Nanoparticle separation in cross-flow filtration by introduction of electrophoresis
Date
2014
Authors
Journal Title
Journal ISSN
Volume Title
Publisher
University of Delaware
Abstract
Membrane fouling is a long-existed problem in filtration industries. Techniques to prevent membrane fouling and improve filter performance in cross-flow filtration have been studied since early twentieth century. The research of cross-flow filtration is a milestone in filtration technique development. By applying a flow parallel to the filter medium, deposited particles on the filter can be effectively washed away. However, when particle sizes become small, like nano-sized particles, it will become much more difficult for the shear flow to remove the depositions away. Filter cakes are formed due to concentration polarization. The superimposition of an external electrical field inside the cross-flow filtration system has been proved to be able to greatly enhance the flux and particle removal efficiency. Cross-flow electro filtration (CFEF) is a hybrid separation process combining both the features of conventional cross-flow filtration and electrophoretic separation apparatus. The technique of CFEF has been applied for separation of nano-sized particles from liquids due to its high selectivity and independence of special membranes although fouling will still occur on the membrane, depending on the conditions applied in different experiments. The major goal of this research is to minimize membrane fouling and maximize nanoparticle removal efficiency. A prototype cross-flow electro filtration module has been designed and constructed for the experiment. The CFEF module is consisted of a peristaltic pump, an external tube, a tubular shaped metal net and a concentric rod as electrodes, a circular membrane placed between the two electrodes, and a D.C. power supply connected to the electrodes. Charged particles can be separated depending on their size distribution and surface charge density. Three kinds of particles with different pHzpc and mean sizes: SiO 2 , TiO2 and γ-Al2 O3 are used for this study. The influences of clogging on membrane can be ignored during each experimental running. Results demonstrate that the CFEF system can separate nanoparticles effectively. Particle removal efficiency is highly related to the electric field strength, filtration flow rate and pH of the feed solution. A mathematical model is also developed to quantify the effects of parameters including particle sizes, solution pH, filtration flow rate and electric field strength on membrane performance. The influences of Coulomb forces among particles are also evaluated in this research, which is proved to be an essential factor affecting the removal rate and has not been considered by previous researchers. The model results suit well with experimental data, which proves that the mathematical model is highly reliable. Further, based on the results of model and experiments, it is possible to separate mixture of nanoparticle solution using the CFEF module by adjusting the pH and applied electric field strength.