Theoretical Studies On Adsorption Behaviors And Microscopic Mechanisms Of Metal Ions At The Surfaces Of Smectite Nanoparticles

Clay minerals are ubiquitous in soil and sedimentary environments,among which smectite is one group of the most widely distributed clay minerals in the natural environment.The adsorption of metal ions at the clay mineral/water interface not only profoundly affects the dissolution-precipitation of soil minerals,ion exchange and catalytic reaction,but also determines the migration and transport of metal ions（especially heavy metal ions）in soil systems.Although significant progress has been made in the adsorption of heavy metals on clay mineral surfaces,the microscopic mechanisms underlying the adsorption selectivity towards heavy metal ions,and mechanisms that are crucial for exploring more efficient adsorbents,are still largely unknown.Therefore,studying the adsorption behavior and mechanism of metal ions at the clay mineral/water interface is helpful for us to deeply understand the properties of clay mineral adsorption and other aspects,and is of great significance for controlling soil heavy metal pollution.In this thesis,the molecular dynamics simulation method is used to deeply and systematically study the adsorption behavior and adsorption mechanism of metal ions on different surfaces of smectite nanoparticles at the molecular/atomic level,as well as their respective effects on controlling heavy metal ions.First,we developed a smectite nanoparticle model with a fully flexible structure,explored the behavior of metal ion adsorption on different surfaces of montmorillonite nanoparticles,and described the metal ions on the external basal plane,interlayer surface and edge surface of montmorillonite nanoparticles,respectively.Especially,elucidating the intrinsic mechanism of efficient adsorption of metal ions at the edge of montmorillonite.Furthermore,due to the ubiquitous isomorphous substitution in the smectite structure,different charge distribution positions may affect the adsorption reaction of metal ions at the clay mineral/water interface.By comparing the adsorption behavior of metal ions on different surfaces of montmorillonite and beidellite,the effect of charge distribution position on the adsorption of metal ions on different surfaces of smectite was clarified.In addition,montmorillonite is used as an efficient heavy metal adsorbent,but both external surfaces and strongly confined bihydrated（2WL）interlayers are known to adsorb heavy metals solely as outer-sphere of inferior stability,which is a contradiction in the effective use of montmorillonite to control heavy metal pollution.By studying the K⁺/Pb²⁺exchange kinetics and exchange thermodynamics on different surfaces of montmorillonite,we elucidate the adsorption and exchange mechanisms of K⁺/Pb²⁺on different surfaces of montmorillonite nanoparticles,providing important new insights for Pb²⁺control,which is beneficial to explore more efficient heavy metal adsorbents.The main results are shown below:（1）Compared with the external basal plane and interlayer surface,the edge surfaces of montmorillonite always showed higher adsorption efficiency.Pb²⁺ions are much more stable than Na⁺ions on the entire surface of montmorillonite nanoparticles,and the inner-spere Pb²⁺ions emerge and predominate only at edge surfaces,manifesting the central role of edge surfaces to remove heavy metals.The peculiar distribution of edge-O atoms causes similar coordination environments for inner-and outer-sphere metal ions,and inner-sphere metal ions are preferred significantly due to bonding with edge-hydroxyls.Metal ions with smaller radii are more favorable to adsorb at edge surfaces,and those with considerable hydration effects（e.g.,heavy metals）can be preferred.（010）rather than（110）edges are superior for adsorption,which is caused mainly by structural differences.（010）edges are more exposed to adsorbates,and cleavage of smectites nanoparticles along（010）edges enhances removal of heavy metals.（2）The charge distribution of smectite significantly affects the adsorption behavior of metal ions on different surfaces.Compared with the external basal surface and interlayer surface of montmorillonite,the charge distribution of beidellite is closer to the surface,and the interaction with metal ions is stronger.The stability of both inner-sphere and outer-sphere adsorbed metal ions are stronger than that of montmorillonite.Compared with montmorillonite,the Na⁺and Pb²⁺ions adsorbed by the inner-sphere appear almost only on the edge surface.On each surface of beidellite,the two metal ions are dominant in the inner-sphere form,and have a higher adsorption capacity,which is significantly improved.The overall adsorption efficiency of beidellite nanoparticles for metal ions was investigated.The metal ions on the edge of montmorillonite are always more stable than the edge of beidellite.Isomorphic substitution sites and octahedral Al³⁺sites represent the main adsorption sites on the edge surfaces of montmorillonite and beidellite.However,compared with the Mg²⁺/Al³⁺substitution sites of montmorillonite,the Al³⁺/Si⁴⁺substitution sites of beidellite have lower capacity for metal ions.In addition,the adsorption stability of metal ions on the octahedral Al³⁺sites of beidellite is also lower than that of montmorillonite.（3）The effect of different montmorillonite surfaces on the control metal was revealed at the molecular level.Competitive adsorption of binary K⁺/Pb²⁺ions indicates that all surfaces are Pb²⁺selective,and the Pb²⁺selectivities at different surfaces decline as edge>2WL-interlayer>external basal plane.1)The K⁺/Pb²⁺exchange kinetics and thermodynamics show that both metal ions on the external basal plane are"exchange adsorption"that has short residence time and high exchange frequency,indicating the K⁺/Pb²⁺exchange is relatively easy and reversible,so the external basal plnae play a minor role for Pb²⁺control;2)Edges cause“specific adsorption”for Pb²⁺,with significantly longer residence time and lower exchange frequency,so the effect of Pb²⁺control by the montmorillonite edge is more important at lower Pb²⁺fractions;3)2WL-interlayers represent the major reservoir for metal ions and play a key role,owing to kinetic blocking of K⁺/Pb²⁺exchange,especially at higher Pb²⁺fractions.Therefore,the selective adsorption and unfavorable exchange kinetics causes montmorillonite nanoparticles to be efficient for Pb²⁺control,especially for 2WL-interlayers.