ADSORPTION OF CADMIUM AND ITS EXCHANGE CHARACTERISTICS IN SOME ISRAELI SOILS

Cd adsorption isotherms were measured on five Israeli soils ranging in specific surface area from 65 to 315m²/g. Retention capacity of Cd ranged from 4.7 × 10³ to 10 × 10³μg/g soil (8.4 to 17.9 mequiv/100 g soil) and was correlated with the specific surface area. Almost all the adsorbed Cd was exchangeable with 0.5 M CaCl₂ and the remaining fraction was extractable in 0.5 M HCl. Plots of K_d, the distribution coefficient of Cd between the solid and solution phases, vs. Γ, the surface density of adsorbed Cd, showed that all soils behave similarly, K_d sharply diminishing with Γ. It was suggested, based on comparison with literature data on Cd adsorption on montmorillonite, that below γ s 0. 5 × 10¹⁷ ions/m², specific adsorption mechanisms prevail whereas above it the adsorption is mainly due to electrostatic interactions and ion exchange.     

LONG-TERM TRANSFORMATIONS AND REDISTRIBUTION OF POTENTIALLY TOXIC HEAVY METALS IN ARID-ZONE SOILS INCUBATED: I. UNDER SATURATED CONDITIONS

Solid-phase transformations of Cd, Cu, Cr, Ni and Zn, added as soluble salts at several levels to two arid-zone soils, were studied over a period of one year. The soils were maintained under a saturated-paste regime and sampled periodically. A selective sequential dissolution procedure was employed to determine the changes in metal distribution among six operationally defined solid-phase fractions. A function,U _ts was introduced to measure the fractional attainment of equilibrium of the soils following a perturbation. The direction and rate of redistribution of the added metals in the soils were affected by the nature of the metal, the soil properties and the metal loading level. Cd added to the soils was transferred from the exchangeable (EXC) into the carbonate (GARB) fraction. When soluble Cu, Cr, Ni and Zn were added at low loading levels, metals were transferred from the reducible oxides(RO) bound and easily reducible oxides (ERO) bound fractions and the EXC fraction, into the CARB fraction. However, at the higher loading level, metals were transferred from the EXC and CARB fractions into the organic matter bound (OM), ERO and RO fractions. TheU _ts function approached lower values as incubation continued but remained removed from 1. The overall flux of metals among fractions was the combined result of the readjustment of the metals in the native soil to changing conditions due to saturation, and the transfer of added soluble metals to the less labile fractions.