Degradation of xanthene-based dyes by photoactivated persulfate: experimental and computational studies.

Dyes are naked-eye detectable even at low concentration levels and can cause environmental damage when released into aquatic effluents; therefore, methods for removing the residual color from the aquatic media are always a current issue. In this paper, degradation of three xanthene dyes, Rhodamine B, Eosin Y, and Sodium Fluorescein, using photoactivated persulfate was evaluated at pH 3.0 and 11.0. The dyes' degradation followed a pseudo-first-order reaction. Although the solution is completely decolorized in 40 min at pH 3.0, achieving 75% mineralization requires a longer reaction time of 180 min. Furthermore, GC-MS analyses indicate that degradation products are mainly low-molecular weight acids, CO2 and H2O. Experiments carried out in dark and under UV irradiation showed substantial contribution of radical (SO4•- and HO•) and non-radical pathways to dye degradation in both pH. Additionally, to get more insights into the degradation pathways, HOMO-LUMO energy gaps of the dyes were calculated by DFT using MPW1PW91/MidiXo level of theory and, in general, the lower the bandgap, the faster the degradation. Fukui functions revealed that the preferential sites to radical attack were the xanthene or the benzoate portion depending on the pH, wherein attack to the xanthene ring provided better kinetic and mineralization results.

Redox cycling of copper mediated by hydrazine: efficient catalyst systems for oxidative degradation of rhodamine B.

Catalytic processes based on Fenton-like reactions on the degradation of organic pollutants have been improved by accelerating the redox cycling of metal ions. This work presents, at first, the results obtained for the heterogeneous degradation of rhodamine B (RhB) by copper ferrite (CuFe2O4) in presence of hydrogen peroxide (H2O2) and hydrazine (N2H4) as redox cycle accelerator. Atomic absorption spectroscopy showed small amounts of Cu2+ are leached from ferrite highlighting the influence of the homogeneous catalysis in the whole process. The data obtained for the homogeneous process using Cu2+ in solution containing both N2H4 and H2O2 indicated such system is highly efficient mineralizing 73% of RhB within only 10 min of reaction and having H2O and CO2 as major products. Using tert-butyl alcohol as radical scavenger, it was confirmed hydroxyl radical (HO•) is the active oxidant species regarding the RhB degradation. The homogeneous catalyst was applied to a real sample of textile effluent spiked with RhB and showed reasonable efficiency, although lower than that obtained for the standard solutions of RhB. This result was assigned to the interference of salts in the medium that react with HO• thus acting as radical scavenger.