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The Interaction of Phosphate with Calcite¹

ABSTRACT

Adsorption isotherms of phosphate on calcite were shown to be described by a two-region Langmuir isotherm equation, i.e., the plot showed two distinct linear portions. The break in the slope of the Langmuir plot was found, on a solubility diagram of the calcium phosphates, to correspond closely to the division between octocalcium phosphate and hydroxylapatite.

The adsorption data were also found to be described by the BET equation. The monolayer capacity computed from the BET equation was found to correspond closely with the adsorption maximum computed from the initial solpes of the Langmuir plots. Comparison of monolayer capacities with the total surface area of the calcite indicated that only approximately 5% of the total surface was covered with phosphate ions.

The differential isosteric heat of adsorption,
was computed from the adsorption data. A plot of
as a function of phosphate sorbed revealed a discontinuity at the region 1 boundary of the adsorption data.

Kinetics of phosphate reaction with calcite were found to be consistent with heterogeneous nucleation theory and are discussed in terms of the previous adsorption data.

It was concluded that the interaction of phosphate with the calcite surface could be described as a heterogeneous nucleation process.

¹ Contribution from the Dep. of Soil Science and Biometerology, Utah State Univ., Logan, Utah 84322. This work was partly supported through the US/IBP Desert Biome program, under Grant no. GB 15886 from the National Science Foundation. Published with the approval of the Director, Agr. Exp. Sta. as Journal Paper 1756.

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 24, 1973. Accepted for publication August 31, 1973.

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