Adsorption of Acid Orange 7 on the surface of titanium dioxide.

The adsorption of a model textile azo-dye, Acid Orange 7 (AO7), on the surface of titanium dioxide was extensively investigated in aqueous TiO(2) suspensions over wide ranges of AO7 concentrations (1 x 10(-4)-3 x 10(-3) M) and pH values (2-10). Results obtained with the use of a variety of techniques, including potentiometric titrations, adsorption isotherms, adsorption edges, and microelectrophoresis, were used for the description of the "AO7 solution/TiO(2) surface" interface. This has been achieved by taking into account the effects of pH on the speciation of the dye in solution and on the nature and population of the surface groups of TiO(2). Results could be modeled very well with the use of the recently introduced CD-MUSIC approach. According to the model employed, the TiO(2) surface is not considered homogeneous but is characterized by the presence of different types of surface groups, namely singly (TiOH(-1/3)), doubly (Ti(2)O(-2/3)), and triply (Ti(3)O(0)) coordinated. Surface complexes are not treated as point charges, but their charge is spatially distributed in the interfacial region. It has been found that adsorption of AO7 on the TiO(2) surface occurs to a significant extent only at pH values lower than 7, via the sulfonic group of the azo-dye, through the formation of a bidentate innersphere surface complex. The determination of the adsorption mode of TiO(2), which is supported by ex situ FTIR results, as well as of the adsorption constant, K(ads), allowed the description of the pH dependency of the AO7 adsorption over large pH and AO7 concentration ranges.