Copper and zinc competitive adsorption: Desorption in calcareous soils

The behavior in competitive adsorption-desorption reactions of Cu and Zn was studied in four calcareous soils. Cu and Zn were added to the soil by Cu, Zn, and Cu + Zn sulfate solutions in a CaSO4 background. Soil sorption of these cations was described by equilibrium isotherms that fitted either Freundlich- or Langmuir-type equations, although Cu desorption data fitted only Freundlich isotherms. Cu and Zn competition was quantified by distribution coefficients, K-d, relating cation distribution between soil and solute and by the competitive Langmuir equation. The competitive Langmuir equation was the better suited to describe the Cu-Zn competitive adsorption in these soils. Distribution coefficients presented lower values when both cations were present, decreasing when the Cu and Zn concentration in solution increased (decreasing soil affinity for these cations), thereby increasing their mobility through the soil. However, the distribution coefficient of specifically adsorbed Cu in equilibrium with cations extracted by a Mg(NO3)(2) solution increased with Cu concentration. Cu adsorption was more depressed by Zn than Zn adsorption by Cu. The different behavior of Cu and Zn seems dependent on the percentage Ca(CO3)(2) and to a lesser degree, on Cu and Zn organic matter complexes, free ir on content, and surface precipitation on oxides and carbonates.