Seven-Step Sequential Extraction Procedure to Evaluate Cd and Zn Mobility in Soil

The DD2005-03 sequential extraction method, developed for the analysis of heavy metals in soil, was applied to 234 soil samples covering the area of the Henan Province representative agriculture soil. On average, the order of Cd in each fraction was Exchangeable (27.3%) > Weakly bound to organic matter (22.7%) > Strongly bound to organic matter (16.4%) > Residual (15.1%) > Carbonate (12.1%) > Fe/Mn oxide bound (4.5%) > Water soluble (2.0%), Zn was Residual (66.3%) > Fe/Mn oxide bound (10.9%) > Weakly bound to organic matter (9.4%) > Exchangeable (6.0%) > Strongly bound to organic matter (3.9%) > Carbonate (2.9%) > Water soluble (0.6%). The accuracy of the sequential extraction was judged by the relative error (RE). RE for Cd ranged from 0 to 45% with mean 16.3%.RE for Zn ranged from 0.1-11.4% with mean 3.4%. On average, bioavailability index (BI) for Cd and Zn is 39.1% and 9.0% respectively. The mobility of the elements in the order Cd > Zn corresponds with the plant- availability of individual elements.