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Nanotechnology is characterized by the use particles between 1 and 100 nanometers (mn) of size. European Union (2011) has defined nano-materials as any material whether natural or artificial where more than 50% of the material is in the dimension of 1-100 nm. Nanopaxticles are used in different fields like energy, medicine, agriculture, personal care products, electronics, textile, food processing and packaging, and environment (Figure 1).
In recent times, a lot of attention is being given to the application of nanornaterials for removing toxic substances from wastewater and air (Yang et al. 2006, Li et al. 2005). Nanoparticles (NP) can be classified in two groups, i.e., organic and inorganic. Organic nanoparticles comprise carbon nanoparticles, while inorganic are with magnetic, noble metal (palladium, gold and silver) and semiconductor nanoparticles (titanium dioxide and zinc oxide) (Tripathi et al. 2018). The materials used in nano-remediation are zeolites, metal oxides, carbon nanotubes, and many noble metals, out of which nano-valent iron is presently the most used material. Naturally occurring NPs are found in volcanoes and forest fires, and as by-products of combustion process, while engineered nanoparticles (ENPs) are manufacmred for industrial use like black carbon, fumed silica, titanium
Figure 1. Use of nanopaiticles in different fields.
Table 1. Characteristics and examples of different engineered nanoparticles.
Among these four types, uses of carbon and metal based nanoparticles are more frequent.
dioxide (Ti02), iron oxide (FeOx), quantum dots (QDs), fullerenes, carbon nanotubes (CNTs), and dendrimers. Table 1 shows the characteristics and examples of different nanoparticles.
Nanobioremediation in pollution control
Nanoremediation methods are unique in the sense that it can treat persistent contaminants for which only few remediation techniques persist; it avoids formation of harmful intermediates and most importantly it remediates a contaminated site in lesser time which reduces the overall remediation cost of a site (Muller and Nowack 2010). Nanobioremediation techniques can be used in solid waste remediation, heavy metal pollution, groundwater and wastewater, uranium and hydrocarbon remediation.
At present time, atmospheric pollution caused due to the anthropogenic activities is one of the major concerns all over the world. In case of India, the problem of air pollution has increased due to the increasing population, industrialization and urbanization since the last few decades (Mina et al. 2013). Gases like carbon monoxide (CO), sulphur dioxide (S02), nitrogen oxides (NOx), as well as particulate matter (PM) are primary air pollutants. Primaiy air pollutants undergo chemical reactions in the air to form secondary' air pollutants like ground level ozone (03), photochemical smog, acid rain, etc. Poor air quality affects vegetation, animals and also human beings. Scientists and engineers have suggested that nanotechnology promises less consumption of resources and energy and also produces less waste and hence causes less pollution (Fleischer and Grunwald 2008). Nanotechnology can be used in controlling air pollution by sensing and detection, remediation and treatment (Yunus et al. 2012). Nanomateiials have large specific surface areas and are highly reactive. There can be many methods through which remediation process can proceed of which some are absorption, absoiption, chemical reactions, filtration and photo-catalysis.
Nano-adsorbents have a high specific surface area with higher absoiption rate and leave smaller footprint on the environment. It is mainly used for removal of organics, bacteria and heavy metals. Nanoadsorbents are broadly classified as oxide based nanoadsorbents, carbon nanotubes, and graphene based nanoadsorbents, depending on their role in adsorption process. Nanoscale adsorbents like carbon nano tubes (CNT) have property to increase adsoiption of metal ions and can be used for treating polluted air (Sugunakala et al. 2017). CNTs and metal oxides are the most commonly used nanoparticles for the removal of heavy metals from aqueous solution (Ray and Shipley 2015). The adsoiption capacity of CNTs is due to their pore structure, pore volume and existence of a broad spectrum of surface functional groups. These nanomaterials have good potential in removing different inorganic and organic pollutants, in both air and aqueous environment (Yunus et al. 2012). CNTs are powerful adsorbents for various organic compounds like dioxin, polynuclear aromatic hydrocarbons, chlorobenzenes and chlorophenols, dyes, etc. Zeolites are effective sorbents and ion- exchange media for metal ions. NaP 1 zeolites can remove heavy metals from acid mine wastewaters (Moreno et al. 2001, Savage and Diallo 2005). Ordinary CNTs were modified into multiwall CNTs which helped in the removal of Pb (II) and Mn (II) (Tarigh and Shemirani 2013), and Cu (II) (Tang et al. 2012) from waste water. Mohamed et al. (2017) also reported the removal of Hg (II) by employing a functionalized-CNTs absorbent. Pharmaceutical and personal care products are source of some of the emerging pollutants which are difficult to detect, measure and remove from wastewater streams. Carbon nanomateiials are effective in their removal from water (Yu et al. 2014). Some compounds which can be effectively removed by carbon nanomaterials are ciprofloxacin (Li et al. 2014), ibmprofen (Clio et al. 2011), estradiols (Kumar and Mohan 2012), triclosan (Cho et al. 2011), etc.
Oxide based nanoadsorbents include titanium oxide/dendrimers’ composites, zinc oxides, magnesium oxide, manganese oxides and feme oxides. Dendrimers are mostly used for removal of organic compound and heavy metals from polluted water. However, dendrimers have found very little use commercially, since its synthesis is very complex. Some of the examples where dendrimers are used at batch scale to examine its effectiveness like silver dendrimers have been found effective to treat and remove bacteria such as Pseudomonas aeruginosa, Escherichia coli and Staphylococcus aureus (Balogh et al. 2001). Gupta et al. (2015) reported that the modified ferric oxide nano-adsorbents showed high affinity for the removal of different pollutants such as Cr3+, Co2+, Ni2+, Cu2+, Cd2+, Pb2+ and As3+ simultaneously fr om wastewater. Modified ZnO nanoadsorbent, i.e., nano-assemblies were used for removal of different kinds of heavy metals (Singh et al. 2013). Kiunar et al. (2013) reported that the use of mesoporous hierarchical ZnO nano-rods was very much effective in removing Pb (II) and Cd (II) from wastewater.
Gr aphene oxides and modification of graphene oxide or graphene with metal oxides or organics produce various nanocomposites which are useful in removal of pollutants from air and water (Wang et al. 2013). Various researchers reported that use of graphene and its other composite for the removal of heavy metals fr om wastewater was getting more attention due to its high surface area, mechanical strength, light weight, flexibility and chemical stability (Azamat et al. 2015, Dong et al. 2015, Vu et al. 2016, Zare-Dorabei et al. 2016, Ding et al. 2014). Besides these, different types of silicon nanomaterial like silicon nanotubes, silicon nanopaiticles and silicon nanosheets are also used as nano-adsorbents. In addition to this, nanoclays, polymer-based nanomaterials, nanofibres, and aerogels are some of the nanomaterials used for adsorption of heavy metals fr om wastewater. Although use of CNTs for pollutant removal have several advantages, high cost of CNTs hinder their commercial use. Therefore, for production and use at commercial scale the technology needs to become economically viable.
Nano-catalysts such as photocatalyst, electrocatalyst, fenton based catalyst, and chemical oxidant have iimnense potential for removing both organic and inorganic contaminants. Usually, these photocatalysts are comprised of semiconductor metals that can degrade variety of persistent organic pollutants in wastewater such as dyes, detergents, pesticides and volatile organic compound (Lin et al. 2014). Furthermore, semiconductor nano-catalysts are also highly effective for degradation of halogenated and non-halogenated organic compounds, and also heavy metals in specific situation (Adeleye et al. 2016). Some materials like titanium dioxide (Ti07), zinc oxide (ZnO), iron oxide (Fe,03) and tungsten oxide (W03) can be used as photocatalysts to oxidize the organic pollutants. The titanium dioxide nanoparticles (Ti02) are capable of removing atmospheric pollutants like NOx, VOCs and other pollutants into less toxic species (Shen et al. 2015). Besides this, TiO, nanoparticles also possess antibacterial properties which are inversely proportional to the particle size (Mohamed 2017). Titanium dioxide (Ti02) nanoparticles are also promising photocatalysts used for purification of water (Adesina 2004). Ti02 nanoparticles can serve both as oxidative and reductive catalysts for organic as well as inorganic pollutants. Kabra et al. (2004) documented that TiO, nanoparticles degrade organic compounds like chlorinated alkanes and benzenes, dioxins, fiirans, PCBs and reduce toxic metal ions like Cr (VI), Ag (I) and Pt (II) in aqueous solutions. Nanotechnology can help in developing new nanocatalysts with enhanced surface area, thereby increasing the reaction efficiency for air pollution remediation (Ozkar 2009). Nanogold based catalysts have excellent treating effect on different toxic air pollutants (Singh and Tandon 2014). ZnO photocatalyst is being developed and it is expected to have both detection and remediation functions (Yadav et al. 2017). When ZnO nanoparticles were used after modification with chitosan for removal of pennethrin from contaminated water, results showed that more than 96% of pennethrin was removed by using a small amount of nanoparticle (Dehaghi et al. 2014). Baneijee et al. (2012) used ZnO nanoparticles for reduction of Cr (VI) under the presence of sunlight. Results showed upto 35% reduction of Cr (VI) in 2 hours aqueous phase with ZnO-Е nanoparticles as compared to only ZnO particles (19%).
Nano-filtration is the process wherein pressure of 3.5-16 bar is applied on the membrane and particles of size less than 0.5-1 mn are rejected by the membranes. These membranes are pressure- driven membranes and have pore sizes between 0.2 mn and 4 mn (Nowack 2008). These membranes are mostly used to remove colours, turbidity, microorganisms, organics, salts, hardness, and inorganic matter from the water. It is also helpful in removal of heavy metal from water (Kumar et al. 2014). Nanostructured membranes can also separate different air pollutants from the exhaust. These membranes remove multivalent ions, pesticides and heavy metals more effectively than conventional treatment methods. Nanofibre coated filter is used to remove dust from air at industrial plants (Muralikrishnan et al. 2014).
When inorganic fillers are added in polymeric nano membranes to improve their performance, they are known as nano-composite membranes. Nano composite membranes mainly find their application in water and wastewater treatment process. There are various nano fillers like carbon nanotubes, graphene, silver, titanium nanoparticles, etc. Advantages of these composites are that they enhance the selectivity process, improve thermal properties of material and moreover enhance the efficiency of filtration process (Ursino et al. 2018).