Chemical forms and transformations of Pb and Cd in granitic latosol of Hainan Island

The transformation processes of Pb and Cd in granitic latosol of Hainan Island were characterized and the effect of exotic heavy metals (Pb and Cd) and three soil amendments on their chemical forms in the soil studied. Results show that the major fractions of Pb in the original tested soils are in bond and residual forms. Lower content of the available form of Pb indicates low environmental risk. The percentage of the different forms of Pb is in the order of: residual > organic > Fe-Mn oxide > carbonate > exchangeable > water soluble form. The major fractions of Cd in the original tested soils are in the form of Fe-Mn oxide bond and carbonate bond. The higher content of exchangeable Cd indicates high environmental risk. The percentage of the different forms of Cd is in the order of: Fe-Mn oxide > carbonate > exchangeable > organic > residual > water soluble form. With increasing loads of exotic heavy metals (Pb and Cd) in the soils, the tendencies are that Fe-Mn oxide Pb > residual Pb > organic Pb > carbonate Pb > exchangeable Pb > water soluble Pb, and that exchangeable Cd > Fe-Mn oxide Cd > carbonate Cd > residual Cd > organic Cd > water soluble Cd. The application of three soil amendments (calcium superphosphate, lime and sodium sulphide) significantly reduces the percentage of Pb and Cd in water soluble and exchangeable forms; and organic Pb, carbonate Pb and Fe-Mn oxide Pb decline. However, residuals of Pb, carbonate-Cd, Fe-Mn oxide bound Cd and organic Cd increase. Cd residual is basically stable.