Preparation and Properties of an Aluminum Hydroxide-Modified Diatomite Sorbent for Removal of Fluorides from Waters

Abstract

A new sorbent (Al-D) for the removal of fluoride ions is prepared by modifying diatomite with aluminum ions. The sorbent is studied using X-ray diffraction, thermogravimetry, X-ray photoelectron spectroscopy, and adsorption structure analysis; its ability to remove fluoride ions from model aqueous solutions is measured under static conditions. The diatomite, which is modified by heterogeneous hydrolysis of an aluminum salt in the presence of diatomite suspension, has a higher specific surface area and a greater sorption pore volume and exhibits excellent characteristics in fluoride adsorption. Fluoride sorption isotherms measured under equilibrium conditions are modeled using the Langmuir, Freundlich, Langmuir−Freundlich, Brunauer−Emmett−Teller, and two-step Langmuir equations. The model parameters are calculated. The two-step Langmuir model gives the best fit to the experimental isotherms (R² = 0.9836). Fluoride adsorption on the sorbent Al-D occurs via ion exchange: surface OH⁻ groups are replaced with fluoride ions from solution to form aluminum fluoride complexes at the sorbent surface.