TEMPERATURE EFFECTS ON THE POINT OF ZERO CHARGE AND ISOELECTRIC POINTOF A RED SOIL RICH IN KAOLINITE AND IRON MINERALS

The effects of temperature (373-1373 K) on the point of zero charge (P ZC) and isoelectric point (IEP) of a red soil rich in kaolinite and ir on minerals were studied. PZC values of the soil treated at 373 and 57 3 K indicated the presence of iron oxide. The soil calcined between 77 3 and 1173 K shows a PZC almost coincident with the respective values of kaolinite. At 1373 K, the PZC of the soil is nearer to the value of iron oxide. In the entire temperature range studied the PZC values we re lower than the IEP values. An approach of PZC and IEP values was ob served after a partial removal of iron oxide by the dithionite-citrate -bicarbonate (DCB) method. The analyses of the PZC and IEP values, of electron probe micro analysis (EPMA) data and of specific surface area s evidence a specific adsorption of iron oxide on kaolinite. Finally, the dissolution sequence of iron and aluminium contained in soil was d etermined using hydrochloric acid.