The formation of mixed complexes between metals, humic acids and small organic and inorganic ligands

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 and presence of other 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, U O22+ as a representative of the hexavalent actinides, Co2+ and Cd2+) do form mixed complexes with humic acid and an additional ligand (e.g. carbonate). It is further demonstrated that the extent of mixed complex formation is a function of the concentration of both the humic acid concentration and the additional ligand. Speciation calculations under real groundwater conditions show that mixed complexes of Eu3+ with humic acid and CO32- or OH- will be present. (authors). 19 refs., 11 figs., 5 tabs