Initial behavior of intraparticle diffusion model used in the description of adsorption kinetics

The intraparticle diffusion model (IPD) proposed by Weber and Morris has been widely applied for the analysis of adsorption kinetics. In this work, the characteristic curves based on this model were plotted with various initial adsorption factors (R_i). Four zones of the initial adsorption according to R_i value from 0 to 1 were classified; that is, approaching completely initial adsorption (zone 4), strongly initial adsorption (zone 3), intermediately initial adsorption (zone 2), and weakly initial adsorption (zone 1). Activated carbons with micropore volume fraction of 0.537 and 0.686 were prepared from oil-palm shells by steam activation. Based on the standard deviations, the kinetics of the adsorption of tannic acid (TA), methylene blue (MB), phenol, and 4-chlorophenol (4-CP) on activated carbons could be best described by intraparticle diffusion model. The initial adsorption of TA and MB belonged to zone 2, and that of phenol and 4-CP mostly belonged to zone 3. Nearly 80% of the 86 adsorption systems surveyed belonged to zones 2 and 3, indicating that the R_i value was smaller when the carbon with smaller particle and steam-activated carbon was used.