Uranium Speciation In Microcosm Experiment Of Reduction Of U(Ⅵ) By Functional Indigenous Microbial Communities Under Anoxic Condition

The uranium containing wastewater which comes from uranium mining andleaching is the main source of uranium contaminated groundwater. In oxic waters,U(VI) forms many soluble hydroxide and carbonate complexes which lead toincreased solubility and mobility, in these phases can flow with groundwater and inlarge the pollution range. In the reducing waters, U(IV) forms insoluble phases andthus is relatively immobile. The bioreduction of soluble U(VI) to sparingly solubleU(IV) species is an attractive remedial technology. Previous laboratory experimentshave figured out that some dissimilatory metal-reducing bacteria or sulfate-reducingbacteria can obtain energy from supplied electron donors for growth by transportingelectrons to U(VI) and result in the precipitation of U(IV) in groundwater underanaerobic conditions. In practical application, it’s difficult to achieve the anaerobiccondition. Therefore, it is of great significance to investigate the bioreduction of U(VI)in groundwater under anoxic conditions. The main purpose of this study is to test thebioreduction is one of the ways of U(VI) being reduced to U(IV).In this study, the microcosm were prepared with the groundwater and sedimentwhich were collected from a uranium mill tailings impoundment affiliated to auranium mill in South China. They were amended with ethanol, lactate and glucoseand incubated under anoxic conditions to investigate the availability of U(VI)bioreduction. The variation of pH, COD, Nitrate, Sulfate and U(VI) was measuredduring the incubation. The centrifuged sediment samples were collected after121days of incubation and used for X-ray adsorption fine structure test to investigate theaverage valence state and uranium species. The results indicated the addition of carbon source under anoxic condition caneffectively stimulate microbial activity by which aqueous U(VI) was reduced. Theconcentrations of U(VI) in the ethanol, lactate and glucose amended microcosms werefound to decrease to0.030,0.025and0.026mg/L after96,88and96days ofincubation, respectively. The U L3-edge X-ray adsorption near edge structure spectrafrom the sediment samples after121days of incubation indicated that approximately(42±10)%,(55±10)%,(53±10)%U(VI) were reduced in the glucose, lactate andethanol amended microcosms, respectively. The analysis of the EXAFS spectra forsample collected from control group indicates that uranyl is predominantly bound tobidentate carbon and monodentate phosphorus ligands, after121days of incubation,uranyl in sediment is predominantly bound to monodentate phosphorus.In conclusion, U(VI) can be reduced to U(IV) under anoxic conditions bymicrobial activity and bioreduction is one of the ways of U(VI) being reduced toU(IV).