Photocatalytic Processes for the Removal of Dye - Green Chemistry for Dyes Removal from Wastewater - page 143

adsorbent for pollutant sequestration [42]. The latter has been one of the
most attractive applications of this waste and will be reviewed in detail.
Han et al. [43] used rice husk without any treatment for the adsorption
of Congo red dye from effluents in a fixed-bed column. They found that the
pH, initial dye concentration, flow rate of the effluent and bed depth have
significant effects on the adsorption capacity of rice husk. They observed
that an increase in the pH level of the solution would lead to a decrease
in the adsorption efficiency of the adsorbent. They attributed this effect
to the change in the surface charge of the adsorbent and a competition
between the dye molecules and OH - ions at higher pH values. Also it has
been pointed out that the existence of salt in the effluent would enhance
the removal capacity of the adsorbents due to the reduced repulsive forces
between the surface functional groups of the rice husk and dye molecules.
However, it should be noted that the maximum capacity of the untreated
rice husk was measured to be only 3 mg.g -1 , which is a very low value.
The adsorption equilibrium and kinetics of two types of dye molecules by
unmodified rice husk have been investigated by Safa et al. [44]. hey con-
sidered the effect of various parameters such as initial dye concentration,
pH, adsorbent dose and adsorbent particle size on its removal efficiency.
Similar to the study by Han et al. [43], they also observed a decrease in the
removal percent by an increase in the pH value. Moreover, the adsorption
capacity of the adsorbent decreases by increasing the rice husk dose, which
was ascribed to the aggregation of the adsorbent particles at high loadings
and the availability of fewer binding sites.
In order to increase the adsorption capacity of the rice husk, some
researchers have investigated its chemical and/or physical modification.
Chemical modification using sodium hydroxide has been carried out for
this purpose [45,46]. However, the maximum adsorption capacity of the
NaOH-modified rice husk for Malachite Green was found to be around
16 mg.g -1 , whereas the same adsorbent material has a capacity of around
45 mg.g -1 for Crystal Violet. These results show that this modification
procedure is not quite useful to increase the adsorption capacity of rice
husk. In contrast to the literature studies conducted by Han et al. [43], an
increase in the pH value of the dye solution resulted in a drastic increase
in the removal amount of the dye molecules (see Figure 5.1). This behav-
ior was rationalized by the fact that the surface of the adsorbent was
positively-charged at very low pH values and thus electrostatic repulsion
occurred between the positively-charged dye molecules and the surface of
the adsorbent. As the pH of the solution increased, deprotonation of the
adsorbent surface sites took place and subsequent attraction between the
negatively-charged surface sites and positively-charged dye molecules led