Experimental Study And Numerical Simulation Of U(Ⅵ)migration In Unsaturated Soil

In recent years,with the development of the nuclear industry,the migration of radionuclides in soil has caused a certain degree of pollution to the ecological environment.Studying the law and and influencing factors of radionuclide migration and diffusion in soil can provide a theoretical basis for the treatment of soil radioactive pollution.In this study,the surface soil of the paddy field around a uranium tailings reservoir in Jiangxi is taken as the research object.Soil colloids and inorganic colloids were separated by Stoke’s Law and continuous extraction methods,and the with scanning electron microscopy（SEM）,specific surface area（BET）,X-ray photoelectron spectroscopy（XPS）and X-ray diffraction characterization（XRD）and other characterization techniques were used to analyze the morphology,structure,and material composition of soil samples.Through batch experiments,the adsorption performance and mechanism of uranium by soil components under different conditions were studied.The turbidimetric method was used to study the stability and mobility of soil colloids under different pH and ionic strength conditions.By studying the competitive adsorption of different co-existing ions with different components of the soil,the adsorption mechanism on the soil surface was analyzed.The desorption performance of different solutions on uranium was studied,and the concentration of uranium in the soil was changed with the change of the physical and chemical properties of the soil solution.At the same time,the migration and diffusion of uranium under the action of matrix flow and preferential flow in unsaturated soils were analyzed by dynamic soil column leaching experiments and Geostudio software numerical simulation.The main conclusions are as follows:（1）The paper Analyzed and characterized the physical and chemical properties of soil.It is concluded that the paddy soil is a slightly acidic silty clay with an organic matter content of 32.91g/kg,the main chemical components of the soil are SiO₂,Al₂O₃and Fe₂O₃,they exist as quartz and kaolinite.（2）The results of static batch experiments showed that pH value,initial uranium concentration,reaction time and reaction temperature all have effects on the adsorption of uranium by soil components.Under the optimal conditions,the adsorption rate and capacity of soil,soil colloid and mineral colloid for uranium reached 76.67%,83.03%,48.87%and 8.53mg/g,9.24 mg/g,5.43 mg/g,respectively.The adsorption process conforms to the quasi-second-order kinetic model,Langmuir model,and Freundlich isothermal model which indicated that the adsorption process is mainly chemical adsorption and belongs to a single-layer adsorption on an uneven surface.According to the thermodynamic parameters,it was found that the adsorption process of uranium by the sample was spontaneous,endothermic,and entropy increasing.（3）The competitive adsorption of coexisting ions in the soil study showed that each component of the soil has good adsorption for uranium,thorium,and lead,and the active adsorption sites of the soil and each component for uranium,thorium,and lead are not consistent.The mixed solution can promote the adsorption,and adsorption of uranium by inorganic colloid is improved about 10%.（4）The stability and mobility of soil colloids and inorganic colloids with different infiltrat in aqueous solutions study showed that the pH of infiltrating water has a great influence on the stability and mobility.With the increase of pH,the stability of colloids is gradually increasing,and the mobility is gradually increasing.Different ions and ion concentrations also have a certain effect on the stability and mobility of soil colloids and inorganic colloids.For both samples of the NaCl and CaCl₂ solutions,the stability of the colloid showed rapid increase and then slow decrease with the increase of ion concentration.（5）The desorption of uranium complexes by different reagents showed that each reagent has the same tendency for the desorption of uranium from soil,soil colloids and inorganic colloids,and the order of desorption ability is inorganic acid,organic acid,sodium carbonate and deionized water.Among them,the inorganic colloid-uranium complex is the easiest to be dissolved indicated that the complex formed by inorganic colloid and uranium is the most unstable.（6）The dynamic leaching experiment found that the saturated adsorption capacity of uranium in unsaturated soils is constant,and the adsorption capacity is about 2.1 mg/g.The desorption of uranium in soil by different infiltration waters showed that in the matrix flow,the soil colloid and mineral colloid solution can improve the desorption of uranium from the soil and the desorption rate reached 1.63 times than the resolution of the aqueous solution.Under the action of preferential flow,the desorption rate of uranium from soil by soil colloid solution was improved about 10%compared with that in matrix flow.（7）Geostudio simulation software was used to analyze the migration and diffusion of uranium under the matrix flow and preferential flow.The results showed that the migration and diffusion of uranium in the soil under the preferential flow are stronger than that of matrix flow.