Research On Construction Of Low-pressure Composite Nanofiltration Membrane Based On Glycosides And Its Separation Performance

Nanofiltration,a pressure-driven membrane separation process,has been widely used in hard water softening,industrial wastewater treatment and biomedicine separation,etc.However,the operating pressures of conventional nanofiltration membranes are 0.5～2.0 MPa which leads to high energy consumption,and the separation selectivity of solutes with different sizes is low.The rational construction and precise regulation of the separation layer structure,and the development of low-pressure composite nanofiltration membrane with high flux and high separation selectivity are of great significance for reducing the energy consumption and promoting its application in the field of water treatment and material separation.Natural extract glycosides have abundant hydroxyl groups which can provide reaction sites for interfacial polymerization.In addition,some aglycones with large volume of glycoside and rigid non-planar contorted structure can reduce the accumulation between polymer chains in the process of interfacial polymerization,which is conducive to the formation of loose separation layer.They are potential materials for the construction of low-pressure composite nanofiltration membrane.Hence,in this paper,stevioside（St）was used as aqueous phase additive and aqueous monomer to construct low-pressure composite nanofiltration membrane by interfacial polymerization.The pore size and porosity of the separation layer were controlled by changing the concentration of St,which was used for efficient separation of dye/Na Cl or antibiotic/Na Cl mixed system.On this basis,St,mohagoside（Mv）,icariin（Ic）and salidroside（Sr）were used as aqueous monomers to prepare low-pressure composite nanofiltration membrane,and the effects of glycoside structure on the structure and separation performance of the composite nanofiltration membrane were studied.Specific contents and main conclusions are as follows:（1）The PA/St composite nanofiltration membranes were prepared by interfacial polymerization on PAN based membrane with Piperazine（PIP）as aqueous phase monomer,St as aqueous phase additive and trimesoyl chloride（TMC）as organic phase monomer.The influence of the incorporation of St on the structure,flux and dye desalination of PA/St composite nanofiltration membranes were studied.The results showed that the introduction of larger molecular volume and non-plane distortion structure of St made the layer structure loose and pore size larger,which led to the increase of flux.When the concentration of St was 0.05wt%,the flux of PA/St-0.05 composite nanofiltration membrane was 40.7 L·m^-2·h^-1·bar^-1,which was increased by 113%compared with the PA nanofiltration membrane.With the addition of St,the surface electronegativity of PA/St composite nanofiltration membrane was enhanced,and the rejection of Na₂SO₄ was improved.The rejection of PA/St-0.05 composite nanofiltration membrane of Na₂SO₄ was 93.2%,which was 13.6%higher than that of PA nanofiltration membrane.In addition,the separation factor of PA/St-0.1 composite nanofiltration membrane for the CR/Na Cl mixed system was 10.4,which was increased by 225%compared with the PA nanofiltration membrane.（2）St/PAN low-pressure composite nanofiltration membranes were prepared by interfacial polymerization on PAN based membrane with St as aqueous phase monomer and TMC as organic phase monomer.The effects of St,Na OH,TMC concentration and interfacial polymerization time on the performance of St/PAN low-pressure composite nanofiltration membranes were investigated.The effects of St concentration on the physicochemical structure of St/PAN low-pressure composite nanofiltration membrane and the separation performance of antibiotic/Na Cl mixed system and anti-compaction performance of St/PAN-0.1 low-pressure composite nanofiltration membrane were studied.The results showed that the pore size of St/PAN low-pressure composite nanofiltration membrane decreased and the thickness of separation layer increased with the increase of St concentration,which led to the decrease of flux from 81.2 L·m^-2·h^-1·bar^-1 to 20.6 L·m^-2·h^-1·bar^-1.The surface electronegativity of membrane surface was increased,which made the rejection of Na₂SO₄ increased from 79.3%to 90.5%.In addition,when Na OH and TMC concentrations increased,the flux of the St/PAN low-pressure composite nanofiltration membranes firstly decreased and then increased,and the rejection of Na₂SO₄ firstly increased and then decreased.With the increase of interfacial polymerization time,the flux of St/PAN low pressure composite nanofiltration membrane decreased,and the rejection rate of Na₂SO₄ increased.When St concentration was 0.1 wt%,TMC concentration was 0.1 wt%,Na OH concentration was 0.05 mol/L,and the interfacial polymerization time was 3min,the St/PAN-0.1 low-pressure composite nanofiltration membrane showed high selectivity for the separation of BAC/Na Cl and TH/Na Cl mixed systems,and the separation factors were 9.0 and 9.3,respectively.In addition,the rigid structure of St made St/PAN low-pressure composite nanofiltration membrane have better compaction resistance and term stability.（3）St,Mv,Ic and Sr were used as aqueous phase monomers respectively,glycoside low-pressure composite nanofiltration membranes were prepared by interfacial polymerization with TMC on the PAN based membrane.The effects of glycoside structure on the structure,flux,and antibiotic desalination performance of low-pressure composite nanofiltration membrane were studied.The results showed that under the same preparation conditions,the fluxes of St/PAN,Mv/PAN,Ic/PAN and Sr/PAN low pressure composite nanofiltration membranes were81.2 L·m^-2·h^-1·bar^-1,121.3 L·m^-2·h^-1·bar^-1,24.5 L·m^-2·h^-1·bar^-1,and 56.4 L·m^-2·h^-1·bar^-1,respectively.The molecular volume of Mv and St was larger than that of Ic and Sr,which made the diffusion velocity of Mv and St molecules slower.Therefore,the separation layers of Mv/PAN and St/PAN low-pressure composite nanofiltration membrane were thinner,with larger pore diameter and higher flux.In addition,the non-coplanar contorted structure of Mv and St molecules made the separation layer structure looser and further improved the flux.Besides,due to the combined action of pore size screening effect and Donnan effect,the separation factors of the four low-pressure composite nanofiltration membranes of St/PAN,Mv/PAN,Ic/PAN and Sr/PAN for BAC/Na Cl mixed system were 9.0,6.1,3.8 and 6.5,respectively.