Investigation On Functionalized Lamellar Graphene Membranes In Seawater Desalination Via Molecular Dynamics Simulation

With the rapid development of science and technology,the gradual deterioration of environment and the increasing shortage of water resources,it is important to find the efficient separation technology for seawater desalination.In recent years,with the rapid development of nanotechnology,membrane separation has shown great potential in seawater desalination,more and more researchers focus on it.In 2010,graphene(GRA)won the Nobel Prize as a two-dimensional nanomaterial,and it was considered as one of the most promising material for nanomembrane owing to its atomic thickness(one or a few atoms thick),large surface area and high tensile strength,fast fluid transport,low pressure requirements and a wider range of operating conditions.However,more issues remain unclear in the experiments,such as the effects of different functional groups on the desalination performance of lamellar graphene membranes,the relationship between water flux and hydrophilicity of graphene membranes,and the transport mechanisms between water molecule and separation mechanism of ion passing through two-dimensional nanochannels are not well understood.With the continuous progress and development of high performance computers,molecular simulation has become an efficient methods of predicting material properties and designing functional materials,which provides the strong support for the experiments.Therefore,the effects of different factors(different functional groups,interlayer spacing,et al.)on the desalination performance was investigated via molecular dynamics(MD)simulation in this thesis,the transport and separation mechanisms for water molecules and ions in the nanochannels were revealed from the viewpoint of molecular structure,transfer behavior and intermolecular forces.The relationship between structural parameters of functionalized graphene membranes and desalination performance was also illustrated in this thesis,the main conclusions obtained in this thesis are as follows:(1)The water flux in the lamellar graphene membrane modified with different functional groups(GRA-X)was influenced both by the interlayer spacing and the hydrophobicity of the functional groups.When the interlayer spacing between functionalized graphene sheets is 0.9 nm,the water flux is mainly determined by the interaction between functional groups and water molecules,the water flux passing through the graphene sheet modified by hydrophilic functional groups is lower than that modified by hydrophobic functional groups.When the interlayer spacing is larger(>1.1 nm),the water flux is mainly influenced by the interlayer spacing due to the reduce of interaction between water molecules and functional groups.The ion rejection rate is determined by the effective interlayer spacing.When the interlayer spacing is 0.9 nm,the ion rejection rate of GRA-X membranes is higher than 95%,and the ion rejection rate decreased gradually with the increase of the interlayer spacing.(2)As discussed above,the water flux rate is closely related to the interaction between water molecules with functional groups and the order of water molecules in two-dimensional nanochannels.When the edges of graphene nanosheets are modified by-COOH and-CH₃,the water flux rate could be change by adjusting the type and content of functional groups on the graphene surface and the water flux rate is the largest when the graphene surface is modified by the epoxy,since water flux is closely related to the interaction between water molecules and functional groups and the orderliness of water molecules in the two-dimensional nanochannels.When the distance of interlayer spacing is 0.9 nm,the effective interlayer spacing is less than0.9 nm and the effective size of the two-dimensional nanochannel is closer to hydration diameter of Na⁺and Cl^-,which results in the relatively higher ions rejection rate(>90%).(3)The distance of interlayer spacing(H),offset(O),and the with of slit gap(d G)in multilayer stacked graphene oxide membranes(MGOM)shows important influences on the desalination performance.The multilayer stacked graphene oxide membrane maintains the best performance under this condition(H=0.8 nm,d G=1.1nm and O=1.26 nm).More interestingly,the rejection of Na⁺is slightly lower than that of Cl^-in different systems due to the different interaction between the ions and the functional groups in the lamellar graphene membrane.