Structural evolution of nickel oxide silica sol-gel for the preparation of interlayer-free membranes

• Interlayer-free nickel oxide silica membranes for desalination applications
• Sols prepared with and without H2O2 formed micro and mesoporous structures.
• Higher silanol/siloxane ratio for sols prepared with H2O2, and smaller pore sizes.
• Membranes gave high water fluxes and salt rejection up to 7.3 kg m− 2 h− 1 and 99.9%.
• Membrane with H2O2 sols gave lower water fluxes and higher salt rejections.

This work investigates a modified sol-gel method for the preparation of interlayer-free nickel oxide silica membranes for desalination applications. The sol–gels were synthesized using TEOS, nickel nitrate hexahydrate, ethanol as solvent and water with and without peroxide (H2O2). The effect of the nickel embedded in the silica matrix as Ni/Si molar ratio was varied from 5 to 50 mol% and systematically studied. The sols prepared with H2O2 resulted in microporous structures and lower pore volume, contrary to the mesoporous structures derived from sols without H2O2. The modified sol-gel method proved to be robust enough for coating directly on α-alumina substrates, as opposed to conventional methods which required substrates with interlayers. All interlayer-free nickel oxide silica membranes delivered high salt rejection ranging from 91.5 to 99.9%, and reaching water flux as high as 7.3 kg m− 2 h− 1. The membranes prepared using sols with H2O2 gave lower water flux and slightly higher salt rejection, attributed to lower pore volume and smaller pore size, respectively. The membranes prepared with Ni/Si molar ratio of 25% achieved the highest water flux, though salt rejection slightly decreased with the increase of feed salt concentration from brackish (NaCl 0.3 wt%) to sea water (NaCl 3.5 wt%).