The adsorption equilibrium of water soluble fatty acids on the surface modified and unmodified active carbon in aqueous solution was examined. An active carbon “Pittsburgh active carbon (Calgon Co.)” was used as an adsorbent, and these surfaces were modified by treatment of various oxidants such as nitric acid, hydrochloric-hydrofluoric acid, air, oxygen, and ozone. Water soluble fatty acid used in this experiment were low and middle fatty acids from acetic acid (C2) to nonanoic acid (C9).
The adsorption isotherms were conformed to Freundlich equation. Traube's rule was applicable to the adsorption of fatty acids on unmodified active carbon. The adsorption capacity increased with a decrease in the solubilities. On the other hand, on the surface modified active carbon by nitric acid the adsorption capacity of fatty acids decreased with an increase in the concentration of nitric acid (1 N, 4 N, and 13.2 N). However, with the active carbon modified by 13.2 N nitric acid Traube's rule was applicable to the adsorption of the middle fatty acids except lower fatty acids.
The adsorption capacity of nonanoic acid (C9) on active carbon modified by various oxidants decreased in the following order
HCl-HF>Unmodified >1 N HNO3>O2>O3 4 N HNO3 13.2 N HNO3>Air
It is considered that the adsorption capacity relates to the surface oxides of active carbon.