A Theory Study Of The Impacts Of Co-ions On The Adsorption Of Metal Ions At The Clay Minerals-aqueous Solutions Interfaces

Clay minerals are widely distributed on the Earth surfaces and usually known to be layered aluminosilicates containing special nanoscale layer structures and high specific surface areas.In the meanwhile,clay minerals are an important constituent of soils,and are natural adsorbents and catalysts in natural environments.Owing to their unique charge properties and three-dimensional structure,clay minerals exhibit the particular physicochemical properties such as adsorption,ion exchange,and swellability.For a long time,ion adsorption at the clay mineral/solution interfaces has become one of the research focuses in the field of colloidal interface chemistry,soil science and environmental science.Adsorption of ions at the clay mineral/solution interfaces not only affects the dissolution of clay minerals,surface precipitation,ion exchange and catalysis,but also largely controls the absorption and utilization of nutrients and the transport of water and contaminants in soil systems.Accordingly,to study the adsorption behaviors and mechanisms of ions at the clay mineral/solution interfaces can help us to understand the physicochemical and adsorption properties of soil clay minerals,and also has significant implications for the management of global environmental pollutions and sustainability of ecological developments.Isomorphous substitution causes clay minerals to carry an abundance of permanent charges,and oppositely charged ions in solutions can be adsorbed.The adsorbed ions are bound tightly to clay surfaces by electrostatic attraction.For clay minerals,the most common isomorphous substitution phenomenon is that the lattice ions are replaced by low-valent ions,and hence clay minerals are generally negatively charged.On the other hand,in real soil systems,metal ions seldom exist alone,and instead multiple metal ions co-exist forming the complicated soil/solution interfaces.A large amount of heavy metal and radioactive ions accumulated in soils have seriously threatened the safety of soil systems and human health.Therefore,study of the adsorption behaviors of multiple metal ions at the clay mineral/solution interfaces help us to understand the complex soil systems and the influence of co-existent ions on the adsorption of metal ions,and also help us to understand the polluting mechanism of heavy metal and radioactive ions and find the remediation methods.In this thesis,molecular dynamics?MD?simulations have been conducted to study at a molecular level the impacts of co-ions on the adsorption of metal ions at the clay mineral/solution interfaces.Firstly,the adsorption of single metal ions at the mineral/solution interfaces was explored,and the effect of charge distribution on the adsorption of metal ions was addressed.Secondly,the effects of co-ions on the adsorption behavior of metal ions at the montmorillonite/solution interfaces were studied,including adsorption type,adsorption amount,adsorption stability and coordination environment,which further results the interpretation of the co-adsorption mechanism of binary metal ions.Finally,the effects of co-ions on the adsorption of binary ions at the beidellite/solution interfaces were demonstrated,and regulation of charge distribution on the adsorption behavior of binary ions at the clay mineral/solution interface was discussed as well.The main results are shown below:?1?Metal ions may correspond to the completely different adsorption behaviors at the montmorillonite/solution interfaces and the beidellite/solution interfaces.For instance,Na⁺and Pb²⁺ions are mainly outer-sphere adsorbed at the montmorillonite/solution interfaces,where the adsorption stability is infirm and metal ions are easily desorbed from the montmorillonite surfaces into bulk solutions.When they are adsorbed at the beidellite/solution interfaces,the results of the atomic density profiles and potentials of mean force indicate that these two metal ions are mainly inner-sphere adsorbed and possess high stability.On the other hand,the Cs⁺ions have the similar adsorption behaviors at the montmorillonite/solution interfaces and the beidellite/solution interfaces,and the predominant adsorption mode is inner-sphere.Compared with montmorillonite,the adsorption amount of Cs⁺ions at the beidellite surfaces is larger and the stability is obviously higher.It can be seen that the adsorption behaviors of metal ions can be significantly affected by the location of surface charges,and the influences on the inner-sphere adsorption are more pronounced than on the outer-sphere adsorption;?2?There are general principles for the co-adsorption of binary metal ions at the montmorillonite/solution interfaces.The adsorption modes of metal ions are not affected by co-ions,but the adsorption amount and adsorption stability are obviously affected.However,different binary systems have particular adsorption characteristics:Co-existence promotes Pb²⁺adsorption while inhibits Cd²⁺adsorption;Co-existence inhibits Pb²⁺adsorption while promotes Cs⁺adsorption;Co-existence promotes each other's adsorption of Pb²⁺and Na⁺ions;Cs⁺ions are more dominant in the competition of adsorption sites at montmorillonite surfaces with K⁺ions,etc.The results of the research have contributed to our understanding of the complex metal ion adsorption mechanisms in real soil systems,as well as the prevention and control of heavy metals and radioactive metal ions in soils;?3?Co-adsorption of different metal ions at the beidellite/solution interfaces also show the different characteristics,but the results are significantly different from those of the montmorillonite/solution interfaces.Competition between Na⁺and Cs⁺ions for the adsorbed sites affects their adsorption structure and causes the discrepancy of adsorption stability and quantity.Na⁺ions become preferred due to co-adsorption.The adsorption of Pb²⁺ions and Na⁺ions on the beidellite surfaces is a trend of wane and wax,and the electric fields significantly affect the adsorption stability.The sequence of stability follows as Pb²⁺>Na⁺under the high electric fields while Pb²⁺<Na⁺under the low electric fields.Furthermore,the presence of Pb²⁺ions completely inhibits the adsorption of Ca²⁺ions and a clear adsorption stability sequence is observed as Pb²⁺>Ca²⁺on the beidellite surfaces.The results of this thesis provide a molecular-level understanding on the co-adsorption mechanism of binary metal ions at clay mineral/solution interfaces at a molecular level,which may help to cognize the real soil systems;Meanwhile,the reason why clay systems are severely contaminated by heavy metal ions is partially explained that helps to understand and remediate the contaminated soils.