PHOTOASSISTED DEGRADATION OF DYE POLLUTANTS - V - SELF-PHOTOSENSITIZED OXIDATIVE TRANSFORMATION OF RHODAMINE-B UNDER VISIBLE-LIGHT IRRADIATION IN AQUEOUS TIO2 DISPERSIONS

Chemical oxygen demand (CODCr) and proton NMR, UV-vis, and spin trappi ng EPR spectroscopic evidence is presented to demonstrate the inverse photosensitized oxidative transformation of tetraethylated rhodamine ( RhB) under visible illumination of aqueous titania dispersions. Both d e-ethylation and oxidative degradation take place with the former proc eeding in a stepwise manner to yield mono-, di-, tri-, and tetra-de-et hylated rhodamine species. Intermediates present after each de-ethylat ion step remain in a fast dynamic equilibrium between the titania part icle surface and the bulk solution. The concentration of (OH)-O-. radi cals, formed from the inverse photosensitization process through the s uperoxide radical anion, increases upon addition of the anionic dodecy lbenzene sulfonate surfactant (DBS) because a larger number of RhB exc ited 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) ar e unable to drive the photosensitized degradation further. A mechanism for the competitive photoreactions between degradation and de-ethylat ion is described, in which de-ethylation {zeta similar to 0.0035} is m ostly a surface occurring process, whereas degradation {zeta similar t o 0.0015} of the RhB chromophore is predominantly a solution bulk proc ess.