Uranium In Groundwater Chemical Speciation And Geochemical Engineering Barrier

The treatment of radioactive wastewater is one of the important concerns and must be seriously considered in the nuclear industry, whereas the geochemical engineering barrier is one of the basic and key issue for the buried disposal of radioactive waste.In this paper, study on geochemical engineering barrier for the buried disposal of very low-level radioactive waste containing uranium was performed. The chemical species of U in groundwater was analyzed by a theoretical calculation method, and the effect of the additives on the engineering barrier was investigated on the basis of the calculated chemical characteristic of U(â…¥)'s species. Then, the appropriate soil medium was selected according to the surface electric charge, and the adsorption of uranium on the soil was investigated in a static way while the migration of U was performed in column experiments.The theoretical calculation for the chemical species and migration behavior of uranium in groundwater were accomplished by combining the thermodynamics balance analysis mode, the geochemical condition and the known thermodynamics data. The results indicated that the overwhelming majority (more than 99%) of U(VI) in the groundwater exit as the complex species, such as UO₂(CO₃)₂^2-, UO₂(CO₃)₃^4-, UO₂CO₃⁰ and UO₂(HPO₄)₂^2-, while the other species as UO₂(OH)₂⁰, UO₂(OH)⁺ or UO₂²⁺, account for less than 1%.It can be shown by the additive experiment that most of the investigated additives can not remarkably improve the adsorption performance of the soil for uranium, whereas some additive have reverse effect. Also, it is very difficult for reductive sediment of U(â…¥) to U(â…£)by adding some reduction additives, such as N₂S. Fortunately, the soil sample â…¢, a kind of arenaceous clay with orange yellow color from the disposal site, has very strong adsorption ability to uranium with a K_d value of up tol228. 4ml. g^-1. It can be conclude that this soil should be the best candidate for the geochemical engineering barrier materials, if the granularity, lithology and commercial availability were comprehensively considered. Then the further experiment was performed.Surface electric charge of the soil was measured, and the result that the soil sample â…¢ has the highest positive charge value of 9. 60mmol/100g among the investigated soil samples was noted. It is thus clear that the K_d value is direct ration to the positive charge of the soil, and the adsorption mechanism of uranium on soil should involve the strong adsorption of uranyl complex anion onto the positive charge colloid.The adsorption behavior of uranium onto the topsoil and deep soil from the soil sample III was evaluated. The topsoil with different granularity has same principle mineral components and contents as the deep soil. The surface positive charge of the topsoil declined with decrease of the granularity, and almost one time less than the topsoil sample. Also, the adsorption rate of the topsoil for uranium dropped along with decrease of the granularity, while no significant change was observed with the deep soil sample. However, the adsorption rate of the topsoil and deep soil both rose with the increasing of pH or ratio of solid to liquid, and increased first, then declined gradually with the increasing of uranium concentration. The adsorption equilibrium with the two samples was achieved within 14 days, while the maximal adsorption rate with the topsoil was observed at normal temperature. Moreover, it could be seen that the absorption rates of the deep soil sample were always several to ten times higher than that of the topsoil sample in each experimental conditions. Absorbed and retarded capacity of Uranyl complex ion direct ratio surface positive charge in soil, and the mechanism is static absorption.The absorption ability of the deep soil sample for uranium could been greatly improved by adding Ca(0H)2, leading to the adsorption rate up to 1. 9X 104ml. g"1. The similar result could be obtained by adding charry gritstone, and the adsorption rate and the quantity of the added charry gritstone was usually in keeping with linear correlation.It should also be paid much attention to that the adsorption rate resulted from dynamic column migration experiments was ten times less than that from static experiments. The main reason is that column migration experiments don' t attain equilibrium. Yet, this condition is similar to the groundwater flow and the Uranyl ion is retarded by the soil with the groundwater flow, so the results of column migration experiments have actual value.In summary, several factors related to the geochemical engineering barrier for the buried disposal of very low-level radioactive waste containing uranium, such as the chemical species and migration behavior of uranium in groundwater, the effect of the additives on the engineering barrier, and the adsorption behavior of uranium on the soil had been investigated in this paper, and enormous expeimrntal data summarized. It is indubitable that all these will be useful to disposition and safety assessment of radioactive waste containing uranium.