Adsorption and precipitation of fluoride on calcite nanoparticles: A spectroscopic study

Adsorption and precipitation of fluoride on the surface of calcite (CaCO3) nanoparticles was investigated. Results from X-ray photoelectron spectroscopy (XPS) showed that calcium fluoride (fluorite, CaF2) precipitated on the surface of calcite nanoparticles at pH 7.2 ± 0.1 and fluoride concentration of 100 mg/L, while no precipitate was found at 10 mg/L. Analysis by high resolution transmission electron microscopy (HRTEM) of calcite nanoparticles reacted with 40 mg/L of fluoride showed a thin layer of fluorite on the surfaces. Interaction of calcite nanoparticles with 100 mg/L of fluoride resulted in damage of single calcite nanoparticle, and more grains of precipitate were observed. Transition from adsorption to precipitation was confirmed by zeta potential of calcite nanoparticles that shifted to negative values when at fluoride concentration of 5 mg/L, and then increased with fluoride concentration. Fluoride at low concentration is mainly removed by surface adsorption via electrostatic interactions, while surface precipitation of calcium fluoride begins at higher fluoride concentration (ca. ≧10 mg/L), and surface precipitation becomes dominant at higher concentration (ca. ≧40 mg/L).