Interfacial Selectivity And Transmission Mechanism Of Charged Polymer Membranes For Mechanism Research And Application

With the development and popularization of membrane separation technology,membrane plays an important role in solute separation as a core component.Charged polymer membranes have become a research hotspot because of their selective permeability to material separation.The interfacial selectivity of charged polymer membranes is mainly related to the sieving effect of pore diameter,the repulsion effect of surface charge and the difference of affinity.In this paper,the dense membrane represented by ion exchange membrane（IEM）and the loose charged polymer membrane represented by ultrafiltration membrane（UF）were studied.The mechanism of interface selectivity and transmission mechanism was explored through experimental characterization,and theoretical support was provided for applications in related fields.In order to explore the relationship between the“pore diameter”of the IEM and the interface selectivity and the solute permeability mechanism.Based on the characterization and definition of molecular weight cut-off（MWCO）of pressure membrane,the concept of apparent“MWCO”of IEM was proposed.The solute membrane permeation mechanism was analyzed by pressure-driven filtration method,concentration difference driven diffusion method and electric field driven electromigration method.The results showed that the pressure driven filtration method was not suitable for the characterization apparent“MWCO”of IEM.It was proved that the interception property of IEM was not only the simple sieving effect of pore diameter,but also the“exchange”process.Using polyethylene glycol as Feed solution by diffusion characterization,the results showed that the diffusion flux was related to the molecular weight of the solute,the degree of porosity and thickness of the membrane.The adsorption capacity per unit area was also related to membrane thickness and water content.The apparent“MWCO”of the same type（homogeneous/heterogeneous）anion exchange membrane was smaller than that of the cation exchange membrane.The monovalent selective IEM showed the smallest apparent“MWCO”due to its high surface density.Using quaternary ammonium salt as Feed solution by electric field characterization,the results showed that the surface group charge and membrane structure affect the apparent“MWCO”of cation exchange membrane.Finally,the three characterization methods were compared,and the apparent“MWCO”of IEM was defined.The effect of interfacial selectivity of IEM on ion separation and concentration was studied.The effects of standard IEM（AM）and monovalent selective anion exchange membrane（MVA）on the separation and concentration of Cl^-/SO₄^2-were investigated by Selectrodialysis（SED）.The results showed that higher initial concentration of monovalent ions had a positive effect on membrane selective separation and was beneficial to the concentration of high-quality salt.The current density affected the ion migration rate and the selective separation of the membrane,thus affecting the salt purity and concentration of the product.At higher current density,AM exhibited reverse selectivity for SO₄^2-.However,under medium current density(200 A·m^-2),high concentration and purity of salts could be obtained in both the Product and Brine compartment.The concentration of Na₂SO₄ and Na Cl could be concentrated to 0.84mol·L^-1 and 1.90 mol·L^-1,respectively,and the purity of both could reach more than94%.Glycerol with COD of 2000 mg·L^-1 was added into the Feed solution.The results showed that the existence of glycerol has little effect on the selective separation of membrane and the quality of product salt,and the retention rate in Feed solution could reach more than 95%.Finally,the energy consumption and current efficiency of SED to obtain two concentrates（Na₂SO₄ and Na Cl）and desalted water（containing organic matter）were evaluated to provide a comprehensive understanding of industrial applications.In order to further study the interaction between pore diameter and surface charge of charged polymer membrane under the action of electric field and the interface effect.In this paper,IEM and charged UF were used in the membrane reactor.The influence of charged UF pore diameter and current density on the membrane interface concentration polarization layer and water migration was investigated.A charged UF made of polyether sulfone in solution had a negative charge on its surface.Under the action of electric field force,anions accumulate on the active surface of charged UF,resulting in concentration polarization.Driven by the permeability difference on both sides of the membrane,water molecules would migrate from the supporting side of the charged UF through the membrane to the side of the concentration polarization layer.So as to achieve the purpose of two compartment solutions were concentrated and desalted respectively,this phenomenon was called Electrical field Induced Forward Osmosis（EIn FO）.The results showed that the concentration polarization was inversely proportional to the pore diameter of the charged UF.The amount of water migration was proportional to the membrane pore diameter.At the same time,the amount of anion migration was also affected by the concentration polarization.With the increase of the applied current density,the concentration polarization of the active side of the charged UF increased,resulting in the increase of water migration.The change of solution p H during EIn FO also affected the surface potential of charged UF.The effect of water migration and salt concentration was affected.Finally,the application of EIn FO process in industrial wastewater recycling was prospected.