One-step preparation of high-performance bilayer α-alumina ultrafiltration membranes via co-sintering process

Compared to the traditional separation technology, membrane separation is particular beneficial for the treatment of contaminated fresh water such as agricultural runoff, and hydraulic fracturing wastewater. However, the development of cost-effective membranes with both high permeability and high rejection performance is still a challenge. In this study, we design one-step preparation of high-performance bi-layer α-alumina ultrafiltration (UF) membranes supported on coarse tubular substrates via co-sintering process. In this approach, boehmite sol and alumina nanoparticles were mixed in different ratios for the fabrication of microfiltration (MF) layer and UF layer, respectively. The effects of dip-coating dispersion formulation, coating time and thermal treatment process on the membrane microstructure and performance were comprehensively investigated. It was found that the presence of boehmite sol in the MF layer could accelerate its sintering process, while the addition of alumina nanoparticles into UF layer could promote the phase transition from γ phase to α phase at low temperature and retard the shrinkage. The optimized doping content of boehmite and alumina nanoparticles were 2 wt% and 4 wt% to achieve a good match of thermal expansion properties between the two co-sintering layers. For MF layer and UF layer, the membrane thickness were controlled to be 40–50 µm and about 1 µm, respectively. After co-sintering at 1000 °C for 2 h with a heating rate and cooling rate of 0.5 °C/min, tubular α-alumina UF membranes were found to display a high pure water permeability of 70 L m⁻ h⁻¹ bar⁻¹ and a small pore size of ~5 nm, which also exhibited good dye removal capabilities in the process of treating dye waste water.