Hydrothermal synthesis of hierarchical hollow hydroxyapatite microspheres with excellent fluoride adsorption property

Abstract

Abstract Hierarchical hollow hydroxyapatite microspheres (HHHMs) were fabricated by a facile hydrothermal process and used as an adsorbent for removing fluoride from aqueous solutions. The adsorbent was characterized by microscopic, spectroscopic, diffractometric and thermal analysis techniques. The adsorption kinetics, adsorption isotherms, and the effects of both the initial solution pH and coexisting anions on fluoride adsorption were evaluated. The results showed that the excellent fluoride adsorption capacity of the HHHMs was related to their hierarchical porous hollow structure and large specific surface area. Furthermore, the adsorption kinetics of the HHHMs were accurately described by the pseudo-second-order kinetic model, the adsorption isotherms were perfectly fitted by the Langmuir isotherms model. The adsorption process was pH dependent. The fluoride adsorption capacity decreased the most in the presence of carbonate, followed by phosphate and sulfate, and then nitrate and chloride, which had equal effect on the fluoride adsorption process. Moreover, the adsorption mechanism was studied by FT-IR, XPS, and pHpzc analysis. The results confirmed that the interaction of fluoride with the HHHMs was mainly dominated by ion exchange, electrostatic interactions, and hydrogen bonding. All these results indicated that the low-cost and scalable production of HHHMs has excellent applicability for wastewater treatment.