Construction Of Hydrophilic/Hydrophobic Janus Membrane Distillation Membrane And Its Application In Rare Earth Wastewater Treatment

A large amount of high-salt wastewater generated in the process of rare earth smelting and separation urgently needs reasonable and effective treatment and disposal.Membrane distillation（MD）is a thermal membrane coupling separation technology with hydrophobic porous membrane as the physical barrier,which has wide application potential in the field of high-salt wastewater treatment and reuse and is expected to play an important role in the treatment of rare earth high-salt wastewater.However,problems such as low membrane flux,easy membrane fouling and wetting have always limited the development and application of membrane distillation technology.Hydrophilic modification of membrane surface can effectively improve the fouling resistance/wettability of membrane distillation membrane,but often at the expense of membrane distillation flux,that is,there is a trade-off effect between fouling/wetting resistance and membrane distillation flux.In addition,existing surface hydrophilic modification strategies still have the disadvantages of complex and time-consuming processes.Therefore,it is of great significance and practical value to develop a convenient and fast hydrophobic membrane surface hydrophilic modification strategy to prepare membrane distillation membranes with high flux and fouling/wetting resistance.In order to break through the trade-off effect and rapidly prepare high-performance membrane distillation membranes,this thesis proposes a new strategy for the construction of hydrophilic/hydrophobic composite membranes based on an improved interfacial polymerization method and systematically investigates the structure-effect relationship of the produced membranes.（1）A modified interfacial polymerization method（mIP）was proposed to construct ultrathin polyamide（PA）hydrophilic layers on the surface of hydrophobic porous polytetrafluoroethylene（PTFE）membranes in one step and quickly,and the prepared PA@PTFE hydrophilic/hydrophobic Janus membranes combined high flux and excellent resistance to oil fouling.The water contact angle（WCA）in air and oil contact angle（OCA）underwater were measured before and after the modification,and the surface of PA@PTFE membrane showed hydrophilic in air and oleophobic under water（WCA and OCA of 77.6±2.4°and 112.5°,respectively）compared with the pristine PTFE membrane which was hydrophobic in athe ir but oleophilic under water（WCA and OCA of 128.5±1.9°and 72.5±1.1°,respectively）.The state of confined water on the surface of PA@PTFE membrane was analyzed by Raman spectroscopy and identified that its PA layer contains both free water（FW）and intermediate transition state water（IW）with low evaporation enthalpy,and the molar ratio of IW to FW reached 1.02.The vacuum membrane distillation（VMD）flux of the pristine PTFE membrane was 13.96±0.62 kg/（m²·h）when treating a 3.5 wt%aqueous NaCl solution at a vacuum of 90 k Pa（60°C）,while also maintaining a desalination rate of more than99.9%due to the presence of low evaporation enthalpy IW,the flux of PA@PTFE increased by 122.3%to 31.04±1.04 kg/（m²·h）.In contrast,the PA@PTFE membrane showed a stable desalination performance with an initial flux of 30 kg/（m²·h）and a flux drop of only 16.3%after 50 hours,and In contrast,the PA@PTFE membrane demonstrated a stable desalination performance with an initial flux of 30 kg/（m²·h）,a flux decrease of only 16.3%after 50 hours,and a total desalination rate of more than99.9%.（2）An ultrathin,hydrophilic,and defect-free dense PA layer was rapidly constructed on the surface of the hydrophobic porous PTFE membrane by a two-step mIP process,and the prepared IP²@PTFE composite membrane was simultaneously high flux,resistant to oil fouling and to wetting by the surfactant sodium dodecyl sulfate（SDS）.Compared with the PA@PTFE membrane prepared by one-step mIP method,the surface of IP²@PTFE membrane exhibited stronger hydrophilicity in air and oleophobicity under water,with WCA and OCA reaching 50.2±3.7°and 141.73±5.2°,respectively;meanwhile,the PA layer on the surface of IP²@PTFE membrane was more dense,smooth and free of obvious defects,where IW/FW was further increased to 1.13.Under the same conditions,the VMD flux of IP²@PTFE membrane reached38.72±1.97 kg/（m²·h）,which was increased by 24.7%and 177.4%than that of PA@PTFE membrane and pristine PTFE membrane,respectively.when treating NaCl solutions containing SDS（0.1-0.4 m M）,IP²@PTFE membranes exhibited When treating NaCl solutions containing both SDS（0.1-0.4 m M）and mineral oil（500 ppm）,IP²@PTFE membranes showed excellent resistance to fouling and wettability,while membrane wettability occurred rapidly for both PA@PTFE membranes and PTFE pristine membranes.By examining the retention rate and liquid penetration pressure of the membrane,we hypothesize that the wettability of IP²@PTFE membrane should be the result of the synergistic effect of size exclusion and capillary effect provided by its dense hydrophilic PA layer.（3）The IP²@PTFE membrane was used to treat the simulated rare earth high-salt oily wastewater by VMD operation,in which the feed solution contained 50 g/L NaCl,15 g/L CaCl₂,0.1 g/L MgCl₂,0.03 g/L P507,and 0.04 g/L kerosene.The desalination rate of IP²@PTFE membrane remained stable during the 30h continuous treatment process,and the membrane flux only decayed by 31.1%at the end,indicating its good potential for application in the field of rare earth high-salt wastewater treatment and reuse.This thesis shows that an improved interfacial polymerization method can easily and quickly construct ultra-thin and defect-free hydrophilic PA coatings on the surface of conventional hydrophobic porous membranes to obtain membrane distillation composite membranes with high flux,oil fouling resistance,and SDS wetting resistance.The results of this thesis are expected to provide new perspectives and methods for the preparation of high-performance membrane distillation membranes for rare earth high-salt wastewater treatment applications.