where C is the intercept that reflects the boundary layer effect; the larger the C value, the greater the contribution of the surface sorption in the rate-controlling step and kid is the intraparticle diffusion rate constant (mg/g min1/2), which can be evaluated from the slope of the linear plots of qt versus t1/2 (Fig. 8). The contribution in adsorption process by intraparticle diffusion can be evaluated if the experimental data fit linearly with qt versus t1/2 plot. As seen from Fig. 8, the adsorption data do not produce the ‘line of best fit’ through the origin over the wide contact time. This deviation
from the origin may be due to differences of mass transfer during initial and final stages of adsorption process. This phenomenon indicates more than one process is regulating the adsorption process. Three separate zones can be clearly seen from Fig. 8. The first (Phase I), linear portion is related to the instant external diffusion due to available mesoporous active sites. The second portion (Phase II) describes the gradual slower adsorption when solutes diffuses
gradually to interior microporous surface of the particles and the third portion (phase III) is the final equilibrium stage, where adsorption process become intraparticle diffusion controlled. Similar observations were also found in the adsorption of bentazon onto AC prepared from Lawsonia inermis wood and banana stalk AC