Membranes have an increasingly important role in alleviating water scarcity and the pollution of aquatic environments. Promising molecular-level design approaches are reviewed for membrane materials, focusing on how these materials address the urgent requirements of water treatment applications.

Materials for next-generation desalination and water purification membranes

Oil spills and industrial organic pollutants have induced severe water pollution and threatened every species in the ecological system. To deal with oily water, special wettability stimulated materials have been developed over the past decade to separate oil-and-water mixtures. Basically, synergy between the surface chemical composition and surface topography are commonly known as the key factors to realize the opposite wettability to oils and water and dominate the selective wetting or absorption of oils/water. In this review, we mainly focus on the development of materials with either super-lyophobicity or super-lyophilicity properties in oil/water separation applications where they can be classified into four kinds as follows (in terms of the surface wettability of water and oils): (i) superhydrophobic and superoleophilic materials, (ii) superhydrophilic and under water superoleophobic materials, (iii) superhydrophilic and superoleophobic materials, and (iv) smart oil/water separation materials with switchable wettability. These materials have already been applied to the separation of oil-and-water mixtures: from simple oil/water layered mixtures to oil/water emulsions (including oil-in-water emulsions and water-in-oil emulsions), and from non-intelligent materials to intelligent materials. Moreover, they also exhibit high absorption capacity or separation efficiency and selectivity, simple and fast separation/absorption ability, excellent recyclability, economical efficiency and outstanding durability under harsh conditions. Then, related theories are proposed to understand the physical mechanisms that occur during the oil/water separation process. Finally, some challenges and promising breakthroughs in this field are also discussed. It is expected that special wettability stimulated oil/water separation materials can achieve industrial scale production and be put into use for oil spills and industrial oily wastewater treatment in the near future.

Biomimetic super-lyophobic and super-lyophilic materials applied for oil/water separation: a new strategy beyond nature

Application and modification of poly(vinylidene fluoride) (PVDF) membranes – A review

A superhydrophobic-superoleophilic PVDF membrane is fabricated via an inert solvent-induced phase inversion for effective separation of both micrometer and nanometer-sized surfactant-free and surfactant-stabilized water-in-oil emulsions solely driven by gravity, with high separation efficiency (oil purity in filtrate after separation > 99.95 wt%) and high flux, which is several times higher than those of commercial filtration membranes and reported materials with similar permeation properties.

Superhydrophobic and superoleophilic PVDF membranes for effective separation of water-in-oil emulsions with high flux.

One of the greatest challenges to the sustainability of modern society is an inadequate supply of clean water. Due to its energy-saving and cost-effective features, membrane technology has become an indispensable platform technology for water purification, including seawater and brackish water desalination as well as municipal or industrial wastewater treatment. However, membrane fouling, which arises from the nonspecific interaction between membrane surface and foulants, significantly impedes the efficient application of membrane technology. Preparing antifouling membranes is a fundamental strategy to deal with pervasive fouling problems from a variety of foulants. In recent years, major advancements have been made in membrane preparation techniques and in elucidating the antifouling mechanisms of membrane processes, including ultrafiltration, nanofiltration, reverse osmosis and forward osmosis. This review will first introduce the major foulants and the principal mechanisms of membrane fouling, and then highlight the development, current status and future prospects of antifouling membranes, including antifouling strategies, preparation techniques and practical applications. In particular, the strategies and mechanisms for antifouling membranes, including passive fouling resistance and fouling release, active off-surface and on-surface strategies, will be proposed and discussed extensively.

Antifouling membranes for sustainable water purification: strategies and mechanisms

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

D. Rana

Papers

Surface Modifications for Antifouling Membranes