Study On Transport Mechanisms In Graphene-Based Materials For Water Desalination

Water is the basic demand for life and the important factor for the development of social civilization.The shortage of fresh water resources is one of the most urgent problems in the world.Desalination is a universally recognized way to effectively and sustainably obtain fresh water.In recent years,the vigorous development of two-dimensional(2D)materials has opened up a new direction of membrane processes for desalination,and its advantages of high efficiency and low energy consumption have received extensive attention.Graphene-based materials are the most representative 2D materials.Invesgating the transport process of ionic solution inside them is of great significance for exploring basic mass transfer phenomena at nanoscale and revealing its physical mechanism.On the other hand,it provides a guiding direction for the design of membranes made by 2D materials,which makes it more applicable to seawater desalination,water treatment,functional ions filtration and other fields.This thesis focuses on two key issues of water transport and ion transport inside graphene-based materials.And emphatically discusses the influence of structural parameters,surface properties,external temperature field and flow mode on mass transfer process.This thesis performs the molecular dynamics simulations of pressure-driven ionic solution flowing through the multilayer graphene oxide channels.Different configuration parameters of nanomaterials(the width of laminate crack,the layer spacing and the distance of gap misalignment)and the solution types(the hydration radius and hydration structure)are discussed to investigate its effect on salt rejection.A physical model based on classical fluid mechanics has been established to describe the relationship between the water transport resistance and different structural parameters in multilayer graphene oxide channels.The slipped flow in the interlayer of graphene oxide have been deeply studied.It can be obtained from the simulations that the slipped velocity decreases with the increase of oxide concetration,and the flow rate of water grows linearly with the slipped velocity.Combined with the kinetic theory and classical fluid mechanics,the nature of the slip flow phenomenon is explained.The flow between the nanolayers is attributed to the comprehensive effect of the "positive effect" of the slip flow and the "negative effect" of the shrinkage of effective flow path.The empirical relationship between the characterization parameters(slip coefficient,contraction of effective flow path)and the oxide concentration of surfaces has been obtained by molecular dynamics simulations.In addition to the nature of the material itself,this thesis also studies the effects of external conditions(temperature field,flow patterns)on the transport process in the nanoscaled graphene channels.For the ionic solution flows through the graphene channel under temperature field,a special phenomenon that water molecules and ions moves in opposite direction is found under the same temperature gradient.The kinetic theory has been introduced to explain the the motion mechanism of water molecules(thermal creep effect)and salty ions(thermophoretic effect).By simulating the flow under different temperature differences,the relationship between the thermal-induced velocity and the temperature gradient in the channel is obtained.Moreover,it is also revealed that there is a transformation process of water between quasi-square ordered structure and chaotic disordered structure under extremely confined conditions.The ordered structure performs anisotropic collective diffusion and the sub-continuum transport phenomenon accompanying the existence of temperature difference.For 2D materials stacked in layers,both simulations and experiments have found that different flow modes also affect the seepage process of the material.This is because the atomic thickness of 2D materials make boundary become transparency,allowing interaction potentials to penetrate the boundary and affect the flow in the channel.The study of the viscosity boundary covering graphene layers demonstrates the existence of boundary transparency would affect the flow in the nanochannel.The hindrance of graphene boundary transparency to the flow of water molecules in adjacent channels is illustrated by simulating the flow in flexible and rigid three-layer graphene channels.It can be concluded that the parallel flow mode in which the water in each channel maintains the same motion state can reduce the mutual interference of the interlayer flow.The transport process of ionic solution in graphene-based materials is of great significance in the fields of seawater desalination,ion seiving and water treatment.Therefore,it is of great academic value to study the special flow phenomena,give the appropriate theoretical explanation and model prediction inside it.Based on these,we can design the proper membrane parameters and system structure to further improve the transmembrane flux,effectively save energy and improve efficiency.