Structure Design Of Transition Layer Of Composite Forward Osmosis Membrane Based On Polyacrylonitrile Nanofibers

Forward osmosis is a very promising membrane separation technology,which has many advantages that other membrane separation processes do not have,such as low energy consumption,high separation efficiency and wide separation range,low membrane pollution tendency and so on.These characteristics make it has great application prospects in desalination,sewage treatment,osmotic membrane bioreactor,food processing,drug release,power generation and other industrial practical fields.The interconnected structure and large porosity of nanofiber mats make electrospun nanofiber membrane potential candidate for forward osmosis membrane support layers to reduce the concentration polarization phenomenon.However,using nanofibers as support layer also has certain disadvantages,such as low mechanical properties,high surface roughness and large pores,which greatly reduce performances of forward osmosis membranes.In order to solve the above problems,this topic will be continued on the basis of the existing researches by using electrospun polyacrylonitrile(PAN)nanofiber mats as substrate of composite forward osmosis membranes.Chitosan layer was used as the sub-layer of the forward osmosis membrane,and interfacial polymerization was performed on the obtained membrane.Through research,it can be concluded that chitosan was attached to nanofiber surface,which effectively inhibited the deformation of nanofibers during the test.The tensile strength of the composite support layer was as high as 30 MPa,which greatly improved the mechanical properties of membranes.Meanwhile,chitosan sub-layer enhanced the bonding strength between the support layer and the interfacial polymerization layer,provided a flat reaction interface for the formation of the polyamide active layer,and formed a uniform,continuous,and dense polyamide thin layer,which improved the selectivity and permeability of membranes.The experimental results showed that when the concentration of chitosan was 3.8%,the rejection rate of NaCl by the forward osmosis membrane was 97.4%.On the basis of the above research,in order to achieve precise regulation of the structure of the sub-layer,polyvinyl alcohol(PVA)nanofibers were prepared by electrospinning,and then PVA nanofibers were dissolved to form an ultra-thin barrier layer by evaporation.The structure of sub-layer was controlled precisely by adjusting the spinning parameters and evaporation parameters.The experimental results showed that the sub-layer of the high-performance composite forward osmosis membrane prepared in this experiment had a flat and smooth surface,and at the same time,the hydrophilicity of the membrane was improved by PVA,which was conducive to the reduction of concentration polarization and the improvement of water flux.According to a series of tests,the composite forward osmosis membrane had a low reverse salt flux(4.2 g MH)while maintaining a high water flux(more than 129 LMH),which had great advantages when compared with nanofiber membranes reported in the literature.Methods of constructing the sub-layer in this study to improve the mechanical properties of the nanofiber-based forward osmosis membrane and improve the interfacial binding reaction lay a foundation for the subsequent improvement of the filtration performances of the nanofiber-based forward osmosis membrane.