**2.4 Nanostructured materials**

In the last decade, carbon nanotubes [91], fullerenes [92] and graphene [93] have occupied an important place in the area of adsorption of heavy metals from effluents. They possess exceptional mechanical and chemical properties, strength, exchange capacity, electrical conductivity and thermal stability. A high surface area along with numerous intermolecular interactions gives them an edge over other adsorbents in remediation of heavy metals.

## *2.4.1 Carbon nanotubes, fullerenes and graphene*

Iijima discovered carbon nanotubes (CNTs) in 1991 [94]. They exist as long carbon cylindrical in shape with a continuous hexagonal graphite sheets. They are of two types: single walled CNT, which have a single graphite sheet and multi walled CNTs which have multiple sheets. They have portrayed excellent potential for heavy metal from wastewater for copper [95, 96] lead, [97, 98], chromium [99, 100], nickel [100, 101] and cadmium [100, 102]. CNTs prove to be excellent adsorbents owing to the advantages such as mechanical and surface properties electrical and semiconductor properties [102, 103]. They also provide a high specific surface area (150-1500m2 /g) and the presence of mesopores increases their adsorption efficiency [104–107]. The presence of different functional groups containing elements such as oxygen, nitrogen and sulfur directly and indirectly affect the adsorption mechanisms that enhance the adsorption of heavy metals [108–111].

Oxidized CNTs also portray exceedingly high adsorption capacity for the removal of Cr6+, Pb2+ and Cd2+ from wastewater [112–115]. Wang et al. (2007a) carried out a study using MWCNTs activated with conc. HNO3 which escalated the adsorption capacity due to creation of more oxygen functional groups. The equilibrium time for Pb2+ adsorption was found to be 120 min at an optimum pH 2.0 [116]. Nanocomposites are also prepared using CNTs with ferrous, zirconium, aluminium oxides by coprecipitation method for removal of Pb, As, Cu, Ni and Cr ions [117–122]. Luo et al. synthesized Fe2O3/MnO2/acid oxidized MWCNT nanocomposites for removal of Cr6+. At an optimum pH of 2.0 a maximum removal capacity of 85% was achieved by the nanocomposite [123]. Ge et al. prepared magnetic Fe@MgO nanocomposites for the removal of Pb2+ from water. A maximum adsorption efficiency of 14746.4 mg/g was achieved for Pb2+ at 120 min contact time [124]. Stafiej and Pyrzynska stated some facts related to adsorption capacity of CNTs and reached a conclusion that pH and concentration of heavy metals significantly affect the CNTs efficiency [125]. CNTs portray excellent adsorption efficiency due to their surface morphology, electrochemical potential and ion exchange capacity [126, 127]. The ability of CNTs to be easily modified makes them selective adsorbents with the merit of enhanced adsorption efficiency [113, 127–130]. They are instituted as great adsorbents in the field of wastewater treatment due to their appreciable mechanical and surface characteristics,

mechanical and magnetic properties and high stability [131]. But the use is restricted due to the accumulation of the active sites by the adsorbate. Hence, activation of CNTs offers the advantage of increasing the sites with functional groups which in turn increases their adsorption efficiency for heavy metal removal from water and wastewater [132–138].

The discovery of fullerenes in 1985 led to another breakthrough in adsorption science [139, 140]. They have a closed-cage structure containing pentagonal and hexagonal carbon rings with the formula C20+m, m being an integer. Their adsorption efficiency can also be attributed to their surface morphology and presence of mesopores which gives then higher ion affinity and higher specific surface area for remediation of heavy metal ions from water and wastewater [141, 142]. Alekseeva et al. conducted a study using fullerenes for the removal of Cu2+ and explained the mechanism through Langmuir model [143]. The maximum adsorption efficiency was found to be 14.6 mmol/g. Spherical fullerene containing 60 carbon atoms is the most explored one. Its striking features comprise of hydroxyl and epoxy functional groups on surface, large surface to volume ratio, hydrophobicity, high electron affinity and low aggregation capacity which make it beneficial for heavy metal removal [144–146]. But their use is often restricted due to their high price. So, research on incorporation of other conventional adsorbents with fullerenes has come up. It was revealed that fullerenes enhance the porous structure of adsorbent leading to increase in the removal efficiency of heavy metals. It was found that adsorption capacity of ACs escalated by 1.5-2.5 times after introduction of fullerenes into their structure [147, 148].

Graphene came into the scene in 2004 and is a 2-D hexagonal lattice of carbon atoms. It also possesses structural, chemical and mechanical properties which aid its use in wastewater treatment. It has a high surface area, active functional groups and sites on its surface which enhance its adsorption capacity [149–151]. Graphene can also be activated by oxidation to increase functional groups which surge the adsorption capacity for removal of heavy metals [114, 152–155]. Deng et al. 2010 conducted a study using functionalized graphene for removal of Pb2+ from aqueous solution. At an optimum pH of 5.0 the maximum adsorption capacity reached was 406.6mn/g within 40 min [156]. Several studies were conducted to study the properties of graphene oxides for adsorption [157–161]. It was revealed that graphene oxides can also be magnetically modified which increases their adsorption capacity [162, 163]. A study by Zhao et al. used layered graphene oxide for removal of Pb2+ from aqueous solution. The adsorbent layers had oxygen functional groups which greatly enhanced the adsorption capacity reaching a maximum of 1850 mg/g [164]. Jian et al. synthesized a bio-adsorbent polyacrylamide/graphene oxide hydrogel grafted with sodium alginate and studied the removal of Cu2+ and Pb2+ from aqueous solution. The maximum adsorption capacity of Cu2+ was 68.76 mg/g at pH 5 and 240.69 mg/g for Pb2+ at 5.5 pH [165].

#### **2.5 Low cost adsorbents**

Although, ACs are the most widely used adsorbents, their use is limited due to their high cost and low regeneration. Same is with other developed adsorbents such as carbon nanotubes, fullerenes and nanocomposites. To make the process of wastewater treatment speed up and effective, it is vital to look for adsorbents that are cost effective as well administer a high adsorption efficiency. Thus, the need for low cost adsorbents came to be realized. Low cost adsorbents comprise of those non-conventional materials that are easily available and cost effective mainly agricultural and industrial waste.

#### *Removal of Heavy Metals from Wastewater by Adsorption DOI: http://dx.doi.org/10.5772/intechopen.95841*


**Table 2.** *Agricultural wastes for heavy metal removal.*


**Table 3.** *Industrial wastes for heavy metal removal.*
