TY  - GEN
ID  - heidok5600
UR  - https://archiv.ub.uni-heidelberg.de/volltextserver/5600/
Y1  - 2005///
TI  - Basic studies for the incorporation of uranium in sediments
AV  - public
A1  - Koroleva, Margarita
N2  - Uranium migration in natural aqueous systems is an ongoing concern in environmental research.  Investigation on sorption interactions with soil, sediments and rocks are important to understand uranium mobility, in order to correct U/Th dating methods in open systems. Uranium immobilization is possible due to reduction U(VI) to U(IV), adsorption or co-precipitation.  Under oxidizing environmental conditions, uranium typically occurs in the hexavalent form as the mobile, aqueous uranyl ion (UO22+). Moreover, depending from environmental conditions, uranium forms carbonate complex such as UO2(CO3)22- or UO2(CO3)34- . Uptake of such dissolved metal contaminants by many fine-grained mineral phases (clays, oxides, and hydroxides) is most commonly achieved by adsorption. For carbonates recent evidence suggests that incorporation into solids (co-precipitation) dominant uptake. Uranium sorption experiments were carried out with lake sediment and aragonite samples. All experiments were conducted using the uranium isotope 232U under ambient pressure and room temperature at different pH. Sediment samples were obtained from an artificial lake (Willersinnweiher, SW Germany). This lake has relatively high uranium concentration in water and sediment columns. The lake has two seasonally different redox conditions in the water column, which were simulated in the lab. To understand uranium behaviour an artificial uranium isotope was added into the water column. The experiments were conducted at ambient pressure and room temperature. After reaction time the uranium concentration was measured in the water column, in pore water and in sediment column. The results indicate in both cases 80 % of uranium saturated into the sediments. But in oxygenated water uranium penetrated deeper into the sediment.  Adsorption experiments show that adsorption of uranium by the lake sediment is strongly pH dependent, and that adsorption at low and high pH is minimal; the maximum adsorption occurs near neutral pH. The experiments of uranium uptake by aragonite powder were conducted at pH range 6-11. These experiments show also the strongly pH dependence and maximum of uranium uptake is at pH 7 (98%). Also experiments regarding (1) the influence of major seawater elements on uranium uptake, (2) uranium transport and (3) kinetic of uranium uptake by aragonite were conducted. Uranium uptake by powdered aragonite is fast (less than 0.3 hours). The content of Mg2+ in solution highly decreases the process of uranium incorporation.    Numerical modeling of the process of U(VI) sorption by sediment was conducted. This study shows that the surface complexation model DLM can predict the major three types of uranium behaviours: (1) the increase of uranium adsorption at pH range between 5 and 6.5, (2) the maximum of adsorption at nearly neutral pH and (3) the decrease of uranium adsorption between pH 8 and 11.
ER  -