Abstract
The complexation relevant for humic acid (HA), one of natural organic matters, is complicated because of the heterogeneity of HA. In this work, the complexation between HA and UO22+, whose environmental behavior is important in the radioactive waste management, was evaluated based on this heterogeneity, especially the presence of the multiple complexing sites in HA. HA is recognized to have a wide distribution of its molecular weight and the structural differences depending on its molecular weight. Therefore, for the better understanding of the effects of these heterogeneities, HA was fractionated by gel permeation chromatography (GPC) and the complexation between the obtaind fractions and UO22+ was examined by ultrafiltration and compared with that by the fluorescence quenching experiment. The stability constant (logK) of unfractionated HA was 6.25, which is similar as the reported values (logK = 6.13～6.75). In case of HA fractions, the obtained stability constants were not different among them, in contrast, the numbers of the complexing sites were largest in the lowest molecular-weight fraction. This indicated the complexing amount between HA and UO22+ could be influenced by the structural difference in the HA fractions. The comparison between their results and those of the quenching experiments revealed that the complexing sites, which do not participate in the fluorescence of HA, were contained in all fractions and these site formed more stable complexes with UO22+ than the fluorescent sites.
