanium dioxide: Effect of pH, ionic strength, and co-existing ions

Abstract

Adsorption of fluoride from water onto titanium dioxide (TiO2) powder was investigated. The sorbent was crystalline TiO2 composed of mostly anatase with a specific surface area of 56m2/g. Adsorption kinetics and isotherm experiments were performed using an aqueous solution with bicarbonate alkalinity representative of natural waters. Adsorption kinetics data showed that maximum adsorption of fluoride occurred within 3h, following a pseudo-second order kinetics model. Adsorption isotherm data followed the Langmuir equation, indicating favorable adsorption of fluoride onto TiO2, while results from the Dubinin–Radushkevich model are indicative of physical adsorption of fluoride. Adsorption of fluoride increased with decreasing solution pH. Maximum adsorption of fluoride occurred within the pH range of 2–5, while approximately 75% of maximum adsorption was obtained in the pH range of 7–8 with rapidly declining adsorption above pH 9. The pHpzc data for TiO2 indicated the preferred adsorption of fluoride onto the acidic surface of TiO2. Higher bicarbonate concentrations in solution increased the solution pH which was conducive to a decrease in adsorption of fluoride onto the surface of TiO2 at higher pH. Overall, the solution pH was the main factor controlling the uptake of fluoride by TiO2.