Computational Simulation Of Interaction Mechanism Between Heavy Metals/radionuclides And Montmorillonite At Different Interfaces

The adsorption and fixation on clay minerals can greatly control the environmental processes and geochemical circulation of heavy metals and radionuclides.Therefore,an in-depth and quantitative characterization of the microscopic adsorption mechanism of heavy metals and radionuclides at the clay-mineral interface is not only helpful for reliable assessment of related environmental risks,but also provides theoretical guidance for the development of environmental mineral nanomaterials.In the past decades,the adsorption behavior of heavy metals and radionuclides on clay minerals have been explored by various experimental techniques（batch adsorption experiments,surface complexation models,and spectroscopy technology）.However,due to the complexity of the interface structure and reactivity of clay minerals,it is difficult to quantitatively reveal the adsorption mechanism of these pollutant ions on clay minerals by only using experimental means,which prevents an accurate and profound understanding of the geochemical circulation process of these elements.In recent years,theoretical calculation methods based on computer simulations have been widely used in environmental science,geochemistry,materials,physical chemistry,and other fields.In this paper,by constructing various heavy metals/nuclides-montmorllonite interface complex system under various environmental conditions,molecular dynamics simulations are carried out.To elucidate the microscopic dynamic processes,ion distribution characteristics and variations,ion adsorption properties,diffusion kinetics,and thermodynamic energy contributions involved in the cation exchange of heavy metals and nuclides into the interlayer region.The reaction characteristics and fundamental mechanism of these cation exchange processes are quantitatively clarified.The main conclusions are as follows:1.The microstructure and kinetics of typical heavy metal ions(Zn²⁺,Cd²⁺and Pb²⁺)in the interlayer region and nano-pore of montmorillonite are revealed.（1）Cd²⁺and Pb²⁺can form inner-sphere and outer-sphere complexes on the basal surface of Wyoming montmorillonite,while Zn²⁺can not form inner-sphere adsorption,and the three ions can only form outer-sphere adsorption on the basal surface of Arizona montmorillonite;（2）the adsorption of heavy metal ions between montmorillonite layers is mainly due to electrostatic interactions;（3）the diffusion abilities of heavy metal ions in montmorillonite interlayer and nano-pore are obviously weaker than in bulk aqueous solution.2.The dynamic characteristics and interface microstructure information of the exchange of a series of heavy metal ions(Ag⁺,Cd²⁺,Cu²⁺,Ni²⁺,Pb²⁺and Cr³⁺)from the external pore solution to the interlayer region are clarified.（1）Isomorphic substitution of Mg for Al in the secondary outermost octahedral position will reduce the coordination number of edge Al;（2）hydrogen bondings between solvent water molecules and edge functional groups result in the formation of water film,and the number of hydrogen bonds at（110）edge is more than that at（010）edge;（3）The cation exchange capacities of different heavy metals are different.The exchange ratio of Ag⁺is the largest（over 50.0%）,followed by Ni²⁺,Cd²⁺,Pb²⁺and Cu²⁺（about 32%to 35%）,and the exchange capacity of Cr³⁺is the smallest（less than 6%）;（4）a small amount of Cl^-is exchanged through associating with heavy metal cations.3.The dynamic exchange process of radionuclides(Cs⁺,Rb⁺,UO₂²⁺,Sr²⁺,Ba²⁺and Eu³⁺)entering into montmorillonite interlayers are investigated,the cation exchange capacity,aqueous species distribution,and diffusion behavior of are revealed.（1）With the increase of the valence state of nuclide cations,the exchange capacity decreases,and the collapse of montmorillonite particles is beneficial to the exchange of foreign nuclides;（2）CO₃^2-has a strong complex ability to UO₂²⁺,and small CO₃^2--UO₂²⁺clusters are formed even in 0.1mol/L UO₂²⁺solution;compared with CO₃^2-,other anions(Cl^-,I^-,Tc O₄^-and SO₄^2-)have slightly weaker complexing ability with UO₂²⁺.（3）The orientation of linear UO₂²⁺between the layers is mostly perpendicular to the basal plane;（4）The diffusion of nuclides in the interlayer of montmorillonite is more constrained than that in the external nanopore solution.The research results deepen the understanding of the migration and stabilization of heavy metals and radionuclides in nature,and provide theoretical guidance for developing efficient and environmentally friendly environmental mineralogical nanomaterials.