Nanoscale Porous Materials for Water Treatment: Advances and Challenges - Nanotechnologies for Water Environment Applications - page 338

Based on the structure, CNM can be subdivided into several subgroups. The
diversity of these CNMs comes from the surface chemistry which mainly depends on the
presence or absence of heteroatoms such as oxygen, nitrogen, hydrogen, and sulfur,
which exist as surface functional groups at the edges of graphene layers. Oxygen-
containing functional groups, such as carboxyls, carbonyls, phenols, lactones, aldehydes,
ketones, pyrones, quinones, hydroquinones, and anhydrides, have a significant effect on
the interactions between an adsorbate and the carbon surface. The oxygen-containing
groups and the delocalized electrons of the graphitic structure can, to a large extent,
determine the charge and acidity/basicity of the porous carbon surface. For example
carboxyl, phenolic hydroxyl and lactonic groups are acidic while carbonyl, pyrone,
chromene and quinine are basic. Hence, pH and temperature affect the surface functional
groups which finally affect the interaction with contaminants. Hence, pH and
temperature also have strong influence on CNM's properties. In this chapter, we have
subdivided CNMs into 4 subgroups: activated carbon, ordered mesoporous carbon,
Fullerenes C 60 /C 70 and carbon nanotubes; each of them will be discussed on the basis of
their advancement for the application in water treatment.
11.2.2.1 Activated Carbon
As the name implies, activated carbon is one of the most active forms of carbon
mostly derived from charcoal with an exceptionally high surface area (e.g., typically
~500 m 2 ) and a large amount of microporosity. Based on their types and properties,
activated carbon can further be classified into different subgroups, such as powdered
activated carbon (PAC), granulated activated carbon (GAC), impregnated carbon,
pelleted activated carbon (EAC), polymers coated carbon, carbon byproduct from
industries (CBI), etc. Since pore size, pore distributions, surface area and pore surface
chemistry are the major factors in the adsorption process, activated carbon has been one
of the most widely used materials for environmental applications, mainly due to
extremely high surface area and large pore size. For example, it has been extremely
useful for environmental remediation both for air or water treatment such as spill
cleanup, groundwater remediation, drinking water filtration, air purification, capture of
volatile organic compounds such as paints or other household products.
Recent interest on nanoscale materials has greatly influenced activated carbon
research as well. Mangun et al. (2001) have synthesized nanoporous activated carbon
fibers (ACFs) with an average pore-size of 1.16 nm and surface areas ranging from 171
to 483 m 2 /g. The sorption isotherm of benzene, toluene, pxylene and ethylbenzene onto
the ACFs at 20 o C was found to follow the Freundlich equation. In all cases, the ACFs
had much higher organic sorption equilibrium constants than granular activated carbon
(Mangun et al., 2001). Granular nanoporous activated carbon prepared from
polyacrylonitrile (PAN) was found to have a surface area of 544 m 2 /g with the total pore
volume, V tot = 0.278 cm 3 /g and the mesopore volume, V micro = 0.266 cm 3 /g (also see