Oxidative decomposition of rhodamine B dye in the presence of VO2+ and/or Pt(IV) under visible light irradiation: N-deethylation, chromophore cleavage, and mineralization.

In order to make clear the roles of dissolved O2 in the photocatalytic decomposition of organic pollutants and to discriminate different degradation pathways (N-deethylation, chromophore cleavage, and mineralization) during the degradation of dye, the photodegradation of rhodamine B (RhB) has been investigated using vanadate and/or platinum species as electron acceptors in the presence or absence of O2 under visible light irradiation. It was found that with VO2+ as electron acceptor, RhB underwent efficient N-deethylation under visible light irradiation and O2 was found to slow down this process significantly. Little mineralization has been observed in the presence and absence of O2 in VO2+ systems. By contrast, Pt(IV) resulted in the cleavage of conjugated chromophore structure (bleaching) of RhB dye under the otherwise identical conditions. In this case, the presence of O2 did not affect the bleaching rate of the dye, but enhanced greatly the mineralization. Both cleavage of conjugated chromophore structure and N-deethylation occurred simultaneously upon the coaction of VO2+ and Pt(IV) under visible light irradiation. The mineralization yield of the combined system was evidently higher than the expected summation of separate ones. TOC, XPS, and ESR results indicate that in the VO2+ and Pt(IV) combined system VO2+ not only oxidized RhB leading to deethylation but also oxidized the reduced Pt(II) to regenerate Pt(IV) leading to the further cleavage of chromophore structure of RhB, which behaved quite different from the separate ones. A mechanism was also proposed to interpret the different pathways for the oxidative photodecomposition of RhB under visible irradiation.