Adsorption of Catechols, 2-Aminophenols, and 1,2-Phenylenediamines at the Metal (Hydr)Oxide/Water Interface: Effect of Ring Substituents on the Adsorption onto TiO2

Environ. Sci. Technol., 30 (5), 1604 -1613 es950615+

Adsorption of Catechols, 2-Aminophenols, and 1,2-Phenylenediamines at the Metal (Hydr)Oxide/Water Interface: Effect of Ring Substituents on the Adsorption onto TiO₂

Dharni Vasudevan and Alan T. Stone*

Department of Geography and Environmental Engineering, G. W. C. Whiting School of Engineering, The Johns Hopkins University, Baltimore, Maryland 21218

Received: August 17, 1995

Revised December 6, 1995

Accepted December 12, 1995

Abstract:

The extent of adsorption and the value of the adsorption equilibrium constant (K^s_intr) for simple organic ligands is influenced by the identity of the ligand donor groups and other substituents on the aromatic ring. Catechols adsorb onto TiO₂ to a significantly greater extent than 2-aminophenols; the adsorption of 1,2-phenylenediamines is negligible. The TiO₂ surface has a high ionic contribution to bonding; ligands possessing donor groups with the highest ionic contribution to bonding adsorb to the greatest extent. A covalent contribution to bonding can increase binding to Ti(IV)-containing surfaces, but only when the ionic contribution is already strong. Within each ligand class, substituents alter the competition between protons and surface sites for binding the deprotonated ligand. For this reason, pK_a1, pK_a2, and log K^s_intr are all important in determining the extent of adsorption. Additionally, substituents that impart hydrophobicity also raise the extent of adsorption and the value of log K^s_intr.

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