Effect of surface functional group on Freundlich adsorption isotherm

This study was designed to investigate the relationship between Freundlich isotherm coefficient l/n and the molecular properties of activated carbon adsorbent. The surface of the adsorbent was modified using nitric acid to produce different types and amounts of oxidized groups. These functional groups changed the electron density of graphite rings on the carbon surface. Isotherm data for the adsorption of PCP and PNP on the regular and modified activated carbon were obtained experimentally. Regression analyses showed that Freundlich coefficient l/n was affected by carboxyl groups stronger than three other groups – lactonized carboxyl, hydroxyl, and carbonyl. The results showed that Freundlich 1/n increased linearly with an increase in the amount of carboxyl groups. Molecular modeling using the CAChE program showed that the HOMO density at the adsorption site decreased linearly with an increase in the number of carboxyl groups. The results suggested that Freundlich 1/n could be estimated a priori based on the molecular structure of the adsorbent surface and the HOMO density of the adsorption site.