Cadmium sorption-desorption by tropical soils as affected by cultivation

Abstract

The sorption of Cd by two soils from Tanzania with different culti vation histories: Mkindo si te and Mafiga site was studied. Samples were collected at both virgin and cultivated sites . The two virgin soils showed similar abi lities to bind Cd. Mkindo soil ( 16.3 g C kg') presented the highest affinity for Cd. Ye t, Mafiga soil (12 .3 g C kg' ) sorbed almost the same amount amount. Mafiga soil had higher cation exchange capacity and Mn and Fe oxide content and these characteristics seemed to counteract the effect of lower organic matter. Once cultivated, the two soils behaved quite differentl y. In Mkindo, 10 years of culti vation have led to a noticeable increase in the maximum Cd sorption capacity. In Mafiga, on the contrary, after 30 years of cultivation the soil Cd retention ability had decreased. These changes in Cd sorption have been related to the effect of cultivation in sorne important soil characteristics affecting metal sorption. In both virgin and cultivated sites, the reversibility of the sorp tion reaction was very low. More than 70% of the sorbed Cd wW' apparently strongly bonded.