Study On The Interaction Between Bentonite Colloids And Eu(Ⅲ)

The safe management and disposal of radioactive waste is one of the key issues to be considered in the process of sustainable development of nuclear fission energy.Study on the migration behavior of radionuclides in the repository environment can provide an important reference for the safety evaluation of the repository and nuclear environment.Besides the solid and liquid phases,colloidal phases are ubiquitous in the aquifer system of underground environment.Due to its large specific surface area and high surface energy,colloidal phases have strong adhesion to radionuclides.Therefore,as carriers of radionuclides,colloidal phases play an important role in the process of radioactive pollutant transport.In china,bentonite from Gaomiaozi region（Inner Mongolia）has been selected as buffer backfill material for high-level radioactive waste geological repository.The stability of colloids is closely related to the migration behavior of radioactive pollutants in the environment.The mobile colloids dispersed in water can be facilitated with the migration of the components with weak mobility,which lead to the fate of radioactive pollutants changed,thus pose a potential threat to environmental safety.Although it has been recognized that colloids play an important role in the radionuclides migration,the current research on the interaction between colloids and radionuclides is not systematic.In addition,the mechanism that how colloids controlling the environmental behavior of radionuclides are still unclear.Therefore,the interaction between bentonite colloids and Eu（Ⅲ）was studied in this thesis,the stability of bentonite colloids,the adsorption of Eu（Ⅲ）on colloids.In this thesis,the co-transport of BC-Eu（Ⅲ）and HA-BC-Eu（Ⅲ）were investigated in detail to further understand the environmental behavior of bentonite colloids and to grasp the transport and fate of radionuclides carried by colloids in environment,which are expected to provide an important reference for the safety evaluation of disposal repository.The thesis is composed of seven chapters.Chapter 1 is the preface,which focuses on the over view of research progress and the significance and content of this work;Chapter 2 is the study of the colloids aggregation kinetics,discussing the stability of bentonite colloids and its influencing factors;Chapter 3 is the study of colloidal deposition kinetics,discussing the deposition behavior of colloids in a long period of time;Chapter 4 is the study of the adsorption of Eu（Ⅲ）on colloids.Chapter5 is about the co-transport behavior of BC-Eu（Ⅲ）.Through the co-transport behavior between bentonite colloids and Eu（Ⅲ）,the factors affecting the colloids migration and co-transport behavior of BC-Eu（Ⅲ）are discussed.Chapter 6 is the co-migration behavior of HA-BC-Eu（Ⅲ）in ternary colloidal system.The factors affecting the humic acid individual migration,the co-transport behavior of HA-Eu（Ⅲ）and the co-migration behavior of HA-BC-Eu（Ⅲ）under different pH conditions are discussed in detail.Chapter 7 is the summary and prospect part,which summarizes the research results of this work and the future research directions are prospected.The main results of this thesis are listed as follows:（1）The stability of bentonite colloids from Gaomiaozi region（GMZ bentonite）considering aggregation kinetic and reversibility were investigated as a function of colloidal concentration,pH,electrolyte concentration,coexisting ions and humic acid by using the photon correlation spectroscopy（PCS）technique.The results showed that GMZ bentonite colloids has good dispersion in deionized water and can be stabilized for at least 30 days.The stability of bentonite colloids was dependent significantly on pH,cations and humic acid,while slightly on colloidal concentration and anions concentration.Aggregation was getting obvious and kinetically fast when increasing either salinity or acidity.The aggregated colloidal bentonite clusters could return to fully dis-aggregated state if environmental conditions became dispersion favorable.In different cation electrolyte systems,the critical coagulation concentration（CCC）is in the following order:Eu³⁺<Ba²⁺<Sr²⁺<Ca²⁺<Mg²⁺<Na⁺,which is consistent with that predicted by Schulze-Hardy rule.In addition,the presence of a small amount of humic acid can effectively reduce the aggregation trend and significantly improve the colloids stability due to electrostatic effect and steric hindrance effect.（2）The effects of pH,temperature and electrolyte concentration on the deposition kinetics of bentonite colloids were investigated by photon correlation spectroscopy.The deposition mechanism of bentonite colloids at different temperatures and electrolyte concentrations was explained by DLVO（Derjajuin Landan Verwey Overbeek）theory.The results showed that that the count rate of photon correlation spectroscopy was proportional to particle concentration,which facilitated the qualitative determination of rate constants of aggregation or sedimentation process.The alkaline conditions was favorable for bentonite colloidal stability,while high temperature and high salinity destabilized colloids due to the decrease of repulsion potential energy as temperature and salinity rising.One possible explanation is that the strong Brownian motion of colloidal particles resulting from the higher temperature increased the collision probability of colloidal particles and make them have enough energy to jump over the energy barrier.In addition,the colloids diffusion double layer is compressed with high temperature and high ionic strength increased,which lead to the decrease of repulsive force between colloids particles and thus colloids aggregation and deposition are formed.（3）The adsorption of Eu（Ⅲ）on bentonite colloids were studied by static batch experiments.The effects of contact time,colloids concentration,pH,Eu（Ⅲ）concentration,background electrolyte,temperature and humic acid on the adsorption of Eu（Ⅲ）were discussed in detail.The adsorption mechanism was explained and the corresponding thermodynamic parameters were obtained.The results showed that the adsorption of Eu（Ⅲ）on bentonite colloids quickly reaches equilibrium and thus the adsorption process is a strong chemical adsorption.The adsorption mechanism is ion exchange and outer complexation under low pH condition,while it is mainly inner complexation or surface precipitation at high pH.Cations and anions can inhibit the adsorption of Eu（Ⅲ）on bentonite colloids to some extent.The order of inhibition was as follows:Sr²⁺>Ca²⁺>Mg²⁺>Na⁺and HSO₄^->Cl^->HCO₃^-,but the presence of PO₄^3-in the system obviously promotes the adsorption of Eu（Ⅲ）on bentonite colloids.Langmuir isothermal model can well describe the adsorption process of Eu（Ⅲ）on bentonite colloids.The adsorption process of Eu（Ⅲ）on colloids is spontaneous endothermic reaction.The rising temperature is beneficial to its adsorption and this process belongs to single layer adsorption.The presence of humic acid can effectively improve the adsorption of Eu（Ⅲ）on colloids.Bentonite colloids and humic acid have synergistic effects on the adsorption of Eu（Ⅲ）.（4）The co-transport behavior and mechanism between colloids and Eu（Ⅲ）were investigated by column experiments.The key physical and chemical factors affecting the environmental behavior of bentonite colloids were emphatically investigated,for instance,flow rate,colloidal particle size,colloidal concentration,and hydro-chemical factors such as pH,ionic strength and humic acid.The results showed that high flow rate can obviously promote the migration of bentonite colloids in quartz sand column,but slow flow rate and colloids concentration have no obvious effect on the colloids migration.When large particles moved in quartz sand porous media,they will be trapped and blocked by medium voids.Although the pore size of medium is small but the colloids size is large with low connectivity,so the blocking effect is obvious.The co-transport of BC-Eu（Ⅲ）is effectively facilitated at high pH conditions,while effectively restrained at high ionic strength.The more obvious the filtration maturation,the more significant retardation is.That is related to the stability of colloids under different conditions.The control of the co-transport of BC-Eu（Ⅲ）is depends on Eu（Ⅲ）concentration,colloids stability and mobility.Colloids can adsorb Eu（Ⅲ）and promote its migration when the Eu（Ⅲ）concentration is low,while the migration of Eu（Ⅲ）in saturated quartz sand columns was blocked,since the colloids stability and mobility is decreased at high Eu（Ⅲ）concentration.（5）The co-transport behavior of HA-BC-Eu（Ⅲ）in ternary colloidal system was studied by means of quartz sand column experiment.The factors affecting the HA individual transport and co-transport behavior of HA-Eu（Ⅲ）and HA-BC-Eu（Ⅲ）in ternary colloidal system at different pH were discussed.The results showed that increasing pH or decreasing ionic strength can can be facilitated with HA migration in quartz sand.The co-transport of HA-Eu（Ⅲ）is significantly determined on Eu（Ⅲ）concentration,pH and ionic strength.In HA-BC-Eu（Ⅲ）ternary colloidal system,Colloids can effectively promote the migration of Eu（Ⅲ）in quartz sand column,which attribute to the synergistic effect between bentonite colloids and humic acid.The migration of Eu（Ⅲ）promoted by colloids at different pH may be due to the preferential adsorption of Eu（Ⅲ）on high HA concentration.