The adsorption of Zn2+, PbZ+. Cu2+, Co2+, and CdZ+ (Mz+) by soils was measured at concentrations ranging from lo-' to M in to M CaCI2. Exchange between Ca2+ and M2+, and solubility products [M2+][OH-]2 indicate that M2+ 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 varicd 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. IntroductionT H E A D S O R P T I O N of heavy metal cations by soils has importance in determining their availability to the plant and their movement through the soil. Earlier we found that selective adsorption of Zn by soils is dependent on their major cation-exchange materials (Wada and Abd-Elfattah, 1979). The present paper reports the results of studies along the same lines on the adsorption of Pb, Cu, Co, and Cd by soils. The main interest is in the effects of kind of heavy metal cation and of cation-exchange material, and the effect of soil pH, on the selective adsorption of heavy metal cations by soils. The evaluation of these effects is important in estimating the heavy metal retention by the soil and in order to elucidate the reaction mechanism.

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The adsorption of Zn, as compared with Mg, on two mineral soils, which differed in their major cation-exchange materials and with and without Ca-saturation, was measured in the presence of free CaCl a •The adsorption of Zn as well as Mg occurred on cation-exchange sites. The Zn adsorption data conformed to a two-term Langmuir equation. The presence of two kinds of adsorption sites and their numbers and bonding energies were deduced. However, the Langmuir approach was not adopted on the basis of comparison between the total number of the adsorption sites for Zn deduced and the CEC of the soils.As an alternative approach, the selectivity coefficient K~~, as defined by the equation:was calculated for each adsorption equilibrium and plotted against the amount of Zn adsorbed. This K~:-[Zn]80i! plot was used to estimate the capacities of the soil to adsorb Zn with specified affinities. The K~: value varied between 1 and 1,000, whereas the corresponding K~arz value varied only between 0.5 and 1. The K~: value was dependent upon the amount of Zn adsorbed, the status of exchangeable cations, and the major cation-exchange materials (montmorillonite VS. allophane-imogolite) in the soils. The importance of surface OH groups in allophane-imogolite as specific adsorption sites for Zn was suggested.

The adsorption of Zn by soils which are different in their major cationexchange materials was measured at equilibrium Zn concentrations up to lo-' M in lo-' to l O -' M CaCl,. The results are interpreted on K$;-[Zn],,il plots, where KE; is the selectivity coefficient defined by the equation All natural samples except those containing halloysite exhibited no or very small specific Zn adsorption. AU Ca-saturated samples exhibited specific Zn adsorption dependent oncation-exchange materials. The cationexchange sites with high selectivities for Zn (K3: > 10) constitute more than 40 per cent of the total exchange sites in soils containing allophane, imogolite, and halloysite, whereas those with moderate to low selectivities for Zn (K$; < 10) predominate in montmorillonitic, vermiculitic, and humic soils. Differences in the contribution of the respective cationexchange materials to specific Zn adsorption are discussed relating t o differences in the origin of their negative charge.

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