Recent Advances in the Development of Novel Iron–Copper Bimetallic Photo Fenton Catalysts

Abstract

Advanced oxidation processes (AOPs) have been postulated as viable, innovative, and efficient technologies for the removal of pollutants from water bodies. Among AOPs, photo-Fenton processes have been shown to be effective for the degradation of various types of organic compounds in industrial wastewater. Monometallic iron catalysts are limited in practical applications due to their low catalytic activity, poor stability, and recyclability. On the other hand, the development of catalysts based on copper oxides has become a current research topic due to their advantages such as strong light absorption, high mobility of charge carriers, low environmental toxicity, long-term stability, and low production cost. For these reasons, great efforts have been made to improve the practical applications of heterogeneous catalysts, and the bimetallic iron–copper materials have become a focus of research. In this context, this review focuses on the compilation of the most relevant studies on the recent progress in the application of bimetallic iron–copper materials in heterogeneous photo–Fenton-like reactions for the degradation of pollutants in wastewater. Special attention is paid to the removal efficiencies obtained and the reaction mechanisms involved in the photo–Fenton treatments with the different catalysts.

1. Introduction

In recent years, the rapid development of the industrial sector has resulted in numerous effluents that contain toxic substances and affect the environment in a dangerous way [1]. Some of these pollutants, due to their chemical nature, are resistant to the conventional physical and biological processes commonly used in wastewater treatment plants [2]. For this reason, advanced oxidation processes (AOPs) have been postulated as viable, innovative, and efficient technologies for the removal of these compounds from water bodies [3]. Among the AOPs, the heterogeneous Fenton and photo–Fenton processes have been shown to be effective for the degradation of various types of organic compounds in industrial wastewater [4,5].

Equations (1)–(5) are the main pathways involved in the Fenton processes. Reaction 1 is a key step because it generates the strongly oxidizing radical HO^•, while regeneration of Fe²⁺ is the rate–determining step under dark conditions and in the absence of alternative Fe³⁺–reducing species (Equation (2)) [6,7].

$$F{e}^{2+}+H_2{O}_2\to F{e}^{3+}+ H{O}^{-}+H{O}^{•}\hspace{1em}(\mathrm{k}=40–80 {\mathrm{M}}^{-1}{\mathrm{s}}^{-1})$$

(1)

$$F{e}^{3+}+H_2{O}_2\to F{e}^{2+}+H{O}_2^{•} + H^{+}\hspace{1em}(\mathrm{k}=9.1\times {10}^{-7} {\mathrm{M}}^{-1}{\mathrm{s}}^{-1})$$

(2)

$$F{e}^{2+}+H{O}^{•}\to F{e}^{3+}+ H{O}^{-}\hspace{1em}\hspace{1em}\hspace{1em}(\mathrm{k}=2.5–5\times {10}^8 {\mathrm{M}}^{-1}{\mathrm{s}}^{-1})$$

(3)

$$F{e}^{3+}+H{O}_2^{•} \to F{e}^{2+}+ H^{+}+ O_2\hspace{1em}\hspace{1em}\hspace{1em}(\mathrm{k}=0.33–2.1\times {10}^6 {\mathrm{M}}^{-1}{\mathrm{s}}^{-1})$$

(4)

$$F{e}^{2+}+ H{O}_2^{•}\to F{e}^{3+}+H{O}_2^{-}\hspace{1em}\hspace{1em}\hspace{1em}(\mathrm{k}=0.72–1.5\times {10}^6 {\mathrm{M}}^{-1}{\mathrm{s}}^{-1})$$

(5)

Equations (1)–(5) are the most important steps in the dark Fenton chemistry because H₂O₂ is consumed and Fe²⁺ is regenerated from Fe³⁺ by these reactions. The HO^• and HO₂^• radicals are the main species involved in the oxidation of pollutants. In addition to Fe(II), other transition metal ions can also promote similar processes, which are then referred to as Fenton–like or Fenton–type. For example, both oxidation states of copper can react with H₂O₂ to form HO₂^• and HO^• radicals (Equations (6) and (7)) [8].

$$C{u}^{2+}+H_2{O}_2\to C{u}^{+}+H{O}_2^{•} + H^{+}\hspace{1em}\hspace{1em}\hspace{1em}(\mathrm{k}=1.15\times {10}^{-6} {\mathrm{M}}^{-1}{\mathrm{s}}^{-1})$$

(6)

$$C{u}^{+}+H_2{O}_2\to C{u}^{2+}+ H{O}^{-}+H{O}^{•}\hspace{1em}(\mathrm{k}=1.0\times {10}^4 {\mathrm{M}}^{-1}{\mathrm{s}}^{-1})$$

(7)

Under suitable conditions, copper can act as a better catalyst compared to iron, as it is able to form temporary complexes with oxidation products and rapidly convert Cu⁺ to Cu²⁺ and vice versa [9,10,11]. Since the oxidation products do not form stable complexes with copper, reactive coordination sites remain available for continuous catalytic cycling. Therefore, copper not only provides a better redox cycle but is also active in the mineralization of organic matter. However, a major disadvantage of copper–based catalysts is the high excess of H₂O₂ required to maintain catalytic activity, which increases treatment costs [12]. This disadvantage has often been mitigated by the use of bimetallic composites of copper and iron [9,13,14,15].

Since Reaction 1 proceeds much faster than Reaction 2, the Fe²⁺ ions are consumed faster than generated, so that a large amount of ferric hydroxide is formed as sludge during the process, which causes additional problems in separation and disposal [6,16,17]. Fenton (H₂O₂/Fe²⁺) and Fenton–like (H₂O₂/Fe³⁺) processes can be significantly enhanced under ultraviolet (UV)/visible irradiation (λ < 600 nm). The UV region of the electromagnetic spectrum extends from about 100 nm to 400 nm, while the visible region extends from about 400 nm to 760 nm [18]. Photoirradiation prevents the accumulation of Fe³⁺ ions in the system since Fe²⁺ ions are regenerated from Fe³⁺ by photoreduction [17,19,20,21]. The improvement in pollutant removal rates achieved with the photo–Fenton process compared to the Fenton process can be explained by the following contributions [6,22,23,24]: (i) the generation of HO^• and Fe²⁺ via the photolysis of iron (III) hydroxo complexes (λ < 580 nm) (Equations (8) and (9)) [25,26], followed by the participation of Fe²⁺ in Equation (1) to generate further HO^• radicals [27], (ii) the photolysis of H₂O₂ when λ < 310 nm is used, and (iii) the photolysis of Fe(III) chelates (Fe₃L) formed between Fe³⁺ and the organic substrate, its degradation intermediates or other ligands present in the reaction medium (Equation (10)) [6]. However, the photo–Fenton process still has some disadvantages, such as the narrow pH range (2.8–3.5), poor switching between Fe²⁺ and Fe³⁺, and sludge formation. Therefore, many efforts have been made to develop catalysts that can operate efficiently at circumneutral pH conditions [28,29,30,31].

$${\left[Fe\left(OH\right)\right]}^{2+}+h\nu \to F{e}^{2+}+H{O}^{•}$$

(8)

$${\left[Fe\left(H_{2}O\right)\right]}^{3+}+h\nu \to F{e}^{2+}+H{O}^{•}+H^{+}$$

(9)

$$[F{e}^{3+}L]+h\nu \to {[F{e}^{3+}L]}^{•}+L^{•}$$

(10)

In the heterogeneous Fenton process, solid iron catalysts (such as the iron minerals magnetite (Fe₃O₄) or hematite (α–Fe₂O₃)) are used instead of aqueous Fe²⁺, to catalyze the generation of hydroxyl radicals in acidic or near–neutral media [32,33]. The mechanism of this process starts with the adsorption of H₂O₂ on the surface of Fe–based catalysts [34,35]. Then, the formation of a surface complex (≡Fe²⁺–H₂O₂ and/or ≡Fe³⁺–H₂O₂) precursor occurs (Equations (11) and (12)). Subsequently, the Fe²⁺–H₂O₂ complex generates ≡Fe³⁺ and the surface–bound HO^• radical by intramolecular electron transfer (Reaction 13) [36]. The ≡Fe³⁺–H₂O₂ complex can be converted to ≡Fe²⁺ and ^•OOH (Equation (14)) [36], and the HOO^• can further regenerate ≡Fe²⁺ (Equation (15)) [37]. The HO^• can either attack the adsorbed organic compounds in the vicinity or oxidize the non–adsorbed organic compounds.

$$\equiv F{e}^{2+}+H_2{O}_2\to \equiv F{e}^{2+}-H_2{O}_2$$

(11)

$$\equiv F{e}^{3+}+H_2{O}_2\to \equiv F{e}^{3+}-H_2{O}_2$$

(12)

$$\equiv F{e}^{2+}-H_2{O}_2\to \equiv F{e}^{3+}+ H{O}^{•}+H{O}^{-}$$

(13)

$$\equiv F{e}^{3+}-H_2{O}_2\to \equiv F{e}^{2+}+ HO{O}^{•}+H^{+}$$

(14)

$$\equiv F{e}^{3+}+HO{O}^{•} \to \equiv F{e}^{2+}+ O_2+H^{+}$$

(15)

Monometallic iron catalysts (ZVI, Fe₂O₃, Fe₃O₄, and Fe(OOH)) are limited in practical applications due to their low catalytic activity, poor stability, and recyclability [38]. On the other hand, copper–based oxides have become the priorities of research into novel photocatalysts, due to their advantages such as strong light absorption, high carrier mobility, non–toxicity, environmental friendliness, long–term stability, and low production cost. In particular, copper has the property of a Lewis acid, which can create a localized acidic microenvironment by interacting with iron species. The effect of expanding the pH range of the system is achieved by the localized acidic microenvironment when bimetallic Fe–Cu photocatalysts are used [39]. In this context, great efforts have been made to improve the practical applications of heterogeneous catalysts, and bimetallic oxides (composite oxides of iron and another metallic element) are an active research area. Similarly, bimetallic catalysts composed of iron and copper are advantageous for the reduction of Fe³⁺ in Fenton–like reactions. Previous reports have suggested that the cooperation between the redox pairs of iron (Fe³⁺/Fe²⁺) and copper (Cu²⁺/Cu⁺) [40,41] can accelerate electron transfer at the interface to promote the rapid reduction of Fe³⁺. Han et al. [42] explained the synergistic effect due to the reduced ΔE for the Fe³⁺/Fe²⁺ redox cycle obtained for the bimetallic Fe–Cu catalyst compared to the monometallic Fe catalyst. Sun et al. [43] suggested that the Fe²⁺ species of the Fe–Cu bimetallic catalyst is mainly regenerated by the reaction between Fe³⁺ and Cu⁺ (Equation (16)) rather than by the reduction of Fe³⁺ by H₂O₂ (Equation (2)). Thus, the coexistence of Fe and Cu on the surface of a catalyst can accelerate the process of electron transfer in the reaction environment and create a suitable condition for the generation of reactive radical species.

$$C{u}^{+}+F{e}^{3+} \to {Cu}^{2+}+ {Fe}^{2+}\hspace{1em}\hspace{1em}\hspace{1em}\Delta {\mathrm{E}}^0=0.6 \mathrm{V}$$

(16)

In summary, for the above reasons, bimetallic iron–copper materials are likely to perform better as heterogeneous Fenton and photo–Fenton catalysts compared to monometallic iron or copper materials. In this context, this review focuses on the compilation of the most relevant studies on the recent advances in the application of bimetallic iron– copper materials as Fe and Cu sources in heterogeneous photo–Fenton–like reactions for the degradation of pollutants in wastewater. Although the differences in the model pollutants and the experimental conditions make an evaluation difficult, we used as central parameters for the comparisons between the different materials, on the one hand, the combinations “achieved efficiency/treatment time” (which can be found in the tables) and, on the other hand, the dominant mechanisms in each study. For clarity, the results in the next sections are classified into the following groups according to the chemical composition of the catalysts (Figure 1): (i) CuFe₂O₄, (ii) mixed ferrites containing Fe and Cu, (iii) CuFeO₂ and CuFeS₂ materials, (iv) Fe–Cu oxide composites, (v) Metal–Organic Frameworks (MOF) based on Fe and Cu.

2. CuFe₂O₄

Ferrites are a broad class of iron–bearing oxides that include spinel ferrites, perovskites, ilmenite (FeTiO₃), and hexagonal ferrites (or hexaferrites). In general, spinel ferrites can be described as ferrites with the formula MFe₂O₄, where M is a divalent metal cation (such as Fe, Co, Zn, Ni, Cu, or others) [44]. Due to their thermal stability [45], unique structural [46], optical [47], magnetic [48], electrical, and dielectric properties [49], spinel ferrites have broad potential technological applications in photoluminescence [50], photocatalysis [51], biosensor development [52], magnetic drug delivery [53], corrosion protection [54], antimicrobial agents [55], and biomedicine (hyperthermia) [56]. In particular, CuFe₂O₄ has an inverse spinel structure with 8 Cu²⁺ ions in octahedral sites and 16 Fe³⁺ ions evenly distributed between the tetrahedral (A) and octahedral (B) sites [57].

The Tauc’s equation was used to determine the optical band gap (E_g) of copper ferrite [58]:

$$\alpha h\nu =B{\left(h\nu -E_g\right)}^m$$

where α is the energy–dependent absorption coefficient, hν is the photon energy, B is an energy independent constant, and the factor m depends on the nature of the electron transition and is equal to 2 or 1/2 for the direct and indirect transition band gaps, respectively. Assuming a direct semiconductor, E_g can be obtained from the extrapolation of the linear section of Tauc’s plot of (αhν)^0.5 vs. hν to the energy axis. In comparison with other spinel ferrites such as CoFe₂O₄ (E_g = 2.7 eV) and NiFe₂O₄ NPs (E_g = 2.2 eV) [59], CuFe₂O₄ NPs have a relatively small band gap of (1.7–1.9 eV) which allows them to use freely available energy in the form of sunlight to degrade pollutants in water [59]. Since CuFe₂O₄ is a narrow band gap material, it can be successfully used in type II or Z–scheme heterojunction photocatalysts [60].

Wei et al. [39] prepared magnetic Fe–Cu materials by the sol–gel method. Solutions containing 0–40% Cu²⁺ were prepared separately to obtain catalysts with different iron and copper ratios. The molar ratio of iron and copper had significant effects on the surface and structural properties of the materials. These materials were used for the removal of pyridine under UV irradiation in a Fenton–like system. The effects of various parameters, such as the molar ratio of Fe and Cu, catalyst and H₂O₂ dosage, initial pH, and initial pyridine concentration were studied. The best degradation performance was obtained for the M–40% material prepared from the 40% Cu²⁺ solution, which consisted of Fe₂O₃ and CuFe₂O₄. This co–doped Fe–Cu catalyst showed high activity, good stability, and easy recovery for the degradation of pyridine when the catalyst loading was 0.90 g L⁻¹, the H₂O₂ dosage was 832.50 mg L⁻¹, the initial pH was 7, and the initial pyridine concentration was 100.00 mg L⁻¹. The degradation efficiency of pyridine was more than 99% within 30 min and the TOC removal efficiency was 97.4% within 50 min. Therefore, the M–40% catalyst could be a potential material for wastewater treatment, which could extend the active pH range to facilitate recycling and reuse. The proposed reaction mechanism involves the reduction of Cu²⁺ to Cu⁺ at the surface of the photocatalyst (Equation (6)), and the reaction of Cu⁺ with H₂O₂ to form hydroxyl radicals and Cu²⁺ (Equation (7)). In addition, HO^• is obtained from Cu²⁺ and water under UV irradiation (Equation (17)). Part of Cu²⁺ is reduced to Cu⁺ by O₂^•− according to Equation (18). Furthermore, the monovalent copper ions can also be oxidized by H₂O₂ to produce Cu³⁺, which reacts with water molecules under UV light to form hydroxyl radicals (Equations (19) and (20)). Direct evidence for the occurrence of Equation (19) was obtained by EPR spectroscopy using the spin–trapping approach [61]. It is very likely that this reaction occurs at the surface of the photocatalyst. The complete mechanism is shown in Figure 2.

$$\equiv C{u}^{2+}+H_{2}O+ h\nu \to \equiv C{u}^{+}+H{O}^{•}+H^{+}$$

(17)

$$C{u}^{2+}+O_2^{•-} \to C{u}^{+}+ O_2$$

(18)

$$C{u}^{+}+H_2{O}_2 \to C{u}^{3+}+ 2 H{O}^{-}$$

(19)

$$\equiv C{u}^{3+}+H_{2}O + h\nu \to \equiv C{u}^{2+}+H{O}^{•}+H^{+}$$

(20)

Silva et al. [62] prepared mixed iron and copper oxides by the modified Pechini´s method. The samples were calcined at different temperatures and used for the removal of Methylene Blue dye (MB) by solar photo–Fenton catalysis. The results of X–ray diffraction analysis showed that hematite and copper ferrite phases were mainly responsible for the high efficiency of the photochemical reaction. MB removal from the aqueous solution was carried out in the presence of H₂O₂ (300 mg L⁻¹), with a 1.0 g L⁻¹ catalyst, at a neutral pH, and under solar irradiation. One of the catalysts was more efficient than pure iron (Fe₂O₃) and copper oxides (CuO), indicating the synergistic effect produced by combining the two metals on the same material. The authors proposed a mechanism for the regeneration of the active sites that satisfactorily explained the experimental results.

Cao et al. [63] prepared CuFe₂O₄ composites via combustion solution synthesis. The authors studied the effect of Cu content on the synthesized composites. The composite with 18 wt.% Cu had the best photo–Fenton activity. This nanocomposite, which was composed of CuFe₂O₄ and CuO, could degrade 40 mg L⁻¹ MB, 20 mg L⁻¹ Rhodamine B (RhB), and 20 mg L⁻¹ methyl orange (MO) in 40, 30, and 50 min, respectively. In addition, the composite showed superparamagnetic behavior and could be recycled.

Guo et al. [40] synthesized self–assembled hollow nanospheres of CuFe₂O₄ using the solvothermal method. The catalytic activity of the nanospheres was evaluated by the degradation of MB. In addition, these authors compared the properties of CuFe₂O₄ particles prepared by different methods. It was found that the good performance of the solid catalyst depends on both the large specific surface area and the degree of the optical response. The increase in photoelectric response and conductivity are beneficial for the improvement of catalytic performance. Transient photo–current experiments showed that the samples obtained by the solvothermal method exhibited a better optical response in the on–off cycle under light conditions.

Leichtweis et al. [64] prepared a novel composite catalyst by doping CuFe₂O₄ nanoparticles in the malt bagasse biochar. Malt bagasse (the malted barley residue) is the main residue obtained in the manufacture of beer. Composites with different ratios of malt biochar and CuFe₂O₄ were produced. The composites had lower band gap energy than CuFe₂O₄, which increased the photocatalytic activity. At 60 min of heterogeneous photo–Fenton treatment (pH 3), visible light tests showed that pure CuFe₂O₄ removed only 39% of the RhB color, while the composites removed up to 88% of the dye. A total of 100% color removal (pH 3) was achieved at 10- and 20-min reaction times for 10 and 50 mg L⁻¹ of dye under solar irradiation. Tests performed in the presence of specific radical scavengers showed that HO^•, O₂^•−, and h⁺ were the predominant reactive species involved in the degradation of the dye.

Jiang et al. [65] prepared a magnetic Bi₂WO₆/CuFe₂O₄ catalyst to be used in the removal of the antibiotic tetracycline hydrochloride (TCH). The obtained catalyst achieved 92.1% TCH (20 mg L⁻¹) degradation efficiency in a photo–Fenton–like system and a mineralization performance of 50.7% and 35.1% for TCH and a raw secondary effluent from a wastewater treatment plant, respectively. The excellent performance was attributed to the fact that photogenerated electrons accelerated the conversion Fe(III)/Fe(II) and Cu(II)/Cu(I), which increased the reaction rates of Fe(II)/Cu(I) with H₂O₂ and generated abundant HO^• radicals for pollutant oxidation. EPR assays confirmed that O₂^•− and HO^• were mainly responsible for THC degradation in dark and photo–Fenton–like systems, respectively.

The low efficiency of photo–Fenton processes at a neutral pH is mainly due to the precipitation of iron and can therefore be prevented by the proper addition of iron complexing agents, such as tartaric acid [66]. Guo et al. [67] prepared CuFe₂O₄ particles by the sol–gel method and investigated the effects of tartaric acid (TA) on the degradation of MB in the presence of CuFe₂O₄ and H₂O₂ under light irradiation. The results showed that the introduction of TA increased the decolorization rate of MB decolorization from 52.0% to 92.1% within 80 min. The contribution of O₂^•− was only 10%, whereas that of HO^• was about 88%. The enhancement of MB degradation in the presence of TA was explained by the complexation of Fe³⁺ and Cu²⁺ with TA (Equation (21)), followed by a photo–induced ligand to metal charge transfer process (Equation (22)). In the literature, similar equations with iron species and other organic radicals have been proposed to explain the photo–Fenton mechanism [66]. The photo–induced intramolecular charge transfer of the copper (II) complex of tartaric acid has already been described in the mechanism of the catalytic reduction of Cr(VI) by tartaric acid under the irradiation of simulated sunlight [68]. The excited species Fe²⁺/Cu⁺–(TA)^• further produced Fe²⁺/Cu⁺ and TA^• radicals according to Equation (23). Moreover, the presence of molecular oxygen also accelerated the transformation of the chemical state of the metal ions (see Equation (24)).

$$F{e}^{3+}/C{u}^{2+}+TA\to F{e}^{3+}\left(TA\right)/C{u}^{2+}\left(TA\right)$$

(21)

$$F{e}^{3+}\left(TA\right)/C{u}^{2+}\left(TA\right)+h\nu \to F{e}^{2+}{\left(TA\right)}^{•}/C{u}^{+}{\left(TA\right)}^{•}$$

(22)

$$F{e}^{2+}{\left(TA\right)}^{•}/C{u}^{+}{\left(TA\right)}^{•} \to F{e}^{2+}/C{u}^{+} + {\left(TA\right)}^{•}$$

(23)

$$F{e}^{2+}{\left(TA\right)}^{•}/C{u}^{+}{\left(TA\right)}^{•} + O_2 \to F{e}^{3+}{\left(TA\right)}^{•}/C{u}^{+}{\left(TA\right)}^{•} + O_2^{•-}$$

(24)

Rocha et al. [69] prepared CuFe₂O₄ from copper recycled from spent lithium–ion batteries. The photocatalytic properties were analyzed by monitoring the decolorization of MB in a heterogeneous photo–Fenton process in the presence of solar radiation. The decolorization efficiency was 96.1% in 45 min of reaction Formic and acetic acids were detected as degradation products of MB by ion chromatography.

Lin and Lu [70] developed CuFe₂O₄ nanoparticles decorated on partially reduced graphene oxides (CuFe₂O₄@rGO), to selectively and efficiently cleave lignin model compounds into value–added aromatic chemicals via a sunlight–assisted heterogeneous Fenton process. Controlled oxidative cleavage of the lignin model compound enabled the production of high–value aromatic chemicals, guaiacol and 2–methoxy–4–propylphenol, in high yields, instead of complete mineralization of the lignin model compound. The partially reduced graphene oxides serve as the large size support to accommodate and immobilize the CuFe₂O₄ nanoparticles for easy recycling of the catalyst, to attract the lignin model compounds through π–π stacking and hydrogen bonding for efficient cleavage reaction, and to accelerate the transport of photo–induced electrons for better charge separation and thus higher photocatalytic activities.

Membrane separation offers many advantages in wastewater treatment, including high efficiency, low energy consumption, and ease of operation [71]. However, membrane fouling always lowers the separation efficiency and shortens the membrane life, which greatly hinders the application of membrane technology. For this reason, Wang et al. [72] combined photo–Fenton and membrane processes for water treatment (Figure 3). These authors synthesized CuFe₂O₄ particles and doped them into the PVDF@CuFe₂O₄ membranes. The photo–Fenton process can degrade various foulants through the generation of hydroxyl radicals, which improves the filtration performance of the membranes.

The PVDF@CuFe₂O₄ membrane (1.0% CuFe₂O₄) showed significant improvement in both permeability and separation, with a tripling of flux and doubling of rejection compared to the values obtained by the membrane filtration process itself. In addition, the PVDF@CuFe₂O₄ nanofiltration membrane showed excellent stability and reusability after repeated tests over fifteen cycles.

Recently reported studies on the use of CuFe₂O₄ as photo–Fenton photocatalysts are summarized in Tables below.

3. Mixed Ferrites Containing Fe and Cu

As already mentioned, among the iron–based materials used as heterogeneous photo–Fenton catalysts, ferrites have attracted much attention because they are chemically and thermally stable magnetic materials [73,74]. Most importantly, ferrites have a narrow band gap, which enables them to efficiently utilize the visible region of solar energy in photocatalysis [75]. MnFe₂O₄ has a large specific surface area, good biocompatibility, and excellent magnetic properties [76,77]. However, mixed ferrites have better catalytic behavior compared to single ferrites [78]. In particular, doping with Cu²⁺ could improve the optical properties of MnFe₂O₄, which is due to the lattice defects in the spinel structure generated as a consequence of the smaller ionic radius of Cu²⁺. In addition, doping with other metals can also introduce oxygen vacancies, leading to a significant improvement in catalytic performance due to structural distortions [79]. Meena et al. [80] prepared Ce–doped MnFe₂O₄ by a low–temperature solution combustion synthesis using Oxalyl Dihydrazine (ODH) as fuel and reported that the band gap decreased after Ce–doping compared to that of pure MnFe₂O₄. Moreover, copper, manganese, and iron have been shown to have synergistic effects [81]. Thus, Cu²⁺–doped MnFe₂O₄ could further improve the catalytic activity because the different radii of metal ions in the spinel structure may lead to some defects and distortions, which result in the production of oxygen vacancies that are beneficial for the formation of reactive oxygen species (ROS) [82].

Yang et al. [83] synthesized porous Cu_0.5Mn_0.5Fe₂O₄ nanoparticles by a co–precipitation process and a subsequent high–temperature annealing treatment method. Cu_0.5Mn_0.5Fe₂O₄ nanoparticles exhibited much higher catalytic activity towards the degradation of bisphenol A (BPA) by the activation of H₂O₂ under UV light irradiation compared with CuFe₂O₄ and MnFe₂O₄ nanoparticles. The authors proved, through radical scavenger and EPR/DMPO experiments, that the hydroxyl radical is involved and plays a critical role in the presence of H₂O₂. They also proposed a possible BPA degradation pathway.

Sun et al. [82] employed Cu_0.8Mn_0.2Fe₂O₄ in the presence of H₂O₂ and achieved a great improvement in the degradation efficiency of TCH, which was due to the fact that H₂O₂ was activated by the Fe, Mn, and Cu ions on the Cu_0.8Mn_0.2Fe₂O₄ surface to generate HO^• radicals. Two possible mechanisms for H₂O₂ activation by Cu_0.8Mn_0.2Fe₂O₄ at pH = 3 and pH = 11 were proposed. Firstly, Cu_0.8Mn_0.2Fe₂O₄ could be excited by visible light to generate electrons (e⁻) and holes (h⁺), then H₂O₂ could react with the generated e⁻ to produce HO^• (Equation (25)). Photoelectrons migrating to the surface of the catalyst immediately react with the metal ions of the catalyst, which not only promotes the circulation of Fe, Mn, and Cu components but also strengthens the synergistic effect between them (Equations (16), (26) and (27)). When H₂O₂ came into contact with Fe, Mn, and Cu ions, the Fenton reaction was triggered immediately and TCH was mineralized by these reactive species into small molecule compounds.

$$H_2{O}_2+ e^{-}\to H{O}^{•}+H{O}^{-}$$

(25)

$$M{n}^{3+}+F{e}^{2+} \to { Mn}^{2+}+ {Fe}^{3+}$$

(26)

$$M{n}^{3+}+C{u}^{+} \to {Mn}^{2+}+ {Cu}^{2+}$$

(27)

Wang et al. [84] used a p–n heterostructured nano–photocatalyst based on the p–type Cu₂ZnSnS₄ (CZTS) nanosheets, which were successfully assembled on the surface of ZnFe₂O₄ (ZFO) nanospheres, forming CZTS/ZFO p–n heterostructures. These p–n heterostructures could not only efficiently expand the spectral response and promote photo–induced charge separation, but also increase the specific surface areas for photocatalytic and photo–Fenton reactions. All these factors resulted in the p–n hetero–structured CZTS/ZFO nano–photocatalyst with significantly enhanced photocatalytic activity for the degradation of MO in the presence of H₂O₂ with visible light irradiation, compared to pure ZFO. This behavior was due to the synergistic enhancement effects of the CZTS/ZFO p–n heterostructure combined with the photo–Fenton mechanism. (Figure 4).

Shi et al. [85] fabricated a multi–functional honeycomb ceramic plate by coating a layer of CuFeMnO₄ on the surface of a cordierite (magnesium iron aluminum cyclosilicate) material. The honeycomb structure was beneficial for light trapping and energy recycling and thus improved the solar–to–water evaporation efficiency. The CuFeMnO₄ coating layer acted as both the photothermal material for the solar–driven water evaporation process and the catalyst for the removal of volatile organic compounds (VOCs) via the heterogeneous photo–Fenton reaction. With the integration of the photo–Fenton reaction into the solar distillation process, clean distilled water was produced with efficient removal of the potential VOCs from the contaminated water sources.

All the results described in this section are summarized in

Table 1. Summary of recently used Fe–Cu ferrites and mixed ferrites as photo–Fenton photocatalysts in wastewater treatment.

Material (a)	Conditions	Contaminant	Degradation Efficiency	Reference
Fe2O3/CuFe2O4(b)	UV light, pH 7, [H2O2] = 832.50 mg L−1	Pyridine (100 mg L−1)	>99% within 30 min; TOC removal of 97% within 50 min	[39]
α–Fe2O3/CuFe2O4 (c)	Natural solar light, pH 7, [H2O2] = 300 mg L−1	MB (MB, 100 mg L−1)	100% removal of the dye in 180 min	[62]
CuFe2O4/CuO (d)	Halogen lamp, pH not indicated, [H2O2] not indicated	MB (40 mg L−1), Rhodamine B (20 mg L−1), Methyl Orange (20 mg L−1)	100% dye degradation within 50 min	[63]
Hollow CuFe2O4 nanospheres (e)	300–W UV curing lamp (λ > 400 nm), pH not indicated, [H2O2] = 0.02 M	MB (30 mg L−1)	96.4% in 60 min	[40]
CuFe2O4/biochar nanocomposites (f)	Fluorescent lamp (395−580 nm)/Sunlight, pH 3–7, [H2O2] from 2.5 to 10 μM	Rhodamine B	100% color removal was obtained at 10 and 20 min of reaction for 10 and 50 mg L−1 of dye with solar radiation	[64]
Bi2WO6/CuFe2O4 (e)	Visible light, pH 2.6–6.3, [H2O2] = 10 mM	Tetracycline hydrochloride (TCH)	After 30 min, 92.1% TCH (20 mg L−1) degradation (pH 2.6) efficiency and 50.7% and 35.1% mineralization performance.	[65]
CuFe2O4/tartaric acid (TA) (b)	UV–curing lamp (365–450 nm), pH 5.0, [H2O2] = 0.02 M	MB (50 mg L−1)	Introducing TA enhanced MB decolorization rate from 52.0% to 92.1% within 80 min.	[67]
CuFe2O4 (f)	Sunlight, pH 7.0, [H2O2] = 0.3 M	MB (10 mg L−1)	Decolorization efficiency was 96.1% in 45 min of reaction.	[69]
CuFe2O4@rGO (g)	Simulated sunlight, pH not indicated	Guaiacylglycerol–β–guaiacyl ether (lignin model compound)	Yields of 72.6% and 52.5% were achieved for guaiacol and 2–methoxy–4–propylpheno, respectively, in 60 min.	[70]
CuFe2O4 nanoparticles doped in polyvinylidene fluoride (PVDF) membranes (h)	Xe arc lamp, pH 3.0–11.0. The H2O2 (30 wt.%) dosage was in the range 50–1200 μL in 50 mL MB solution	MB (100 mg L−1)	MB was thoroughly degraded in 30 min when the pH is 3.0, while there is still 15.6% of the MB left at solution pH of 11.0.	[72]
Cu0.5Mn0.5Fe2O4 (i)	Cu0.5Mn0.5Fe2O4 0.08 g L−1 pH: 4.2 [H2O2] = 10 mM	BPA (10 mg L−1)	100% degradation and 47.6% mineralization 5 min	[83]
Cu0.8Mn0.2Fe2O4 (e)	Catalyst 0.100 g L−1 pH 3 and 11 300 W Xe lamp with a 420 nm UV–cut off filter	TC–HCL 100 mL 80 mg L−1	99% 30 min	[82]
CZTS/ZFO p–n heterostructures (e)	Catalyst 0.5 g L−1 pH 3 to 9 [H2O2] = 10 mM. 500 W Xe high intensity discharge lamp > 450 nm	MO 10 mg L−1	91% 120 min At optimum pH: 6	[84]
CuFeMnO4 on the surface of a honeycomb ceramic substrate (j)	0.05g of catalyst pH = 6.71. [H2O2] from 0 to 0.1 M Solar light	Phenol as a VOC model 10, 20,50 and 100 mg L−1 MB 10 mg L−1	∼99.18% COD and MB was removed 20 min	[85]

^(a) The synthesis method is shown in the footnotes. ^(b) Sol–gel method. ^(c) Modified Pechini method. ^(d) Solution combustion synthesis method. ^(e) Solvothermal method. ^(f) Co–precipitation. ^(g) Solvent–assisted interfacial reaction process. ^(h) Non–solvent induced phase separation method. ⁽ⁱ⁾ Chemical co–precipitation method followed by high–temperature annealing treatment. ^(j) See original papers for detail.

.

4. CuFeO₂ and CuFeS₂ Materials

Bimetallic Cu(II) and Fe(II) oxides and sulfides capable of activating H₂O₂ for the degradation of organic pollutants have become a focus of research. Among these catalysts, CuFeO₂ has gained wide concern due to its simple synthesis, low cost, interesting catalytic activity, and high chemical stability. In addition, due to its narrow band gap and cooperative impact of Fe³⁺/Fe²⁺ and Cu²⁺/Cu⁺ redox cycles, CuFeO₂ materials usually exhibit significant visible–light absorption and a good performance for H₂O₂ activation.

Schmachtenberg et al. [86] obtained delafossite–type powders by conventional hydrothermal (CuFeO₂) and microwave–assisted hydrothermal (CuFeO₂–MW) routes. Both materials were tested as potential catalysts in the photo–Fenton reaction under visible light. An experimental design was used for optimizing the degradation efficiency of the dye Reactive Red 141 and assessing the effects of the operating variables pH, catalyst loading, and oxidant concentration. Both a substantial reduction in the treatment times and a significant efficiency improvement were observed for the catalyst prepared by microwave irradiation (i.e., about 98% of dye degradation at 30 min) in comparison with the material obtained by the conventional method (i.e., 84% at 150 min). The latter difference was ascribed to the fact that CuFeO₂–MW samples presented higher values of surface area and pore volume, as well as smaller band–gap energy in comparison with conventional CuFeO₂. In addition, under optimal conditions (i.e., pH = 3.0, [CuFeO₂–MW] = 0.25 g L⁻¹, and [H₂O₂] = 8 mmol L⁻¹) 80% of TOC removal was achieved at 180 min. Finally, reusability tests conducted with the CuFeO₂–MW catalyst showed only a marginal loss of decolorization efficiency after four cycles (from 98% to 93%) and total concentrations of leached Fe and Cu ions of less than 1.0 wt.% of the catalyst content.

Liu et al. [87] prepared stoichiometric CuFeO₂ microcrystals with high crystallinity and single crystal phase via the optimized hydrothermal process. The authors presented a detailed characterization including the analysis of crystal structure, optical properties, and photoelectrochemical behavior. TCH was used as a model compound to evaluate the activity of CuFeO₂ under different conditions. Since the microcrystals showed a nonnegligible photocatalytic activity in the absence of H₂O₂, the authors proposed that the photo–generated electrons in the CB of CuFeO₂ could interact with dissolved oxygen and the photo–generated holes in the valence band react with H₂O, thus forming active radicals such as O₂^•− and HO^•, respectively. On the other hand, in the presence of H₂O₂, the material showed some catalytic activity for the Fenton reaction under dark conditions, which was enhanced more than four times upon illumination. The heterogeneous photo–Fenton–like treatment of TCH, carried out at pH 8 and using a Xe–Lamp in a quartz cell, showed conversion degrees of about 80 and 90% within the first 60 and 120 min, respectively. Photo–Fenton assays performed in the presence of different radical scavengers indicated that HO^• is not directly responsible for the degradation, but that the photogenerated holes and O₂^•− are the predominant reactive species.

Da Silveira Salla et al. [88] evaluated the catalytic properties of CuFeS₂–MW through BPA degradation tests conducted at a near–neutral pH and using simulated visible light. Results showed a remarkable enhancement of the catalytic efficiency (i.e., about 90% in the first 15 min) for the catalyst obtained in the presence of citrate, with a rate about 10 times faster than that of CuFeS₂ prepared without citrate. This behavior suggested that the presence of citric acid accelerates the photo–conversion of Fe³⁺ to Fe²⁺, thus increasing the generation of HO^• and the efficiency of the overall process. Reusability assays showed that, after the fourth cycle, the degradation rate remained higher than 95% and the TOC decrease was 76.6% after 60 min of reaction. In addition, concentrations of Fe and Cu leached into the solution were lower than 0.5% of the total amounts of Fe and Cu present in the catalyst at the beginning of the reaction. To assess the main reactive species, tests were conducted in the presence of different ROS scavengers (i.e., t–butanol for HO^• and p–benzoquinone for O₂^•−). Results showed that BPA degradation may be ascribed to the generation of HO^•. As expected, due to the low selectivity of the HO^• radicals, the degradation efficiency was affected by the presence of other chemical species in the real effluent such as carbonates/bicarbonates, nitrates/nitrites, sulfates, phosphates, and chlorides. Using LC–MS analysis, the authors identified several hydroxylated derivates resulting from the initial reaction stages. These primary intermediates are likely to undergo ring–opening reactions to form typical aliphatic acids as degradation by–products, which may further undergo a series of oxidation steps that finally lead to complete mineralization.

More recently, Da Silveira Salla et al. [89] prepared a chalcopyrite powder by a microwave–assisted method (CuFeS₂–MW), which exhibited higher catalytic activity than that obtained with the material prepared by the conventional method (CuFeS₂). Both materials were synthesized in the presence of citric acid, which is capable of enhancing the photoreduction of Fe(III). Tartrazine dye was used as a model compound for the degradation by the photo–Fenton reaction under visible irradiation at pH 3.0. CuFeS₂–MW reached 99.1% of tartrazine decolorization at 40 min and 87.3% of mineralization at 150 min, the decolorization rate being twice as fast as that obtained with CuFeS₂. The latter difference was attributed to the synergy between higher crystallinity and increased amount of Fe²⁺ on the CuFeS₂–MW surface when compared to CuFeS₂. Reusability tests performed on both CuFeS₂–MW and CuFeS₂ showed that, after five cycles, the decolorization efficiencies remained at 93.6 and 69.6%, respectively. Moreover, the highest concentrations of leached iron were less than 1% of the total amount of iron present in the catalysts. The addition of t–butanol decreased 20–fold the tartrazine decolorization rate, while p–benzoquinone did not inhibit tartrazine decolorization. These results show that HO^• radicals are the key species for tartrazine removal and that O₂^•− radicals do not play a significant role in the degradation mechanism.

Cai et al. [90] anchored CuFeO₂ on a Manganese residue (MR) through mechanical activation (MA) for obtaining Fe–Cu@SiO₂/starch–derived carbon composites. The material was tested as a heterogeneous catalyst for the photo–Fenton treatment of TC using H₂O₂ and visible light. Under optimal conditions (i.e., 15 mM of H₂O₂ and pH 7.0) 100% of TC conversion (50 mg L⁻¹) was achieved within 40 min. Moreover, the observed decay constants were 4.00, 2.77, and 2.14 times higher than those of Cu/SC, MAMR–Fe₃O₄@SiO₂/SC, and MR–Fe–Cu@SiO₂/SC, respectively. Reutilization tests showed good material stability since the photocatalytic performance of MAMR–Fe–Cu@SiO₂/SC changed from 99.2% to 96.3% after five cycles and metal leaching was below 0.1 mg L⁻¹ for Cu, Fe, and Mn. Based on XPS analyses before and after material usage, the authors proposed that the efficiency of the prepared catalyst is closely related to the interaction of Cu²⁺/Cu⁺, Fe³⁺/Fe²⁺, and Mn³⁺/Mn²⁺ couples. The study of reactive species through the use of scavengers and ESR showed that both HO^• and O₂^•− contribute to TC degradation in the MAMR–Fe–Cu@SiO₂/SC + H₂O₂ + visible light system, while surface photogeneration of electrons and holes seems to play a negligible role.

Xin et al. [38] synthesized CuFeO₂/biochar composites for heterogeneous photo–Fenton–like processes (HPF–like) via the hydrothermal method. Biochar prevented agglomeration and avoided the usage of added reductants. Moreover, the introduction of biochar had several advantages, such as enhancing visible–light absorption, narrowing the bandgap of CuFeO₂, and partially suppressing the recombination between photoelectron and hole pairs. By applying the design of experiments and the surface response methodology the authors studied the effects of the operating conditions on TC degradation efficiency. The results showed that the optimum parameters were 220 mg L⁻¹ of catalysts, 22 mM of H₂O₂, and pH 6.4, with a pH dependence relatively gentle in the range of 4.0 to 8.0. Under optimal conditions, the efficiency of TC degradation in HPF–like systems was 96.7% after 120 min of treatment. Furthermore, the catalyst exhibited excellent stability since the activity only decreased by 3.9% after five utilization cycles and the total dissolved concentrations of both iron and copper ions at 120 min were below 0.02 mg L⁻¹. Active species scavenging experiments were carried out by using p–benzoquinone, silver nitrate, methyl alcohol, and ammonium oxalate as the scavengers of O₂^•−, photoelectrons, HO^•, and photogenerated holes, respectively. Interestingly, ammonium oxalate did not decrease the catalytic performance but accelerated the TC degradation rate. The latter effect was attributed to a lower photoelectron quenching rate, which could promote Fe³⁺/Fe²⁺ and Cu²⁺/Cu⁺ cycles, thus significantly improving H₂O₂ activation. In addition, ESR experiments allowed the identification of HO^• as the predominant active species, whereas photoelectrons and O₂^•−, were auxiliary species for TC degradation. The mechanistic findings are schematically summarized in Figure 5.

It is worth mentioning that the authors also explored the TC degradation intermediate products by HPLC–MS and proposed plausible transformation pathways. On the other hand, Xin et al. [91] also studied the heterogeneous visible–light Photo–electro–Fenton (H–VL–PEF) treatment of TC using an undivided photoelectrochemical cell in the presence of CuFeO₂/biochar particles. A nitrogen/oxygen self–doped biomass porous carbon cathode was used as a gas diffusion electrode (GDE) to efficiently produce H₂O₂, while a Xe–Lamp (filter > 420 nm) was used as a visible irradiation source for CuFeO₂/biochar particles. The performances of different treatments (including electro–catalysis, photo–catalysis, photo–electro–catalysis, electro–Fenton, and H–VL–PEF) were evaluated for comparable experimental setups. The H–VL–PEF showed excellent performance, achieving the highest TC degradation rate and the best TOC removal with the lowest energy consumption. The authors analyzed the effect of several operational parameters and found that the full decomposition of TC (20 to 200 mg L⁻¹) was attained within 60–70 min under optimal conditions (i.e., 100 mg L⁻¹ CuFeO₂/biochar, 80 mA cm⁻² of current density, and pH 5.0). Reutilization tests showed negligible metal leaching and small efficiency losses after 10 cycles. Experiments performed in the presence of selective scavengers of active species as well as ESR measurements indicated that HO^• radicals are the main species responsible for TC degradation, whereas O₂^•− radicals play a subsidiary role. The schematic representation of the involved processes is presented in Figure 6. In addition, based on the intermediates detected by HPLC–MS analyses, the authors proposed general pathways for TC transformation and used a software tool for predicting, through QSAR techniques, the evolution of sample toxicity with the treatment time.

More recently, Xin et al. [92] achieved a further enhancement in the performance of photo–electro–Fenton (PEF) degradation of TC by introducing modifications to the latter treatment strategy. Three GDE composites were fabricated by mixing different weight ratios of a nitrogen/oxygen self–doped porous biochar (NO/PBC) cathode for H₂O₂ electrogeneration with a NO/PBC–supported CuFeO₂ (CuFeO₂–NO/PBC) catalyst for H₂O₂ activation. The NO/PBC was prepared by the pyrolysis method, whereas CuFeO₂–NO/PBC was prepared by the hydrothermal method without additional chemical reductants. The PEF treatment of TC was carried out in an undivided quartz reactor with a Pt anode and using visible light (Xe–Lamp, cutoff > 420 nm). The authors analyzed the effects of the NO/PBC to CuFeO₂–NO/PBC ratio, the current density, and initial pH on system performance, which depends on the compromise between H₂O₂ formation, parallel reactions such as 4–electron oxygen reduction reaction (ORR) and H₂ evolution, and H₂O₂ decomposition. Using a NO/PBC to CuFeO₂–NO/PBC ratio of 1:1, a current density of 80 mA cm⁻², and pH 5, an efficiency of 96.1% was achieved for TC (20 mg L⁻¹) degradation at 180 min in the PEF treatment. The latter value is twice the rate observed in EF and an order of magnitude higher than that recorded under anodic oxidation. The degradation efficiency at 180 min of PEF treatment steadily declined from 96.1% at pH 5.0 to 46.8% at pH 11.0. After five cycles of material use, the PEF treatment efficiency was higher than 80%, the amounts of leached Fe and Cu were negligible, and surface hydrophobicity remained practically constant. Assays in the presence of radical scavengers showed that HO^• and O₂^•− played the primary and auxiliary roles for tetracycline degradation, respectively. The authors proposed that despite the photoinduced hole could not effectively participate in TC degradation, the photogenerated electrons reacted with H₂O₂, O₂, and Fe³⁺/Cu²⁺ to form HO^•, O₂^•−, and Fe²⁺/Cu⁺, respectively, thus promoting an efficient TC transformation. Finally, from the reaction intermediates detected by HPLC–MS, QSAR tools, and experiments of E. coli growth inhibition, a general picture of TC oxidation pathways and toxicity evolution was outlined.

All the results described in this section are summarized in

Table 2. Summary of recently used CuFeO₂ and CuFeS₂ as photo–Fenton photocatalysts in wastewater treatment.

Material (a)	Conditions	Contaminant	Degradation Efficiency	Reference
Delafossite–type CuFeO2 (b,c)	visible light, pH from 2.4 to 3.6, H2O2 from 3 to 13 mM, catalyst from 0.13 to 0.33 gL−1	Reactive Red 141 dye, 50 mg L−1	about 98% at 30 min	[86]
3R–delafossite CuFeO2 microcrystals (b)	200 mL reactor, 20 mM of H2O2, 1 g L−1 catalyst, initial pH 8	Tetracycline hydrochloride 20 mg L−1	96.1% in 180 min	[87]
CuFeS2(c)	visible light, 20 mM of H2O2, 0.2 g L−1 catalyst, pH 6	bisphenol A (BPA) 20 mg L−1	97% in 60 min	[88]
CuFeS2 chalcogenide powders (b,c)	visible light, pH 3.0, 8.33 mM of H2O2, 0.2 g L−1 of catalyst	tartrazine, 100 mg L−1	99.1% of tartrazine decolorization after 40 min and 87.3% of mineralization after 150 min	[89]
MAMR–Fe–Cu@SiO2/SC, a core–shell structure of CuFeO2@SiO2/starch–derived carbon anchored on a Manganese Residue (d)	Xe–lamp with UV cut–off, pH from 2.5 to 9.5, 15 mM of H2O2, 0.7 g L−1 of catalyst	Tetracycline 50 mg L−1	100% in 40 min	[90]
CuFeO2/biochar (b)	Xe Lamp with UV cutoff, pH 4 to 8, 20 mM of H2O2, 0.2 g L−1 of catalyst	Tetracycline 20 mg L−1	97.6% in 120 min	[38]
CuFeO2/biochar (b)	photo–electro–Fenton, Xe–Lamp with UV cutoff, pH from 3 to 11, H2O2 generated by a NO–doped/porous carbon cathode	Tetracycline (20 to 200 mg L−1)	100% in 60–70 min	[91]
Nitrogen/oxygen self–doped porous biochar (NO/PBC) and NO/PBC–supported CuFeO2 (CuFeO2–NO/PBC) (b)	undivided quartz reactor, visible light, pH from 3 to 11, H2O2 by a gas diffusion electrode	Tetracycline 20 mg L−1	98% at 30 min	[92]

^(a) The synthesis method is shown in the footnotes. ^(b) Hydrothermal method. ^(c) Microwave–assisted hydrothermal method. ^(d) Reductive roasting and mechanical activation.

.

5. Fe–Cu Oxide Composites

In this section, we present the results obtained with other types of heterogeneous catalysts consisting of mixtures of iron and copper oxides and mixed oxides of defined composition. Heterojunctions formed by different copper and iron oxides and/or sulfides decrease the recombination rate of photogenerated e⁻ and h⁺ [93,94].

Mansoori et al. [95] prepared an iron–copper oxide impregnated NaOH–activated biochar (Fe–Cu/ABC) through the pyrolysis of activated biochar, followed by the impregnation method. The catalytic activity of the bimetallic catalyst was tested for ciprofloxacin (CIP) degradation through a heterogeneous photo–electro–Fenton process) at a natural pH. The heterogeneous catalyst exhibited remarkable catalytic activity and showed great stability and structural integrity for five cycles. Furthermore, from a practical point of view, the catalyst exhibited an acceptable performance by oxidizing CIP dissolved in various water matrices such as tap water, river water, and a real sample of wastewater. The intermediates by–products of CIP were determined, and a plausible degradation mechanism was proposed. The adsorption of as–generated H₂O₂ onto the well–developed surface of the mesoporous bimetallic catalyst facilitated the reduction of Fe³⁺ to Fe²⁺ and Cu²⁺ to Cu⁺. The co–existence of Fe and Cu on the surface of activated biochar accelerates the process of electron transfer in the reaction and enhances the production of reactive species.

Iron oxide NPs supported on the surface of hydroxylated diamond exhibited photocatalytic activity and stability for the visible light–assisted Fenton reaction even working at pH values around 6 and demonstrated superiority as supports compared to other alternative solids such as activated carbon, graphite, carbon nanotubes, and even the benchmark semiconductor TiO₂ [96]. In particular, Manickam–Periyaraman, et al. [97] prepared bimetallic Fe₂₀Cu₈₀ NPs supported on various surface hydroxylated diamond NPs (D) and compared them to analogous catalysts based on graphite. The assays were conducted at pH 6 for the heterogeneous Fenton degradation of phenol (Ph) assisted by natural or simulated sunlight irradiation, achieving a mineralization degree of 90% at an H₂O₂ to Ph molar ratio of 6. The authors explained this result based on the high activity of reduced copper to form hydroxyl radicals and the favorable redox process of Fe²⁺ maintaining a pool of reduced Cu⁺ species. Thus, a heterogeneous catalyst based on abundant iron or copper metals allowed the promotion of H₂O₂ activation to yield hydroxyl radicals under visible light irradiation according to Equations (28)–(36).

$$F{e}_{20}C{u}_{80}/D_3 + h\nu \to e^{-}+ h^{+}$$

(28)

$$H_2{O}_2+ e^{-}\to H{O}^{•}+H{O}^{-}$$

(29)

$$F{e}^{3+}+e^{-} \to { Fe}^{2+}$$

(30)

$$C{u}^{2+}+e^{-} \to {Cu}^{+}$$

(31)

$$C{u}^{+}+e^{-} \to { Cu}^0$$

(32)

$$h^{+}+ H_2{O}_2 \to H{O}_2^{•}+H^{+}$$

(33)

$$h^{+}+ F{e}^{2+} \to F{e}^{3+}$$

(34)

$$h^{+}+ C{u}^{+} \to C{u}^{2+}$$

(35)

$$h^{+}+ organic compunds \to oxidized products$$

(36)

Khan et al. [98] compared the photocatalytic activity of novel peculiar–shaped CuO, Fe₂O₃, and FeO nanoparticles (NPs) to that of the iron(II)–doped copper ferrite, Cu^II_0.4Fe^II_0.6Fe^III₂O₄, through the degradation of MB and RhB. The catalysts were synthesized via the simple co–precipitation and calcination technique. The highest degradation efficiency was achieved by CuO for RhB and by Cu^II_0.4Fe ^II_0.6Fe^III₂O₄ for MB. The CuO/FeO/Fe₂O₃ composite proved to be the second–best catalyst in both cases, with excellent reusability.

Asenath–Smith et al. [99] used iron oxide (α–Fe₂O₃, hematite) colloids synthesized under hydrothermal conditions as catalysts for the photodegradation of MO. To enhance the photocatalytic performance, Fe₂O₃ was combined with other transition–metal oxide (TMO) colloids (e.g., CuO and ZnO), which are sensitive to different regions of the solar spectrum (visible and UV, respectively), using a ternary blending approach for compositional mixtures. For a variety of ZnO/Fe₂O₃/CuO mole ratios, the pseudo–first–order rate constant for MO degradation was at least twice the sum of the individual Fe₂O₃ and CuO rate constants, indicating that an underlying synergy governs the photocatalytic reactions for these combinations of TMOs. The increased photo–catalytic performance of Fe₂O₃ in the presence of CuO was associated with the hydroxyl radical, consistent with heterogeneous photo–Fenton mechanisms, which are not accessible by ZnO. Then, the authors proposed a mechanism where CuO plays a supportive role in Fe₂O₃ photocatalysis by decomposing H₂O₂ to generate HO^• radicals. With additional ROS available, the reaction kinetics are accelerated. The bandgap energies of the TMOs used in this study further corroborated the obtained results. Smaller bandgap materials, such as Fe₂O₃ and CuO, have valence–band edges above the oxidation potential of H₂O. As a result, it is energetically favorable for a photogenerated h⁺ on Fe₂O₃ or CuO to participate in the generation of HO^• radicals by H₂O oxidation. Larger band gap materials, such as ZnO, have valence–band edges that are below the H₂O oxidation potential for HO^• formation, but their conduction band edges are below the reduction potential of O₂.

Lu et al. [100] used a magnesium aluminum silicate known as attapulgite (ATP) supported Fe–Mn–Cu polymetallic oxide as a catalyst with UV irradiation in the photocatalytic oxidation (photo–Fenton) treatment of a synthetic pharmaceutical wastewater. Fe–Mn–Cu@ATP had good catalytic potential and a significant synergistic effect since it removed almost all heterocyclic compounds, as well as humus–like and fulvic acid. The degradation efficiency of the nanocomposite only decreased by 5.8% after repeated use for six cycles (COD removal reached 64.9%, and the BOD ₅/COD increased from 0.179 to 0.387 after 180 min of reaction time). The proposed mechanism involves: (i) the direct reaction between the catalyst and hydrogen peroxide, which generates HO^• radicals and oxidized ions of the three metals, (ii) the UV photoreductions of these metal ions, and (iii) indirect reactions between photogenerated free radicals and the catalyst. In this mechanism, the consumed metal ions are effectively regenerated under UV light, and HO^• finally reacts with the organic matter.

Davarnejad et al. [101] synthesized alginate–based hydrogel–coated bimetallic iron–copper nanocomposite beads through a green method and used them as heterogeneous catalysts for metronidazole elimination from wastewater. These authors employed the response surface methodology (RSM) based on the Box–Behnken design (BBD) to assess both the individual and interaction effects of five main variables involving catalyst loading, initial pH, reaction time, metronidazole, and H₂O₂ concentrations. The data obtained from the model were in good agreement with the experimental ones. The optimum conditions (for 95.3%, based on model, and for 95.0%, based on experiments) were found at a catalyst (Fe₂O₃–CuO@Ca–Alg) loading of 44.7 mg L⁻¹, an initial pH of 3.5, a metronidazole concentration of 10 mg L⁻¹, a H₂O₂ concentration of 33.17 mmol L⁻¹, and a reaction time of 85 min.

Zhang et al. [102] used Fe₃O₄@Cu₂O/carbon quantum dots (CQDs)/nitrogen–doped carbon quantum dots (N–CQDs) (FCCN) for the degradation of azo compounds, such as MO, acid orange II, and mordant yellow 10, even at a neutral and alkaline pH (pH: 7–12) with a shortest time for complete degradation of 15 min. There is a cooperative interaction between each component, so the synergism from all the components of FCCN enhances the photocatalytic properties. The authors proposed a mechanism where in the first place, the up–conversion characteristics of CQDs and N–CQDs improve the light utilization of FCCN. Secondly, the load of CQDs and N–CQDs can separate and transfer photo–generated charges between Fe₃O₄ and Cu₂O more efficiently, accelerating the circulation of Fe³⁺/Fe²⁺ for the decomposition of H₂O₂ to yield the active species: HO^• radical. Thirdly, the light–induced protons of CQDs change the partial pH, so that the FCCN system is able to work in alkaline solutions. Finally, the insoluble layers of CQDs and N–CQDs are beneficial to enhance the stability of the composite (see in Figure 7 the schematic diagram of the catalytic mechanism proposed by the authors). The magnetism of Fe₃O₄ made this catalyst easily separable and the insoluble layers of CQDs and N–CQDs allowed it to be repeatedly used without activity change even after 10 cycles. The degradation rate constant of MO in the FCCN/H₂O₂/light system was 5.4 times higher than that of the Fe₃O₄@Cu₂O/H₂O₂/light system, indicating that the loaded quantum dots greatly enhanced the photocatalytic activity. The degradation rate constant in the FCCN/H₂O₂/light system was 2.5 times higher than that of the simple mixture (Fe₃O₄@Cu₂O + CQDs + N–CQDs)/H₂O₂/light system, which suggests that these excellent catalytic performances should come from the synergism effect of all components in FCCN.

All the results described in this section are summarized in

Table 3. Summary of recently used Fe–Cu oxides composites as photo–Fenton photocatalysts in wastewater treatment.

Material (a)	Conditions	Contaminant	Degradation Efficiency	Reference
Iron–copper oxide impregnated NaOH–activated biochar (FeCu/ABC catalyst) (b)	Heterogeneous PEF process, pH = 5.8 catalyst dosage of 1 g L−1, electrical current of 200 mA	CIP (45 mg L−1)	100% removal 2 h	[95]
Fe20Cu80(0.2 wt.%)/D3 (c)	catalyst ~200 mg L−1, [H2O2] (200 mg L−1; 5.88 mM), 20 °C, simulated sunlight. pH = 4	Phenol (100 mg L−1)	90% removal 2 h	[103]
NP–3 (CuII0.4FeII0.6FeIII2O4) (d)	NP–3 = 400 mg L−1, [H2O2] = 1.76 × 10−1 mol L−1, pH = 7.5 Optonica SP1275 LED lamp (GU10, 7 W, 400 Lm, 6000 K, Optonica LED, Sofia, Bulgaria)	MB (1.5 × 10−5 mol L−1) RhB (1.75 × 10−5 mol L−1)	100% 140 min	[98]
Iron oxide (α–Fe2O3, hematite) colloids combined with other transition–metal oxide (TMO) colloids (e.g., CuO and ZnO) (e)	750 mg L−1 catalyst, [H2O2] = 0.025 mol L−1. Tungsten halogen lamps	MO (25 µM)	7 to 78% 60 min	[99]
Fe–Mn–Cu@ATP (f)	500 mL of Fe–Mn–Cu@ATP dosage (1–12 g L−1), pH = 3 [H2O2] (0.1–0.6 mol L−1) UV 40 W UV lamp	pharmaceutical wastewater	COD removal: 64.9% 180 min	[100]
Fe2O3–CuO@Ca–Alg hydrogel (g)	4.7 mg L−1 of catalyst, pH = 3.5 [H2O2] = 33.17 mmol L−1 UV light	[MNZ]0 10 mg L−1	Removal = 95% 85 min	[101]
FCCN (h)	Catalyst 1 g L−1 pH: 7–12 [H2O2] = 15 mM 500W Xe lamp 564 nm cut–off filter	MO, acid orange II and mordant yellow 10–20 mg L−1	100% 15 min	[102]

^(a) The synthesis method is shown in the footnotes. ^(b) Pyrolysis of activated biochar followed by impregnation. ^(c) Deposition of the NPs (Fe, Cu, or Fe–Cu) onto the surface of commercial carbonaceous supports ^(d) Co–precipitation. ^(e) Hydrothermal method. ^(f) Aeration–coprecipitation method. ^(g) Green method using walnut green shells. ^(h) See Figure 7A.

.

6. Metal–Organic Frameworks Based on Fe and Cu

Metal–organic frameworks (MOFs) are inorganic–organic porous polymers composed of metal ions or metal clusters linked to each other by bridging organic ligands to form three–dimensional structures with a high specific surface area. In particular, Fe–based MOFs, made by polydentate organic ligands as linkers and inorganic Fe ions or Fe–oxo clusters as nodes, have recently attracted great attention in various applications such as gas adsorption, catalysis, and sensor development [104]. Within the applications, Fe–based MOFs have been considered promising catalysts in photo–Fenton processes due to their interfacial electron transfer properties and the cycling of the Fe(III)/Fe(II) redox pair. Additionally, the band gaps of Fe–based MOFs are suitable for visible light photoactivation and thus, Fe–O clusters could be directly excited to generate electron hole pairs (e⁻ − h⁺), which subsequently degrade organic pollutants or lead to the generation of reactive oxygen species [105]. Likewise, it has been shown that the separation efficiency of photogenerated carriers in the Fe–O cluster remains low due to fast electron–hole recombination [105,106]. Taking this into account, different modifications in the structure of MOFs have been proposed to delay the recombination rate and improve pollutant degradation efficiencies [106,107]. In this sense, the combination of MOFs with metals or metal oxide nanoparticles may form a new interface or heterojunction that efficiently couples the catalytic process of photo–induced electrons and radicals’ generation [108,109]. Moreover, the addition of a second metal ion into the nodes of frameworks could significantly improve the catalytic properties of MOFs [41]. Recently, different reports have incorporated Cu into Fe–based MOFs structure yielding Fe–Cu bimetallic MOFs in order to improve the performance of these materials in photo–Fenton processes.

Do et al. [110] reported the preparation of Fe–doped Cu 1,4–benzenedicarboxylate MOFs (Fe–CuBDC) and its application as a heterogeneous photo–Fenton catalyst for the degradation of MB in aqueous solution under visible light irradiation. In this work, the authors compared the efficiency of the bimetallic Fe–CuBDC with each single metal MOF. The degradation efficiency of MB with Fe–CuBDC was much higher than those obtained with the others, which indicates that the partial substitution of Cu by Fe significantly improves the photocatalytic activity of the material. Complete removal of MB was achieved after 70 min under light irradiation with 1.0 g L⁻¹ of Fe–CuBDC (

Table 4. Summary of recently used Fe–Cu bimetallic MOFs as photo–Fenton photocatalysts in wastewater treatment.

Material (a)	Conditions	Contaminant	Degradation Efficiency	Reference
Fe–CuBDC(b)	Simulated sunlight, pH 6, [H2O2] = 50 mM, [Catalyst] = 1 g L−1	Methyl Blue (50 mg L−1)	100% removal of the dye in 70 min	[110]
MCuFe MOF (b,c)	300 W xenon lamp equipped with a UV cut–off filter (λ > 400 nm), pH 4–9, [H2O2] = 5 mM, [Catalyst] = 0.05 g L−1	Methyl Blue (50 mg L−1)	100% removal of the dye in 40 min	[111]
Cu2O/MIL(Fe/Cu) (b)	500 W xenon lamp, pH 7.47, [H2O2] = 49 mM, [Catalyst] = 0.5 g L−1	Thiacloprid (80 mg L−1)	100% removal of TCL in 20 min; 82% TOC removal in 80 min	[112]
L–MIL–53 (Fe, Cu) (b)	300 W xenon lamp equipped with a UV cut–off filter (λ > 420 nm), pH 7, [H2O2] = 5 mM, [Catalyst] = 0.1 g L−1	Ciprofloxacin (20 mg L−1)	60% removal of CIP in 30 min	[113]

^(a) The synthesis method is shown in the footnotes. ^(b) solvothermal method. ^(c) Hydrothermal method.

). The degradation performance of the Fe–CuBDC catalyst was relatively constant with a slight reduction in removal efficiency from 99.9% to 97.2% after five reuse cycles.

Shi et al. [111] prepared a binary bimetallic heterojunction, which consisted of a core–shell magnetic CuFe₂O₄@MIL–100(Fe, Cu) metal–organic framework, via an in–situ derivation strategy. The synthesis methods, which involved the in situ surface pyrolysis of CuFe₂O₄ nanoparticles and complexation with trimesic acid to derive MIL–100(Fe, Cu), allowed the formation of a bimetallic core–shell CuFe₂O₄@MIL–100(Fe, Cu) heterojunction (MCuFe MOF). A schematic diagram of the synthesis process of MCuFe MOF is shown in Figure 8. These materials showed notable catalytic performance in a photo–Fenton process towards the degradation of various organic pollutants by increasing H₂O₂ activation efficiency and decreasing the required dosage of MCuFe MOF (0.05 g L⁻¹) over a wide pH range (4–9) (Table 4). Furthermore, the MCuFe MOF showed a high stability for the degradation of organic contaminants, with almost no decrease in activity and negligible metal leaching after five successive cycles. According to the proposed mechanism, the combination of the photothermal conversion effect with the formed heterojunction cannot only accelerate the generation and separation of photogenerated e⁻ and h⁺ but also improve the continuous and efficient transformation of ≡Fe(III)/Fe(II) and ≡Cu(II)/Cu(I) redox couples, leading to enhanced photo–Fenton efficiency (see Figure 9).

Zhong et al. [112] performed Cu₂O growth on the surface of Cu–doped MIL–100 (Fe) to obtain a Cu₂O/MIL(Fe/Cu) composite, which showed an enhanced interfacial synergistic effect and was successfully used as photo–Fenton catalysts for thiacloprid (TCL) degradation. The authors reported that Cu doping into MIL(Fe) led to the reduction in the band gap, and a boost of the redox cycle Fe²⁺/Fe³⁺. Likewise, the growth of Cu₂O extended the light absorption range of MIL(Fe/Cu) from UV to the visible region. A proposed TCL degradation mechanism is shown in Figure 10. Cu₂O/MIL(Fe/Cu) composite exhibited a TCL degradation rate nearly 10 times faster than those of Cu₂O and MIL–100(Fe), separately, achieving a complete TCL degradation within 20 min of reaction. Moreover, TOC removal reached 82.3% within 80 min under neutral conditions, which highlights the good performance of the Cu₂O/MIL(Fe/Cu) composite. Catalyst reuse tests showed no significant efficiency loss in reaction rates even after the tenth cycle, reaching complete TCL degradation within 20 min in each cycle.

Low–crystalline MOFs, with long–range disorder but local crystallinity, allow the availability of more active sites and defects than highly crystalline materials. These materials could improve the activation capacity of Fe–based MOFs towards H₂O₂ by increasing the number of metallic coordinately unsaturated active sites (CUS) within the frameworks. Wu et al. [113] prepared low–crystalline bimetallic MOFs of MIL–53(Fe, M) (M: Mn or Cu), via a one–pot solvothermal method, as photo–Fenton catalysts for the degradation of CIP. The results showed a significant improvement in photo–Fenton performance for the low–crystallinity catalyst compared to the crystalline counterparts, which was mainly attributed to the enhancement of the synergism between the hetero–metal nodes. In particular, low crystallinity MIL–53(Fe, Cu) exhibited a much higher removal efficiency and a faster reaction rate than that of crystalline MIL–53(Fe, Cu) within 30 min (Table 4). Besides the increased metal CUSs in the low–crystalline state, both Cu and Mn could increase the specific surface area and promote the visible–light absorption and separation/transportation of carriers in the low–crystalline state, thus leading to the acceleration of Fe(II)/Fe(III) and M(red)/M(ox) cycles.

All the results described in this section are summarized in Table 4.

7. Conclusions and Future Perspective

This review has shown the present trends in the development of bimetallic iron–copper materials and their application as catalysts in heterogeneous photo–Fenton–like reactions for the degradation of wastewater pollutants. Bimetallic Fe–Cu catalysts have shown better performance than monometallic Fe catalysts.

We have compared many materials with very different chemical compositions and physicochemical properties. In general, bimetallic Fe–Cu catalysts have shown better performance than monometallic Fe catalysts. In the works summarized here, it is difficult to find a common explanation of the synergistic effect of iron and copper on the catalytic activity of the materials. Some works attribute the improvement to changes in the band–gap, others to the decrease in the electron–hole recombination rates, and even to the reduction of H₂O₂ by electrons, or the beneficial effect of the binary redox couples of Fe(II)/Fe(III) and Cu(I)/Cu(II) on the decomposition efficiency of H₂O₂. However, the common factor in all the materials seems to be that the coexistence of both metals evidently favors the redox cycles of Fe and Cu, resulting in higher catalytic activity. The co–existence of Fe and Cu on the surface of a catalyst accelerates the process of electron–transfer in the reaction environment, providing an appropriate condition for the activation of hydrogen peroxide and the generation of reactive radical species. This hypothesis is supported by the excellent degradation and mineralization results obtained when aqueous solutions of the contaminant are treated with these catalysts. In particular, lower degradation times and catalyst dosages are required to achieve complete pollutant degradation. Most of the bimetallic Fe–Cu catalysts have shown very good results at a circumneutral pH, which represents one of the major advantages of these materials since the development of an efficient and sustainable photo–Fenton treatment at neutral pH values remains a challenge for the scientific community working in the field. In addition, many of the works summarized here report the stability of the bimetallic catalysts even after having been used in four or five cycles.

From the analysis of the collected research, we can conclude that further efforts should focus on the preparation of new catalysts by green synthesis routes, that is, using bioactive agents such as plant materials, microorganisms, or biological waste. This approach would help to reduce the overall production cost and to limit environmental impact. Only a few of the compilated articles employed renewable energies such as natural solar light as the photoirradiation source. Most of them used either Xe lamps with or without cut–off filters for simulating sunlight or other light sources, such as UV lamps or visible light from lamps or LEDs. In this context, further studies devoted to addressing the effect of using solar light as the photoirradiation source on the performance of the catalysts are certainly encouraged.

Although excellent degradation and mineralization results have been obtained by applying this technique at the laboratory scale, its effectiveness needs to be evaluated in continuous flow systems for real industrial effluents. Most of the published research focused on the degradation of single pollutants (mainly dyes and antibiotics), disregarding the effects of the presence of humic–like dissolved organic matter and other contaminants. Moreover, to increase the efficiency and decrease the cost of the treatment of recalcitrant contaminants in real systems, the integration of photo–Fenton methods with biological technologies should be explored.