Influence of iron oxides on cobalt adsorption by soils

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Abstract
SUMMARY: Eleven soils from Denmark and Tanzania were extracted with ammonium acetate (controls), EDTA, and dithionite‐EDTA (DE) to fractionate iron (and manganese) oxides. The amounts of cobalt adsorbed and acidity desorbed by the extracted soils as well as by two synthetic iron oxides were determined, using 0.85 mM cobalt in 0.2 m NaNO3.No significant correlations were found between cobalt adsorption and the contents of extractable manganese or organic matter, presumably because of their low contents. The major part of the cobalt adsorption (by DE‐extracted samples) was due to the clay fraction and was associated with the release of approximately one proton per adsorbed Co. The remaining cobalt adsorption 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. Approximately 1.7 protons were released per Co adsorbed by these iron oxide fractions and by the synthetic iron oxides.The amounts of cobalt adsorbed by the soil iron oxides were well predicted from the contents and specific surfaces of the two iron oxide fractions together with the specific cobalt adsorption of synthetic iron oxides.