Development of a smart plasma-treated nano-filter using carbon nitride nanostructures for oil–water emulsion separation: experimental and theoretical studies

Abstract

In response to global concerns over oil and water separation, the research and development of super-wetting filters with high permeation flux has increased. In this study, an oxygen plasma treatment was applied to modify the surface of bulk-graphitic carbon nitride (b-CN) nanoparticles (g-C₃N₄) and carbon nitride nanosheet (CN-NS) coated stainless steel meshes to achieve highly efficient oil/water separation. The coated filters were characterized by X-ray diffraction, and several spectroscopy and microscopy techniques, in order to discern the structural changes, as well as the molecular changes, that is, the functional groups induced on the surface. Under 5 minutes of O₂ plasma treatment, b-CN coated meshes achieve outstanding oil/water separation in concentrated mixtures, with an approximately 225-fold increase in permeate water flux of 34 394.9 L m⁻² h⁻¹ compared to untreated mesh. In contrast, the water separation flux changes slightly for CN-NS mesh using plasma treatment. The plasma-treated b-CN coated mesh also works efficiently for oil-in-water emulsions. By alternating plasma and heat treatments, the b-CN coated mesh can act as an intelligent filter, and switch between super-hydrophilicity and hydrophobicity. This occurs because the plasma and heat treatments induce the creation and loss of surface functional groups (mainly OH), respectively. Furthermore, by alternating plasma and heat treatments, it is possible to switch between super-hydrophobicity and initial water wettability of the coated mesh, enabling us to handle both options (water- and oil-removal) in one single separation device. Density Functional Theory (DFT) calculations reveal that the presence of the adsorbed OH, COOH, and NH₂ groups induces the breaking of C–N bonds linking triazine rings on the surface of the C₃N₄ nanostructures. As a result of the defect created by the scission of the bond, one C atom and two N atoms of the triazine rings bonded to the functional groups protrude above the layer, and the vertical distances of the functional groups to the g-C₃N₄ layer increase substantially. The protruding functional groups then build a physical barrier preventing the approach of large hydrocarbon molecules to the surface of the filter, but allowing for water wetting.