Influence of surface functional groups and solution pH on removal of organic compounds and a heavy metal by activated carbon

Influence of surface functional groups and solution pH on the adsorption of dissolved organic substances (aniline andnitrobenzene) and a heavy metal (Cd (II)) on activated carbon has been studied. Commercially available activated carbon (AC) and oxidized one with nitric acid (Ox-AC) were employed as adsorbents for the experiments. Adsorption ability of the adsorbents was evaluated by Langmuir- and Freundlich-type adsorption isotherms. Point of zero charge (pHpzc) of the activated carbon was dropped from 9.2 to 3.2 by the oxidation. Decrease in organic matter uptake was also observed for Ox-AC due to decrease in π electron density and formation of water molecular clusters on the carboxyl groups located on the pore mouth at higher pH region. When aniline was used as adsorbate, decline of adsorption probably owing to repulsive force between anilinium ion and graphene layer occurred at lower pH value. On the other hand, Cd (II) uptake by Ox-AC was rapidly increased as the equilibrium solution pH rose, whereas it was not for AC. Since Cd (II) adsorption sites were considered to be carboxyl groups at the edge of the graphene layers, the Cd (II) adsorption capability was greatly influenced by the charge of them on the Ox-AC.