Molecular Simulation Study On The Application Of Nanoporous Materials In Desalination

With the growth of population and the development of industry,the lack of fresh water resources has become more and more prominent,and desalination is expected to be one of the effective methods to solve water shortage.At present,the reverse osmosis（RO）technology is commonly used for seawater desalination.With the development of nanotechnology,people’s understanding of nanomaterials has gradually deepened.Therefore,it is of great significance to screen or design suitable nanomaterials and combining them with reverse osmosis membrane technology for the development of desalination technology.The membranes constructed by carbon nanotube arraysas well as the layered graphene oxide have been synthesized in the experiment,which exhibits excellent desalination performance,this indicates that the nanoporous materials show the excellent application prospect in the field of seawater desalination.However,the separation mechanism and interaction mechanism between water/ions and nanoporous materials are still unclear.With the development of computer technology,molecular dynamics simulation has been widely employd in biology,materials and other fields.The desalination mechanism of nanoporous materials can be explored from the molecular level via molecular simulation,which provides some theoretical guidance for experimenters.In this thesis the desalination performance of nanomaterials with different structure was investigated by non-equilibrium molecular dynamics simulation.The research materials include:Boron Nitride Nanotubes（BNNT）,Graphene Oxide（GO）and Metal Organic Framework（MOF）.In this paper,the effects of pore structure,functional groups and other factors on desalination performance were investigated,which reveals the separation mechanism and interaction mechanism between water molecules and nanoporous materials,it provides effectively guidance for experimental synthesis of membrane materials with superior performance.The main conclusions of this thesis are as follows:（1）The ion rejection would be improved significantly by introducing some functional group to the entrance of BNNT（8,8）,while the water flux is slightly reduced.The steric hindrance of functional group and the attractive interaction between ions and charged groups lead to the decrease of the effective diameter of the nanotube,so that the water flux would be decreased.The transfer bebavior of ions in BNNT（8,8）modified with charged functional groups is unfavorable in terms of thermodynamics and dynamics owing to the larger free energy gap between water molecules and ions as well as the decrease of effective diameter caused by the trapped phenomena,therefore,the salt rejection and desalination performance would be improved.Salt rejection can reach 100%by introducing 1 NH₃⁺group or 1 COO^-group to BNNT（8,8）.（2）The water flux and ion rejection would be influenced both by steric hindrance and charge interaction,but it is mainly deterimined by the electronic interaction,When the left and right ends of the BNNTs have different charges,an electric field is formed in the tube,which reduces the ion rejections.（3）As the content of epoxy groups increases,the order of water molecular chains increases in GO_O system,and the water flux increases gradually.The water flux in GO_OH system tends to decrease first and then increase with the increase of oxygen content,which can be attributed to the hydrogen bonds between water molecules and hydroxyl groups.The water flux in GO_O systems are higher than that in GO_OHH systems under the same conditions.When the hydroxyl content is high,the hydroxyl group easily replaces the water molecules in the hydration layer of Na⁺,which causes Na⁺to fall into the hydroxyl group and improves the Na⁺rejection of GO_OH system.（4）The water flux in FMOF-1 is higher than that in FMOF-CH₃,because the-CF₃group is more hydrophobic than the-CH₃ group,the interaction between FMOF-1 and water molecules is weaker than that of FMOF-CH₃,which maks water molecules more easily to transport in the FMOF-1 tunnel.