Study On The Regulation Of Carbon Nanotube Phospholipid Vesicles On The Structure And Separation Performance Of Reverse Osmosis Composite Membrane

Freshwater is crucial for human survival.Currently,membrane separation technology,represented by reverse osmosis(RO),has become one of the main methods to solve the freshwater crisis,playing a huge role in fields such as seawater desalination,brackish water desalination,and wastewater reuse.At present,polyamide composite membranes(PA-TFC)prepared by interfacial polymerization method occupy the dominant position in the RO membrane market.However,due to the trade-off effect between selectivity and permeability,although polyamide composite membranes have a high rejection rate,the water flux has always been at a low level.Carbon nanotubes(CNT)are considered to be the ideal nanomaterial to break through the "trade-off" effect of RO membranes due to their one-dimensional tubular structure,smooth inner wall,and superhydrophobicity,which have a water molecule transport rate several orders of magnitude higher than other nanomaterials.Constructing highperformance thin film nanocomposite(TFN)membranes based on CNT as water channels and different structures has become a research hotspot.Although related research has achieved fruitful results,it still faces two problems: poor compatibility between CNT and interfacial polymerization layer and disordered arrangement of CNT in the interfacial polymerization layer.This article addresses the issues of poor compatibility and disordered arrangement of CNTs in TFC membranes.Inspired by carbon nanotube pore proteins(CNTPs)-short CNT fragments that can be precisely and spontaneously inserted into phospholipid vesicles-we propose to prepare carbon nanotube liposomes(CNT liposomes)through thin film hydration to introduce them as nanospheres into the polyamide selective layer of TFC membranes to prepare modified TFN membranes.This fully unleashes the potential of CNTs,provides additional water channels,and increases the water flux of the membrane without affecting its selectivity to break the "tradeoff" effect.To further increase the loading of CNTs in phospholipid vesicles,the structural effects of positively charged phospholipids on CNT phospholipid vesicles and reverse osmosis membranes were studied.The main conclusions are as follows:(1)Neutral phospholipid DOPC was selected as the carrier for CNTs,and DOPC-CNT liposomes and pure DOPC liposomes were prepared by thin film hydration method,followed by the preparation of corresponding TFN-CNT and TFN-DOPC reverse osmosis membranes.FTIR,Raman,Stopped-flow,and Cryo-TEM characterization from different angles confirmed the successful preparation of DOPC-CNT liposomes,and fitting calculations showed that the embedding of CNTs increased the water permeability of DOPC-CNT liposomes by 10 times.The structure,surface element composition,morphology,and hydrophilicity of the reverse osmosis membrane were characterized by FTIR,XPS,SEM,TEM,AFM,hydrophilic contact angle,and surface free energy analysis.Lower crosslinking degree,increased surface roughness,enhanced hydrophilicity,and low water molecule resistance channels provided by CNT liposomes jointly promoted the increase of water flux.In the separation test of Na Cl solution,TFN-DOPC and TFN-CNT reverse osmosis membranes showed 36.5% and 71.4% flux improvement,respectively,while maintaining the same salt rejection rate.After calculation,CNTs loaded in liposomes provided 25.6% flux improvement for TFN-CNT reverse osmosis membrane.(2)In order to increase the load of CNTs,positively charged DOTAP phospholipids and neutral DOPE phospholipids were mixed in different ratios to prepare CNT phospholipid vesicles,and the effect of positively charged phospholipids on the structure of CNT phospholipid vesicles and reverse osmosis membranes was studied.Similarly,DOPE:DOTAP=4:1(2:1)mixed liposomes were prepared by thin film hydration method,and corresponding TFN-M and TFN-M-CNT reverse osmosis membranes were prepared for a series of characterization tests.FTIR,Raman,Stopped-flow,and Cryo-TEM characterization confirmed the successful preparation of DOPE/DOTAP-CNT liposomes from different angles,and Stopped-flow fitting water permeability values and near-infrared absorption spectroscopy(NIR)qualitatively verified that DOPE/DOTAP-CNT-liposomes 2:1 have a larger CNT load.FTIR,XPS,SEM,AFM and other characterizations were used to analyze the TFN-M reverse osmosis membrane,and the results showed that the TFN-M-CNT reverse osmosis membrane exhibited higher cross-linking degree,larger surface roughness and stronger hydrophilicity,which was attributed to the higher CNT load and the effect of DOPE phospholipids on the crosslinking degree of the PA layer of the reverse osmosis membrane.In the separation test of Na Cl solution,TFN-M and TFN-M-CNT reverse osmosis membranes showed 59.5% and 152.4%increase in flux,respectively,with CNT phospholipid vesicles providing 58.2% of the flux increase for TFN-M-CNT reverse osmosis membranes,which is due to the positive effect of positively charged phospholipids on CNT load compared to the increased flux increase of TFNCNT.(3)TFN-DOPC,TFN-CNT,TFN-M,and TFN-M-CNT reverse osmosis membranes all showed satisfactory stable rejection rates and acceptable fluctuating water flux in the 48-hour stability test,with at least 85.7% membrane flux recovery rate after two pollution cycles,and up to 91.0% membrane flux recovery rate with increasing CNT load,fully demonstrating the excellent potential of CNT phospholipid vesicles for reverse osmosis membranes.In summary,this study used DOPC-CNT liposomes and DOPE/DOTAP-CNT mixed liposomes with different phospholipid ratios as nanoscale particle-modified reverse osmosis membranes,fully exploiting the potential of CNTs as water channels,providing new ideas for membrane desalination for the purpose of obtaining drinkable fresh water,and promoting the development of nanoscale water channels in the field of water treatment.