Complexation of samarium(III) and americium(III) with humic acid at very low metal concentrations

The complexation of samarium(III) with humic acid was studied at concentrations between 10-5 M and 10-10 M by the separation of complexed from uncomplexed species with electrophoretic ion focusing or cation exchange chromatography, followed by the radiometric determination of the element in both fractions. The influence of humic acid concentration, pH-value, metal concentration and competing cations was investigated. Formation of a 1:1 complex between Sm(III) and humate is observed with a pH-independent complexation constant log β = 7.1 ± 0.2 derived from electrophoretic data. With cation exchange separations lower log β values are found, 4.8 ± 0.2 with weakly and 4.2 ± 0.2 with strongly acidic resins, indicating a competition between samarium binding to the ion exchanger and to weak and strong binding sites in the humate. A slowly established equilibrium is observed between samarium humate binding modes of different strength. Variation of the samarium concentration from 10-6 to 10-10 M has only a slight influence on the stability constant, and magnesium interferes only above 10-4 M concentrations. For the complexation of americum(III) with humic acid at 10-11 M metal concentration log β = 6.6 ± 0.2 is obtained from electrophoretic separations, and 4.7 ± 0.2 with weakly, 4.2 ± 0.2 with strongly acidic resins. This is, within the errors, in agreement with the samarium data. (orig.)