# A Review of Adsorbents for Heavy Metal Decontamination: Growing Approach to Wastewater Treatment
## 6 Factors Affecting the Adsorption Process
### Commercially Available Adsorbents
#### 6.9.2 Carbon Nanotubes  
Carbon has the ability to exist in a variety of molecular forms, which are referred to as allotropes of carbon. These allotropes can be thought of as various carbon structural variations. Carbon nanotubes (CNTs) are made up of cylindrical graphite sheets (an allotropic type of carbon) that have been rolled into a tube-like structure. Carbon nanotubes (CNTs), which belong to the fullerene structural family, are being studied for their ability to remove heavy metals from water (lead, chromium, cadmium, arsenic, copper, zinc, and nickel) [259]. Single-walled carbon nanotubes (SWCNTs) are cylindrical carbon nanotubes with a single graphene shell. Multi-walled carbon nanotubes (MWCNTs), on the other hand, are made up of many layers of graphene sheets. The structure of (a) multiwall carbon nanotubes (MWCNTs) and (b) single-walled carbon nanotubes is shown in Figure 12.  
[PERSON] et al. (2013) found that MnO\({}_{2}\) functionalised MWCNTs have a 41.6 mg/g adsorption capability for cadmium removal [261]. [PERSON] et al. (2015) studied the adsorptive removal of cadmium on alumina-decorated MWCNTs and discovered a 27.21 mg/g adsorption capacity [262]. Furthermore, practically all studies show that the acid modified CNTs have a higher adoption capacity than the raw CNTs. This could be owing to an electrostatic interaction between the divalent heavy metal ions and the negative charge on the CNTs surface following acid treatment. CNTs are sometimes used in combination with other nanoparticles to boost the effectiveness of removal. Heavy metal ions have been successfully removed by modified carbon nanotubes [263; 264]. Another technique to improve the surface features of CNTs is to graft functional groups or molecules onto them. Several studies have shown that CNTs grafted with various functional groups may remove heavy metals from aqueous solutions [265; 266; 267]. Figure 13 depicts a schematic of typical CNT surface grafting and subsequent metal ion adsorption.  
Figure 12: The structure of (**a**) MWCNT and (**b**) SWCNT [260].  
In some research, the combined effect of electrostatic attraction and sorption-precipitation was shown to absorb metal ions [269]. Figure 14 showed possible methods for interacting cadmium ion adsorption on the surface of CNT/Al2O3. Cadmium ions are adsorbed on the surface of modified CNTs via physical interactions with the Cd (II) and hexagonally arranged carbon atoms in the graphite sheet. Cadmium ions are adsorbed in the graphite sheet. [PERSON] et al. [270] study's yielded promising results, demonstrating that MWCNTs treated with nitric acid may effectively remove metal ions from multicomponent systems. Maximal adsorption capacities, calculated by applying the equation of Langmuir for adsorption isotherms for the individual ions were 97.08 mg/g for Pb2+, 24.49 mg/g for Cu2+, and 10.86 mg/g for Cd2+.  
Figure 14: Synthesis routes of activated carbons from bio-waste.  
Figure 13: Schematic representation of CNTs grafting and metals adsorption [268].