Phosphorus adsorption by selected clay minerals
The structure of the ionic atmosphere surrounding the clay particles has been presented to show the theoretical effect of certain cations on the adsorption of phosphorus by clay minerals. The various diffuse double layer and Donnan equilibrium theories were used in this theoretical treatment. Three standard clay minerals, a kaolinite, an illite and a montmorillonite were used in the investigation of the effect of exchangeable cations on phosphorus adsorption in dilute phosphorus solutions. The clays were first treated to remove the surface coatings and impurities with the least possible alteration of the clay lattice. The purified clays were then saturated with Li⁺, Na⁺, K⁺, Mg⁺⁺, Ca⁺⁺, Sr⁺⁺, Ba⁺⁺ by salt leaching techniques. The clays were then equilibrated with very dilute phosphorus solutions. Preliminary experiments with representative clay samples showed that equilibrium between adsorbed phosphorus and the phosphorus in solution was reached in 72 hours. Each of the three clays saturated with a specific cation was shaken with dilute NaH₂P0₄ solutions (2-40 μg P/ml). The pH of these suspensions were measured and were found to vary in the range of 6.0-7.2. From the final and the initial phosphorus concentration in solution, the amount of phosphorus adsorbed per unit weight of clay was determined. Phosphorus adsorption maxima were calculated from the Langmuir adsorption isotherm. The values of the constants derived from the plot of these isotherms was shown to follow the general order predicted from theoretical considerations. Phosphorus adsorption changed with the saturating cation in the following order: for kaolinite Li⁺ < K⁺ < Na⁺ « Mg⁺⁺ < Ca⁺⁺ = Sr⁺⁺ < Ba⁺⁺ for illite Li⁺ < NA⁺ < K⁺ « Mg⁺⁺ < Ba⁺⁺ < Ca⁺⁺ for montmorillonite Mg⁺⁺ < Sr⁺⁺ = Ba⁺⁺ < Ca⁺⁺ Phosphorus adsorption maxima were smaller than values previously obtained for similar clay-phosphorus mixtures. Theoretical and practical considerations proved that Al and Fe ions were responsible for higher phosphorus adsorption values.
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