CO₂ adsorption behavior and kinetics on chemically modified activated carbons

CO₂ adsorption capacities of activated carbon-based adsorbents subjected to different treatments, such as HNO₃ oxidation, air oxidation, alkali impregnation, and heat treatment under helium gas atmosphere, were determined by gravimetric analyses and reported previously by our group. In the current work, the experimental adsorption isotherms of these modified activated carbon samples were fitted to Langmuir, Freundlich, and Dubinin-Radushkevich (D-R) models. The best fits were obtained to the D-R equation, indicating competitive or multilayer CO₂ adsorption occurring in the micropores of the adsorbents. Air oxidation followed by alkali impregnation led to the highest micropore volume, ranging between 0.259 and 0.298 g.CO₂/g.adsorbent, which resulted in the highest CO₂ adsorption capacity of 8.87% at 1 atm and 25°C. The CO₂ adsorption kinetic plots revealed a two-step adsorption process for all the adsorbents: a relatively fast kinetic region phase followed by a slow one until reaching the equilibrium. Pseudo-first and pseudo-second order kinetics explain adsorption for the kinetic region for most of the samples. When the whole adsorption data range is considered, the adsorption cannot be explained by any model at 25°C due to the complex nature of the adsorbents, but the adsorption behavior fits rather well to pseudo-first order kinetics at 120°C for the alkali-impregnated samples.