Molecular-scale investigation of fluoride sorption mechanism by nanosized hydroxyapatite using ¹⁹F solid-state NMR spectroscopy

Hydroxyapatite (Hap) has been shown to be an excellent sorbent for F⁻ removal of elevated levels of fluoride in groundwater worldwide; however, the molecular mechanisms of this process have not been clearly addressed. Herein, we used ¹⁹F solid-state NMR spectroscopy to investigate F⁻ sorption mechanisms by nanosized Hap combined with ¹H NMR and ¹H{¹⁹F} Rotational Echo DOble Resonance (REDOR) technology in addition to other characterization methods such as Transmission Electron Microscopy (TEM), X-ray Diffraction (XRD) and Nanoscale Secondary Ion Mass Spectroscopy (NanoSIMS). Our experimental results showed that F⁻ sorption mechanisms depend on solution pH and fluoride concentration ([F⁻]). At pH 7 and [F⁻] ≤ 50 mM, a single ¹⁹F NMR peak at −103 ppm was observed, which could be assigned to fluorapatite [Ca₅(PO₄)₃F] (Fap) or fluoro-hydroxyapatite solid solution [Ca₅(PO₄)₃F_x(OH)_1−x; x = 0–1] (F-Hap). A simultaneous formation of fluorite (CaF₂) precipitates (δ_F-19 = −108 ppm) was observed at higher [F⁻] (e.g., 100 mM), which was further confirmed by TEM and XRD analysis. The NanoSIMS and ¹H{¹⁹F} REDOR analyses indicated that a dissolution-precipitation process was involved in the F⁻ sorption on Hap. Our results strongly support the efficacy of Hap for F⁻ removal even after several instances of regeneration, making it a cost-effective strategy for fluoride treatment.