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.