SOLUBILITY AND ADSORPTION OF ZINC IN A SODIC SOIL

Abstract

The aim of this study was to evaluate the solubility and adsorption of Zn as a function of suspension pH ranging from 4.5 to 11.0 at a constant level of Zn in a sodic soil. To eludicate if Zn adsorption followed the Langmuir isotherm, Zn adsorption, as affected by levels of Zn up to 40 μg/ml at a constant pH of 7.0 and 0.1 M/L ionic strength, was also evaluated for Na- and Ca-saturated soils.

Zinc solubility at pH higher than 7.9 was controlled by precipitation of Zn as Zn(OH)2 or ZnCO3 in sodic soils. In the pH range of 6.0 to 7.9, Zn solubility was highly pH dependent, and the chemisorption reactions controlled the Zn concentration of the equilibrium solution. At low pH, 4.2 to 6.0, a reduction in the adsorption of Zn may be attributed to an increase in concentration of competing cations, such as Al, Mn, Fe, and Ca, and to the partial dissolution of soil mineral carbonates.

Zinc adsorption conformed to the Langmuir isotherm for both Ca- and Na-saturated soils. However, the relationship for the Ca-soil showed two distinct portions. Zinc adsorption for Ca-soil followed the Langmuir isotherm up to an equilibrium concentration of 20 μM/L for the lower part and 62 μM/L for the upper part of the curve, whereas for an Na-soil the Langmuir isotherm was a single curve up to 42 μM/L. At low levels of Zn application, Ca-soil adsorbed 2.7 times less Zn, though held with a higher affinity, than at higher Zn concentrations and 1.7 times less than that of Na-soil.