| Abstract: | Leaching of aluminium from the weathering profile of the African surface in Malawi was deduced from the chemistry of a vadose profile. Its loss occurs where congruent kaolinite dissolution results in saprolite collapse, leaving a weakly lateritized, sandy residual mantle. This leaching is a contemporary process, as shown by the precipitation of aluminosilicate evaporites in the dambos (seasonally waterlogged bottomlands). This paper presents the results of determinations, by ICP-OES and AAS-graphite furnace, of aluminium levels in groundwaters, examining the effects of a variety of pretreatments. Filtration (0.45 μm) substantially reduces the levels of total aluminium, showing it to be in particulate form. Subsequent acidification causes precipitation of < 0.45 μm forms of aluminium, further reducing the apparent levels to below ICP-OES detection limit. This is consistent with mobilization in organically bound forms which are destabilized at low pH, and accords with earlier work which showed that indigenous micro-organisms can cause dissolution of kaolinite. Organically bound mobilization provides an explanation for aluminium leaching at near-neutral pH in conjunction with the presence of silica in solution. The identification of this mechanism of kaolinite dissolution and aluminium evacuation from vadose profiles provides a realistic process for the levelling of extensive planation surfaces; the contingent collapse of the kaolinized saprolite would lower the interfluves, progressively narrowing the vadose zone until the landsurface approaches the planar form of the stable, regional water-table. |
