Editorial for themed issue on “Advanced Materials in Water Treatments”
Water is the most important chemical compound on earth, essential for the survival of all known forms of life. Although we as humans recognize water as the most precious natural resource, we frequently disregard it by polluting rivers, lakes and oceans. Pollution changes the environment, alters the eco-systems, and makes water undrinkable. It is estimated that nearly 1.5 billion people worldwide lack safe drinking water, and at least 5 million deaths per year can be attributed to waterborne diseases. Water pollution, therefore, is a serious global issue. In addition, many large regions such as Africa, Australia and the Middle East are facing severe fresh water shortages because of drought. In this case, desalination can be regarded as one of the important water treatments to produce fresh water for human consumption and other uses.Water pollutants usually contain: (1) disease-causing agents, such as bacteria, viruses, protozoa and parasitic worms that enter the water sources through sewage systems and untreated wastes; (2) oxygen-demanding wastes, which can deplete oxygen levels in the water and therefore cause other organisms to die; (3) organic pollutants, including aryl chlorides, dyes, oil, plastics and pesticides, which are harmful to humans and all living things in water; (4) water-soluble inorganic pollutants, such as acids, salts and toxic metals, large quantities of which will make water undrinkable and cause the death of aquatic life; (5) nutrients (water-soluble nitrates and phosphates), which can deplete the water's oxygen supply and kill fish and, when found in drinking water, can cause death in young children; (6) suspended sediments, which can cause depletion in the water's light absorption; and (7) water-soluble radioactive compounds, which can cause cancer, birth defects and genetic damage.
Various treatments have been investigated for water decontamination and desalination, especially for drinking and fresh water usages. These treatments include adsorption, chemical coagulation, photodegradation, biodegradation, active sludge, and so on. All of these methods are closely related to advanced materials, such as porous materials, polymer membranes, inorganic/organic flocculating agents, oxidants, photocatalysts, microbial carries/releasing materials, and so on. Without question, the development of water treatment processes relies much on the research progress in advanced materials.
Porous materials, especially porous carbons (e.g. activated carbons, carbon blacks, carbon fibers) and functional polymers with selective adsorption properties have been extensively used in industrial processing of pollutant removal due to their large specific surface areas, high porosity, chemical inertness and good mechanical stability. For example, activated carbon can efficiently capture organic dyes from waste water, which are difficult to be biodegraded. However, the applications of these porous carbons can be restricted to their micropores, which are inaccessible to large-size pollutants. Accordingly, many methods have been used to enlarge the pore size in activated carbons from micropore to mesopore range. These methods include the addition of metal salts, and various treatments using acids, bases, ammonia, low-pressure plasma, oxygen plasma, as well as high temperature, which all can result in improving the performance towards removing large-size pollutants. Ordered mesoporous materials show great potential in replacing conventional porous materials because their porosity, structure and surface properties can be easily tailored. Recently, a large spectrum of ordered mesoporous materials with different structures, compositions and morphologies has been made available routinely. Many researchers have been focusing on the applications of ordered mesoporous materials in water treatments and, as expected, many of these materials were found to be very effective and useful. Generally, one of the most important advantages of ordered mesoporous materials in water treatments is their very large adsorption capacities, due to the large surface areas and pore volumes with available binding sites for different species. For instance, properly functionalized mesoporous silicas exhibited high adsorption affinity towards many heavy metal ions. Besides metal ions, organic dyes can also be effectively removed by surface-modified mesoporous materials. The adsorption properties of these materials are governed by their pore sizes, pore structures, amount and distribution of active sites, etc. The use of mesoporous silica and carbon materials for the removal of large organic dyes and biomolecules, therefore, becomes one of the most active research areas.
Membranes used for water treatments consist of a wide range of materials, including inorganic porous membranes (such as zeolites, and microfiltration and ultrafiltration ceramics), organic polymer membranes (such as dense desalination membranes and porous polymer membranes), and inorganic-polymer composite membranes. The related processes include microfiltration (MF), ultrafiltration (UF), nanofiltration (NF), reverse osmosis (RO), forward osmosis (FO), and ion exchange (e.g., electrodialysis). Currently, RO desalination is generally used to desalinate seawater and brackish water, though the energy consumption of this process is very high. The energy efficiency is in dire need of improvement, while advanced membranes may also lead to significant energy reduction in desalination process.
Semiconductors, such as titania, are one kind of promising photocatalyst for water purification. Photocatalysts function by generating powerful but short-lived oxygen-based radicals upon irradiation, and therefore are capable of inactivating various organic pollutants, bacteria and viruses without generating harmful by-products. Advanced oxidation processes, unlike conventional chemical disinfectants, do not consume antimicrobial nanomaterials, and are therefore much more efficient to use in industrial water treatments. As a result, many recent research activities have focused on the development of new photocatalysts by controlling nanostructure, composition, morphology, and physicochemical properties for water treatments.
Without question, due to the urgency of many existing and emerging water issues on earth, a moment has been formed to test and use new advanced materials that have not been available previously, in varying water treatments. However, there has been no systemic and integrated discussion forum of this important topic thus far. The scope of this themed issue thus focuses on the synthesis and functionalization of advanced materials as adsorbents and catalysts for water treatments, the studies of relations between the pore structure and surface chemistry of these materials and their ability for water purification, the interaction between pollutants and materials with variable morphologies such as membranes, fibers, and monoliths, and the distribution of pollutants inside pores, but not restricted to these. The aim of this effort is to present the potential of advanced materials for water treatments, to increase our awareness about such opportunities, and to promote the transition from fundamental research to industrial applications of these materials.
The themed issue on “Advanced Materials in Water Treatments” provides a snapshot of the current viewpoints of research and development in this rapidly growing field. The issue contains twenty-three articles, including six reviews and seventeen full research papers, which cover a broad range of advanced materials from porous materials, layered materials, membranes, thin films, to nanofibers for water-purification processes, including the removal of heavy metal ions, organic dyes, organic amines, and bacteria, as well as desalination of water. Significant progress made recently in the design and synthesis of advanced materials has been highlighted, such as ordered carbon nanostructures, functional ordered mesoporous silicas, periodic mesoporous organosilicas, organic–inorganic hybrid composites, nanofibrous cellulose composites, hybrid titania, multi-layered core–shell nanospheres, reverse osmosis membranes, ultrafiltration membranes, composite ion-exchange membranes, and zeolite membranes. These promising materials provide new, efficient and practical ways for water purification, which may spark interest for a broad readership.
Materials chemistry will be essential for creating new advanced materials for water treatments. However, we are still at the beginning of this exciting journey as only a small spectrum of advanced materials has been used in real world. The major challenge is to find low-cost and facile treatment processes of advanced materials that can be afforded by the poorest region of earth. We hope that this themed issue of Journal of Materials Chemistry on “Advanced Materials in Water Treatments” will stimulate further research and development in this fascinating area.
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