If the intra-particle diffusion is involved in the
adsorption processes, then the plot of the square root
of time versus the uptake (q ) would result in a linear t
relationship and the intra-particle diffusion would be
controlling step if this line passed through the origin.
When the plots do not pass through the origin, this
indicative of some degree of boundary layer control
and this further shows that the intra-particle diffusion
is not the only rate controlling step, but also other
processes may control the rate of adsorption [32]. As
The effect of initial pH on bioadsorption of dyes
was examined over a range of pH values from 2 to 10
and the results are presented in Fig. 1. As elucidated in
Fig. 1, the dye removal was minimum at pH 2 for both
adsorbents. The dyes adsorbed increased as the pH
was increased from 2 to 7 for APR and from 2 to 8 for
RPR. Then, beyond pH 7 and 8 there was no notable
change. For this reason, pH 7 and pH 8 was selected
for future experiments for APR and RPR respectively.
After adsorption experiments, it was found that at low
pH and at high pH, the dye become protonated, the
electrostatic repulsion between the protonated dyes and
positively charged adsorbent sites results in decreased
adsorption. Higher adsorption at pH 7 and 8 may be
due to increased protonation by the neutralization of
the negative charges at the surface of the adsorbent,
which facilitates the diffusion process and provides
more active sites for the adsorbent. Furthermore, the
solution pH is above the zero point charge (pH = 4.8 zpc
for APR and 4.4 for RPR) and hence the negative
charge density of the surface of the adsorbents increases
which favors the adsorption of cationic dye