| Water is an indispensable resource for human survival.However,water shortage caused by water pollution is one of the major problems to be solved urgently.Therefore,the development of convenient and efficient water treatment technology has become the primary task for human beings.Nanofiltration membrane separation technology is considered as one of the most promising water purification technologies because of its advantages of simple operation,energy saving and environmental protection.The main factors affecting the separation performance of nanofiltration membranes include the material and structural characteristics of the membrane.Therefore,nanofiltration membranes with high separation performance and stability can be obtained through the development of new materials and the regulation of the membrane structure.In this study,as a classic two-dimensional layered anionic clay material,the layered double metal hydroxide(LDH)was used as the material to prepare the separation layer of the composite membrane.It has the characteristics of strong hydrophilicity,anion exchange between layers,and adjustable layer spacing.The new porous materials Fe-cage and carboxylated carbon nanotubes(CNT)were used to functionally modify LDH,and assembled on the surface of polyacrylonitrile ultrafiltration membrane by vacuum-assisted method.In this way,a high-performance nanofiltration membrane can be obtained which can be used for removing small molecular organic matters in water.Firstly,exfoliated CoAl-LDH nanosheets were prepared by solvothermal method in one-step.CoAl-LDH/Fe-cage composites were self-assembled with zero-dimensional material Fe-cage by electrostatic interaction,and the ratio of them was regulated by the Zeta potential.CoAl-LDH/Fe-cage composite was deposited on the surface of hydrolyzed polyacrylonitrile(PAN)substrate to form a composite membrane by vacuum-assisted method.By characterizing the micromorphology and surface properties of CoAl-LDH/Fe-cage and its composite membrane,it was found that the composite membrane had a typical two-dimensional layered structure,and the addition of Fe-cage increased the interlayer distance of CoAl-LDH.At the same time,because Fe-cage has a hydrophilic surface and a hydrophobic inner cavity,the mass transfer efficiency of water molecules in the membrane can be improved.By changing the ratio and concentration of exfoliated CoAl-LDH and Fe-cage,the different stacking modes of CoAl-LDH/Fe-cage were controlled.The CoAl-LDH/Fe-cage composite membrane was used for the removal of dyes and humic acids in water.Under the optimal membrane forming conditions,the permeance and rejection of the composite membrane to Congo red were 88 kg/(m~2·h·MPa)and 98%,respectively.The permeance was 40%higher than pure LDH membrane with the same retention,and the composite membrane can maintain stable operation for 100 h when removing humic acid from water.Next,the same strategy was used to intercalate one-dimensional material CNT between CoAl-LDH layers to form composite membrane.Due to the negative charge of carboxylated CNT,it can form complex with exfoliated CoAl-LDH nanosheets by electrostatic interaction,and then deposited on the surface of the hydrolyzed PAN-based membrane by vacuum-assisted method to prepare CoAl-LDH/CNT composite membranes.At the same time,the water permeance of PAN-based membrane with different cut-off molecular weights and different states were investigated.The results showed that the wet base membranes with high cut-off molecular weight were more conducive to the preparation of nanofiltration membranes for small molecule organic removal.Through the characterization of the morphology and structure of the CoAl-LDH/CNT composite membrane,it was found that the addition of CNT created more nano-mass transfer channels between CoAl-LDH layers.Due to the low friction and rapid flow of water molecules in the CNT tube,the permeance of the membrane was greatly improved.The prepared CoAl-LDH/CNT composite membrane was used to remove dyes and humic acid from water,and the influence of preparation conditions on separation performance was investigated.The results showed that when separating Evans blue from water,the rejection of the CoAl-LDH/CNT composite membrane was98%,and the permeance can reach 2600 kg/(m~2·h·MPa),which improved by 36%compared with pure LDH composite membrane.Moreover,the CoAl-LDH/CNT composite membrane has better anti-fouling performance to humic acid than that of CoAl-LDH/Fe-cage composite membrane.In this study,the electrostatic effects of positive and negative charges were used to assemble Fe-cage and CNT with exfoliated CoAl-LDH nanosheets,respectively.Vacuum-assisted assembly was used to form intercalated LDH composite membrane on the surface of PAN-based membrane.The intercalation of Fe-cage and CNT not only increased the interlayer distance of CoAl-LDH,but also regulated the interlayer chemical properties,which formed a rapid transmission channel for water molecules.Therefore,the permeability of the LDH membrane was improved.The method is easy to operate with cheap raw materials.Moreover,the prepared composite membranes have excellent small molecule organic matter removal performance,which exhibited great application potential in the field of loose nanofiltration membranes. |
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