Carbon Nanotubes for Environment Purification
Transportation, manufacturing, construction, petroleum refining, mining, and other industrialization and urbanization processes are responsible for depleting natural resources and are directly involved in producing large amounts of hazardous waste, which in return contributes to the pollution of air, water, and soil, posing a threat to public health and environmental security. Toxic gases, which consist of carbon oxides, nitrogen oxides, sulphur oxides, and ozone, etc., other suspended particles, and VOCs are some of the examples of atmospheric pollutants, while other organic and synthetic substances, including phenols, hydrocarbon, and pesticides, along with heavy metals are direct contributors of soil and water pollution. Search environmental contaminants possess a high potential to produce harm to the human help as they have the ability to invade the human health system through inhalation, absorption, or ingestion. Nanotechnology is considered to be one of the most promising ways of revolutionizing the environmental purification process. Nanomaterials possess a stronger reactivity, large surface contact area, and a superior disposal capacity (Ibrahim et al., 2016).

Carbon nanotubes (CNTs) are one such nanomaterial that has been widely employed for environmental cleansing and remediation. The structure of CNTs allows for the addition of one or more functional groups (OH, COOH, C=O) on the surface, which has the capability of improving stability and selectivity while influencing the maximum adsorption capacity of the system. Its unique features include high conductivities (thermal and electrical), increased strength, high levels of hardness, and specialized adsorption capacity, CNTs in particular, have a huge potential for applications. Carbon atoms and the adsorbents interact with each other on the surrounding walls of CNTs, which feature cylindrical pores. “Single-walled carbon nanotubes (SWCNTs) and multiwalled carbon nanotubes (MWCNTs)” are the two prominent types of carbon nanotubes (Mohammed, 2017). These nanoparticles have proved to have great promise as excellent adsorbents for removing a variety of inorganic and organic contaminants from air, water and soil.

CNTs for adsorption of toxic gases

Dioxin adsorption
CNTs comprise of graphene sheets with carbon atoms having a hexagonal arrangement surrounding the tube axis. The benzene rings of dioxin have significant contact with the surface of CNTs. Furthermore, the whole surface of the Carbon nanotube and dioxin molecules interact with each other having the potential for overlapping the processes, which in turn are used for boosting the absorption potential inside the pores. CNTs' enhanced and excellent oxidation resistance is also considered to be effective for adsorbent renewable at high temperatures. Because of dioxin's severe toxicity, more effective adsorbents than activated carbon are necessary to decrease dioxin admission to a lower level. Dioxin's interaction with CNTs is roughly three times greater than dioxin's contact with activated carbon in this situation. This enhancement is most likely going to the curved surface of nanotubes in comparison to flat sheets, which provide a greater interaction force between the CNTs and dioxins.

NOx adsorption
The distinctive structures, surface functional groups, and electronic characteristics of CNTs have the probability of being linked with NOx adsorption. Oxidization of NO to NO2 occurs and is subsequently adsorbed on the surface of nitrate solution when O2 and NO pass through CNTs.

CO2 capture
Carbon nanotubes have a substantially lower CO2 adaptive capacity. The chemical modification of carbon nanotubes has a high potential for CO2 capture. Modification and combination of CNT with various other chemical solutions, including ethylenediamine (EDA), 3-aminopropyltriethoxysilane (APTS), and  polyethyleneimine (PEI),  the adsorption efficiency of CO2 is enhanced. In the absence of water, the solution includes amine groups that can react with CO2 to create carbamate, increasing CO2 adsorption effectiveness. The efficiency and effectiveness of CO2 adsorption on modified CNTs improve with increasing relative humidity.
Isopropyl alcohol adsorption (IPA)

SWNTs that have been oxidized by a solution of NaClO and HNO3 can be employed as adsorbents for IPA vapour adsorption. After it is oxidized by HNO3, NaClO, and HCl, solution, the physicochemical characteristics of SWNTs improved, resulting in a pore size reduction, while an increase in the surface area of micro-pores; the active surface of the base and the surface of functional groups. As a result, SWCNTs may absorb more IPA vapour from the atmosphere (Yunus et al., 2012).

Water remediation
Some heavy metals can be absorbed by functionalized carbon nanotubes, and some organic colours can be removed from the water. To boost the solubility and reactivity of CNTs, many procedures, including reaction with a diazonium salt, cycloaddition, oxidation, fluorination, and free radical polymerization, can be applied. Microwave-assisted MWCNTs have been considered to be efficient for removing Zn (II) from solution in water with a clearance rate of more than 99%. There are also some studies about carbon nanotubes degrading the harmful contaminants in water. Water and phenolic compounds, for example, are poorly attached to the external surface of CNTs, but they can be highly adsorbing on the functionalized CNTs. Because of the synchronized access of H-bonding and π-π stacking in the system, the binding of phenol to CNT-OH is considered to be more than that of molecules of water. Additionally, the composite materials for the manufacture of CNTs, and other chemicals are utilized for removing the contaminants from water (Zhu et al., 2019).
Soil remediation

CNTs have been researched in heavy metal polluted water with excellent outcomes; conversely, their application in polluted soils has been virtually unmapped. According to Matos et al. (2017), the potential of carbon nanotubes in soil purification, particularly in mobilizing the heavy metal ions for example “lead (Pb2+), nickel (Ni2+), copper (Cu2+), and zinc (Zn2+)”, are typically found in the polluted soil. The addition of definitely scattered MWCNTs had little influence on the heavy metals Pb2+ and Cu2+ since they were virtually completely immobilized by the soil particles. The inclusion of MWCNTs, on the other hand, resulted in a 30% increase in the immobilization of various heavy metals, including Ni2+ and Zn2+, as compared to the reference test utilizing the soil. The findings lead to the conclusion that adding the tiny quantity of scattered CNTs to the soil can successfully boost its absorption capacity and, as a result, can be used to improve the immobilization of the heavy metal in the soil matrix. There is a difference in immobilization as it fluctuates with heavy metals.

Ibrahim, R. K., Hayyan, M., AlSaadi, M. A., Hayyan, A., & Ibrahim, S. (2016). Environmental application of nanotechnology: air, soil, and water. Environmental Science and Pollution Research, 23(14), 13754-13788.
Matos, M. P., Correia, A. A. S., & Rasteiro, M. G. (2017). Application of carbon nanotubes to immobilize heavy metals in contaminated soils. Journal of Nanoparticle Research, 19(4), 126.
Mohamed, E. F. (2017). Nanotechnology: future of environmental air pollution control. Environmental Management and Sustainable Development, 6(2), 429-454.
Yunus, I. S., Harwin, Kurniawan, A., Adityawarman, D., & Indarto, A. (2012). Nanotechnologies in water and air pollution treatment. Environmental Technology Reviews, 1(1), 136-148.
Zhu, Y., Liu, X., Hu, Y., Wang, R., Chen, M., Wu, J., ... & Zhu, M. (2019). Behaviour, remediation effect and toxicity of nanomaterials in water environments. Environmental Research, 174, 54-60.