The paper presents the results of research on the methylene blue (MB) degradation with sodium persulfate (PDS) activated with visible light (Vis) in the presence of glucose and sucrose. The following test were examined: absence and presence of sugars in concentrations of 50–300 mM, presence and absence of Vis radiation, pH of the model solution (6.0–12.0), identification of free radicals, kinetics of the pseudo-first-order reaction. The degradation of the MB ranges from 58 to 84% depending on the pH of the model solution. The highest degradation degree (about 83–84%) was observed after 90 min of visible light irradiation for the process conducted in the presence of persulfate (6.5 mM) and glucose/sucrose (100.0 mM) at pH = 12.0 (PDS-12/100G/Vis and PDS-12/100S/Vis). The mathematically calculated half-life of MB (t/2) has been reduced in the visible-light system irradiated from 111.8 min (PDS in pH = 6.0) to 68.0 min (PDS-6/100S/Vis) and from 58.7 min (PDS in pH = 12.0) to 36.9 min (PDS-12/100S/Vis). Decolorisation of the methylene blue takes place with the participation of three radicals: sulfate SO∙−4, hydroxyl OH• and superoxide O∙−2. The dominant radicals were superoxide radicals. In a slightly acidic environment (pH = 6.0), there were hydroxyl and sulfate radicals. At pH = 12, hydroxyl radicals were mainly responsible for the degradation of methylene blue. The results of this study showed that method may be an interesting alternative for the treatment of coloured wastewater containing low dye concentrations.
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