Photoassisted Degradation of Dye Pollutants. V. Self-Photosensitized Oxidative Transformation of <i>Rhodamine B</i> under Visible Light Irradiation in Aqueous TiO₂ Dispersions

Chemical oxygen demand (CODCr) and proton NMR, UV−vis, and spin trapping EPR spectroscopic evidence is presented to demonstrate the inverse photosensitized oxidative transformation of tetraethylated rhodamine (RhB) under visible illumination of aqueous titania dispersions. Both de-ethylation and oxidative degradation take place with the former proceeding in a stepwise manner to yield mono-, di-, tri-, and tetra-de-ethylated rhodamine species. Intermediates present after each de-ethylation step remain in a fast dynamic equilibrium between the titania particle surface and the bulk solution. The concentration of •OH radicals, formed from the inverse photosensitization process through the superoxide radical anion, increases upon addition of the anionic dodecylbenzene sulfonate surfactant (DBS) because a larger number of RhB excited states are able to inject an electron into the conduction band of the TiO2 particles. Also, intermediates that can no longer absorb the visible light, (i.e., once the dye solution is competely bleached) are unable to drive the photosensitized degradation further. A mechanism for the competitive photoreactions between degradation and de-ethylation is described, in which de-ethylation {ζ ∼ 0.0035} is mostly a surface occurring process, whereas degradation {ζ ∼ 0.0015}of the RhB chromophore is predominantly a solution bulk process.