Structure And Property Optimization Of Polyamide Composite Forward Osmosis Membrane

With the intensification of industrial pollution and the increase of the world’s population,the problem of water pollution is becoming more and more serious,and freshwater resources are becoming increasingly tense.As a membrane separation technology driven by internal osmotic pressure,forward osmosis technology has the advantages of no additional operating pressure and low membrane fouling trend.It has shown great potential in many applications such as seawater desalination and wastewater treatment.However,the problems of concentration polarization and reverse solute diffusion still limit the development and application of forward osmosis technology.Therefore,the optimization and improvement of the performance of forward osmosis membranes is still an important research direction.In this paper,the interfacial polymerization method is used to prepare a composite forward osmosis membrane.The structure and performance of the forward osmosis membrane are optimized to alleviate the problems of concentration polarization and reverse solute diffusion,and improve the permeability of the forward osmosis membrane.The modified forward osmosis membrane was analyzed by Fourier transform infrared spectroscopy,water contact angle,scanning electron microscope and other characterization methods to explore the mechanism of membrane permeability improvement.First,in order to reduce the effects of internal concentration polarization and reverse solute diffusion on the membrane flux,a forward osmosis membrane with dual active layers was prepared.The experiment found that compared with the single active layer membrane,the reverse solute flux of the double active layer membrane was significantly reduced,the selectivity became stronger,and the anti-membrane pollution performance was greatly improved,but the water flux performance was slightly reduced.In order to increase the water flux of the double active layer membrane,the method of reserving the solute in the support layer is adopted.The results show that this method does increase the water flux of the double active layer membrane.And when the reserved solute concentration is 0.45 M,the water flux is increased by 14.58%compared with the blank double active layer membrane.In the water contact angle test,it was found that the water contact angle of the modified membrane decreased with the increase of the concentration of the reserved draw solute,indicating that the presence of the reserved draw solute improved the hydrophilicity of the double active layer membrane.In the anti-pollution experiment,the single active layer membrane in the active layer facing the draw solution(AL-DS)mode,6 hours of water flux decreased by more than 70%;in the active layer facing the feed solution(AL-FS)mode,the water flux also exceeded 20%,and the water flux of dual active layer membranes did not decrease by more than 10%within 6 hours,and the reproducibility was significantly higher than that of single active layer membranes.Under longer running time,the membrane reserved solute is more stable than the blank double active layer membrane.The water flux of the membrane is one of the most important parameters in forward osmosis technology.The structure of the double active layer has not significantly improved the water flux.In order to further improve the water flux performance of the forward osmosis membrane,a new type of covalent organic framework(COF)were incorporated into the active layer during the interfacial polymerization reaction to improve the permeability of the single active layer forward osmosis membrane.The analysis of the surface of the modified forward osmosis membrane by scanning electron microscopy can be observed that the COF material is introduced into the active layer.At the same time,the experimental results show that the COF material incorporated into the polyamide active layer can indeed significantly increase the water flux of the composite membrane.Within a range,the improvement of water flux performance has a tendency to increase with the increase of COF addition.When the amount of COF added is 0.5 mg,the efficiency of water flux improvement can reach up to 35%at different concentrations of draw fluid,and the average increase efficiency exceeds 30%.At the same time,it has a low reverse solute flux,which shows that after the COF material is incorporated,the forward osmosis membrane still maintains good selectivity.At the same time,it has a low reverse solute flux,indicating the addition of COF material.