KINETICS AND PATHWAYS OF APATITE FORMATION - IN SITU TIME-RESOLVED STUDIES

Heavy metal contamination in soils is a serious public health hazard. A new and promising method for in situ metal sequestration and environmental remediation, based on the formation of co-precipitated apatite in contaminated soils, has gained considerable amount of attention in the past decade, mainly due to its cost-efficiency and high effectiveness. However, the role of precursor phases (expected to form in near surface conditions) on the sequestration and fate of metals is unknown.

In situ time-resolved X-ray diffraction studies of apatite formation pathways from aqueous solutions were carried out at the X7B beamline of the National Synchrotron Light Source at Brookhaven National Laboratory, Upton, NY. A series of experiments with different Ca/P (corresponding to stoichiometric ratios of Ca and P found in the precursor phases proposed in the literature) and liquid/solid ratios in the starting material, in the presence and absence of contaminant species and range of temperatures were performed. In all experiments, two starting solutions of calcium acetate and ammonium phosphate were mixed instantaneously at room temperature. The resulting precipitate was then vacuum-filtered and the remaining slurry, of the desired liquid/solid ratio, was placed inside a heating cell and analyzed within 10 min of the initial precipitation.

In all experiments, the initial precipitate was identified as a mixture of amorphous calcium phosphate and brushite (CaHPO₄ • 2H₂O), independent of the initial Ca/P ratio in solution, and were the only phases present in the solution throughout the duration of the analysis in the experiment conducted at ambient temperature. Under the conditions of elevated temperature and in the absence of contaminant species, initial brushite transformed to intermediate monetite and then to apatite. The rate of the phase transformation was dependent on the temperature and Ca/P ratio of the starting solutions, being the fastest and most complete at the ratio corresponding to OCP stoichiometry. The presence of Mn²⁺ ions influenced the pathway of the phase transformation, resulting in the formation of a different final product of the reaction – whitlockite.