Effect of rheological properties of AlOOH sol on the preparation of Al2O3 nanofiltration membrane by sol-gel method

In order to directly prepare an High Flux Al2O3 nanofiltration membrane on an Al2O3 support with an average pore size of 4 μm, AlOOH sol was prepared with aluminum isopropoxide as the precursor, The effect of rheology on the dip-coating of AlOOH sol and the effect of viscoelasticity on the heat treatment of AlOOH gel film to prepare defect-free Al2O3 nanofiltration membrane were studied. The results indicate that AlOOH sol will increase its viscosity with the increase of the standing time. When the viscosity increases to a certain extent, the colloidal particles will gradually transform into gels, and change from Bingham fluid to Herschel-Bulkley pseudoplastic fluid. The thickness of the AlOOH gel film is related to the viscosity of the AlOOH sol. The flow viscosity of AlOOH sol should be about 0.0025∼0.005 Pa·s, while the thickness of the AlOOH gel film after dip-coating is about 6.5∼12 μm. The storage modulus and loss modulus of AlOOH gel film increase with the increase of aging time. Only when the storage modulus of the AlOOH gel is greater than the saturated vapor pressure of the solvent under normal pressure (0.1 MPa), it will not crack due to the evaporation of the sol during the heat treatment process. After the storage modulus exceeds 0.1 MPa, the surface of the heat-treated Al2O3 ceramic membrane is smooth and crack-free, the rejection rate for crystal violet dye is 99.8%, and its average pore size is 2.75 nm, that has the capability of nanofiltration. Due to the lack of intermediate layer, the pure water flux of the Al2O3 nanofiltration membrane is as high as 201.7 l.m-2bar-1h-1, and the steady-state filtration flux is 48.7 l.m-2bar-1h-1 when filtering 20 mg/l crystal violet solution. By controlling the rheological properties of AlOOH sol, a high flux Al2O3 nanofiltration membrane was prepared.