Chemical forms and transformations of Pb, Cu and Cd in agricultural soils of Changjiang and Zhujiang deltas

The transformation processes of Pb,Cu and Cd in ten agricultural soils from Changjiang and Zhujiang deltas were characterized and the effect of Cd,Cu and Pb loads on their chemical forms in the soil studied. Results show that a major fraction of heavy metals in unpolluted soils is in residual form.The proportion of extractable fraction of the heavy metals varies with soil type,but follows a general order of Cd>Cu>Pb. With increasing loads of heavy metals in the soils,the proportion of exchangeable form of the metals increases while that in residual form decreases,leading increased availability and impact of the metals on the environment.When heavy metals are added in low levels,the metals are preferen tially transformed into oxides and organic matter fraction bonds. However,the higher proportion of metals leads to exchangeable and carbonate fraction bond transformation.The proportion of exchangeable Pb,Cu and Cd fractions in the soil increase with decreasing pH.