Accelerated methylene blue (MB) degradation by Fenton reagent exposed to UV or VUV/UV light in an innovative micro photo-reactor

• UVC light activates Fenton mediated dye degradation in a micro-photoreactor.
• VUV was more effective compared to UV light leading to dye degradation.
• The radicals interveneing in the dye degradation were idientified.
• The HO2 radical seems to be main ROS-species leading to the dye degradation.

This study presents the accelerated discoloration/degradation of methylene blue (MB) in solution by Fenton reagent under exposure to ultraviolet (UV) or Vacuum-UV/UV (VUV/UV) light in an innovative micro photo-reactor. The MB degradation was kinetically faster when using VUV/UV light at 254/185 nm compared to UV-irradiation at 254 nm. Oxidative radicals produced by the photo-Fenton process were identified with appropriate scavengers. The addition of benzoquinone (BQ) at millimolar (mM) concentrations to MB solutions precluded completely the MB photo-induced bleaching, while tert-butanol hindered to a lesser extent the MB degradation suggesting that HO2 was the predominant intermediate leading to MB degradation. The VUV/UV micro photo-reactor comprised mercury resonance lines at 185 and 254 nm. The photon percentages absorbed by water were estimated to be 16.8% at 254 nm and 78.3% at 185 nm by water, while those absorbed by H2O2 were 0.9% and 2.2%, correspondingly. The solution parameters affecting the features of MB degradation, such as MB, Fe, and H2O2 concentrations, were explored and reaction mechanism was proposed. The lifetimes of OH and HO2 were estimated to be 2 ns and 0.38 s under an optimized solution with 0.016 mM MB, 0.147 mM H2O2 and 0.05 mM Fe3+. Moreover, an estimation of the mean free paths of these radicals in solution provided the evidence that it was the radical lifetimes and mean-free paths, not their oxidation potentials that controlled the MB degradation kinetics. This study shows the potential of this VUV/UV assistant photo-Fenton process for the degradation of diluted organic compounds in aqueous solution.

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