Low-cost Preparation And Separation Performance Of Spinel Ceramic Membrane For Oily Wastewater Treatment

In recent years,efficient treatment techniques have attracted increasing attention for oily wastewaters,which are produced from domestic and industrial processes such as petrochemical,textile,pharmaceutical and metal processing.Ceramic membranes separation technologies exhibited unique merits such as high separation efficiency,facile operation,low maintenance cost and environmental sustainability,especially higher thermal,chemical and mechanical stabilities and lower fouling propensity,extending their applied range.Hazardous heavy-metal-bearing waste sludges,as water treatment by-products,pose greater threat to our ecological environment if discharged directly,which urgently needs to be safely recycled with high value added.It remains critically challenging for highly efficient recycling of hazardous solid-state sludges and economic treatment of wastewaters in one protocol.Herein,we proposed a waste-to-resource strategy for rational design of spinel ceramic membranes,simultaneously addressing the issues of heavy metal sludge recycling and oily wastewater treatment.The spinel ceramic membranes were prepared by dry/wet spinning technology combined with high temperature thermal conversion methods.Separation performance of oil-in-water（O/W）emulsions was systematically investigated at various emulsion properties（emulsion types and pH values）.First,an almost complete conversion of ZnAl₂O₄was achieved.The oil-water separation performance of ZnAl₂O₄hollow fiber membranes was studied preliminary.Secondly,a super-hydrophilic CuAl₂O₄membrane was prepared from simulated copper-containing sludge with optimized membrane structure,properties and permeability.Also,the thermal conversion process mechanism of spinel phase was proposed.Separation performance of O/W emulsions was systematically investigated at various emulsion properties.Moreover,separation performance of real oil wastewater was studied.Finally,spinel membranes were prepared with actual sludge as raw materials,and then tested for not only synthetic oily emulsion but real oily wastewater.（1）Preparation of ZnAl₂O₄ hollow fiber membrane and its oil-water separation performance.ZnAl₂O₄hollow fiber membranes were prepared by bauxite and ZnO（simulated zinc sludge）via dry-wetting spinning technique combined with subsequent sintering.The permeability,mechanical properties,surface morphology and chemical properties of the optimized spinel membrane were also characterized.The completed phase conversion of ZnAl₂O₄membrane was obtained at 1200°C,with proper pore size（0.525μm）,water flux（～14773 L/（m²·h·bar））,mechanical strength（～20 MPa）and hydrophilicity（water contact angle（WCA）=20.1°）.Stable permeability（～73 L/（m²·h·bar））and high flux recovery rate（～100%）were realized for ZnAl₂O₄membrane during O/W emulsion separation.（2）Preparation and characterization of CuAl₂O₄ hollow fiber membrane.We first elucidated the formation mechanism of acid-stable spinel phase via qualified and quantified X-ray diffraction（XRD）techniques during membrane preparation using a mixture of synthetic copper-laden sludge（CuO）and naturally abundant bauxite mineral.The CuAl₂O₄spinel rapid formation in the first stage（800°C and 970°C）,while CuO and Al₂O₃were simultaneously decreased.With sintering temperature increasing,an almost complete conversion of CuAl₂O₄spinel was achieved in the second stage（970-1060°C）.The minor decomposition of CuAl₂O₄spinel occurred in the third stage（1060-1100°C）.Results indicated that CuAl₂O₄was steadily formed through the reactions between copper oxide and alumina（major）or mullite（minor）at 970-1060°C.Also,spinning and sintering parameters were systematically studied to rationally tailor membrane structure,and properties such as morphology,porosity（～22.35%）,pore size（～0.6μm）,water permeability（～8525.15L/（m²·h·bar））and mechanical strength（～32 MPa）.With rational structure design,the CuAl₂O₄membrane surface showed superhydrophilicity（WCA=0.98°）and underwater superoleophobicity（UWOCA=156°）,endowing good hydrophilicity and consequently anti-fouling properties of spinel ceramic membrane for O/W emulsions separation.（3）Separation performance of CuAl₂O₄ hollow fiber membranes for O/W emulsions.Three different types of O/W emulsions were used to explore the separation performance of CuAl₂O₄membrane.The results showed the membrane exhibited high flux recovery rate（～100%）and oil rejection（>94%）for all the O/W emulsions.The spinel membranes exhibited enhanced permeate flux with mitigated membrane fouling especially for larger oil droplets or lower viscosity oil.The performance under different pH conditions showed that the membrane was more suitable for alkaline condition（pH=8-10）.Even for higher viscosity oils（i.e.,engine oil）,permeate flux（～188-210 L/（m²·h·bar））was enhanced with mitigated fouling at alkaline condition.Moreover,sufficient rejection（94.36%）and flux（37-47 L/（m²·h·bar））were achieved for the CuAl₂O₄membrane when treating real alkaline oily wastewater.（4）Preparation,characterization and oil-water separation performance of hollow fiber membrane made from actual copper-containing sludge.The CuFe₂O₄hollow fiber membrane was prepared from actual heavy metal sludge and Fe₂O₃.Then phase inversion process was studied at different sintering temperatures,where the CuFe₂O₄spinel membrane achieved complete conversion at 900-1100°C.The water flux of CuFe₂O₄spinel membrane sintered at 1000°C was 320 L/（m²·h·bar）with average pore size of 0.43μm.The high oil rejection（99.69%）and permeate flux（177 L/（m²·h·bar））,demonstrated the effective separation for O/W emulsion.In addition,the membrane was used to efficiently separate actual oily wastewater.