Abstract
CaO/Ca12Al14O33/Ca5Al6O14 composites were synthesized through calcination of Ca-Al layered double hydroxides (CaAl-LDHs). The structures of the CaAl-LDHs and the composites were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and nitrogen adsorption–desorption isotherms. Batch experiments were carried out to study the influence of various experimental parameters such as initial fluoride concentration, contact time, pH and the coexisting anions on the adsorption of fluoride. The fluoride adsorption follows the Freundlich isotherm model. With the World Health Organization (WHO) guideline of 1.5 mg L−1, the adsorption capacity was 52.78 mg g−1. The kinetic data were fitted to the pseudo-second-order model. The change of pH (3–10) has almost no influence on the fluoride removal. High concentration of the coexisted sulfate, carbonate and phosphate anions slightly inhibited the fluoride removal. The fluoride removal mechanism study suggested that CaO transformed into CaF2 after fluoride adsorption, which play key roles in fluoride removal. The hydrolysis of Ca12Al14O33 and Ca5Al6O14 resulted the formation of Ca3Al2(OH)12. Fluoride anions participated in this hydrolysis process and resulted in the formation of Ca3Al2(OH)12-xFx, which was revealed by XRD, X-ray photoelectron spectroscopy analysis (XPS) and SEM elemental mapping images.
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