VUV oxidation

Tab. 5.2 Current examples of research related to photo-initiated AOPs in aqueous phase. Ox intermediary oxidation products formed, detected during treatment and characterized. Research examples concerning VUV oxidation in aqueous phase and UV oxidation in the gas phase are presented in Chapter 7 (Table 7-2 and Table 7-4, respectively). For O3-UV based AOPs refer to Cottschalk et al. (2000)... 

Unlike the H2O2-UV process the VUV oxidation of organic substrates in water (H2O-VUV) is initiated by the photochemical homolysis of water producing a series of reactive species (cf Fig.s 5-9 and 7-3, and Equations 5-18 to 5-20). In many cases water itself is the prominent absorber of VUV radiation at 172 nm (cf Ex-... 

Fig. 7-13) led to a chloride ion mass balance of 80% (Oppenlander and Baum, 1995a). This work confirmed the slower VUV oxidation of saturated chlorohydrocarbons compared with the fast VUV oxidation of unsaturated derivatives. This difference in reaction behavior can be predicted from the higher second order reaction rate constants of the latter with hydroxyl radicals (c.f Fig. 6-24 and Tab. 6-6). 

Certainly, the efficiency of large-scale water treatment systems using incoherent Xe2 excimer lamps seems to be limited due to the extremely small reaction zone of VUV oxidation. However, VUV photolysis using efficient Xe2 excimer lamps has a great potential for the treatment of humidified polluted gaseous waste... 

Extremely inhomogeneous conditions are found in oxidative degradation processes induced by vacuum ultraviolet (VUV) irradiation of aqueous reaction systems. In fact, the absorption cross section of water for an almost monochromatic excitation at 172 nm (Xe excimer lamp, vide infra) being very... 

L. Jakob, T. M. Hashem, M. M. Kantor, and A. M. Braun, Oxidative Degradation Processes by Vacuum Ultraviolet (VUV) Photolysis, Division of Environmental Chemistry, ACS Meeting, Proceedings, San Francisco, 1992. 

In this chapter some of the presently known optical properties of zinc oxide are reviewed. In particular, the anisotropic dielectric functions (DFs) of ZnO and related compounds from the far-infrared (FIR) to the vacuum-ultraviolet (VUV) spectral range are studied. Thereupon, many fundamental physical parameters can be derived, such as the optical phonon-mode frequencies and their broadening values, the free-charge-carrier parameters, the static and high-frequency dielectric constants, the dispersion of the indices of refraction within the band-gap region, the fundamental and above-band-gap band-to-band transition energies and their excitonic contributions. 

Oppenlander T, Gliese S (2000) Mineralization of Organic Micropollutants (Homologous Alcohols and Phenols) in Water by Vacuum-UV-Oxidation (H2O-VUV) with an Incoherent Xenon-Excimer Lamp at 172 nm, Chemosphere 40 15-21. 

Measurements of the total carbon content (TC) of water, which consists of 1C and TOC (TC = IC-i-TOC), are usually based on the catalytic combustion of the water sample in the presence of oxygen to yield the TC value. Alternatively, the determination of TOC is possible by a photochemical method, i.e. the VUV-initiated oxidation of organic matter in the low pg L concentration range with formation of CO2 (Huber and Frimmel, 1991). The CO2 resulting from both procedures is analyzed quantitatively in a nondispersive infrared analyzer (NDIR). 

Photo-initiated AOPs are subdivided into VUV and UV oxidation that are operated in a homogeneous phase, and in photocatalysis (Fig. 5-15). The latter can be conducted in a homogeneous aqueous phase (photo-enhanced Fenton reaction) or in a heterogeneous aqueous or gaseous phase (titanium dioxide and certain other metal oxide catalysts). These techniques apply UV-A lamps or solar UV/VIS radiation and they are in pre-pilot or pilot status. According to Mukhetjee and Ray (1999) the development of a viable and practical reactor system for water treatment with heterogeneous photocatalysis on industrial scales has not yet been successfully achieved. This is mainly related to difficulties with the efficient distribution of electromagnetic radiation (UV/VIS) to the phase of the nominal catalyst. 

Owing to the different and distinct absorption properties of the individual auxiliary oxidants or photocatalysts, the photo-initiated AOPs presented in Fig. 5-15 must be utilized at specific spectral bands covering the VUV, UV-C, UV-B, UV-A and parts of the visible range of the electromagnetic spectrum. This is outlined in Fig. 5-16. The photo-Fenton process using Fe(III) oxalate is probably the most favorable for solar photochemistry, since the quantum yield 0 is high (cf Tab. 6-4), and ferrioxalate absorbs up to X of 500 nm. 

Well studied primary reactive species in radiation- or photo-initiated reactions of auxiliary oxidants in an aqueous phase are hydrated electrons (eaq), hydrogen atoms (H ) and hydroxyl radicals ( OH), the last being by far the most important ones in photo-initiated AOPs. The formation and reactivity of ejq and of H were described by Hart and Anbar (1970) and by Buxton et al. (1988). Hydrated electrons can be produced by VUV photolysis of water, by photolysis of aqueous solutions of [FelCNlq]" or of V with formation of [Fe(CN)5] and il2, respectively (cf. Buxton et al, 1988). 

In the H2O-VUV AOP no additional auxiliary oxidants must be used to produce sufficient amounts of OH radicals. The technical development of incoherent exci-mer VUV and UV sources of electromagnetic radiation (see Chapter 4.3) led to a renaissance of research related to water photolysis. Hence, the experimental conditions and results of several recent investigations that deal with the applications of VUV and UV incoherent excimer lamps in the field of AOP research (mainly in aqueous media) are briefly summarized in Tab. 7-2. 

Several examples selected from the current research literature will demonstrate the enormous potential of the H2O-VUV AOP for oxidizing and mineralizing organic water contaminants. 

Fig. 7.14 Oxidation and mineralization of 2,4-dichlorophenol (2,4-DCP) in aqueous solution by the H2O-VUV AOP using a Xe2 excimer flow-through photoreactor evolution of 2,4-DCP, TOC and chloride ion concentrations. Conditions photoreactor see Figure 4-16, Pel (lamp) = 142 W, irradiated volume V = 2L,...

The VUV-initiated oxidation of 2,4-dichlorophenol (2,4-DCP) led to an excellent cr mass balance of 98% and to the diminution of the total organic carbon content (TOC) of the solution (Fig. 7-14). Clearly, the TOC diminution kinetics was much slower than that of the substrate oxidation. It was shovm that the observed kinetics of the TOC degradation of 2,4-DCP was strongly dependent on the electric input power of the Xe2 excimer lamps used (Baum and Oppenlander, 1995). 

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