| Seawater desalination technology has considered as promosing method to alleviate the shortage of water resource owing to its advantages of energy conservation and environmental protection.Currently,reverse osmosis membrane technology has been widely applied in seawater desalination.Two-dimensional nanomaterials(such as graphene,hexagonal boron nitride,molybdenum disulfide),as a new efficient nano membranes have been employed in desalination in recent years by taking their advantages of excellent chemical stability and ultra-fast water transfer characteristics,and these membranes show excellent desalination performance.Nevertheless,the transport and separation mechanisms for water molecules/ions passing through nano membranes were still poorly understood.With the development of high-performance computers,molecular dynamics simulation has become an effective method to predict material performance and design material.It is easy to explain the desalination mechanism of water molecules/ions passing through two-dimensional nanomaterial from molecular level via molecular dynamics simulations,which provides effective supplement for the experiment and gives theoretical guidance for design and synthetic two-dimensional membranes.In this thesis,non-equilibrium molecular dynamics simulations were employed to explore the desalination performance of membranes constructed by different two-dimensional nanomaterial(Multilayer Graphene,Boron Nitride and Molybdenum Disulfide).The influence factors,such as,the distance of interlayer spacing,functional groups,charge distribution and so on,on the desalination performance were explored.Meanwhile,the transport and separation mechanisms for water molecules/ions passing through different two-dimensional nanomembanrs were also illustrated.The major findings of this paper are as follows:(1)The salt rejection rate and the water flux in multilayer graphene membrane would be affected by the distance of interlayer spacing(H)and the width of gap(dG)and H=0.8nm,dG=1.0nm was the optimal desalting conditions of the membrane,this can be attributed to the size of hydration layer around ions are nearly the same as the thickness of water layer confined the interlayer spacing when H=0.8nm.It is difficult to dehydrate some water molecules around ions.Meanwhile,the largest free energy for ions in H=0.8nm system indicates that sodium ion need to overcome higher energy barrier if they want to pass through interlayer spacing,so it is difficult for ions to pass through multilayer graphene membrane.Additionally,when the salt rejection is lower,the salt rejection rate and the water flux could be increased remarkably by adding the number of graphene sheets and gaps.(2)Boron Nitride(BN)membranes constructed with nanosheets comtain boron and nitride atoms in their ports(BN-BN membranes)showed excellent desalination performance.Some hydrogen bonds were formed between water molecules and functional group when the functional group was introduced to the port of the BN-BN membranes,water molecules need to adjust their configuration at the port constantly,which results in the decrease of water flux.it’s difficult for ions passing through the the functionalized BN membrane owing to the steric effect of the functional group,which results in the increase of rejection rate.The salt rejection rate decreases with the interlayer spacing increasing,while the water flux decreases firstly and then increased,and the water flux is lowest when the interlayer spacing is 1.1 nm.Since the hydrated ions could constantly shuttle back and forth between water molecules chains,and hydration layer around ions would exchange with the water molecules between interlayer spacing,which results in the lowest water flux.(3)Water molecules arranged disorder in the lamer MoS2 membrane,water flux would be influenced little by charge distributon in MoS2 nanosheets.When MoS2 membrane was discribled with chemical model,in which the charge information was consider for every atoms,the rejection rate value for Cl-is higher than that for Na+,this can be attributed to the repulsion interaction between chloride ion and nanosheets as well as the attraction between sodium ions and MoS2.Sodium ions rejection rate increases when the charge of Mo atoms around the pores in the single-layer nanopore MoS2 membrane increases.Meanwhile chloride ion would be adsorbed in pore due to electrostatic attraction,which results in the decrease of radius,correspondingly,water flux decrease.When some functional group was introduced to nanopore in MoS2,water flux would decrease while the rejection rate increase... |
PDF Full Text Request