SPATIAL-RESOLUTION OF OXYGEN MEASUREMENTS DURING VUV-PHOTOLYSIS OF AQUEOUS SYSTEMS

Water homolyses upon vaccum-ultraviolet (VUV) excitation into hydroxyl radicals and hydrogen atoms (1). These primary species induce a serie s of reactions which may belong to either a reductive or oxidative pro cess manifold [1]. Applying the VUV-photolysis of water to degrade oxi datively organic compounds in water (AOP) [2-4], hydroxyl radicals wil l react with the organic substrate (RH). Resulting C-centered radicals of reactions (1) and (2) react efficiently with dissolved molecular o xygen generating peroxyl radicals (3) of relative long lifetimes. Thes e peroxyl radicals initiate thermal chain reactions enhancing the over all rate of mineralization. H2Okv-->H-.+OH. (1) HO.+RH-->H2O+R(.) (2) R(.)+O-2-->RO(2)(.) (3) Within the emission spectra of a Xe-excimer li ght source (lambda(max):172 nm), the absorption cross section of water is very high (total absorption within less than 0,1 mm). Taking into account (a) the relative high quantum yield of (1) (0.45) and (b) the short lifetime of hydroxyl radicals (< 1 mu s), relative high local co ncentrations of C-centered radicals may be produced which will effect the local concentration of dissolved molecular oxygen, as replacement of consumed oxygen (3) is diffusion controlled and relatively slow. Un der such conditions, a strong heterogeneity between a volume in which primary reactions will take place predominantly and a non-irradiated v olume restricted to the thermal oxidation reactions may be observed wi th appropriate analytic tools.