Adsorption of Lead, Copper, Zinc, Cobalt, and Cadmium by Soils That Differ in Cation‐exchange Materials

Summary The adsorption of Zn 2+ , Pb 2+ , Cu 2+ , Co 2+ , and Cd 2+ (M 2+ ) by soils was measured at concentrations ranging from 10‐ 7 to 10‐ 2 M in 10‐ 3 to 10‐ 2 M CaCI 2 . Exchange between Ca 2+ and M 2+ , and solubility products [M 2+ ][OH − ] 2 indicate that M 2+ is not precipitated as hydroxide but is adsorbed on cation‐exchange sites. The proportion of selective adsorption sites with specified values of the selectivity coefficient calculated using Ca as reference ion, increased in the order montmorillonite < humus, kaolinite, < allophane. imogolite < halloysite, iron oxides. Raising the soil pH by Ca‐saturation increased both the amount and affinity of adsorption. Selectivity of adsorption increased in the order Mg, Ca < Cd, Co < Zn < Cu, Pb, and the selectivity coefficient varied from < 1 to > 10 000. The formation of the coordination complexes of heavy metal with deprotonoted OH and COOH groups as ligands is suggested as a possible mechanism of selective adsorption.