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Effect of the Alcohol Cosolvent in the Removal of Caffeine by Activated Carbons

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journal contribution
posted on 25.07.2012, 00:00 by Ana S. Mestre, Susana C. R. Marques, Ana P. Carvalho
The effect of the addition of methanol and 2-propanol as cosolvents on the caffeine adsorption onto activated carbons was evaluated by both kinetic and equilibrium studies. The kinetic results presented a good adjustment to the pseudo-second-order kinetic model and reveal that the alcohol cosolvent has a double negative effect on caffeine adsorption: it enhances caffeine’s solubility lowering its affinity toward the adsorbent and it seems to compete with caffeine for the adsorption active sites. The change in the isotherm shape from L to F type, when the mixture 2-propanol/water was used, was justified in terms of the higher interaction between the caffeine and 2-propanol molecules. This solvent presents the most similar Hansen three-dimensional solubility parameters when compared to the ones of caffeine. The overall adsorption results point out that when the sample with acidic surface chemistry is used, the water adsorption is favored and consequently the effect of the cosolvent is less marked and is mainly explained by the solubility. For the carbons with basic surfaces the addition of the alcohol cosolvent reflects in the adsorption mechanism due to the competitive adsorption of the alcohol, especially when the cosolvent is 2-propanol.