Effects of pCO2 on hydroxyapatite formation: X-ray diffraction study

Hydroxyapatite is an important phosphorus (P) sink in calcareous soils. The activity of carbonate in soil pore water, however, is often underestimated because soil respiration and solution-calcite equilibria could elevate CO₂(g) concentration much greater than 415 ppmv (i.e., pCO₂: ∼0.3 mm Hg). Thus far, the role of CO₂(g) or pCO₂ in the hydroxyapatite formation in calcareous soils has not been extensively investigated. Accordingly, the effects of CO₂ concentration (415, 8000, and 20,000 ppmv) on hydroxyapatite formation were investigated at pH 8 using experimental geochemistry and X-ray diffraction (XRD) analysis. XRD analyses showed the formation of hydroxyapatite under all CO₂ concentrations, but the extent of calcite formation increased with increasing CO₂ concentration. The formation of calcium (Ca) carbonate phosphate was also observed after 30 days under [CO₂(g)] up to 8000 ppmv. This is attributed to an increase in calcium carbonate formation. Scanning electron microscopy showed rounded hydroxyapatite particles. The variability of [CO₂(g)] in subsoils should be considered in the P cycle in calcareous soils.