THE ADSORPTION AND PRECIPITATION OF PHOSPHATE ONTO CALCITE

Summary Scanning electron micrographs show that the reaction products of solution phosphate and calcite are hemispherical, coral-like growths on calcite surfaces. Electron probe micro-analysis indicates that these are a calcium phosphate, and solution data plotted against solubility isotherms suggest that dicalcium phosphate (DCP) is formed rapidly and slowly changes to octa-calcium phosphate (OCP). X-ray diffraction shows that DCP and OCP are present with DCP predominating. The ratio of Ca:P in the treated calcite, after allowing for the CaCO3 present, is that in DCP, and a surface coating of OCP on DCP is likely. The exchangeability of the reacted P falls from 100 per cent for small amounts (0–10 μg P per g calcite) to a constant 30 per cent when larger amounts are present (200–1000 μg/g) resulting from the porous structure of the coral-like growths. These cover only a small fraction of the calcite surface even when large amounts of P are present so allowing calcite to control solution pH.