Influence of humic colloids on the migration of U(VI) in compacted clay

Clay minerals and organic substances are widespread components of many soils, sediments, and rocks. As humic acids (HA) possess a strong ability for metal complexation, they can influence the migration of radioactive and toxic metal ions in clayey environments. However, the migration behavior of humic colloids and its impact on the actinide migration in clay formations is poorly understood.
Former studies discuss the influence of HA on the radionuclide migration in clays controversially. Wold and Eriksen [1] reported the diffusion of humic colloids through compacted bentonite without strong physical hindrance mobilizing metal ions. In contrast, Wang et al. [2] found a significant immobilization of Eu(III) by humic substances. Maes et al. [3] stressed the reversibility of the interaction between radionuclide, organic matter, and clay. The competition of the adsorbents for radionuclides is kinetic and determines their distribution between mobile and immobile phase.
In the present study we investigated the diffusion behavior of humic colloids in compacted clay as a function of compaction and pH. The impact on the uranium transport was studied by comparing the uranium diffusion in absence and presence of HA.
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The migration of HA in clay is governed by diffusion. It is influenced by the colloidal behavior of HA. At higher clay bulk densities, size fractionation affects the diffusion parameters. In presence of HA colloids U(VI) is immobilized in association with HA near the high concentration boundary.
The German Federal Ministry of Economics and Technology funded this study under contract 02E9673. The technical assistance of Ch. Müller is gratefully acknowledged.
REFERENCES
[1] Wold, S., Eriksen, T.E. (2005) Proceedings of the 2nd International Symposium Clays in natural and engineered barriers for radioactive waste confinement. Tours, France, 14-18 March 2005, 547.
[2] Wang, X. et al. (2005) Adsorption Science and Technology, 23, 801-811.
[3] Maes, N. et al. (2004) Report SCK•CEN-BLG-988. SCK•CEN, Mol, Belgium.
[4] Sachs, S. et al. (2004) Report FZR-399, FZ Rossendorf, Dresden, Germany.
[5] Lead, et al. (2000) Environ. Sci. Technol. 34, 1365-1369.