Heavy metal adsorption by humic umbrisols: selectivity sequences and competitive sorption kinetics.

This work examines sorption kinetics of Cd, Cr, Cu, Ni, Pb, and Zn adsorbed simultaneously by four humic umbrisols, as well as the influence of the soil components on the adsorption rate. Starting from the value of K(d), the metals most adsorbed by the soils are Cu, Cr, and Pb. The soils that adsorb the largest amounts of metals have also the highest organic matter, oxide, and clay content. In general, the most common selectivity sequence of adsorption is Pb > Cr > Cu > Cd approximately Ni approximately Zn. Sorption kinetics is rapid for Cr, Cu, and Pb in all the soils studied and competes favorably for the adsorption sites with Cd, Ni, and Zn. Various rate equations (zero-order, first-order, second-order, Elovich, and power function) were applied to the kinetic data to obtain the corresponding rate coefficients. A second-order reaction best represents the kinetic data for Cr in most of the soils.