CADMIUM SORPTION AND TRANSPORT IN VARIABLE CHARGE SOILS - A REVIEW

Alfisols, Entisols, Inceptisols, Ultisols, Vertisols, and Oxisols are all commonly found in tropical and subtropical regions receiving more than 500 mm mean annual rainfall. Landscapes throughout the tropics an d subtropics are, however, dominated by Oxisols and Ultisols occupying extensive areas of potentially highly productive soils. The mineral f ractions of these soils consist primarily of low-activity clays having variable surface charge that differs from high activity clays in the origin of that charge. Low activity clays are dominated by iron (Fe) a nd aluminium (Al) oxyhydroxides and 1:1 layer silicates (kaolin). Much research has been conducted on the effects of pH, organic matter (OM) , and cation composition of the soil solution on the surface charge ch aracteristics of variable charge soils from the tropics. In general, n et negative surface charge increases with increasing soil pH and OM co ntent. Adsorption of metal ions by variable charge soils and minerals also increases as their pH, clay, and OM contents increase. Although t he precise mechanisms for the change in net negative charge of soil an d mineral surfaces with increasing pH are not fully understood, the ge neration of negative charge either through dissociation of H+ ions fro m surfaces or consumption of OH- ions by soils is generally accepted. In soils dominated by permanent charge surfaces, heavy metals are not mobile but in variable charge soils, the low surface charge density cr eates conditions conducive to increased mobility. Consequently, the ad sorption of heavy metals, in particular, cadmium (Cd) by strongly weat hered soils in relation to the effects of inorganic and organic ligand s and the implications for metal transport are reviewed.