We have identified compound physical-chemical properties and structural criteria required to facilitate the adsorption of hydroxypyridines, quinolines, and pyrimidines onto aluminum(III), iron(III), and titanium(IV) oxides. We achieved this by measuring the adsorption behavior of a number of structurally related compounds, each with a nearly isolated variation in molecular structure, and systematically evaluating the effect of structural change on the nature and extent of adsorption. We find that the adsorption at the metal oxide-water interface is significant only when the oxo-hyroxy (keto-enol) tautomeric equilibrium in aqueous solution favors the hydroxy tautomer. Lack of adsorption of the oxo tautomer can be attributed to the absence of favorable electrostatic interactions between the compound and the surface, the absence of ligand groups capable of surface complexation, and the presence of strong intermolecular hydrogen bonding between ligand groups (carbonyl and amide) and water molecules. We find that adsorption of hydroxy tautomers with ortho-substituted cyclic N and -OH groups (such as trichloropyridinol) likely occurs via nonspecific electrostatic interactions, while non-ortho-substituted hydroxy tautomers (such as 8-hydroxyquinoline) and compounds such as (2-hydroxy-methylpyridine) likely adsorb via surface complexation. This study demonstrates that for compounds classified as tautomeric heterocycles it is imperative to accurately represent the predominant tautomer in aqueous solution so that the appropriate physical-chemical properties and reactivities are utilized in the evaluation of environmental fate.
展开▼
