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 μm) and dissolved PO4 3– (0.5–5.3 mM) as a function of pH (6–8) and temperature (25–80 °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 PO4 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.