Influences of internal coagulant composition on microstructure and properties of porous YSZ hollow fibre membranes for water treatment

•Effect of internal coagulant on microstructure of hollow fibre membranes was explored.•Addition of NMP into the internal coagulant decreased the coagulation power greatly.•Highly asymmetric structure was obtained with ⩾90% NMP in the internal coagulant.•YSZ hollow fibre MF membranes with high water permeability were fabricated.•YSZ hollow fibre MF membranes exhibit good oil/water separation properties. Porous yttria-stabilized zirconia (YSZ) hollow fibre membranes were prepared by the combination of phase inversion and sintering technique. The mixtures of pure water/N-methyl-2-pyrrolidinone (NMP) with different NMP content were used as the internal coagulant and tap water was used as the external coagulant. The coagulation power of the internal coagulant was characterized by Hanson solubility parameters. The influences of internal coagulant composition on the microstructure and properties of YSZ hollow fibre membranes were investigated. Results show the addition of NMP decreases the coagulation power of the internal coagulant significantly, and leads to more porous inner surface, diminishing sponge-like layer and larger finger-like pores penetrating from the outer side to the inner surface. When the content of NMP reaches more than 90%, a highly asymmetric structure composed of outer thin skin layer and supporting finger-like structure layer can be obtained. The increase of NMP content in internal coagulant contributes to higher porosity, larger separation pore size, and thus higher pure water permeability and lower bending strength. The YSZ hollow fibre membrane, with pure NMP as internal coagulant, shows an average pore size of 0.58μm, pure water permeability of 10.89m3/(m2hbar) and bending strength of 121.8MPa sintered at 1320°C for 5h. The steady permeate flux in oil/water separation of the prepared hollow fibre membranes decreases with the increase of NMP content, while the oil rejection rate up to 99.5–99.8% shows no obvious change.