The Sorption And Transport Of Se(Ⅳ) And Sr(Ⅱ) In Granite And The Impact Mechanism Of Fracture Filling Materials

Nuclear energy will inevitably produce nuclear waste while bringing economic benefits.The deep geological disposal method is widely accepted as the disposal scheme of high-level nuclear waste.During nuclear waste storage,radionuclides may be released into the surrounding rock and transported outward along the cracks in the bedrock,threatening ecological safety and human health.Granite is the first choice of the surrounding rock of the geological disposal reservoir,and its mineral components have obvious differences in the adsorption ability of nuclides.Under the influence of tectonic action,weathering action,and groundwater action,secondary minerals are formed in granite fractures,accumulating continuously and becoming fracture fillings.Granite matrix,fracture,and fracture filling together constitute the granite system.Compared with the mineral components of the granite,the clay minerals,carbonate,and metal oxides contained in the fracture fillings have stronger cation exchange ability and perform a more significant influence on the radionuclides’transport process.Therefore,studying the adsorption mechanism of radionuclides on granite and fracture fillings is essential to accurately predict the adsorption and transport characteristics of radionuclides in the surrounding rock and fracture system,which is related to the selection of geological disposal repository of nuclear waste and site safety performance evaluation.In this paper,granite and fracture filling samples were first collected in the study area through field investigation.The physical and chemical properties of granite and fracture filling were analyzed using X-ray diffraction,X-ray fluorescence spectroscopy,scanning electron microscopy,and other means.The stable isotopes ⁸⁰Se and ⁸⁵Sr of radionuclides ⁷⁹Se and ⁹⁰Sr were selected as representative nuclides.The distribution characteristics of Se（IV）and Sr（II）in the groundwater system of the study area were investigated using hydrogeochemical simulation software.Then,a series of batch adsorption experiments and dynamic adsorption experiments were conducted to investigate the adsorption and transport mechanism of Se（IV）and Sr（II）on granite,fracture filling,and mineral components.The main contribution minerals to Se（IV）and Sr（II）adsorption in granite and fracture filling were identified,and the relative contribution rate of each mineral component was quantified.Finally,the adsorption and transport processes of Se（IV）and Sr（II）in granite fracture were investigated by dynamic core fracture transport experiment.The adsorption and transport effect mechanism of nuclides was revealed based on coupled hydrogeochemical process transport model.The correlation of the multi-level adsorption characteristic parameters in the granite system is discussed based on the component additivity model.Through the above research,the following conclusions are obtained:（1）The mineral components of granite and fracture filling and the distribution characteristics of Se and Sr in the study area were determined.The main mineral components of granite are potassium feldspar,albite,quartz,and biotite.The mineral composition of fracture fillings is more complex,including potassium feldspar,albite,quartz,biotite,calcite,illite,and montmorillonite.There is a small amount of Sr elements in the groundwater of the study area,mainly in the form of Sr²⁺and Sr SO₄.The nuclide Sr（II）does not change the form of Sr elements after entering the groundwater.The nuclide Sr（II）is mainly Se O₃^2-and HSe O^3-after entering the groundwater.（2）The static and dynamic adsorption characteristics of Se（IV）and Sr（II）in granites and mineral components were analyzed,and the relative contributions of each mineral component were quantified.The batch adsorption experiments show that biotite has the strongest adsorption capacity for Se（IV）and Sr（II）,followed by potassium feldspar,albite,and quartz.The coexistence of Se（IV）and Sr（II）can significantly promote the adsorption capacity of each other on granite and its mineral components.The dynamic adsorption and transport experiments show that the adsorption of Se（IV）by granite and its mineral components is mainly non-equilibrium adsorption,and the retention capacity of feldspar minerals is significant.The fraction contribution rates of potassium feldspar,albite,biotite,and quartz are 26.38%,43.72%,17.05%,and 12.86%,respectively.（3）The static and dynamic adsorption characteristics of Se（IV）and Sr（II）in fracture fillings and mineral components were investigated,and the contribution capacity of each mineral component was quantified.The batch adsorption experiments show that montmorillonite has the strongest adsorption capacity for Se（IV）and Sr（II）,followed by illite,potassium feldspar,albite,quartz,and calcite.The dynamic adsorption and transport experiments show that Se（IV）adsorption by fracture filling is mainly non-equilibrium adsorption.The minerals that contribute the most to the adsorption are montmorillonite and illite,whose fractional contribution rates are 49.49%and 26.62%.The adsorption capacity of Se（IV）and Sr（II）by fracture fillings change with the type and content of fillings.（4）A transport model coupled with hydrogeochemical processes was developed to reveal the adsorption mechanism of Se（IV）and Sr（II）on the core fracture surface.The former is mainly controlled by surface complexation reaction,while the latter is mainly controlled by surface complexation reaction and ion exchange reaction,and ion exchange reaction is dominant.The effects of water complexation and mineral solubilization precipitation reaction on Se（IV）and Sr（II）transport were weak.The coexistence of Se（IV）and Sr（II）can promote the adsorption of each other on minerals.The increase in flow velocity can promote the surface complexation reaction and ion exchange reaction,leading to a significant increase in the adsorption amount of Se（IV）and Sr（II）on minerals.The influence of initial concentration on Se（IV）and Sr（II）adsorption and transport behavior was relatively weak.（5）The influence mechanism of fracture filling was revealed based on the coupled model and dynamic core fracture transport experiments.The montmorillonite and illite in the fracture filling have large surface site density,cation exchange capacity,and specific surface area,which promotes the surface complexation reaction and ion exchange reaction of the nuclides Se（IV）and Sr（II）on the mineral surface.In the presence of fracture filling,the adsorption growth rates of Se（IV）and Sr（II）are 41%～54%and 104%～170%,respectively.After long desorption,the residual adsorption capacity of Se（IV）and Sr（II）in the core with fracture filling is about 2～6 times that of the core without fracture filling.In summary,the study results of this paper expand the scientific understanding of the adsorption and transport mechanism of nuclides in the granite system,which provides model and parameter support for accurate simulation and prediction of the transport process and spatial and temporal distribution of nuclides in the surrounding rock system.It also provides a theoretical basis for the safety assessment of deep geological repositories of high-level radioactive waste.