The Penetration of Phosphate into the Kaolinite Crystal

HE silicate minerals in the clay fraction of soils A have been considered for several years to be capable of fixing phosphate. Mattson (2) and Steele (8) showed that bentonite and the colloidal materials extracted from various soils would fix phosphate. Later, Scarseth (5) studied in some detail the fixation of phosphate by bentonite. These experiments did not offer conclusive evidence as to the fixation of phosphate by the silicate portion of the clay because the materials used were not treated to remove free iron oxides, and the exchangeable calcium and magnesium were not extracted in all cases. Toth (10) found that the removal of the free iron oxides by the procedure of Drosdoff and Truog (i) produced a significant decrease in the power of the clays from several soils to fix phosphate. However, it is possible that a part of the fixation by the treated clays was still due to iron. Since the method used for removing free iron oxides does not remove as much iron as does the improved procedure (13), some of the iron not removed from the treated clays might have become soluble and caused the precipitation of phosphate. The experiments of Murphy (3) and Stout (9) left little doubt that certain minerals of the kaolinite group can fix phosphate independently of the other constituents commonly found in the clay fraction of soils. The free iron oxides were not removed from the kaolinite which they used. However, the low iron content of good kaolinite samples and the extremely high amounts of phosphate fixed make it probable that the fixation was caused largely by kaolinite. The results obtained by these workers indicate that the fixation of phosphate by kaolinite is an exchange reaction in which the phosphate in solution replaces the hydroxyl groups exposed on the surface of the particles. The reaction of phosphate with kaolinite is similar to those of phosphate with substances in solution in that the reaction conforms to predictions based on the mass law. The higher the concentration of phosphate in solution and the more acid the solution (within limits), the more phosphate is fixed. An excess of hydroxyl ions causes the reaction to be reversed, and brings the phosphate back into solution. A consideration of the nature of the substances involved suggests that the differences between the reaction of phosphate with kaolinite and the precipitation of phosphate by some substance in solution may be ascribed mainly to the crystalline condition of the kaolinite. Because of the non-expanding kaolinite lattice, the hydroxyl groups on layers inside the crystals are not readily replaced by phosphate ions in solution around the clay particles. The phosphate fixation is increased when more hydroxyl ions are exposed on the surface by fine grinding. Several observations made during preliminary experiments with kaolinite led to the thought that not all the phosphate fixed had replaced surface hydroxyl groups, and that perhaps a part of it had penetrated into the clay particles and replaced hydroxyl groups on the lattice layers inside the crystals. The results of experiments to test this theory are presented below.