High performance interlayer-free mesoporous cobalt oxide silica membranes for desalination applications

• Acid–base catalysed sol-gel method to prepared cobalt oxide silica membranes • Interlayer-free deposition directly onto macroporous ceramic substrates • High water fluxes of 31.5 kg m− 2 h− 1 and N 90% salt rejection for seawater at 60 °C • Increasing the cobalt loading from 0 to 35 mol% improved silica membrane stability. • Membranes also performed well for high NaCl concentrations of 7.5 wt.%.

By employing an acid–base catalysed sol-gel method, cobalt oxide silica membranes were successfully deposited directly onto the macroporous ceramic substrates without any interlayer, which is an attractive research avenue. The effect of cobalt loading ranging from 0 to 35 mol% into the silica matrix was investigated for the physical and chemical properties of xerogels followed by membrane morphology and desalination performance. Silanol concentration was promoted when cobalt concentration was increased due to the shift of sol pH to a more acidic region of ~4.44 (initial sol pH ~ 6) which accelerated the hydrolysis reaction. Additionally, the formation of tricobalt tetroxide was found to systematically increase with increasing cobalt loading with a minimum threshold of 5 mol%. Four membranes (Co-1, 5, 20 and 35) were tested in various feed saline solutions (0.3–7.5 wt.%) from temperatures ranging from 22 to 60 °C. The water fluxes for seawater feed concentrations increased with temperature from 5.4 to 31.5 (Co-1), 1.3 to 15.8 (Co-5), 0.6 to 25.8 (Co-20) and 4.6 to 11.3 kg m−2 h−1 (Co-35) with high salt rejections (90 to 99%), except for the Co-1 membrane which failed at 60 °C with 24.6% salt rejection. This was attributed to the high concentration of non-hydrostable silica, whilst increasing the concentration of tricobalt tetroxide led to the formation of stable silica derived membranes. In addition, the membrane film structure contains a well-dispersed tricobalt tetroxide phase within the silica matrix, forming mesoporous percolative pathways which were able to achieve excellent separation of the hydrated salt ions in all the testing conditions. Therefore, the formation and concentration of tricobalt tetroxide within the silica matrix provided overall structural stabilising enhancement for the preparation of interlayer-free silica derived membranes, with superior stability and performance.