Polymer-matrix nanocomposite membranes for water treatment
Introduction
Water is the foundation of life. However, due to the rapid growth of world population, abuse of water resources, and water pollution, water shortage problem becomes more and more serious. Worldwide, around 780 million people still lack access to improved drinking water sources (WHO, Progress on Drinking Water and Sanitation, 2012). Hence, cost-effective technologies must be developed to extend water resources and solve water pollution problems. Membrane water treatment is expected to play an increasingly important role in areas such as drinking water treatment, brackish and seawater desalination, and wastewater treatment and reuse, because it is simple in concept and operation, does not involve phase changes or chemical additives, and can be made modular for easy scale up [1], [2].
Polymeric membrane is currently the most widely used membrane type for water treatment due to its straightforward pore forming mechanism, higher flexibility, smaller footprints required for installation and relatively low costs compared to inorganic membrane equivalents [3]. However, it is still restricted by several challenges such as trade-off relationship between permeability and selectivity (also called Robeson upper boundary in membrane gas separation), and low resistance to fouling. The development of membranes with high permeability and rejection, and good antifouling property is much needed for water purification under the context of energy efficiency and cost effectiveness.
Polymer-matrix nanocomposite membranes are advanced membranes with nanomaterials dispersed in their polymer matrices. They could be used for gas–gas, liquid–liquid, and liquid–solid separations. The concept of making nanocomposite membranes was originally developed to overcome the Robeson upper boundary in the field of gas separation in the 1990s [4], [5], where highly selective zeolites were incorporated into polymers to improve both permeability and selectivity [6], [7]. Besides gas separation [8], [9], [10], many other applications have been examined by using nanocomposite membranes, such as direct methanol fuel cells [11], [12], proton exchange membrane fuel cells (PEMFCs) [13], sensor applications [14], [15], lithium ion battery [16], [17], pervaporation (PV) [18], [19], [20], organic solvent nanofiltration (OSN) [21], [22], and water treatment. Due to its promise of overcoming the trade-off relationship between permeability and selectivity as well as mitigating membrane fouling problem during water treatment applications, it has gained considerable attention and is considered as the cutting edge of creating the next generation of high performance membranes.
The aim of this review is to summarize the recent scientific and technological advances in the development of nanocomposite membranes for water treatment. Challenges and future research directions will also be discussed. Readers interested in gas separation are referred to two excellent reviews recently published on nanocomposite gas separation membranes [5], [8].
According to membrane structure and location of nanomaterials, nanocomposite membranes can be classified into four categories: (1) conventional nanocomposite; (2) thin-film nanocomposite (TFN); (3) thin-film composite (TFC) with nanocomposite substrate; and (4) surface located nanocomposite. The typical structures of these membranes are illustrated in Fig. 1. It is worth noting that the red spheres used in the figure not only stand for nanoparticles (NPs), but also could represent nanotubes, nanofibers or nanosheets. The publication numbers related to each type of the nanocomposite membranes for water treatment are also depicted in Fig. 1, where the data are obtained based on searching and screening using the key words “nanocomposite and membrane” or “mixed matrix and membrane” in the database, Scopus.
Section snippets
Conventional nanocomposite
In the conventional nanocomposite membranes, nanofillers fall into one of the four categories: 1) inorganic material; 2) organic material; 3) biomaterial, and 4) hybrid material with two or more material types. Fabrication of nanocomposite membranes is mostly based on phase inversion (PI) method in which nanofillers are dispersed in polymer solution prior to the PI process, and can be prepared in either flat sheet or hollow fiber configurations (Fig. 2). This type of membrane is mainly used in
Thin-film nanocomposite (TFN)
Thin film composite (TFC) membrane consists of an ultra-thin barrier layer (commonly made of PA) atop a more porous supporting layer. It has been the major type of RO/NF membrane since being first developed by Cadotte in the 1970s [158], and widely used to desalinate seawater/brackish water or remove heavy metals, hardness, organic micropollutants such as pesticides, disinfection by-products (DBPs), endocrine disrupting compounds (EDCs), and pharmaceutically active compounds. Recently, the
TFC with nanocomposite substrate
This class of membranes was first developed to investigate the effects of nanofiller on membrane compaction behavior, which is listed along with other studies in Table 3. In that study by Pendergast et al. [215], silica or zeolite NPs were embedded into the PSU substrate, which was then used in the IP process to prepare TFC membranes for RO. The prepared membranes showed a higher initial permeability and experienced less flux decline during the compaction when compared with the original TFC
Surface located nanocomposite
In addition to membrane structure, porosity and thickness, membrane surface properties such as hydrophilicity, pore size, charge density, and roughness have a major impact on the membrane performance in terms of separation and antifouling characteristics. Modification of surface properties, therefore, could significantly improve the efficiency of membrane water treatment, as for surface-located nanocomposite membranes listed in Table 4. The process of preparing this type of membranes has
Conclusions and perspectives
Progress in the development of polymer-matrix nanocomposite membranes for water treatment has been tremendous in recent years. Besides tuning the physicochemical properties of membranes (hydrophilicity, porosity, charge density, thermal, and mechanical stability), the incorporation of nanomaterials can provide membranes with some unique properties of nanomaterials and also possibly induce new characteristics and functions based on their synergetic effects. It provides a new dimension to design
Acknowledgments
Financial support of this research was partially provided by the United States Geological Survey (G11AP20089) through Missouri Water Resources Research Center (MWRRC).
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