Novel Apatite-Based
Sorbent for Defluoridation: Synthesis
and Sorption Characteristics of Nano-micro-crystalline Hydroxyapatite-Coated-Limestone
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Abstract
Elevated levels of fluoride (F<sup>–</sup>) 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<sup>–</sup>; 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<sub>4</sub><sup>3–</sup> (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 PO<sub>4</sub><sup>3–</sup>. The HA crystal morphology varied with the above variables. The
sorption kinetics and magnitude of F<sup>–</sup> sorption by
HA-coated-fine limestone are comparable to those of pure HA, and the
F<sup>–</sup> levels dropped to below the World Health Organization’s
drinking water limit of 79 μM for F<sup>–</sup> 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<sup>–</sup> suggest
that the mode of sorption is through the formation of fluoridated-HA
or fluorapatite at low F<sup>–</sup> levels and fluorite at
high F<sup>–</sup> loadings