The role of dispersive and electrostatic interactions in the aqueous phase adsorption of naphthalenesulphonic acids on ozone-treated activated carbons

The adsorption of 1-naphthalenesulphonic (NS), 1,5-naphthalenedisulphonic (NDS) and 1,3,6-naphthalenetrisulphonic (NTS) acids on ozonated activated carbons was assessed. Commercial activated carbon (Filtrasorb 400) was treated with different ozone doses in order to study the effect of ozone treatment on their surface properties, and investigate the behaviour of these carbon samples in the adsorption of the above naphthalenesulphonic acids. Experimental results showed that the adsorption capacity sharply decreased as the number of sulphonic groups in the aromatic ring increased. As the concentration of oxygenated electron-withdrawing groups on the carbon surface increased, a significant reduction in adsorption capacity of aromatic sulphonic compounds was observed. These results indicate that the adsorption process takes place mainly by π-π dispersion interactions between the aromatic ring electrons of the naphthalenesulphonic acids and the basal plane of the activated carbon. Moreover, in all cases, the adsorption capacity of aromatic sulphonic acids decreased as the pH increased. This indicates that electrostatic interactions between the adsorbate and the carbon surface also took place, as a result of the ionisation of oxygenated surface groups on the carbon surface. Nevertheless, the dispersive (non-electrostatic) forces govern these adsorption processes.