Comparisons of pore properties and adsorption performance of KOH-activated and steam-activated carbons

Carbonaceous adsorbents with controllable pore sizes derived from carbonized pistachio shells (i.e., char) were prepared by the KOH activation and steam activation methods in this work. The pore properties including the BET surface area, pore volume, pore size distribution, and pore diameter of these activated carbons were characterized by the t-plot method based on N2 adsorption isotherms. Through varying the KOH/char ratios from 0.5 to 3, the KOH-activated carbons exhibited BET surface areas ranging from 731 to 1687 m2/g with a similar micropore content (80-92%). The carbons activated by steam at 830 °C for 2 h had a BET surface area of 821 m2/g with the micropore content of 42%. The micropore/total pore volume ratio (Vmicro/Vpore) and average pore size (Dpore) were independent of the KOH/char ratio, revealing that KOH activation is a powerful method in developing and controlling the number of micropores with a very similar pore size distribution. The adsorption equilibria and kinetics of methylene blue, basic brown 1, acid blue 74, 2,4-dichlorophenol, 4-chlorophenol, and phenol from water on all activated carbons at 30 °C were investigated to demonstrate the fact that adsorption of organics is not only dependent upon the BET surface area but is also determined by the relative size between pores and molecules. The adsorption isotherms were subjected to the model fitting according to Langmuir and Freudlich equations. By comparing the projected area of adsorbates, the surface coverage of phenols is about 3.6 times of that of dyes (based on unit gram of activated carbon). The Elovich equation was found to suitably describe the adsorption process of all KOH-activated carbons while the adsorption behavior on the steam-activated carbon was reasonably fitted with the intraparticle diffusion model.