Fenton-based Processes

First, it is necessary to introduce the "classical" Fenton reaction. This reaction was discovered in 1894 by H. J. H. Fenton, who observed the oxidation of tartaric acid in the presence of ferrous cation (Fe2+) and hydrogen peroxide (H2O2). Indeed, in the presence of H2O2 and Fe2+, the generation of hydroxyl radical (HO•) occurs (Eq. 1) and subsequently, the oxidation of organic compounds can be achieved. Alternatively, ferric cation (Fe3+) can also reacts with H2O2 in order to produce hydroperoxyl radical (HOO•); this reaction is called “Fenton-like” (Eq. 2). Therefore, from both Fenton and Fenton-like reactions, the cycle of iron is observed since, on one hand, Fe2+ is transformed into Fe3+ (Fenton) and, on another hand, Fe3+ is transformed into Fe2+ (Fenton-like).

Fenton:         Fe2+ + H2O2 →  Fe3+ + HO- + HO•  [k ≈ 101 M-1·s-1]       (Eq. 1)

   Fenton-like:  Fe3+ + H2O2 →  Fe2+ + H+ + HOO•   [k ≈ 10-3 M-1·s-1]            (Eq. 2)

However, the iron cycle is not catalytic. In addition to the significant difference in the kinetic constants, iron species are not stable in homogeneous systems. Indeed, Fe2+ tends to oxidize into Fe3+ while Fe3+ tends to aggregate, coagulate and precipitate, thus acidic conditions (pH < 3) are required to limit these drawbacks. For example, non-toxic and natural complexing agents can be used (like ascorbic acid, polyphenols, etc.) to improve the regenaration of Fe2+ from Fe3+, which is the limiting step in Fenton reactions. But heterogeneous systems appear the best solution since a wide pH range (e.g. environmental conditions) can be used although the reactions take place slower. The mechanisms of heterogeneous Fenton and Fenton-like are the subjects of intense discussions in the scientific community since it is still unclear whether ROS are generated at the surface of the material or at the close surroundings of the surface through possibly leached iron.

Photo-Fenton process is considered as viable alternative to the Fenton reactions, since there is no need to add hydrogen peroxide to the system. Indeed, in homogeneous system, the photo-Fenton process involves the photolysis of aqua complexes of iron (III), i.e. the absorption of light by the complex, thus leading to a ligand-to-metal charge transfer (LMCT). It is worth noting that the most photoactive iron (III) complex is [Fe(H2O)5(OH)]2+. In heterogeneous system, the mechanism is still not fully resolved since the photolysis can occur either between adsorbed water molecule (surface hydroxyl group) and Fe (III) or at the close surroundings of the surface of iron based material (similar to homogeneous system). The latter is favored for iron oxides/hydroxides because photo-dissolution of iron occurs. The photo-Fenton process leads to the generation of ROS, especially hydroxyl radicals, according to the following reaction (simplified chemical equation):

Fe3+ + H2O + hv  →  Fe2+ + H+ + HO•

Recently, a growing interest is focused on iron-free Fenton based processes. Indeed, not only iron but other metals can also trigger the generation of ROS in the presence of H2O2. This is the case for Al, Ce, Cu, Mn and V. Intense research is actually performed to elucidate these iron-free Fenton based mechanisms both in presence of H2O2 and under light, so the resulting innovative and photoactive materials could trigger multi-Fenton processes.