Remediation for Heavy Metal Contamination 285

of pollutants from soil and water. The specificity for a particular hazardous chemical, the adsorption

efficiency and the benefit-cost ratio are important factors in selecting adsorbents. In the recent past,

different nanomaterials viz. carbon analogues, carbonaceous nanostructured composites, nano­

magnetic materials, microporous glasses, adsorbent sieves ceramics, clay polymers, etc. have been

synthesized (Kumar and Guleria 2020). In comparison to the materials utilized historically and

commercially as adsorbents, they have been found to have a high adsorption capacity for heavy

metals from wastewater. Among nanoabsorbents, carbon nanomaterials are most extensively used

for the removal of heavy metals. The carbon nanomaterials can be classified as (i) Zero-dimensional

(having all the three dimensions less than 100 nm); for example, are fullerene and quantum dots.

(ii) One-dimensional (1 D) (having only one dimension larger than 100 nm and two dimensions

smaller than 100 nm), e.g., nanotubes of carbon and titanium. (iii) Two-dimensional nanomaterials

(with two dimensions greater than 100 nm), e.g., graphene. (iv) Three dimensional (3-D) (all

dimensions are greater than 100 nm), e.g., graphite and some nono-composites. Highly efficient

nano absorbents are described next.

16.4.2.1 Carbon Nano Tubes (CNTs)

Engineered materials with unique characteristics such as electrical conductivity, optical activity,

mechanical strength and surface morphologies are known as carbon nanotubes (CNTs). Due to

their high porosity, light mass density, large specific area, hollow structure and strong interactions

with pollutant molecules, they are used as effective adsorbents. Internal sites, interstitial channels,

grooves and the outer surface of CNT bundles are all adsorption sites. When compared to internal

sites, exterior sites attain equilibrium faster. The adsorption process is largely driven by chemical

interactions between the surface functional groups of CNTs and metal ions. CNTs often include

–OH, –C=O and –COOH groups, depending on the synthesis and purifying procedure. However,

some other functional groups can also be added to CNTs through oxidation using Pd, Ni or Pt or

removed from CNTs through heat treatment at 2200ºC. Cu, Zn and Pb were removed using CNTs

manufactured using the microwave irradiation technique. Kaushal and Singh (2017) reported removal

efficiency of 111 mg g^–1, 71 mg g^–1, and 85 mg g^–1 for Cu (II), Pb(II) and Zn(II), respectively by these

types CNTs. Carbon nanotubes can be classified based on their dimensions into zero-dimensional

(0-D, e.g., fullerene and quantum dots), one dimensional (1-D, e.g., nanotubes and titanium), two

dimensional (2-D, e.g., graphene), or multi-dimensional. The one-dimensional single wall CNTs

(SWCNTs) are promising for their nano-sized porous structure, high surface area and easy surface

functionalization. Thus, SWCNTs are widely used for the removal of metal pollutants. Researchers

demonstrated SWCNT-based nanocomposite could efficiently adsorb Hg (more than 99.56% within

7 min). In an experiment conducted by Anitha et al. (2015) to evaluate the adsorption capacity of

unadorned SWCNTs and their functionalized SWCNTs (viz., SWCNTs-OH, SWCNTs-NH2 and

SWCNTs-COOH) to remove meta(loids). It has been observed that SWCNTs-COOH have the

highest adsorption capacities (150–230%) compared to unadorned SWCNTs. However, SWCNTs-

OH and SWCNTs-NH have resulted in poor adsorption capacity. SWCNTs-COOH have also shown

the adsorption capacity of 96, 77, and 56 mg g^-1 for Pb²⁺, Cu²⁺, and Cd²⁺, respectively. There is

another type of CNTs developed by researchers called multiwall carbon nanotubes (MWCNTs).

Zhang et al. (2012) created MWCNTs-TiO2 and used them to photodegrade methylene blue. Zhang

et al. (2012) reported MWCNTs based nano-adsorbent can efficiently removal Pb (II) and Hg (II)

to the extent of 65.40 mg g^–1 and 65.52 mg g^–1, respectively. Moreover, sulfonated multi-walled

CNTs (s-MWCNTs) were synthesized by Ge et al. (2014) through processing p-MWCNTs with

strong sulfuric acid at high temperatures to adsorb Cu (II). However, since their discovery in the

early 1990s, CNT toxicity has been a significant issue. Another disadvantage of employing CNTs

is their low removal effectiveness and limited selectivity. Furthermore, because the walls of carbon

nanotubes are not reactive, the sorption capacities for metal ions are relatively low (Mukhopadhyay

et al. 2020).