Recent Advances in Iron Oxide-Based Heterojunction Photo-Fenton Catalysts for the Elimination of Organic Pollutants

Abstract

Organic pollutants released into water bodies have posed a serious threat to aquatic ecosystems. The elimination of organic pollutants from water through the photo-Fenton process has attracted extensive attention. Among various photo-Fenton catalysts, iron oxides have been intensively studied due to their environmentally benign characteristics and abundance. However, the rapid recombination of photogenerated charge carriers (e⁻–h⁺) and slow Fe(III)/Fe(II) cycling of iron oxides restrict their catalytic performance. Thus, this state-of-the-art review focuses on the recent research development regarding iron oxide-based heterojunctions with enhanced catalytic performance to eliminate organic pollutants. This review provides a fundamental understanding of the iron-based heterogeneous photo-Fenton reaction. In addition, various heterojunctions for photocatalytic applications are comprehensively summarized. A thorough discussion is held on the material design for iron oxide-based heterojunctions with improved photo-Fenton catalytic performance. Ultimately, the challenges and prospects of iron oxide-based heterojunction catalysts for photo-Fenton water decontamination are outlined.

1. Introduction

In recent decades, global water security is facing serious threats due to organic pollutants generated by the rapid advancement of industry, such as endocrine disruptors (EDCs), pharmaceuticals and personal care products (PPCPs), perfluorinated compounds (PFCs), brominated flame retardants (BFRs), pesticides, etc. [1,2]. To address the above issues, numerous physical, biological, and chemical techniques have been investigated to treat organic pollutants in water, including membrane filtration [3], precipitation [4], adsorption [5], activated sludge processes [6], and chemical degradation [7]. Nevertheless, physical techniques can only remove organic pollutants from water; and they cannot effectively degrade them [8]. Biotechnologies are hardly suitable for the treatment of organic pollutants with poor biochemical characteristics and high relative molecular mass. As a result, chemical technologies have attracted considerable attention for water decontamination. Among the various kinds of chemical technologies, advanced oxidation processes (AOPs) are one of the most reliable for pollutant detoxification by generating a variety of reactive oxygen species (ROS) that degrade and further fully mineralize the organic pollutants [9]. AOPs include the Fenton process, ozone and photocatalytic reactions, etc. [10], which can generate highly reactive ROS including hydroxyl radicals (HO•), superoxide radicals (O₂^•−), singlet oxygen (¹O₂), and sulfate radicals (SO₄^•−) for removing organic pollutants [11,12].

The Fenton process is an efficient method due to its strong oxidation effects [13]. In the conventional Fenton process, Fe²⁺ and H₂O₂ react to generate HO• by Equations (1) and (2) [14,15]. However, the homogeneous Fenton process has the inherent drawbacks of a large amount of iron sludge generation [16]. Heterogeneous Fenton instead of homogeneous Fenton has been fully developed, effectively overcoming the limitations of homogeneous Fenton [17]. Nevertheless, the slow regeneration of Fe²⁺ (k = 0.02 M⁻¹ s⁻¹)in the reaction between Fe³⁺ and H₂O₂ restricts the decontamination efficiency of the heterogeneous Fenton process [18,19,20]. Recently, introducing light into the heterogeneous Fenton process can dramatically accelerate the transformation from Fe³⁺ to Fe²⁺, which has received significant attention [21]. The heterogeneous photo-Fenton catalytic reaction is mainly dominated by the photocatalytic reaction of semiconductors. Under visible light irradiation, the catalyst is readily excited to induce photogenerated electron and hole (e⁻–h⁺) pairs when the catalyst receives energy equal to or above its band gap energy. Subsequently, the separation of e⁻–h⁺ occurs; the e⁻ transfer from valence band (VB) to conduction band (CB) and leave h⁺ in the VB. The Fe³⁺ can be reduced to Fe²⁺ by photogenerated e⁻ (Equation (3)), which is conductive to the H₂O₂ activation. The O₂ can be reduced by e⁻ in the CB to generate O₂^•−, while the h⁺ in the VB can react with surface hydroxyl (–OH) to produce HO• for oxidizing organic pollutants. Moreover, the presence of H₂O₂ will directly react with e⁻ into HO• (Equation (4)). As a result, the synergy of photocatalysis and Fenton can solve the sluggish cycle of Fe³⁺/Fe²⁺.

$${\mathrm{Fe}}^{2+}+{ \mathrm{H}}_2{\mathrm{O}}_2\to {\mathrm{Fe}}^{3+}+\mathrm{H}\mathrm{O}•+{\mathrm{OH}}^{-} \mathrm{k}=76 {\mathrm{M}}^{-1} {\mathrm{s}}^{-1}$$

(1)

$${\mathrm{Fe}}^{3+}+{ \mathrm{H}}_2{\mathrm{O}}_2\to {\mathrm{Fe}}^{2+}+\mathrm{H}{\mathrm{O}}_2•+{\mathrm{H}}^{+} \mathrm{k}=0.02 {\mathrm{M}}^{-1} {\mathrm{s}}^{-1}$$

(2)

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

(3)

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

(4)

Natural iron oxides are of great interest to the scientific community because of their ability to adsorb pollutants and activate H₂O₂ [22]. In addition, iron oxide has unique properties such as being abundant, low-cost, non-toxic, environmentally friendly, and magnetic [23]. In general, α-Fe₂O₃ (hematite), γ-Fe₂O₃ (maghemite), Fe₃O₄ (magnetite), and FeO (wurtzite) are the most widely used and promising iron oxides for water treatment [24]. However, the iron oxides are less active in heterogeneous Fenton reactions due to the extremely slow rate of reduction in Fe³⁺ [25]. Moreover, leaching under acidic conditions similarly causes a decrease in the catalytic activity of iron oxides [26]. At the same time, the use of pure-phase iron oxide only as a photocatalyst has several drawbacks, including poor electrical conductivity, short hole diffusion length, and rapid light-induced electron and hole recombination [27,28]. Transient absorption spectroscopy (TAS) is generally performed to directly measure the lifetimes of photogenerated charge carriers. Various TAS studies carried out on α-Fe₂O₃ have revealed significant electron–hole recombination on ultrafast time scales. An ultrafast decay time constant of <1 ps and <100 ps was shown for various nanostructures of α-Fe₂O₃, corresponding to the relaxation of hot electrons to the CB edge and the subsequent recombination with the hole and trap state, respectively, which confirmed that the ultrafast recombination property is inherent to α-Fe₂O₃ [29]. Kronawitter et al. designed an α-Fe₂O₃/WO₃ core–shell heterostructure [30]. TAS analysis revealed that constructing α-Fe₂O₃/WO₃ heterojunction interfaces can effectively promote surface-trapped hole removal on ps timescales, thereby significantly improving photoanode performance through enhanced charge separation efficiency. Hence, one of the best options is to form heterojunction composites with other nanocomposites, which exhibit high conductivity and appropriate energy band structure, improving the photo-Fenton performance due to heterojunction interfaces facilitating charge separation and active species generation. In this aspect, the published papers have seen rapid growth over the past decade according to the databases from Web of Science and Scopus using the photo-Fenton process and heterojunctions as topics (Figure 1a,b), indicating that heterojunctions in the photo-Fenton system have gained tremendous attention. Herein, we focus on recent advances in iron oxide-based heterojunctions that eliminate organic contaminants in the photo-Fenton systems, which provide readers with insight and inspire new solutions for their further development in practical applications (Figure 2).

2. Classification of Heterojunctions

The photo-Fenton reaction can be regarded as a synergy of the photocatalytic reaction and the Fenton reaction. In the photocatalytic process, semiconductor photocatalysts can generate photogenerated electrons to reduce Fe³⁺ to Fe²⁺, thereby promoting the activation of H₂O₂ in the Fenton process. The photo-Fenton reaction can be regarded as a combination of photocatalytic reactions and Fenton reactions. In the photocatalytic process, semiconductor photocatalysts can generate photogenerated electrons to reduce Fe³⁺ to Fe²⁺, thereby promoting the activation of H₂O₂ in the Fenton process. However, it is difficult for single-component semiconductor photocatalysts to simultaneously possess a broad light absorption range and strong redox capabilities. Additionally, photogenerated electrons in the CB tend to return to the valence band VB and recombine with photogenerated holes in single-component semiconductors, significantly reducing solar energy utilization efficiency. The heterojunction refers to a special structure formed at the interface between two semiconductor materials with different band structures. The presence of this structure modulates the energy band alignment while promoting photogenerated charge carrier separation and Fe³⁺/Fe²⁺ redox cycling, thereby enhancing the photo-Fenton activity. Therefore, designing suitable heterojunction-based photocatalytic systems to suppress the recombination of photogenerated electrons and holes is crucial for photo-Fenton catalysis. The conventional heterojunctions consist of two different single-crystal semiconductors and are categorized as homotypic heterojunctions (n–n junctions and p–p junctions) and antipode heterojunctions (p–n junctions) [31]. In recent years, with the quick advancement in photocatalytic technology, heterojunctions are mainly divided into Schottky junction, type-I, type-II, type-III, Z-scheme, and S-scheme heterojunctions based on the difference in energy band structure and charge transfer mechanism [32].

2.1. Schottky Junction

The Schottky junction consists of a semiconductor and a metal and can accelerate the transfer of carriers from the semiconductor (SC) to the metal, effectively preventing the recombination of e⁻–h⁺ pairs [33]. The performance of a Schottky junction is determined by the Fermi energy level (E_F) difference between the semiconductor and the metal. Upon contacting a semiconductor with a metal, due to the gap in E_F, electrons will flow from the material with the higher E_F to the lower E_F, thus inducing an internal electric field (IEF) at the interface [34]. This IEF effectively separates photogenerated e⁻–h⁺ pairs and facilitates e⁻ migration [35]. As shown in Figure 3a, when the E_F of the semiconductor (E_FS) is above the E_F of the metal (E_FM), the photogenerated electron is captured by the metal, and the Schottky barrier formed captures the photogenerated hole; when the E_FS is lower than the E_FM, the h⁺ are captured by the metal, and the Schottky barrier formed prevents the transfer of the photogenerated electron to the metal (Figure 3b); and when the E_F is equal between the semiconductor and the metal, the Schottky barrier is not formed.

2.2. Type-I, Type-II, and Type-III

Conventional heterojunction photocatalysts are classified into three types according to the energy band positions: type-I, type-II, and type-III. Type-I is straddling the gap (Figure 4a), where the CB of semiconductor A (SC-A) is higher than that of semiconductor B (SC-B), and the VB of SC-A is lower than that of SC-B [36,37]. Therefore, both electrons and holes of SC-A will transfer to SC-B, thus increasing the possibility of recombination. As depicted in Figure 4b, type-II consists of two semiconductors with relatively staggered band positions, and, thus, is coupled in a staggered configuration [38,39]. The CB of material SC-B is more negative than that of SC-A, whereas the SC-A exhibits a more positive VB position [40]. Under light irradiation, photogenerated e⁻ are transferred from CB of SC-B to CB of SC-A, and h⁺ are transferred from the VB of SC-A to the VB of SC-B [41]. As a result, the photogenerated e⁻–h⁺ pairs are efficiently separated, which improves the photocatalytic performance [42], leading to Type-II heterojunctions becoming the most widely used heterojunctions. Moreover, the energy band gap of the type-III heterojunction is a broken gap, where the charge carrier transfer is apparently not feasible (Figure 4c) [43].

2.3. Z-Scheme and S-Scheme

Z-scheme heterojunctions include all-solid-state Z-scheme and direct Z-scheme heterojunctions [44]. Z-scheme heterojunctions achieve efficient photogenerated carrier separation and migration by simulating natural photosynthesis with superior redox capabilities [45]. The all-solid-state Z-scheme heterojunction consists of two differential semiconductors using carbon-based materials or noble-metal nanoparticles as the electron mediators (Figure 5a) [46,47]. The heterojunction overcomes the limitation that the conventional Z-scheme is only built in the solution phase, effectively shortening the electron transfer distance and improving the photocatalytic performance. The all-solid-state Z-scheme uses an electron mediator to achieve the recombination of e⁻ in CB of SC-A and h⁺ in VB of SC-B. The strongly reducing e⁻ and strongly oxidizing h⁺ remaining in the CB of SC-B and the VB of SC-A contribute to reduction and oxidation reactions, respectively [48]. The direct Z-scheme heterojunction has not initially been explored in depth for its electron transfer mechanism since it was first reported by Grätzel in 2001 [49,50]. Yu et al. proposed the concept of a direct Z-type heterojunction in 2013, which effectively reduces the recombination of the e⁻–h⁺ pairs, thus enhancing the photocatalytic activity (Figure 5b) [51,52].

The S-scheme heterojunction was proposed based on the direct Z-scheme heterojunction [53]. As shown in Figure 6, the S-scheme heterojunction consists of an oxide photocatalyst (OP) and a reduced photocatalyst (RP) [54]. Meanwhile, the RP has a higher E_F, CB, and VB than the OP. When the RP and OP are in contact, e⁻ are transmitted from the RP to the OP owing to the different E_F. In the interface, the E_F of the RP is bent upward at the edge of the energy bands because of e⁻ depletion, whereas that of OP curves downward due to the accumulation of electrons. As a result, the E_F of the RP and OP equalizes at the contact point. Simultaneously, an IEF is established at the interface from the RP to OP. The IEF, band bending, and Coulomb force cooperatively attract photogenerated e⁻ from the CB of the OP to the VB of the RP under light irradiation. Ultimately, the photogenerated e⁻ in the CB of the RP and h⁺ in the VB of the OP are preserved, manifesting robust reducing and oxidizing capabilities, respectively. As a result, the S-scheme heterojunction reinforces the charge carries separation and retains the largest redox potential [55]. In addition, the S-scheme heterojunction not only provides excellent redox capacity, but also inhibits charge recombination.

3. Synthesis Methods of Iron Oxide-Based Heterojunction

Synthesis methods play an important role in improving the catalytic performance of heterojunction photo-Fenton catalysts by affecting the microstructure, size, specific surface area, and electronic structure of the catalysts. Iron oxide-based heterojunction photo-Fenton catalysts can be prepared via synthesis methods such as the hydrothermal, solution combustion, co-precipitation, and sol–gel methods. The hydrothermal method is currently the most widely reported synthesis method for the catalysts. The high temperature and pressure conditions in hydrothermal reactions promote the controlled particle size, shape, and morphology of the catalyst, thereby improving the crystallinity of the catalyst and reducing defects, which leads to the superior photo-Fenton activity of the catalysts. Meanwhile, due to their simplicity, low cost, and controllable composition, the solution combustion method and co-precipitation methods are extensively used for the preparation of iron oxide-based heterojunction photo-Fenton catalysts. Additionally, the sol–gel method has been widely employed because of its ability to control the composition, particle size, and morphology of the catalyst and to allow the incorporation of dopants and co-catalysts.

Table 1. Synthesis of iron oxide-based heterojunction photo-Fenton catalysts.

Catalysts	Fabrication Method	Condition	Reference
FeIn2S4/Fe2O3	Hydrothermal	90 °C, 6 h	[56]
ZnFe2O4/α-Fe2O3/Pt	Hydrothermal	180 °C, 24 h	[57]
Fe2O3@PPy/PB	Reactive template	90 min	[58]
α-Fe2O3-Fe3O4/CeO2	Solution combustion	Sunlight, 1 h	[59]
P-CN/NCDs/Fe3O4	Co-precipitation	90 °C, 1 h	[60]
PVA/CNF/Fe3O4	Co-precipitation	50 °C, 24 h	[61]
γ-Fe2O3/FeTiO3	Hydrothermal	150 °C, 5 h	[62]
α-Fe2O3/CdS/SiO2	Hydrothermal	120 °C, 24 h	[63]
Fe2O3@A1−xRx-TiO2	Sol–gel	100 °C, 5 h	[64]
Fe2O3/TiO2	Sol–gel	60 °C, 3 h	[65]
Fe3O4@FeS2@C@MoS2	Hydrothermal	200 °C, 7 h	[66]
MoS2/C@Fe3O4	Hydrothermal	180 °C, 16 h	[67]
Cr2Bi3O11-Bi2O3/Fe3O4@PCs	Hydrothermal	160 °C, 8 h	[68]
TiO2/Fe2TiO5/Fe2O3	Ion exchange	Roon temperature, 2 h	[69]

summarizes the synthesis methods of iron oxide-based heterojunction photo-Fenton catalysts.

4. Iron Oxide-Based Heterojunction with Metal Oxides

4.1. Iron Oxide-Based Heterojunction with TiO₂

TiO₂, as one of the most extensively studied photocatalysts, continues to attract significant interest on account of its availability, nontoxicity, low cost, and chemical stability [70,71]. However, TiO₂ only exploits UV light, which significantly limits its photocatalytic performance [72]. Coupling TiO₂ with visible light-responsive SC can effectively overcome the limitations. The α-Fe₂O₃/TiO₂ (FT-200) heterojunction was successfully synthesized using the simple approach of the in situ phase transformation method (Figure 7) [73]. The incorporation of α-Fe₂O₃ onto TiO₂ nanoparticles not only enhanced the absorption of visible light but also improved the separation of photogenerated e⁻–h⁺ pairs. The α-Fe₂O₃/TiO₂ heterojunction demonstrated superior photo-Fenton performance for various chlorophenol pollutants at a low H₂O₂ concentration, achieving complete degradation (100%) within 12 min and a mineralization rate that exceeded 98% within 30 min. The α-Fe₂O₃-TiO₂ catalysts, synthesized via the sol–gel method, exhibited the anatase crystalline phase of TiO₂ [74]. The incorporation of Fe₂O₃ into TiO₂ increased the specific surface area and crystal dimension, while reducing the band gap energy. The complete degradation of cefuroxime sodium salt (CFX) and 71.9% mineralization were achieved in the heterogeneous photo-Fenton system. Moreover, the catalyst demonstrated high stability against corrosion and was proven to be reusable for at least five degradation cycles. Hence, the integration of TiO₂ with α-Fe₂O₃ to form a TiO₂/α-Fe₂O₃ heterojunction not only enhances the absorption of visible light but also accelerates the Fe(III)/Fe(II) redox cycle, thereby achieving efficient H₂O₂ activation.

4.2. Iron Oxide-Based Heterojunction with ZnO

ZnO is an n-type semiconductor that has garnered significant attention for its exceptional physico-chemical properties, widespread availability, high chemical stability, and environmental sustainability [75]. Nevertheless, achieving sufficient photocatalytic activity of ZnO for actual applications remains challenging because of the limited capability of ZnO in the UV region and the photogenerated e⁻–h⁺ pairs recombination [76]. Liu et al. prepared ZnO@Fe₃O₄ composites (ZFCM-5) by intelligent layer-by-layer construction with excellent photo-Fenton degradation properties, capable of completely degrading p-nitrophenol in 60 min [77]. ZFCM-5 exhibited a high specific surface area, providing more active sites. Meanwhile, the band gap of the composite was narrowed to 2.35 eV, resulting in a substantial enhancement in visible light absorption. Furthermore, ZFCM-5 is superparamagnetic, enabling rapid separation under the outside magnetic field as well as keeping strong catalytic activity even after six cycles. Uma et al. synthesized Ag-ZnO/α-Fe₂O₃ composites (Ag-2S) via the sonochemical method, which demonstrated excellent catalytic performance in the photo-Fenton system, enabling the efficient elimination of organic contaminants [78]. The incorporation of silver nanoparticles notably enhanced both the light absorption and separation efficiency of charge carriers. Moreover, the Ag-2S composite achieved near-complete degradation of methylene blue (MB) dye within 30 min.

Table 2. Summary of iron oxide-based heterojunctions with metal oxides for photo-Fenton removal of organic pollutants.

Catalysts	Pollutants	Condition	Removal Efficiency	Reference
α-Fe2O3@TiO2	TCH (50 mg L−1)	300 W Xe lamp (λ > 420 nm), pH = 5.45, [H2O2] = 120 μL, [Catalyst] = 0.1 g	100%, 90 min	[79]
TiO2/Fe2TiO5/Fe2O3	MO (10 mg L−1)	300 W Xe lamp (λ > 420 nm), pH = 4, [H2O2] = 130 μL, [Catalyst] = 50 mg	87%, 10 min	[69]
α-Fe2O3/TiO2	2,4-DCP (10 mg L−1)	300 W Xe lamp (λ > 420 nm), pH = 3, [H2O2] = 1 mM, [Catalyst] = 50 mg	100%, 12 min	[73]
Fe2O3/TiO2/rGO	MO (20 mg L−1)	5 W LED lamp (λ = 420 nm), pH = 7.3, [H2O2] = 19.8 mM, [Catalyst] = 1 g L−1	97%, 20 min	[80]
Fe2O3/TiO2	RbX (50 mg L−1)	65 W mercury lamp (λ = 365 nm), pH = 3, [H2O2] = 60 mM, [Catalyst] = 2 g L−1	90.57%, 180 min	[81]
Fe3O4-TiO2	RhB (30 mg L−1)	Ultraviolet analyzer (365 nm, 6W), pH = 7, [H2O2] = 485 mM, [Catalyst] = 1 g L−1	98.12%, 120 min	[82]
Ag-ZnO@α-Fe2O3	MB (5 ppm)	350 W mercury lamp (λ > 380 nm), pH = 3, [H2O2] = 0.5 mL, [Catalyst] = 30 mg	99.3%, 30 min	[78]
Ag/Fe3O4/WO3	p-NP (5 mg L−1)	cool white LED, pH = 3, [H2O2] = 1 M, [Catalyst] = 2 mg	100%, 90 min	[83]
ZnO/Co3O4/CoFe2O4/Fe3O4	TC (15 mg L−1)	λ = 500 nm, pH = 7, [H2O2] = 4.4 mM, [Catalyst] = 150 mg	90.9%, 6 min	[84]

summarizes the research results regarding the removal of iron oxide-based heterojunctions with metal oxides for the photo-Fenton removal of organic pollutants in the last decade, from which we can compare the removal efficiency of different catalysts under different reaction conditions. In summary, the photogenerated charge recombination in pure-phase iron oxide photo-Fenton catalysts is not unfavorable for efficient catalytic reactions. Therefore, coupling iron oxides with other metal oxides to construct a heterojunction system has been demonstrated to improve the structural stability and quantum efficiency of the catalyst, thus enhancing its photo-Fenton performance.

5. Iron Oxide-Based Heterojunction with Bismuth-Related Semiconductors

Bismuth-based semiconductors are non-toxic, abundantly available, and exhibit high chemical stability [85]. Additionally, bismuth-based semiconductors (e.g., BiOCl, Bi₂MoO₆) possess a narrow bandgap (~2.5–3.0 eV), enabling effective visible light absorption. The formation of heterojunctions optimizes the energy band alignment, thereby enhancing the utilization of the full light spectrum through interfacial charge transfer [86]. Moreover, the internal electric field at the heterojunction interface efficiently separates photogenerated e⁻–h⁺ pairs [87]. Iron oxide-based heterojunctions containing bismuth have been studied previously. The FeO_x/Bi₄TaO₈Cl (denoted as FeO_x/BTOC-10) photo-Fenton catalyst was synthesized using a straightforward in situ loading method [88]. The UV-vis diffuse reflectance spectra (UV-vis DRS) revealed that the introduction of FeO_x reduced the band gap, improved light absorption, and promoted the photogenerated e⁻–h⁺ pairs separation. FeO_x/BTOC-10 degraded 83.5% of TC-H within 20 min, an efficiency nearly four times that of pure BTOC. After five cycle experiments, FeO_x/BTOC-10 still degraded 87.4% of tetracycline hydrochloride (TC-H), demonstrating excellent stability and reusability. Next, we will primarily focus on BiOX (X = Cl, Br, I), Bi₂WO₆, and Bi₂MoO₆ as examples to evaluate the photo-Fenton activity of the composite formed by iron oxides and the bismuth-based materials.

5.1. Iron Oxide-Based Heterojunction with BiOX (X = Cl, Br, I)

BiOX-based materials are chemically stable, non-toxic, and highly resistant to corrosion in aqueous environments [89]. The materials contain several compounds that are sensitive to visible light due to their narrow optical band gaps, making them suitable for photocatalytic applications. The special layered structure of BiOX creates sufficient space for the polarization of atoms and orbitals. The IEF of the materials can extend the lifetime of photoinduced charge carriers, effectively inhibiting their recombination. Xu et al. successfully fabricated a p–n type heterojunction by anchoring Fe₃O₄ nanoparticles onto floral BiOI (FOB5) using the in situ precipitation method [90]. UV-vis DRS revealed that the introduction of Fe₃O₄ significantly broadens for BiOI the visible absorption range. Furthermore, vibrating sample magnetometer (VSM) analysis demonstrated that the FOB5 heterojunction exhibited ferromagnetic properties. Photoluminescence (PL) spectroscopy revealed that the FOB5 heterojunction effectively suppressed the recombination of photogenerated e⁻–h⁺ pairs, thereby enhancing catalytic performance. In the photo-Fenton system, Rhodamine B (RhB) was completely degraded within 240 min, demonstrating that the synergistic photocatalytic oxidation and PDS oxidation effectively enhanced degradation efficiency. Furthermore, the FOB5 heterojunction showed superior adsorption capacity and photocatalytic activity in RhB degradation compared to individual BiOI and Fe₃O₄. In another study, the AgBr/BiOBr/Fe₃O₄ (ABF) heterojunction was synthesized using solvothermal and in situ precipitation methods [91]. Fe₃O₄ nanoparticles were homogeneously distributed on the AgBr/BiOBr surface, forming strong interfacial contact that facilitated the efficient electron transfer and separation efficiency of photogenerated charge carriers. The presence of Fe₃O₄ not only improved the magnetic property of material for recycling, but also provided additional active sites, enhancing its degradation performance. ABF-5 (with 5 wt% Fe₃O₄ content) exhibited an excellent photo-Fenton degradation performance, achieving a 96.84% removal of carbamazepine (CBZ) within 30 min. In reservoir water and hospital wastewater, ABF-5 achieved CBZ degradation efficiencies of 85.84% and 96.84%, respectively, demonstrating its promising potential for practical applications.

In addition, the combination of BiOX-based materials with Fe₂O₃ has also been extensively studied. Wang et al. designed a novel catalyst (FFB-3) with a Z-scheme heterojunction structure to improve the transfer of electrons and separation efficiency of the photogenerated e⁻–h⁺ pairs, thus enhancing the catalytic performance of the system (Figure 8a,b) [92]. BiOBr was synthesized using the solvothermal method, and the FFB-3 catalyst was synthesized by dispersing FeCl₃·6H₂O and BiOBr in deionized water with dropwise addition of KBH₄ solution at room temperature. The experimental results showed that the optimal composite catalyst achieved a 98.22% removal efficiency and a 59.48% mineralization rate of TC in 90 min at natural pH. In addition, the possible degradation pathways of TC were raised by LC-MS analysis, and the toxicity of TC and its intermediates was assessed using the Toxicity Estimation Software Tool (T.E.S.T) (version 5.1), which demonstrated that the TC toxicity was significantly reduced following degradation. The researchers prepared a photo-Fenton catalyst (b-BOC/FO) by the hydrothermal method (Figure 8c,d) [93]. The characterization results showed that the b-BOC/FO composite successfully formed a 2D/2D heterojunction structure. Upon visible light irradiation, b-BOC/FO 0.5 showed a 92% removal efficiency of TC, which was remarkably superior to that of b-BOC (40%) and FO (55%) alone. The catalytic performance of the b-BOC/FO 0.5 was 4.96 and 3.5 times better than that of b-BOC and FO, respectively, indicating that the heterojunction structure could provide a considerable contact surface area, which enhanced the absorption of visible light and the separation efficiency of photogenerated charge carriers in the b-BOC/FO 0.5. A series of novel Ag/Fe₂O₃/BiOI Z-scheme heterojunction photo-Fenton catalysts with different compositions were synthesized by the simple hydrothermal method [94]. Ag/Fe₂O₃/BiOI-2 (2 wt% Ag and 15% Fe₂O₃) showed the highest removal efficiency of 96% for TC in the photo-Fenton system. The removal efficiency of the catalyst was significantly higher than that of BiOI and Fe₂O₃ alone, indicating that the Z-scheme heterojunction structure notably enhanced the photo-Fenton activity. The rate constant of Ag/Fe₂O₃/BiOI-2 was 11 and 3 times higher than that of Fe₂O₃ and BiOI, respectively, which further proved the efficient photo-Fenton catalytic performance. This study suggests that the Z- scheme heterojunction structure promotes the separation of photogenerated carriers and the transfer of electrons, accelerating the Fe²⁺/Fe³⁺ cycling, which enhances the photo-Fenton catalytic performance.

5.2. Iron Oxide-Based Heterojunction with Bi₂WO₆

Bi₂WO₆ is a typical layered Aurivillius material with a reduced e⁻–h⁺ recombination rate and tunable band gap (Eg = 2.6–2.9 eV), which makes it increasingly promising for photocatalysis [95,96]. It has been reported that the existence of a suitable band gap match between Fe₂O₃ and Bi₂WO₆ facilitates the formation of heterojunction photocatalytic systems [97,98,99]. Wang et al. synthesized a 2D/2D S-scheme heterojunction photo-Fenton catalyst α-Fe₂O₃/Bi₂WO₆ (FO/BWO) by the hydrothermal method, which exhibited significant degradation performance for organic pollutants under visible light (Figure 9a) [100]. The FO/BWO (0.5) exhibited a specific surface area of 27.44 m² g⁻¹, which exceeded that of FO and BWO individually, indicating that the 2D/2D heterojunction structure increased the specific surface area. Moreover, the FO/BWO composite exhibited a red shift in its absorption edge according to the UV-vis DRS, indicating its enhanced absorption of visible light. Under visible light irradiation, the MB removal efficiency of FO/BWO (0.5) was nearly 100%, outperforming that of FO and BWO alone by factors of 11.06 and 3.29, respectively. The FO/BWO (0.5) exhibited a rate constant of 0.1895 min⁻¹, significantly surpassing that of FO (0.0171 min⁻¹) and BWO (0.0574 min⁻¹). Furthermore, the researchers investigated the matching of Fe₃O₄ and Bi₂WO₆ to form a heterojunction for photo-Fenton catalysis. Within 120 min, DMPBP [5]-BWO-Fe₃O₄ (F₂₀-BWO-D₇) generated an H₂O₂ concentration of 5.506 μM, which was 90.3-fold and 9.25-fold greater than that observed for BWO and BWO-D₇, respectively (Figure 9b–d) [101]. The removal efficiency of RhB by F₂₀-BWO-D₇ reached 98.3%. F₂₀-BWO-D₇ achieved a TOC removal efficiency of 85.7%, markedly surpassing that of both BWO and BWO-D₇. In addition, density functional theory (DFT) calculations showed that DMPBP [5] exhibited a much higher adsorption energy for RhB (−0.061 eV) compared to Fe₃O₄ and BWO, indicating a stronger pollutant adsorption capacity.

5.3. Iron Oxide-Based Heterojunction with Bi₂MoO₆

Bi₂MoO₆ is a direct bandgap semiconductor with a narrow bandgap ranging from 2.6 to 2.8 eV, enabling its response to visible light [102]. Meanwhile, Bi₂MoO₆ has excellent optical properties. In addition, the alternating arrangement of layers improves the efficiency of photogenerated carrier separation. In recent years, there have been more and more reports on the degradation of organic pollutants by Bi₂MoO₆ as photocatalysts [103,104,105]. Xiu et al. prepared 3D hierarchically structured Fe₃O₄/Ag/Bi₂MoO₆ magnetic microspheres with good light transmittance and absorption by the hydrothermal-photoreduction strategy (Figure 10a–c) [106]. In addition, the introduction of Fe₃O₄ and Ag extended the light absorption spectrum of the materials into the visible and near-infrared regions and improved the solar energy utilization. Meanwhile, the introduction of Fe₃O₄ realized the cooperation of photocatalysis and the Fenton reaction, which significantly improved the removal efficiency through reacting H₂O₂ generated from the photocatalytic reaction with Fe₃O₄ to generate HO•. The removal efficiencies of Fe₃O₄/Ag/Bi₂MoO₆ (A2) for Aatrex and bisphenol A (BPA) reached 98.9% and 99.2%, respectively, in 150 min. Moreover, the mineralization rate for highly toxic organic pollutants was significantly higher than that of Bi₂MoO₆ alone. MOF-derived spherical shell-structured Fe₂O₃@C-Bi₂MoO₆ heterojunction catalyst (FO@C/BMO) achieved the efficient photo-Fenton elimination of TC (Figure 10d) [107]. The experimental results showed that the catalyst achieved a 93.2% removal efficiency of TC within 50 min. Owing to the synergistic interaction between the Fenton process and photocatalysis, the photo-Fenton reaction exhibited a significantly higher rate constant for TC removal compared to either the Fenton reaction or photocatalysis alone. The photogenerated electrons promoted Fe³⁺/Fe²⁺ cycling, which improved the reaction performance of the catalyst. In addition, the large generation of active species such as HO•, O₂^•−, h⁺, and ¹O₂, which acted cooperatively in the TC removal, was confirmed in the FO@C/BMO-60 photo-Fenton system through radical scavenging tests and electron paramagnetic resonance (EPR) spectroscopy. Moreover,

Table 3. Summary of iron oxide-based heterojunction with bismuth-related semiconductors for photo-Fenton removal of organic pollutants.

Catalysts	Pollutants	Condition	Removal Efficiency	Reference
FeOx/Bi4TaO8Cl	TC-H (20 mg L−1)	300 W Xe lamp, (λ ≥ 420 nm), pH = 6, [H2O2] = 99 mM, [Catalyst] = 0.4 g L−1	83.5%, 20 min	[88]
α-Fe2O3/Bi2WO6	MB (5 mg L−1)	300 W Xe lamp, (λ ≥ 400 nm), pH = 7, [H2O2] = 100 μL, [Catalyst] = 20 mg	100%, 25 min	[100]
Fe2O3@C-coupled Bi2MoO6	TC (25 mg L−1)	500 W Xe lamp with a 420 nm UV cut-off filter, pH = 3, [H2O2] = 1 mmol, [Catalyst] = 40 mg	93.2%, 50 min	[107]
black-BiOCl/F2O3	TC (10 mg L−1)	300 W Xe lamp, (λ < 400 nm), [H2O2] = 75 μL, [Catalyst] = 50 mg	92%, 25 min	[93]
α-Fe2O3/BiOI	MO (10 mg L−1)	Xenon lamp, (λ < 420 nm), pH = 6.5, [H2O2] = 10 mM, [Catalyst] = 0.05 g	99.5%, 15 min	[108]
(H2PO4−, Ni2+)–α-Fe2O3/Bi2S3	p-NP (0.2 mM)	Visible light, pH = 3, [H2O2] = 1.3 mM, [Catalyst] = 0.04 g	91.3%, 30 min	[109]
Fe3O4@rGO@CdS/Bi2S3	MB (25 ppm)	solar light, pH = 7, [H2O2] = 0.0536 M, [Catalyst] = 10 mg	99%, 70 min	[110]
Fe3O4@La-BiFeO3	NOR (20 mg L−1)	500 W Xe lamp, (400 nm < λ < 780 nm) pH = 5, [H2O2] = 20 mM, [Catalyst] = 0.7 g L−1	96.6%, 60 min	[111]

summarizes the research results on the removal of the iron oxide-based heterojunction with bismuth-related semiconductors for the photo-Fenton removal of organic pollutants.

In conclusion, coupling iron oxides with bismuth-based semiconductor photocatalysts provides a significantly large contact surface area, ensuring the efficient separation of photogenerated carriers and faster charge transfer efficiency. It can enable the continuous reduction of Fe³⁺ to Fe²⁺, generating HO• to sustain the reaction. Therefore, it is feasible to couple iron oxides with bismuth-based semiconductor photocatalysts to construct a 2D heterojunction structure for enhancing photo-Fenton activity, thus developing a more advanced photo-Fenton catalytic system.

6. Iron Oxide-Based Heterojunction with Carbon-Based Materials

Carbon-based materials are widely used in heterojunction photocatalysts due to their strong adsorption capacity, electrical conductivity, wide light absorption range, and tunable surface properties [112,113,114,115]. Carbon-based materials can enhance the photocatalytic activity of organic pollutants by increasing the quantity of catalytic sites, expanding the light absorption region, and promoting charge carrier separation [116,117]. Meanwhile, carbon-based materials in heterojunctions are usually used as electron transfer channels to enhance the separation and transport of photogenerated charge carriers while suppressing the recombination of electron–hole pairs. In addition, carbon-based materials can extend the light absorption range, increasing light utilization and improving catalyst stability and reusability through enhancing light absorption and structural stability of the materials. We will discuss in detail the use of heterojunctions formed by a variety of carbon-based catalysts (such as metal–organic frameworks, activated carbon, biochar, carbon quantum dots, carbon nanotubes, reduced graphene oxide, etc.) with iron oxides for the removal of organic pollutants in the photo-Fenton system.

6.1. Iron Oxide-Based Heterojunction with Metal–Organic Frameworks (MOFs)

Metal–organic frameworks (MOFs), made up of metal nodes linked by organic ligands, are promising in catalysis due to their excellent catalytic activity, tunable crystal structure, and high porosity [118]. The adaptable nature of metal centers and organic ligands in MOFs contributes to enhanced photocatalytic efficiency and makes them suitable candidates for heterojunction construction [119,120]. Fe₂O₃-on-ZrO₂ polypod catalysts were prepared by an MOFs derivatization strategy by compositing UiO-66(Zr)-NH₂ and MIL-88B(Fe) via solvothermal reaction and high temperature calcination (Figure 11a–c) [121]. The catalysts were prepared by a strategy involving MOFs derivatization coupled with reflux thermal treatment and the calcination method for the degradation of TC-HCl in wastewater. The Fe₂O₃-on-ZrO₂ achieved a removal efficiency of more than 95% for 20 mg L⁻¹ of TC-HCl in 25 min, and the removal efficiency could still reach 91% after five cycles, which showed excellent stability and reusability. In addition, Fe₂O₃-on-ZrO₂ had abundant Fe₂O₃-on-ZrO₂ heterojunctions, which promoted the transfer of photogenerated electrons from ZrO₂ to Fe₂O₃ and accelerated the cycle of iron. The low photogenerated carrier recombination efficiency and high photogenerated carrier separation efficiency of Fe₂O₃-on-ZrO₂ were confirmed by PL and photocurrent response tests. Meanwhile, the important roles of HO• and O₂^•− in the degradation mechanism were confirmed by free radical trapping experiments and ESR techniques. In addition, the scanning Kelvin probe (SKP) technique was used to measure the surface work function of the samples, which further revealed the photogenerated carrier transfer mechanism. In addition, a novel heterogeneous photo-Fenton catalyst, α-Fe₂O₃@g-C₃N₄@NH₂-MIL-101(Fe) (FGN), incorporating a dual Z-scheme heterojunction structure, was synthesized by Hu et al. via hydrothermal approach for the removal of TC (Figure 11d,e) [122]. The FGN removed 98.33% of 10 mg L⁻¹ TC in 12 min, and the removal efficiency was maintained at 92.27% after five cycles. In addition, the H₂O₂ consumption test and free radical quenching experiment confirmed that O₂^•− served as the principal active component within the system. Meanwhile, the semiconductor type and flat band potentials were determined by the Mott–Schottky test, and the transfer path of photogenerated carriers in FGNs, i.e., the dual Z-scheme heterostructure mechanism, was proposed.

In addition, the researches successfully synthesized Fe₃O₄@MIL-100(Fe) composites by the in situ growth method and systematically investigated their ability to degrade levofloxacin (LEV) in the photo-Fenton system [123]. The experimental analysis revealed that the Fe₃O₄@MIL-100(Fe) composite exhibited the highest removal efficiency of 93.4% for LEV at a mass ratio of 1:4. The high catalytic performance was mainly ascribed to the cooperative interaction between Fe₃O₄ and MIL-100(Fe), which enhanced the effective separation of photogenerated electron–hole pairs in MIL-100(Fe) and promoted the rapid reduction of Fe³⁺ to Fe²⁺ during the photo-Fenton process. In addition, the composite had good recyclability with no significant decrease in photo-Fenton activity. In practical applications, the composite successfully removed LEV from LEV-containing wastewater with removal efficiencies ranging from 77.9% to 85.5%. HO• and h⁺ play a dominant role as reactive species in the degradation of LEV.

6.2. Iron Oxide-Based Heterojunction with g-C₃N₄

Owing to its distinctive electronic band structure, cost-effectiveness, and simple synthesis process, graphite-phase carbon nitride (g-C₃N₄) has emerged as a focal point in photocatalysis research [124,125,126]. With a narrow bandgap of 2.7 eV, g-C₃N₄ can be excited by visible light and demonstrates remarkable efficiency in the photocatalytic removal of pollutants [127]. However, g-C₃N₄ suffers from a fast rate of photogenerated charge complexation, which limits the further improvement of its photocatalytic performance [128]. One of the most efficient approaches to address the above issue is the formation of heterojunctions. The rational selection of semiconductor materials to form heterojunctions can achieve effective separation and migration of photogenerated charges while preserving high photocatalytic activity [129].

Table 4. Summary of iron oxide-based heterojunction with g-C₃N₄ for photo-Fenton removal of organic pollutants.

Catalysts	Pollutants	Condition	Removal Efficiency	Reference
Fe2O3/g-C3N4	Amaranth (0.03 mM)	300 W Xe lamp with a 420 nm UV cut-off filter, pH = 3, [H2O2] = 1 mL, [Catalyst] = 50 mg	97.6%, 10 min	[130]
Fe2O3/g-C3N4	TC (20 mg L−1)	300 W Xe lamp, pH = 4, [H2O2] = 50 mM, [Catalyst] = 50 mg	96.4%, 180 min	[131]
Fe2O3/g-C3N4	TC (5 mg L−1)	500 W Xe lamp with a 420 nm UV cut-off filter, pH = 5.5, [H2O2] = 50 mM, [Catalyst] = 20 mg	90.7%, 120 min	[132]
α-Fe2O3@g-C3N4@NH2-MIL-101(Fe)	TC (10 mg L−1)	300 W Xe lamp, pH = 5, [H2O2] = 20 mM, [Catalyst] = 10 mg	98.33%, 120 min	[122]
α-Fe2O3@g-C3N4	TC (40 mg L−1)	100 W LED lamp (λ = 420 nm), pH = 5.5, [H2O2] = 10 mM, [Catalyst] = 0.05 g	92%, 60 min	[133]
α-Fe2O3/g-C3N4/SiO2	RhB (10 ppm)	100 W LED lamp (λ = 420 nm), pH = 3, [H2O2] = 7 × 10−4 M, [Catalyst] = 0.06 g	97%, 120 min	[134]
Fe2O3/S doped g-C3N4	Norfloxacin (5 mg L−1)	Visible light, pH = 4, [H2O2] = 6 g L−1, [Catalyst] = 50 mg	100%, 25 min	[135]
α-Fe2O3/g-C3N4	Phenol (50 mg L−1)	350 W Xe lamp, pH = 2.5, [H2O2] = 45 mM, [Catalyst] = 10 mg	90%, 70 min	[136]
α-Fe2O3/g-C3N4	Acetaminophen (ACT) (20 mg L−1)	35 W Xe lamp with a 420 nm UV cut-off filter, pH = 5, [H2O2] = 5 mM, [Catalyst] = 0.1 g L−1	100%, 25 min	[137]
TiO2/α-Fe2O3/Fe-g-C3N4	LEV (15 mg L−1)	Xe lamp, pH = 2, [H2O2] = 8 mM, [Catalyst] = 0.5 g L−1	93.8%, 90 min	[138]
Fe2O3–Fe–CN	TC (20 mg L−1)	300 W Xe lamp with a 400 nm UV cut-off filter, pH = 7, [H2O2] = 300 μL, [Catalyst] = 0.02 g	99.88%, 40 min	[139]
Fe2O3/g-C3N4	MO (20 mg L−1)	1000 W tungsten/halogen lamp (Philips) with a 420 nm UV cut-off filter, pH = 3, [H2O2] = 50 μL, [Catalyst] = 50 mg	90%, 90 min	[140]
Fe2O3/Boron-doped g-C3N4	Amoxicillin	Sunlight, pH = 4, [H2O2] = 30 μL, [Catalyst] = 0.02 g	93%, 30 min	[141]
g-C3N4/Fe2O3	TC (20 mg L−1)	300 W Xe lamp, [H2O2] = 50 mM, [Catalyst] =20 mg	87%, 60 min	[142]
γ-Fe2O3/g-C3N4	Oxytetracycline (OTC) (20 mg L−1)	300 W Xe lamp with a 420 nm UV cut-off filter, pH = 4.89, [H2O2] = 150 μL, [Catalyst] = 0.1 g	85.7%, 60 min	[143]
α-Fe2O3/NCDs/g-C3N4	Indole (50 mg L−1)	350 W Xe lamp, pH = 5, [H2O2] = 90 mM, [Catalyst] = 0.15 g L−1	85%, 150 min	[144]
MLD/CN/Fe3O4	TC (20 mg L−1)	300 W Xe lamp with a 420 nm UV cut-off filter, pH = neutral, [H2O2] = 80 mM, [Catalyst] = 0.5 g L−1	95.8%, 80 min	[145]
g-C3N4/Fe3O4@MIL-100(Fe)	Ciprofloxacin (CIP) (200 mg L−1)	Visible light with a 420 nm UV cut-off filter, pH = 3, [H2O2] = 2.64 g L−1, [Catalyst] = 0.67 g L−1	94.7%, 120 min	[146]

summarizes the research results on the removal of iron oxide-based heterojunctions with g-C₃N₄ for the photo-Fenton removal of organic pollutants in the last decade.

Wang et al. constructed a Z-scheme heterojunction of three-dimensional flower-like Fe₂O₃ and two-dimensional g-C₃N₄ nanosheets via a simple solvothermal method for efficient and stable degradation of azo dyes in the photo-Fenton system [130]. Fourier transform infrared spectroscopy (FTIR) analysis revealed the presence of Fe–N coordination bonds and Fe–O bonds in the Fe₂O₃/g-C₃N₄ composites, indicating that the interface between Fe₂O₃ and g-C₃N₄ interactions was conducive to the formation of heterojunctions and the enhancement of photocatalytic activity. UV-vis DRS spectra showed that the Fe₂O₃/g-C₃N₄ composites exhibited a noticeable red shift in their absorption edge toward the visible light region, indicating that the incorporation of Fe₂O₃ enhanced the visible absorption property and was beneficial to the production of photogenerated charge carriers. The experimental findings indicated that the optimized 8% Fe₂O₃/g-C₃N₄ composite demonstrated outstanding photocatalytic activity with a removal efficiency of amaranth (97.6%) within 10 min. Free radical scavenging experiments revealed that ¹O₂ and O₂^•− were the dominant reactive species in the Fe₂O₃/g-C₃N₄ photo-Fenton system, primarily contributing to the degradation of amaranth. Combined with the experimental results, a potential mechanism for the photocatalytic reaction was suggested. In addition, the S-scheme heterojunction photocatalyst constructed by 2D α-Fe₂O₃ and 2D g-C₃N₄ was investigated for the photo-Fenton removal of TC (Figure 12a–d) [131]. Field emission scanning electron microscopy (FESEM) and high-resolution transmission electron microscopy (HRTEM) analyses revealed that α-Fe₂O₃ nanosheets were evenly distributed across the surface of g-C₃N₄ nanosheets, resulting in the formation of a tightly bound heterojunction. The experimental results showed that the heterojunction containing 5.26 wt% α-Fe₂O₃/g-C₃N₄ achieved 96.4% removal efficiency of TC within 180 min, which was significantly higher than that of the Fenton reaction (59.6%) and the photocatalytic reaction (74.1%) alone. The photo-Fenton reaction involved HO•, O₂^•− and h⁺ as the main reactive species. Efficient charge carrier separation and transfer upon photoexcitation was further confirmed by PL spectroscopy and EPR measurements. DFT calculations exhibited that the migration of electrons from g-C₃N₄ to α-Fe₂O₃ led to the establishment of an IEF, which facilitated photogenerated charge separation and transfer. The heterojunction indicated excellent photo-Fenton activity and stability through efficient charge separation and accelerated Fe³⁺/Fe²⁺ cycling. A novel α-Fe₂O₃/g-C₃N₄ material was developed via loading α-Fe₂O₃ hexagonal sheets on the exterior of g-C₃N₄ microspheres for the photo-Fenton degradation of TC (Figure 12e,f) [132]. The composite exhibited highly efficient photo-Fenton activity under visible light, with a 90.7% removal efficiency of TC within 120 min, which was significantly better than that of the photocatalytic and Fenton alone. The results suggested that the efficient degradation performance of the composite was caused by the synergistic impact between α-Fe₂O₃ and g-C₃N₄ as well as the formation of a Z-scheme heterojunction. The intermediates of TC degradation were analyzed by UPLC-MS, and three possible degradation pathways involving N-methyl loss, ring cleavage, dehydration, demethylation, deoxygenation, and hydroxylation reactions were proposed.

In addition to Fe₂O₃, the composite of Fe₃O₄ and g-C₃N₄ is also widely used in the photo-Fenton removal of organic pollutants in water [147,148,149]. Zhang et al. synthesized a collection of carbon bridge-modified g-C₃N₄/Fe₃O₄ magnetic heterogeneous catalysts (MLD/CN/Fe₃O₄) by the copolymerization method and explored the effect on TC removal during the photo-Fenton process (Figure 12g) [145]. Under the optimal conditions, the 0.8 MLD/CN/Fe₃O₄ catalyst achieved 95.8% TC degradation and 55.7% mineralization. The possible mechanism of TC degradation in the photo-Fenton system was proposed via EPR analysis and free radical quenching experiments: The catalyst promoted the decomposition of H₂O₂ and generated more hydroxyl radicals through the enhanced charge transfer ability and the Fe³⁺/Fe²⁺ cycle, which were conductive to the efficient degradation of TC. Furthermore, a novel ternary heterojunction catalyst (g-C₃N₄/Fe₃O₄@MIL-100(Fe)) (CFM) was investigated, which efficiently degraded ciprofloxacin (CIP) via a photo-Fenton process, demonstrating excellent catalytic activity and reusability, and providing a new strategy for the treatment of highly concentrated antibiotics [146]. CFM degraded 200 mg L⁻¹ of CIP by 94.7% in 120 min, indicating excellent photo-Fenton catalytic performance. The reaction rate constant of CFM was 0.0656 min⁻¹, which was far superior to the performance of g-C₃N₄ and FM alone. Meanwhile, the proposed mechanism of CIP degradation in the CFM/H₂O₂/vis system was put forward: The photogenerated electrons generated by g-C₃N₄ were transferred to Fe₃O₄ and MIL-100(Fe), which accelerated the cycling of Fe(III)/Fe(II) and, thus, enhanced the removal efficiency of CIP. Mass spectrometry and frontier electron density (FED) theory were used to propose the degradation pathways of CIP, mainly including the cleavage of piperazinyl and quinoline rings.

6.3. Iron Oxide-Based Heterojunction with Graphene Derivatives (GO/rGO)

Graphene is a single-atom-thick 2D nanomaterial with carbon atoms organized in a hexagonal pattern [150,151]. Owing to its excellent electrical conductivity and extensive surface area, graphene is widely used in photocatalytic applications as an electronic mediator and photocatalyst carrier [152,153,154]. However, graphene is a semiconductor with a zero band gap, making the pristine form of graphene inappropriate for the formation of heterojunctions with iron oxides for photo-Fenton reactions [155]. Nevertheless, derivatives of graphene, including graphene oxide (GO) and reduced graphene oxide (rGO), are rich in surface functional groups and have excellent semiconducting properties, endowing them with a high pollutant-adsorption capacity [156,157]. In addition, GO and rGO allow further chemical modification and can form heterojunctions with iron oxides for the catalytic degradation of pollutants [29,158]. Wang et al. prepared a TiO₂/γ-Fe₂O₃/GO ternary composite by a one-step self-assembly method and explored its CIP degradation performance driven by visible light [159]. The results demonstrated that the composites displayed outstanding photocatalytic performance and stability in CIP degradation. The 0.03TiO₂/γ-Fe₂O₃/GO composite achieved 99% removal efficiency of 10 mg L⁻¹ CIP in 140 min, which was significantly higher than that of the other components and the pure γ-Fe₂O₃/GO. The 0.03 TiO₂/γ-Fe₂O₃/GO composite reached a 49% TOC removal rate in 160 min, indicating that CIP was effectively mineralized. The main roles of HO•, h⁺, and electrons in CIP degradation were confirmed based on free radical trapping experiments, with electrons playing an important role.

A range of binary (α-Fe₂O₃/rGO) and ternary (g-C₃N₄/Fe₂O₃/rGO) (FerGCN) composite photo-Fenton catalysts were synthesized using a straightforward hydrothermal method, and their photochemical oxidative degradation of SMX was examined (Figure 13) [160]. The removal efficiency of SMX by 0.4-FerGCN-3 reached 99.9% with a rate constant value of 0.060 min⁻¹. In addition, TOC analysis showed that 0.4-FerGCN-3 mineralized SMX up to 73%. According to the EPR and quenching experiments, the degradation process of SMX involved the participation of HO•, O₂^•−, and ¹O₂, and both radical and non-radical routes were involved in the degradation process. The proposed degradation mechanism included the following processes: photogenerated electrons were migrated from the CB of α-Fe₂O₃ to the VB of g-C₃N₄, leaving abundant holes; the holes interacted with water molecules to generate HO•; and, the photogenerated electrons interacted with dissolved oxygen to generate O₂^•−, which was further converted into ¹O₂.

Consequently, carbon-based materials, characterized by their cost-effective fabrication, excellent conductivity, high surface area, tunable functional groups, and controllable physicochemical properties, serve as ideal supports for photo-Fenton catalysts. The coupling of iron oxides with carbon-based materials can effectively enhance the Fe³⁺/Fe²⁺ redox cycling, thereby significantly improving the catalytic performance in photo-Fenton systems.

7. Conclusions and Outlook

In conclusion, this work summarizes the recent advances in iron oxide-based heterojunctions for the elimination of organic pollutants in photo-Fenton systems. Firstly, the review describes the various types of heterojunctions that have been developed for the study and the application of photo-Fenton reactions, such as Schottky junctions and type-I, type-II, type-III, Z-scheme, and S-scheme junctions. In addition, the heterojunctions are classified according to the properties, energy band structures, and transfer mechanisms of carrier, while the photogenerated electron separation and transfer mechanisms are discussed in detail. Next, a series of iron oxide-based heterojunction photo-Fenton catalysts are divided into three sections: metal oxides, bismuth-related semiconductors, and carbon-based materials. Having a suitable band gap to form heterojunctions with iron oxides is important for the successful preparation of heterojunctions. Therefore, the iron oxide-based heterojunction photo-Fenton catalysts have the following features: a compact heterojunction contact interface; excellent light-absorption capacity; a wide light-absorption range; abundant active sites; high photogenerated electron-hole separation efficiency, and excellent H₂O₂ activation ability. The above properties enable the iron oxide-based heterojunction catalysts to rapidly degrade organic pollutants in water under photo-Fenton conditions. However, there are still several pressing challenges: secondary contamination due to iron leaching, the difficulty of designing materials with broad spectral response, and the competing effects of natural organic matter (NOM) in practical water bodies. Consequently, for the removal of organic pollutants from water by iron oxide-based heterojunctions, atomic-level interfacial modulation (i.e., single-atom catalysis, defect engineering), machine learning-assisted material screening and reaction condition optimization, and optical–electrical–magnetic multi-field coupled reactor development are important research directions.