Study On The Preparation And Salt/Water Separation Property Of High-flux Nanofiltration Membrane Based On Spin Coating

China occupies a low per capita availability of freshwater resources,while the increase in population has enlarged the freshwater demand and the water pollution has become serious,which contributed to the severity of the freshwater shortage.In addition to saving,protecting freshwater and recycling wastewater,exploring new freshwater resources is worth to study.Our country is rich in seawater and brackish water and the crisis of freshwater resources in coastal areas and inland arid areas can be effectively alleviated by rational use of desalination technology.Nanofiltration（NF）has many advantages such as low energy consumption in salt/water separation and can be used to remove trace organic compounds from water bodies.NF is now widely used in water treatment,but the intrinsic weakness is that the water permeance and the retention performance are mutually constrained and balanced.Improving the water flux of the polyamide composite NF membranes without sacrifice of the salt rejection rate is of great value for producing freshwater more efficiently.There have been various methods for reducing the thickness of NF membranes to improve the permeability,but few focus on the effects of the deposition method and the distribution location of diamine monomers upon the substrate in membrane thickness and performance.In this study,a spin-coating assisted interfacial polymerization（SCIP）method was proposed to prepare ultrathin high-flux polyamide NF membranes and the effect of spin coating on the performance of NF membranes was systematically explored.In traditional interfacial polymerization process,the piperazine was loaded onto the substrate film by dip coating,and then reacted with trimesoyl chloride（the organic phase monomer）to form a polyamide nanofilm.In this study,the deposition method was changed from dip coating to spin coating to improve the NF membranes performance.The sodium fluorescein can exhibit fluorescence at the light of 488 nm with laser scanning confocal microscopy.And the piperazine solution mixed with sodium fluorescein was loaded to the substrate and the loading of piperazine was evaluated by the distribution of sodium fluorescein indirectly.As for dip coating,the fluorescent thickness on the surface of the substrate was relatively thin,the fluorescence intensity value is small and the back surface of the prepared polyamide nanofilm has longer protrusions extending to the porous substrate.It demonstrated that the piperazine would be dispersed in the pores and on the surface of the substrate via dip coating method.As for spin coating,the fluorescent thickness on the surface of the substrate was relatively thick,the fluorescence intensity value was relatively bigger with the relatively small fluorescence intensity fluctuation and the back surface protrusions of the polyamide nanofilm were smaller.It indicated that the diamine monomer would be enriched on the surface of the substrate via spin coating method.The results showed that continuous polyamide nanofilms could be successfully formed on the substrate via SCIP and the effects of rotational speed,spinning volume and concentration of piperazine were studied.As the rotation speed increased from 500to 3500 rpm,the membrane thickness decreased and the flux of the nanofiltration membrane increased with the decrease of salt rejection.When the rotation speed increased to 5000 rpm,the increase of water permeance was small and the rejection rate of Na₂SO₄ was less than 95%.When the spinning volume of piperazine increased from 0.2 m L to 1 m L,the water permeance of the polyamide NF membrane via SCIP gradually increased.However,when the spinning volume of piperazine increased to 2m L,the water permeance decreased,the decline trend of which was sharp and then turned to even.With the increase of the piperazine concentration,the water permeance of the NF membrane via SCIP gradually decreased,and the salt rejection increased.The optimized membrane preparation conditions obtained from the above research based on polysulfone substrates were 3500 rpm and 1 m L piperazine of 0.3 w/v%.The NF membranes fabricated at such condition had Na₂SO₄ rejection of 96.2%,and water permeance of 36.1 L m^-2 h^-1 bar^-1,which was 4.7 times the water permeance of the NF membrane via traditional interfacial polymerization(7.7 L m^-2 h^-1 bar^-1).Compared with dip coating,spin coating method could consume less diamine monomers and finish piperazine deposition and removing the residual diamine solution in one step,making the operation simple and fast.Applying the above optimized parameters to substrates of other materials,better performance can be achieved.The water permeance of the NF membranes based on Kevlar substrates is 37.9 L m^-2 h^-1 bar^-1 with the Na₂SO₄ rejection rate of 97.5%,while that of the NF membrane based on polyacrylonitrile substrates is41.7 L m^-2 h^-1 bar^-1 with the Na₂SO₄ rejection rate of 96.2%,This indicated the scalability of SCIP and such three membranes had better performance compared with commercial NF membranes NF 90,NF 270 and membranes prepared by most existing studies.The diamine monomer loaded by dip coating would be randomly dispersed in the pores and on the surface of the substrate,leading to the formation of thick and dense polyamide nanofilms and longer back surface protuberance.Thus,the NF membrane had larger hydraulic resistance and the water permeance was smaller.However,the diamine monomer would be enriched on the surface of substrate via spin coating,which contributed to quickly form a complete and initial polyamide nanofilm.The initial film would inhibit the diffusion of piperazine to the organic phase and hinder the further growth of the initial nanofilm,thereby reducing the membrane thickness.Under the above optimized membrane fabrication parameters,the thickness of the polyamide nanofilm could be reduced less than 10 nm,resulting in obvious improvement of the water permeance.Moreover,the spin coating could homogeneously enrich the diamine monomer on the surface of the substrate,which was conducive to the formation of a relatively defect-free polyamide composite nanofiltration membrane and reduces the length of the protuberances of the polyamide nanofilm back surface,thus reducing the hydraulic resistance.and maintaining good mechanical stability and retention performance.Herein,the thickness of the polyamide NF membranes was significantly reduced by SCIP,which effectively increased the water permeance.Our study provided a new approach for improving the permeability of the nanofiltration membrane and devoted efforts to seawater and brackish water desalination.