Effect Of Chlorite Colloids And Biotite Colloids On U(Ⅵ) Transport Through Granular Granite Columns

The safe disposal of high-level radioactive waste（HLW）is one of the critical missions for the sustainable development of China’s nuclear industry.Among many disposal options,deep geological disposal repository is a safe and effective approach to the permanent disposal of HLW.China has selected the Beishan area of Gansu province as the main pre-selected site for the geological disposal repository of HLW.China has begun constructing its first underground research laboratory in the Beishan site in 2021.Thus,the study of radionuclide migration is of great significance both practically and in terms of evaluating the safety of geological disposal of HLW.This study focuses on the clay colloids that are formed from chlorite,a fracture filler commonly found in granite,and biotite,which is the primary constituent mineral of granite.Batch-type adsorption experiments were performed to investigate the adsorption of U（Ⅵ）on chlorite.Additionally,laboratory-scale column experiments were conducted to examine the impact of two colloids on the transport behavior of U（Ⅵ）in a Beishan granite particle column.The obtained adsorption data were quantitatively described by the surface complexation model（SCM）,and a two-site kinetic attachment/detachment model was applied to simulate the transport experimental data.The results of the study include the following aspects:（i）The results of potentiometric titration showed that the acid-base titration curves of sodium-based chlorite under different ionic strengths were close to each other.The titration experimental results are successfully quantitatively explained by considering the deprotonation reaction of a type of edge site.The results of batch experiments showed that the adsorption of sodium-based chlorite on U（Ⅵ）was significantly affected by p H,U（Ⅵ）concentration,solid-to-liquid ratio（m/V）,and CO₂concentration,independent of ionic strength.The adsorption data of U（Ⅵ）under different experimental conditions can be quantitatively explained by a non-electrostatic surface complexation model with three surface complexes,≡SOUO₂⁺,≡SO（UO₂）₃（OH）₅and≡SO（UO₂）₃（OH）₇^2-.（ii）Chlorite colloids were prepared and well characterized.The individual and co-transport behavior of U（Ⅵ）and chlorite colloids in particulate Beishan granite was studied as a function of important in-situ factors,such as ionic strength,p H and U（Ⅵ）concentration.The results indicated that in the context of individual transport systems,the transportation of U（Ⅵ）is found to be sensitive to its concentration,whereas the transport of chlorite colloids is hindered by an increase in ionic strength and a decrease in p H.The effect of chlorite colloids on U（Ⅵ）transport in the co-transport system is dependent on the transport ability of the colloids.When the chlorite colloids are highly stable,they can facilitate the transport of U（Ⅵ）.However,if the chemical conditions are unfavorable for the transport of chlorite colloids,the adsorbed U（Ⅵ）may be trapped in the granite particle column along with the chlorite colloids.（iii）Laboratory column experiments were conducted to study the effect of biotite colloids on U（Ⅵ）transport in saturated granite particle columns.The results showed that biotite colloids had a more significant impact on U（Ⅵ）transport compared to chlorite colloids.This suggests that the properties of clay colloids play a crucial role in U（Ⅵ）transport.（iv）The variation of particle size and zeta potential of representative colloid effluents during transport was analyzed.A size exclusion effect was demonstrated during colloid transport in the absence/presence of U（Ⅵ）.The individual and co-transport of two colloids in the column could be described by a two-site kinetic attachment/detachment model.The results of CFT and DLVO calculations were consistent with the colloid transport experiment results.