Photo-initiated AOPs

In photochemistry, and especially in photo-initiated AOP applications, it is obvious that the appropriate selection of the lamp type largely determines the effectiveness of the desired process. The evaluation has to take into account technical and economic considerations. First, the radiation source must emit radiation that is absorbed by the reactant, i.e. the emission spectrum of the lamp must match the absorbance spectrum of the reactant to a maximum extent. Secondly, the electric input power Pef/W of a radiant source determines the radiant power P/W of... 

Status of Technical Realization of Photo-initiated AOPs and Photochemical Treatment Strategies 127... 

Fig. 5.15 State of realization of photo-initiated AOPs for water and air treatment. LP low-pressure, MP medium-pressure.

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. 

UV oxidation of organic water pollutants in the presence of hydrogen peroxide, ozone or both with powerful medium-pressure mercury lamps is performed on industrial scales (Gottschalk et al., 2000). The synergistic combination of ozone and UV is especially suited for water sanitation, i.e. treatment of swimming pool and spa water (Rice, 1997). Many full-scale remediation applications of photo-initiated AOPs using hydrogen peroxide or ozone are already in operation (Chem-viron Carbon, 1997, Freeman and Harris, 1995). 

The developments in the field of photo-initiated AOPs over the last decade undoubtedly indicate their large-scale feasibility with some restrictions concerning the costs of these processes as stand-alone systems, the long-term reliability of the system components (lamps etc.) and the possible formation of undesirable byproducts in some cases. However, photo-initiated AOPs have many similarities with nature s own self-repair mechanisms of natural pollution. Many of them can be adapted, in particular solar AOPs, for use in developing countries. 

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. 

Fig. 5.16 Classification of photo-initiated AOPs according to active wavelength ranges and spectral domains of excitation. designates modified titanium dioxide, for...

Tab. 5-2 presents several examples of recent research concerning photo-initiated AOPs that are related to the photooxidation or photominerahzation of specific compounds or to the diminution of global water parameters. It is a long way from being a comprehensive list (this would inevitably go beyond the scope of this book), and it was randomly selected from the current Hterature. Nevertheless, it demonstrates the enormous amount of research activity in the field of photo-initiated AOPs in aqueous systems that elucidate reaction mechanisms and that establish the broad application potential of these processes. Examples of O3-UV AOPs are not included. They were reviewed by GoUschalk et al. (2000). 

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)... 

The UV absorbance spectra of the transient species H (Xo get 250 nm), OH ( max - 230 nm), HO ( max 231 nm) and 02 ( max 248 nm) were reported by Getqff (1996). These data suggest that the direct photolysis of the intermediary species must also be considered for the evaluation of photo-initiated AOPs. However, this is not important for the OH radical because its concentration is always so low (cf Fig. 6-13) that its photolysis is negligible. 

However, the comparison of the data collected in Tab. 6-4 offers a realistic estimate of process efficiencies. The quantum yield of OH radical formation on Ti02 is only 4 to 10% at best. This is the major reason why Ti02 processes have, in general, not been commercially successful. Pure economic analysis (favored by Bolton) results in an estimate that hydrogen peroxide based photo-initiated AOPs are 50 to 100 times more efficient in their use of electricity than Ti02 photocata-... 

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). 

Fig. 6.14 Experimental evidence for the formation of OH radicals generated in photo-initiated AOPs mechanism of the DMPO spin trapping technique.

The existence of free hydroxyl radicals in photo-initiated AOPs can be proven by applying a well-established method, the so-called spin trapping technique. The diamagnetic spin trap 5,5 -dimethyl-1-pyrroline N-oxide (DMPO) forms a stable paramagnetic spin-adduct with OH radicals. Its formation can be detected by electron paramagnetic resonance (EPR) spectroscopy. The underlying chemistry of... 

In conclusion, it is obvious from Fig.s 6-16 to 6-23 that the concentration of dissolved molecular oxygen [(02)aq] during the application of photo-initiated AOPs (and others) must be maintained at a sufficiently high level to guarantee the formation of organic peroxyl radicals (R-O ) on the way to mineralization of organic water contaminants. 

Tab. 7.2 Selected examples of applications of incoherent excimer lamps in photo- initiated AOPs in aqueous and gaseous media... 

Several examples of comparative studies of photo-initiated AOPs are collected in Tab. 7-3. 

From this table the following conclusions may be drawn. Firstly, the effectiveness of photo-initiated AOPs is strongly dependent on the nature of the substrates secondly, phenol and its halogenated derivatives are very sensitive to the photo-Fmton reaction and thirdly, this table demonstrates that the photo-Fenton, the H2O2-UV and the O3-UV AOP seem to be very efficient in oxidative degradation of various organic substrates. 

However, it must be stated that the reaction pathways are different during the various treatment routes. This fact manifests itself in the different number and types of intermediate oxidation products that have been identified with identical substrates during different AOPs (cf Rajeshwar, 1996). Further, the optimum conditions for a specific photo-initiated AOP treatment depend mainly on the nature of the waste or model water. For instance, H2O2-O3 treatment (without irradiation) can have advantages in the treatment of waters with high inherent UV ab-... 

Non-thermal plasma processing (e. g. Hsiao et al., 1996, Yamamoto et al., 1996) and photo-initiated AOPs are promising and irmovative developments in this... 

Fig. 7.23 Photo-initiated AOPs and their applications in air pollution control.

Several recent research papers on photo-initiated AOPs in the gas phase are collected in Tab. 7-4. It is obvious from the table that the main activities of gas phase... 

Tab. 7.4 Examples of research related to homogeneous and heterogeneous photo-initiated AOPs in the gas phase. Examples of applications of incoherent excimer lamps are collected in Table 7-2. Ox intermediary oxidation products formed and detected during treatment... 

Levenspid earlier presented, in 1972, a qualitative discussion about the product distribution related to photochemical reactions comparing batch and batch recirculation photochemical reactors. The essentials of this discussion can be transferred to photo-initiated AOPs (at least to the H2O2-UV process), which at low concentrations of a pollutant M ([M] <100 mg L ) usually follow an overall first order reaction kinetics (Bolton et al, 1996). 

Fig. 8.15 Summary of factors influencing the reaction kinetics and the process efficiency of photo-initiated AOPs.

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