Effects of pCO2 on Hydroxyapatite Formation and its Stability: Implications for Understanding Phosphorus Availability in Calcareous Soil
Hydroxyapatite is an important phosphorus (P) sink in calcareous soils, controlling P solubility. The activity of carbonate in soil pore water, however, is often underestimated because soil respiration and solution-calcite equilibria could elevate pCO2 much greater than 415 ppmv. Thus far, the role of pCO2 in hydroxyapatite formation in calcareous soils has not been extensively investigated. Accordingly, the effects of pCO2 levels (415 ppmv to 20,000 ppmv) and aging time (7 days to 1 month) on hydroxyapatite formation were investigated using experimental geochemistry and X-ray diffraction (XRD) analysis. X-ray diffraction (XRD) analyses showed a decrease in the quantity of hydroxyapatite with increasing pCO2. This is attributed to decreased calcium (Ca) activity and an increase in calcium carbonate formation. Scanning Electron microscopy (SEM) showed rounded particles of hydroxyapatite, and zeta potential values of these particles remained positive (2-112 mV) at pH 8 in all samples regardless of pCO2 and aging times. Dissolution experiments showed that higher pCO2 values facilitated the release of P from hydroxyapatite, suggesting the carbonate ligand-promoted dissolution mechanism. The hydroxyapatite was stable at pH 6, but dissolution was enhanced by decreasing pH from 6 to 4. Hydroxyapatite prepared under higher pCO2 released more phosphate than that prepared under pCO2 = 415 ppmv, indirectly suggesting higher solubility in carbonate substituted hydroxyapatite. The variability of pCO2(g) should be considered to better understand the P cycle in calcareous soils.
