|Other Abstract||With the peaceful utilization of nuclear energy, considerable uranium-containing wastewater originated from the nuclear industry was discharged into sub- or near- surface and groundwater, resulting in serious radioactive pollution and threatening public safety. Hereby, understanding the complex biogeochemical reactions of uranium, including its speciation, migration and transformation has profound significance for the safety assessment of the geological disposal of radioactive waste, recycling of radioactive elements and the bioremediation of radioactively contaminated environments. Fungi are widely distributed in nature and are also an important part of biological chain that participates in energy metabolism and element cycle. It has been proved that fungi affect the behavior of uranium via multiple pathways, such as biosorption, bioaccumulation, and biomineralization. Therefore, this dissertation systematically evaluate the interaction between uranium and the ascomycete fungus Saccharomyces cerevisiae, whom is widely existed in nature, as well as the fungus Pyrenochaetopsis sp., whom is isolated from radioactive contaminated sites. The main results are as follows:
Part I: Adsorption behavior of uranium on methanal modified S. cerevisiae
1. Methanal modification drastically improve the U(VI) adsorption capacity of S. cerevisiae cells, which is even 6 times than that of living cells under the same condition (1 mg/L U). The kinetic study suggests that the biosorption of uranyl ions is a rapid process, which equilibrated within ~90 minutes, and there is electron sharing and transfer in the adsorption process. The optimal pH for adsorption is 5.8, which might be related to the ionic ligands on the cell surface, the pI values of amino acids, and the hydrolysis of uranyl ions. The Langmuir and Freundlich isotherm models fitting results shows that the adsorption of uranyl ions by methanal modified S. cerevisiae is homogeneous (mono-layer coverage only).
2. Scanning electron microscopy(SEM) and Fourier transform infrared spectroscopy(FTIR) showed that methanal modification change the surface morphology of S. cerevisiae cells and protein structure. Aminomethylation lead the hydroxymethyl derivatives. A layer of scale-like uranium precipitation is attached on the cell surface after uranyl adsorption. Uranium associates with -OH, -PO43-, -COO, C=O and other functional groups, where U(VI) complexes -COO into bidentate ligand. The interaction mechanisms between methanal modified S. cerevisiae and uranyl ions might be including complexation, precipitation and electrostatic interaction. X-ray diffraction spectroscopy(XRD) shows that methanal modified S. cerevisiae cells unable to transform amorphous U(VI) into crystals.
Part II: The mineralization process of uranium by S. cerevisiae
1. The U(VI) toxicity experiments on S. cerevisiae cells show that more than 90% of S. cerevisiae cells stay alive in oligotrophic water for more than 8 days, while in different U concentration solutions, the cells show totally different survival curves, who survive at least 4 days in a 100 mg/L U(VI) solution based on 60% of the living cells.
2. With the rapid adsorption of U(VI) on cell surface, the U(VI) concentration in the solution drops rapidly, however the Pi concentration and the pH value of the solution gradually increase, and the pH value of solution finally stabilize at ~7.0, After exposure for 1 d, the solution was over-saturated with respect to Chernikovite. As cells expose to 100 mg/L U less than 2 days, the U(VI) on the cell surface keeps in an amorphous state, while the diffraction peaks appears over time, which proved that the amorphous U was transformed into tetragonal Chernikovite. Mesoscopic studies have confirmed that in the initial stage of the interaction, U(VI) is embedded in the cell surface as irregular scale-like precipitations, these irregular precipitations on the cell surface of S. cerevisiae gradually disappears, yet the crystalline precipitations (uranyl phosphate) appear over time. The crystallographic information indicates that the thickness of newly formed Chernikovite is gradually thinning from nearly a hundred nanometers to tens of nanometers, and the S. cerevisiae cells even mediating UO22+ into a nearly perfect crystal.
3. The biomineralization process of U(VI) by S. cerevisiae including three stages: (1) surface adsorption, (2) amorphous precipitation, and (3) crystallization.
Part III: The interaction between Pyrenochaetopsis sp. and uranium
1. The growth of the Pyrenochaetopsis sp. on agar is affected by the gradient U concentrations. The maxium growth rate of the fungus is in 0.01 mM-U amended agar and the lowest is in 0.1 mM U counterpart. When U concentration exceeds 0.1 mM, the fungus stops growing. Low concentration of U (<0.01 mM) can effectively promote the growth of fungi. When 0.01 and 0.1 mM U are amended to the liquid medium, the Pyrenochaetopsis sp. showed a totally different trend in the enrichment of U: the adsorption equilibrium reachs after 10 d in 0.1 mM U, and the U level in the solution remains unchange, while in 0.01 mM U, U(VI) desorption occurs after the adsorption equilibrium reached.
2. During the growth of fungi in solid or liquid phase, the cells enrich both nutrients and pollutants (U) from the surrounding environment to themselves through the hyphae, which even accumulate 10-times of U(VI) than surrounding environment. According to XANES and XAFS spectrum, Pyrenochaetopsis sp. grows in both planktonic and solid-phase exhibit the same complexation form with respect to U(VI). Within 30 days timescales, Pyrenochaetopsis sp. is unable to use electron donors and mediate the bioreduction of U(VI) in aerobic environment, yet hydrolyze phosphate from its own phosphate pool under the function of phosphatase, enventually form the low solubility meta-autunite.
3. The concentration of Na+ barely affects the species of uranyl phosphate. High concentration Fe3+ may form the secondary iron minerals, thus making U(VI) precipitate on its surface and hindering the complexation of U-P. Pi has a significant concentration effect on the precipitation of U(VI). Only when the molar concentration of Pi is several times than that of U(VI), can uranyl phosphate complexes or minerals be formed.
The above results enhance our understanding of the mechanism of uranium transformation by environmental fungi, which provide a fundamental theory for future application of fungi to the recovery of uranium in wastewater bioremediation of radioactive contaminated soil and groundwater at pilot- or field-scales.|