Dissolution behavior of kaolinitic tropical soils

Batch experiments at controlled pH and ionic strength were conducted with representative kaolinitic soils from Brazil to investigate their short-time dissolution behavior. The release of Al and Fe from the soils showed similar pH dependence, exhibiting a minimum value (p.m.d.) just above the point of zero net charge for the soils. Both Al and Fe release rates below the p.m.d. were correlated positively with soluble organic C, proton concentration, and net proton surface charge, the last two relationships being power-law functions similar to those observed for the dissolution rates of specimen metal oxides and aluminosilicates. Below the p.m.d., release of Si from the soils exhibited a pH dependence very much like that observed for specimen kaolinites and the molar 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 in both dissolved and microparticulate forms.