3.7. Adsorption kinetics
Calculated parameters of the various kinetics model and the validation results have been reported (Table 6). Low X2 and
Δqe characterized the pseudo second order and the Elovich models suggesting that these two models describe the kinetics
of RhB uptake onto RH. The initial adsorption rate (h) was found to increase as the initial adsorbate concentration increased
suggesting that there was increase in driving force for mass transfer hence more dye molecule reached the adsorbent
surface within a short period of time. βEl is a constant related to the extension of surface coverage and decreased with
increased in initial adsorbate concentration, the surface area of RH was obtained to be very low (Table 2) hence uptake of
RhB onto RH may have been more through functional groups. However, the constant αEl that is related to chemisorption rate
was found to increase as the initial dye concentration increased, this suggests that more than one mechanism governs the
uptake of RhB onto RH [39]. Although the values of constant v obtained for the fractional power kinetic model was observed
to be less than unity suggesting that fractional power may describe the adsorption kinetics of RhB-RH system. The large
values of X2 and Δqe obtained for fractional power and Avrami kinetic models validates that these two kinetic model does
not describe the kinetics of RhB uptake onto RH. The Bangham and the intraparticle diffusion models clearly show that
multiple adsorption stages occurred. Correlation coefficient obtained from the Bangham model ranged between 0.9069 and
0.9797 suggesting that pore diffusion was involved in RhB uptake onto RH. However, the linearity of the Bangham plotsuggests that adsorbate pore diffusion is not the only rate controlling step. Multilinear profile was obtained for intraparticle
diffusion model plot and this plot did not pass through the origin (Fig. 8(f)), this suggests that boundary layer diffusion also
occurred in the uptake of RhB onto RH. While ki1, C1 and R2
1 are the slope, intercept and correlation of the first steeper
portion, ki2, C2 and R2
2 indicates the slope, intercept and correlation of the second linear portion respectively. Since the
values of Ki1 are greater than the observed values of ki2, this indicates that intraparticle diffusion model mainly controlled
the uptake of RhB onto RH.