| As a high-efficiency and energy-saving membrane separation technology,nanofiltration has been widely used in many fields such as water treatment,seawater desalination,and chemical industries.The nanofiltration membrane fabricated by the interfacial polymerization(IP)is composed of an ultra-thin active layer and a support layer.The optimization of the active layer by changing the IP conditions has been extensively studied,but there are few reports on the influence of the structure and properties of support layer on the nanofiltration membranes made by IP.In recent years,researchers have gradually realized that the property of the support layer also has an important impact on the IP process and the performance of the nanofiltration membrane,but there is still a lack of systematic research.In this study,support layers with different pore size,hydrophilicity,hydrophobicity,and surface charge properties are used for IP to systematically explore the influence mechanism of the physical and chemical properties of support layer on the structure,properties and performance of nanofiltration membranes,and provide new ideas for the preparation and modification of nanofiltration membranes.At the same time,it is convenient to explore a suitable support layer and expand the application of nanofiltration membranes.In this study,PES ultrafiltration membrane was used as the support layer to optimize IP conditions and explore the influence of process parameters on the performance of nanofiltration membrane.The optimal IP conditions were determined as follows:piperazine concentration 0.2 wt%,TMC concentration 0.1wt%,reaction time 30 s,post-treatment temperature 60°C.As pressure increases,the water flux and salt rejection of the nanofiltration membrane increase;the salt solution permeability increases with the increase of the cross-flow intensity;as the concentration of salt solution increases,the solution permeability and the monovalent salt rejection decrease.The flux of the nanofiltration membrane prepared under optimized conditions is about 12.92 LMH/bar,the rejection of sodium sulfate is about 98.8%,and the performance is better than the commercial NF270 nanofiltration membrane.To study the influence of support layer pore size and distribution on the properties and performance of nanofiltration membrane,PES support with different pore size was used to prepare nanofiltration membranes,Substrates porosity,pore size and distribution were tested.The surface morphology,chemical composition,pore size distribution of membrane were characterized,and the permeability/salt rejection properties of the nanofiltration membrane were tested.The study showed that increasing the subatrate porosity helps to increase the permeation flux of nanofiltration membrane;the larger pore size of substrate,the thicker and rougher the active layer formed by IP,which reduces the permeability of the nanofiltration membrane;Support layer with more uniform pore size distribution is easier to form a defect-free dense active layer structure,which helps to improve the salt retention of nanofiltration membrane,and the permeability of the membrane is reduced.In order to analyze the effect of substrates hydrophilicity,interfacial polymerization was performed on polyacrylonitrile(PAN),polyethersulfone(PES),and polyvinylidene fluoride(PVDF)UF membranes with approximately the same pore size.The hydrophilicity,surface morphology,pore size and other properties of nanofiltration membrane were Characterized,and the permeability and salt retention were tested.The results show that the stronger the hydrophilic support layer have the stronger hydrophilicity of the nanofiltration membrane prepared by IP,which helps to improve the permeability.Permeability of the TFC-PAN membrane reaches18.55 LMH/bar.It is about 1.8 times that of TFC-PVDF membrane.In addition,if the support layer is too hydrophilic or too hydrophobic,it will hinder the adsorption and diffusion of monomers in the water phase,thereby affecting the formation of the active layer and reducing the salt retention of nanofiltration membrane.Therefore,the preparation of high-performance nanofiltration membranes by IP need suitable hydrophilicity support layer.The surface charge of PES substrate was modified with polyethyleneimine(PEI)deposition.Explored the impacts of substrates surface charge on the nanofiltration membrane surface charge,hydrophilicity,surface morphology and permeability/salt rejection performance.The results showed that the lower surface Zeta potential of the support layer formed the stronger negatively-charged nanofiltration membrane,and the salt retention was significantly improved.The sodium sulfate salt rejection of the nanofiltration membranes made from the positively charged substrates were less than 80%,while the salt rejection of the TFC membrane made by the negatively charged support layer is as high as 98.74%.At the same time,the lower potential support layer helps to improve the hydrophilicity of nanofiltration membrane and promotes the improvement of permeability.Correlation analysis between the properties of different support layers and the performance of nanofiltration membrane was carried out respectively.Through the determination coefficient R~2 value and the slope of the fitting equation,the influence mechanism of substrate properties on the performance of nanofiltration membrane was systematically analyzed.The results showed that the influence degree order between support layer property and the permeability of nanofiltration membrane is:hydrophilicity>pore size distribution>porosity>surface charge>pore size;the degree order of influence on salt retention performance of nanofiltration membrane is:pore size distribution>Surface charge>hydrophilicity>pore size≈porosity.Compared with other properties of support layer,the pore size distribution has the most obvious impact on permeability/salt rejection of the nanofiltration membrane,while the pore size has a weak effect. |
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