Facile fabrication of super-wettable mesh membrane using locally-synthesized cobalt oxide nanoparticles and their application in efficient gravity driven oil/water separation

A functional cobalt (II, III) oxide (Co₃O₄) nanoparticle coated stainless steel mesh membrane for the application of oil-water separation was fabricated by facile spray coating, where the coating material Co₃O₄ nanoparticle was locally synthesized by co-precipitation method. The fabricated membrane exhibited the unique surface wettability of in-air superhydrophilicity with static contact angle, θ_WA= 0°, in-air superoleophilicity with static contact angle θ_OA= 0°, and underwater superoleophobicity with static contact angle θ_OW= 160°. The contrasting water and oil wettability of the membrane surface is quite congenial for the separation of oil and water in immiscible oil-water mixture. When this membrane was used as a filtering medium in gravity driven oil water separation system, oil phase retained on the feed side of the membrane, while the water phase permeated through the membrane. In this process, the oil-water separation efficiency as high as 99 % was achieved, which is quite significant considering the simplicity of the membrane fabrication. Also, the membrane remained quite intact in terms of retaining the same level of separation efficiency even after 10 consecutive oil-water separation cycles, which shows the mechanical, structural and chemical robustness of the membrane. The results of this oil-water separation process is explained in the light of the observed surface wettability, that emerge from the three different interfacial energies and hierarchical surface roughness brought about by the spray coating. FESEM images of the membrane surface confirmed the uniform distribution of Co₃O₄ nanoparticles with average particle size of ∼ 30–50 nm, and the XRD pattern revealed the typical rhombohedral crystal shape of Co₃O₄. The elemental and chemical compositions of the surface were further substantiated by XPS and FTIR spectra respectively.