Designable Fabrication And Desalination Performance Of Ceramic-based Forward Osmosis Membranes

With exponential growth in world population and rapid development of economics and industrialization,water resources scarcity and water pollution have been increasingly aggravated in recent years.As a new membrane separation technology,osmotically-driven forward osmosis?FO?has great prospects in the desalination and wastewater treatment since it offers potential advantages of great tolerance to membrane fouling,high available water recovery and low energy consumption,in addition to its nearly complete rejection to a wide range of contaminants.Nevertheless,the existing organic membranes often suffer from their low mechanical strength,insufficient thermal and chemical stability,and poor hydrophilicity,which prevent their uses for more extensive applications in FO process,especially in high pressure,acid and alkali,organic solvents and other harsh applications.Therefore,research on the structural design and performance enhancement of novel FO membrane is urgently pursued for practical application.The selection of membrane material and design of membrane structure are highlighted to form a high-quality selective layer and enhance FO performance.A high-performance ceramic-based thin-film composite?TFC?FO membrane with low-cost and stable hollow fiber mullite ceramic as the membrane substrate was prepared by engineering a novel nano-composite interlayer of titanium dioxide and carbon-nanotube?TiO₂/CNT?.The structural morphologies and properties were systematically characterized for different substrates?without interlayer,with TiO₂ interlayer,or with TiO₂/CNT interlayer?and the corresponding ceramic-based TFC-FO membranes,which focused on the nano-composite interlayer affecting on the structure and performance of the membrane.A formation mechanism of the polyamide?PA?selective layer and water and reverse salt transport mechanism was also proposed.The main conclusions are shown as follows:First,with low cost bauxite and fly coal ash as starting materials,mullite hollow fiber membrane substrate was prepared,which exhibited a typical asymmetric macro-porous structure.Effects of sintering temperature on membrane surface morphology,pore size,porosity and mechanical strength were investigated systemically.The results show that when sintering temperature was 1250°C,the mullite substrate with finger-like macro-voids?74%?,moderate pore size?475 nm?,high porosity?22%?,excellent hydrophilcity?29°?and high mechanical strength?58 MPa?can be attained.Then M-PA membrane with the hollow fiber membrane substrate was fabricated.The effect of mullite substrate on the structure and performance of M-PA membrane were investigated.From SEM image results,it was observed that the PA layer of M-PA membrane had great thickness and larger pores?possibly defects?in its surface due to the mullite substrate with very large surface pores?475 nm?and roughness?R_a=132±33 nm?.In the PRO mode,the M-PA membrane exhibited a water flux of 10.6±1.4 L/m² h and the reverse salt flux of M-PA membrane was 15.9±0.7 g/m² h,which may suggest that the surface properties of the mullite substrate are not conducive to the formation of thin and defect-free PA layer.A ceramic-based TFC-FO membrane with TiO₂ interlayer?denoted as M-T-PA?was prepared for forming a defect-free and thin PA layer.The effect of TiO₂ interlayer on the substrate,PA layer and performance of M-T-PA membrane was investigated.The surface pore size and roughness of the mullite substrate with TiO₂ interlayer were decreased after introduction of TiO₂ interlayer.A thin and less defect PA layer was formed on the TiO₂interlayer,which resulted in the improvement of FO performance.In the PRO mode,the M-T-PA membrane exhibited a water flux of 18.6±1.7 L/m² h and the reverse salt flux of M-T-PA membrane was 7.7±0.5 g/m² h.Besides,the S value of M-T-PA membrane is lower than M-PA membrane.The results show that introduction of TiO₂ interlayer on mullite substrate was not only able to finely tailor desirable properties such as surface pore size and roughness to provide a versatile platform for PA formation via interfacial polymerization reaction,but also decreased S value and thereby reducing internal concentration polarization?ICP?effect.A ceramic-based TFC-FO membrane with TiO₂/CNT interlayer?denoted as M-T/CNT-PA?was prepared to further improve FO performance of TFC-FO membrane.The effect of nano-composite TiO₂/CNT interlayer on the substrate,PA layer and the performance of M-T/CNT-PA membrane was investigated,and the effect of CNT thickness and various process parameters on the membrane performance were studied.The results show that the TiO₂/CNT interlayer further decreased the surface pore size and roughness of mullite substrate.This results in the formation of much thinner PA layers featuring less defects,larger effective filtration area and better selectivity on the TiO₂/CNT interlayer.At the optimum condition of membrane fabrication,the M-T/CNT40-PA membrane exhibited a water flux of 28.5±1.8L/m² h and a reverse salt flux of 7.4±0.6 g/m² h.Finally,a formation mechanism of the PA layer and water and reverse salt transport mechanism was proposed.Mechanism analysis indicates that a special nano-composite interlayer not only provided more active cites for the formation of thinner defect-free nano-voids-containing PA layer,but acted as a highly porous three dimension network structure for rapid water transport,which resulted in the greatest improvements in FO performance.This paper provides a novel protocol for rational design and fabrication of high performance inorganic FO membrane as well as environmental engineering applications with enhanced performance.Considering that the ceramic-based FO membranes offer excellent chemical stability and mechanical strength,they are expected to be further developed and used in harsh applications?such as pressure retarded osmosis and organic solvent filtration?.