Ultrafast photosensitized degradation of xanthates by using methylene blue as photosensitizer under simulated and natural solar light irradiation: Operational parameters, mechanism and toxicity evolution

Xanthates have attracted considerable attention due to their broad applications in mineral flotation process and unique natures including environmental persistence and toxicity. This study proposed an innovative approach for the efficient xanthates treatment from mineral processing wastewater. Methylene blue (MB) with good water solubility and strong light absorption in visible range was used as the photosensitizer to degrade potassium ethyl xanthate (PEX) under homogeneous conditions. MB, even at a very low concentration (0.5 mg/L), was found to rapidly remove PEX in only 10 min whether under simulated or natural solar light irradiation. Besides, the influences of various operational parameters, including photosensitizer dosage, PEX concentration, pH, light intensity, common ions, and water source on the photosensitized oxidation process was evaluated and analyzed. The possible degradation path of PEX was investigated depending on the intermediates detected by Liquid chromatography-mass spectrometry (LC-MS), and the photosensitized degradation mechanism was further speculated and validated by the radicals quenching experiments and electron paramagnetic resonance (EPR) spectroscopy. Furthermore, the toxicity of PEX was assessed against Bullfrog tadpole and Lemna minor, and it was found to be greatly reduced after treating with MB photosensitization. Overall, it is the first time that MB is proved to be an outstanding candidate for actual flotation wastewater treatment, providing a more efficient, economical and greener alternative to conventional methods.