Kinetics of Fluorapatite\nPrecipitation in Fluoride-Contaminated\nWater

Calcium phosphate (Ca-PO<sub>4</sub>) solids, especially\napatites,\nare considered potential amendments to remediate aquifers impacted\nby inorganic pollutants such as fluoride (F) due to their low solubilities.\nHowever, the precipitation kinetics of these solids at room temperature\nare poorly constrained, which limits their application. Specifically,\nto evaluate the extent and kinetics of F remediation through fluorapatite\n(FA; Ca<sub>5</sub>(PO<sub>4</sub>)<sub>3</sub>F) precipitation, the\nquantification of the fluorapatite formation constant is important.\nIn this contribution, flow-through experiments were conducted to estimate\nthe surface-normalized rate constant of FA precipitation. Experiments\nwere performed in the presence of a typical Ca source, calcite (4\ng·L<sup>–1</sup>), with a fixed influent phosphate dosage\n(1 mM) at variable influent F concentrations (0.1–0.42 mM)\nand flow rates (0.01–0.04 mL·min<sup>–1</sup>).\nReactor effluents were collected using a fraction collector at specific\ntime intervals and analyzed for pH and concentrations of F, Ca, and\nPO<sub>4</sub> until a steady-state was reached. The reacted solids\ncollected after the experiments were characterized for the identification\nof precipitated phases. Process-based elemental mass balance equations\nwere used to model the eluent concentrations. The surface normalized\nrate constants of precipiation of fluorapatite and hydroxyapatite\n(HA; Ca<sub>5</sub>(PO<sub>4</sub>)<sub>3</sub>OH) were determined\nto be 10<sup>–28.24±1.24</sup> μmoles·m<sup>–2</sup>·min<sup>–1</sup> and 10<sup>–21.74±6.85</sup> μmoles·m<sup>–2</sup>·min<sup>–1</sup>, respectively. However, the modeled critical supersaturation ratio\nfor fluorapatite was comparable to the ratio for hydroxyapatite. Quantification\nof these fundamental rate constants fill a key knowledge gap that\nwould likely help in predicting the fate and transport of F and PO<sub>4</sub> in contaminated aquifers or in the treatment of F-contaminated\nsurface and aquifer waters.