Adsorption of cobalt by soil iron oxides at low solution concentration

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Abstract
Eleven selected soils from Denmark and Tanzania were treated with ammonium acetate (controls), EDTA, and dithionite-EDTA (DE) to fractionate iron and (manganese) oxides. The amounts of cobalt adsorbed were determiend from a 3 .mu.M equilibrium cobalt solution, corresponding to the cobalt level in nautral soil solutions using sodium nitrate (0.2 M) to suppress non-specific adsorption, by the extracted soils as well as by two synthetic iron oxides. No significant correlations were found between cobalt adsorption and the contents of organic matter and extractable manganese, presumably due to their low contents in the siols investigated. Close correlations were, however, found between the amounts of cobalt adsorbed, especially fractions thereof, and the contents of iron oxides. The amounts of cobalt adsorbed by the DE-extracted soils, void of iron (and manganese) oxides, were attributed to the clay silicates. The remaining cobalt adsorption, i.e. the difference between cobalt adsorbed by acetate-extracted and DE-extracted samples, was attributed to the iron oxides. This portion of adsorbed cobalt was well described by considering soil iron oxides composed of only two fractions, an EDTA-extractable fraction of high reactivity and a less reactive fraction corresponding to the difference between DE-extractable iron and EDTA-extractable iron. The amounts of cobalt adsorbed by the soil iron oxides were well predicted from the contents and specific surface of the two iron oxide fractions in soil together with the amount of cobalt adsorbed by the synthetic iron oxides.