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Phosphorus Adsorption by Kaolinite and Montmorillonite: I. Effect of Time, Ionic Strength, and pH

Dep. of Soil Chemistry and Plant Nutrition, Agricultural Res. Organization, Inst. of Soils and Water, Bet Dagan 50–250, Israel

U. Kafkafi

Dep. of Field Crops, Faculty of Agriculture, Hebrew Univ. of Jerusalem, POB 12, Rehovot, Israel

G. Sposito

Dep. of Soil and Environmental Sciences, Univ. of California, Riverside, CA 92521

*Corresponding author.

ABSTRACT

Phosphate reactions in soils are affected by various factors operating simultaneously. The objective of this work was to study specific effects of total P concentration, pH, ionic strength, and equilibration time on P partitioning between solid and solution phases of Ca- and K-kaolinite and -montmorillonite suspensions as model systems of soils containing these minerals. Phosphorus concentration in the solution phase (C_p) stabilized within 24 to 48 h after introducing P into the system. Increased electrolyte concentration, which elevated the electrical conductivity (EC) from 0.2 to 5.0 dS/m, decreased C_p in both clay systems. Further increase to about 40 dS/m had a negligible effect on C_p. Phosphorus adsorption by Ca-clay exceeded that by K-clay. This, and the effect of EC on C_p are suggested to operate via their effect on the extent of the diffuse double layer, which determine the accessibility of P to adsorption sites on clays' edges. Phosphorus solution concentration as a function of pH was minimal in the studied clays around pH 6. In the investigated pH range (5–9), P partitioning was controlled by adsorption, which could be described by a modified Langmuir model. The model accounts for effects of C_p and pH on adsorption and considers retention of individual P species by the adsorbent.

Contribution from the Agricultural Res. Organization, Inst. of Soils and Water, no. 1968-E, 1987 series.

Received for publication June 2, 1987.

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