Preparation Of High Performance MXene-based Composite Nanofiltration Membrane And Their Performance Control

Nanofiltration(NF)technology can effectively reject multivalent ions and organic small molecules with molecular weight higher than 200 Da,and is widely used in seawater desalination and wastewater purification,which is one of the key common technologies to alleviate the vital challenges such as resource shortage,environmental degradation and energy tension.In recent years,MXene has been widely used to construct high-performance NF membranes because of its graphene-like layer structure,rich surface functional groups,excellent antibacterial properties,adsorption properties and high carrier mobility.Since MXene nanosheets are usually charge-negative leading to disordered stacking,and MXene-based NF membranes mainly perform molecular sieving and selective mass transfer through the stacked inter-nanosheet channels,this work prepares MXene based NF membranes with high separation performance by regulating the charge-negativity of MXene based NF membranes and constructing nano mass transfer channels.Firstly,a series of pure Ti3C2Tx MXene membranes with different loading amounts and the same lateral size of 400-600 nm were fabricated by vacuum filtration using commercial polyethersulfone(PES)microfiltration membranes with a pore size of 0.45 μm and a diameter of 50 mm as the substrate.The morphology and the separation performance for Rhodamine B(RhB)of pure Ti3C2Tx MXene membranes prepared using different loading amounts were investigated.The results showed that the Ti3C2Tx MXene nanosheets uniformly covered the surface of PES substrate,and the number of micron-level defects decreased gradually with the increase of MXene loading.Based on the "trade-off" effect,the rejection of RhB increased with the increase of Ti3C2Tx MXene membrane thickness,but the corresponding permeability decreased,in which the rejection of Ti3C2Tx MXene membrane with the loading of 0.40 mg·cm-2 was 99.61%with the permeability of 20.07 L·m-2·h-1·bar-1.Then,MXene-based NF membrane with high water permeability and stability by intercalating Ag nanoparticles(AgNPs)into the laminar structure of Ti3C2Tx MXene with hyperbranched polyethyleneimine(HPEI)crosslinking.The pure Ti3C2Tx MXene membrane,the AgNP-intercalated Ti3C2Tx MXene(AgNP@Ti3C2Tx MXene)membrane and the AgNP@Ti3C2Tx MXene membrane with HPEI crosslinking(HPEI-AgNP@Ti3C2Tx MXene)were comparatively investigated regarding their physicochemical properties and NF performance.The prepared HPEI-AgNP@Ti3C2Tx MXene membrane exhibited competitive water permeabilities(24.64-30.73 L·m-2·h-1·bar-1)and nearly 100%rejections for Congo red,methyl orange,methyl blue and RhB solutions for long-term operation.Moreover,the HPEIAgNP@Ti3C2Tx MXene membrane also showed excellent NF performance for MgSO4,MgCl2,Na2SO4 and NaCl solutions with the rejections of 84.15%,77.01%,74.67%and 56.58%,respectively.The NF separation mechanisms for the HPEI-AgNP@Ti3C2Tx MXene membrane were elucidated based on its unique physicochemical structure.Finally,the photocatalysis and membrane separation technology were combined to explore the antifouling of Ti3C2Tx MXene based NF membranes without affecting the separation performance.A series of ZnO@Ti3C2Tx MXene photocatalytic self-cleaning membranes with different ZnO loadings were fabricated by vacuum filtration using ZnO nanoflowers to modify Ti3C2Tx MXene membranes,and the NF performance and photocatalytic performance of the membranes were systematically characterized.The results showed that the prepared ZnO@Ti3C2Tx MXene membranes exhibited excellent permeability(138.81-157.19 L·m-2·h-1·bar-1)at a ZnO loading of 5 mg,the rejection of CR,MO,MB and RhB solutions exceeded 91%.Moreover,the degradation rates of MO,RhB,MB and CR within 270 min were 28.88%,83.08%,92.38%and 89.31%,respectively,which demonstrated that the membrane could achieve photocatalytic self-cleaning.In addition,the photocatalytic degradation mechanism of ZnO@Ti3C2Tx MXene photocatalytic self-cleaning NF membrane was explored and elucidated based on the Schottky barrier formed by the ZnO@Ti3C2Tx MXene membrane.