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Advanced Oxidation Processes

General Aspects

Advanced Oxidation Processes are distinct processing methods that make use of the formation of hydroxyl radicals •OH. Hydroxyl radicals are a highly reactive, non-selective species with a lifetime in the µs-range. •OH radicals are strong oxidation agents that react even with very stable chemical compounds. Hence, •OH radicals are suited as reagent for degradation reactions. Although all AOPs make use of the same reactive species, generation of the radicals can be achieved differently. Popular AOPs are the Fenton-process using H2O2 together with Fe2+ and photocatalytic methods using TiO2 and UV-light.[4]

Fenton Reaction and Photo-Fenton-Process

The Fenton reaction yields hydroxyl radicals from the reaction of Fe2+ with H2O2. The Fe2+ ion is oxidized to Fe3+ and hydrogen peroxide dissociates to a hydroxyl radical •OH and a hydroxyl anion OH. Subsequently, Fe3+ reacts with hydrogen peroxide to Fe2+, forming HO2• radicals. These radicals can reduce Fe3+ again. H2O2 reacts with hydroxyl radicals to H2O and HO2• radicals.[5]    AOPeq1

Hydroxyl radicals can be generated by irradiation with UV-light as well. The combination of the Fenton reaction with irradiation is called photo-Fenton-reaction. Combining two different initiation steps provides high local •OH concentrations and with this high reaction rates. This is ensured by two ways: Firstly, by photochemical reduction of Fe3+ and secondly by homolytic cleavage of H2O2.[5]AOPeq2

Fenton and photo-Fenton-reactions have to be operated at pH values of 3 to provide optimal conditions for hydroxyl radical formation. Above this value precipitation of Fe3+ ions occurs. Below a pH value of 3, H+ ions react with hydroxyl radicals under formation of water. Both reactions decrease the efficiency of the overall degradation process.[5;6]

Photocatalytic processes

AOPs using photocatalysts are very attractive options for the mineralization of dissolved pharmaceuticals in waste water. Usually metal oxides and especially semiconductors such as TiO2 are used for this purpose. TiO2 is the most prominent photocatalyst, because it is a cheap, pH stable compound with a high photocatalytic activity. Usually TiO2 P25 from Evonik is used as a suspension. This is a white powder with an average particle size of 21nm and a specific surface of 50m2 g−1. It consists of Anatas (70%) and Rutil (70%).[7;8]AOPeq3

During the photocatalytic process electrons are excited from valence band to conduction band by absorption of UV light. In this way an electron hole in the valence band and a highly reactive and mobile electron in the conduction band are formed. Via oxidation of adsorbed H2O and OHmolecules by the electron hole hydroxyl radicals are generated. Meanwhile, oxygen is reduced by electrons in the conduction band to superoxide radical.[[7;8]                                                   AOPeq4

This radical can react with a nearby proton to form a perhydroxylradical. Two perhydroxylradicals can combine to oxygen and hydrogen peroxide. Finally, hydrogen peroxide can be reduced to one hydroxyl radical and one hydroxyl anion.[9]

AOPeq5                             

  1. http://www.wasser-wissen.de/abwasserlexikon/a/abwasser.htm.
  2. SUREK, D.: Pumpen fuer Abwasser- und Kläranlagen, 2014, Springer Fachmedien Wiesbaden.
  3. MEISSNER, M.: Arzneimittel in der Umwelt: Natur als Medikamentendeponie, Dtsch Arztebl International, 2008, 105 (24), A–1324–, http://www.aerzteblatt.de/ int/article.asp?id=60535.
  4. ANDREOZZI, R.: Advanced oxidation processes (AOP) for water purification and recovery, Catalysis Today, 1999-October, 53 (1), 51–59, DOI: 1016/s0920-5861(99) 00102-9.
  5. PIGNATELLO, J. J.; LIU, D.; HUSTON, P.: Evidence for an Additional Oxidant in the Photoassisted Fenton Reaction, Environmental Science & Technology, 1999-June, 33 (11), 1832–1839, DOI: 1021/es980969b.
  6. FAUST, B. C.; HOIGNÉ, J.: Photolysis of Fe (III)-hydroxy complexes as sources of OH radicals in clouds, fog and rain, Atmospheric Environment. Part A. General Topics, 1990January, 24 (1), 79–89, DOI: 1016/0960-1686(90)90443-q.
  7. LEGRINI, O.; OLIVEROS, E.; BRAUN, A. M.: Photochemical processes for water treatment, Chemical Reviews, 1993-March, 93 (2), 671–698, DOI: 1021/cr00018a003.
  8. WINKLER, J.: Titandioxid, 2013, Vincentz Network GmbH & C, ISBN 3866308930.
  9. MARTIN, S. T.; LEE, A. T.; HOFFMANN, M. R.: Chemical mechanism of inorganic oxidants in the TiO2/UV process: increased rates of degradation of chlorinated hydrocarbons, Environmental Science & Technology, 1995-October, 29 (10), 2567–2573, DOI:10.1021/es00010a017.

 
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