Design And Preparation Of Temperature Self-responsive Nanocontainers And Performance Study Of Their Modified Reverse Osmosis Composite Membranes

Currently,the shortage of drinking water remains a global challenge.The supply-demand imbalance between the availability and use of fresh water has led to a greater reliance on purified water.Reverse osmosis technology plays a key role in providing an alternative source of water for desalination and water recovery.The most advanced reverse osmosis（RO）membrane is thin film composite（TFC）prepared by interfacial polymerization of amines and chlorides.The polyamide（PA）layer on the surface of the membrane selectively removes ions and small molecules while allowing water permeation.However,the poor chlorine resistance of the polyamide layer significantly reduces the service life of the membrane,increases the energy consumption and operating costs of the desalination process,and becomes the Achilles’heel of reverse osmosis polyamide membranes in practical applications.Although much research has been conducted to improve the performance of thin film composite membranes,achieving high chlorine resistance and desalination of membranes remains a major challenge.In this paper,temperature-responsive nanocontainer were innovatively designed and prepared to produce a chlorine-resistant reverse osmosis membrane with intelligent temperature-responsive properties by combining the nanocontainers with polyamide reverse osmosis membrane.The surface morphology of this thin film nanocomposite（TFN）membrane was changed by the addition of nanocontainers,which resulted in a larger"ridge and valley"structure,increasing the specific surface area of the separation layer and providing more water channels for water molecule transport.In addition,the surface roughness of the membrane increases significantly,which increases the hydrophilicity and facilitates the dissolution of water molecules on the membrane surface.In the chlorination cycle experiments,the polyamide structure broke due to the attack of active chlorine,resulting in a faster decrease of salt interception rate of the TFC membrane.The TFN membrane with the addition of temperature-controlled smart nanocontainer continues to respond to the temperature of the membrane after chlorination treatment,and the nanocontainer will dissolve the shell after thermal stimulation,releasing the inner repair material,which can precisely repair the amide bond and effectively fill the broken polyamide layer network structure,slowing down the degree of chlorination breakage of the polyamide membrane.The study consists of three main parts.（1）The Ag@SA@CS@PVA（ASCP）nanocontainers were synthesized using a layer-by-layer self-assembly method.Silver nanoparticles（Ag NPs）are used as the nucleus of the nanocontainer,sodium alginate（SA）and chitosan（CS）as the restorative agents,and 2488-polyvinyl alcohol（PVA）as the shell material of the nanocontainers.The structure,morphology and temperature control performance of the synthesized nanocontainers were investigated.The nanocontainers were added inside polyamide reverse osmosis membranes to prepare TFN membranes,and since 2488-PVA is a cold-soluble polymer material,it possesses certain solubility at room temperature,and the best solubility is at 37°C.Therefore,its incorporation into the polyamide membrane as a shell material can effectively achieve the continuous repair effect of the polyamide membrane after chlorination breakage by changing the external environment.The PVA shell is basically consistent with the actual working environment for response temperature,which can effectively synchronize membrane chlorination with temperature response and release the inner material of the nanocontainer to realize the self-healing property of the reverse osmosis membrane.The surface morphology,roughness,elemental content and contact angle of the TFN membrane were systematically investigated to explore the mechanism of the repair action of the nanocontainer on the polyamide separation layer in the chlorination cycle experiment of the reverse osmosis membrane.In addition,anti-fouling,anti-bacterial and 72 h stability experiments were conducted on the prepared temperature-controlled TFN membranes.After the addition of ASCP nanocontainers to the polyamide layer,the NaCl rejection of the TFC membrane was reduced by 15.64%after 9 chlorination treatments（1000 ppm,sodium hypochlorite（NaCl O）solution）,while the NaCl rejection of the TFN membrane was reduced by only 8.35%.It was demonstrated that ASCP nanocontainers have good chlorine resistance and good stability,and have great potential as membrane doping materials for targeted remediation of polyamide reverse osmosis membranes.（2）In the preparation of polyamide reverse osmosis membranes by interfacial polymerization,various reaction conditions of the membranes were optimized to enhance the separation performance of the polyamide reverse osmosis membranes.The aromatic polyamide reverse osmosis membranes with homogeneous surface structure were prepared by systematically investigating the infiltration time of aqueous and oil phases,the temperature and time of heat treatment,and the concentration of m-phenylenediamine（MPD）,homophthalic trichloride（TMC）and dimethyl sulfoxide（DMSO）.Scanning electron microscopy（SEM）and Fourier transform infrared spectroscopy（FTIR）were also used to explain the changes in surface morphology and functional groups of the membranes.Finally,the permeability and salt ion retention capacity of the polyamide reverse osmosis membrane were significantly improved by optimizing the process conditions.（3）The same electrostatic self-assembly method is used to prepare Si O₂@SS@PVA（SSP）nanocontainers,with silica（Si O₂）nanoclusters were used as the inner core center of the container,and silk glue molecules（SS）with carboxyl and amino groups as characteristic groups are loaded on its surface,and finally 1799-polyvinyl alcohol（1799-PVA）is used for encapsulation,and the nanocontainer structure is prepared through layer-by-layer self-assembly technology.SS is a water-soluble spherical protein composed of various amino acids,which is biocompatible and is often coated or deposited on the surface of polyamide membranes for fouling and chlorine resistance studies due to its rich amino and carboxyl groups.We have added temperature-responsive nanocontainers to polyamide reverse osmosis membranes to obtain smart responsive self-healing integrated chlorine resistant reverse osmosis membranes without the need for additional reagents and procedures.By releasing the inner SS material after responsive dissolution of the outer shell material,the amino and carboxyl groups on the surface of the SS molecules can effectively bond with the broken amide bonds and thus fill the structural damage areas of the polyamide layer caused by chlorination,which greatly enhances the chlorine resistance of the membrane and prolongs its service life.The modified membrane surface has enhanced hydrophilicity and can form a hydrated layer on its surface,which effectively prevents the adhesion of bovine serum protein molecules from contamination.SSP nanocontainers of 0.008 wt.%were added inside the polyamide layer and 9 chlorination cycles were conducted for TFN-Cl-T（2000 ppm,NaCl O,70°C temperature response）,TFN-Cl（2000 ppm chlorination,no temperature response）and TFC（2000 ppm chlorination,no temperature response）membranes,respectively,and the results showed that the TFN-Cl-T membrane maintained 90.38%chlorination resistance after 9 cycles.The results showed that the TFN-Cl-T membrane still maintained 90.38%NaCl rejection after 9 chlorination cycles,while the TFC and TFN-Cl membranes had dropped to 71.24%and 62.15%,respectively,and the NaCl rejection of both membranes had dropped to below 80%,which fully demonstrated the stronger oxidation resistance of the TFN-Cl-T membrane.Through the national standard conversion,the modified membranes can effectively continue to operate for 60,000 h under the specified chlorination conditions and still maintain good salt interception performance.