Preparation Of Graphene And Carbon Nanotube Based Nanofiltration Membranes And Electro-Enhanced Performance In Desalination

Nanofiltration,as an important membrane separation technology,can effectively reject organic molecules above 300 Da and multivalent ions.It has great application potential in drinking water and sewage treatment,industrial wastewater treatment,seawater and brackish water desalination.However,nanofiltration membranes are subject to the trade-off between permeability and selectivity(rejection performance):increased permeability results in decreased selectivity,and vice versa.This trade-off limits the simultaneous achievement of high water permeation and high solute rejection.Besides,current nanofiltration membranes also exhibit low monovalent ion rejection rate,generally less than 60%.Thus,developing novel nanofiltration membranes with high flux and high rejection and seeking new strategies for better separation performance are the focus of current researches of nanofiltration technologies.Carbon nanomaterials possess special physicochemical properties and excellent water transport characteristics,which have great potential in fabricating high-performance membranes.In this thesis,innovative nanocarbon-based nanofiltration membranes are designed and constructed by using graphene and carbon nanotube.Moreover,the strategies of controlling membrane surface charge and pore size and conducting external-electric-voltage assistance are employed to achieve enhanced water permeability and salt rejection performance.Furthermore,the enhancement mechanisms of membrane performances are explored.The main research contents and results are shown as follows:(1)Reduced graphene oxide-oxidized carbon nanotube(RGO-OCNT)nanofiltration membranes were prepared by vacuum filtration.The OCNT content was tuned for controlling the membrane surface charge and pore size.The permeability and selectivity of the membrane were tested using two salt(Na₂SO₄ and NaCl)solutions as the feed.The results showed that the membrane exhibited co-enhanced permeability and ion selectivity.As the OCNT content increased from 0 to 83 wt.%,the pure water permeance improved from 1.20 to 11.3 L m^-2 h^-1 bar^-1.Meanwhile,the Na₂SO₄ rejection rate improved from 58.8 to 78.1%and the NaCl rejection rate increased from 12.1 to 35.3%at the transmembrane pressure of 2 bar.Besides,the rejection rate was increased from 84.2 to 90.1%for Na₂SO₄ and 28.4 to 59.1%for NaCl at 5 bar.The analysis based on Donnan steric pore model revealed that the enhanced permeability was attributed to the enlarged pore size and increased hydrophilicity,while the improved selectivity was mainly dependent on the increase of membrane surface charge.The Na₂SO₄ rejection was simultaneously influenced by the membrane pore size and surface charge,whereas the NaCl rejection was mainly dominated by the surface charge.(2)A conductive polystyrenesulfonate-doped polyaniline/carbon nanotube(PANI-PSS/CNT)nanofiltration membrane was fabricated via vacuum filtration followed by chemical oxidative polymerization.By applying external electric voltage on the PANI-PSS/CNT membrane cathode,the permeability and ion rejection performance of electrically-assisted PANI-PSS/CNT membrane were investigated.With increasing the voltage(cell voltage)from 0 to 2.5 V,the pure water permeance of the membrane remained at around 16.0 L m^-2 h^-1 bar^-1,while the rejection rate increased from 81.6 to 93.0%for Na₂SO₄ and 53.9 to 82.4%for NaCl.In addition,the membrane surface charge density at 2.5 V was-73.0 mC m^-2,which was 6.1 times as much as that without voltage(-11.9 mC m^-2).Donnan steric pore model analysis revealed that the electrical assistance could increase the surface charge density of the membrane,which led to higher cation concentration but lower anion concentration in the membrane.Such a change enlarged the ion concentration difference and the Donnan potential difference between the membrane and bulk solution,resulting in increased ion transfer resistance.Therefore,the ion rejection performance was improved.(3)An ethylenediamine-and polystyrenesulfonate-intercalated graphene oxide/carbon nanotube(GO&EDA-PSS/CNT)asymmetric membrane was constructed by vacuum filtration,and a new strategy for enhancing water permeation and ion rejection by electric-field-assisted membrane filtration was presented.The membrane filtration performance with electric-field assistance was investigated by using the CNT layer as cathode and a titanium mesh as anode and applying an external voltage across the GO&EDA-PSS separation layer.With increasing the voltage from 0 to 3.0 V,the flux of the membrane increased from 9.13 to 17.4 L m^-2 h^-1 at 1 bar.Simultaneously,the NaCl rejection rate improved from 52.4 to 78.3%.The changes of the flux and rejection rate were in good agreement with the results of numerical simulation.The simulation investigation indicated that when applying the voltage,the electric field force driven the fluid in the separation layer to move,generating electroosmotic flow and increasing the velocity of water transport.Thus the flux was enhanced.Meanwhile,the electric-field effect also increased the ion partitioning between the membrane and bulk solution,which declined the anion concentration and increased the cation concentration in the membrane.This change led to increased ion transfer resistance and the ions in the feed solution were less likely to transfer through the membrane.As a result,the NaCl rejection rate was improved.In summary,the permeability and rejection performance of the membranes were effectively enhanced by controlling membrane surface charge and pore size and applying external voltage.The studies in this thesis are helpful to understand nanofiltration membrane processes and have great guide significance in the design of high-performance nanofiltration membranes and the development of nanofiltration technologies in water treatment.