Hydrophobic Membrane Cathode Via Carbon Nanotube Coating And Its Anti-oil-fouling Mechanism In Membrane Distillation

Membrane distillation,a typical thermally driven hydrophobic membrane-based membrane separation process,has great potential in desalination and separation of oily wastewater.However,the fouling and wetting phenomena of hydrophobic membrane by low surface energy substances such as oils seriously restrict its wide application.Hydrophobic and electrostatic interactions between membrane interfaces and foulants work together to regulate the membrane fouling behavior.Investigation on the hydrophobic interactions of membrane surface has made great progress in preventing and controlling membrane fouling;but little attention has been paid to the contribution of electrostatic interaction.The antifouling effect of electrostatic interaction in the membrane distillation process needs further exploration.Here,we use oil-in-water emulsions as target foulants,the conductive membranes were adopted as a cathode based on carbon nanotube interfaces medication strategy,with their electrical potential modified by the applied electrical field,this project explores the effect and regularity of the antifouling of membrane distillation by the co-adjusting of hydrophobic electrostatic interactions and the interface hydrophobicity.Results are shown as follows:Based on carbon nanotube(CNT)materials,a novel Janus composite membrane(PVDF-M-CNT)was prepared via the interface loading and hydrophilization of conventional PVDF hydrophobic membrane by spraying.The optimal loading values of CNT and PVA are confirmed to be 0.57 and 0.0057 mg/cm~2,respectively.Interestingly,despite the 30%smaller effective pore size and the～15μm increased thickness than the original membrane,the vapor flux was not compromised in the modified membrane with5%～8%flux enhancement,probably due to the improved mass-heat conduction(high heat conductivity and increased gas-solution evaporation area,etc.)of the CNT layer,as suggested by the mass-heat transfer studies.Moreover,antifouling properties of the modified membrane were noted for treating a 1000 mg/L hexadecane emulsion with a flux decrease of<10%,and its antifouling performance was significantly improved compared to the controlled group(flux decreased by 60%).The force analysis shows that the maximum adhesion force of the modified membrane to oil droplets was less than 20%of the original membrane,and the amount of oil adhered to the interface was less,which was in line with the XDLVO analysis results.Overall,our results confirmed the hypothesis that CNT may be a promising candidate for fabricating a new-generation permissive and fouling-resistant Janus membrane to treat oily wastewater.By taking advantage of the conductivity of CNT and improve the electrically conductive hydrophobic membrane cathode.The modified membrane PVDF-M-CNT exhibited a less flux decline of<5%in dealing with an extremely high-concentration oil emulsion(2000 ppm)at cell potential of 3.0 V.The anti-oil-fouling robustness was further confirmed over a continuous three-cycle operation(flux recovery by>94%).A modeled fouling rate constant was negatively associated with the calculated capacitive surface charge of the membrane cathode.Upon increasing of cell potential from 1.0 to 3.0 V,the increasing trend of total force from negative value to positive value implies the highly repulsive interaction between oil emulsion and membrane cathode,which is in accordance with the anti-fouling performance of e-DCMD.Interestingly,the increasing sliding dynamics of oil droplet along the interface of membrane cathode was found by the applied weak cell potential,which could also contribute to the reducing of fouling compared to the oil droplet adhered controlled membrane.Our results indicate that the applied electoral field could help modulate the electrostatic interaction between Janus conductive membrane cathode and foulant,and the oil fouling resistance was enhanced.By the Janus conductive membrane cathode prepared via three hydrophobic membranes,the universality of anti-oil-fouling was verified.Moreover,the analyzation of interface characteristic of the layers(namely,membrane,CNT layer and fouling layer)by the electrical impedance spectroscopy(EIS)helps providing more mechanistic insights on the anti-fouling potential of membrane interface under applied electrical field.The flux of all CNT composite membranes was not compromised,while decreased flux was observed when electrical field is applied.Compared with the controlled membrane,PVDF-0.22,PVDF-0.45 and PTFE flux declines 34%,100%and 100%,respectively,and the CNT modified membrane decreased 4%,11%and 18%at cell potential of 3.0 V,respectively.The Nyquist plots show that the semi-circle increases along with the increased applied potential,which indicates the enhanced impedance.The EIS equivalent circuit fitting results show that the resistance of the base coupons and CNT layer of the modified film increases gradually with the cell potential,and the resistance of the fouling layer decreases with the voltage.Electrostatic repulsion plays an important role in controlling membrane fouling.In order to reveal the anti-oil fouling mechanism of the conductive membrane cathode under the coupling of hydrophobic interactions and electrostatic interactions,a CNT layer was loaded on PVDF-0.22 membrane and its interface wettability was regulated to obtain hydrophilic(water contact angle of 33±2°)and superhydrophobic(water contact angle of 180°)conductive membrane cathodes,they were further applied to treat the charged hexadecane emulsions for their fouling performance and mechanism.For the negative charged oil droplets stabilized by anionic surfactants,the conductive membrane cathode declined by 10～20%at 3.0 V.For the positively charged oil droplets stabilized by cationic surfactants,the hydrophilic membrane flux decreases by 33%and the hydrophobic membrane flux loss<5%with the cell potential at 3.0 V.With the increased hydrophobicity of the membrane,oil droplets tend to change from membrane surface fouling to membrane pore fouling.The proposed mechanism could be a useful rule for the design of conductive membrane.Statistical analysis showed that the contribution of electrostatic interactions to regulate membrane fouling behavior should not be ignored(36%),which was slightly lower than hydrophobic interactions(the explanatory degree was about 50%).The results of this project are of great significance to the mechanism of membrane fouling prevention and control,and provide a scientific basis for the preparation of new anti-fouling materials and the development of anti-fouling technology originating from electrostatic interaction in future membrane technology.