Adsorption of Fluoride Ions Using Si–Al–Mg Layered Double Hydroxides

Abstract

ABSTRACT Removal of fluoride ions from water environments has become a research topic because high concentrations of fluoride ions in drinking water are harmful to the human body. The adsorption of fluoride ions by a Si – Al – Mg layered double hydroxide (LDH) was investigated in this study. The prepared adsorbent had a hydrotalcite-like LDH structure, which was decomposed by calcination and reconstructed upon contacting water. The adsorption process followed pseudo-second-order kinetics. Granulation of the adsorbent for application to chromatographic operation decreased the adsorption rate constant. Adsorption of fluoride ions was high at 7 < pHeq <10 and decreased at higher pHs. The adsorption mechanism involved ion exchange between the fluoride ions in aqueous solution and chloride ions in the adsorbent as well as physisorption via van der Waals forces. The adsorbent was selective towards fluoride ions over nitrate and sulfate ions, although the adsorption of fluoride ions was suppressed in the presence of chloride, carbonate, and phosphate ions. Chromatographic operation using the granulated adsorbent with an alumina-based binder was characterized by slow kinetics, necessitating a low flow rate for the feed solution. The prepared adsorbent was also used to selectively adsorb fluoride ions from simulated wastewater produced by the semiconductor manufacturing process.