EFFECT OF REMOVAL OF ORGANIC MATTER AND IRON- OR MANGANESE-OXIDES ON ZINC ADSORPTION BY SOIL

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Abstract
To determine the relative importance of various soil fractions in zinc (Zn) adsorption, six topsoils were equilibrated with seven Zn solutions to study Zn adsorption by whole soils and soils with the following fractions removed: organic matter, amorphous iron (Fe) oxides, crystalline Fe oxides, manganese (Mn) oxides, and organic matter plus crystalline Fe oxides. The low-Zn portion of all adsorption curves conformed to the Langmuir isotherm, and Langmuir coefficients were used to evaluate Zn adsorption capacities. The finer textured whole soils, which were also higher in organic matter than the sandy soils, had the highest Zn adsorption capacities. Organic matter removal lowered adsorption capacities for all soils except one and did so more for the higher organic matter soils. Removal of either the amorphous or crystalline Fe oxides increased Zn adsorption capacity. A significant reduction in adsorption capacity resulted when the Mn oxides were removed from three soils high in total Mn. The lowest Zn adsorption capacities were for soils where both the organic matter and crystalline Fe oxides had been removed. Although all colloidal-sized particles have a role in Zn adsorption, these results show that organic matter and Mn oxides can be as important as clays and Fe oxides in Zn adsorption.