Distribution of Chemical Forms for Co, Cr, Ni and V in Typical Soils of China

Co, Ni, Cr and V in 25 typical soils of China were fractionated into exchangeable, carbonate bound (calcareous soils), Mn oxide bound, organically bound, amorphous Fe oxide bound, crystalline Fe oxide bound and residual forms using a seven-step sequential extraction procedure, so as to study the distribution of chemical forms of these metals as well as the effects of soil properties on the distribution. The results showed that most of soil Co, Ni, Cr and V were present in residual forms, and the distribution ratio averaged 48.2% for Co, 53.0% for Ni, 81.5% for Cr and 68.7% for V. The speciation of heavy metals was greatly influenced by soil physico-chemical properties and the chemistry of elements. The results also indicated that the recovery of metal elements by the sequential extraction procedure was satisfactory, with the relative difference between the sum of seven forms and the total content in soils averaging 9.5% for Co, 12.8% for Ni, 6.6% for Cr and 7.2% for V.