Feng et al.
[31] synthesized CuFe
2O
4 nanoparticles to be used as a Fenton catalyst in the degradation of sulfanilamide. They suggest that CuO is more reactive and effective than Fe
3+ for the activation of H
2O
2 and, which is more important, can work in a higher pH range than conventional iron oxides. Moreover, Zhang et al.
[32] observed that the leaching of Cu
2+ is 30 times lower in CuFe
2O
4 than with CuO. The promising performance of the copper spinel ferrite as a Fenton catalyst was pointed out by several authors. Suraj et al.
[33] synthesized CuFe
2O
4 by the chemical coprecipitation method and used it as a heterogeneous Fenton catalyst for the treatment of pulp and paper wastewater, obtaining a 78% elimination of the chemical oxygen demand. Ding et al.
[34] demonstrated that the morphology of the spinel is also very important. They synthesized hollow CuFe
2O
4 spheres with oxygen vacancies (
Figure 1), which demonstrated greater degradation of ciprofloxacin than normal CuFe
2O
4 particles. This better performance of hollow spheres was attributed, among other factors, to the synergistic oxygen vacancies and confinement effects on the catalyst surface. The oxygen vacancies produce highly active electron-rich Cu
+ species, which enhanced the H
2O
2 activation and, thus, the hydroxyl radical generation. In turn, the hollow structure is responsible for concentrating the organic pollutants near the
•OH-generator active sites, improving the organic pollutant molecules/
•OH radicals contact, and accelerating the degradation. According to Yu et al.
[35], the particle size and surface area are more important factors than a crystalline structure for improving the catalytic efficiency of CuFe
2O
4. López-Ramón et al.
[36] evaluated the effect of calcination temperature on the catalytic activity of CuFe
2O
4 synthesized by the sol–gel method, finding that the calcination temperature has two opposite effects: the activity decreases with increasing temperature due to the increase in crystalline size and cubic-to-tetragonal transformation of ferrite and appearance of hematite; however, the metal leaching decreases with increasing calcination temperatures.
Sing and Singhal
[42] demonstrated that the transition-metal doping of cobalt ferrites is a promising method for tuning the physical characteristics of catalysts and thus, enhancing their catalytic and magnetic properties. For that, the authors synthesized a series of Ru-doped cobalt ferrite nanoparticles by the sol–gel method for the photo-Fenton degradation of red Remazol textile dye, achieving a degradation of approximately 90% within 120 min. The mechanism proposed is based on the photocatalytic and Fenton character of the Ru-modified ferrite. An electron-hole pair is created by the irradiation of cobalt ferrite nanoparticles with visible light. The photogenerated electrons are responsible for the OH
• generation from H
2O
2 and also the reduction of the Fe
3+ cation on cobalt ferrite to Fe
2+, which further generates OH
• radicals in the reaction with hydrogen peroxide. Vinosha et al.
[43] also analyzed the photo-Fenton performance of CoFe
2O
4 nanoparticles obtained by means of chemical coprecipitation, achieving almost total degradation of methylene blue (~99.3% in 75 min) under visible light irradiation. As an outstanding result, they proposed that the pH used in the synthesis was not an impact parameter that affected the morphology of the catalyst; however, it significantly affects the particle size (a more alkaline (pH > 9) medium, larger crystallite size). It has been proposed that the reactions that lead to the formation of CoFe
2O
4 by the chemical coprecipitation method in an aqueous medium, are those presented in Equations (9) and (10)
[44].
In turn, Iron cobaltite (FeCo
2O
4) has been also studied in environmental remediation and energy storage, thanks to its electrical properties and electrochemical performance
[45]. In the energy storage field, Mohamed et al. demonstrated that iron cobaltite nanorods show a better capacity and lower overpotential as the cathode of lithium−O
2 batteries than other metal cobaltites (Mn, Ni, and Zn)
[46] because the FeCo
2O
4 surface has the highest number of electropositive Co
3+ active sites that improve the oxygen adsorption and Fe
2+ in the tetrahedral site that favors the release of electrons to reduce oxygen. Yadav et al.
[47] demonstrated that iron cobaltites are also efficient for supercapacitive and photocatalytic applications due to the valence states of the Fe
3+/Fe
2+ and Co
3+/Co
2+ species, which are considered active catalytic sites. These nanoflake-like iron cobaltites present a capacitance as high as 1230 F g
−1 (5 mV s
−1) with a good rate capability and superior cycling stability and also show a good photocatalytic performance achieving up to 94.19% degradation of crystal violet dye under sunlight illumination. However, despite this, very little work has been carried out with reference to their evaluation as a Fenton-like catalyst
[48][49]. Zhang et al.
[48] synthesized nitrogen-containing carbon/FeCo
2O
4 composites and analyzed their performances as Fenton catalysts for the degradation of methylene blue obtaining almost 100% removal in 10 min without pH adjustment, which was attributed to the uniform distribution of bimetals and nitrogen doping, which ensured the exposure of sites with high catalytic activity. Zhao et al.
[49] analyzed the behavior of FeCo
2O
4/g–C
3N
4 as a photo-Fenton catalyst in the degradation of rhodamine B (RhB), obtaining 98% degradation in 45 min, which was attributed to a synergetic interaction between photocatalytic and Fenton-like reactions and the effective separation of the photogenerated charges. Therefore, it is proposed that the use of iron cobaltite as a pristine catalyst (without doping or support) in the Fenton reaction may be promising for future applications.
In general, spinel ferrites turn out to be attractive materials for catalytic activities in Fenton processes, mainly because they manage to improve the generation reaction of Fe
2+ and achieve a synergistic effect between metal ions with valences similar to those of iron involved in the process (M
2+, M
3+, M: Metal).
Perovskites are other types of materials with promising catalytic activity that can be synthesized by the abovementioned methods (Chemical coprecipitation, sol–gel, and hydro/solvothermal)
[50][51][52][53]. Perovskites can be defined as a type of mixed oxide with different formulations, binary (ABO), ternary (AA’BO or ABB’O), and quaternary (AA’BB’O), where A and B are cations sites occupied by alkali metals, alkaline-earth metals or rare-earth metals and transition metals, respectively
[54].
Some perovskites have been studied in different Fenton-type reactions, for example, Carrasco-Díaz et al.
[55] removed paracetamol by Fenton reaction using LaCu
1−xM
xO
3 (M = Mn, Ti) as the catalyst and determined that the most active catalyst was the one that contained the highest amount of Cu
2+ at the surface. Moreover, they found that the titanium and manganese species seem not to be responsible for the improvement of activity with respect to the sample LaCuO
3. Li et al.
[56] synthetized a Ca
1−xFeO
3−δ perovskite and determined that the A-site cation can distort the FeO
6 octahedra in the perovskite and regulate the oxygen vacancies (OV) concentration; in this way, an A-site cation deficient of Ca
0.9FeO
3-δ results in an improved H
2O
2 activation for the degradation of tetracycline by a Fenton-like process. Similarly, Xie et al.
[57] found that the copper incorporation in LaCoO
3 perovskite improved the electro-Fenton activity due to the enhancement of redox activity and oxygen vacancies, but in this case, by the substitution of B-site elements, which synergistically promoted the activation of hydrogen peroxide to a hydroxyl radical (
•OH). On the other hand, Rusevova et al.
[58] degraded phenol via heterogeneous Fenton-like reactions using iron-containing LaFeO
3, and BiFeO
3 perovskites, and made a comparison with data reported in the literature using, as a catalyst, nano-sized Fe(II, III) oxide particles, determining that the perovskites synthetized had a higher catalytic activity. Zhao et al.
[59] determined that BiFeO
3 supported in carbon aerogel (BFO/CA) with a three-dimensional (3D) structure improves the catalytic activity and stability of BiFeO
3, resulting in an interesting strategy for the development of advanced catalysts for its possible application in Fenton processes.
In summary, perovskites are materials similar to spinel ferrites, and the contribution of two different metal species (formation of OVs) can be of interest to the Fenton process.