Neptunium(V) Sorption onto Granite and its Mineral Constituents in the Absence and Presence of Humic Acid


Neptunium(V) Sorption onto Granite and its Mineral Constituents in the Absence and Presence of Humic Acid

Schmeide, K.; Sachs, S.; Reich, T.; Hennig, C.; Heise, K. H.; Bernhard, G.

In Germany, besides salt formations also granitic subsurface environments are taken into account as potential host formations for the geological disposal of radioactive waste [1]. For safety assessments of radioactive waste repositories, amongst others, the sorption behavior of radionuclides onto the host rock has to be known. It is determined by various factors including pH, Eh, the presence of inorganic and organic ligands as well as the speciation of radionuclides in the aqueous and solid phase.
In the present work, the sorption of neptunium(V) onto granite and its main mineral constituents quartz, orthoclase, biotite, albite, and muscovite is studied under anaerobic conditions as a function of pH, ionic strength and solid/solution ratio in a series of batch equilibrium experiments. Furthermore, the effect of humic acid on the neptunium sorption is studied for different ligand concentrations. A 14C-labelled synthetic humic acid (type M42, [2]) is used for the experiments. Its sorption behavior is comparable to that of natural humic acids. The oxidation state of neptunium in the presence of humic acid and minerals is followed at selected pH values over a period of three months by NIR absorption spectroscopy. The neptunium sorption is affected by both the pH and the presence of organic material.
Neptunium species on the mineral surface are identified using neptunium LIII-edge extended X-ray absorption fine structure (EXAFS) spectroscopy. The structural parameters of the near-neighbor surrounding of sorbed neptunium are compared to those of neptunium hydrate, hydroxide and humate complexes.

[1] Papp, R.: Gegenüberstellung von Endlagerkonzepten in Salz und Hartgestein. FZKA-PTE Nr. 3, Forschungszentrum Karlsruhe GmbH, Karlsruhe 1997.
[2] Bubner, M., et al.: Isotopically Labelled Humic Acids for Heavy Metal Complexation. J. Labelled Compounds and Radiopharmaceuticals XLI, 1017 (1998).

Keywords: Neptunium; humic acid; sorption; granite; muscovite; orthoclase; quartz; albite; biotite; EXAFS

  • Poster
    8th International Conference on Chemistry and Migration Behaviour of Actinides and Fission Products in the Geosphere: Migration '01, Bregenz, Austria, 16.-21.09.2001

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