Interactions of xanthines with activated carbon II. The adsorption equilibrium

In the present work, we have studied the adsorption of xanthine derivatives by activated carbon sorbents in aqueous solutions. The study comprised both kinetic, equilibrium and thermodynamic aspects. The kinetic results were reported in a previous paper; the equilibrium-related results are discussed here. The two types of carbon used exhibit some differences but the equilibrium isotherms obtained are all of the H-3 type in the classification of Giles. This suggests a high affinity of the sorbents for the sorbates.We also found that the overall adsorption process comprises more than one individual adsorption–desorption process of which one leads to the formation of a ‘‘monolayer’’ and the other to the ‘‘precipitation’’ of the sorbate on the sorbent surface (multilayer adsorption); the amount of sorbate adsorbed in monolayer form was seemingly greater in C-A14.