Cost-efficient natural apatite–metakaolin supported TiO2 mesoporous membrane for dye removal applications

Abstract

The synthesis of a TiO₂-tight ultrafiltration membrane supported on a low-cost ceramic substrate based on metakaolin and natural apatite was proposed in this study. To optimize the substrate composition, three samples were prepared by combining metakaolin and natural apatite with weight ratios of 25:75 (MK25), 50:50 (MK50), and 75:25 (MK75), and then sintered for 2 h at 850, 900, 950, and 1000 °C. The effect of sintering temperature and metakaolin content on the final properties of the prepared samples was studied in terms of microstructure, porosity, and mechanical strength. According to the results, the mechanical strength increased with increasing temperature and metakaolin content, but at the same time, the porosity decreased. MK50 composition and the temperature of 950 °C were chosen as optimal conditions, and the support was prepared by extrusion. The prepared substrate presented a compressive strength of 4.4 MPa, porosity of 33%, and an average pore size of 2.2 μm. To prepare a crack-free ultrafiltration membrane, the deposition of the TiO₂ top layer was favoured over an alumina intermediate layer. The obtained membrane has a pore size of 6.8 nm and water permeability of 5.6 L h⁻¹ m⁻² bar⁻¹. The membrane performance was tested for the removal of cationic and anionic dyes. The ultrafiltration experiments have shown a high removal rate for anionic and cationic dyes. Specifically, it was found that the removal rates of these dyes exceeded 75% without adjusting the pH of the solution.