Importance of surface chemical properties of carbons for their use as adsorbents in aqueous media

An investigation of the influence of the surface chemical properties of activated carbons on their use as adsorbents in aqueous media was conducted.;Commercial activated carbon samples were modified by aqueous oxidation with nitric acid, reaction in ammonia, and heat treatments in nitrogen or hydrogen. A monotonic decrease in point of zero charge with oxygen content of activated carbon was evident. This trend was attributed to the behavior of activated carbons as Bronsted acids or Lewis bases. The differences between isoelectric point and point of zero charge values were attributed to differences in the properties of the external and internal surfaces. The electrokinetics of carbon surface and solution equilibria were invoked to provide a satisfactory explanation for the effect of pH on adsorption on carbons.;A theoretical model which describes the adsorption of weak electrolytes in terms of fundamental intermolecular forces was employed to predict the adsorption of weak electrolytes on carbons. It was noted that the model did not predict the existence of a clear adsorption maximum for pKa ;Nitric acid oxidation of carbon enhanced the adsorption of oxalic and fumaric acids formed by hydrolysis of the corresponding sodium salts in aqueous solution. Adsorption of both benzoate and phenolate anions decreased as surface acidity increased. The reduced adsorption of aromatic anions on oxidized carbons was attributed to electrostatic repulsion between the anionic adsorbate species and the negatively charged carbon surface, as well as reduced dispersion interactions between the aromatic rings of both adsorbate and adsorbent. Reaction of carbon with ammonia lowered benzoate anion adsorption but had no significant effect on the adsorption of phenolate anions. Nitric acid oxidation and reaction of carbons with ammonia were observed to enhance the adsorption of selected aliphatic anions at high pH.