Preparation Of Electrically Enhanced Forward Osmosis Membrane (FO) For Membrane Fouling Mitigation

As a membrane separation technology driven by osmotic pressure,forward osmosis（FO）technology has outstanding advantages such as good effluent water quality,low membrane fouling trend,and low energy consumption.It has shown great applications potential in the fields of seawater desalination,sewage treatment and reuse.However,the serious membrane fouling during the long-term operation is the key to restricting the popularization and application of FO technology at this stage.Membrane fouling will cause the attenuation of water flux,resulting in a decrease in membrane operating efficiency,and frequent membrane cleaning will increase operating costs and reduce membrane service life.Therefore,mitigating membrane fouling is of great significance for improving the operating performance of the FO membrane.Organic fouling is one of the important types of FO membrane fouling.The fouling caused by the ubiquitous organic substances in the water body represented by sodium alginate,bovine serum albumin and humic acid has attracted widespread attention.Aiming at the problem of organic fouling of FO membrane,this work proposes a control strategy to reduce membrane fouling by introducing a conductive nanomaterial layer in the preparation process of FO membrane,which provides a new idea for future FO membrane fouling control.In this paper,a conductive thin film composite forward osmosis（TFC-FO）membranes were prepared by introducing carbon nanotubes（CNTs）and two-dimensional transition metal carbon/nitrogen/carbonitride MXenes conductive layer on a polyethersulfone（PES）base membrane.The influence of nano-material layers on the structural and performance of the FO membrane and the feasibility of alleviating organic fouling under the condition of an external electric field were investigated.The main research contents and results are listed as follows:（1）The CNTs intermediate layer was vacuum filtered on a porous polyethersulfone（PES）membrane to prepare a conductive TFC-FO membrane,and the performance evaluation and anti-organic fouling analysis were performed.The research results showed that the introduction of CNTs increases the surface roughness of the conductive TFC-FO membrane and also greatly reduces the resistance of the membrane.Compared with the conventional TFC-FO membrane,the pure water flux of the conductive TFC-FO membrane after the introduction of CNTs is increased by2.33 times,and the J_W/J_Svalue is increased by about 4.72 times.The CNTs intermediate layer effectively regulates the PA active layer and the membrane structure,reduces the resistance of water transmission,and effectively improves the membrane performance.In addition,the anti-fouling experiment of the membrane using sodium alginate as a simulated pollutant shows that in the absence of an electric field,a large amount of contaminants aggregated on the surface of conventional TFC-FO membrane and conductive TFC-FO membrane;when a 2 V electric field is applied,The water flux decay rate of the conductive TFC-FO membrane is significantly reduced,and the contaminants on the membrane surface are greatly decreased.The repulsive force between the negatively charged membrane and the negatively charged pollutants under an external electric field greatly improves the anti-fouling performance of the conductive membrane.（2）A high-performance conductive TFC-FO membrane was prepared by vacuum filtering the MXenes layer on a porous PES membrane,and the performance evaluation and anti-organic fouling analysis were carried out.The results of the study found that the introduction of MXenes greatly reduced the resistance of the TFC-FO membrane,but had no significant effect on the surface morphology and basic properties of the active layer of the TFC-FO membrane.In terms of separation performance,compared with conventional TFC-FO membranes,the introduction of MXenes increases the J_W/J_S ratio of conductive TFC-FO membranes and improves the permeability,and when the the load amounts of MXenes is 0.47±0.06 g·m^-2,the conductive TFC-FO membrane shows the best performance.Organic fouling experiments represented by sodium alginate and bovine serum albumin show that in the absence of an external electric field,there is no significant difference between the water flux level and change trend of the conductive TFC-FO membrane and the conventional TFC-FO membrane;After a 2 V electric field,the flux decay rate of the conductive TFC-FO membrane is significantly decreased,and the number of pollutants on the membrane surface is significantly reduced.Similar to the role of the first part of the CNTs intermediate layer,the MXenes layer alleviates membrane fouling under electric field conditions because of the repulsive force between the membrane and the pollutants.（3）Based on the use of MXenes to prepare conductive TFC-FO membranes,in order to enhance the membrane resistance to organic fouling,the fouling flux changes of the conductive TFC-FO membrane under the six operating conditions of no aeration,aeration,2 V electric field,2 V electric field+aeration,4 V electric field and4 V electric field+aeration are studied.The research results indicate that when 4 M Na Cl is used as the draw solution and deionized water is used as the feed solution for performance evaluation,the J_W/J_S ratio of the conductive TFC-FO membrane does not change significantly under the six conditions,which shows that for the same conductive TFC-FO membrane,the different operating conditions did not affect the permeability of the conductive membrane itself.For the sodium alginate fouling experiment,compared with the two operating conditions of no aeration and aeration,after applying a 2V electric field,the flux attenuation rate dropped from 31.00%to about 26.90%,and the water flux was significantly increased.The flux decay rate of the conductive FO membrane under 4 V voltage is significantly lower than that of 2 V voltage,and the drop rate is 4.40%;while under 4 V voltage,the flux decay rate of the conductive FO membrane under aeration is only 2.20%lower than that of no aeration.The electric field mainly improves the anti-fouling performance of the membrane through the action of electrostatic repulsion.