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 and subtropics are, however, dominated by Oxisols and Ultisols occupying extensive areas of potentially highly productive soils. The mineral fractions 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) and 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 characteristics of variable charge soils from the tropics. In general, net negative surface charge increases with increasing soil pH and OM content. Adsorption of metal ions by variable charge soils and minerals also increases as their pH, clay, and OM contents increase. Although the precise mechanisms for the change in net negative charge of soil and mineral surfaces with increasing pH are not fully understood, the generation of negative charge either through dissociation of H+ ions from 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 creates conditions conducive to increased mobility. Consequently, the adsorption of heavy metals, in particular, cadmium (Cd) by strongly weathered soils in relation to the effects of inorganic and organic ligands and the implications for metal transport are reviewed.