Experimental Studies On Bioreduction Of U(?)in Groundwater In A Uranium Tailings Reservoir By Indigenous Functional Microbial Communities

U(?)bioreduction is a new kind of U(?)remediation method.Electronic donors,such as ethanol,acetate,lactate and vegetable oil,are added into groundwater,and microbial communities which have the ability of U(?)bioreduction can be stimulated.High mobility of U(?)can be reduced to low mobility of U(IV).Thus,the risk of uranium entry into the human body has been reduced.Carbonate and bicarbonate concentrations can arrive at 0.1~1.8 g L-1 in uranium tailings reservoirs.High concentration of bicarbonate can affect the abundance of microbial communities capable of reducing U(?)in groundwater,thereby affect U(?)bioreduction.The reduction efficiency of U(?)is often not high in the process of in situ U(?)bioreduction.U(?)bioreduction often lags behind nitrate reduction,and accompanies with sulfate and Fe(?)reduction.Nitrate reducing bacteria(NRB),sulfate reducing bacteria(SRB)and Fe(?)reducing bacteria(FRB)have been induced enrichment by high concentrations of nitrate,sulfate and Fe(?)in sediments,and then the sediments have been added to the remediation site to study their effects on U(?)reduction in groundwater.In view of these problems,the following researches have been carried out in this paper:(1)Sediment and groundwater samples were collected from a uranium tailings reservoir in South China.The anaerobic microcosms amended with 30 m M bicarbonate and amended without bicarbonate were established,respectively.The reduction of U(?)in the microcosms by functionalized indigenous microbial communities were investigated.It was found that the time when the reduction of U(?)completed in the microcosm amended with 30 m M bicarbonate was 10 d later than in the microcosm amended without bicarbonate,the concentration of the U(?)in the former microcosm finally maintained at the level of 0.2 mg L-1,and the concentration of the U(?)in the later microcosm decreased to the level lower than the limit concentration of uranium waste water(0.05 mg L-1,<Regulations on radiation protection and environmental protection of uranium mining and metallurgy(GB 23727-2009)>).The chemical extraction and XANES analysis showed that the proportions of U(IV)in sediments at day 46 were 45%±5% and 55%±5%,respectively in the microcosm amended with 30 m M bicarbonate and the microcosm amended without bicarbonate.The Miseq sequencing of the sediment samples collected at day 10 and day 25 from the microcosm amended with 30 m M bicarbonate and the microcosm amended without bicarbonate showed that Desulfovibrionaceae and Veillonellanceae were more easily stimulated in the microcosm amended with 30 m M bicarbonate than in the microcosm amended without bicarbonate,but the reduction of U(?)by them were found to be inhibited by the high concentration bicarbonate,and U(?)reduction speed became slower.The experimental results show that high concentration bicarbonate has a significantly inhibitory effect on the reduction of U(?).(2)In order to investigate the influence of different concentrations of bicarbonate on the abundance of microbial communities capable of reducing U(?)in groundwater,7 groups of experiments incubated with lactate and amended with 7 initial concentrations of bicarbonate(0,5,10,15,20,25,and 30 m M,respectively)were conducted.It was found that U(?)concentration cannot decrease lower than 0.05 mg L-1 when the initial bicarbonate concentration was higher than 10 m M.The main functional microbial communities capable of reducing U(?)in the group amended with 0 m M initial concentration of bicarbonate were Cellulomonas,Desulfosporosinus,and Desulfovibrio,whose total abundance were as high as 36%.The main microbial community capable of reducing U(?)in the group amended with 10 m M initial concentration of bicarbonate was Desulfitobacterium,and its abundance was higher than 14%.The main functional microbial communities capable of reducing U(?)in the group amended with 15 m M initial concentration of bicarbonate were Desulfovibrio and Geobacter,and their total abundance was over 13%.The overall experimental results indicate that the higher initial bicarbonate concentration leads to the lower abundance of microbial communities capable of reducing U(?)in groundwater.(3)In order to study how sediments which induced enrichment by high concentrations of nitrate,sulfate and Fe(?)affect U(?)bioreduction,sedment-groundwater microcosms were conducted,and 3 groups of experiments were designed: A.sediment which induced enrichment by high concentration of nitrate was added into sediment-groundwater microcosms and amended with ethanol,which contrasted with microcosms injected NRB and amended with ethanol,microcosms amended with ethanol only(control group),and microcosms amended without ethanol(blank);B.sediment which induced enrichment by high concentration of sulfate was added into sediment-groundwater microcosms and amended with ethanol,which contrasted with microcosms injected SRB and amended with ethanol,control group,and blank;C.sediment which induced enrichment by high concentration of Fe(?)was added into sediment-groundwater microcosms amended with ethanol,which contrasted with microcosms injected FRB and amended with ethanol,control group,and blank.Misq high throughput sequencing was used to analyze the diversity and abundance of the microbial communities in the experimental groups and the control group.Results indicated that microbial communities capable of reducing U(?)in all experimental groups and the control group were Geobacter,Desulfosporosinus,Desulfotomaculum,Desulfitobacterium and Desulfovibrio.The total abundance of these microbial communities in the control group was 13.1%;these microbial communities were not detected in blank;the total abundance of these microbial communities in sediments which induced enrichment by high concentrations of nitrate,sulfate and Fe(?)groups were 27%,35.3%,and 30%,respectively.U(?)concentrations of the three experimental groups reduced under 0.05 mg L-1 at day 22,19,and 22,respectively.However,it needs 28 days in the control group.All of the three experimental groups have promoted the reduction of U(?)in the groundwater.The method “Sediment which induced enrichment by high concentration of sulfate” has the best promotion effect on U(?)bioreduction.