Nowadays, the water crisis is one of the greatest challenges. Contaminated water is a ubiquitous problem around the world. Demand for water is growing rapidly because of the rapid increase in population and urbanization. However, resources are limited in arid and densely populated areas. So, shortage of water resources in the world makes scientists aware to find efficient technologies for the treatment of contaminated water and desalination of seawater. This is not only the problem of water shortage, pollution is also a big issue of shortage of water. The level of water pollution has reached at peak and society is also concerning about the detrimental effects of pollution on the environment, aquatic, and aerial. Due to the increase in industrialization, urbanization, domestic and agricultural waste, and deforestation, water resources as lakes, rivers, aquifers, oceans and groundwater are vulnerable to pollution. Human activities are producing many harmful pollutants that are very toxic to living organisms.¹ These pollutants can be divided into different types as organic (e.g., agricultural pesticides and herbicides, dyes, alcohols, carboxylic acid, aliphatic and aromatic compounds, and oils), inorganic (e.g., silver (Ag), lead (Pd), cadmium (Cd), nickel (Ni), arsenic (As), mercury (Hg) and salts), nutrients (e.g., ammonia (NH₃), phosphate (PO₄^3-), nitrite (NO₂^-) and nitrate (NO₃^- )), radioactive pollutants (e.g., iodine (¹³¹I), sulfur (³⁵S), phosphorous (³²P), calcium (⁴⁵Ca), cobalt (⁶⁰Co), and carbon (¹⁴C)) and pathogens (e.g., bacteria, fungi and viruses).² Due to the increase in the world population, the water demand is increased by 64 billion cubic meters per annum.³,⁴ A report by United Nations (UN) depicted that almost 2 billion population in the world did not have clean water in 2018 and it would be nearly 1.8 billion by 2025.⁵,⁶ So, treatments of water to get clean water has become an interesting topic of the public and government. There is a need to use effective and cost-effective technologies to disinfect the water, especially in rural areas. Several conventional techniques are used as filtration, sedimentation, flocculation, coagulation, centrifugal separation to clean the water. Despite this,other methods as evaporation, distillation, crystallization, precipitation, solvent extraction, ion exchange, adsorption, reverse osmosis (RO), forward osmosis (FO), microfiltration (MF), ultrafiltration (UF), electrolysis, and electrodialysis are used to clean water.⁷ Among these, engineering expertise are required that is not cost-effective. From the above techniques, adsorption is considered as the best technique especially onto nanomaterials and this is a proficient technique to remove the heavy metals from the wastewater as compared with other methods that are not efficient in the treatment of water.^8–10 Several adsorbents as chelating minerals,¹¹,¹² activated carbon,¹³ biopolymers,¹⁴,¹⁵ engineered nanomaterials¹⁶,¹⁸ and nanocomposites^19–22 have been utilized for the removal of pollutants.

Nanomaterials have unique properties which are absent in bulk-sized materials. Nanomaterials are used due to their large surface area, small size, reactivity, and active aromaticity. The high surface area-to-volume ratio of the nanomaterial helps to increase the sorption efficiency which can be improved more by the surface modification through the use of functional groups on the surface of the material without the interaction with the bulk properties.^23–26 The surface modified materials have been explored immensely and resulted in enhanced performances in wastewater treatments.²⁷,²⁸ Several nanomaterials like nanospheres,²⁹,³⁰ nanorods³¹,³² and nanotubes³³,³⁵ are used for the removal of distinct pollutants in the wastewater. Carbon nanotubes (CNTs) have attracted enormous attention of the researchers and scientists since the first discovery in 1991 due to the unique physical, chemical, electrical, and structural properties of CNTs.³⁶ CNTs are utilized in several technologies to mitigate water scarcity by cleaning water in the world. Various carbon nanotube nanotechnologies have immense water treatment to clean wate...