Effect of solution pH on the removal of clofibric acid by cork-based activated carbons

Clofibric acid adsorption from the aqueous phase was studied using cork-based activated carbons (CAC: chemically activated with K2CO3; CPAC: physical activation of sample CAC with steam). CPAC outperformed the uptake of water treatment commercial carbons. Results highlight the importance of pH in clofibric acid adsorption: the highest removals were obtained for pH 2.0 and decrease for higher pHs. The sigmoidal adsorption isotherms obtained were fitted to the Dubinin–Astakhov equation. The characteristic adsorption energy revealed that CAC has the highest affinity for the solute, in accordance with its narrow micropore size distribution. The molecular and electronic structure showed that the solvation energy of the undissociated and dissociated forms of clofibric acid is the key factor to explain the isotherm shape and the dependence of the pH. For pH 3.6 the dissociated form is dominant and the uptake significantly decreases, showing that the solvent shields the interaction of the dissociated specie (higher solvation energy) with the carbon surface. The results show that once the solvation energies of the undissociated and dissociated forms of clofibric acid are known, a complete characterization of an activated carbon allows one to predict with confidence its behavior for the adsorption of this compound from the aqueous phase.