MIXED COMPLEX-FORMATION OF EU3+ WITH HUMIC-ACID AND A COMPETING LIGAND

The assessment of the influence of humic substances on the migration of radionuclides upon the burial of high radioactive waste in a geological repository and on the transport of toxic transition metals in the aquatic and terrestrial environment involves speciation calculations which simulate the behaviour under in situ conditions of for example pH, carbonate concentration, presence of complexing ligands. Such calculations assume that complexation reactions of metal ions with humic substances and with other ligands, either natural (carbonate) or man made (e.g. EDTA) act independently. In this contribution it is experimentally proven that metal ions of environmental concern (Eu3+ as a representative of trivalent lanthanides and actinides, UO22+ as a representative of the hexavalent actinides, Co2+ and Cd2+) do form mixed complexes with humic acid and an additional ligand (e.g. carbonate). Mixed complex interaction constants could be calculated in case of Eu3+, with acetylacetone (log beta(EuAA(2)HA) = 15.4+/-0.4), iminodiacetic acid (log beta(EuIDA(2)HA) = 18.1+/-0.5), carbonate (log beta(Eu(CO3)(2)HA) = 16.4+/-0.3) and hydroxyl (log beta(EuOH(2)HA) = 18.0+/-0.4). It is proven that mixed complexes of EU(3+) with humic acid and CO32- or OH- will be present in most groundwater conditions and should therefore be incorporated in speciation and transport models.