Internalization of Fluoride in Hydroxyapatite Nanoparticles.

Hydroxyapatite (HAP) is a cost-effective material to remove excess levels of fluoride from water. Historically, HAP has been considered a fluoride adsorbent in the environmental engineering community. This paper substantiates an uptake paradigm that has recently gained disparate support: assimilation of fluoride to bulk apatite lattice sites in addition to surface lattice sites. Pellets of HAP nanoparticles (NPs) were packed into a fixed-bed media filter to treat solutions containing 30 mg-F/L (1.58 mM) at pH 8, yielding an uptake of 15.97 ± 0.03 mg-F/g-HAP after 864 h. Solid-state 19F and 13C magic-angle spinning nuclear magnetic resonance spectroscopy demonstrated that all removed fluoride is apatitic. A transmission electron microscopy analysis of particle size distribution, morphology, and crystal habit resulted in the development of a model to quantify adsorption and total fluoride capacity. Low- and high-estimate median adsorption capacities were 2.40 and 6.90 mg-F/g-HAP, respectively. Discrepancies between experimental uptake and adsorption capacity indicate the range of F- that internalizes to satisfy conservation of mass. The model was developed to demonstrate that F- internalization in HAP NPs occurs under environmentally relevant conditions and as a tool to understand the extent of F- internalization in HAP NPs of any kind.

Atom Probe Tomography of Encapsulated Hydroxyapatite Nanoparticles.

Hydroxyapatite nanoparticles (HAP NPs) are important for medicine, bioengineering, catalysis, and water treatment. However, current understanding of the nanoscale phenomena that confer HAP NPs their many useful properties is limited by a lack of information about the distribution of the atoms within the particles. Atom probe tomography (APT) has the spatial resolution and chemical sensitivity for HAP NP characterization, but difficulties in preparing the required needle-shaped samples make the design of these experiments challenging. Herein, two techniques are developed to encapsulate HAP NPs and prepare them into APT tips. By sputter-coating gold or the atomic layer deposition of alumina for encapsulation, partially fluoridated HAP NPs are successfully characterized by voltage- or laser-pulsing APT, respectively. Analyses reveal that significant tradeoffs exist between encapsulant methods/materials for HAP characterization and that selection of a more robust approach will require additional technique development. This work serves as an essential starting point for advancing knowledge about the nanoscale spatiochemistry of HAP NPs.