Novel Carbon-Based Nanoadsorbents for Removal of Synthetic Textile Dyes from Wastewaters - Green Chemistry for Dyes Removal from Wastewater - page 38

centimeters, or multi-walled (MWCNTs) consisting of nested tubes with
outer diameters ranging from 5 to 100 nm and lengths of tens of microns
(Figure 2.1a-c) [27,28]. The constituent cylinders within MWCNTs are
separated by ~0.35 nm, similar to the basal plane separation in graphite,
and may possess different chiral structures (Figure 2.1b). A special case
of MWCNTs is the double-walled CNT composed of just two concentric
cylinders (Figure 2.1c).
Carbon nanotubes have extremely high tensile strength (≈150 GPa,
more than 100 times that of stainless steel), high Young's modulus
(≈1 TPa), large aspect ratio, low density (1.1-1.3 g cm -3 , one-sixth of that
of stainless steel), good chemical and environmental stability, high ther-
mal conductivity ( 3000W m -1 K -1 , comparable to diamond) and high
electrical conductivity (comparable to copper) [30]. Thus CNTs are very
promising in field emitters [31], nanoelectronic devices [32], electrodes
for electrochemical double-layer capacitors [33], hydrogen storage [34],
and many more areas.
In environmental engineering, CNTs are envisaged as a promising new
class of adsorbent for the remediation of toxic pollutants because of their
highly porous and hollow structure, large specific surface area, light mass
density, surface functional groups and hydrophobic surfaces [22,35]. Due

( a )

( b ) ( c )

Figure 2.1 Structural representation of (a) a SWCNT, (b) a MWCNT made up of three
shells of different chirality and (c) a double-walled CNT. Reprinted from [26]; Copyright
2004, with permission from John Wiley and Sons, and from [29]; Copyright 2009, with
permission from the American Chemical Society.