Study On The Migration Behavior Of Nuclides Uranium In The Soil-water System Around A Uranium Tailings Pond And Its Impact Mechanism

Uranium resources provide the raw material guarantee for the rapid development of nuclear energy.However,a large number of tailings produced by mining,processing and hydrometallurgy may cause certain radioactive pollution to the surrounding soil and water.Due to the particularity of the research site and object,some properties of radionuclides in the soil-water environment surrounding the tailings pond are still unclear,such as occurrence form,migration process and formation mechanism.Therefore,it is necessary to reveal the process of nuclide migration and transformation in soil-water system and its impact mechanism.These work can provide basic data for predicting the migration law of radionuclides and evaluating the radiation safety of uranium tailings ponds,and also provide theoretical basis for pollution prevention and control.Based on the National Natural Science Foundation of China,the key issues such as nuclide migration behavior and its influencing mechanism in soil-water system around uranium tailings ponds area were systematically studied.The research methods included field sampling,laboratory testing,static experiment,dynamic column experiment and geochemical simulation,combined with SEM,XPS,XRD analysis and characterization.The spatial distribution of typical nuclides and uranium occurrence form in soil-water system around the tailings pond were found out.The adsorption,migration and influence mechanism of uranium in vadose zone and saturated zone were obtained.Finally,the migration behavior and influencing mechanism of uranium in the soil-water system were summarized.The research results are as follows:（1）Distribution and pollution characteristics of the main nuclides in soil-water systemBased on the quantitative detection of the specific activity of typical radionuclides such as²³⁸U,²²⁶Ra,²³²Th and ⁴⁰K in soil and water samples,the spatial distribution characteristics of the radionuclides were analyzed,and the potential pollution level of the tailings pond to the surrounding environment was evaluated.As a result,²³⁸U and ²²⁶Ra were the main radioactive pollutants in soil-water system.The distribution characteristics of radionuclides ²³⁸U,²²⁶Ra and²³²Th were significantly affected by tailings pond,while ⁴⁰K was less affected by the tailings pond.The low content of nuclide in a rice field was related to the strong cultivation activity of paddy soil.²³⁸U and ²²⁶Ra tend to accumulate near the surface.The vertical distribution of ²²⁶Ra content varied greatly,and ²³²Th content changed least with depth,which is related to its source and migration performance.²²⁶Ra pollution was obvious in different types of water bodies.The contents of ²³⁸U and ²²⁶Ra changed greatly along the surface water flow,with a similar trend and strong migration ability.The content of ²³²Th was low in soil-water system.In addition,the distribution of nuclides in groundwater was uniform.（2）The characteristic of uranium forms in soil-water systemUranium was selected as the research object and the uranium occurrence form was analyzed by using Tessier five-step extraction method.The results showed that uranium was dominated by"active uranium"in tailing sand and river sediment,and the carbonate-bound uranium was the most with strong biological activity and release ability.The"Inert uranium"was dominant in paddy soil,with residue uranium accounting for about 50%,which is not easy to activate and diffuse,and has poor migration ability.From tailing sand to paddy soil surface,the uranium form changed from"active uranium"to"inert uranium",with improved stability and reduced mobility.Meanwhile,the software PHREEQC was used to simulate the uranium forms in different types of water samples.The results showed that the proportions of uranyl carbonate and hydroxyl carbonate gradually increased,while the proportions of uranyl fluoride and uranyl sulfate gradually decreased from the leachate water to the surface water,the groundwater.It was mainly related to the p H value and the concentration of F^-、SO₄^2-and HCO₃^-.（3）Adsorption and migration characteristics of uranium in vadose environmentTaking paddy soil and sediment widely distributed as the research object,the static batch experiment of uranium were carried out.The results showed that the adsorption rate of uranium from paddy soil was faster than that from sediment,and the adsorption amount was also higher than that from sediment,which is mainly related to the particle size and material composition of the medium.The adsorption kinetics of uranium in soil was in good agreement with Elovich’s fitting results,indicating that the adsorption was the coupling result of physical adsorption and chemical adsorption,and belongs to complex heterogeneous diffusion process.The Langmuir and Temkin isothermal adsorption models were used in the thermodynamic adsorption process,which indicated that the adsorption of uranium on paddy soil and sediment existed in the way of monolayer adsorption and multilayer molecular adsorption.The soil-water distribution coefficient of uranium was 57.27 m L/g by dynamic column experiment.It indicates that uranium was easily adsorbed by paddy soil and slowly migrated.Based on laboratory experiments,the migration distance of uranium with 1.5 mg/L continuous point source was predicted by software Hydrus-1D.It takes about 40 years for uranium to migrate to the bottom of 4 m thick vadvate zone.（4）The influence mechanism of environmental factors on uranium migration under saturated conditionsThe dynamic experiment and solute migration models were used to analyze the migration mechanism of uranium in saturated conditions under the control of particle size,p H value,initial concentration,cation type and organic matter.The experiment results showed that the smaller the particle size,the slower the uranium migration rate.The cation Ca²⁺has obvious competitive adsorption with uranium.The higher the atomic number and valence state of the cation,the stronger the competitive adsorption capacity,and the slower the migration of uranium.With the increase of p H value,the R_d increased first and then decreased.So the uranium migration ability decreased first and then increased,mainly affected by the uranium forms.At the same time,humic acid can improve the mobility of uranium,and reduce the influence of particle size and initial concentration on uranium migration.SEM-EDS and XPS analysis showed that uranyl ions were attached to the surface of quartz sand by adsorption during the migration process,but the adsorption did not change the structure of quartz sand.In addition,the two-point non-equilibrium solute transport model can describe the penetration process of uranium in saturated quartz sand column.（5）Uranium migration in soil-water system around tailings pond and its main controlling factorsBased on the influence of soil and water properties on uranium distribution,the migration mechanism of uranium in soil-water system was revealed in combination with the migration characteristics of uranium in vadose and saturated zone.The leachate water was the source of uranium pollution in surrounding surface water,and adsorbed into bottom mud through sediment-water distribution in flow system.The dissolved uranium in the tailings pond penetrated into the aquifer through the vadose zone,which was mainly affected by the p H,organic matter in the soil and water properties.The p H of soil was positively correlated with the specific activity of ²³⁸U,and had a great influence on the adsorption of uranium,which was related to the uranium form and the surface charge of soil.The organic matter content in paddy soil and sediment were high,but the specific activity of ²³⁸U were low.Because organic matter occupies more adsorption sites on the surface of solid phase medium,the effective adsorption sites on sediment surface are reduced,and the competitive adsorption with uranium occupies a dominant position.The p H,HCO₃^-,F^-and SO₄^2-are the main controlling factors on uranium migration.The p H mainly affects the uranium form through hydrolysis and dissociation.The HCO₃^-,F^-and SO₄^2-can act as ligands and form different complexation complexes with uranyl ions,which affect the uranium form in water.