A modified Schubert method for the measurement of the stability of europium humic acid complexes in alkaline conditions

The interaction constant of small Eu concentrations with soil organic matter of different origin is measured at pH=9 in presence of 0.1 M HCO3- using Schubert's ion exchange method. Only a 1:1 complex is formed in the considered soil organic matter concentration range (<10-3 M). The interaction constant increases only slightly (0.3 logK units) with a decrease in the Eu occupancy range from 1 to 10-4% and tends to a steady value of 1013.9. The interaction constants are identical (1014±100.5) for Podzol and Boom Clay humic acid extracts, commercial humic acid (Fluka), and for various organic matter subfractions of a Podzol Soil (humic+fulvic acid). The presence of a 103 excess of Pb++ with respect to Eu does not affect the Eu-interaction constants. (orig.)