| In this paper,the unconfined groundwater in a uranium tailings area was taken as the research object.Based on the investigation of the distribution characteristics of uranium concentration in the tailing pond,static adsorption experiments and dynamic migration experiments were used to study the migration and transformation of uranium in the aquifer medium.Adsorption of uranium in the aquifer medium was investigated by several methods,such as adsorption kinetics model,adsorption isotherm model and sample characterization before and after migration.The progressive extraction method was used to explore the morphological characteristics of uranium and the mechanism in the process of uranium migration and transformation in the aquifer medium.The main conclusions are following list.?1?Uranium concentration at each sampling point in groundwater did not change with the seasons in the study area,basically.The range of uranium concentration in the groundwater at each sampling point in wet,normal,and dry periods varied from9.05×10-41.15×10-1mg/L,1.2×10-39.59×10-2mg/L and 5.92×10-47.85×10-2mg/L,respectively.Only uranium concentrations at the S7 sampling point exceed US and Canadian standard limits.?2?Adsorption time,initial uranium concentration,initial pH value,addition of metal ions and particle size were the main factors affecting the uranium adsorption in groundwater aquifer media.Adsorption time had obvious influence on the effect of uranium adsorption.The adsorption saturation time of medium sand weighted 1 g was 8h when the initial uranium concentration was 10 mg/L and pH was 5.Then,the increase of adsorption time had no effect on the adsorption rate and the adsorption capacity.The adsorption capacity increased with the increase of the initial uranium concentration,but the adsorption rate showed a decreasing trend,which might be mainly due to the imbalance of the amount of supersaturated uranyl ions in the solution and the limited adsorption potential of the adsorption medium.The initial pH value had a significant effect on the uranium adsorption on the aquifer medium.The optimum pH value for adsorbing uranium was 5.Too high or too low pH values were not conducive to the adsorption of uranium,which was mainly related to the uranium form and H+content in solution.The addition of metal ions inhibited the adsorption of uranium in groundwater aquifers and decreased the adsorption rate and adsorption capacity.The main reason was that positively charged metal ions and uranyl ions together competed for a negative charge.The smaller the particle size meant the larger the specific surface area,which could be beneficial to achieve the faster the adsorption equilibrium,the higher the adsorption rate and the adsorption capacity of adsorbed uranium?3?The process of uranium adsorption by aquifer media could be described by the pseudo-second-order kinetics model?R2=1?,Elovich?R2=0.977?model and the Langmuir?R2=0.988?,Freundlich?R2=0.995?equation,And the maximum adsorption capacity obtained was 0.196 mg/g,which was consistent with the experimental results.The results showed that the adsorption of uranium in aquifer media could coexist with multiple adsorption modes such as monolayer adsorption,multilayer adsorption,and chemical adsorption.?4?The migration of uranium in the medium of groundwater aquifers showed that the uranium concentration decreased with the increase of the migration depth.The main reason might be that the aquifer medium absorbed uranium and caused the decrease of uranium concentration in the solution during the migration.The speed of uranium migration increased with the increase of particle size.The main reason might be that the larger the particle size meant the smaller the adsorption of uranium on the aquifer medium and the lower the retardation of uranium adsorption.?5?The form of uranium showed that active uranium,latently active uranium,and inert uranium were present in all aquifer mediums.And active uranium and inert uranium accounted for a large proportion.?6?Scanning electron microscopy and energy dispersive spectroscopy analysis showed that the surface of the aqueous medium was filled with uranium after adsorption,the surface became flat,the morphology change,and the uranium content could be detected,indicating that the groundwater aquifer media could adsorb uranium. |
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