Transform of lead, copper, and cadmium added in agricultural soils in the Changjiang and Zhujiang Deltas

Chemical forms of heavy metals in soils can affect mobility and bio-availability of the heavy metals in the soils. Understanding evolution of chemical fractions of the heavy metals in the soil pollution process is helpful to predict effects of heavy metal accumulation on soil environments. In this study, the transform processes of Pb, Cu and Cd in ten agricultural soils, collected from the Changjiang and Zhujiang deltas were characterized, and effects of Cd, Cu and Pb loads on their chemical forms in the soils were studied. The results showed that the major fraction of heavy metals in the unpolluted soils was residual forms, proportions of extractable fractions were CdCuPb and varied with the soils. With increasing loads of heavy metals in the soils, the proportions of metals as exchangeable form increased, those as residual form decreased, and thus increased availability of the metals and their impacts on the environments. When heavy metals were added in low levels, the metals were preferentially transformed into oxides- and organic matter bound fractions. However, the higher proportions of metals were transformed into exchangeable and carbonate bound fractions when metals were added in higher levels. The proportions of exchangeable Pb, Cu, and Cd fractions in the soils increased with decreasing pH. Application of amendments such as limestone, fly ash, organic manure and furnace slag could decrease the proportion of exchangeable metal fractions.