Effects Of Electric Fields At The Surfaces Of Clay Minerals On The Adsorption Of Metal Ions And Coupling Between Metal Ions And Anions

Clay minerals are one of the main constituents of soils,accounting for more than 90 wt.%of the solid phase.The composition,structure and properties of clay minerals show profound impacts on the physical properties,chemical properties,adsorption,catalysis and other performances of soils.Clay minerals are usually characterized by the large surface area,outstanding adsorption capacity and high surface reactivitiy,and thereofere are widely used in a number of fields such as adsorption,separation,catalysis,pharmaceuticals and materials.In addition,clay minerals can be used in the industrial and enviormental remediation aspects,as adsorbents for the management of heavy metals contaminants and the inhabitation of radioactive nuclides.Adsorption of ions at clay minerals always occurs and it has been acknowledged as one of the most important chharateristics.Ion adsorption is the primary cause for soils to have the environmental capacity to supply nutrients,store nourishing elements,self-purify pollution elements and pollutants.Adsorption is the first step for many chemical reactions,and thus they play a critical importance role in dissolution of soil minerals,surface speciation,ion exchange and polymerization.Ion adsorption is also the first steps for the numerous reactions in soil systems,and plays a critical role during the dissolution of clay mienrals,precipation,ion exchange,clay aggregation and other processes.Accoingdly,ion adsorption is one of the focusing issues for soil,colloid,interface and environment sciences.Different clay minerals may have distinct amounts of isomorphic substitutions.The previous results clearly indicate the critical importance of electric fields resulting from isomorphic substitutions during the adsorption of ions,while the mechanisms remain largely elusive;moreover,the ion-specific effects implicated during these processes require to be clarified at a molecular level.Ion-specific effects exist ubiquitiosuly in colloids,soils and biomolecules and profoundly affect the interfacial adsorption,clay aggregation,clay particle stability,enzyme activity,supper molecular aggregation,pH determination,surface tension and so on.In consequence,the investigations of ion adsorption at clay surfaces and ion-specific effects only only provide important theoretical basis for the ion exchange,surface wettability,clay swelling and other fundamental topics in soil science,but also facilitate the understanding of the mechanisms of related industrial processes and environmental remediation.In this thesis,molecular dynamics（MD）simulations are conducted to study,at a molecular level,the adsorption behaviors of the various metal ions at the interface of clay minerals/aqueous solutions.Firstly,the adsorption behavior and ion-specific effects of metal ions at the surfaces of clay minerals with relatively fewer isomorphous substitutions（i.e.,low electric fields）are investigated.Secondly,the adsorption behavior and ion-specific effects of metal ions at the surfaces of clay minerals with relatively more isomorphous substitutions（i.e.,high electric fields）are invesitigated.Then the adsorption behaviors and ion-specific effects corresponding to different strengths of electric fields are compared,and the reason for the reversal of Hofmeister sequence is explored.In the end,the adsorption behaviors of cations and anions at the interfaces of clay minerals and electrolyte solutions are investigated.The differences of ion pairs in different regions are analyzed,and the impacts of clay minerals to the adsorption behaviors of ion pairs and their distribution are addressed at a molecular level.The main findings are summarized as follows:（1）For relatively low electric fields,there are two adsorption structures for Na⁺,K⁺and Cs⁺at clay surfaces:inner-sphere and outer-sphere modes.The adsorption numbers of inner-and outer-sphere modes are small and the increase of ion concentrations bring about a very limited increase of the adsorption amounts while the elevation of electric fields contribute obviously greater.The inner-sphere species is situated in vicinity of substituted tetrahedral sites with relatively high mobility and has larger coordination numbers with water molecules than with mica surfaces.Combined with other analyses,it is deduced that the interaction with water molecules plays a more important for the interfacial adsorption at relatively low electric fields.The stability of inner-sphere species reduces with the increase of ionic radius.Hofmeister series follows as Na⁺>K⁺>Cs⁺and hydration effect is attributed to arouse the ion-specific effects therein.（2）For relatively high electric fields,the inner-sphere Na⁺,K⁺and Cs⁺ions are detected at mica surfaces,especially at low ionic concentrations.Compared with the low electric fields,the numbers of inner-sphere ions show a remarkable increase,and the distances of metal ions with mica surfaces are obviously smaller.The inner-sphere species lies above the center of ditrigonal cavities with the low mobility,and the stability improves significantly due to the increase of electric fields.The coordination numbers with water molecules reduce sharply and the inner-sphere species forms more direct bonds with mica surfaces rather than water molecules.The Hofmeister sequence is reversed and follows as Cs⁺>K⁺>Na⁺for high electric fields.Thus,it has been observed that mechanisms of ion-specific effects are distinct for different electric fields:hydration（Na⁺>K⁺>Cs⁺）and polarization（Cs⁺>K⁺>Na⁺）effects are respectively the driving forces for the low and high electric fields,which further result in the reveral of Hofmeister sequences.（3）Metal ions and anions at the interface of clay minerals and aqueous solutions construct several types of ion pairs,and the types of ion pairs rely strongly on the identity of metal ions.Ca²⁺can form three types of ion pairs:contact ion pairs（CIP）,solvent-shared ion pairs（SIP）and solvent-separated ion pairs（2SIP）.The SIP is the main type for Ca²⁺;For Na⁺,the main types of ion pairs are CIP and SIP;Cs⁺are more likely to form direct contact ion pairs（CIP）than Na⁺,and rarely form solvent-shared ion pairs（SIP）.The effects of clay surface on the cation-anion coupling are significant:for Na⁺and Cs⁺,the probability of ion pairs decreases when approaching mica surfaces;When the ionic concentrations are low,no CIP for Na⁺can be detected in R1（inner-sphere）and R2（outer-sphere）,while CIP in R2（outer-sphere）can still be observed for Cs⁺;The Ca²⁺in R2（outer-sphere）and R3（diffusion layer）can construct CIP in concentrated solutions,due to that the outer-sphere Cl-ions are drived by the stable inner-sphere Ca²⁺ions that increases the probability of forming ion pairs.