The microbial reduction of U(VI) by Bacillus sp. dwc-2, isolated from soil in Southwest China,was explored using transmission electron microscopy(TEM), X-ray photoelectron spectroscopy(XPS) and X-ray absorption near edge spectroscopy(XANES). Our studies indicated that approximately 16.0% of U(VI) at an initial concentration of 100 mg/L uranium nitrate could be reduced by Bacillus sp. dwc-2 at pH 8.2 under anaerobic conditions at room temperature.Additionally, natural organic matter(NOM) played an important role in enhancing the bioreduction of U(VI) by Bacillus sp. dwc-2. XPS results demonstrated that the uranium presented mixed valence states(U(VI) and U(IV)) after bioreduction, which was subsequently confirmed by XANES. Furthermore, the TEM and high resolution transmission electron microscopy(HRTEM) analysis suggested that the reduced uranium was bioaccumulated mainly within the cell and as a crystalline structure on the cell wall.These observations implied that the reduction of uranium may have a significant effect on its fate in the soil environment in which these bacterial strains occur.
The biosorption mechanisms of uranium on an aerobic bacterial strain Streptomyces sporoverrucosus dwc-3,isolated from a potential disposal site for(ultra-)low uraniferous radioactive waste in Southwest China,were evaluated by using transmission electron microscopy(TEM),energy dispersive X-ray(EDX)analysis,Fourier transform infrared spectroscopy(FT-IR),X-ray photoelectron spectroscopy(XPS),proton induced X-ray emission(PIXE)and enhanced proton backscattering spectrometry(EPBS).Approximately60% of total uranium at an initial concentration of 10 mg/L uranium nitrate solution could be absorbed on 100 mg S.sporoverrucosus dwc-3 with an adsorption capacity of more than3.0 mg/g(wet weight)after 12 hr at room temperature at p H 3.0.The dynamic biosorption process of S.sporoverrucosus dwc-3 for uranyl ions was well described by a pseudo second-order model.S.sporoverrucosus dwc-3 could accumulate uranium on cell walls and within the cell,as revealed by SEM and TEM analysis as well as EDX spectra.XPS and FT-IR analysis further suggested that the absorbed uranium was bound to amino,phosphate and carboxyl groups of the cells.Additionally,PIXE and EPBS results confirmed that ion exchange also contributed to the adsorption process of uranium.
The complexation, precipitation, and migration behavior of uranium in the presence of humic acid (HA) or fulvic acid (FA) were investigated by cation exchange, ultrafiltration and dynamic experiment, respectively. The results showed that (i) complex equilibrium between the uranium and humic substances was achieved at approximately 72 h, (ii) the coordination number varied from 1:1 to 1:2 ( U(Ⅵ) : humic acid) as pH increased from 3 to 6; and (iii) , while the complex stability constant decreased when temperature increased, but increased with pH value. We found that the precipitation of uranyl could only be observed in presence of HA, and the precipitation was influenced by conditions, such as pH, uranium concentration, temperature, and the HA concentration. The maximum precipitation proportion up to 60% could be achieved in the condition of 40 mg/L HA solution at pH 6. We further observed that the migration behavior of uranium in soil in the presence of humic acid (HA) or fulvic acid (FA) was different from that in the presence of inorganic colloid, and the effect of humic substances (HS) was limited.
LIAO JialiWEN WeiLI BingYANG YuanyouZHANG DongKANG HoujunYANG YongJIN JiannanLIU Ning
To develop a microbe-based bioremediation strategy for cleaning up thorium-contaminated sites, we have investigated the biosorption behavior and mechanism of thorium on Bacillus sp. dwc-2, one of the dominant species of bacterial groups isolated from soils in Southwest China. Thorium biosorption depended on the p H of environment, and its rapid biosorption reached a maximum of up to 10.75 mg Th per gram of the bacteria(wet wt.) at pH 3.0. The biosorption agreed bettter with Langmuir isotherm model than Freundlich model, indicating that thorium biosorption was a monolayer adsorption. The thermodynamic parameters, negative change in Gibbs free energy and positive value in enthalpy and entropy, suggested that the biosorption was spontaneous,more favorable at higher temperature and endothermic process with an increase of entropy. Scanning electron microscopy(SEM) indicated that thorium initially binded with the cell surface, while transmission electron microscopy(TEM) revealed that Th deposited in the cytoplasm and served as cores for growth of element precipitation(e.g., phosphate minerals) or by self-precipitation of hydroxides, which is probably controlled by ion-exchange, as evidenced by particle induced X-ray emission(PIXE) and enhanced proton backscattering spectrometry(EPBS). Fourier Transform Infrared(FTIR) further indicated that thorium biosorption involved carboxyl and phosphate groups and protein in complexation or electrostatic interaction. Overall results indicated that a combined electrostatic interaction-complexation-ion exchange mechanism could be involved in thorium biosorption by Bacillus sp. dwc-2.
Uranium removal efficacy of fullerence multi-macrocyclic polyamine derivatives(C60-MMP),a novel chelating agent,was evaluated in mice.C60-MMP was administrated intravenously into mice at 30 min after the uranium contamination.The molar ratio of chelating ligand/uranium was about 1:1.The results indicate that C60-MMP can effectively prevent accumulation of uranium in liver at 8 h after C60-MMP injection.At 48 h after the last injection,uranium deposition in liver of C60-MMP treated mice is approximately 65%less than that of the control group.C60-MMP reacted positively in promoting the removal of uranium from kidney,and the urinary uranium excretion increased significantly,compared with the control and DTPA-treated mice.However,repeated administration of C60-MMP,and combined injection of DTPA and C60-MMP,did not show desirable effects on uranium removal from mice.It implies that more investigations are needed for the treatment protocols and clinical applications of C60-MMP.
