Fenton's reaction

 H.J.H Fenton discovered in 1894 that several metals have a special oxygen transfer properties which improve the use of hydrogen peroxide. Actually, some metals have a strong  catalytic power to generate highly reactive hydroxyl radicals (.OH). Since this discovery, the iron catalyzed hydrogen peroxide has been called Fenton's reaction. Nowadays, the Fenton's reaction is used to treat a large variety of water pollution such as phenols, formaldehyde, BTEX, pesticides, rubber chemicals and so on.

Applications of the Fenton's reaction:

This process may be use to wastewater, contaminated soils and sludges with the following actions:

• Organic pollutant destruction
• Toxicity reduction
• Biodegradability improvement
• BOD/COD removal
• Odor and color removal
• Destruction of resin in radioactive contaminated sludge

How does the Fenton's reaction work ?

After addition of the iron and the hydrogen peroxide, they are going to react together to generate some hydroxyl radicals as it shows in the following equations:

Fe²⁺ + H₂O₂ ---->  Fe³⁺ + .OH + OH^-

Fe³⁺ + H₂O₂ ---->  Fe²⁺ + .OOH + H⁺

The typical range for the iron dose is 1 part of Fe per 5-25 parts of H₂O₂.

After that the hydroxyl radicals are going to react with the pollutants to oxidize its. Actually the hydroxyl radicals can react according 4 kinds of reactions with the pollutants:

• Addition: ^.OH + C₆H₆  ---->  (OH)C₆H₆
• Hydrogen Abstraction: OH + CH₃OH  ---->  CH₂OH + H₂O
• Electron Transfer: ^.OH + [Fe(CN)₆]^4-  ---->  [Fe(CN)₆]^3- + OH^-
• Radical Interaction: ^.OH + ^.OH  ---->  H₂O₂

During the Fenton's reaction all the parameters are adjusted to promote the two first kind of reaction between the pollutant and the hydroxyl radicals. 

Requirements of the reaction:

• pH adjustment to 3-5 : if the pH is too high the iron precipitate in Fe(OH)₃ and will decompose the H₂O₂ to oxygen. Basically, the optimal pH occurs between 3 and 6. It's really important to pay attention to the double pH drop due to the addition of Iron and H₂O₂ as you can see in the following chart. Indeed, FeSO4 catalyst which contains residual H2SO4 and the H2O2 addition is responsible for the fragmentation of organic material into organic acids.

• addition of the iron catalyst as a solution of FeSO₄ 

• Adding slowly the H₂O₂ : in order to control the increasing of the pH and the temperature during the reaction it's better to complete the reaction step by step with a continuous adjustment. 

Resource: US Peroxide