THE ADSORPTION AND PRECIPITATION OF PHOSPHATE ONTO CALCITE

Abstract

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 CaCO 3 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 μ g P per g calcite) to a constant 30 per cent when larger amounts are present (200–1000 μ g/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.