The intraparticle diffusion model commonly divides the
adsorption process into three stages: the rapid surface adsorption
stage, the gradual inward diffusion stage and the final equilibrium
stage. Plotting the adsorption capacity (qt) verses square root of
adsorption time (t1/2) gave characteristic curves with three
periods. These periods are attributed to the adsorption stages of
the exterior surface, interior surface, and equilibrium, respectively,
as shown in Fig. 5.