| Nanofiltration(NF)membranes,whose properties are intermediate to ultrafiltration and reverse osmosis,have the advantages of low operating pressure,excellent effluent quality,and no phase change in the separation process.Owing to these features,NF membranes have been used in many applications involving drinking water purification,brackish water treatment,industrial wastewater reclamation,and seawater desalination.However,there exist some issues during the applications of NF membranes,such as low water flux,easy to be fouled,and sensitivity to chlorine.In the case of the low flux,it can be solved by adopting a high-flux NF membrane module.As a result,energy consumption can be reduced,thereby leading to much lower operating costs.Although the membrane fouling and poor chlorine stability of NF membranes can be greatly improved by using pretreatment procedures,it also increases the process and increases the operating cost.To ensure the continuous and efficient operation of the NF system,it is of great urgency to fabricate an NF membrane module with high flux,anti-fouling and chlorine resistance.Considering the fact that the performance of an NF membrane has a great relationship with its membrane structure,the preparation of NF membranes with various functions can be realized by the rational design of the NF membrane structure.In this work,to solve the issues in the application of NF membranes,functionalized NF membranes with high-flux,anti-fouling and chlorine-resistant performance were designed and fabricated individually through tailoring the features of the polyamide layers based on optimizing interface characters.In order to solve the problem of low flux for NF membranes,a triple-layered NF membrane with an intermediate layer was prepared by polyvinyl alcohol(PVA)assisted interfacial polymerization method.The pristine polyethersulfone(PES)ultrafiltration support was firstly coated with PVA layer,and a novel membrane(PES-PVA)with a more hydrophilic surface,higher surface porosity and stronger stability was prepared.Then,the PES-PVA membrane was used as the support and a polyamide film was deposited on the surface of the PES-PVA support via interfacial polymerization.The PES-PVA membrane hydrophilic and porous features promoted the formation of an ultrathin polyamide membrane with nanostrip-textured morphology.The thickness of the polyamide layer was 97.8 nm and 9.6 nm for the control membrane and modified membrane,respectively.Compared with the control membrane,the PVA-interlayered NF membrane had a thinner and denser separation layer with a larger filtration area,thus exhibiting excellent separation performance.For the PVA-interlayered NF membrane,the water permeability and Na2SO4rejection were 31.4 L·m-2·h-1·bar-1 and 99.4%,respectively.The separation performance of the triple-layered NF membrane prepared in the pilot production line was close to that of the NF membrane prepared in the laboratory,indicating that the NF membrane with a PVA intermediate layer has great potential for large-scale production.Aiming at the problem of poor anti-fouling performance for NF membranes,anti-fouling NF membranes were constructed using surface modification strategies.Firstly,the anti-fouling behavior of the PVA surface-modified NF membrane and PVA-interlayered NF membrane were compared.The results confirmed that the PVA-interlayered NF membrane possessed better anti-fouling performance than that of the control NF membrane,and the PVA surface-modified NF membrane owned the best antifouling property.Secondly,the effect of Fe?/TA coating on the anti-fouling performance of the NF membrane was studied,and the results showed that the Fe?/TA coating enhanced the anti-fouling behaviors of the NF membrane.The regeneration of the Fe?/TA layer realized the efficient removal of the foulants and the fouled layer,which further improved the anti-fouling characteristics of the NF membrane.Importantly,the regeneration process resulted in a sustainable anti-fouling ability for the modified NF membranes.After 3 cycles of fouling and regeneration tests,the flux recovery rate of the Fe-TFC-3 membrane was above96.0%,and the separation performance was close to that of the original membrane.Though the surface modification strategy had greatly enhanced the anti-fouling behaviors of NF membranes,the water permeability was slightly reduced.To simultaneously improve the anti-fouling and water flux,the NF membrane was modified via in situ incorporation of calcium bicarbonate during the fabrication process.The combination of carboxyl shielding of calcium ions and the nanobubble effect induced by carbonate contributed to the fabrication of high-performance NF membranes with both anti-fouling and high water flux.The TFC-Ca4.0 membrane exhibited a 90%higher flux recovery rate during 5 cycles of fouling tests.To address the problem of chlorine sensitivity of NF membranes,chlorine-resistant NF membranes were prepared via an in-situ grafting strategy.Firstly,the Fe?/TA supramolecules were in situ grafted onto the surface of an NF membrane.The presence of the grafted layer shielded the chlorination sites on the surface of the NF membranes,which greatly improved the chlorine resistance of the NF membranes.After being soaked in Na Cl O solutions at different p H values,the modified NF membranes showed much stronger chlorine stability than the control membranes.Impressively,the Fe?/TA supramolecular modified NF270 membrane also exhibited enhanced chlorine resistance,indicating that this modification method can be used in actual processes.Secondly,melamine was in situ grafted onto the surface of the NF membrane,and the chlorine torrent behaviors of the modified NF membranes were systematically evaluated.The static chlorine immersion experiment showed that grafting of melamine could significantly improve the chlorine stability of the NF membranes.As stated above,NF membranes with different functionalities can be prepared through the design and control of interface characteristics,which had solved the problems of low flux,easy to be fouled and poor chlorine stability encountered in the actual operation of NF membranes.However,the water quality faced by NF membranes was more complicated than expected during the actual operation process.Therefore,different modification strategies can be combined to further fabricate multifunctional NF membranes. |
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