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

Abstract

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.