Kinetics of Fluorapatite Precipitation in Fluoride-Contaminated Water

Abstract

Calcium phosphate (Ca-PO₄) solids, especially apatites, are considered potential amendments to remediate aquifers impacted by inorganic pollutants such as fluoride (F) due to their low solubilities. However, the precipitation kinetics of these solids at room temperature are poorly constrained, which limits their application. Specifically, to evaluate the extent and kinetics of F remediation through fluorapatite (FA; Ca₅(PO₄)₃F) precipitation, the quantification of the fluorapatite formation constant is important. In this contribution, flow-through experiments were conducted to estimate the surface-normalized rate constant of FA precipitation. Experiments were performed in the presence of a typical Ca source, calcite (4 g·L^–1), with a fixed influent phosphate dosage (1 mM) at variable influent F concentrations (0.1–0.42 mM) and flow rates (0.01–0.04 mL·min^–1). Reactor effluents were collected using a fraction collector at specific time intervals and analyzed for pH and concentrations of F, Ca, and PO₄ until a steady-state was reached. The reacted solids collected after the experiments were characterized for the identification of precipitated phases. Process-based elemental mass balance equations were used to model the eluent concentrations. The surface normalized rate constants of precipiation of fluorapatite and hydroxyapatite (HA; Ca₅(PO₄)₃OH) were determined to be 10^{–28.24±1.24} μmoles·m^–2·min^–1 and 10^{–21.74±6.85} μmoles·m^–2·min^–1, respectively. However, the modeled critical supersaturation ratio for fluorapatite was comparable to the ratio for hydroxyapatite. Quantification of these fundamental rate constants fill a key knowledge gap that would likely help in predicting the fate and transport of F and PO₄ in contaminated aquifers or in the treatment of F-contaminated surface and aquifer waters.