Gaomiaozi Bentonite Colloid:Interactions With Uranium And Thorium

As an important part of the “carbon peak and neutrality targets” strategy,nuclear power plays an irreplaceable role in the transformation of the energy structure.In order to ensure the sustainable development of nuclear power and enable the public to correctly understand and receive nuclear power,it is necessary to properly handle and dispose of radioactive waste generated during the development and use of nuclear power.The deep geological repository for high-level radioactive waste is internationally recognized as a feasible high-level waste disposal technology at present,and China also started to build the first underground laboratory for high-level radioactive waste in Beishan,Gansu Province in 2021,which marks a new chapter in the treatment and disposal of high-level radioactive waste in China.However,the long half-life of high-level radioactive waste has brought unprecedented challenges to the construction,evaluation and supervision of the repository.How to ensure that the radionuclide energy is safely locked in the repository within tens of thousands of time scales is the focus of scientific researchers.The potential area in Beishan,Gansu Province is a repository with granite as the surrounding rock material and Gaomiaozi bentonite as the alternative buffer/backfill material.Bentonite is responsible for preventing nuclide leakage.However,some studies have pointed out that bentonite will produce colloid after being erosion by groundwater,which has the risk of carrying radionuclides to the biosphere.It can be seen that understanding the physical and chemical behavior of bentonite colloid itself and its interaction with nuclides is very important.In this work,natural Gaomiaozi Na-bentonite is selected as the research object.The bentonite colloid with a mass fraction of 0.5% is extracted by gravity sedimentation method in a pure water.The mass fraction of the colloid is quantitatively analyzed by ultraviolet visible spectroscopy,and small angle X-ray scattering technology is used,combined with dynamic light scattering technology and X-ray photoelectron spectroscopy technology,the microstructure characteristics of Gaomiaozi bentonite colloid and its lamellar structure and aggregation morphology after interaction with U(VI)and Th(IV)were studied from the following aspects:(1)Gaomiaozi bentonite block presents an irregular lamellar stacking structure in the natural environment,and has three interlayer hydrated structures,namely 0 interlayer water(0W,dspace=0.92 nm),1 interlayer water(1W,d-space=1.27 nm),and 2 interlayer water(2W,dspace=1.46 nm).The d-space of 0W structure can be approximately regarded as the lamellar thickness of GMZ.In the process of water absorption and swelling,the defective 0W structure swells to 1W and precipitates.In addition,mineral components such as kaolinite or quartz also precipitate,so the main mineral component of the bentonite colloid formed after swelling is montmorillonite(93.4%).After swelling,the bentonite still presents a lamellar structure,with a three-layer hydration(3W,d-space=2.12 nm)structure.(2)Gaomiaozi bentonite colloidal particles can be stably suspended in water,with hydrodynamic diameter of 170~336 nm,negative charge on the surface,zeta potential between-31.5 m V and-49.16 m V.The smaller the size of particles,the higher the zeta potential on thesurface,and the more irregular the stacking.The charges on the surface and the edge of colloidal particles are different.With the decrease of p H,the edge charges are covered by hydrogen ions and become positive,and electrostatic interaction with the still negatively charged surface causes particles to agglomerate.The addition of metal cations to the colloidal system will preferentially exchange ions with colloidal particles,disrupt the stacking structure of the layers,and electrostatic interaction will occur with the particle surface after the interlayer cations are saturated,resulting in the reduction of zeta potential and the agglomeration of colloidal particles.In the ionic strength of groundwater environment,all bentonite colloidal particles are agglomerated and precipitated,but still have a lamellar stacking structure with a spacing of 2 nm.(3)As the colloid has a higher montmorillonite content,the adsorption capacity of the colloid to U(VI)and Th(IV)is stronger than that of the block.In a neutral water environment,U(VI)and Th(IV)are hydrolyzed.The hydrolysate of U(VI)is a plate-like particle with a size of 30~50 nm.The hydrolysate of Th(IV)evolves from a mononuclear polymer to a random long chain polymer over time and finally evolves into a single thorium dioxide particle with a diameter of~5 nm.The reaction between hydrolysates of U(VI)and Th(IV)and colloidal particles of Gaomiaozi bentonite occurs on the particle surface,The hydrolysates adhere to the particle surface,resulting in the reduction of zeta potential and the aggregation of colloidal particles.At 25 ℃,about 15 % colloidal particles precipitated after the reaction.With the temperature rising to 85 ℃,the reaction shifted to the right,but the precipitation did not increase further.The lamellar structure and aggregation morphology of colloidal particles were not permanently affected.Zr(IV)was selected as the analog of Pu(IV)to study the interaction with GMZC.At neutral p H,the hydrolysate of Zr(IV)attached to the surface of colloidal particles,reducing its stability and causing particle agglomeration.