Pollution Characteristics and Migration-transformation Mechanism of Uranium in Soil Around Uranium Tailings Pond

Author:Chen Jing Ying

Supervisor:qiu zu min


Degree Year:2019





Uranium ore forms the basis for the development of national nuclear energy and nuclear military undertaking.With the gradual opening up of China’s nuclear weapon test site,the increase in uranium mining and metallurgical activities and the decommissioning of uranium mines,the problem of soil treatment and remediation in uranium mines and surrounding areas has gradually attracted the attention of scholars.In particular,nuclide in radioactive waste may be weathered or rainwater impregnated,leached or underground permeated and then enters soil and groundwater,posing a potential threat to ecological environment of the mining area and the safety of people’s lives.Therefore,studying occurrence characteristics of nuclides pollution in uranium mines and the surrounding soils,and understanding the effects of migration pathways,migration speeds,migration laws and its transformation mechanism is undoubtedly necessary for ensuring ecological environment and food security of uranium mining areas.In this paper,radionuclide uranium in the soil around tailings pond of a uranium mine is taken as the research object.Research methods combining field survey,indoor analysis,uranium form classification,batch experiment and column experiment were used to determine the pollution characteristics of radionuclide uranium in the soil.The uranium adsorption behavior of farmland soil in uranium mining area was studied to determine the influencing factors in uranium migration in soil.HYDRUS software and Visual Minteq software were adopted to respectively study migration and transformation mechanism of uranium in farmland soil under sewage irrigation and in soil under rainfall conditions.(1)Using research methods of field survey,indoor analysis,uranium form classification,batch experiment and column experiment,pollution,enrichment characteristics and source of uranium in soil of uranium mining area were analyzed to determine occurrence form distribution characteristics of uranium in soil of the mining area and factors influencing the dynamic process of exogenous uranium form transformation.The correlation between uranium content in surface soil-rhizosphere soil-rice root was analyzed,and the enrichment of uranium in the rhizosphere was confirmed.In addition,uranium was more easily enriched in soils with more Fe2O3,Al2O3 and P2O5,and organic matter would strengthen uranium enrichment by soil.The multivariate statistical analysis of soil uranium pollution sources revealed that there may be a homologous relationship between U,Cu,Pb and Th elements in the tailings area,which leads to synergistic effect.The surface soil uranium has the highest residual form content,which is the main occurrence form of uranium in the soil.The ratio of carbonate-bound and organic-bound states is obviously high in the rhizosphere soil.Soil with high uranium content also has high proportion of active uranium,pH and organic matter are the main factors affecting occurrence form of uranium.The addition amount of exogenous uranium also affects form distribution of uranium in the soil.After exogenous uranium in high concentration enters the soil,the process of transition to residue state is slower.Farmland soil has gradually decreased surface porosity after absorption of uranium.The characteristic absorption peak of uranium appears in the range of 800-1100cm-1,and with the extension of culture time,strong absorption peaks appear near 1030 cm-1 and 800 cm-1.pH increased slowly with the prolongation of culture time during the waterlogged incubation,and the results of morphological distribution simulation with pH changes were consistent with the experimental results.(2)The influencing factors of uranium migration in the soil were determined by studying uranium adsorption behavior of farmland soil in uranium mining area.The adsorption process of uranium by soil is best described by Langmuir adsorption isotherm model.The correlation coefficient R2 of quasi-second order kinetic model is0.9977.The fitting results of internal diffusion kinetics model show that the first half of adsorption is mainly controlled by membrane diffusion,and the later stage is mainly controlled by internal diffusion.It is known from adsorption thermodynamics fitting curve that the adsorption process of uranium,which can be spontaneously carried out at normal temperature and is not affected by temperature.There are a large number of polar groups in soil organic matter,which results in a large amount of negative charges on the soil surface,forming a complex with UO22+,and improving the adsorption capacity of the soil for uranium.The specific surface area and pore volume,pore diameter analysis indicate that the experimental soil belonged to silt,the isothermal adsorption curve type is type II,the hysteresis loop type is type C,and pore diameter is in the range of 250 nm,which belongs to porous material.(3)Using HYDRUS software,the migration,accumulation and simulation,prediction of uranium in farmland soil under sewage irrigation were analyzed.The indoor soil column leaching experiment and HYDRUS software were adopted to simulate uranium-containing sewage irrigation conditions.The migration and accumulation behavior of uranium in the vertical direction of the soil indicate that soil adsorption of uranium is a decisive factor for uranium concentration in soil column leachate and uranium distribution in soil column profile.The soil in the study area showed penetration during the leaching process with different leaching amount.The saturated hydraulic conductivity and dispersity of the soil microscopically affect the time when uranium begins to penetrate the soil and the shape of the breakthrough curve.HYDRUS software was used to fit the uranium migration in soil at different flow rates and uranium accumulation in the soil.The migration of uranium content in soil column was well fitted for different time and different sections.The migration distance of uranium in the soil was predicted under sewage irrigation condition.Under uranium concentration of 0.5mg·L-1 in irrigation water,the pollutant migrates about 10cm in 120 days and 50cm in 1200 days.Under sewage irrigation,the risk of uranium to environmental groundwater cannot be ignored.(4)Visual Minteq software was used to systematically study leaching characteristics and transformation mechanism of uranium in soil under rainfall conditions.By simulating release process of uranium in soil under different pH rainfall conditions,it was found that pH reduction significantly aggravated desorption of uranium.The release process has better fitting effect when two-constant rate equation is used,indicating that release of uranium from the soil surface reflects non-uniformity of the energy.The profile distribution characteristic of various forms of uranium after leaching shows that the proportion of active uranium is generally increasing,and the proportion of residual uranium is decreasing continuously.Where,the rate of decline is the highest in soil column with leachate at pH4.Uranium form in three soil solutions with pH4.0,pH5.6 and pH7.0 was simulated by Visual Minteq.The simulation results were consistent with the experimental data.It can be seen that uranium in the soil has certain environmental risks under acid rain conditions.