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

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 center dot 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 254nm 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)-O-center dot and HO2 center dot 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. (C) 2016 Elsevier B.V. All rights reserved.