Effect of nano-sized Al2O3-particle addition on PVDF ultrafiltration membrane performance
Introduction
Ultrafiltration has been used extensively in many membrane-separation processes, especially in the wastewater treatment field, including oil–water and protein effluent separation [1], [2], [3], [4], [5], [6], [7]. The hydrophilicity of the membrane and its porous structure play important roles in membrane-separation processes. A suitable porous membrane must have high permeability, good hydrophilicity, and excellent chemical resistance to the feed streams. In order to obtain high permeability, membranes should have high surface porosity and good pore structure. An asymmetric membrane is ideal for this purpose. Polyvinylidene fluoride (PVDF) is one material that can form, such asymmetric membranes. This polymer is thermally stable and resistant to corrosion by most chemicals and organic compounds. PVDF-based membranes show outstanding anti-oxidation activities, strong thermal and hydrolytic stabilities, and good mechanical and film-forming properties. PVDF membranes can thus be used in many ultrafiltration processes through various modifications.
Many studies have attempted to improve the performance of PVDF membranes using various techniques, including physical blending, chemical grafting, and surface modifications. The blending of polymers has the advantage of easy preparation using phase inversion. The addition of hydrophilic materials to the dope solution increases the water permeability of a membrane with similar pore size and pore distribution, due to an increase in pore density as well as in the hydrophilicity of the membrane surface and inside the pores [8], [9]. Polymethyl methacrylate (PMMA) is an organic material that is commonly blended with PVDF. The addition of hydrophilic PMMA not only improves the membrane pore size distribution and pore structure, but also enhances its permeation performance without a loss of retention [10], [11]. Additional organic materials that can improve PVDF membrane properties include polymethy acrylate (PMA) [12], [13], polyvinyl acetate (PVAC) [14], and cellulose acetate (CA) [15]. The addition of organic hydrophilic materials can improve some properties of membranes, but reduces membrane strength.
Recent studies of PVDF-blending modifications have focused on blending the polymer with inorganic materials. The addition of inorganic fillers has led to increased membrane permeability and improved control of membrane-surface properties [16], [17], [18]. Inorganic materials that can be blended with PVDF include silica [18], zirconium dioxide (ZrO2) [19], and some small molecule inorganic salts, such as lithium salts [20]. The common feature of these modifications is the addition of a higher proportion of inorganic materials. SiO2–PVDF, ZrO2–PVDF, and LiClO4–PVDF have ratios of inorganic particles to PVDF of 0.2, 1.0, and 0.128, respectively [18], [19], [20]. The PVDF-membrane morphology and elasticity are both significantly affected by the mass of inorganic materials added.
In the present study, PVDF UF membranes were modified by inorganic nano-sized Al2O3 particles. This research aimed to prepare Al2O3–PVDF composite membranes using the phase-inversion method by including a small proportion of Al2O3 particles but effectively improving the membrane performance. The effects of the Al2O3-particle concentration in the casting solution on the membrane hydrophilicity, permeation flux, morphology, mechanical properties, and anti-fouling performance were examined.
Section snippets
Materials
The PVDF used was a commercial product (FR904). Dimethylacetamide (DMAC; >99%, reagent) was employed as the solvent. Alumina (Al2O3) particles (10 nm in size) were added to PVDF solutions. The other additives were sodium hexad-phosphate and polyvinyl pyrrolidone (PVP). A mixture of distilled water and ethanol was used as the nonsolvents for the polymer precipitation.
Membrane preparation
Al2O3–PVDF composite membranes were prepared by the phase-inversion method. Casting dopes were prepared by dissolving the PVDF in
Effect of the addition of nano-sized Al2O3 particles on membrane flux
Membrane fluxes were affected by the addition of nano-sized Al2O3 particles. Fig. 1 shows that increased Al2O3 concentrations led to increased water permeate fluxes, although this trend cease when the quantity of the nano-sized Al2O3 particles reached a certain level. This finding can be interpreted as follows. PVDF is a hydrophobic polymer. Its hydrophilicity can be improved significantly by the addition of nano-sized Al2O3 particles, which have some favorable characteristics, such as
Conclusions
Al2O3–PVDF composite membranes were synthesized using the phase-inversion process. The addition of nano-sized Al2O3 particles did not affect the pore size, pore numbers, or crystal formation of the PVDF membranes. Although the membrane-surface morphology was altered by increased of surface roughness, it had no negative effects on membrane permeability or anti-fouling performance. The permeation-flux increase of the modified membrane was attributed to increases in the surface hydrophilicity and
Acknowledgements
The paper was supported by the 973 National Basic Research Program, the Teaching and Research Award Program for Outstanding Young Teachers in Higher Education Institutions of the Ministry of Education, and the Daqing Science and Technology Bureau, China.
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