Molecular Dynamics Simulation Study On Desalination Performance Of Two-Dimensional Material (Molybdenum Disulfide,Graphene) Nanofiltration Membrane

In recent years,two-dimensional materials have been a research hot topic,showing good application prospects in medical,aerospace,electronic information,water treatment and other fields.As representative two-dimensional materials,one is graphene and its derivatives,and the other is molybdenum disulfide（MoS₂）.Both graphene and MoS₂have excellent performance in water treatment.They have been designed into many different types of nano-membranes,and multi-layer and porous graphene and MoS₂films have been reported.Although single-layer nanoporous materials have great application potential in water treatment,it is very difficult to obtain single-layer membrane materials in experiments.Relatively speaking,multilayer membrane materials are easier to prepare,but there are few studies on their water treatment capabilities;on the other hand,although both graphene and MoS₂can be used as nanofiltration membranes for water treatment,their performance is not as good.No systematic comparison was performed.In this paper,using molecular dynamics simulations,the adsorption properties of graphene and MoS₂membranes were first studied,then the pore size was determined by comparing the water flux and rejection,and finally,the porous single-layer and double-layer MoS₂and graphite Water flux and Na Cl ion rejection of olefinic membranes.The simulation results show that the single-layer nanoporous film has higher water flux than the double-layer nanoporous film,but at the same time the ion rejection rate will be lower;the water flux of the MoS₂film is better than that of the graphene film;the further mechanism Exploration shows that due to the outer hydrophobic edge and inner hydrophilic edge of MoS₂pores,water molecules in MoS₂pores have faster passing speed and higher mass density than those in graphene pores.As the charge of molybdenum atoms around the pores becomes higher and higher,the rejection rate becomes higher but the water flux becomes lower and lower.That is,increasing the polarity of the pore edges leads to a decrease in water flux while improving ion retention.These findings may provide a theoretical reference for the design of MoS₂membranes in water treatment.Based on the excellent water flux and rejection of porous MoS₂membranes,this paper then investigated the effects of pore size and pore content on the desalination performance of the membranes.That is,under the premise of a given total pore area,the effect of the size of the pores on the desalination performance of the membrane is studied,and when the size of the pores is determined,the effect of the ratio of the area of the pores to the membrane is studied on the desalination performance of the membrane.At the same time,the intercepted ions are expanded from monovalent sodium ions to divalent calcium ions and ferric ions,making the rejection rate more general.In this study,the total pore area of MoS₂was set to 200?²,and the single pore area was 20?²,30?²,40?2,50?²,and 60?²respectively,and the size of the pores was determined by comparing the water flux and retention rate.In order to further understand the mechanism,the membrane surface normal velocity of water molecules,the average force potential,and the number of water molecules in the pores were counted.The simulation results show that as the area of the pores increases,the speed of water molecules passing through increases,the energy barrier decreases,the number of molecules in the pores increases,and the water flux also increases.On the basis of keeping the total pore area constant,this study explored the interception effect of different single pore areas on different ions.Due to the more stable hydration radii of calcium and iron ions and the slower speed of the ions themselves,the ion rejection is higher.The summary is as follows:1)With the increase of the single pore area,the water flux increases,but the rejection rate decreases;2)When the pore area transitions from 30?²to 40?²,the water flux increases significantly;2)The concentration of different ion solutions The law of water flux is:Na⁺>Ca²⁺>Fe³⁺,and the law of interception rate of different ions is:Na⁺<Ca²⁺<Fe³⁺.On the other hand,applying pressure results in higher water flux but lower ion rejection.This study explored the effect of changing the single pore size and pore content of MoS₂on the desalination performance,laying a theoretical foundation for the future application of MoS₂ porous film materials in water treatment.