Adsorption of 4-Nitrocatechol, 4-Nitro-2-Aminophenol, and 4-Nitro-1,2-Phenylenediamine at the Metal (Hydr)Oxide/Water Interface: Effect of Metal (Hydr)Oxide Properties

Aromatic ligands possessing three combinations of phenolate and amino ligand donor groups ortho to one another have been used to compare the adsorption properties of six (hydr)oxide mineral preparations as a function of ligand concentration, surface loading, pH, and ionic strength. Strong adsorption of 4-nitrocatechol onto Ti(IV)-, Fe(III)-, and Al(III)-bearing surfaces, moderate adsorption of 4-nitro-2-aminophenol onto Ti(IV)- and Fe(III)-bearing surfaces and weak (to negligible) adsorption onto Al(III)-bearing surfaces, and negligible adsorption of 4-nitro-1,2-phenylenediamine onto all surfaces was observed. Stoichiometries of surface complex formation and intrinsic equilibrium constants for adsorption (Ksintr) were determined using the computer program FITEQL. The charge-to-radius ratios of these +III and +IV surface-bound metals ions are high, yielding a strong ionic contribution to bonding and higherKsintrvalues for anionic phenolate ligand donor groups than for neutral amino groups. In the case of Ti(IV) and Fe(III) surface metals, significant polarizability of the d orbital LUMO yields a weak-to-moderate covalent contribution to bonding with neutral amino groups. Even when differences in surface area, site density, and protonation equilibria are taken into account, extents of aromatic ligand adsorption onto (hydr)oxide minerals composed of the same metal ion are substantially different. Hence, differences in crystal and surface structure and differences in the stoichiometry of the solid (OH−vs O2−) significantly affect adsorption.