ADSORPTION OF PHOSPHATE BY CLAY MINERALS PART I. KAOLINITE AND HALLOYSITE

Adsorption of phosphate and other ions by kaolinite and halloysite, which are present in certain soil types, has been investigated. Adsorption of phosphate is appreciable, bat of other ions, negligible. The remits indicate that phosphate adsorption or anion exchange, in general, by kaolinite is rendered complicated by the presence of aluminium, either in the free state or present in the laterally broken down lattice of clay minerals when the latter are finely powdered. Removal of free alumina by means of dilute acid and/or treatment of phosphatised kaolinite with ammonium oxalate solution bring down the p.c a. to its normal low value. Analysis of the oxalate leachate shows the formation of definite aluminium phosphates on phosphatisation of kaolinite. This has been further confirmed with the help of electron diffraction pictures. The agreement between the p.a.c. and b.e c. of kaolinite has been explained by assuming that the surface OH groups act as the centre of both cation and anion exchange, depending on the experimental conditions. B.e.c. of halloysite has been found almost the same as of kaolinite, but p.a.c. is higher, particularly in acidic solution.