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 F-19 solid-state NMR spectroscopy to investigate F- sorption mechanisms by nanosized Hap combined with H-1 NMR and H-1{F-19}) 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 [ca(5)(PO4)(3)F] (Fap) or fluoro-hydroxyapatite solid solution [Ca-5(PO4)(3)F-x(OH)(1-x); x = 0-1] (F-Hap). A simultaneous formation of fluorite (CaF2) precipitates (delta(F-19) = -108 ppm) was observed at higher [r] (e.g., 100 mM), which was further confirmed by TEM and XRD analysis. The NanoSIMS and H-1{F-19) 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. (C) 2019 Elsevier Inc. All rights reserved.