Preparation Of PAN-MWCNTs/PEI Composite Forward Osmosis Membrane And Study On Ammonia Nitrogen Removal Performance

In recent years,water shortage is becoming a serious global problem due to rapid population growth,rapid industrialization and destruction of available fresh water resources.Membrane separation technology plays a key role in wastewater treatment and water resource recovery.The forward osmosis technology（FO）has the advantages of low energy consumption,low operating pressure,simple equipment and less scaling.Forward osmosis technology and the preparation of forward osmosis membrane have been rapidly developed,and the preparation of high-performance forward osmosis membrane is the key issue of the development of forward osmosis technology.The support layer prepared by electrostatic spinning method has high porosity and interconnected pore structure.The addition of intermediate layer makes up for the disadvantages of the nanofiber membrane which is not conducive to the growth of defection-free PA layer and the fiber mechanical strength is not high.However,the problems such as easy shedding of the intermediate layer and poor stability in practical application should be overcome accordingly.In this paper,a high porosity support layer was prepared by electrostatic spinning technology.The hydrophilic and positively charged interlayer solution of multi-walled carbon nanotubes/polyvinyl imine（MWCNTs/PEI）was laid on the PAN nanofiber membrane by suction filtration method,and then the polyamide active layer was synthesized by interfacial polymerization to prepare the film composite forward osmosis membrane（TFC-FO）.The specific research contents are as follows:（1）Four concentrations of PAN solution were spun to explore the optimal preparation concentration of the support layer.The intermediate layer solution of different MWCNTs/PEI quantities was laid on the nanofiber membrane,and the forward osmosis performance was evaluated after the preparation of TFC-FO membrane.Through SEM,mechanical strength and pore size characterization,it was found that when PAN concentration was 13 wt%,fiber diameter was uniform and smooth,and fiber film pore size and mechanical strength were appropriate.By SEM,AFM,XPS,contact Angle and other characterization of the interlayer,it was found that when the interlayer loading was 0.849 g·m^-2,the PA layer presented a vermicular structure with the highest cross-linking degree and the lowest surface roughness and excellent hydrophilicity.Using pure water as raw liquid and 1 mol·L^-1 Na Cl as extraction liquid,the water fluxes were 36.23±1.81 LMH（FO mode）and 42.06±2.10 LMH（PRO mode）,which were2.2 times higher than those of CTA membrane（FO mode）and 2.43 times higher than those of CTA membrane（PRO mode）.The specific salt flux values were 0.32±0.050 g L^-1（FO mode）and 0.29±0.017 g L^-1（PRO mode）,which were 64.83%（FO mode）and 104.16%（PRO mode）lower than those of CTA membrane,respectively.At the same time,the treatment performance is stable in the 20 h long-term water treatment process.（2）TFC-FO membrane was prepared by changing the content of hydrophilic polyamine PEI in MWCNTs/PEI interlayer solution,and the effect of TFC-FO membrane on the selective separation performance of ammonia nitrogen was investigated.The characterization results of contact Angle,SEM and Zeta potential test showed that the positive electroconductivity of the solution with increased PEI content was enhanced,and the hydrophilicity of the membrane was enhanced after extraction and filtration,and the surface morphology of the membrane did not change significantly.In the FO concentration of single ammonia nitrogen simulated wastewater,f0-PEI_0.05 and F0-PEI_0.1 have 91.14%and 89.43%rejection rates of 500 mg/L NH₄⁺-N,respectively.In the initial effluent process of the concentrated pig wastewater bioreactor,the interception rates of PO₄^3--P of the three membranes are all higher than 98%,and the interception rates of FO-PEI_0.05 and FO-PEI_0.1 membranes for NH₄⁺-N are all higher than 83%,far higher than the interception rates of FO-PEI_0.02 membranes 63.11%,indicating that appropriately increasing PEI content in the intermediate layer solution is conducive to improving the selective separation performance of ammonia nitrogen in the actual wastewater treatment of FO membrane.