The sorption of Cd, Zn and Ni by soil clay fractions: Procedures for partition of bound forms and their interpretation

Abstract

This work is the first of several projects concerned with the study of higher-affinity reactions of Cd, Zn and Ni ions with soil clay fractions. Procedures for the separation of sorbed metals into fractions of lower and higher affinity for soil surfaces are described and evaluated. Various concentrations of Cd, Zn and Ni were allowed to react in the presence of 0.01 M Ca(NO 3) 2 with soil clays for 1 week after stabilization of suspension pH. The adsorbed metals were partitioned by a brief extraction with 0.01 M Ca(NO 3) 2 and the resultant fractions, called specifically and non-specifically sorbed metals, were measured by radioisotopic procedures. Measured separation factors showed that the fraction of sorbed metals that was desorbed by a rapid Ca(NO 3) 2 extraction still had a preference, sometimes marked, over Ca on the soil clay fraction. Separation of fractions of sorbed metals on the basis of affinity was reproducible, but the boundary conditions defined by separation factors vary appreciably between adsorbents, with values in the range 3–20 for amounts sorbed equivalent to ≦ 0.05% of cation exchange capacity and for pH values < 7. The proportions of Cd, Zn and Ni bound at high-affinity sites were strongly dependent on experimental conditions of pH, equilibrium time and surface saturation in relation to each soil clay. Hence, comparisons of affinities of trace metals for soils by reliance on measures of total sorption only, without assessing the contribution of lower-affinity forms, may prejudice conclusions and predictions arising from studies of the possible retention of metal pollutants in soils and fixation of micronutrients from fertilizers.