Fenton's reagent

Fenton's reagent is a solution of hydrogen peroxide (H₂O₂) and an iron catalyst (typically iron(II) sulfate, FeSO₄).^[1] It is used to oxidize contaminants or waste water as part of an advanced oxidation process. Fenton's reagent can be used to destroy organic compounds such as trichloroethylene and tetrachloroethylene (perchloroethylene). It was developed in the 1890s by Henry John Horstman Fenton as an analytical reagent.^[2][3][4]

Reactions

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.^[5]

Fe^2+ + H2O2 → Fe^3+ + HO^• + OH^−

(1)

Fe^3+ + H2O2 → Fe^2+ + HOO^• + H^+

(2)

2 H2O2 → HO^• + HOO^• + H2O

(net reaction: 1+2)

The free radicals generated by this process engage in secondary reactions. For example, the hydroxyl is a powerful, non-selective oxidant.^[6] 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.^[7]

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.^[8]

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 + (byproducts)

(3)

An example hydroxylation reaction involves the oxidation of barbituric acid to alloxane.^[9] 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.^[10] 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.^[11]

Effect of pH on formation of free radicals

pH affects the reaction rate due to a variety of reasons. At a low pH, complexation of Fe+2 also occurs, leading to lower availability of Fe+2 to form reactive oxidative species (OH^•).^[12] Lower pH also results in the scavenging of ^•OH by excess H⁺,^[13] 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 Fe+3 species in solution.^[11] Solubility of iron species is directly governed by the solution's pH. Fe+3 is about 100 times less soluble than Fe+2 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.^[14] Higher pH also decreased the redox potential of ^•OH thereby reducing its effectiveness.^[15] 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.^[16]

Impacts of operation pH on reaction rate

Low pH	Formation of [Fe(H2O)6]^2+ complex, hence reducing Fe^2+ 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

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.^[17] 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.^[18] 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.^[19]

Applications

See also: Advanced oxidation process

Fenton's reagent is used as a sewage treatment agent.^[20]

Fenton's reagent can be used in different chemical procesess that supply hydroxyl ion or oxidize certain compounds:^[21]

• The first stage of Fenton’s reaction (oxidation of Fe³⁺ with hydrogen peroxide) is used in Haber–Weiss reaction
• Fenton's reagent can be used in organic synthesis reactions: e.g. hydroxylation of arenes via a free radical substitution
• Conversion of benzene into phenol by using Fenton's reagent
• Oxidation of barbituric acid into alloxan.
• Coupling reactions of alkanes

Fenton-like reagent

Mixtures of Fe+2 and H₂O₂ are called Fenton reagent. If Fe+2 is replaced by Fe+3, it is called Fenton-like reagent.

Numerous transition metal ions and their complexes in their lower oxidation states (LmMn⁺) were found to have the oxidative features of the Fenton reagent, and, therefore, the mixtures of these metal compounds with H₂O₂ were named "Fenton-like" reagents.^[22]

See also

• Carbon monoxide-releasing molecules

References

1. Hemond, Harold (2015). Chemical Fate and Transport in the Environment (3rd ed.). Elsevier. p. 287. ISBN 9780123982568.
2. Koppenol, W. H. (1 December 1993). "The centennial of the Fenton reaction". Free Radical Biology and Medicine. 15 (6): 645–651. doi:10.1016/0891-5849(93)90168-t. PMID 8138191.
3. Fenton, H. J. H. (1894). "Oxidation of tartaric acid in presence of iron". Journal of the Chemical Society, Transactions. 65 (65): 899–911. doi:10.1039/ct8946500899.
4. Hayyan, M.; Hashim, M. A.; Al Nashef, I. M. (2016). "Superoxide ion: Generation and chemical implications". Chemical Reviews. 116 (5): 3029–3085. doi:10.1021/acs.chemrev.5b00407. PMID 26875845.
5. Tang, Zhongmin; Zhao, Peiran; Wang, Han; Liu, Yanyan; Bu, Wenbo (2021). "Biomedicine Meets Fenton Chemistry". Chemical Reviews. 121 (4): 1981–2019. doi:10.1021/acs.chemrev.0c00977. PMID 33492935. S2CID 231712587.
6. Cai, Q.Q.; Jothinathan, L.; Deng, S.H.; Ong, S.L.; Ng, H.Y.; Hu, J.Y. (2021). "Fenton- and ozone-based AOP processes for industrial effluent treatment". Advanced Oxidation Processes for Effluent Treatment Plants. pp. 199–254. doi:10.1016/b978-0-12-821011-6.00011-6. ISBN 978-0-12-821011-6. S2CID 224976088.
7. 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.
8. Casado, Juan; Fornaguera, Jordi; Galan, Maria I. (January 2005). "Mineralization of aromatics in water by sunlight-assisted electro-Fenton technology in a pilot reactor". Environmental Science and Technology. 39 (6): 1843–1847. Bibcode:2005EnST...39.1843C. doi:10.1021/es0498787. PMID 15819245.
9. Brömme, H. J.; Mörke, W.; Peschke, E. (November 2002). "Transformation of barbituric acid into alloxan by hydroxyl radicals: interaction with melatonin and with other hydroxyl radical scavengers". Journal of Pineal Research. 33 (4): 239–247. doi:10.1034/j.1600-079X.2002.02936.x. PMID 12390507. S2CID 30242100.
10. Jenner, E. L. (1973). "α,α,α′,α′-Tetramethyltetramethylene glycol". Organic Syntheses; Collected Volumes, vol. 5, p. 1026.
11. Cai, Q. Q.; Lee, B. C. Y.; Ong, S. L.; Hu, J. Y. (15 February 2021). "Fluidized-bed Fenton technologies for recalcitrant industrial wastewater treatment–Recent advances, challenges and perspective". Water Research. 190: 116692. Bibcode:2021WatRe.19016692C. doi:10.1016/j.watres.2020.116692. PMID 33279748. S2CID 227523802.
12. Xu, Xiang-Rong; Li, Xiao-Yan; Li, Xiang-Zhong; Li, Hua-Bin (5 August 2009). "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.
13. Tang, W. Z.; Huang, C. P. (1 December 1996). "2,4-Dichlorophenol Oxidation Kinetics by Fenton's Reagent". Environmental Technology. 17 (12): 1371–1378. Bibcode:1996EnvTe..17.1371T. doi:10.1080/09593330.1996.9618465.
14. Szpyrkowicz, Lidia; Juzzolino, Claudia; Kaul, Santosh N (1 June 2001). "A Comparative study on oxidation of disperse dyes by electrochemical process, ozone, hypochlorite and fenton reagent". Water Research. 35 (9): 2129–2136. Bibcode:2001WatRe..35.2129S. doi:10.1016/s0043-1354(00)00487-5. PMID 11358291.
15. Velichkova, Filipa; Delmas, Henri; Julcour, Carine; Koumanova, Bogdana (2017). "Heterogeneous fenton and photo-fenton oxidation for paracetamol removal using iron containing ZSM-5 zeolite as catalyst" (PDF). AIChE Journal. 63 (2): 669–679. doi:10.1002/aic.15369.
16. Cai, Qinqing; Lee, Brandon Chuan Yee; Ong, Say Leong; Hu, Jiangyong (9 April 2021). "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. 1 (4): 847–858. doi:10.1021/acsestwater.0c00192. S2CID 234110033.
17. 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.
18. Robbins; Cotran (2008). Pathologic basis of disease (7th ed.). Elsevier. p. 16. ISBN 978-0-8089-2302-2.
19. Lapointe, Marc (14 June 2004). "Iron supplementation in the intensive care unit: when, how much, and by what route?". Critical Care. 8 (2): S37-41. doi:10.1186/cc2825. PMC 3226152. PMID 15196322.
20. Chen, Yan-Jhang; Fan, Tang-Yu; Wang, Li-Pang; Cheng, Ta-Wui; Chen, Shiao-Shing; Yuan, Min-Hao; Cheng, Shikun (2020-02-18). "Application of Fenton Method for the Removal of Organic Matter in Sewage Sludge at Room Temperature". Sustainability. 12 (4): 1518. doi:10.3390/su12041518. ISSN 2071-1050.
21. "Fenton's Reaction - Reaction Details, Reagent, Applications, FAQs". BYJUS. Retrieved 2022-07-25.
22. Goldstein, S.; Meyerstein, D.; Czapski, G. (October 1993). "The Fenton reagents". S Goldstein et al. Free Radic Biol Med. 15 (4): 435–445. doi:10.1016/0891-5849(93)90043-t. PMID 8225025. Retrieved 19 November 2023.

Further reading

• Goldstein, Sara; Meyerstein, Dan; Czapski, Gidon (1993). "The Fenton reagents". Free Radical Biology and Medicine. 15 (4): 435–445. doi:10.1016/0891-5849(93)90043-T. PMID 8225025.
• Barbusiński, K. (2009). "Fenton Reaction – Controversy concerning the chemistry" (PDF). Ecological Chemistry and Engineering. 16 (3): 347–358.

External links

• Reference Library Peroxide Applications
• Companies that use Fenton's Reagent for chemical remediation: ORIN