Preparation Of One-dimensional Carbon Nanomaterials Membranes And Their Enhanced Water Treatment Performance Under Electro-assistance

As an efficient water treatment technology,membrane separation has been widely used in drinking water purification and wastewater treatment.Traditional membrane separation suffers from the problems of serious membrane fouling,and the trade-off relationship between membrane permeability and selectivity.Carbon nanomaterials possess atomically smooth surface and high specific surface area.Thus the separation membranes constructed by carbon nanomaterials are expected to achieve much higher water transport and separation capabilities than traditional separation membranes.In addition,the excellent electrical conductivity of carbon nanomaterials makes it possible to combine membrane separation technology with electrical assistance,which could further enhance membrane performance and be a promising method to solve the bottleneck problem of membrane separation technology.This work utilizes carbon nanofiber and carbon nanotube（CNT）to construct scalable high-performance conductive membranes.The stability and mechanical strength of the carbon nanomaterial membranes are improved through chemical crosslinking and structure control.Besides,the system for coupling membrane separation and electro-assistance has been constructed by taking advantage of the excellent electrical conductivity of carbon nanomaterials.The enhanced separation performance of carbon nanomaterial membranes under electro-assistance is investigated by treating oil-water mixture,surface water and actual wastewater.The main research contents and results are shown as follows:（1）Poly-3-methyl thiophene/carbon membranes（PCMs）were prepared by electrospinning-carbonization-electrodeposition process.The effects of electro-assisted wettability conversion on membrane permeability and separation performance were investigated.The results revealed the composition of poly-3-methylthiophene on the surface of PCMs could be changed by doping and de-doping perchlorate under electro-assistance,which caused the reversible conversion of membrane wettability.The doped PCMs were superhydrophilic and the de-doped PCMs were superhydrophobic.The wettability was one of the key factors affecting membrane permeability and separation performance.When the pressure was 0.1 bar,the pure water flux of doped PCMs was 942 L·（m²·h）^-1,which was 4.7 times of the pure water flux of de-doped PCMs.Driven by gravity,water could penetrate through doped PCMs and oil was intercepted,while oil could penetrate through the de-doped PCMs and water was intercepted.The results indicated that electro-assisted wettability conversion could effectively control the membrane permeability and separation performance of oil-water mixture.（2）In view of the relatively complicated preparation process of carbon nanofiber membranes,carbon nanotubes were employed as membrane materials due to their simple preparation methods.Carbon nanotubes/polyvinylidene fluoride composite hollow fiber membrane（CNTs/PVDF HFM）was prepared by vacuum filtration and chemical crosslinking.The anti-fouling performance under electro-assistance was investigated.The results showed that the crosslinking between carboxyl groups of CNTs and hydroxyl groups of polyvinyl alcohol（PVA）could enhance the stability of conductive CNTs layer.When the concentration of PVA was 0.2%,the tensile stress and elastic modulus of CNTs/PVDF HFM were respectively 40.9 MPa and 822 MPa,which were comparable to those of commercial polyvinylidene fluoride membrane.Furthemore,the CNTs functional layer could remain intact after 0.5 h ultrasonic oscillation,which indicated that the crosslinked CNTs/PVDF HFM had good tensile strength and stability.In the treatment of secondary effluent(COD:92 mg-L^-1),the average flux of membrane separation process with 2 V cell voltage assistance（membrane as cathode）could accomplish 91.5%of the initial flux,which was 1.5 times as high as that of membrane separation process alone.After backwashing,the flux recovery ratio of membrane separation process with 2 V cell voltage assistance（membrane as cathode）was 97.9%,which was 1.7 times as high as that of membrane separation process alone,evidencing the effective membrane f-ouling mitigation under electro-assistance.（3）In order to improve the poor permeability of composite hollow fiber membrane,nanofibers prepared by electrospinning were used to construct the support layer with a three-dimensional network structure.Carbon nanotubes/nanofiber composite hollow fiber membrane（CNC-HFMs）was made up of crosslinked CNTs conductive separation layer and nanofibers support layer.The separation performance of CNC-HFMs for surface water under electro-assistance was investigated.Owing to the 94%porosity and three-dimensional network pore structure,the pure water flux of CNC-HFMs（average pore diameter:221 nm）reached 5800 L·（m²·h·bar）^-1,which was 7.3 times higher than that of CNTs/commercial polyacrylonitrile hollow fiber membrane（average pore diameter:243 nm）.When CNC-HFM was employed as cathode for surface water treatment under 2 V cell voltage assistance,the removal efficiencies of turbidity and total organic carbon were respectively 1.3 and 1.4 times as high as those of membrane separation process alone,which indicated the electro-assistance could effectively improve removal efficiency of membrane separation.（4）Based on vacuum filtration-chemical crosslinking,a method for the large-scale preparation of carbon nanotube/alumina flat ceramic membranes（CA-FCMs,average pore diameter:90 nm）had been developed.An electro-assisted membrane separation device（1 m3·d-1）based on CA-FCM was designed for surface water treatment.The water treatment performance of electro-assisted membrane separation was evaluated by comparing with those of membrane separation alone and commercial polyvinylidene fluoride（PVDF）flat membrane（average pore diameter:100 nm）separation.Results showed that average water production of electro-assisted CA-FCM filtration was 427 L·（m²·h-bar）^-1,which was 1.7 times and 5.3 times higher than that of CA-FCM filtration alone and commercial PVDF membranes,respectively.At the same time,the effluent quality had been significantly improved,indicating that the electro-assisted membrane separation could effectively enhance the membrane separation performance for water treatment.In summary,the conductive membranes with excellent application prospects can be fabricated by the chemical modification and crosslinking of one-dimensional carbon nanomaterials.And the electro-assisted membrane separation can improve the separation performance in water treatment.This work provides new ideas for the preparation of high-performance conductive separation membranes,which is conducive to promote the development and practical application of electro-assisted membrane separation.