Design And Performance Of Whisker-structured Mullite-based Ceramic Membranes For Water Treatment

With the rapid development of population and industrialization,freshwater resource shortage and water pollution become critical environmental challenges.State economy and energy rely on petrochemical industry,where wastewater is produced from its production processes with key components such as emulsified oil and inorganic salt.If not well treated,it will pose a serious threat to the environment and human beings.Inorganic ceramic membrane separation technique could be used to treat the characteristic pollutants such as emulsified oil and inorganic salt in petrochemical wastewaters due to its advantages such as simple operation,good separation efficiency,as well as excellent mechanical,thermal and chemical stability of ceramic membranes.In spite of promising robustness and separation performance,current ceramic membranes still suffer from some technical bottlenecks such as insufficient permeability and high cost.Therefore,for efficient removal of emulsified oil and inorganic salt,this work proposed a novel protocol for rational design and application of whisker-structured ceramic membranes.Firstly,we fabricated mullite whisker-structured membranes.Different from conventional ceramic membranes,such membranes with lower processing cost（sintering cost）exhibited higher porosity and permeability.Particularly,enhanced underwater-oleophobicity was realized for this novel structure with higher surface roughness,improving the possibility of anti-oil-fouling ability for the treatment of oily wastewaters via pressure-driven filtration.Membrane fouling models were further employed to understand the complex interaction between oil droplets and membrane surface at various conditions.Then,carbon nanotube networks in situ was constructed on mullite whisker-structured ceramic membrane surface via chemical vapor deposition.The super hydrophobic mullite-CNTs composite membrane showed a good desalination performance in membrane distillation application,providing a broad application prospect in wastewater treatment applications for this novel whisker-structured membrane.The main research topics,results and conclusions are as shown as follows:（1）Design and preparation of whisker-structured mullite ceramic hollow fiber membrane.Whisker-structured mullite ceramic hollow fiber membrane with"long finger-like pore and thin-sponge layer"was prepared with natural mineral bauxite and industrial waste fly ash as raw materials and 20 wt.%tungsten oxide as additive.The surface morphology,phase composition,surface properties,pore size distribution,water/gas permeability,porosity were compared between traditional particle-packing-structured（W0）and novel whisker-structured（W20）membranes at various sintering temperatures（1100-1500℃）.The addition of tungsten oxide during membrane fabrication not only accelerated the formation of more mullite phase at lower temperature,but resulted in the enhancement of porosity,pore size,N₂ permeance and water permeability（especially at higher temperature）.At the same time,enhanced surface roughness of such whisker-structured membrane exhibited a better hydrophilicity in air（water contact angle:12.0±1.6°）and underwater-oleophobicity（oil contact angle:146.3±2.9°）,showing a stronger ability of repelling oil droplets（i.e.,better anti-oil-fouling property）during O/W emulsions separation.（2）Separation performance and membrane fouling mechanism of whisker-structured mullite membrane.The whisker-structured mullite membrane sintered at 1200℃（average pore size:0.7μm）was used for the separation of oily wastewater,which can be removed by a pressure-driven microfiltration process.The results showed that it exhibited good permeability and oil rejection for the oil-water emulsion treatment under different emulsion properties and operating parameters.After simple backwashing operation,the initial permeability flux of the membrane could be recovered by 80-90%,indicating its good regeneration ability.Under optimum operating conditions(cross-flow velocity:1.02m·s^-1,trans-membrane pressure:0.15 bar),a steady permeability of 880 L·m^-2·h^-1·bar^-1 was obtained with high oil rejection（97%）,which were higher than most existing ceramic membranes in reported literatures.Even for real acidic oily wastewater with high oil concentration,they also showed promising permeability(steady permeability:180 L·m^-2·h^-1·bar^-1)and oil rejection（98.2%,risk-free discharge）,but lower energy consumption.In addition,the classical membrane fouling model fitting results showed that cake filtration model dominated membrane fouling for the treatment of high concentration O/W emulsions at high cross-flow velocity and trans-membrane pressure.In comparison,combination of two models dominated membrane fouling for treatment of low oil concentration O/W emulsions at low cross-flow velocity and trans-membrane pressure.The flux variation at various p H values might be attributed to the electrostatic interaction between the oil droplets and membrane surface.（3）Fabrication and desalination performance of mullite-CNTs composite membrane.For inorganic salt in petrochemical wastewaters,membrane distillation process was used for its efficient separation to achieve zero-liquid discharge.Considering the mechanical strength,whisker-structured mullite membrane sintered at 1500℃was selected as the support,and CNTs were in situ constructed on mullite support via chemical vapor deposition.The obtained superhydrophobic mullite-CNTs composite membrane presented a good liquid entry pressure （1.13 bar）.With the transmembrane temperature difference of 50℃,the feed velocity of 0.77m·s^-1,it achieved stable permeate flux of 13 L·m^-2·h^-1 and 6-7 L·m^-2·h^-1 for 3.5 wt.%（simulated seawater）and 7.0 wt.%（high salinity wastewater）,respectively,while the flux and salt rejection（99.9%）also were stable.The whisker-structured ceramic membrane fabricated in this study showed low-cost,high permeability and anti-oil fouling ability for microfiltration process due to the formation of whisker interlocking structure and enhancement of surface roughness.After superhydrophobic modification,the obtain composite membrane also showed good anti-wetting ability and permeability in membrane distillation.Both good performance and stability were achieved for separation of emulsified oil wastewaters and hypersaline wastewaters.Membrane application strategy proposed in this work can also be extended to other water treatment or more microfiltration separation applications.