Experimental Study On Restoration Groundwater In Decommissioned Mining Area Of Acid In-Situ Leaching Uranium Mine By Sediments Enriched With Reduction-Functional Microbial Community

With low production cost,high resource utilization and high degree of automation,acid in-situ leaching uranium mining technology was the main mining process in the early days of sandstone uranium mining in China.Some of the acid in-situ leaching uranium mines in Xinjiang are about to be or have been decommissioned after more than 20 years of mining.The decommissioning of acid in-situ leaching uranium mines has caused pollution problems such as lower pH value,low microbial abundance,and excessive uranium and SO₄^2-concentrations in groundwater.Therefore,it is important to find an efficient,economical and environmentally friendly remediation method for the radioactively contaminated groundwater in the decommissioned mining areas of acid in-situ leaching uranium mine to restore the local groundwater ecosystem and protect the personal safety of the residents.In this paper,a sediment-simulated groundwater microcosms of uranium tailing pond with high abundance of functional microorganisms in Hunan was used to enrich the sediment with a reduction-rich microbial community,and then the sediment was added to the groundwater-core sample microcosms obtained from the decommissioned mining area of acid in-situ leaching uranium mine in Xinjiang province,forming the experimental group A.As a comparison,sulfate-reducing bacteria and electron donors were added separately to the groundwater-core sample microcosms,which were denoted as experimental groups B and C,respectively.The changes of pH value,total uranium,nitrate,sulfate,COD and total iron concentration in each experimental group during the reaction were detected,and the changes in the valence and binding morphology of uranium in the sediment and the microbial community were analyzed.The results showed that the pH in group A increased to 5.5 within 15days,which reached the conditions suitable for the survival of microorganisms.The concentrations of uranium and nitrate dropped to 0.039and 16.17 mg/L at 15 d,respectively.The concentration of sulfate dropped to107.31 mg/L at 60 d.The concentrations of uranium and nitrate have reached the relevant national emission standards.When the repair time reached 60 d,the removal rate of uranium was close to 100%,and the removal rates of nitrate and sulfate were 95.98%and 91.06%,respectively.In group B,sulfate-reducing bacteria can survive in acidic groundwater containing radionuclides and have a certain reducing effect,but its reducing effect is significantly lower than that of the microbial community in group A.Therefore,the sulfate and uranium concentrations of group B still did not meet the relevant national standards when the restoration time was 60 d,and the reducing ability of nitrate was not as good as that of group A.The pH value of groups C and D increased slightly,and the concentrations of nitrate,sulfate and uranium decreased slightly,but the fluctuation range of groups C and D was much lower than that of groups A and B.In conclusion,compared with groups B,C,and D,the reduction efficiency of U（VI）in group A was higher.The results of XPS analysis showed that the ratio of U（VI）to U（IV）in the sediment of group A was about 1:2,indicating that the reduction-functional microbial community had reduced 2/3 of U（VI）in the groundwater to U（IV）.Meanwhile,the analysis of binding morphology of uranium also proved that the sediments enriched with reduction functional microbial communities could transform uranium from unstable forms（Exchangeable and carbonate bound states）to stable（Uranium in Fe-Mn oxide bound,organic bound and residual states）forms.Compared with group B and C,the total abundance of microorganisms with reduction function such as Desulfosporosinus spp.,Geobacter spp.and Pseudomonas spp.in group A was the highest,and the reduction efficiency of U（VI）in group A was more obvious than that in other groups.Thus,it is a remediation method with potential application prospects to restore groundwater in the decommissioned mining area of acid in-situ leaching uranium mine by using the sediments enriched with reduction-functional microbial community.