VUV-PHOTOLYSIS OF AQUEOUS SYSTEMS - SPATIAL DIFFERENTIATION BETWEEN VOLUMES OF PRIMARY AND SECONDARY REACTIONS

Rate and reaction pathway of the degradation of organic compounds in a queous solution by VUV-irradiation (Xe-excimer: 172 nm) are strongly i nfluenced by the concentration of dissolved molecular oxygen in the vo lume of primary reactions. Due to the very short lifetimes of the inte rmediates generated by the homolysis of water, this volume is almost i dentical with the irradiated fraction of the total reactor volume. Giv en the fact, that VUV-radiation is totally absorbed within less than 0 .1 mm due to the high absorption cross-section of water, investigation s on the concentration of dissolved molecular oxygen in the volume of primary reactions require a very high spatial resolution. This spatial resolution is reached by using a very small oxygen optode connected t o the photoreactor. The measurements show that VUV-irradiation of a ho mogenous aqueous system is leading to pronounced heterogeneity: a very thin volume, close to the surface of the light source, which is chara cterised by the diffusion controlled reactions of primary radicals of very short lifetimes with the organic substrate and the trapping of th e C-centered radicals by molecular oxygen, may be differentiated from the remaining major part of the reactor volume in which slower thermal reactions initiated by peroxyl radicals dominate. (C) 1997 IAWQ. Publ ished by Elsevier Science Ltd.