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

Abstract

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 TiO 2 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.