Preparation And Characterization Of Al₂O₃ Ultrafiltration Membranes

In recent years,ceramic membranes have been widely used in the fields of dye retention and gas separation because of their unique properties,such as high mechanical strength,good permeability,high permeability,thermal stability,acid and alkali corrosion resistance and long life.Alumina ultrafiltration membrane was usually prepared by the traditional aluminum alkoxide hydrolysis process,but this method was not conducive to the low cost,rapid and large scale alumina ultrafiltration membrane preparation because of some disadvantages,such as high raw material cost,complex process,long reaction time and long production cycle.In this work,Al OOH sols were prepared conveniently and quickly at room temperature by using industrial pseudo-boehmite powder（Al OOH·x H₂O）as the precursor and nitric acid as the solvent with colloidal sol-gel route.The Al OOH sols were coated on the porous alumina ceramics and porous alumina ceramic microfiltration membranes by dip coating method.After drying and calcination,γ-Al₂O₃ ultrafiltration membranes with intact microstructure and uniform pore size were obtained.The properties of Al OOH sols andγ-Al₂O₃ultrafiltration membrane were analyzed by rotary viscometer,nano-laser particle size analyzer,field-emission scanning electron microscope（FE-SEM）,N₂ adsorption-desorption analyzer（BET）and solute retention method.The main results were as follows:（1）When the H⁺/Al³⁺molar ratio was 0.2～0.25 and the solid content was4%～8%,the Al OOH sols with moderate viscosity and good stability were obtained,which was conducive to prepare sol coating slurry.When the H⁺/Al³⁺molar ratio was 0.2 and the solid content was 8%,the average particle size of Al OOH sol was about 74 nm.（2）The formation process of Al OOH sols was detailedly analyzed.It’s proved that there were a large number of hydroxyl groups on the surface of Al OOH colloidal particles.With nitric acid addition,the nitrate ion was absorbed by the positive charge Al OOH sol core to form the electric double layer structure.With superfluous nitric acid addition the dispersed sol nanoparticles adhered with each other and aggregated.The aggregated sol particles formed bigger particles and 3-dimension net structure.（3）When the PVA content was 1.25%～1.5%and the solid content was2%～4%in the liquid system,smooth,complete and non-crackedγ-Al₂O₃ultrafiltration membranes can be prepared.When the PVA content was between 1.25%and 1.50%,the thickness of the membrane decreased from3μm to 1.8μm,the MWCO of the membrane was 14.4～41.3 k Da,and the average pore diameter of the membrane was 6.98～11.34 nm.The pure water flux was 9.45～10.33 L·m^-2·h^-1·bar^-1.When the solid content was in the range of 3%～4%,the thickness of the membrane became thicker with solid content.The MWCO of the membrane was 43.4～53.2 k Da.The average pore diameter of the membrane decreased with the solid content increasing,and ranged from 11.59～12.74 nm.The pure water flux was 18.07～28.83L·m^-2·h^-1·bar^-1.（4）Congo red can be effectively rejected by theγ-Al₂O₃ ultrafiltration membrane.The results showed that the retention rates of the membrane after 120 min was still good,and the filtration flux of the membrane for congo red was up to 20.37 L·m^-2·h^-1·bar^-1;After four cycles using,the congo red retention rates was over 95%,the filtration flux was 13.58L·m^-2·h^-1·bar^-1,indicating that the ultrafiltration membrane with good reusability.