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Phenolic Wastewater Treatment: Development and Applications of New Adsorbent Materials 353

of p electrons or p-complexation, indicate the potential for fabricating new
and highly selective adsorbents, which is very much desirable in the indus-
trial wastewater treatment process with adsorption methodology where
selective removal of hazardous chemicals is required. The substitution of
electron-withdrawing groups such as the nitro group reduces the electron
density of the p-system of the benzene ring, which acts as an electron accep-
tor; the carboxyl oxygen on the carbon surface acts as an electron donor.
On the basis of the FTIR spectrum of ATW (Ahmaruzzaman and Laxmi
Gayatri, 2010b), it is seen that the surface of ATW is enriched with groups
such as dCOOH, dOH, and dC]C. Consequently, it is assumed that
there is a possibility of pdp interaction between C]C double bonds or the
benzene ring of phenol molecules and the C]C of ATW surface (Chen
et al., 2007; Ma et al., 2011). In addition to this pdp interaction, the
presence of a typical electron acceptor group, the dCl group in the case
of 4-CP, can contribute toward electron-donor acceptor interaction
between 4-CP and the ATW surface (which can act both as an acceptor
and a donor group). Thus, the most likely mechanism for 4-CP adsorption
by ATW is electron donor-acceptor interaction. Similarly, the presence
of the electron-withdrawing dNO 2 group causes the aromatic ring of
the solute molecule (4-NP) to act as an electron acceptor. The details of
the mechanism of interaction of phenols on the ATW surface may also
be supplemented from pH and pH ZPC (value of 2.40) studies where the
uptake of phenols was observed to be higher in the acidic pH range with
the surface remaining mostly negatively charged with a fall in percent
adsorption at higher pH above the pK a value of the respective dissociated
phenol molecules. This therefore suggests the participation of neutral phenol
molecules toward the diffusion into the porous surface of ATW.

8.5.1 Adsorption Mechanism in AES
The nature of adsorption in AES may similarly be explained on the basis of
FTIR. The presence of carboxylic, carbonyl, and hydroxyl groups in the
adsorbent is expected to play a role in the adsorption mechanism. The usual
pH factor also plays a significant role in explaining the adsorption mecha-
nism. Again, properties of the adsorbate molecule, like their molecular sizes,
solubility factor, pK a , and nature of the substituents, also influence the
adsorption process. The molecular size determines the accessibility of
the adsorbate to the pores of carbon, while aqueous solubility is related to
the degree of hydrophobic interaction between the adsorbate and the