[Contribution of kaolin to phosphate adsorption by some Indonesian red soils]

The contribution of kaolin to phosphate adsorption by some Indonesian red soils from Java and Sumatra is investigated in this research. Soil kaolin was concentrated from the clay fraction by removal of iron oxides in buffered dithionite-citrate-bicarbonate solution. P sorption measurements were carried out by shaking 100 mg of soil kaolin in 20 ml 0.2 M KCl containing 0-10 mgP ml-1, centrifuging and determining P concentration in the supernatant by the molybdate blue method. The mineralogical composition of the clay fraction was surprisingly uniform with kaolin group minerals (generally b-axis disordered kaolinite) being dominant. Electron microscopy indicated that halloysite occurs in some samples but it is much less abundant than kaolinite and does not give a 10 A reflection in XRD patterns. Mica, goethite, gibbsite, hematite, quartz and anatase have also been identified in minor amounts. The various properties of kaolinite showed little variation indicating that there is a normal or equilibrium form of kaolinite in these soils. This mineral is best described as microcrystalline (median crystal size = 16 nm) highly b axis disordered (HB index 5.4), with appreciable Fe2O3 (2.5 percent), moderate CEC (10 cmol (+) kg-1) and small AEC (1 cmol(-) kg-1). All kaolin samples showed strong P sorption and the sorption data conform to single Langmuir and Freundlich equations, however the majority of data are best described by the Langmuir equation. The P adsorption capacity of the kaolins as indicated by the values of Langmuir P maximum (Xm) range from 875-1450 mgP g-1. The Freundlich coefficient k, which is a measured of the number of P adsorption sites in samples, ranged from 140-400 with a median value of 345. There is a strong (r=0.71**] positive relationship of P adsorption maximum (Xm) with anion exchange capacity. This work has shown that kaolin in some tropical soils has a considerable P sorption capacity with Xm values of about 1 mg g-1. Coexisting iron oxides in these red soils have Xm values of about 10 mg g-1. Thus kaolin is likely to contribute substantially to P sorption by these tropical red soils where kaolin is the major constituent. The Langmuir P adsorption maximum of this kaolinite is closely related to its anion exchange capacity but the median value of Xm of 1015 mgP g-1 clay corresponds to about 10 ceqP kg-1 which is about 10 x greater than the AEC. Thus P adsorption by kaolin is not predominantly by the anion exchange sites but rather occurs by specific adsorption at favourable surface sites which are likely to include crystal edges and other defects