| Fresh water resources threaten and restrict human existence and development seriously because of its characteristics of low reserve,large consumption,waste and heavy pollution.Seawater desalination is an effective way to solve this crisis,among which reverse osmosis technology is the most advanced and energy-efficient technology.Because of its excellent physical properties,graphene is a new generation of ideal membrane material and has a good application prospect in the field of membrane separation technology.The time-dimensional ion selectivity is a breakthrough theory in the field of reverse osmosis,which overcomes the limitation of the size of sub-nanopore on reverse osmosis membrane and partly breaks through the permeabilityselectivity trade-off.However,the temporal reverse osmotic salt filtration mechanism of multilayer reverse osmosis membrane is still unclear.In this paper,molecular dynamics method was used to reveal the salt filtration mechanism of temporal selectivity of porous graphene membrane,and a gradient nanopore reverse osmosis membrane model with ultrahigh water flux and high ion retention rate was proposed,which provided important theoretical support for temporal selectivity multilayer reverse osmosis technology and its application.Firstly,the influence of interfacial slip velocity of monolayer porous graphene reverse osmosis film on saltwater reverse osmosis characteristics was studied.The results show that the salt rejection increases and the water flux first increases and then decreases with the increase of the interface slip velocity of porous graphene reverse osmosis membrane.Secondly,salt filtration models of double-layer and three-layer porous graphene reverse osmosis were constructed.The analysis shows that: the influence of interface slip velocity on the characteristics of saltwater reverse osmosis is consistent under the porous graphene reverse osmosis models with different thickness.In particular,the increase of reverse osmosis membrane thickness increases the salt rejection,but hinders the increase of water flux.In addition,the concept of “temporal selectivity”is closely related to solid-liquid interface conditions,and the influence of nano-pore diameter change in the innermost membrane on reverse osmosis behavior is further analyzed.The results show that water flux increases rapidly with the increase of the nanopore size in the innermost layer of the supply end,and high salt rejection is also guaranteed.Finally,the mechanism of temporal selectivity reverse osmosis was analyzed and explained in detail from the perspective of solution velocity,ion density and water molecule density in the model,combining with the reverse osmosis behavior under the change of interface slip velocity,reverse osmosis membrane thickness and innermost nano-pore size.Then,the nanopore structure of the three-layer graphene reverse osmosis membrane was innovatively designed and optimized for the high water flux requirements of the nanopore channel.Based on the gradient pore design principle of "large inside and small outside",five kinds of gradient nanopore structures are obtained.Then,combined with the filtration study of the change of the innermost nanopore,the results showed that the gradient nanopore structure can guarantee the high selectivity and improve the permeability significantly.In addition,the diameter contrast shows that the change of nanometer pore size of the innermost reverse osmosis membrane has the greatest influence on water flux.In the case of the same rotation speed of graphene membrane,the faster the salt solution rotates,the smaller the salt rejection corresponds to the salt filtration model.Based on the trajectory characteristics of salt ions selected by time,the trajectories of salt ions and water molecules around gradient pore are discussed,and the variation rules of water flux in reverse osmosis process with different gradient pore structures are analyzed in detail.In order to further promote the practical application and development of the principle of temporal selectivity,a three-layer single-hole graphene salt filter model was constructed,and the influence of the shear rate of reverse osmosis membrane on the reverse osmosis behavior was analyzed using the principle of temporal selectivity.Finally,a reverse osmosis model with optimal benefits is obtained by comparing the benefits of the centrifuge salt filtration model and those of previous studies.The research results of this paper further elucidated and verified the time selectivity reverse osmosis salt filtration mechanism.At the same time,the research on the influence of gradient nanopore structure on salt filtration was studied,and the general rule that the reverse osmosis membrane thickness is inversely proportional to the water flux has broken to a certain extent.These results provide a theoretical basis for the design and development of large-scale salt filtration equipment. |
