DISSOLUTION BEHAVIOR OF KAOLINITIC TROPICAL SOILS

Batch experiments at controlled pH and ionic strength were conducted w ith representative kaolinitic soils from Brazil to investigate their s hort-time dissolution behavior. The release of Al and Fe from the soil s showed similar pH dependence, exhibiting a minimum value (p.m.d.) ju st above the point of zero net charge for the soils. Both Al and Fe re lease rates below the p.m.d. were correlated positively with soluble o rganic C, proton concentration, and net proton surface charge, the las t two relationships being power-law functions similar to those observe d for the dissolution rates of specimen metal oxides and aluminosilica tes. Below the p.m.d., release of Si from the soils exhibited a pH dep endence very much like that observed for specimen kaolinites and the m olar ratios, Si/Al and Fe/Al released, were smaller than 1.0, probably because of the rapid solubilization of organic Al forms. Above the p. m.d., colloidal dispersion occurred and Al, Si, and Fe were released i n both dissolved and microparticulate forms.