Fabrication Of Thin Film Composite Reverse Osmosis Membranes And Intensive Studies In Microstructure And Property

In order to solve the global water crisis,which is the most serious problem facing human beings,membrane technology has been widely used in fields such as water purification,wastewater recovery and seawater desalination,which is playing an important role in solving water shortage crisis.Reverse osmosis（RO）membrane process,which can provide high quality water through removing all the impurities in aqueous such as dissolved salts and organic compounds,has become one of the most popular desalination technologies because of its high energy efficiency to alleviate the water crisis.Therefore,it is of great practical significance to improve the separation efficiency of RO process through enhancing the water permeability,rejection ability of the membrane used.The high efficiency of the RO process arises from high water permeability and high salt rejection of polyamide thin film composite（PA-TFC）membranes.The PA-TFC RO membrane has continually developed over the last forty years.However,the trade-off effect between permeability and selectivity limits the application of conventional polymer membrane,so it still needs improvement in terms of its permselectivity.In this study,in order to improve the permeability of RO membranes,regulations of free volumes or constructions of water channels via varying approaches are used during interfacial polymerization through the addition of organic additives,supercritical critical carbon dioxide treatment and incorporation of layered or porous fillers.And m-phenylenediamine（MPD）and trimesoyl chloride（TMC）are used as monomers.1.To regulate free volume in polyamide active layer,the DMSO is added PA layer during the interfacial polymerization and the free volume properties are investigated using pulsed slow positron beam.The water flux of TFC membrane increases consistently with increasing DMSO content.When the DMSO content increases from 0 to 1.0 wt.%,the salt rejection of the membrane maintains above 98%.And it decreases remarkably when the DMSO content increases from 1.0 to3.0 wt.%.Therefore,it is obvious that the DMSO has a pronounced impact on the separation performance of TFC membranes.It is believed that the changes of microstructure in membranes play a key role.The thickness of the PA active layer on the membranes can be estimated through fitting the results of depth dependence of o-Ps intensity（I₃）.The o-Ps lifetime（or mean free volume radius）is appeared to increase gradually with increasing DMSO from 0 to 1.0 wt.%,and it increases little with further incorporation of DMSO of 2.0,and 3.0 wt.%.The enlarged mean free volumes are responsible for the enhanced water permeation.However,the salt rejection is significantly decreased with only a slight increase in mean free volume size around a critical value of～88.0（?）³.The result is well interpreted for that although almost no change in mean free volume size is found,the broadening of free volume distribution enables amount of larger free volumes to be involved in salt permeation.It is conclud that not only the free volume size but also the free volume distribution plays a key role in the permeation performance of PA active layer.Considering the practical application of RO membranes,to improve water permeation and salt selectivity,it is important to fabricate membranes with free volumes larger than critical size of water clusters and smaller than the average size of hydrated penetrants,i.e.,a narrow distribution of free volumes.2.Being treated in ScCO₂ at 50℃and 20 MPa for one hour,free volumes in PA12/PVA blends expand.After CO₂ depressurization,the expanded free volume size decreases as a function of the elapsed time due to the molecules relaxation.The enlarged free volumes in the ScCO₂ treated PA12/PVA blends show various longer relaxation times depending on the weight ratios of PVA.Much longer relaxation time is found for PA12/PVA with higher PVA weight ratio.Similarly,being treated in ScCO₂ at 50℃and 20MPa for six hour,free volumes in TFC membrane expanded,and the water flux increases17.3%.But simultaneously,the rejection is high.Expanded free volumes offer short path for water molecular and block the passage of salt.In summary,be treated in ScCO₂,TFC has a higher water flux and a still high rejection.ScCO₂ is a kind of green,non-pollution solvent,providing a new thinking and way to enhance the performance of TFC.3.To improve the water permeability of the membrane,a few layered graphene oxide（GO）nanosheet is added during the interfacial polymerization（IP）and the GO@TFC membranes are fabricated.Combining the structure characteristic of TFC membrane and the interlayer spacing of two-dimensional（2D）GO nanosheets,extra nanochannels that facilitate the transport of water molecules are provided.Roughness and thickness of the PA layer decrease because of the retardation of MPD diffusion into the organic solvent by the GO nanosheets during the IP process.The hydrophilicity and the crosslinking degree of the PA active layer are enhanced because of the oxygen functional groups present on GO nanosheets.GO@TFC membranes maintain high Na Cl rejection and enhanced water permeability because of the interlayer spacing of GO nanosheets facilitating the transport of water molecules.Compared with TFC membrane,GO@TFC membrane with proper content of GO have high water flux as well as high rejection.4.MIL-101-Cr,water-stable Metal-organic Frameworks（MOFs）,as well as the MIL-101-Cr-NH₂,the amination modification MOFs,and MIL-101-Cr-SO₃H,the sulfonated MOFs,are used to fabricate TFN-MOFs membranes via IP process,i.e.TFN-MIL-101-Crmembrane,TFN-MIL-101-Cr-NH₂membraneand TFN-MIL-101-Cr-SO₃H membrane.Compared with the TFC membrane,TFN-MOFs membranes have small dense leaf-like structure.Hydrophilicity of TFN-MOFs membranes are enhanced due to the presence of hydrophilic MOFs and the pore structure.The MOFs nanoparticles provide unique channels which facilitates the transport of water molecules,increasing the water permeability of TFN-MOFs membranes.When equal content of MOFs is incorporated,water fluxes of all TFN-MOFs membranes are higher than TFC membrane.The TFN-MIL-101-Cr membrane possesses the highest water flux.Even the channel size in MIL-101-Cr-SO₃H is smaller than that in MIL-101-Cr-NH₂,and TFN-MIL-101-Cr-SO₃H possess higher water permeability because of the strong hydrophilcity of the sulfonic group.As for salt rejection,due to the channel size in MIL-101-Cr-NH₂and MIL-101-Cr-SO₃H is narrow for occupation of the amine and sulfonic group,TFN-MIL-101-Cr-NH₂ and TFN-MIL-101-Cr-SO₃H membrane possess relative higher salt rejection.And they maintain the salt rejection above 97%when the MOFs addition below 0.05w/v%.5.MIL-53-Al and the amination modification MIL-53-Al-NH₂ are embedded in PA layer through IP process,respectively.And TFN-MIL-53-Al and TFN-MIL-53-Al-NH₂membranes are fabricated.The TFN-MIL-53-Al membrane possesses dense small leaf-like structure and rough surface when the MOFs content is low.While with the increase of MOFs content,the surface of the membrane becomes much smoother.Compared with TFC membrane,the existence of pore channels in MOFs nanomaterials,coupled with unique surface characteristics,can potentially achieve synergistic effect for rapid diffusion of water molecules,resulting in much greater water permeability in TFN-MOFs membranes.TFN-MIL-53-Al-NH₂ membrane has relatively higher salt rejection（above 97%）for much narrower channel structure in MIL-53-Al-NH₂.It is because of the occupation by amino group.For different TFN-MOFs membranes based on different pore structure in MOFs,when equal content of MOFs is incorporated,water flux of TFN-MOFs membrane is related on the pore structure.Water flux is gradually lower according to TFN-MIL-101-Cr,TFN-MIL-101-Cr-SO₃H,TFN-MIL-101-Cr-NH₂,TFN-MIL-53-Al,TFN-MIL-53-Al-NH₂and TFC membrane.Compared with TFC membrane,the salt（i.e.Na Cl）permeance is gradually lower according to TFN-MIL-101-Cr,TFN-MIL-101-Cr-NH₂,TFN-MIL-101-Cr-SO₃H,TFN-MIL-53-Al,TFN-MIL-53-Al-NH₂membrane.This proves that the desalting effect is enhanced successively because of the different pore structure in MOFs.