Study On Preparation And Performance Of Reverse Osmosis Composite Membranes Based On The Selective Layer Modification

As the dominant type of reverse osmosis(RO)membrane in seawater desalination applications,aromatic polyamide(PA)thin-film composite(TFC)membranes have aroused tremendous attention among oversea and domestic researchers owing to their many advantages such as high separation performance,low energy input,low fouling,and environmentally friendly.However,challenges still remain for the application of PA-TFC membranes.The trade-off between permeability and selectivity,high fouling potential as well as low boron removal are major obstacles for the efficient operation of membrane systems.RO membranes with high performance are the key to solve the issues and play a decisive role in practical applications.With the Aim of preparing high-performance PA-TFC membranes,two novel RO membranes were synthesized by applying different approaches of fabrication and modification in this thesis.The main research contents and conclusions are as follows:Firstly,due to its superior hydrophilicity,high stability and low cost,cerium oxide(CeO2)nanoparticles(NPs)were incorporated into the PA selective layer via interfacial polymerization to obtain a novel thin film nanocomposite(TFN)membrane.A series of characterization techniques indicated that CeO2 NPs was successfully introduced into the PA layers.The data of contact angle,root-mean-squared(RMS)roughness and zeta potential showed that the hydrophilicity,roughness and surface electronegativity of TFN membranes were enhanced with the increase of the loading amount of CeO2 NPs.During the RO test,TFN membranes with high selectivity and permeability were obtained through the introduction of CeO2 NPs.TFN-CeO2100 membrane synthesized at optimized condition exhibited a water flux of 44.0 L·m-2.·h-1,which was 50%higher than the pristine TFC membranes with a similar NaCl rejection(98%).CeO2 NPs also endowed the TFN membranes with antifouling properties.Compared with the control TFC membranes,the TFN-CeO2100 membrane gave much lower flux reduction and displayed a relatively high flux recovery ratio.This improved antifouling property was mainly attributed to the increased surface hydrophilicity and electronegativity of TFN membranes,which helped to inhibit the nonspecific adsorption of organic foulants on the membrane surface.Additionally,the carboxyl groups on the commercial RO membrane surface were activated by N-(3-Dimethylaminopropyl)-N’-ethylcarbodiimide hydrochloride(EDC)and N-hydroxysuccinimide(NHS).Two monomers containing amino groups such as 3-Amino-1,2-propanediol(APD)and tobramycin(TOB)were grafted in-situ onto the RO membrane surface by amidation to synthesize a novel TFC membrane with high separation performance.Fourier transform infrared spectrometer(FTIR)and X-ray photoelectron spectroscopy(XPS)analysis demonstrated that APD and TOB could be successfully grafted to the membrane surface.The grafting of APD and TOB resulted in the enhanced hydrophilicity,decreased surface electronegativity as well as reduced surface roughness.The obtained membranes were measured using 2000 ppm NaCl and 5 ppm H3BO3 solution.The separation performance of TFC-TOB/APD2 membrane had a water flux of 4.0 L·m-2·h.1·bar-1 and a NaCl rejection of over 98%.When compared with TFC membrane,water flux of TFC-TOB/APD2 membrane was 29%higher than the unmodified membrane and simultaneously had excellent rejection.It was worth noting that the grafting of APD and TOB can restore membrane boron rejection 84.2%to 91.0%,which was ascribed to the complexation of boric acid with the vicinal diol groups of APD molecules.In addition,TFC-TOB/APD2 membrane had prominent bactericidal capacity against both S.aureus(98.2%)and E.coli(99.9%),attributed to TOB-mediated bacterial cell inactivation.During cross-flow fouling tests,the flux recovery ratio of TFC-TOB/APD2 membrane was 82.7%in the filtration of bovine serum albumin(BSA)solution and 87.3%in the filtration of humic acid(HA)solution,respectively.Compared with the control TFC membrane,TFC-TOB/APD2 membrane always exhibited improved antifouling propertiesTo sum up,a novel TFN membrane with high perm-selectivity,superior stability and improved antifouling properties was synthesized via the introduction of CeO2 in PA layers.Not only that,due to the in-situ grafting of APD and TOB with attractive functions on the PA layers,boron removal capacity,antifouling and antibacterial properties of RO membranes were greatly improved.These modified membranes can overcome the intrinsic drawbacks of conventional RO membranes,and provide a valuable insight to the research area.