Mechanisms of fluoride uptake by surface-modified calcite: A ¹⁹F solid-state NMR and TEM study

Fluoride pollution in groundwater is a serious problem threatening millions of people worldwide. Calcite is considered an ideal adsorbent for defluoridation owing to its widespread presence and low cost. To further enhance its performance, we synthesize a series of phosphate-modified calcites with varying phosphate concentrations. The surface modification led to the formation of a nanosized hydroxyapatite (HAP) coating on the calcite surface. With increasing concentrations of phosphate used for modification, the BET specific surface area of the adsorbents was dramatically enhanced, resulting in a great enhancement of F uptake. At low F concentrations (i.e., <1 mM), surface-modified calcite can achieve up to 25 times higher F removal efficiency than calcite. The ¹⁹F solid-state MAS NMR spectra yielded three distint peaks at δ(¹⁹F) = -86 ppm, -99 ppm, and -122 ppm, representing the formation of carbonate fluorapatite (CFA), fluorapatite (FAP), and coprecipitated F, respectively. This provides strong evidence for the contribution of newly formed HAP to F removal. In contrast, at high F concentrations (e.g., >2 mM), surface modification did not enhance F uptake by calcite. The ¹⁹F solid-state MAS NMR analysis revealed that the predominant deflurodation mechanism is the formation of CaF₂ precipitates (δ(¹⁹F) = -108 ppm) for both pristine and modified calcite at high F concentrations. Under this condition, the contribution of the newly formed nanosized HAP to F uptake is insignificant. Taken together, our results demonstrated the potential of surface modification of calcite as a cost-effective technique for defluoridation for most F-rich groudwater.