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Res. J. Chem. Environ., Volume 12, No. (1), March (2008) | ||||||||
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From Editor's Desk: | ||||||||
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Advanced Photo-oxidation Processes in Technology and Medicine Klementova Sarka Our Editor from University of South bohemia, Czech Republic | ||||||||
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In advanced photo-oxidation processes, electromagnetic radiation of ultraviolet or visible region and reactive oxygen species, ROS, such as ozone, hydrogen peroxide, hydroxyradicals, or singlet oxygen are used for oxidation reactions. In some applications, combination of reactive oxygen compounds (ozone, hydrogen peroxide) and ultraviolet light is used, in other applications reactive oxygen species (hydroxyradicals, peroxyradicals, singlet oxygen) are generated in situ by ultraviolet or visible light in photocatalyzed or photosensitized reactions. These types of processes can be encountered in a broad spectrum of applications from oxidation of environmental contaminants to biomedical procedures such as photodynamic therapy. The wavelength required to carry out one of these processes is generally determined by the principle involved in production of ROS. Visible light is used for photosensitized reactions, visible or near UV (UV-A, see Table1) is used for photocatalyzed reactions and short-wave radiation of UV-C is used for vacuum photolysis. Table I Arbitrary division of UV radiation.
The photooxidation processes can be broadly divided into the following groups: vacuum UV photolysis, oxidation agent /UV processes, photocatalyzed processes (photo-Fenton homogeneous catalysis, heterogeneous photocatalysis on semiconductors) and photosensitized processes. Vacuum UV photolysis. The high energy associated with UV radiation of wavelengths shorter than 190 nm is able to cause photolysis of water which yields hydroxyradicals (·OH) and hydrogen radicals (H·). This process is referred to as VUV photolysis 1. The method is used for contaminant degradation in water and in a high humidity air; the contaminant degradation is accomplished through oxidation by hydroxyradicals or reduction by hydrogen radicals since both types of radicals are powerful redox species. The process is particularly useful in treating waste waters contaminated with compounds that are difficult to oxidize 2, 3. Oxidation agent / UV processes Processes using combination of conventional oxidants (hydrogen peroxide and ozone) and UV irradiation involve generation of hydroxyradicals through UV photolysis of the respective oxidant. Eq. 1 represents the photolytic formation of hydroxyradicals from hydrogen peroxide. Eqs. 2, 3, 4 show the hydroxyradicals generation scheme if ozone in water is irradiated by UV radiation whereas Eqs. 5 and 6 represent the production of hydroxyradicals in wet air. The technique combining irradiation with addition of oxidant is used for removing dyes, pigments and fuel oxygenates from waters4,5. Photo-Fenton process (Homogeneous oxidation) Photo-Fenton process belongs to reactions classified as photocatalytic reactions in homogeneous phase, since the catalytically active form of the catalyst is generated photochemically in situ in the reaction mixture. Fenton´s agent consisting of the hydrogen peroxide plus ferrous ions - has been known for over a century6; the Fenton´s reaction, the dark (non-photochemical) reaction of hydrogen peroxide with organic substances catalyzed by ferrous ions, is still used for degradation of pollutants present in water bodies. The rate of organic pollutants removal and the extent of their mineralization (oxidation of organic carbon to CO2) has been considerably improved by irradiation of the reaction mixtures by the near-UV and blue visible radiation 7. In this process, called photo-Fenton reaction, the reaction is fuelled by: a) continuous generation of the photocatalyst in situ by photochemical photoreduction of ferric ions to ferrous ones; b) by increasing the catalyst amount available for the reaction by release from the complexes by photochemical degradation of some ligands; c) production of hydroxyradicals in the sample by direct photolysis of hydrogen peroxide. Photocatalysis on semiconductors (Heterogeneous oxidation) The semiconductor photocatalysis uses solid catalytic systems where five discrete stages associated with conventional heterogeneous catalysis can be distinguished: a) transfer of liquid or gaseous phases reactant to the catalytic surface; b) adsorption of the reactant on the catalyst surface; c) reaction in the adsorbed phase; d) desorption of products; e) removal of products from the interface region. The photocatalytic reaction occurs in the stage when the reactants are absorbed on the catalyst surface, the activation of the reaction is a photonic activation. Semiconductor is activated by the irradiation from a light source of appropriate wavelength depending on the band gap energy of the semiconductor. The activation generates the pair of charge carriers, a hole, h+, and an electron, e-; the photogenerated charge carriers can react with molecules adsorbed on the surface of the semiconductor. The most important reaction is oxidation of adsorbed water molecules by holes which generate hydroxyl radical as in Eqs . 7 and 8:
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The most common substance naturally present in water, which can react with electrons, is oxygen; oxygen is thus an additional source of hydroxyradicals as shown in Eqs. 9 - 12 :
The generated reactive oxygen species facilitate oxidative degradation of pollutants to render end-products such as water, CO2 and mineral acids. Various metal oxides (i.e. TiO2, ZnO, MoO3, CeO2, ZrO2, WO3, a-Fe2O3 and SnO2) and metalhalogenides (i.e. ZnS, CdS, CdSe, WS2 and MoS2) have been used as catalysts in semiconductor photocatalysis reactions 8. Besides applications dealing with pollutant degradation, photocatalytic technologies have been succesfully tested in biological and medical areas, such as treatment of bacteria, viruses, fungi, yeast, cancer cells, self-cleaning tiles etc.9. Dye-sensitized photo-oxidation, photodynamic therapy Dye-sensitized photoreactions have been used for many purposes in the biomedical sciences. In photodynamic therapy (PDT), a combination of non-toxic dye (photosensitizer), light absorbed by the photosensitizer, and dissolved molecular oxygen are involved in the process leading through the photochemical production of singlet oxygen to destroying of tumor cells or killing the pathogenic microorganisms10. The reaction scheme of PDT process can be described as follows:
1O2+tumor cells ® necrosis of tumor tissue (15) where, 1Sens is the ground (singlet) state of the sensitizer, 1Sens* is the excited (singlet) state of the sensitizer, 3Sens* is the excited triplet state of the sensitizer produced by intersystem crossing, (ISC) from the excited singlet state to the triplet state, 3O2 is the ground (triplet) state of molecular oxygen, 1O2 the singlet oxygen. In PDT therapy, a photosensitizer is administered (intravenously or superficially) to the patient. Within a specific time frame, the drug selectively cumulates in diseased cells, while being eliminated from normal cells. The tumor tissue is then exposed to visible, tissue-penetrating light for in situ activation. Energy transfer from the excited sensitizer to molecular oxygen produces a short-lived (microseconds) activated form of molecular oxygen, singlet oxygen. This highly reactive form of oxygen is the cytotoxic agent; its interactions with cellular components elicit direct oxidation as well as formation of subsequent reactive species such as radicals or peroxides, possibly superoxides. The damaging oxidative process results in necrosis of the tumor tissue. In antimicrobial applications of PDT, Gram-(+) organisms are effectively killed by PDT, while Gram-(-) bacteria are resistant to PDT with many commonly used photosensitizers. The available evidence suggests that multi-antibiotic resistant strains are as easily killed by PDT as naïve strains and that bacteria will not readily develop resistance to PDT. Recently, there have been reports of PDT used to treat infections in selected animal models and some clinical trials for viral lesions, acne, some gastric infections and brain abscesses 11. Acknowledgement The author thanks to David Klement for the cover page design. Cover Page : |
