EFFECTS AND APPLICATIONS OF PHOTO DEGRADATION
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CHAPTER ONE
INTRODUCTION
1.1 CONCEPT OF PHOTODEGRADATION
Photodegradation is the alteration of materials by light. Typically, the term refers to the combined action of sunlight and air and it is usually a combination of oxidation and hydrolysis (Walter 2011). Often photo-degradation is avoided, since it destroys paintings and other artifacts (Walter 2011). It is however partly responsible for re-mineralization of biomass and is used intentionally in some disinfection technologies.
For instance, photodegradation of water pollutants addresses the naturally occurring self-cleaning of some pollutants in sunlit surface waters, as well as several alternative non-photochemical approaches to water treatment (Martin 1995). Photodegradation does not apply to how materials may be aged or degraded via infrared light or heat, but does include degradation in all of the ultraviolet light wavebands (Martin 1995).
These degradation processes can be the accelerated in the presence of a catalyst. In catalyze photodegradation, light is absorbed by a photocatalytic substrate and the photodegradation depends on the ability of the catalyst to create electron–hole pairs, which generate free radicals (e.g. hydroxyl radicals: OH) able to undergo secondary reactions (Anne 2003)
1.2 SOME EFFECTS AND APPLICATIONS OF PHOTO DEGRADATION
Catalytically induced photodegradation has numerous effects, hence applications, on our everyday life. Some of these effects and applications are as stated below:
1.2.1 Effect on Foodstuffs: The protection of food from photodegradation is very important. Some food materials, such as beer, are affected by degradation when being exposed to sunlight. In the case of beer, the UV radiation causes a process, which entails the degradation of hop bitter compounds to 3-Methyl-2-buten-1-thiol and therefore causing changes in the taste. As amber glass has the ability to absorb UV radiation, beer bottles are often made from this glass type to avoid this process.
1.2.2 Effect on Paints, Inks and Dyes: Paints, inks and dyes that are organic are susceptible to photo degradation. To this end, ceramics are almost universally coloured with non-organic origin materials so as to allow the materials retain theircolour even with the most relentless photo-degradation.
1.2.3 Effect on Pesticides and Herbicides: The photo degradation of pesticides is of great interest because of the scale the intensive use of chemicals in agriculture. Pesticides are however selected in part not to photo degrade readily in sunlight. This is so because this will allow them to exert their biocidal activity.
Therefore, additional modalities are implemented to enhance their photo degradation, including the use of photosensitizers, as well as reagents that would generate hydroxyl radicals that have the capacity of attacking the pesticides.(Burrows, H.D. et al; 2002).
1.2.4 Effect on Pharmaceuticals: The photo degradation of pharmaceuticals is of interest because they are found in many water supplies. They have deleterious effects on aquatic organisms including toxicity, endocrine disruption, genetic damage.(Boreen, Anne L.;et al; 2003). But also in the primary packaging material the photo degradation of pharmaceuticals has to be prevented. For this, amber glasses like Fiolax amber and Corning 51-L are commonly used to protect the pharmaceutical from UV radiations. Iodine and collodial silver are universally used in packaging that lets through very little UV light so as to avoid degradation.
1.2.5 Effect on Polymers: Common synthetic polymers that can be attacked include polypropylene and LDPE, where tertiary carbon bonds in their chain structures are the centers of attack. Ultraviolet rays interact with these bonds to form free radicals, which then react further with oxygen in the atmosphere, producing carbonyl groups in the main chain. The exposed surfaces of products may then discolour and crack, and in extreme cases, complete product disintegration can occur (De Laat, J. 1999).
In fibre products like rope used in outdoor applications, product life will be low because the outer fibres will be attacked first, and will easily be damaged by abrasion for example. Discolouration of the rope may also occur, thus giving an early warning of the problem.
Polymers which possess UV-absorbing groups such as aromatic rings may also be sensitive to UV degradation. Aramidfibres like Kevlar, for example, are highly UV-sensitive and must be protected from the deleterious effects of sunlight.
1.3 PHOTOCATALYSIS BYZINC OXIDE
Zinc oxide, with a high surface reactivity owing to a large number of native defect sites arising from oxygen nonstoichiometry, has emerged to be an efficient photocatalyst material compared to other metal oxides (Ali, et al., 2010 – Qamar, et al., 2009). ZnO exhibits comparatively higher reaction and mineralization rates (Poulios, 1999) and can generate hydroxyl ions more efficiently than titanium oxide (TiO2) (Carraway, 1994). ZnO has been often considered a valid alternative to TiO2 because of its good optoelectronic, catalytic and photochemical properties along with its low cost. ZnO has a band gap of 3.0 eV that is lower than that of anatase. Due to the position of the valence band of ZnO, the photo generated holes have strong enough oxidizing power to decompose most organic compounds (Miyauchi, et al., 2002). ZnO has been tested to decompose aqueous solutions of several dyes (Akyol, et al., 2005 – Comparelli, et al., 2005) and many other environmental pollutants (Kandaveluet al., 2004 – Khodja, et al., 2001).
ZnO has been reported to be more efficient than TiO2 (Chen, 2006 – Gouvêaet al., 2000) but the occurrence of photo corrosion and the susceptibility of ZnO to facile dissolution at extreme pH values, have significantly limited its application in photocatalysis. Studies of ZnO hexagonal nanorod structure represent a significant research area, and it is reported that one dimensional (1D) ZnOnanorods arrays could enhance photocatalytic efficiency (Wang, 2008).
Several studies have shown that ZnO was quite active under UV illumination for the photo degradation of some organic compounds in aqueous solution (Sharma, et al., 1995).
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