Reverse Osmosis Characteristics And Mechanism Of Graphene-based Membranes

Graphene-based materials are ideal next-generation reverse osmosis membranes due to their atomic thickness and excellent mechanical properties.Two kinds of reverse osmosis membranes are designed based on hydrated porous graphene and pillared graphene.Molecular dynamics method was used to study the reverse osmosis characteristics of brine under pressure driven.By theoretical analysis and calculation of seepage law,water flux and benefit trade-off,the research results can be compared with the existing technologies,and the great superiority of graphenebased membrane is finally found.The results of this study provide a theoretical basis for the research on the reverse osmosis properties of graphene materials and can be used as a reference for the design of nano-scale reverse osmosis membranes.First,the effects of system temperature,driving pressure and membrane pore size on the permeability and selectivity of reverse osmosis membrane were mainly studied,and the rules of reverse osmosis characteristics and appropriate theoretical analysis methods were obtained.With the increase of temperature and driving force,the permeation of water molecules can be promoted,but the selectivity of the large-aperture reverse osmosis membrane would be reduced.Taking into account the need for a comprehensive analysis of this contradictory effect,a method of benefit trade-off was proposed.According to the selective emphasis on water flux or rejection rate,the effectiveness of each system parameter can be assessed by adjusting the benefit value obtained from the benefit coefficient.When selectivity and permeability are regarded as equally important,the penetration rate of water molecules boosts with the increase of pressure,but the ion interception rate decreases rapidly,resulting in a decrease in benefit value.Small aperture membranes can improve the efficiency by adjusting the thermodynamic coupling field,and those with large apertures need to be weighed among the elements to obtain the most suitable conditions.By comparing the two semi-permeable membranes,it is found that pillared graphene not only inherited the excellent percolation characteristics of graphene and carbon nanotubes,but also had a better effect on ion interception.Then,the reverse osmosis mechanism of the thermal-mechanical coupling field and the effect of pore size is analyzed.The seepage nephogram from various angles show us ions and water molecules with a stable transport trend during the reverse osmosis process.In the reverse osmosis process,ions only pass through the semi-permeable membrane in the center of the pores,while water molecules are in a state of layered seepage.The hydration characteristics are presented through the seepage nephogram and the radial distribution function,and the mechanism of ion hydration shell and pore size effect is revealed.Through the statistics of the number of molecules and hydrogen bonds of different hydration layers,the strength of hydration is obtained under various factors.The stronger the hydration has the more obvious effect of the pore size nanopores on ion rejection rate.The hydration model in different states also reflects the effect of pore size on the hydration state.The hydrated shell is relatively flexible,and the ionic hydrated shell will be squeezed,deformed and even peel off the water molecules through the nanopore.In addition,the hydrogen bond effect and energy barrier distribution at the inlet of the seepage are also studied,and we get the regular rule that the thermal-mechanical coupling field changes the reverse osmosis characteristics.In particular,the cloud image shows a more uniform distribution of ions by applying shear action to the reverse osmosis membrane.This can reduce concentration polarization and prevent scaling on the surface of reverse osmosis membrane from reducing the water flux.Increasing the shear rate leads to the improvement of energy barrier and hydrogen bond,which will hinder the penetration of water molecules to a certain extent,but it can effectively enhance hydration and increase the ion rejection rate.