Effect of CO2 activation of carbon xerogels on the adsorption of methylene blue

The effect of physical activation with CO₂ of carbon xerogels, synthesized by pyrolysis of a resorcinol-formaldehyde aqueous gel, on the adsorption capacities of Methylene Blue (MB) was studied. The activation with CO₂ lead?to carbon materials with micropore volumes ranging from $0.28\ \mathrm{to}\ 0.98~\mathrm{cm}^{3}\,\mathrm{g}_{\mathrm{C}}^{-1}$ . MB-adsorption isotherm studies showed that the increase of micropore volume and corresponding surface area led to: (i) a significant improvement in the capacity of MB-adsorption at monolayer coverage, from $212\ \mathrm{to}\ 714~\mathrm{mg}\,\mathrm{g}_{\mathrm{C}}^{-1}$ , and (ii) an increase of the binding energy related to Langmuir isotherm constant up to 45 times greater than those of commercial microporous activated carbons used as reference (NORIT R2030, CALGON BPL and CALGON NC35). It is proposed that the increase of the binding energy results from chemical cleaning of the O-groups onto carbon surface as a consequence of CO₂-activation, increasing the π–π interaction between MB and graphene layers of the carbon xerogels. Finally, a series of batch kinetics were performed to investigate the effect of CO₂-activation conditions on the mechanism of MB-adsorption. Experimental data were fitted using pseudo-first-order, pseudo-second-order and intraparticle diffusion kinetic models. From pseudo-second-order kinetic model, one observes an increase in the initial rate of MB-adsorption from 0.019 to 0.0565 min^?1, by increasing the specific surface area from $630\ \mathrm{to}\ 2180~\mathrm{m}^{2}\,\mathrm{g}_{\mathrm{C}}^{-1}$ via CO₂-activation. Depending on the activation degree of the carbons, two different mechanisms control the MB-adsorption rate: (i) at low activation degree, the intraparticle diffusion is the rate-limiting phenomenon, whereas (ii) at high activation degree, the reactions occurring at the solid/liquid interface are the rate-limiting steps.