What is the actual mechanism of Fenton reaction?

Question

Fenton reaction was supposedly discovered by Fenton as Tartaric Acid oxidation by hydrogen peroxide in presence of ferrous cation in 1894. Research Paper link.

I have worked on this reaction for years, not an exaggeration. Whenever I mixed $\ce{H2O2}$ with $\ce{Fe^2+}$ solution, or directly solid ferrous sulphate hepta hydrate, I instantaneously got a dark beer-like color, and a vigorously reacting mixture without adjusting $\mathrm{pH}$. $\pu{10 ml}$ of 35% $\ce{H2O2}$ and $\pu{150 mg}$ of $\ce{Fe2SO4.7H2O}$ is mostly what I used. This much peroxide generally defined the reaction as Catalyzed Hydrogen Peroxide Propagations by RJ Watts group. When mixing with 1g soil in first 12 seconds temperature escalated to $\pu{84^\circ C}$, then slowly calmed. Overall bubbling ceased in 60 seconds, what kind of soil did not make any difference. And for $\mathrm{pH}$ measurement, it is very problematic for many reasons so I simply could not get any meaningful results. This just increased my curiosity.

There are experimental and computational studies over its mechanism of reactive oxygen species generation and organic matter degradation. The second one is a bit easier since somehow catching the end products gives the idea, especially with spin trap compounds.

I have lost all of references so I can only give the categories I have encountered.

• Majority went for radical species mechanisms and chain reaction, involving hydroxyl, perhydroxyl, superoxide radicals and uses as a base Haber & Weiss,
  Haber, F. & Weiss, J. J. (1934) Proc. R. Soc. London Ser. A 147, 332-351.
  Yet these reactions are inherently fast, they are just proposed mechanisms.

• There is a considerable number of people stand for ferryl ion pathway, which mostly does not involve radicals but unstable iron (IV) oxidation state compound.
  Usually computational studies propose this idea, as hydroxyl radicals lifetime is very short for making it as an efficient reactant.

The reaction of Fenton I am asking is reaction between aqueous hydrogen peroxide and $\ce{Fe^2+}$ ion without any ligand other than water. If it does not help for explanation you may very well ignore more than 30 variations like sonofenton.

Answer 1

The Fenton reaction

The oxidation of organic substrates by iron(II) and hydrogen peroxide ($\ce{H2O2}$ ) is called the Fenton Reaction. It was first described by Henry John Horstman Fenton in 1894 who first observed the oxidation of tartaric acid by $\ce{H2O2}$ in the presence of $\ce{Fe^{2+}}$ ions [1]. After more than 100 years, the mechanism of the reaction is still unsettled. The recent review by Dunford (2002) stated that:

The mechanism of reaction of hexaquo iron(II) with hydrogen peroxide has been unresolved for 70 years. Most scientists, perhaps by default, have accepted the free radical chain mechanism of Barb et al. (1957)[3]. However an earlier proposal involved formation of the ferryl ion, $\ce{FeO2+}$ [4]. Recent work has favored a mechanism involving $\ce{FeO2+}$ and $\ce{FeOFe^{5+}}$ species. Similarly there are differences of opinion on the mechanism of reaction of iron(III), both hexaquo and chelated, with hydrogen peroxide. These differences have fostered a recent burst of activity, with claims on one hand that hydroxyl radicals play a key role, and on the other, that there is a non-free radical mechanism. In contrast, the mechanism of reaction of the heme-containing peroxidase and catalase enzymes with hydrogen peroxide, orders of magnitude faster than reactions of iron(II)/(III), now appears to be well established.

According to the free radical chain mechanism [3], the formation of radicals $\ce{HO^.}$ and $\ce{HO2^.}$ have been proposed. Although, this mechanism has some flaws, it has still been used to explain result in current research as recently as 2014 [5]. The proposed mechanism was depicted in Figure 1, which is followed by several other well received mechanisms for the reaction discussed in reviews sighted.

Mechanism of the redox reaction involved in the Fenton’s reagent (Figure 1)[5]:

Basic reactions and intermediates involved in the classic Fenton and the metal centered Fenton reactions (Figure 2)[6]:

Proposed non-radical mechanism for the Fenton reaction (Figure 3)[6]:

Possible reaction pathways for the Fenton reaction in absence of organic substrates (Figure 4)[6]:

Requirements of the reaction:

$\mathrm{pH}$ should be adjusted to 3-5: if the $\mathrm{pH}$ is too high the iron precipitate in $\ce{Fe(OH)3}$ and will decompose the $\ce{H2O2}$ to oxygen. Basically, the optimal $\mathrm{pH}$ occurs between $3$ and $6$. It's really important to pay attention to the double $\mathrm{pH}$ drop due to the addition of $\ce{FeSO4}$ ($\ce{Fe^{2+}}$ source) and $\ce{H2O2}$. Indeed, $\ce{FeSO4}$ catalyst which contains residual $\ce{H2SO4}$ and the $\ce{H2O2}$ addition is responsible for the fragmentation of organic material into organic acids. Thus, the iron catalyst should be added as a solution of $\ce{FeSO4}$ with slow addition of $\ce{H2O2}$, you may able to control the suddern variation of $\mathrm{pH}$ and the temperature during the reaction. It's better to complete the reaction step by step with a continuous adjustment of these factors.

References:

[1] LXXIII.—Oxidation of tartaric acid in presence of iron: H. J. H. Fenton, J. Chem. Soc., Trans., 1894, 65, 899–910 (DOI: 10.1039/CT8946500899).

[2] Oxidations of iron(II)/(III) by hydrogen peroxide: from aquo to enzyme: H.Brian Dunford, Coordination Chemistry Reviews, 2002, 233–234, 311-318 (https://doi.org/10.1016/S0010-8545(02)00024-3).

[3] Reactions of ferrous and ferric ions with hydrogen peroxide. Part I.—The ferrous ion reaction: W. G. Barb, J. H. Baxendale, P. George, and K. R. Hargrave, Trans. Faraday Soc., 1951, 47, 462-500 (http://dx.doi.org/10.1039/TF9514700462).

[4] Ferryl ion, a compound of tetravalent iron: William C. Bray, and M. H. Gorin, Journal of American Chemical Society, 1932, 54(5), 2124-2125 (DOI: 10.1021/ja01344a505).

[5] Design Polysaccharides of Marine Origin: Chemical Modifications to Reach Advanced Versatile Compounds: Nathalie Chopin, Xavier Guillory, Pierre Weiss, Jean Le Bideau, and Sylvia Colliec-Jouault, Current Organic Chemistry, 2014, 18(7), 867-895 (DOI: 10.2174/138527281807140515152334).

[6] Fenton reaction - Controversy concerning the chemistry: Krzysztof Barbusinski, Ecological Chemistry and Engineering S, 2009, 16(3), 347-358.

Answer 2

I think that the reaction of hydrogen peroxide with ferrous sulfate under acidic conditions (Fenton type reaction) is rather similar to radiation chemistry in water. In both cases the $\ce{HO}$ radical is important.

$$\ce{H2O2 + H+ + e- -> H2O + HO}$$

While when water it irradated it forms H atoms and HO radicals.

I used to react pivalic acid ($\ce{Me3CCOOH}$) with ferrous sulfate, sulfuric acid and hydrogen peroxide as the first step of the synthesis of $\ce{CyMe4BTBP}$. This forms $\ce{HOOCCMe2CH2CH2CMe2COOH}$ which is formed by dimerisation of the $\ce{Me2C(CH2)(COOH)}$ radical. This radical in turn is formed by the abstraction of a hydrogen atom from the pivic acid.