Nanotechnology is known to be a research field that is related to the making of such devices which are based upon either atoms or molecules.

The nanoparticles or materials that are involved in nanotechnology are so small that they cannot be seen with a naked eye and rather microscopes are used to observe them. Nanomaterials are now taking place of conventional materials for treatment of water.

Introduction

Despite their size being so small, they possess outstanding properties as a result of which they exhibit efficient applications which are highly beneficial for the industries. One of the major applications of nanotechnology is the purification of water. The properties of nanomaterials involved enable the purification of certain means which are explained in this article.

A research field that's concerned with the building of devices, materials, and things on an atomic to molecular scale is known as nanotechnology. A meter's one-billionth is a nanometer and it is 10 times a hydrogen atom's diameter. Human hair has an average diameter of 80,000 nanometres. The ordinary rules of chemistry and physics don't apply at such scales anymore. For example, the characteristics of the material like their reactivity, conductivity, strength, and color, can be substantially different between the macroscale and the nano. As compared to steel, carbon nanotubes are 6 times lighter but 100 times stronger.

Increased Efficiency

Nanotechnology is assumed to have the potential for solving significant health problems, helping in environmental cleaning, and increasing energy consumption efficiency. It is capable of increasing the manufacturing production massively at very lessened costs. According to the advocates of nanotech, nanotechnology products will be more functional, lighter, cheaper, smaller, needing less energy, and less raw materials for manufacturing.

Nanoscale Technologies

The definition of the National Nanotechnology Initiative reflected that at this quantum-realm scale, quantum mechanical effects have a lot of significance, shifting the definition to a research category from a particular technological goal. Such research category is inclusive of technology and research of all types that deal with the matter's special characteristics if the matter occurs below the given size threshold. Thus, when it comes to referring to a wide range of applications and research having size as the common trait, the plural form "nanotechnologies" is as good as the term "nanoscale technologies".

Nanotechnology's future implications are currently being debated over by scientists. Nanotechnology can produce various new devices and materials having applications in a broad range, for instance in consumer products, production of biomaterials energy, nano-electronics, and nano-medicine. However, many similar issues were raised by nanotechnology comparatively, which includes the concerns about the nanomaterial's environmental impact, toxicity, and potential effects on global economics. Due to these concerns, there has been a debate among the governments and advocates about the warranty of nanotechnology's special regulations.

Properties of Nanoparticles

When it comes to respecting the safety of nanoparticles, the main parameters of interest are;

Physical characteristics

• Solubility
• Structure, for instance, defect structure and crystallinity.
• Topography/morphology of the surface.
• Size distribution.
• Aggregation/Agglomeration state.
• Aspect ratio, specific surface area, shape, and size.

Chemical characteristics

• Lipophilicity/hydrophilicity
• Chemistry of the surface (zeta potential, photocatalytic characteristics, physical structure, reactive sites, tension, charge, and composition)
• Phase identity.
• Nano material's composition (for instance degree of impurity, known additives, or impurities)
• Molecular structure/Structural formula.

In order to use nanomaterials in test systems, one should know that some of the significant characteristics are determined by the nano-materials temporal evolution and the surrounding media. Therefore, assessing the nanomaterials should be our major aim but we should assess them in their manufactured composition/form, and in the formulation in which they were delivered to the environment or the end-user if free nanoparticles are involved in the formulation.

The nanomaterials can exist in the form of nanopowders. They are incorporated in solids, suspended in liquids (colloids), and suspended in air (aerosols, nanoparticles, ultrafine particles). The dispersion of manufactured nanomaterials should be done in a suitable media for biological safety evaluation. Suspension’s behavior can be profoundly influenced by the interaction between the nanomaterials and these media.

Potential Dissolution Kinetics

The significance of potential dissolution kinetics should be emphasized more as the amount of recently developing nanomaterials increases. Nanomaterials are capable of dissolving faster as compared to materials of larger size because of the dissolution kinetics being normally proportional to the surface area. This also implements on the silver nanoparticles as they are being more and more utilized for the release of their silver ions as the anti-bactericidal agents. There are no proper studies on dissolution kinetics yet. It is not necessary, neither is it possible to determine all of the characteristics in every situation.

Water Purification

Being a mythical substance, water's material existence is secondary to the symbolic value as in our lives, it is manifested as life's symbol. We need sustainable supplies of clean water as it is very important to the health of the economy, environment, and the world. Right now, the freshwater's available supplies are reducing, which is making human society face a huge problem in fulfilling the increasing requirements of potable water. The reason for a decrease in freshwater's available supplies is an increase in demands from numerous competitor users, unabated flooding, reduction in the quality of water specifically groundwater because of an increase in surface water and groundwater pollution, growth of population, and extended droughts.                                    

The usage of water should require suitable management, development, and planning as it is a precious natural asset, a basic need of every human, and a prime natural resource. Water scarcity has been caused in the world's numerous parts because of the increasing population and groundwater and surface water's overexploitation over the past few decades. There has been a major increase in the wastewater and due to no proper measures of managing and treating this problem, it is also polluting the current freshwater reserves. Water consumption has been increased in cities and towns because of the increased urbanization. Therefore, it’s time to know how to manage the existing water reserves so that we can avoid having a water strain in the future.                                                              

Having safe drinking water available is a concern nowadays. By far, the most significant water resource is groundwater, for approximately all of the country's water requirements. According to the study of UNEP (United Nations Environment Programme), aquifers have over 2 billion people dependent on them for their drinking water. Irrigated agriculture produces the world's 40% of food and largely depends on the groundwater. About 95% of the planet's freshwater makes up the groundwater, resulting in the groundwater being important for the development of the economy and human life. Although when groundwater's increasing scarcity coupled with the reducing quality of water, it posed a serious problem for the rural population, therefore forcing all of them to look at groundwater's treatment since the clean safe water is now turning into an endangered commodity.

Due to the unabated usage of groundwater resources, the world is now looking at a major problem in the shape of the reduced availability of groundwater. Thus, there's no option other than to move to the management of groundwater from the development of groundwater, meaning that we should move toward groundwater's optimal usage. Now, it's everybody's responsibility to offer water that is safe to drink, and to achieve that, there should be some processes of water treatment that should be sustainable, cost-effective, and easy to be implemented in the longer run.

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Role of Nanomaterials in Water Purification and Treatment

Defeating the conventional technologies because of them being time-consuming and expensive, Nanomaterials are now emerging very rapidly as the potential candidates for the treatment of water. Developing countries like Bangladesh and India would specifically benefit from this as the cost of implementation of any new removal process there could result in a significant criterion in predicting its success. If we speak qualitatively, nanomaterials can be substituted for conventional materials which are harmful to the environment, takes more intensity of energy to be produced, or needs more raw materials.

Producing nanoparticles by using green chemistry principles may result in a lessening in hazardous chemical synthesis, a huge amount of reduction in generation of waste, and generally safer inherent chemistry. Although, more quantitative data is needed for substantiating these claims and whether the replacement of the traditional materials with nanoparticles leads to lower consumption of material and energy, whereas preventing unanticipated or unwanted side effects is open for debate. Nanoparticles' safety and their potential influence on the environment have caused a broad debate. A major role is played by nanotechnology in offering clean water in a sustainable, cheap, and efficient way to developing countries.

One can't overlook nanoparticles' potential side effects. For example, when utilized for pollutant degradation, a nanoparticle's catalytic activity can be beneficial but when a cell takes it up, it can also cause a toxic response. In broad-spread adoption of nanoparticles, this nanotechnology's Janus face can be a problem. Although, the cost of nanotechnology can be lowered, thus making it more effective and efficient as compared to the current methods to remove the contaminants in a long run from the water. Nanoparticles can be utilized as catalysts for contaminants' photochemical destruction, as separation media, and as potent sorbents. In order to remove organic compounds and metals from nanofiltration membranes and water, nanosized zerovalent iron is utilized.

Mechanisms of Removing Pollutants from Wastewater by Nanomaterials

Nanosorbents

Nanoparticles are made highly lubricative as solvents due to two vital characteristics. Nanoparticles have way larger surface areas than macroparticles, on a mass basis. Various reactor groups can be used to enhance them for increasing their chemical affinity towards the target compounds. Workers are exploiting these characteristics for the development of highly efficient and highly selective absorbents to remove inorganic and organic pollutants from the contaminated water. The properties of many materials are determined by their size. With 7 nm diameter hematite particles, for instance, adsorbed Cu ions at lower pH values as compared to the particles which have a diameter of 25-88nm, referring to the improved surface reactivity for iron oxide particles with a decrease in diameter.

A novel sorbent has been developed by Peng et al. in 2005. It has a high surface area of 189 m2/g and it consists of cerium oxide supported on the carbon nanotubes. It was seen that the efficient sorbents for As(V) are the CeO2-CNT particles. All of this demonstrates that how the traditional substance's adsorption capacity can be enhanced by chemically modified nanomaterials. A novel As(V) sorbent was synthesized and characterized in 2003 by Deliyanni et al. and it consists of the nanocrystals of akaganeite [β-FeOOH], to remove inorganic ions and metals, for instance, nanosized metal oxides.

Within 4 hours, As (V) and As (III) equilibrium adsorption by nanocrystalline TiO2 takes place and the adsorption followed pseudo-second-order kinetics. In 2005, the equilibrium was obtained by Bang et al. in 63 minutes. As (III) was removed by utilizing a synthesized crystalline hydrous titanium dioxide in 2004 by Manna et al. Within contact time’s first 30 minutes, 70% of As (III) adsorption takes place. There have been successful synthesis and employment of the mixed oxides’ nano-agglomerates, for instance, cerium manganese, iron-chromium, iron-titanium, iron-zirconium, iron-manganese, and iron-cerium, etc., for removing the pollutant (fluoride, arsenic, etc.) from the aqueous solutions.

Indulgence of Zinc and Tin

Reduction capabilities like those of iron are possessed by other metals too for instance tin and zinc. In the process of decontamination, these metals are converted like iron into metal oxides. Similar results can be produced on the combination of other metals with iron. There have been demonstrations on the degrading of trichloro-ethene and trichloro-ethane by both the iron-copper and iron-nickel bimetallic particles. Iron-platinum particles is another example as it has the same capabilities when it comes to degrading chlorinated benzene.

Carbon as an Adsorbent

Carbon is utilized in a huge amount to remove numerous pollutants including the removal of heavy metals from the aqueous solutions as it is a versatile adsorbent. In research, the carbon family's latest member is graphene and it is one of the most potential materials for the processes of water treatment. With the thickness of a single carbon atom, graphene is an sp-2 hybridized, flat, with carbon atoms' 2-D honeycomb arrangement. The utility is offered by graphene and its composites in various applications because of its associated band structure and remarkable 2-D nature. Graphene is made an attractive absorbent candidate for processes of water purification due to characteristics like the presence of the surface functional groups and large surface area. Arsenic was removed from water by using the RGO-magnetite and GO-ferric hydroxide composites.

Effectiveness of Iron-based Oxides

When it comes to eliminating arsenic from drinking water, hydroxides and iron-based oxides have already been proved as efficient materials. The materials supported by GO and RGO have higher binding capacity than the free nanoparticles. It is a matter of interest that the antibacterial characteristic is possessed by the reduced graphene oxide and that characteristic aids in preventing the biofilm's development on the surface of the filter because of the growth of bacteria. The growth of bacteria can result in prematurely clogging of filters or unwanted odors and tastes.

Nanofiltration

Nanofiltration (NF) and other membrane processes are having an uprise as the key contributors to water purification. NF membranes (nanofiltration membranes) are utilized broadly for the treatment of wastewater or drinking water. Nanofiltration is a low-pressure membrane process, separating materials in the size of 0.001-0.1 micrometer. The pore sizes of Nanofiltration membranes are between 0.2-4 nm. They are pressure-driven membranes and they have characteristics between the characteristics of ultrafiltration membranes and reverse osmosis membranes. Nanofiltration membranes eliminate inorganic ions (Na and Ca), microorganisms, and turbidity.

Groundwater Softeners

Groundwater softeners are utilized for pretreatment in seawater desalination, for wastewater treatment (elimination of organic carbon and inorganic and organic pollutants), for the elimination of the dissolved trace pollutants and organic matter from the surface water, and to soften the groundwater by reducing the hardness of the water. Nanofiltration’s usage has been studied by Bruggen & Vandercasteele in 2003 for removing arsenic, nitrates, organic pollutants, biological contaminants, natural organic matter, and cations from surface water and groundwater.

Removal of Minute Materials

Nanofiltration is capable of being utilized for removing U(VI) minute quantities from seawater according to Favre-Reguillon et al. in 2003. Nanofiltration’s usage for desalinating water was evaluated by Mohsen et al. in 2003 too. It was observed that when in combination with reverse osmosis, nanofiltration can efficiently render the brackish water potable. Peltier et al. showed an enhancement in the quality of water in 2003 for a large water distribution system by utilizing nanofiltration. Moreover, huge prominence is being gained by the carbon nanotubes filters in the water treatment processes. Recently in 2004, the successful fabrication of carbon nanotube filters was reported by Srivastava et al..

New Filtered Membranes

They contain hollow cylinders with carbon nanotube walls radially aligned with them. The filters were efficient at eliminating bacteria like S. Aureus and E. Coli from the contaminated water. Autoclaving and ultrasonication readily clean the carbon nanotube filters.

Nanoceramic Filters

A mixture of micro glass with a high positive charge and nano alumina fiber is known as nanoceramic filters and they can retain the negatively charged particles. High efficiency is possessed by the nanoceramic filters when it comes to eliminating bacteria and viruses. Nanoceramic filters have a high capacity for less clogging and for particulates. Dissolved heavy metals can be chemisorbed by the nanoceramic fillers.

Removal of Nanoparticles After Water Treatment

In environmental applications, the nanoparticles’ usage will invariably result in the release of nanoparticles into the environment. Their persistence, toxicity, bioavailability, and mobility, should be understood in order to assess their potential risks in the environment. There is not much information yet on what will happen on the terrestrial and aquatic life's exposure to the nanoparticles in the soil and water. Engineered nanoparticles' rapidly growing usage in numerous industrial scenarios and their potential for the treatment of drinking water and wastewater purification leads to the unavoidable question that how such nanoparticles can be eliminated in the urban water cycle.

Traditional Methods

Filtration and sedimentation are the two traditional methods to remove particulate matter during wastewater treatment. Although, the sedimentation velocities are comparatively low and there won’t be any major sedimentation until the production of larger aggregates because of the nanoparticles’ small sizes. Usual technologies like flocculation are not suitable for removing nanoparticles from the water, which results in the need of discovering the problem’s new solutions. Membrane filtration (reverse osmosis and nanofiltration) has been used already to remove pathogens from the water. Therefore, to eliminate the nanoparticles, this method can be utilized too.

Today, most of the nanoparticles in technical applications are functionalized in nature and thus virgin nanoparticles are being used in the studies, making those studies irrelevant to assess the behavior of the particles that were actually used. Functionalization is utilized for increasing the particle’s mobility by decreasing the agglomeration. However, until now, not enough is known about how functionalization influences the nanoparticles' behavior in the environment.

If you are interested in the applications of graphene,

you can read our blog post here.

Consclusion

There are certain mechanisms that are responsible for the purification of water through nanotechnology. It is indeed a great technological way to purify the contaminated water as it hazardous and can cause serious health problems. However, nanotechnology is one such great approach that is paving the way for industries to avail the maximum number of health benefits that it provides.

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References

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