Reactive Yellow 2 and C.I. Reactive Red 4 dyes, respectively.
The values of q indicate that the formation of a complete molecular
layer (q ¼ 1) was not achieved for the three dyes and
a large fraction of the adsorbent surface remains unoccupied,
particularly in the case of C.I. Reactive Red 4. The incomplete
formation of a monolayer may be attributed to large molecular
diameter of the dye molecules which cannot fully access the
sorbent micropores that account for about 87% of the total surface
area of the adsorbent (see Table 3).
3.3.1.2. Freundlich model. The model parameters and r2
values are presented in Table 4, which indicate that this model
showed lower correlation with the experimental adsorption
data compared to the Langmuir model. The extent of adsorption
for C.I. Reactive Blue 2 was approximately three times
that of C.I. Reactive Red 4, as inferred from the values of
KF, 0.30 and 0.10, respectively. The high adsorption capacity
of C.I. Reactive Blue 2 compared to C.I. Reactive Red 4 can
also be illustrated from the values of Qmax using the Langmuir
model. The n values for all adsorption systems studied were
less than unity, which reflects the favorable adsorption of the
reactive dyes over the entire concentration range used in this
study (1.0 104e1.2 103 mol/dm3) [31]. Furthermore,
the surface of activated carbon is known to be highly heterogeneous
and the energies of active sites are highly variable,
which would also tend to make the values of n less than unity
[22,32].
3.4. Effect of solution pH and ionic strength on
dye adsorption and identification of the
adsorption mechanism
The effect of solution pH on dye removal from solution was
studied under identical conditions for the three dyes chosen for
this study. The data are presented in Fig. 5, which indicate that
the adsorption behavior of each of the reactive dyes was similar,
from pH 2 to pH 10, (e.g., the removal of C.I. Reactive
Blue 2 decreased from 70% to 56%, when the pH was increased
from 2 to 4, with the percentage removal then remaining
almost constant up to pH 8). A large decrease in adsorption
capacity for this dye was observed under basic conditions (i.e.,
a decrease to 40% removal at pH 10). Similar adsorption behavior
with variation in solution pH has been reported in the
literature [2,33]. If electrostatic interaction was the only mechanism
for the dye adsorption, then the removal capacity should