Molecular Mechanism of Surface Recognition. Azo Dyes Degradation on Fe, Ti, and Al Oxides through Metal Sulfonate Complexes

The adsorption of azo dyes (Orange II, Orange I, and Orange G) on α-Fe 2 O 3 and α-FeOOH has been carried out, and the results are compared with those for adsorption on TiO 2 and Al 2 O 3 . Adsorption of azo dye was less favored when the sulfonic groups were on the naphthalene ring, as in the case of Orange G, suggesting an inner sphere mechanism of complex formation between the dye and α-Fe 2 O 3 . The crystalline face of the oxide and the appropriate metal−metal atomic distance rather than the density of surface sites (surface area) seem to control the extent of the adsorption. The latter observations imply a surface molecular recognition mechanism active during the adsorption process of different azo dyes on the particular oxide surface. The adsorption of Orange II (taken as a model dye) from solution at pH ≤ 7 could be understood in electrostatic terms taking into account the species found at the surface of the oxides and the Orange II ionization as a function of solution pH. Modeling of the adsorption processes was carried out taking into account the number of adsorption sites, the equilibria constants, and the surface area of the different azo dyes. The most favorable condition for adsorption was the closest matching of the M−M atomic distance of the oxide to the O−O bond distance in the sulfonic group: −O−S−(O−O) of Orange II. The adsorption of azo dyes was found to occur via the sulfonic group of Orange II through the formation of a bridged bidentate complex. A variety of techniques have been used to follow the adsorption, such as diffuse reflectance infrared fourier transform spectroscopy (DRIFT), high-pressure liquid chromatography (HPLC), surface zeta potential (ζ), UV−vis spectrophotometry, and N 2 (BET) adsorption.