Formation of hydrogen peroxide by VUV-photolysis of water and aqueous solutions with methanol

Abstract

The hydrogen peroxide production upon vacuum ultraviolet (VUV) irradiation of water is reviewed, because published results from the last 10 years lead to conflicting mechanistic interpretations. This work confirms that in pure water, hydrogen peroxide is only produced in the presence of molecular oxygen. Mechanistic schemes explain these findings and confirm earlier statements that recombination of hydroxyl radicals is kinetically disfavoured. In agreement with other recent publications, this work confirms that enhanced hydrogen peroxide production takes place upon VUV irradiation of aqueous solutions of organic compounds. For these investigations, methanol was chosen as an organic model compound. During photolyses, hydrogen peroxide, dissolved molecular oxygen, pH-value of the reaction system, methanol and its products of oxidative degradation were analyzed, and kinetic studies were undertaken to explain the evolution of the concentrations of these components.

VUV/UV light inducing accelerated phenol degradation with a low electric input

“…In addition, VUV also generates H˙ and e aq – in aqueous solutions due to homolysis and water ionization. 43,44 Fig. 2a shows that UV (254 nm) by itself did not lead to phenol degradation.…”

“…The VUV/UV in the absence of the Fenton's reagent (Fe 2+/3+ /H 2 O 2 ) induced phenol degradation due to the VUV (185 nm) light generating HO˙ from the photolysis of water. 43,44 Without UV and VUV irradiation, little phenol degradation was found although both Fe 3+ and H 2 O 2 were present. UV and VUV/UV photo-Fenton processes led to a complete phenol degradation within 6–8 min.…”

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

“…The data at pH 5.4 for light activated MB-degradation shows that no MB degradation was observed at this pH. The reason for this Generation of H 2 O 2 during VUV/UV photolysis has been reported in the presence of dissolved O 2 and organic compounds in solution [30,34,35]. The mechanism involves H • trapping by O 2 yielding HO 2 • .…”

“…In addition, in the VUV/UV tube, the exposed fluence ratio of UV (254 nm) to VUV (185 nm) was determined to be 9.2, which was similar to the ratio provided by the manufacturer of the low pressure (LP) mercury lamps. [28][29][30][31][32][33][34][35]. Eqs.…”

“…VUV is able to induce homolysis and ionization of H 2 O at 185 nm, as shown below in Eqs. (7) and (8) [34,35], b H 2 O 2 /UV direct photolysis of many organic compounds can be neglected due to their low molar absorption coefficients since they absorb light < 200 nm [3,4].…”