Structural Regulation Of Polyamide Forward Osmosis Membranes For Enhanced Desalination

Forward osmosis technology is a key research topic in solving the shortage of fresh water resources.It has many advantages such as low energy consumption and high stability.However,the lack of high-performance membranes restricts the application and development of forward osmosis.As a kind of forward osmosis membrane,the thin film composite membrane has the advantages of simple preparation process and easy modification,but it faces internal concentration polarization and membrane fouling,which has an adverse effect on water flux,screening performance and stability during forward osmosis process.In this paper,the forward osmosis membranes were designed and prepared with respect to three aspects:modification by adding intermediate layer,changing the structure of the thin film composite membrane,and modifying the active layer to intensify the seawater desalination process.The mixed cellulose ester microfiltration membrane was selected as the porous substrate for thin film composite membrane,and it was modified by using polyvinyl alcohol（PVA）as an intermediate layer.The mini-type in-situ tensile testing system and contact angle meter（CAM）were used to test the tensile properties and water contact angle for substrates and the synthesized forward osmosis membranes before and after modification.The forward osmosis membranes before and after modification were observed using field emission scanning electron microscope（SEM）and atomic force microscope（AFM）.The performance tests were conducted under the same conditions to verify the effect of modification on performance improvement.In general,the use of PVA as an intermediate layer effectively improves the mechanical properties,hydrophilicity and surface structure of the forward osmosis membrane,leading to a significant enhancement in desalination performance.The single-skinned and double-skinned polyamide（PA）forward osmosis membranes were prepared on the modified porous substrate using the same method,and the two kind of membranes were tested for forward osmosis performance and a long-time test to verify the effect of double-skinned structure on desalination performance and stability.Using deionized water as the feed solution and 1 M Na Cl aqueous solution as the draw solution,the water flux of the PA-2PVA-MCE was 11.32L m^-2 h^-1 and the reverse salt flux was as low as 0.58 g m^-2 h^-1.the J_s/J_w value was only 0.05 g/L.Moreover,the performance remained stable in a 24 h continuous test.It is proved that the double-skinned structure could improve the stability of thin film composite membranes and the screening performance effectively,however,it reduces the water flux to a certain extent due to the increased mass transfer resistance.Copper hydroxide nanowires（CHNs）were used to adjust the properties of PA active layer.The CHNs were prepared and mixed for interfacial polymerization to prepare a modified polyamide active layer.Through the CAM and SEM test,the effect of the modification of CHNs on hydrophilicity and morphology for the PA active layer was explored.The forward osmosis performance tests and a long-term stability tests were performed on the membranes before and after modification to verify the effect of modification on the performance improvement and stability of PA membranes.Finally,the prepared CHNs modified PA membrane exhibited an ultra-high J_w（36.79 LMH）and a lower J_s（5.76 g MH）under the conditions of using deionized water as the feed solution and 1 M Na Cl as the draw solution.The J_s/J_wvalue was only 0.16 g/L.It was verified that the modification of PA active layer using CHNs could improve the performance of forward osmosis membranes by increasing both surface hydrophilicity and free volume.