Study On Mechanism And Fundamental Application Of U(â…¥) Reduction By Shewanella Oneidensis

The large amounts of uranium residue, tailings and acidic wastewater were produced in uranium mining resulted in pollution from radioactive uranium for humankind’s survival and ecological environment. It is a key issue of security and environment to be solved to seek a cost-effective biological technology to remove the radioactive nuclide from polluted water,even to recycle the metal resources, such as uranium. This effect, main influencing factors and mechanism of U（Ⅵ） reduction were studied by S.oneidensis or ZVI or ZVI and S. oneidensis or the embedding biological microspheres. The main research results are as following:U（Ⅵ） could be reduced effectively by S. oneidens sis mediated by anthraquinone-2-sulfonate（AQS） at anaerobic environment.The reductive removal rate of U（Ⅵ） reached 99.0% during 96 h at p H 7.0, 30 °C, and 1.0mmol/L AQS. The effective concentration of AQS as accelerator for U（Ⅵ）bioreduction was 0.5–1.0 mmol/L. Coexistence of ions, such as Cu²⁺, Cr⁶⁺, or Mn²⁺（2.0 mmol/L）, showed a remarkable negative effect on U（Ⅵ） reduction,and Zn²⁺ showed less influence on the process compared with other tested ions.U（Ⅵ） reduction was remarkably inhibited when the concentration of nitrate ion exceeded 2.0 mmol/L. Sulfate ion（<5.0 mmol/L） slightly promoted U（Ⅵ）reduction. Toxic organic compounds（such as toluene, three- and maleic acid,maleic acid） were available to reduce U（Ⅵ） efficiently by S. oneidensis MR-1and get degraded at the same time. Characterizations with Scanning Electron Microscope（SEM） and Energy dispersive spectroscopy（EDS）indicated the deposition of U element on the cell of S. oneidensis. FTIRspectra analysis indicated that the main groups of S. oneidensis were association of hydroxyl, phenolic hydroxyl, amidogent, halogenated hydrocarbon, carboxyl in the process of U（Ⅵ） reduction.In acidic environment, U（Ⅵ） can be effectively removed by zero valent iron. U（Ⅵ） removal rate reached 96% at 150 min, p H 3.0 and the initial concentration of U（Ⅵ） 20 mg/L. p H has a powerful influence on U（Ⅵ）removal, and EDTA and temperature have less influence. The reductive mechanisms of U（Ⅵ） is precipitation, adsorption and coprecipitation.A cooperative action was observed in ZVI with the bacteria in the reductive process of U（Ⅵ） under anaerobic conditions. With the p H 7.0, 30 ℃,the initial concentration of U（Ⅵ） 20 mg/L and ZVI of 1.0 g/L, the reductive removal rate of U（Ⅵ） was up to 96.9% by S.oneidensis during 24 hrs. The reduction rate of U（Ⅵ） improved with increasing amount of ZVI in the range of 0.5-2.0 g/L. Fe3O4, Fe2O3 and ZVI were effective accelerators for bioreduction of U（Ⅵ）, and soluble Fe（Ⅲ） could remarkably inhibit U（Ⅵ）bioreduction. The reduction rate of U（Ⅵ） was positive correlated with its initial concentration in the range of 5.0-50.0 mg/L. In the range of 30 mg/L,coexistence ions of Cu2+, Ca2+ and Zn2+ show a remarkable negative effect on U（Ⅵ） reduction and among these ions, the negative effect of Cu2+ was the greatest and that of Ca2+ was the second. However, Mn2+ of 30 mg/L shows slightly promote U（Ⅵ） reduction. When the concentration of nitrate ion exceeded 5.0 mmol/L U（Ⅵ） reduction was remarkably inhibited, SO42- ion had less influence on U（Ⅵ） bioreduction. XPS result indicated the deposition of U element were in forms of U（Ⅵ） and U（IV） on iron filings surface, and the reduction product U（IV） was most likely to be in the form of UO2.Optimal process parameters imbedded microspheres are 10% of polyvinyl alcohol, 0.5% of Sodium alginate and crosslinking time at 4 h.The microspheres has a good removal effect for U（Ⅵ）,the removal rate ofU（Ⅵ） by imbedded microspheres could reach 95.8% With the p H 7.0, 30 ℃,the initial concentration of U（Ⅵ） 20.0 mg/L, VMR-1=6m L and VMR-1:VPVA=1:2. Coexistence of ions, such as Cu2+ and Mn2+, showed a remarkable negative effect on U（Ⅵ） reduction, and Ca2+showed a weakly Positive effect on U（Ⅵ） reduction. SEM and EDS indicated that U（Ⅵ） was reduced and precipitated in imbedded microspheres.