Fluoride adsorption onto granular ferric hydroxide: Effects of ionic strength, pH, surface loading, and major co-existing anions

Abstract

Fluoride adsorption onto granular ferric hydroxide (GFH) was investigated using batch methods, under various ionic strength, pH, surface loading, and major co-existing anion conditions. Adsorption of fluoride on GFH included an initial fast adsorption phase followed by a slow adsorption phase. Within the pH range of 2–11, fluoride adsorption equilibrium was not affected by ionic strength, but was significantly affected by pH. Maximum adsorption was achieved in the pH range of 3–6.5. Under the same pH condition, fluoride adsorption followed the Freundlich isotherm, indicating that the GFH surface was heterogeneous. X-ray photoelectron spectroscopy (XPS) and attenuated total reflection-infrared (ATR-IR) spectroscopy data showed evidence for fluoride sorption on the GFH surface via inner-sphere complexation accompanying increased hydrogen bonding and surface hydroxylation. Major anions, including phosphate, bicarbonate, sulfate, and chloride, reduced fluoride adsorption in the following order: H 2PO 4 − > HCO 3 − > SO 4 2− > Cl −.