PHOSPHATE SORPTION AS RELATED TO MINERALOGY OF A HYDROSEQUENCE OF SOILS FROM THE CERRADO REGION (BRAZIL)

Phosphate sorption by minerals having surface Fe-OH or Al-OH groups is an important process in highly weathered tropical soils. In this study, P sorption in 12 samples of soils of a typical hydro-sequence of the Cerrado region (Brazil) was predicted by means of combining data on: (1) the amount and properties of gibbsite, goethite, hematite and kaolinite, and (2) the P-sorption characteristics of these minerals, according to existing data in the literature. The concentration, crystal morphology and surface area of these four minerals were studied by X-rav diffraction (XRD), thermogravimetry, transmission electron microscopy (TEM) and N2-adsorption. Gibbsite crystals were subrounded with its longest axis between 50 and 100 nm. Kaolinite platelets had a diameter of 40-200 nm and frequently had euhedral faces. According to XRD data, the mean crystallite dimension (MCD) perpendicular to the (110) plane of goethite was about 30 nm; hematite crystals appeared to be platy (i.e., MCD110 > MCD104) and slightly larger than the goethite crystals. For most samples, the P sorption. measured at an equilibrium concentration of 10(-5) M, was markedly lower than P sorption obtained by the sum of the sorptions estimated for each of the four minerals. The difference between measured and predicted P sorption became smaller when the soils were treated with H2O2. Moreover, the increase in P sorption with this treatment was positively correlated with the organic matter content. These results suggest that organic compounds, by probably competing for sorption sites. can substantially hinder P sorption of tropical soils. For soils low in organic matter, P sorption can be reasonably well predicted through the mineralogical characteristics of the soil.