High fluoride (&gt;2 mg/L) in drinking water causes chronic fluorosis. Many aquifers contain toxic fluoride (F-) levels that disproportionately affect poor, rural populations who lack access to technically involved, financially prohibitive, or otherwise inappropriate treatment technologies. As of 2008, an estimated 200 million people worldwide have or risk incurring fluorosis because they consume water above the World Health Organization’s recommended level of 1.5 mg/L. Calcium hydroxyapatite (HAP, Ca5(PO4)3OH), synthesized via an aqueous precipitation route, is a promising ionic nanocrystal (NC) technology that is easy to use, selective for fluoride uptake, and a cost-effective treatment option. Moreover, there is significant room for enhancing HAP’s effective sorption capacity. Given that fluorapatite (FAP, Ca5(PO4)3F) has a significantly higher thermodynamic stability than HAP, and that the large phosphate anions conserve the apatite NC framework (a well-studied phenomenon in ionic NC systems), anion exchange of aqueous F- ions with lattice hydroxide ions, OH(l)-, ought to result in full, facile conversion to FAP. However full conversion does not occur on a relevant time-scale, leading researchers to conclude that F- is adsorbed by various suggested (but unobserved) routes. Understanding HAP's F- removal mechanisms could lead to appropriating the HAP material system for full HAP to FAP conversion, which would lead to a tenfold increase in F- removal. In this work, we use quantitative 1H and 19F as well as 2D 1H solid-state nuclear magnetic resonance spectroscopy in conjunction with bench-scale batch testing and conventional characterization techniques to show that host-guest lattice ion exchange does occur. However, the rate of long term ion exchange is hampered by loss of structural water, H2O(st), controlled by heating the material up to 300 °C without incurring other structural changes. We show direct evidence that H2O(st) exists in close proximity (~2-3 Å) to OH(l)- ions located along a 1-dimensional channel in the crystallographic [001] direction. This body of evidence suggests that H2O(st) fills OH- vacancies and facilitates the transport of OH(l)- ions along the channel for exchange with F- ions at the solid-water interface. Further, upon the loss of H2O(st) there is no longer exchange of F- with OH(l)-. Yet, most F- ions that are sorbed end up in the FAP chemical state, leading to the conclusion that these F- ions are filling OH- vacancies.

Role of structural water and hydroxide vacancies in defluoridation mechanisms of calcium hydroxyapatite

256th National Meeting (2018)

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