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
shown in the Fig. 5, the slope of the first linear portion
characterizes the rate parameter corresponding to the
intra-particle diffusion, whereas the intercept of this
portion is proportional to the boundary layer thickness.
The R2 value for this diffusion model was 0.9899 for
APR and 0.953 for RPR. This indicates that the adsorption
of MB onto APR and RPR can be followed
by intra-particle diffusion model. However, the lines
do not pass through the origin, indicating that intraparticle
diffusion is not the only rate limiting mechanism
and that some other mechanisms also play an important
role. Surface adsorption and interaparticle diffusion
were likely to take place simultaneously; both
processes control the kinetics of dye-adsorbent interaction