Laboratory and Field Experiments to Study the Mobilization and Preconcentration of Uranium from Soils and Various Rock Materials by Fungi.
Colette MUNIER-LAMY and Jacques BERTHELIN. LIMOS UMR 7137 CNRS-UHP Nancy I Faculté des sciences, BP239, Vandoeuvre lès Nancy, 54506, France
Plants and microorganisms are directly involved in the solubilization, absorption, accumulation of major and trace elements including radionuclides. Uranium is one of these elements that can be transferred to the liquid phase and to the plant or accumulated under the influence of different microbial processes including oxido-reduction, production of complexing compounds, and biosorption. In natural and disturbed environments, accumulation of uranium has been observed in plants, mushrooms and fungal mycelium. In old mining wastes, where uranium content was from 10 to 645 mg. Kg-1 of soil, contents of 3.5, 235, and 230 mg. Kg-1 dry matter were determined for spruce, gramineous and mushrooms, respectively. Large contents have also been observed in mycelia by coupling of microscopic and spectrometric methods. To progress in knowledge of the role of microorganisms and in particular fungi, different experiments have been performed in the fields or in laboratory devices. In field experiments, fungal mycelium or organic substrates (lignin, cellulose), which supported microbial growth, were placed in porous bags (1 or 5 µm pore size) and then introduced in soil upperlayers. After different periods of exposure, they were collected for analysis, especially for their U content. In laboratory experiments, mainly in batch conditions, U bearing materials were placed in nutrient media where different types of fungi (mycorrhizal and non mycorrhizal fungi) were growing. In field experiments performed in an old mine site on mining wastes, uranium was fixed by fungal mycelium. After 6 to 10 months exposure, U contents were from 125 to 300 mg. Kg-1 dry matter. During cellulose biodegradation, amounts of fixed uranium were from 4 to 25 mg. Kg-1 dry matter. Results of experiments in batch devices also showed the ability of different types of fungi to accumulate uranium supplied as minerals or rock materials. Furthermore, U contents can reach large amounts when fungal growth was done in deficient conditions or in the presence of different types of metabolic inhibitors. Uranium uptake ranging from 2 or 3 to 2800 mg. Kg-1 dry biomass was noted. Such processes which have to be better understood, may be involved in uranium cycling (solubilization and preconcentration), in the transfer to water and plants but also in its recovery and removal from contaminated waters.