Page 380 - Industrial Wastewater Treatment, Recycling and Reuse

352 Industrial Wastewater Treatment, Recycling, and Reuse

8.4.3.1 Binary Mixed Oxide of Iron and Aluminum
The thermal treatments of these hydroxides lead to the formation of a mixed
oxide with high surface area. Preliminary adsorption experiments with a
known amount of the sample for the BMO samples prepared at different tem-
peratures showed that BMO M and BMO H have relatively low adsorption
capacity; thus the batch studies on BMO M and BMO H were not investigated
for these forms. The lower adsorption capacity of BMO H and BMO M is
attributed to the destruction of oxygen surface groups at higher temperatures.
The equilibrium adsorption experiments studied in detail for BMO L exhibited
a comparatively higher adsorption capacity among the three mixed oxides.
1
BMO L showed a distinctly stronger adsorption of 4-NP (20.33 mg g ) than
1
1
4-CP (13.13 mg g ) and phenol (11.23 mg g ). Only slight differences in
removal capacity were observed for the latter two phenols. The relatively
low adsorption capacity of BMO from the activated adsorbents may be attrib-
utedtothehighlycrystallinephaseasexhibitedbytheSEMandXRDimage.It
may be recalled that in wastewater treatment, a higher crystalline phase is less
desirable because it provides a comparatively smaller surface area.

8.5 ADSORPTION MECHANISM

Understanding the mechanism of adsorption is useful in order to optimize
adsorptive wastewater treatment. In the case of carbon-based adsorbents, the
carboxyl groups are mainly responsible for the uptake of phenol by the
donor-acceptor complex mechanism (Mattson et al., 1970). Considerable
research has been done to identify mechanisms of adsorption of phenolic
compounds, and it is suggested that carbonyl oxygens of the carbon surface
act as the electron donor and the aromatic ring of the solute as an acceptor.
However, the real mechanism in AC adsorption is complex and difficult to
explain. The mechanism of irreversible phenol adsorption is proposed based
on the analysis of the average hysteresis on adsorption-desorption isotherms.
Terzyk (2003) suggested that the irreversibility is caused by two factors:
the creation of strong complexes between phenol and surface carbonyl
and lactones as well as by the polymerization. Accordingly, the mechanism
of phenol adsorption is not only determined by so called “p-p interactions”
and “p donor-acceptor complex formation” but also by (strongly depending
on temperature) the “solvent effect” balancing the influence of the two
mentioned factors on this mechanism.
Adsorbate-adsorbent interactions are governed by dispersion, electro-
static interaction, and chemical bonds. Weak chemical bonds, such as those