Properties of soils and clays after acid treatment. I. Clay minerals

Phosphoric acid was more efficient than nitric acid of the same molarity in dissolution of clays in a closed system. Phosphoric acid released considerably more aluminium from kaolinite than did nitric acid. Acid attack released equimolar quantities of aluminium and silicon from kaolinite, but with illite and bentonite the tetrahedral silica layer was preferentially dissolved compared with the octahedral layer. When the reaction products from acidification with phosphoric acid were precipitated in the presence of the clays, they were shown to exist as an aluminium phosphate containing 2 moles of aluminium to each mole of phosphorus and probably amorphous silica. The material existed as small (0.025 µm diameter) particles with a negative charge approaching 20Cg-1. The new materials increased the surface area of a kaolinite consisting of large particles, but decreased the surface area of an illite and montmorillonite. The material was responsible for aggregating all three clays. The results indicate that phosphoric acid treatment of soils should decrease water dispersible clay and improve aggregation of soils by supplying intersititial cements of aluminium phosphate and silica.