Mechanism of phosphate ion adsorption by the clays montmorillonite and kaolinite

Anion adsorption by clays, especially the negatively charged bentonitic clays, is a surprising phenomenon, since the negative charge of the clay is believed to be inherent in the lattice of the clay crystal.^1 The mechanism by which negatively charged ions are adsorbed by negatively charged surfaces is of considerable interest. The study reported in this thesis was carried out to gain an understanding of the mechanism of phosphate ion adsorption by the clays montmorillonite and kaolinite. To test the theory of phosphate ion adsorption by kaolinite that was proposed by Stout^2 and Kelley and Midgley^3, in which the surface layer of hydroxyl ions in the kaolinite lattice are considered to be exchangeable with phosphate ions, phosphated kaolinite was treated with varying amounts of sodium hydroxide and the solution in equilibrium with the clay was analysed for phosphate ions. Contrary to what one would predict from the anion exchange mechanism of Stout and Kelley and Midgley, less phosphate was replaced in solutions or high hydroxyl ion content than in low hydroxyl ion content. In order to determine if phosphate ion adsorption is affected by the type of cation adsorbed on the clay, samples or hydrogen saturated, phosphated montmorillonite were titrated to the equivalence point with sodium hydroxide, potassium hydroxide, and lithium hydroxide. This reduced sodium-saturated, potassium-saturated, and lithium-saturated clays. No differences in the amount or phosphate ion held by the clay whether saturated with hydrogen ion, sodium ion, potassium ion, or lithium ion was observed. It was concluded that phosphate ion adsorption is not a function of the type of cation (hydrogen ion, sodium ion, potassium ion, lithium ion) adsorbed by montmorillonite. To determine if adsorbed phosphate ions are exchangeable with other anions, samples of the phosphated montmorillonite and kaolinite were placed in solutions of varyings concentrations of eight different anions, chosen to represent a variety of sizes, structures, and valencies. Failure of adsorbed phosphate ions to exchange with any of these anions was interpreted as meaning that phosphate ion adsorption by montmorillonite and kaolinite is not a physical adsorption, but rather a specific chemical reaction. The hypothesis that phosphate ion adsorption by clays is a chemical precipitation of aluminum phosphate both by aluminum ions in solution (from dissolution of the clay) and aluminum ions exposed on the broken edges of montmorillonite and kaolinite crystals was proposed.