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

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 mu m) and dissolved PO43- (0.5-5.3 mM) as a function of pH (6-8) and temperature (25-80 degrees C) 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 PO43-. 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 mu M 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.