ReviewRole of nanomaterials in water treatment applications: A review
Introduction
Clean water is one of the most important elements for all living organisms to sustain life. However, due to the rapid pace of industrialization and tremendous increase in the population, the contamination of water resources has occurred globally [1], [2]. Besides other needs, the demand for water has increased tremendously in agricultural, industrial and domestic sectors consuming 70, 22 and 8% of the available fresh water, respectively and this has resulted in the generation of large amounts of wastewater [3], [4], [5] containing a number of ‘pollutants’. Some of the important classes of aquatic pollutants are heavy metal ions and dyes, and once these enter into the water, water is no longer safe for drinking purpose and sometimes it is very difficult to completely treat the contaminated water [6], [7]. Aquatic pollutants are often very dangerous for living beings, and also affect the ecosystem. Therefore, the removal of these pollutants from contaminated water is an urgent need in order to prevent the negative effects on the human health and to the environment.
From past few decades, various techniques have been developed for treating the waste water [8], [9], [10], [11], [12], [13]. Among them, most important methods are solvent extraction, micro and ultra-filtration, sedimentation and gravity separation, flotation, precipitation, coagulation, oxidation, evaporation, distillation, reverse osmosis, adsorption, ion exchange, electrodialysis, electrolysis, etc. From above mentioned techniques, adsorption is one of the considerable techniques for treating the waste water, because of its easy operation, low cost and the availability of a wide range of adsorbents. Besides, adsorption can also be applied for the removal of soluble and insoluble organic, inorganic, and biological pollutants. Additionally, adsorption can also be used for source reduction and reclamation for potable, industrial, and other water purposes. In spite of these facts, adsorption has certain limitations such as it could not achieve a good status at commercial levels. Probably, it is due to the lack of suitable adsorbents with high adsorption capacity and limited use of adsorbents on commercial scale columns. Besides, a single adsorbent cannot be used for removing all kind of pollutants. Different adsorbents are used for different pollutants based on their properties. A comparison of adsorption method was carried out with other water treatment technologies. The order of cost effectiveness is adsorption > evaporation > aerobic > anaerobic > ion exchange > electrodialysis > micro- and ultra-filtration > reverse osmosis > precipitation > distillation > oxidation > solvent extraction [14]. It was observed that, in spite of some limitations, adsorption will be considered a good water treatment technology in the near future. Much work has been conducted on the removal of different pollutants from water by using adsorption in batch mode process [14], [15]. Initially, activated carbon was used for the removal of pollutants from water, which has been replaced by some cost-effective adsorbents [16], [17], [18]. In the last two decades, nanotechnology has emerged significantly with its applications in almost all branches of science and technology. As a matter of fact, various nanomaterials have been prepared and used for the removal of aquatic pollutants [19]. In view of the importance of water quality and emerging utilities of nanotechnology, attempts have been made to discuss various aspects of water treatment by adsorption using nanomaterials. In this regard, promoting nanomaterials presents opportunities to develop local and practical solutions for tackling global water pollution. This review article presents a brief overview of the technical applicability of different nanomaterials for removing various aquatic pollutants.
Although many excellent review articles have been published so far discussing the importance of nanomaterials in water treatment and environmental remediation, but some of them are only a material and/or adsorbent-specific (e.g., CNTs, graphene-based nanomaterials, nano metal oxides, nano zerovalent iron, cellulose nanomaterials [20], [21], [22], [23], [24], [25], [26]) or an adsorbate specific (e.g., metals [22], [24], [27], dyes [28], pharmaceuticals and personal care products [29]). One of the aims of the present review is to compile the important findings of different types of nanomaterials, used in water treatment either as adsorbents, photocatalysts and/or antibacterial agents, for the removal of important aquatic pollutants. A summary of relevant published data with some of the latest important findings, and a source of up-to-date literature is presented and the results have been discussed.
Section snippets
Nanomaterials as adsorbents for water treatment
Nanoadsorbents are nanoscale particles from organic or inorganic materials that have a high affinity to adsorb substances. Because of their high porosity, small size, and active surface, nanoadsorbents not only are capable of sequestering contaminants with varying molecular size, hydrophobicity, and speciation behavior, but also enable manufacturing process to consume raw materials efficiently without releasing its toxic payload [30]. Nanoadsorbents not only work rapidly, but also have
Nanomaterials as photocatalysts
During recent decades, the photocatalytic degradation of various toxic organic compounds has been proposed as a viable process to detoxify water. Much attention has been paid to the photocatalytic degradation of dyes with TiO2 particles under UV or visible light since conventional biological treatment processes are not effective in degrading these pollutants in wastewater [245], [246]. There are various proposed mechanisms for the degradation of dyes using the photocatalyst materials. One
Nanomaterials as antibacterial agents
One of the serious problems world is facing today is infectious diseases and increasing resistance of the microorganisms towards antibiotics [265]. Most of the infection causing bacteria are with strong resistant to at least one of the antibiotics that are generally used to eradicate the infection [266]. For preventing such type of infectious microorganisms, nanoantimicrobials have been proved as effective treatment option [267]. The bacterial growth control is a challenging aspect in every
Conclusions and future perspectives
In the present review article, various nanomaterials are reviewed which have been used for water decontamination. The special emphasis in the review has been given on adsorption, photocatalytic and antimicrobial properties of nanomaterials. As evident from the reviewed literature, a wide range of nanomaterials have been tested for the removal of inorganic and/or organic pollutants. Many nanomaterials eventually present a potent alternative to conventional treatment methods due to increased
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