Fenton's reagent

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Fenton's reagent is a solution of hydrogen peroxide (H2O2) with ferrous iron (typically iron(II) sulfate, FeSO4) as a catalyst that is used to oxidize contaminants or waste waters. Fenton's reagent can be used to destroy organic compounds such as trichloroethylene (TCE) and tetrachloroethylene (perchloroethylene, PCE). It was developed in the 1890s by Henry John Horstman Fenton as an analytical reagent.[1][2][3]

Overview[edit]

Iron(II) is oxidized by hydrogen peroxide to iron(III), forming a hydroxyl radical and a hydroxide ion in the process. Iron(III) is then reduced back to iron(II) by another molecule of hydrogen peroxide, forming a hydroperoxyl radical and a proton. The net effect is a disproportionation of hydrogen peroxide to create two different oxygen-radical species, with water (H+ + OH) as a byproduct.

Fe2+ + H2O2 → Fe3+ + HO• + OH

 

 

 

 

(1)

Fe3+ + H2O2 → Fe2+ + HOO• + H+

 

 

 

 

(2)

2 H2O2 → HO• + HOO• + H2O

 

 

 

 

(net reaction: 1+2)

The free radicals generated by this process then engage in secondary reactions. For example, the hydroxyl is a powerful, non-selective oxidant.[4] Oxidation of an organic compound by Fenton's reagent is rapid and exothermic and results in the oxidation of contaminants to primarily carbon dioxide and water.

Reaction (1) was suggested by Haber and Weiss in the 1930s as part of what would become the Haber–Weiss reaction.[5]

Iron(II) sulfate is typically used as the iron catalyst. The exact mechanisms of the redox cycle are uncertain, and non-OH• oxidizing mechanisms of organic compounds have also been suggested.[citation needed] Therefore, it may be appropriate to broadly discuss Fenton chemistry rather than a specific Fenton reaction.

In the electro-Fenton process, hydrogen peroxide is produced in situ from the electrochemical reduction of oxygen.[6]

Fenton's reagent is also used in organic synthesis for the hydroxylation of arenes in a radical substitution reaction such as the classical conversion of benzene into phenol.

C6H6 + FeSO4 + H2O2 → C6H5OH

 

 

 

 

(3)

An example hydroxylation reaction involves the oxidation of barbituric acid to alloxane.[7] Another application of the reagent in organic synthesis is in coupling reactions of alkanes. As an example tert-butanol is dimerized with Fenton's reagent and sulfuric acid to 2,5-dimethyl-2,5-hexanediol.[8] Fenton's reagent is also widely used in the field of environmental science for water purification and soil remediation. Various hazardous wastewater were reported to be effectively degraded through Fenton's reagent.[9]

Effect of pH on formation of free radicals[edit]

pH affects the reaction rate due to a variety of different reasons. At a low pH, complexation of Fe2+ also occurs, leading to lower availability of Fe2+ to form reactive oxidative species (OH•).[10] Lower pH also results in the scavenging of OH by excess H+,[11] hence reducing its reaction rate. Whereas at high pH, the reaction slows down due to precipitation of Fe(OH)3, lowering the concentration of the Fe3+ species in solution.[12] Solubility of iron species is directly governed by the solution's pH. Fe3+ is about 100 times less soluble than Fe2+ in natural water at near-neutral pH, the ferric ion concentration is the limiting factor for the reaction rate. Under high pH conditions, the stability of the H2O2 is also affected, resulting in its self-decomposition.[13] Higher pH also decreased the redox potential of OH thereby reducing its effectiveness.[14] pH plays a crucial role in the formation of free radicals and hence the reaction performance. Thus ongoing research has been done to optimize pH and amongst other parameters for greater reaction rates.[15]

Impacts of operation pH on reaction rate
Low pH Formation of [Fe(H2O)6] 2+ complex, hence reducing Fe2+ for radical generation
Scavenging of OH by excess H+
High pH Lower redox potential of OH
Self-decomposition of H2O2 due to decreased stability at high pH
Precipitation of Fe(OH)3 species in solution

Biomedical implications[edit]

The Fenton reaction has different implications in biology because it involves the formation of free radicals by chemical species naturally present in the cell under in vivo conditions.[16] Transition-metal ions such as iron and copper can donate or accept free electrons via intracellular reactions and so contribute to the formation, or at the contrary to the scavenging, of free radicals. Superoxide ions and transition metals act in a synergistic way in the appearance of free radical damages.[17] Therefore, although the clinical significance is still unclear, it is one of the viable reasons to avoid iron supplementation in patients with active infections, whereas other reasons include iron-mediated infections.[18]

See also[edit]

References[edit]

  1. ^ Koppenol, W.H. (1993). "The centennial of the Fenton reaction". Free Radical Biology and Medicine. 15 (6): 645–651. doi:10.1016/0891-5849(93)90168-T. ISSN 0891-5849. PMID 8138191.
  2. ^ Fenton H.J.H. (1894). "Oxidation of tartaric acid in presence of iron". J. Chem. Soc., Trans. 65 (65): 899–911. doi:10.1039/ct8946500899.
  3. ^ Hayyan M., Hashim M.A., AlNashef I.M., Superoxide ion: Generation and chemical implications. Chem. Rev., 2016, 116 (5), pp 3029–3085. DOI: 10.1021/acs.chemrev.5b00407
  4. ^ Cai, Q. Q.; Jothinathan, L.; Deng, S. H.; Ong, S. L.; Ng, H. Y.; Hu, J. Y. (2021-01-01), Shah, Maulin P. (ed.), "11 - Fenton- and ozone-based AOP processes for industrial effluent treatment", Advanced Oxidation Processes for Effluent Treatment Plants, Elsevier, pp. 199–254, doi:10.1016/b978-0-12-821011-6.00011-6, ISBN 978-0-12-821011-6, retrieved 2021-04-08
  5. ^ Haber, F.; Weiss, J. (1932). "Über die katalyse des hydroperoxydes" [On the catalysis of hydroperoxides]. Naturwissenschaften. 20 (51): 948–950. Bibcode:1932NW.....20..948H. doi:10.1007/BF01504715. S2CID 40200383.
  6. ^ Juan Casado; Jordi Fornaguera; Maria I. Galan (January 2005). "Mineralization of aromatics in water by sunlight-assisted electro-Fenton technology in a pilot reactor". Environ. Sci. Technol. 39 (6): 1843–47. Bibcode:2005EnST...39.1843C. doi:10.1021/es0498787. PMID 15819245.
  7. ^ Brömme HJ, Mörke W, Peschke E (November 2002). "Transformation of barbituric acid into alloxan by hydroxyl radicals: interaction with melatonin and with other hydroxyl radical scavengers". J. Pineal Res. 33 (4): 239–47. doi:10.1034/j.1600-079X.2002.02936.x. PMID 12390507. S2CID 30242100.
  8. ^ E. L. Jenner (1973). "α,α,α',α'-Tetramethyltetramethylene glycol". Organic Syntheses.; Collective Volume, 5, p. 1026
  9. ^ Cai, Q. Q.; Lee, B. C. Y.; Ong, S. L.; Hu, J. Y. (2021-02-15). "Fluidized-bed Fenton technologies for recalcitrant industrial wastewater treatment–Recent advances, challenges and perspective". Water Research. 190: 116692. doi:10.1016/j.watres.2020.116692. ISSN 0043-1354.
  10. ^ Xu, Xiang-Rong; Li, Xiao-Yan; Li, Xiang-Zhong; Li, Hua-Bin (2009-08-05). "Degradation of melatonin by UV, UV/H2O2, Fe2+/H2O2 and UV/Fe2+/H2O2 processes". Separation and Purification Technology. 68 (2): 261–266. doi:10.1016/j.seppur.2009.05.013. ISSN 1383-5866.
  11. ^ Tang, W. Z.; Huang, C. P. (1996-12-01). "2,4-Dichlorophenol Oxidation Kinetics by Fenton's Reagent". Environmental Technology. 17 (12): 1371–1378. doi:10.1080/09593330.1996.9618465. ISSN 0959-3330.
  12. ^ Cai, Q. Q.; Lee, B. C. Y.; Ong, S. L.; Hu, J. Y. (2021-02-15). "Fluidized-bed Fenton technologies for recalcitrant industrial wastewater treatment–Recent advances, challenges and perspective". Water Research. 190: 116692. doi:10.1016/j.watres.2020.116692. ISSN 0043-1354.
  13. ^ Szpyrkowicz, Lidia; Juzzolino, Claudia; Kaul, Santosh N (June 2001). "A Comparative study on oxidation of disperse dyes by electrochemical process, ozone, hypochlorite and fenton reagent". Water Research. 35 (9): 2129–2136. doi:10.1016/s0043-1354(00)00487-5. ISSN 0043-1354.
  14. ^ Velichkova, Filipa; Delmas, Henri; Julcour, Carine; Koumanova, Bogdana (2016-06-28). "Heterogeneous fenton and photo-fenton oxidation for paracetamol removal using iron containing ZSM-5 zeolite as catalyst". AIChE Journal. 63 (2): 669–679. doi:10.1002/aic.15369. ISSN 0001-1541.
  15. ^ Cai, Qinqing; Lee, Brandon Chuan Yee; Ong, Say Leong; Hu, Jiangyong (2021-01-14). "Application of a Multiobjective Artificial Neural Network (ANN) in Industrial Reverse Osmosis Concentrate Treatment with a Fluidized Bed Fenton Process: Performance Prediction and Process Optimization". ACS ES&T Water. doi:10.1021/acsestwater.0c00192.
  16. ^ Matavos-Aramyan, S; Moussavi, M; Matavos-Aramyan, H; Roozkhosh, S (2017). "Cryptosporidium-contaminated water disinfection by a novel Fenton process". Free Radical Biology and Medicine. 106: 158–167. doi:10.1016/j.freeradbiomed.2017.02.030. PMID 28212822. S2CID 3918519.
  17. ^ Robbins & Cotran (2008). Pathologic basis of disease (7th ed.). Elsevier. p. 16. ISBN 9780808923022.
  18. ^ Lapointe, Marc (2004-01-01). "Iron supplementation in the intensive care unit: when, how much, and by what route?". Critical Care. 8 (2): S37–41. doi:10.1186/cc2825. ISSN 1364-8535. PMC 3226152. PMID 15196322.

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