2.1. Air pollution

The entry of any particle, biological molecule, or harmful compound of solid, liquid, or gas into the atmosphere that poses a risk to the system, harms or diseases living organisms and affects the ecosystem of an area is known as air pollution. This type of pollution can originate from human or natural resources and is divided into two primary and secondary categories. Primary pollutants are usually produced by a natural process such as a volcanic eruption or an abnormal process such as the combustion of fossil fuels and include substances such as carbon monoxide, sulfur and nitrogen oxides, volatile organic compounds, and so on. Secondary pollutants, on the other hand, do not enter the atmosphere directly and are formed by the reaction of primary pollutants with each other, such as peroxyacetyl nitrate, which is produced by the reaction of nitrogen oxides and volatile organic compounds [12].

One of the most important and basic needs in relation to environmental pollution control is continuous monitoring of air pollution [13]. With the use of nanosensors, effective progress has been made in controlling air pollution [10]. With the invention of the first samples of smart dust, the production of such sensors came closer to the stage of scientific application [14]. The main purpose of making smart dust is to produce a set of advanced sensors in the form of very light nanocomputers [15]. These nanosensors easily stay suspended in the air for hours [16]. These tiny particles are made of silicon and can send the collected information to a central base via their own wireless. The data transfer rate of the prototypes is about one kilobyte per second [17].

2.2. Emission of toxic gases

Emission and distribution of deadly and toxic gases is one of the dangers of everyday industrial life. Unfortunately, warnings in the industry are often too late to detect such leaks [18]. The carbon nanotube (CNT) sensors are made of single-layer nanotubes about 1 nm thick and can absorb toxic gas molecules [19]. They are also able to detect a small number of deadly gas molecules in the environment [20]. The researchers claim that these sensors will be used to detect war biochemical gases, air pollutants and even organic molecules in space [21].

New three-dimensional nanostructures are other sensors that have been widely used due to their small dimensions and high precision. For example, ultra-thin ${}_{\mathrm{}}$ films in the 3D structure nanosensor can detect very toxic gases (${}_{\mathrm{}}$ and ${}_{\mathrm{}}$) with high sensitivity [22].

The use of different metals in the structure of multi-walled carbon nanotubes offers sensors with high capabilities for selective detection of toxic gases, so that the presence of platinum nanoparticles in the structure of carbon nanotubes increases the sensitivity of the sensor to ${}_{\mathrm{}}$. Also, the presence of silver and copper detects ${}_{\mathrm{}}$ and ${}_{\mathrm{}}$ gases, respectively [23].

2.3. Heavy metal ions pollution

Scientists have long realized that exposure to particulate matter and heavy metals can cause health problems and diseases such as heart disease, lung cancer and more. In urban areas, the size of airborne particles is typically around 100–300 nm; While heavy metals can be found in different concentrations. Also, heavy metals cannot be broken down by microorganisms, meaning they are not biodegradable [24]. The many problems caused by the presence of heavy metal ions in water, soil, and air make it even more tangible that sensors need to be developed that can detect heavy metal ions before their concentrations reach hazardous levels [25].

Nanomaterials based on quantum dots are one of the most accurate and advanced sensors for detecting heavy metals. Quantum dot nanomaterials can be introduced as a sensor due to their special physicochemical properties, high specific surface and high reactivity [26]. By using quantum dots nanomaterials and connecting to optical or chemical sensor converters, powerful detection devices can be assembled that can detect multiple metals in complex situations [27]. For example, zero-dimensional graphene dots are used because of their interesting properties, including excellent optical properties, adjustable surface groups for absorption, good stability, as well as easy fabrication and preparation process, by doping in optical detector species for heavy metal ion nanosensors [28]. Some biomass is also used in the technology of using quantum dot nanomaterials. These nanobiosensors are used as attractive fluorescent nanosensors due to their high environmental compatibility and biocompatibility. For example, carbon quantum dots extracted from green algae waste are used to detect Fe (III) in effluents [29].

Nanocomposites with a core-shell structure of gold-silica have a more advanced surface plasmon resonance bond than gold nanoparticles; Therefore, it is more sensitive to detect very small amounts of heavy metals in drinking water. Nanocomposites with a gold-silica core-shell structure accumulate when heavy metal ions collide to detect the presence of these contaminants. Also, with the presence of these ions, changes occur in the location of the plasmon adsorption bond due to the chemical adsorption of these ions on the surface of the nanocomposite containing gold [30]. Similar results were obtained for cadmium ions when zinc and lead ions were present in water. In other words, with the presence of these ions, a partial transition in the bands towards longer wavelengths will occur. Suspensions containing gold nanoparticles show a strong red color due to the adsorption of surface plasmon. The presence of zinc and lead ions does not change its red color. Color change occurs only when electrons are transferred from adsorbed ions to metal particles. This phenomenon increases the density of free electrons in the metal conduction band and increases the plasma frequency of the metal [31].

Metal nanoparticles such as gold and silver have very strong and desirable absorption properties in the ultraviolet–visible region of the electromagnetic spectrum, which is due to the cumulative electron interactions between metal atoms and electrons. Nanocomposites with a gold-silica core-shell structure have a more advanced surface plasmon resonance bond than gold nanoparticles and are therefore more sensitive to detecting very small amounts of heavy metals in drinking water. Nanocomposites with a gold-silica core-shell structure accumulate when heavy metal ions collide to detect the presence of these contaminants [28].

3.1. Adsorption of dioxin

Dioxins, which are known as persistent environmental pollutants, can remain in the environment for many years [36]. Dioxins and their associated compounds (such as polychlorinated dibenzofurans and polychlorinated biphenyls) are persistent and highly toxic pollutants [37]. Dibenzo-p-dioxin is a family of compounds consisting of two benzene rings joined together by two oxygen atoms and zero to eight chlorine atoms are attached to the ring [38]. Dibenzofuran is a similar compound except that there is only one oxygen interface between the two benzene rings [39]. The toxicity of various dioxins depends on the number of chlorine atoms in them. Dioxin has no chlorine atom or has a chlorine atom without toxicity; Dioxin contains more than one toxic chlorine atom [40]. 2.3.7.8-Tetraklorodibenzeo-p-dioxin (TCDD) is a compound known to be carcinogenic to humans [41]. Dioxin also affects the immune and endocrine systems and fetal growth. These compounds are mainly produced by the combustion of organic compounds in the incinerator [42]. The concentration of dioxin compounds formed by combustion is about 15–555 ${}^{\mathrm{}}$. Regulations for dioxin emissions are complex and vary from country to country. However, it is generally necessary for the dioxin concentration to fall below 1 ${}^{\mathrm{}}$. Since 1991, activated carbon adsorption has been widely used to eliminate dioxins from incinerators in Europe and Japan [43]. The removal efficiency of dioxin using activated carbon adsorbent is much higher than other adsorbents such as clay, ${}_{\mathrm{}}{}_{\mathrm{}}$ and zeolites; Due to the very high toxicity of dioxin, a more efficient adsorbent than activated carbon is required, so that it reduces the production of dioxin emissions to a lower level [44]. The researchers showed that the relationship and interaction of dioxin with CNTs is about three times stronger than the interaction of dioxin with activated carbon. The results showed that CNTs [35] were significantly better than activated carbon and ${}_{\mathrm{}}{}_{\mathrm{}}$ [45,46] for eliminating dioxins. This improvement is probably due to the curved surface of the nanotubes compared to their smooth surface, which leads to stronger interaction forces between dioxins and CNTs [37,47].

3.2. Adsorption of $\mathbf{}{\mathbf{}}_{\mathbf{}}$

Incomplete combustion of the fuel-air mixture produces pollutants such as carbon monoxide (CO), unburned hydrocarbons (HC) and nitrogen oxides (${}_{}$) [48]; Because methane is a high percentage of natural gas, unburned hydrocarbons in gas engines mostly contain methane [49]. The ratio of carbon to hydrogen (C/H) in methane is lower than any other hydrocarbon, which reduces the emission of carbon monoxide and carbon dioxide during gas combustion [50]. However, in real life, the amount of carbon monoxide and carbon dioxide can not be ignored. Methane is a greenhouse gas and has a far greater greenhouse effect than carbon dioxide. For years, catalytic convertershave played an important role in reducing emissions from internal combustion engines. These converters reduce carbon monoxide, nitrogen oxides and unburned hydrocarbons in car exhaust [51]. Precious metals including palladium, rhodium and platinum are used to convert pollutant gases, but efforts have always been made to eliminate or reduce the consumption of noble metals to reduce prices and improve the efficiency of catalytic converters [52]. Using suitable catalytic converters to reduce pollutant gases is an effective solution. In the case of gasoline-burning vehicles, precious metals such as platinum, palladium and rhodium are used to remove contaminants in catalytic converters, but the high price of these metals and the gradual reduction of their resources limit their use [53]. In recent years, many attempts have been made to propose alternative compounds such as perovskites instead of precious metals [54]. The combination of metal oxides, perovskites and spinels have been acceptable options, and the use of perovskites as catalysts in the removal of pollutants from vehicles has been studied and tested [55]. A major effort has been made to develop technology to eliminate ${}_{}$ emissions from fossil fuel combustion. Common adsorbents used to remove ${}_{}$ at low temperatures include zeolite ion exchange, activated carbon, and dispersed $$ in activated carbon fiber [56]. The amount of ${}_{}$ uptake in these adsorbents is low [57]. The results show that CNTs have the ability to adsorb ${}_{}$ [58]. For example, $$and ${}_{\mathrm{}}$ passes through the substrates of CNTs, then ${}_{\mathrm{}}$ is synthesized and adsorbed on the surface of CNTs [59].

3.3. Adsorption of $\mathbf{}{\mathbf{}}_{\mathrm{}}$

In recent years, the uptake and storage of ${}_{\mathrm{}}$ produced by fossil fuel power plants has received considerable attention. The various technologies of ${}_{\mathrm{}}$ recycling are used, including adsorption, cryogenic adsorption, membrane and other methods. Among these technologies, adsorption-desorption technologies are known as the most developed process. This process is based on amine adsorption or ammonia adsorption process. However, these technologies require a lot of energy for the adsorption process. Researchers are developing membranes based on CNT and nano-silica and zeolite that can collect ${}_{\mathrm{}}$ on a large scale from factory chimneys and reduce greenhouse gases [60].

3.4. Removal of volatile organic compounds

In addition to nitrogen and sulfur oxides, many chemicals are formed by atmospheric reactions: such as soot formation, nitric acid, polyaromaticcompounds, and volatile compounds (VOCs). Clean air regulations are increasingly focusing on particles that are potentially harmful to human health [61]. Most advanced air purification systems were based on photocatalysts and adsorbents such as activated carbon and the ozonolysis process. However, conventional systems are not useful for removing organic pollutants at room temperature [62]. Researchers have now developed new materials that are effective in removing VOCs, sulfur and nitrogen oxides from the air at room temperature [63]. For example, a manganese oxide-based catalyst offers a highly porous space that is also coated with gold nanoparticles, which can decompose acetaldehyde, toluene and hexane and remove it from room air [64].

3.5. Absorption of isopropyl alcohol

In addition to being used as a solvent, isopropyl alcohol (IPA) is also used in the manufacture of optical and semiconductor electronics [65]. Due to the lack of control over air pollution, IPA vapor is released into the atmosphere without treatment. The release of IPA vapor can harm human health as a stimulant and carcinogen [66]. The study of single-walled carbon nanotubes (SWNT) oxidized with sodium hypochlorite and ammonia solution to adsorb IPA vapor showed that oxidation with sodium hypochlorite and ammonia solution reduces the pore diameter of carbon nanotubes and increases the pore surface area, surface functional groups and base active surface area [67,68].

4.1. Nanofilters

Another important application of nanotechnology in the environment is the use of nanofilters in water and wastewater treatment. The membrane used in the nanofiltration process usually repels large molecules and, compared to other methods, is able to purify well water or surface water well with less energy. This process is able to remove a variety of bacteria, viruses, pesticides, pollutants of organic origin and calcium and magnesium salts from the water [70].

Due to the fact that no chemicals are used in the nanofiltration process to soften the water, so its negative environmental effects are far less than conventional chemical methods. In addition, nanoparticles have a great deal of flexibility in the treatment of pollutants. For example, nanostructured particles are used for the immediate treatment of soil, sediments, solid waste, water treatment and liquid waste. Research shows that nanostructured bimetallic particles such as iron-palladium, iron-silver and zinc-palladium have found many applications in the treatment and purification of environmental pollutants, such as chlorinated pesticides of organic origin and halogenated organic solvents. Experience has shown that the use of bimetallic nanostructured particles converts all hydrocarbons containing chlorinated compounds that are highly toxic to environmentally safe hydrocarbons [71].

In addition, there is ample evidence that iron-based nanostructured particles are capable of degrading highly stable contaminants such as perchlorates, nitrates, heavy metals (nickel and mercury), and radioactive materials such as uranium dioxide. In addition, nanostructures can be used to decolorize drinking water. The dye in drinking water should be removed not only because of its appearance but also because it can be the source of trihalomethane production, which is very dangerous. When combined with chlorine, it forms chloroform and other harmful and carcinogenic halogen compounds [72]. Most conventional water treatment methods are not able to separate the acid azo dye, but with the use of nano-membranes, up to 88% of such materials can be easily separated from the water [73]. Research also shows that the use of nanotechnology in water treatment can greatly reduce treatment costs [74]. Alcohols such as ethanol are widely used in industry as solvents or detergents. These substances absorb large amounts of various impurities while consuming. Due to the fact that disposing of them after consumption has harmful effects on the environment, must be treated for reuse. Conventional methods such as distillation waste a lot of energy while polluting the environment [75].

The use of nanofilters is an effective step in protecting the environment and saving energy in this area. Filters are classified according to the size of their pores and are classified into microfilters, ultrafilters and nanofilters. Nanofiltration is basically lower pressure filtration than reverse osmosis, so the cost of nanofilters is more reasonable. In addition, nanofilters have the ability to remove viruses and bacteria so they can be used to remove contaminants in human drinking water and agricultural waters [76,77].

4.2. Ceramic membranes with zeolite coating

One of the major challenges in the field of ceramic membranes is the fabrication of membranes for which the water permeability in the range of ultrafiltrationmembranes and the selectivity of contaminant species for them is similar to nanofiltration membranes and reverse osmosis membranes [78]. In 2001, the results of molecular dynamics simulations showed that reverse osmosis zeolite membranes could be used to desalinate saline water. Since then, extensive studies on saline water desalination and aqueous waste treatment have begun. The main advantages of using zeolite in the manufacture of reverse osmosis membranes are good chemical resistance, good mechanical stability at high pressures and high resistance to pore clogging [79,80].