ADSORPTION AND IMMOBILIZATION OF ZINC IN CALCIUM- AND SODIUM-SATURATED SOILS FROM A SEMIARID REGION, INDIA

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Abstract
The adsorption and immobilization of Zn was studied on 6 Ca- and Na-saturated soil samples that differed in their organic carbon content (OC), cation exchange capacity (CEC) and calcium carbonate (CaCO3). Homoionic (Ca- or Na-saturated) soils were equilibrated with chloride solutions of Zn2+ + Ca2+ and Zn2+ + Na+ at 10 meq l-1 total electrolyte concentration. The data on adsorption equilibria were analyzed, using the thermodynamic approach, the Freundlich adsorption isotherm, and exchange selectivity coefficients. The values of standard free energy change of exchange reaction (.DELTA.Gr0) for the Ca2+-Zn2+ system were negative and for the Na+-Zn2+ system positive, anomalously suggesting a preference for Zn2+ over Ca2+ and a strong preference for Na+ over Zn2+. A plot between log x (adsorbed Zn2+) versus log c (solution Zn2+) showed that these isotherms may be roughly divided into 3 parts: the 1st part showing a proportionately larger increase in the log x per unit increase in log c, the second part with a relatively reduced slope, and the 3rd part with a steep increase in log x for a given increase in log c. The values of the Gapon selectivity coefficient (KG) for the Na+-Zn2+ system were exceptionally high and gradually decreased with the increase in Zn2+ concentration in the equilibrium solution. For the Ca2+-Zn2+ system, the value of the exchange selectivity coefficient (K) did not follow any definite increasing or decreasing trend. Although the soil samples taken for this study differed considerably in their OC (0.1 to 1.9%), CaCO3 (0 to 18.2%), and CEC (5.5 to 17.0 meq 100 g-1), the trend and magnitude of "apparently" adsorbed Zn in these soils differed only slightly. Possible conversion of a part of added Zn2+ in soil to unidentified soluble or precipitated species, such as (Znx)+, Zn(OH)2, ZnCO3, Zn3(PO4)2, etc., seemed to mask the effects of OC, CaCO3, and CEC on adsorption and immobilization of Zn in these soil samples.