No abstract

Spectroscopic extended X-ray absorption fine structure (EXAFS) evidence was obtained on the chemical environment of 99Tc(IV) atoms formed upon introduction of TcO4- into four types of laboratory-scale synthetic and natural systems which mimic in situ natural reducing conditions in humic-rich geochemical environments: (a) magnetite/pyrite in synthetic groundwater in the absence of humic substances (HSs), (b) magnetite/pyrite in natural Gorleben groundwater in the presence of HSs, (c) Boom clay sediment mixed with synthetic groundwater, and (d) Gorleben sand mixed with natural Gorleben groundwater. The investigated systems obey to pH 8-9 conditions, and all measured samples show similar EXAFS spectra for Tc, which could be fitted by a hydrated TcO2 x xH2O phase. The results are interpreted as follows: upon introduction of high concentrations (millimolar to micromolar) of TcO4-to chemically reducing environments, small Tc(IV) oxidic polymers are formed, which either may aggregate into larger units (colloids) and finally precipitate or may interact in their polymeric form with (dissolved and immobile) humic substances. This latter type of interaction--Tc(IV) colloid sorption onto HSs--differs significantly from the generally accepted metal--humate complexation and therefore offers new views on the possible reaction pathways of metals and radionuclides in humic-rich environments.

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 (Eu 3+ as a representative of trivalent lanthanides and actinides, UO| + as a representative of the hexavalent actinides, Co 2+ and Cd 2+ ) do form mixed complexes with humic acid and an additional ligand (e.g. carbonate). Mixed complex interaction constants could be calculated in case of Eu 3+ , with acetylacetone (log/?(EuAA 2 H A) = 15.4±0.4), iminodiacetic acid (log/?(EuIDA 2 HA) = 18.1 ±0.5), carbonate (log£(Eu(C0 3 ) 2 HA) = 16.4±0.3) and hydroxyl(logy3(EuOH 2 HA) = 18.0±0.4). It is proven that mixed complexes of Eu 3+ with humic acid and CO!" or OH~ will be present in most groundwater conditions and should therefore be incorporated in speciation and transport models.

No abstract