X-ray Diffraction and X-ray Absorption Near-Edge Structure Spectroscopic Investigation of Hydroxyapatite Formation under Slightly Acidic and Neutral pH Conditions

Precipitation of hydroxyapatite is one of the important geochemical processes controlling the concentration of phosphorus (P) in alkaline soils, though its formation at below neutral pH is not clearly understood. In this study, the precipitation of hydroxyapatite at pH 5.8 and 6.8 was investigated under environmentally relevant Ca/P molar ratios (1, 50, and 100) as a function of aging time (1–40 days) using conventional X-ray diffraction (XRD), synchrotron-based XRD, and P K-edge X-ray absorption near-edge structure spectroscopy (XANES). The study showed (1) poorly crystalline hydroxyapatite can form under mildly acidic pH, (2) the transformation of dicalcium phosphate dihydrate into a mixture of octacalcium phosphate and poorly crystalline hydroxyapatite at a Ca/P ratio of 10 at pH 5.8, and (3) the formation of poorly crystalline hydroxyapatite and a small quantity of octacalcium phosphate at a Ca/P ratio of 50–100 at pH 5.8 and at a Ca/P ratio of 10–100 at pH 6.8. After 40 days, an increase in crystallinity of poorly crystalline hydroxyapatite was observed in the XRD analysis. However, the XANES analysis was not sensitive in differentiating dicalcium phosphate dehydrate from hydroxyapatite. Our study showed that the precursor of hydroxyapatite is dependent on the Ca/P ratio and pH < 7. The study shows that the formation of hydroxyapatite can potentially control the concentration of P, as long as the activity of P is high enough, in pore waters under slightly acidic and near neutral pH conditions.