Adsorption isotherms: illusive capacity and role of oxygen

Abstract

This paper examines the influence of molecular oxygen on the adsorptive capacity of GAC. A new experimental procedure for determining adsorption isotherms is introduced. This procedure, denoted as “anaerobic”, differs from the currently used techniques, denoted as “aerobic”, in that oxygen is repeatedly purged from the test environment. The results show that the capacity of GAC for the retention of o-cresol can increase up to 3-fold in the presence of oxygen when compared to the anaerobic capacity. The same trend is observed for the adsorption of phenol and 3-ethylphenol. It is shown that this increase in capacity cannot be attributed to biological degradation of these adsorbates in the presence of oxygen. It is speculated that this phenomenon is due to some chemical reactions between the adsorbates and molecular oxygen that are catalyzed by the activated carbon surface and occur at a different time scale than physical adsorption. Initial portions of breakthrough curves for o-cresol are very accurately predicted using capacities depicted by the anaerobic isotherm, while the total GAC adsorptive capacity for o-cresol, as determined from breakthrough experiments, appears to agree closely with the capacity predicted from the aerobic isotherm.