Yuanzineng kexue jishu (May 2024)
Experimental Study on Influencing Factors of GMZ Bentonite Generating Colloids
Abstract
Bentonite, as a buffering and backfilled material in high level radioactive waste disposal repository, will produce colloids when interacting with groundwater in near field. Colloids play an important role on the diffusion and migration behavior of radioactive nuclides. Therefore, study on bentonite releasing colloids is one of important issues in disposal safety assessment. GMZ bentonite, short for Gaomiaozi bentonite, is identified as the most promising backfill material in China. GMZ bentonite was taken as research object in this paper, and factors and mechanisms affecting bentonite releasing colloids were studied using turbidimetric method because of good linear relationship between turbidity and colloid concentration. The effects of ion strength, ion types, acid-base property and temperature on releasing colloids were researched. Sample bottle of turbidimeter was special customized in order to avoid disturbance caused by frequent sampling which would affect measurement results. Bentonite was pressed by tableting firstly, then placed in a restricted container made of granite for simulating the real situation of disposal repository. Relationship curves of ion type, ionic strength, pH and temperature with turbidity were obtained, and the mechanisms of all factors affecting colloids releasing were analyzed. The results show that order of cationic species inhibiting the release of colloids from bentonite is Li+＜Na+＜K+＜NH4+＜Zn2+＜Mg2+＜H+＜Ca2+. The cations with higher charge have stronger suppression ability except H+ which could protonate the edge of montmorillonite and reduce the swelling ability of bentonite significantly. The effect of cations on bentonite releasing colloid depends on the ion hydration radius, agglomeration capacity, ability to neutralize negative charges, and adsorption capacity. OH- among four kinds of anions could promote the release of colloids significantly, while others have little effect on the release of colloids basically. The ability of solution to inhibit bentonite releasing colloids would be stronger, when the ionic strength of the solution is higher. Bentonite generates colloids much easier in neutral condition, weak acid and weak base conditions take second place, while strong acid and alkali conditions have the strongest inhibitory effect on colloid release. Acidic environment is more effective in inhibiting colloid release from bentonite than alkaline environment. Increasing the temperature is conducive to the release of bentonite colloid under 20-50 ℃. The reason lied in raising temperature could increase the internal energy of montmorillonite, improve probability of collision between colloidal particles,and reduce the ability of neutralizing negative charges which is beneficial for the swelling of montmorillonite. The results have important guiding significance for analyzing the retardation of nuclide migration by bentonite colloid and understanding the geochemical behavior of bentonite in the near-field correctly.
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