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Kinetics of the Phosphate Interaction with Calcite¹

ABSTRACT

The kinetics of the phosphate interaction with calcite were studied. The results showed that the reaction did not proceed in the absence of the calcite surface. The kinetics of interaction could be described by two simultaneous reactions. The first reaction was second-order and was ascribed to the adsorption of phosphate on the calcite surface. The second reaction was first-order and was considered to be associated with the surface arrangement of phosphate clusters into calcium phosphate heteronuclei.

Solubility criteria were used to show that at low phosphate concentrations the ultimate calcium phosphate surface mineral formed was hydroxylapatite.

Desorption kinetics were studied by using an anion exchange resin technique. The desorption process could be described as two simultaneous first-order reactions. The desorption mechanism was postulated to correspond to the dissolution of a surface nucleated calcium phosphate mineral, with the second reaction step being the desorption of phosphate from the calcite surface sites.

The rate constants for adsorption and desorption were determined at four temperatures between 0C and 40C. The rate constants were used to compute the activation energies of adsorption and desorption. In addition, the thermodynamic parameters for the enthalpy of activation (H), the entropy of activation (S), and the free energy of activation (G), for both the adsorption and desorption processes were computed and discussed.

¹ Contribution from the Dep. of Soil Science and Biometeorology, Utah State Univ., Logan, Utah 84322. This work was partly supported through the US/IBP Desert Biome program, under Grant # GB 15886 from the National Science Foundation. Published with the approval of the Director, Agr. Exp. Sta. as Journal Paper 1757.

² Post-doctoral fellow and Professor of Soil Chemistry, respectively. The senior author is currently Research Associate, Illinois Geologic Survey, Urbana, Illinois 61801.

Received for publication May 29, 1973. Accepted for publication September 7, 1973.