The recent exponential growth of nanotechnology and numerous applications of nanotechnology-based products resulted in water pollution by engineered nanoparticles. Over the last few decades, membrane technology has emerged as one of the most promising and reliable techniques in water purification. Therefore, it is an obvious candidate to remove manufactured nano-sized contaminants and to purify the water. Nanoparticle properties play a crucial role in the performance and effectiveness of membrane filtration. This experimental study investigates the role of nanoparticle size and polydispersity on fouling and rejection development during dead-end microfiltration of electrostatically stabilized silica nanoparticles. Our work on filtration of monodisperse silica nanoparticles (11 nm, 25 nm and 92 nm) smaller than the membrane pore size (~200 nm) demonstrates that an increasing nanoparticle diameter accelerates pore blockage and development of cake. The specific cake resistance of the filtration cake formed decreases with increasing nanoparticle diameter. Filtration of polydisperse nanoparticles (obtained by mixing monodisperse suspensions in various ratios) shows that increasing the fraction of smaller nanoparticles results in delayed pore blockage, and cake filtration occurring at a later stage. The specific cake resistance of the polydisperse nanoparticles is always found to be in between that obtained for the monodisperse nanoparticle suspensions. An increasing weight fraction of larger nanoparticles results in faster development of nanoparticle rejection due to accelerated pore blockage. However, because of the highly porous structure of the filtration cake originating from strong surface charges, the moderate transmembrane pressure applied and cake imperfections, the smallest (11 nm) nanoparticles were rejected only to a low extent, even during the cake filtration stage. An increase in applied transmembrane pressure during filtration of the polydisperse suspension resulted in faster pore blockage and higher specific cake resistance. Nevertheless, rejection of the nanoparticles in the cake filtration stage improved only slightly with increasing transmembrane pressure.
- Nanoparticle size