Novel Apatite-Based Sorbent for Defluoridation: Synthesis and Sorption Characteristics of Nano-micro-crystalline Hydroxyapatite-Coated-Limestone

摘要

Elevated levels of fluoride (F~- ) in groundwaters of granitic and basaltic terrains pose a major environmental problem and are affecting millions of people all over the world. Hydroxyapatite (HA) has been shown to be a strong sorbent for F~-; however, low permeability of synthetic HA results in poor sorption efficiency. Here we provide a novel method of synthesizing nano- to micrometer sized HA on the surfaces of granular limestone to improve the sorption efficiency of the HA-based filter. Our experiments with granular limestone (38-63, 125-500 μm) and dissolved PO_4~(3-) (0.3-5.3 mM) as a function of pH (6-8) and temperature (25-80 ℃) indicated rapid formation of nano- to micrometer sized HA crystals on granular limestone with the maximum surface coverage at lower pH and in the presence of multiple additions of aqueous PO_4~(3-). The HA crystal morphology varied with the above variables. The sorption kinetics and magnitude of F~- sorption by HA-coated-fine limestone are comparable to those of pure HA, and the F~- levels dropped to below the World Health Organization's drinking water limit of 79 μM for F~- concentrations commonly encountered in contaminated potable waters, suggesting that these materials could be used as effective filters. Fluorine XANES spectra of synthetic HA reacted with F~- suggest that the mode of sorption is through the formation of fluoridated-HA or fluorapatite at low F~- levels and fluorite at high F~- loadings.