The surface regulation of calcite for defluoridation by fluorapatite-induced crystallization

Fluoride can be removed from potable water by induced crystallization of fluorapatite (FAP); however, the properties of the original crystal seed, using calcite as a continuous source of Ca2+, are not well-defined. This paper describes the effect of calcite surface regulation by phosphate groups on the form of FAP that serves as the original crystal seed for the induced crystallization defluoridation. The results show that, when dissociated from phosphate groups, H+ promotes Ca2+ dissolution from the surface of calcite, although the kinetics of calcium dissolution by phosphate acid and phosphate salt are different. The results of molecular dynamic (MD) simulations show that phosphate has a strong binding effect on the surface of calcite, which suits the subsequent FAP coating. The sodium ions (Na+) supplied by sodium dihydrogen phosphate (NaH2PO4) inhibit the recrystallization of calcium carbonate (CaCO3) and promote the continuous release of Ca2+ from calcite. The Ca2+ ions then promote the nucleation and growth of FAP on the surface of the calcite. Scanning electron microscope (SEM) images indicate that rose petal-shaped FAP forms on the surface of calcite with a coating time of 6–24 h, which provides growth sites for induced crystallization defluorination. The fluorapatite-coated calcite (FCC) with a coating time of 6 h and an FAP coverage of the calcite surface of 82–86 % was selected as the seed crystal. The fixed-bed induced crystallization reactor was run for 700 bed volumes (BV) with this seed crystal and successfully removed 84.3–99.8 % of the fluoride.