The dye-sensitized photodegradation of uracil (UR), the parent compound of several profusely employed herbicides, has been studied as a model of their environmental fate. In order to mimic conditions frequently found in nature, aqueous solutions of UR have been irradiated with visible light in the presence of the natural sensitizer riboflavin (Rf). The results indicate that UR is photostable in acid media, but is quickly degraded in pH 7 or pH 9 solutions, where singlet molecular oxygen [O2(1Δg)] and, to a lesser extent, superoxide radical anion (O2•−)—both species photogenerated from triplet excited Rf, 3Rf*—participate in the photodegradation. At pH 7, UR is slowly degraded through an O2•−-mediated mechanism, whereas Rf disappears through its reaction with O2(1Δg) and, in the form of 3Rf*, with UR. On the contrary, at pH 9 Rf is photoprotected through two processes: its regeneration from the formed Rf radical species—a back electron transfer that also produces O2•−—and the elimination from the medium of O2(1Δg) by its reaction with UR. The overall result of the preservation of ground state Rf is the continuity of the photosensitized process and, hence, of the UR degradation. Media with higher pH values could not be employed due to the fast photodegradation of Rf. With rose bengal (RB) as photosensitizer, the rate constants found for the overall interaction between UR and the photogenerated O2(1Δg) were in the range 5 × 105 M−1 s−1 (at pH 7) to 1.3 × 108 M−1 s−1 (in 1 M NaOH aqueous solution, mainly physical quenching). The maximum O2(1Δg)-mediated photooxidation efficiencies with RB were reached at pH 11, where only the O2(1Δg)-reactive quenching with UR was observed.
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