SORPTION OF HEAVY-METALS IN STRONGLY WEATHERED SOILS - AN OVERVIEW

Current knowledge of sorption processes in tropical soils is reviewed. Landscapes throughout the tropics are dominated by oxisols which occu py extensive areas of potentially highly productive soils. These soils are dominated by low-activity sesquioxide minerals and clays that hav e variable charge surfaces. The limited information on tropical soils available suggests that the composition of the ambient soil solution c an influence sorption through changes in particle surface-charge densi ty. Thus the observed decrease in sorption in the presence of divalent index cations may be related to the effect of ionic charge on the dou ble-layer thickness which is manifested through a change in surface-ch arge characteristics. However, much work needs to be done to different iate the effect of cation charge on surface-charge density from the co mpetitive effect between the index cation and heavy-metal ions for the sorption sites. The effects of inorganic and organic ligands on adsor ption of Cd by variable charge surfaces are also reviewed.