Preparation of alumina membranes comprising a thin separation layer and a support with straight open pores for water desalination

A green tape was formed by phase inversion tape casting using an alumina slurry and a graphite slurry. After firing the graphite layer was burned out, and a ceramic membrane was obtained consisting of a two-layered structure: a skin layer of thickness ~8 µm and a finger-like bulk layer of thickness ~730 µm. The skin layer contained randomly distributed pores with diameter 0.8 µm, and thus could act as a functional layer for separation, while the bulk layer contained finger-like straight open pores with diameter 10–100 µm, acting as a support with low transport resistance. The overall porosity of the membrane was ~62%, as determined by the Archimedes method, while the porosity of the skin layer was ~25%, as determined by SEM image analysis. For comparison, another membrane was derived from only an alumina slurry resulting a three-layered structure: a finger-like bulk layer sandwiched by a low porosity layer at each side. The two membranes were modified with fluoroalkylsilane (FAS) to convert the surface from hydrophilic to hydrophobic. Water desalination was determined by exposing the FAS-modified skin layer side to a hot aqueous solution of 4 wt% NaCl and sweeping the other side with dry nitrogen. A salt rejection of over 99% was attained for both membranes. A water permeation flux of 21 L m−2 h−1 was obtained for the two-layered membrane with the NaCl feed solution of 90 °C, which was twice as that of the three-layered membrane. The preparation method for the two-layered structure alumina membrane can be generalized for other membrane systems and porous materials.