Experimental Study Of Bioremediation Simulating Uranium-contaminated Groundwater By Designed And Indigenous Consortia

After the end of in situ leaching uranium mining,uranium contaminated groundwater in uranium mining area poses a threat to residents’health and ecological environment.Adding electron donors to uranium contaminated groundwater can stimulate indigenous flora to reduce high solubility U（VI）to insoluble U（IV）by bio-reduction mechanism,so as to achieve the purpose of repairing uranium contaminated groundwater.Some researches showed that the abundance of nitrate-reducing bacteria（NRB）,sulfate-reducing bacteria（SRB）and iron-reducing bacteria（FeRB）was high in the process of reduction and fixation of uranium by indigenous consortia,and played an important role in this process.Although indigenous consortia is well adapted to the environment through long-term environmental domestication,and there is synergy among functional consortia,it can effectively improve the efficiency of uranium reduction and fixation of uranium.However,the lack of some key functional groups in indigenous consortia may hinder the process of microbial uranium fixation,which makes the study of uranium fixation mechanism more complex.Therefore,how to combine them artificially and effectively through the synergistic effect of functional consortia to construct efficient uranium-fixing bacteria is the key to bioremediation of uranium-contaminated groundwater.The simulation results show that the designed consortia with NRB,SRB and FeRB mixed in equal proportion and indigenous consortia were put into the simulated uranium contaminated groundwater with pH 7.2,25°C,initial concentration of U（VI）20.00 mg/L,initial concentration of microbial 1.0*10⁷cells/mL.The removal rate of NO₃^-in the solution of designed consortia group and indigenous consortia group were more than 90.00%in the first four days,and the removal rates of U（VI）and SO₄^2-in the solution were 93.00%,98.60%and 32.18%,38.93%in the fourth to 19 days.After that,the concentrations of NO₃^-,U（VI）and SO₄^2-in the solution did not change significantly.The pH values of the two groups of solutions increased from 7.2 to 7.60～7.83.The results of scanning electron microscopy-X-ray spectroscopy（SEM-EDS）,Fourier transform infrared spectroscopy（FTIR）,X-ray photoelectron spectroscopy（XPS）and X-ray diffraction（XRD）showed that the uranium fixation process of designed and indigenous consortia was divided into three stages:First,designed and indigenous consortia adsorbed and fixed U（VI）in the solution on the surface of the consortia,and microbial NO₃^-reduction created favorable conditions for the reduction of U（VI）.The functional groups interacting with uranium included hydroxyl,carboxyl,amide and other oxygen-containing functional groups.Second,layered precipitation appeared on the cells surface of designed and indigenous consortia.Microorganisms reduced U（VI）and SO₄^2-synchronously.The relative proportions of U（VI）and U（IV）in the precipitation of artificial flora group were 50.44%and49.56%,respectively.The relative proportions of U（VI）and U（IV）in the precipitation of indigenous flora group were 74.41%and 25.59%,respectively.U（IV）was mainly UO₂.Third,the layered precipitation on the cell surface of designed and indigenous consortia was more compact,and the relative proportions of U（VI）and U（IV）in the designed consortia group were 52.69%and 47.31%,respectively.In the indigenous consortia group,the relative proportions of U（VI）and U（IV）were 64.35%and36.65%,respectively.In this stage,microorganisms promoted the transformation of amorphous U（IV）into UO₂,and improved the stability of the reduction products.The microbial community analysis of designed and indigenous consortia groups showed that the efficient uranium-fixing bacteria should include Fe RB,NRB and SRB.Adding efficient strains such as Clostridium＿sensu＿stricto＿13 or other key microorganisms to uranium contaminated groundwater may be a feasible method to further improve the effect of microbial in situ remediation of uranium contaminated groundwater.The results of this study provide theoretical guidance for the construction of efficient flora in the process of microbial remediation of uranium-contaminated groundwater,and are of great significance for in-situ remediation of uranium-contaminated groundwater.