Surface Modifications for Antifouling Membranes

TLDR: This Review surveys the latest efforts in which the reduction of irreversible fouling is attempted by the modification of the membrane surface.

Abstract: Fouling is the deposition of retained particles, colloids, macromolecules, salts, etc., at the membrane surface or inside the pore at the pore wall. Fouling reduces the membrane flux either temporarily or permanently. While the initial flux can be restored by washing the membrane or by applying back-pressures to the temporarily fouled membrane, it cannot be restored when the membrane becomes permanently fouled. The main focus of this Review is on the permanent flux decline. The fouling is caused by the interaction between the membrane surface and the foulants, which include inorganic, organic, and biological substances in many different forms. The foulants not only physically interact with the membrane surface but also chemically degrade the membrane material. For example, colloidal particles, such as natural organic matter (NOM), are considered as the main reason for membrane fouling, which could be controlled by the permeation hindrance and electric double layer repulsion. The formation of biofilms with extra-cellular polymeric substances (EPSs) and microbial cells matrix is the example of biofouling.1 Biofilms are developed by the microbial cell adhesion and subsequent colonization on the membrane surfaces through EPS, which may account for 50-90% of total organic carbon. The biofouling could be minimized by periodical washing with chemicals such as sodium hypochlorite solution, but it will result in the simultaneous degradation of the membrane material’s lifetime. It is a severe problem for membranes used in pressure-driven processes such as reverse osmosis (RO), nanofiltration (NF), ultrafiltration (UF), and microfiltration (MF) and also for other membrane processes, seriously hampering the applications of membrane processes. Hence, membrane fouling as well as its reduction has been a subject of many academic studies and industrial research and development efforts since the early 1960s when industrial membrane separation processes emerged. Selection of an appropriate membrane, pretreatment of the process fluid, adjustment of operating design, and conditions are all known to control fouling to some extent. On the other hand, development of absolutely nonfouling membranes seems extremely difficult, if not totally impossible. This Review surveys the latest efforts in which the reduction of irreversible fouling is attempted by the modification of the membrane surface. The separation process by membrane is essentially a surface phenomenon. More specifically, the skin layer or top surface layer plays the vital role. Therefore, it is a natural consequence to modify membrane surface for reducing the fouling. It is generally accepted that an increase in hydrophilicity offers better fouling resistance because protein and many other foulants are hydrophobic in nature. Most nanofiltration membranes are electrically charged, which significantly reduces the scale-formation. During the past decade, the emergence of atomic force microscopy (AFM) enabled us to study the effect of the surface roughness in nanoscale on the membrane fouling. It is believed that the membrane fouling with particulate substance is enhanced by an increase in the surface roughness. It is shown in this Review that all of the above concepts, except for the membrane surface charge, are based on correlation of data, which are, at best, valid within a limited range of surface property parameters. * To whom correspondence should be addressed. Phone: (613) 562-5800, ext 6085. Fax: (613) 562-5172. E-mail: rana@eng.uottawa.ca. Dipak Rana is presently a Research Scientist in the Industrial Membrane Research Institute, Department of Chemical and Biological Engineering, University of Ottawa, Ottawa, Canada. Dr. Rana has been a member of various prestigious organizations, such as the Indian Chemical Society, Society for Polymer Science, India, Society of Plastics Engineers, USA, American Chemical Society, etc., for a long period. He was awarded a Ph.D. in Science from Jadavpur University, Calcutta (presently Kolkata), when he was working at the Indian Association for the Cultivation of Science, Calcutta, India. He received his Master in Chemistry with specialization in Physical Chemistry as well as his Bachelor with Honors in Chemistry from the University of Calcutta, India. Dr. Rana has published over 50 professional papers and book chapters. Chem. Rev. 2010, 110, 2448–2471 2448