Solar AOPs

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. 

Application of heterogeneous photocatalysis to water and air purification appears a particularly convenient method in the cases of easily accessible UV light (mainly solar, but also artificial) and diluted pollutants. H02-based photocatalysis offers unique possibilities of mineralization of most organic compounds and, in addition, sterilization. This type of AOP useful for water and air purification has been studied and developed extensively over the last few decades. UV light, especially as a part of solar radiation, must be used to activate this process. In addition oxygen and water are indispensable. 

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. 

Photochemical oxidations are traditionally linked to the AOP discussed earlier (Section 10.1.1.2), because they normally involve irradiation of a sample with UV light in the presence of H2O2 and/or ozone that produces OH radicals. Interestingly, solar radiation can also effect photochemical degradations on its own, with half-lives even on the order of a few minutes. Examples include the almost complete degradation of hormone steroids and the photooxidation of herbicides and ligands. 

All AOP s are based largely on hydroxyl radical chemistry, generated in situ normally by using UV lamps or solar energy. The hydroxyl radical (HO ) has a high reduction potential (2.8 V) and is able to react rapidly and non-selectively with a wide range of organic compounds [4]. 

Semiconductors excited by appropriate energy photons. Two weU-defined AOP s systems which have special interest, because natural solar light can be used, are heterogeneous photocatalysis with Ti02 and homogeneous photocatalysis by the photo-Fenton process. These processes are covered in detail in the following sections. 

Solar photocatalytic degradation of water contaminants with titania and photo-Fenton catalysts has been carried out on a pilot-plant scale at the solar photochemical facilities of the Plataforma Solar de Almeria (PSA) in Spain, and show how solar photocatalysis is likely to become important within the next few decades in wastewater treatment and development of new AOP technologies [93, 94]. 

Heterogeneous Ti02 photocatalysis and homogeneous photo-Fenton, are the AOPs for which the solar technologies have been most extensively studied and developed (13). In the last few years, solar photocatalysis processes have shown to be an important alternative for pesticide degradation in the field of AOPs (14, 15) with extended research performed at pilot-plant scale (16, 17). The renewed interest of researchers (18) in the photo-assisted Fenton processes (classic old reactive system, discovered by Fenton (19) in the last century) is today underlined by a significant number of studies devoted to wastewater treatment... 

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