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354 Industrial Wastewater Treatment, Recycling, and Reuse
adsorbent surface and pK a to the dissociation of the adsorbate. The substit-
uents present on the benzene ring of the adsorbate also influence the adsorp-
tion process due to their electron withdrawing/releasing abilities, which
finally affects the nonelectrostatic interactions between the adsorbate and
the adsorbent. Mattson et al. (1970) proposed that the adsorption of phenolic
compounds on an AC surface was due to the p-electron system of the aro-
matic ring, which can act as an electron acceptor in a donor-acceptor com-
plex, while Coughlin and Ezra (1968) suggest dispersion forces acting
between the p-electrons of the graphitic basal planes of AC and the phenol
molecule. However, further research is needed to gain better insights into
the real mechanism of these adsorbents, including the one for untreated
adsorbents where there is more likely participation of weak van der Waals
forces.
8.6 RESULTS FROM BATCH ADSORPTION
The performances of the prepared adsorbents were compared using the
maximum adsorption capacities. Table 8.5 lists data for the adsorption of
4-CP, 4-NP, and phenol by both the charred and activated forms.
A comparatively good adsorption capacity was noted for the prepared
adsorbents for which the value of q max is quite notable. Therefore, this assess-
ment provides a suitable option for further commercial exploitation of the
product, which is also expected to be sustainable. It is also seen from
Table 8.5 that kinetic data in the studied cases followed the pseudo-second
order model more favorably. It may be summarized that activated adsorbents
with higher surface area and porosity adsorb phenols to a greater extent,
whereas untreated adsorbents having low surface area show little or negligi-
ble adsorption. Overall, adsorption capacity follows 4-NP>4-CP>phenol.
The solubility factor also attributed to explaining this observed result. It may
be noted that the solubility of the three phenols in aqueous media is of the
order phenol>4-CP>4-NP. A comparison of the solubility and adsorption
clearly indicates an inverse relationship between the extent of adsorption and
solubility, i.e., phenols with lesser affinity for water have a higher tendency
to get adsorbed at the solid-liquid interface.
Adsorption of 4-nitrophenol, 4-CP, and phenol on different activated
and nonactivated adsorbents indicated significant performance differences,
and the transport mechanism was found to differ widely for different adsor-
bents. The activated adsorbents were efficient for removal of 4-CP, 4-NP,
and phenol. The capacity of the adsorbents increased tremendously after