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

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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 (Cp) 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 Cp in both clay systems. Further increase to about 40 dS/m had a negligible effect on Cp. Phosphorus adsorption by Ca‐clay exceeded that by K‐clay. This, and the effect of EC on Cp 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 Cp and pH on adsorption and considers retention of individual P species by the adsorbent.