Formation of separate phases of calcium phosphates in calcareous and some alluvial soils of Egypt [electronic resource].

Includes references.

A ctivities of Ca and P species of H20 extract of30 surface soil samples, representing alluvial, calcareous and sandy soils of Egypt, showed the dominance of HP04u species overall calculated ones. The predominance area diagram of various Ca - P forms reveals that P activity in the tested soils was regulated by hydroxyapatite (HAP), the most stable compound according to the thermodynamic rules. Values of the equilibrium constants (log k) calculated for various Ca-P compounds revealed that both dicalcium phosphate dihydrate (DCPD) and dicalcium phosphate unhydrate (DCP) have been precipitated at the beginning of the reaction between soil and solutions of high P concentration. Except few samples, the precipitated compounds transformed rapidly to octacalcium phosphate (OCP), then to tricalcium phosphate (TCP). On the other hand, the common applied low rate of P didn't initiate formation of a separate phase of Ca-P whether the soils were exposed to alternating wetting and drying cycles, or kept at a constant moisture level. Concentrations (10.4, 1.67 x 10.4 and 10-78 M) of P, Ca and H, respectively, expected to produce supersaturated solutions with respect to hydroxyapatite (HAP) were kept constant throughout the reaction period. Results showed that the solutions attained equilibrium with TCP, and at the same time proved super saturation with HAP. These postulations have been confirmed and examined using X-ray diffractio

Summary in Arabic.

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