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• The expansion of dye kinds is occurring progressively due to the rapid advancement of printing and dyeing technology [1,2]. The industrial manufacturing of dyes involves various complex processes, including sulphuration, nitrification, diazotization, reduction, oxidation, and others. Consequently, a substantial volume of wastewater is generated during these processes. The disposal of dyes wastewater has emerged as a critical environmental concern [3–5]. The effluent from dyeing processes exhibits a multifaceted composition, characterized by a high mass concentration of chemical oxygen demand (COD), elevated levels of organic pollutants, intense coloration, and significant toxicity. These properties provide a severe risk of causing carcinogenic, teratogenic, and mutagenic effects [6,7]. Recent studies have indicated that Rhodamine B (RhB) exerts a significant adverse influence on the environment. Hence, it is imperative to employ efficacious approaches to curb the negative effects of contaminants of RhB [8–11].

  Previous research has demonstrated the extensive utilization of physical, chemical, and biological techniques, such as adsorption, ion exchange, and biodegradation, with the purpose of eliminating RhB [12]. Nevertheless, the dyes in question possess chemically stable aromatic molecules and a substantial quantity of color-inducing groups, characterized by numerous rings and extended carbon chains. However, conventional methods such as adsorption, electrolysis, biodegradation, and chemical oxidation are inadequate for their efficient removal [13]. In recent years, researchers have shown a growing preference for advanced oxidation technology due to its rapidity, high efficiency, and lack of secondary pollution. This technology is considered efficient and environmentally friendly, and has found widespread application in various production fields, including environmental protection, chemistry, and materials [14–17]. Fenton catalytic oxidation is classified as an advanced oxidation technology that relies primarily on the catalytic decomposition of H₂O₂ by Fe²⁺ to generate a substantial quantity of hydroxyl radicals (^•OH). With an oxidation electrode potential of 2.80 V, Fenton catalytic oxidation exhibits remarkable oxidation capabilities, surpassed only by fluorine. Consequently, it effectively breaks down numerous organic pollutants into smaller molecules such as H₂O, carbon dioxide, and inorganic salts [18–20]. So, it is essential to note that the conventional Fenton method exhibits a significant reliance on the pH level of the solution [21]. But, a growing number of scholars have shifted their focus towards employing inhomogeneous Fenton system as opposed to the conventional homogeneous Fenton catalysts [22–24]. Aiming to expand the pH response range of the reaction system. The traditional homogeneous Fenton technology has some problems, such as narrow pH response range, difficult recovery and separation of catalyst active components, and secondary pollution caused by iron slime. However, non-homogeneous Fenton technology can effectively overcome the above defects and has a good industrial application prospect [25,26]. Regrettably, the breakdown of organic contaminants in inhomogeneous systems is hindered by sluggish reaction rates, prolonged degradation durations, and excessive H₂O₂ dosages [27–30].

  Based on prior research, the utilization of microwaves has been found to induce the generation of oxygen vacancies and electron-hole pairs within the catalyst, which can either participate in the degradation in the reaction system as oxidants, or the holes can react to generate more hydroxyl radicals to degrade the organic pollutants [31–34]. Beyond that, microwaves have the capability to create "hot spot" phenomena and non-thermal effects on the catalyst, which can further enhance the degradation of organic pollutants by Fenton [35–37]. The utilization of microwave shock technology has emerged as a rapid, straightforward, and effective approach for synthesizing materials, which can attain the rational regulation of the chemical composition and morphology of crystals, and then achieve the enhancement of material properties through its "structure-effect" relationship [38–41]. The utilization of microwave-induced oxidation technology relies heavily on the presence of microwave absorbing materials. Fe₃O₄ is a significant transition metal oxide that holds a major role as a microwave-induced oxidizing material [42–44]. This is primarily attributed to the uniqueness of its outer d-electrons and the fact that the degrees of freedom of charge, spin, orbitals as well as lattice are closely correlated with each other, which gives the transition metal oxides a very rich set of structural and physical properties, in particular excellent microwave absorbing properties [45–47].

  In this work, we synthesized three-dimensional spherical Fe₃O₄ catalysts using the hydrothermal method and subsequently modified their crystal structure through the application of microwave shock method. Using the representative pollutant RhB as a target, the process conditions such as microwave power, initial concentration and catalyst dosage were optimized to achieve efficient degradation of RhB. More importantly, the elucidation of the mechanism behind the enhancement of Fenton catalysis by microwave irradiation was conducted in a comprehensive manner. Additionally, a potential pathway for the degradation of RhB through microwave-assisted Fenton catalysis was proposed. This research has the potential to enhance our understanding of the mechanism underlying the synergistic effect of it. Furthermore, it offers a new thinking for the removal of dyes from water through synergistic catalytic processes.

  Fe₃O₄ microspheres were synthesized through the hydrothermal method using a solution containing 4 mmol of ferric chloride hexahydrate (FeCl₃·6H₂O) and 0.7 mmol of trisodium citrate dihydrate (Na₃C₆H₅O₇·2H₂O) in 40 mL of ethylene glycol and stirring for 30 min, followed by the addition of 15 mmol of sodium acetate (CH₃COONa) for 30 min. The above mixed solution was transferred to a stainless-steel autoclave containing 100 mL of polytetrafluoroethylene liner and heated at 200 ℃ for 10 h. The resulting black precipitate was washed several times with ethanol and deionized water by centrifugation and then dried in an oven at 60 ℃ for 12 h. The dried sample was named Fe₃O₄-Pristine for the prepared sample.

  The modulation of the chemical composition of Fe₃O₄ crystals was mainly achieved by the microwave impact method. A quartz crucible was utilized to contain 500 mg of the prepared Fe₃O₄-Pristine. The crucible is subsequently introduced into a quartz glass container, accompanied by a layer of quartz sand positioned at the base, within a controlled environment glove box that is filled with an argon atmosphere. The uppermost part of the balloon was sealed. Subsequently, the aforementioned apparatus was reacted in a microwave oven at 800 W for 20 s. The designated sample was denoted as Fe₃O₄-Microwave.

  To more accurately assess the benefits of the microwave impact method, the Fe₃O₄-Pristine underwent calcination heat treatment for comparative analysis. The Fe₃O₄-Pristine prepared above was placed in a tube furnace under argon atmosphere at 500 ℃ for 4 h with a temperature increase rate of 5 ℃/min, and the sample that underwent the calcination process was designated as Fe₃O₄-Calcination.

  Microwave assisted Fenton catalysed degradation experiments: The factors affecting the catalytic degradation of Rhb by Fe₃O₄ under MW conditions were investigated by controlling various variables such as catalyst dosage, Rhb solution concentration, H₂O₂ dosage, MW power and cycle life. A 150 mL Erlenmeyer flask was used as the reaction vessel and a certain amount of Fe₃O₄ and 50 mL of Rhb solution were added to the Erlenmeyer flask each time. The conical flask containing the mixed solution was placed in a laboratory microwave oven and reacted at a specified power for a total of 30 min. Every 5 min during the reaction, 4 mL of the solution was removed from the Erlenmeyer flask and placed in a 7 mL centrifuge tube. The solution in the centrifuge tube was centrifuged at 10,000 rpm for 5 min, after which the concentration of the supernatant was determined and calculated.

  To investigate the effect of catalyst dosage on the experiments, five conical flasks containing 10 mg/L Rhb and 0.4 mol/L H₂O₂ solutions with 0.2, 0.4, 1 and 2 g/L Fe₃O₄ were added at 500 W, respectively. To investigate the effect of H₂O₂ dosage on the experiments, five conical flasks containing 10 mg/L Rhb and 1 g/L Fe₃O₄ solutions with 0.1, 0.2, 0.4, 0.6 and 0.8 mol/L H₂O₂ were respectively added at 500 W. To study the effect of Rhb solution concentration on degradation, each conical flask contained 1 g/L Fe₃O₄ and 0.4 mol/L H₂O₂, the MW power was 500 W, and the Rhb concentration was set to 5, 10, 20, and 40 mg/L, respectively. The effect of MW power on the experiments was investigated with 10 mg/L Rhb solution, 1 g/L Fe₃O₄ catalyst and 0.4 mol/L H₂O₂ in each conical flask, and the MW power was set to 200 W, 500 W and 800 W, respectively. To test the lifetime of the catalyst, the reaction power was 500 W and the Erlenmeyer flask contained 10 mg/L Rhb solution, 1 g/L Fe₃O₄ and 0.4 mol/L H₂O₂.

  Simultaneously, this study additionally investigated the impact of catalysts in various reaction systems on the degradation of RhB. The catalyst Fe₃O₄ microwave alone, H₂O₂ alone and H₂O₂-Fe₃O₄ microwave systems were selected for the catalytic degradation of RhB in the absence of microwave irradiation. When microwave irradiation was introduced, MW irradiation alone, MW-Fe₃O₄ microwave system, MW-H₂O₂ system and MW-H₂O₂-Fe₃O₄ microwave system were selected for the catalysis of Rhb.

  Furthermore, an investigation was conducted to examine the impact of active species produced by the catalysts on the degradation of Rhb. The main active species were first verified by quenching experiments, and IPA, BQ, silver nitrate and EDTA-2Na were chosen as radical bases for scavenging hydroxyl radicals (·OH), superoxide radicals (O²⁻), electrons (e⁻) and holes (h⁺), respectively, and DMPO as a trap for ·OH and O²⁻, and TEMPO as a trap for h⁺ and e⁻, respectively.

  Fe₃O₄-Microwave was synthesized by the hydrothermal method. The crystal structure composition of the microspheres was modified using the microwave impact method and calcination heat treatment, and the findings are displayed in Fig. 1. The X-ray diffractograms of all the samples displayed characteristic peaks that matched to the characteristic peaks of the standard card Fe₃O₄ (JCPDS No. 19–629) at specific relative positions. These positions included 18° (111), 30° (220), 35.4° (311), 37° (222), 43° (400), 53.4° (422), 56.9° (511), and 62.5° (440). Additionally, other relative positions of the characteristic peaks were also found to be a match. No substantial variations in peak position were detected, except for the relative intensities of these peaks.

  Figure 1

  

  Figure 1.  XRD patterns of Fe₃O₄ prepared by different methods.

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  Rietveld refinement is a method that can be employed to ascertain the crystal chemical composition in Fe₃O₄ and get crystal structure data of the sample. The Fe₃O₄ (ICSD: 28664) served as the initial model for Rietveld refinement, and the degraded Fe₃O₄ was then subjected to structure refinement using the TOPAS 4.2 software. The outcomes of this process are presented in Fig. S1 (Supporting information). Where the orange point represents the original diffraction peak intensity, the red line represents the fitted intensity, the blue line represents the difference between the original and fitted intensities, and the green vertical line represents the Bragg position where the diffraction peaks are located for the samples. Fig. S1a displays the Rietveld refinement profile of Fe₃O₄-Pristine, along with the fitting results of the structural refinement in Table S1 (Supporting information), in which the values of R_wp, R_exp, and R_p are found to get 3.315, 1.847, and 2.500, respectively. These results confirm the reliability of the structural refinement and indicate that the hydrothermally prepared Fe₃O₄ retains its cubic space group. In the crystal structure of Fe₃O₄ which has an antispinel structure, half of the Fe³⁺ are theoretically occupied in tetrahedral coordination (Fe³⁺-tet.), another half of the Fe³⁺ are occupied in octahedral coordination (Fe³⁺‑oct.), and all of the Fe²⁺ are occupied in octahedral coordination. The occupancies of Fe³⁺-tet., Fe³⁺‑oct. and Fe²⁺ obtained after Fe₃O₄-Pristine fitting were 0.8511, 0.4090, 0.4090, respectively, and the corresponding ratio of Fe²⁺/Fe³⁺ was 0.4901and the degraded Fe₃O₄ was then subjected to structure refinement using the TOPAS 4.2 software.

  These results confirm the reliability of the structural refinement and indicate that the hydrothermally prepared Fe₃O₄ retains its cubic space group. In the crystal structure of Fe₃O₄, which has an antispinel structure, half of the Fe³⁺ are theoretically occupied in tetrahedral coordination (Fe³⁺-tet.), another half of the Fe³⁺ are occupied in octahedral coordination (Fe³⁺‑oct.), and all of the Fe²⁺ are occupied in octahedral coordination. The occupancies of Fe³⁺-tet., Fe³⁺‑oct. and Fe²⁺ obtained after Fe₃O₄-Pristine fitting were 0.8511, 0.4090, 0.4090, respectively, and the corresponding ratio of Fe²⁺/Fe³⁺ was 0.4901.

  The Rietveld refinement figures of Fe₃O₄-Microwave and Fe₃O₄-Calcination are displayed in Figs. S1b and c, respectively, and the fitting results are listed in Table S1, from which it can be seen that the modulation of the chemical composition of the crystals has a negligible effect on the space group, whereas the occupancy of Fe³⁺-tet. gradually decreases from 0.8511 to 0.8323 (0.8084), the occupancy of Fe²⁺ increased from 0.4090 to 0.4273 (0.4191), and the corresponding ratio of Fe²⁺/Fe³⁺ increased from 0.4901 to 0.5066 (0.5090). Hence, the microwave shock method may efficiently and rapidly modify the crystal chemical composition of Fe₃O₄, comparable to the effects of calcination heat treatment. The Rietveld refinement profile in Fig. S1d exhibits refinement factors of R_p = 4.169, R_wp = 5.482, and R_exp = 2.099 after the Fe₃O₄-Microwave degradation process. These values provide evidence of the reliability of the refinement results. The Rietveld refinement analysis confirms that the Fe₃O₄-Microwave, after undergoing the degradation reaction, still maintains its cubic crystal structure with the space group Fd-3 m, and the lattice parameter a = 8.3968 Å, indicating that there is no alteration in the physical phase compared to before the degradation reaction.

  The SEM analysis results of the hydrothermally synthesized Fe₃O₄-Pristine are shown in Fig. 2, from which it can be found that the Fe₃O₄-Pristine morphology presents a microsphere structural morphology with a diameter of about 100 nm (Fig. 2a). The examination of higher magnification TEM images of spherical Fe₃O₄-Pristine demonstrated that the microspheres were formed through the self-assembly of nanoparticles measuring 8–10 nm in size (Figs. 2b and c), which were well facilitated by the use of sodium citrate dihydrate and sodium acetate, respectively, as surfactants and precipitants. Following the application of microwave shock treatment, the morphology of Fe₃O₄-Microwave microspheres exhibited little alterations (Fig. 2d), and the size of the microspheres remained around 100 nm and comprised nanoparticles measuring 8–10 nm.

  Figure 2

  

  Figure 2.  (a, d, g) Scanning electron microscope images and (b, c, e, f, h, i) transmission electron microscope images of three Fe₃O₄ before degradation reaction.

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  In contrast, the microsphere morphology of the calcined heat-treated Fe₃O₄-Calcination did not change much after being heated, and their size remained approximately 100 nm, but the size of the constituent nanoparticles gradually changed from 8-10 nm to 20–30 nm. This change could be due to the particles melting during the heating process and the growth of new particles, resulting in the gradual enlargement of the crystalline particles.

  The N₂ adsorption-desorption test method was used to study the specific surface area and pore size distribution characteristics of Fe₃O₄ prepared under different conditions, and the results are shown in Fig. 3. The N₂ adsorption-desorption isotherms show that Fe₃O₄ prepared under different conditions has an H3 hysteresis loop at relative pressures of P/P₀ = 0.8–1.0 (Fig. 3a), which is typical of Type Ⅳ adsorption behaviour [48], suggesting the presence of mesopores formed by the interparticle spacing between the nanoparticles. Furthermore, the BET specific surface areas of Fe₃O₄-Pristine, Fe₃O₄-Microwave, and Fe₃O₄-Calcination were calculated to be up to 88.71, 66.31, and 33.33 m²/g, respectively. The pore size distribution curves of Fe₃O₄-Pristine, Fe₃O₄-Microwave and Fe₃O₄-Calcination were obtained according to the Barrette-Joynere-Halenda (BJH) method to obtain the pore size distribution curves are shown in Figs. 3d-f, most of the size distribution of Fe₃O₄-Pristine is between 2 nm and 10 nm, with an average pore size of 3.80 nm, and most of the pore size distribution of Fe₃O₄-Microwave is around 30 nm, with an average pore size of 31.05 nm, and most of the pore size distribution of Fe₃O₄-Calcination is also around 30 nm, with an average pore size of 29.14 nm. The Fe₃O₄ microspheres exhibit a mesoporous structure that is advantageous for catalytic degradation. This is due to the increased number of active sites resulting from the larger specific surface area. Additionally, the mesoporous structure acts as a channel for efficient mass transfer, facilitating the swift transportation of reactants and products during the catalytic reaction. Still, the specific surface area of the catalysts decreased during microwave impingement or calcination heat treatment, and the effect of specific surface area on the results of the subsequent experiments can be excluded based on the difference in the subsequent degradation performance of RhB.

  Figure 3

  

  Figure 3.  N₂ adsorption-desorption curves of Fe₃O₄ prepared under different conditions: (a) Fe₃O₄-Pristine, (b) Fe₃O₄-Microwave, (c) Fe₃O₄-Calcination. Corresponding pore size distribution curves of Fe₃O₄ prepared under different conditions: (d) Fe₃O₄-Pristine, (e) Fe₃O₄-Microwave, (f) Fe₃O₄-Calcination.

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  X-ray photoelectron spectroscopy was used to further examine the chemical composition and electronic structure of Fe₃O₄ produced under various conditions. The findings are presented in Fig. 4. The Fe 2p energy level spectra of Fe₃O₄-Pristine may be acquired from Fig. 4a, revealing two prominent peaks located at approximately 724.5 eV and 710.5 eV, which correspond to the Fe 2p_1/2 and Fe 2p_3/2 spin-orbit components of the Fe 2p peak of Fe₃O₄, accordingly. The Fe 2p peak can be resolved into individual peaks corresponding to Fe³⁺, Fe²⁺, and satellite peaks. The binding energies of 710.39 eV and 723.54 eV are associated with Fe³⁺, while the binding energies of 713.13 eV and 726.25 eV are associated with Fe²⁺. Also, the peaks seen at energy levels of approximately 716.33 and 729.48 eV correspond to vibrational satellites of Fe³⁺, while the peaks at 719.36 and 733.14 eV correspond to vibrational satellites of Fe²⁺. The presence of both Fe²⁺ and Fe³⁺ indicates the production of Fe₃O₄, which aligns with the findings of the XRD study [49,50].

  Figure 4

  

  Figure 4.  XPS patterns of Fe₃O₄ prepared under different conditions: (a) Fe 2p and (b) O 1s for Fe₃O₄-Pristine. (c) Fe 2p and (d) O 1s for Fe₃O₄-Microwave. (e) Fe 2p and (f) O 1s for Fe₃O₄-Calcination.

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  The results of Fe 2p fitting for Fe₃O₄-Microwave and Fe₃O₄-Calcination are shown in Figs. 4c and e, respectively, and the content and ratio of Fe³⁺ and Fe²⁺ in the corresponding crystal structures are shown in Table S2 (Supporting information). From the table, it can be found that compared with Fe₃O₄-Pristine, the ratio of Fe²⁺/Fe³⁺ in Fe₃O₄-Microwave increased from 0.4817 to 0.4980, which, firstly, indicates that the microwave treatment under argon atmosphere can modulate the crystal structure composition of Fe₃O₄. And secondly, it demonstrates that microwave treatment can achieve a crystal chemical composition similar to that of the calcination treatment, confirming the microwave's ability to naturally and efficiently modulate the crystal structure. This provides additional evidence that microwaves are uncomplicated and effective in controlling the crystal structure of Fe₃O₄. In addition, as shown in Fig. 4b, the peak of O 1s is fitted into two sub-peaks, O_ads. is a water molecule or hydroxyl group adsorbed on the surface, which corresponds to a binding energy of 531.26 eV. O_lat. is a characteristic peak of Fe₃O₄ lattice oxygen, with a corresponding binding energy of 529.90 eV, and further presents more evidence for the presence of Fe₃O₄.

  In conjunction with the fitting outcomes, it becomes evident that hydrothermal heating increases the amount of adsorbed oxygen in Fe₃O₄. This can be ascribed to the presence of residual organic matter in the raw materials (trisodium citrate dihydrate and sodium acetate) or to the organic molecules that bonded during the reaction, as further corroborated by the diffraction peaks in the XRD pattern.

  Mössbauer spectroscopy is widely employed in the investigation and determination of the chemical state of Fe. This is primarily attributed to the correlation that exists between the chemical state of the compound and one of the Mössbauer parameters, the homogeneous isoenergetic shift [51,52]. The Mössbauer spectrum can be analysed to extract specific parameters, such as the isomass energy shift (I.S.), quadrupole splitting (Q.S.), linewidth (Γ), and the area of resonance absorption (Area). This analysis provides information about the electrons and structure of the iron in the sample [53]. The ⁵⁷Fe Mössbauer spectra of Fe₃O₄ prepared under different conditions are shown in Fig. 5.

  Figure 5

  

  Figure 5.  ⁵⁷Fe Mössbauer spectra of Fe₃O₄ prepared under different conditions: (a) Fe₃O₄-Pristine, (b) Fe₃O₄-Microwave, (c) Fe₃O₄-Calcination, and (d) the Fe²⁺/Fe³⁺ ratio obtained from the fitting.

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  In contrast to the six-line spectrum documented in the literature [54], the six-line spectrum of Fe₃O₄-Pristine exhibits a notable broadening and the emergence of a series of double peaks (Fig. 5a). In Mössbauer spectroscopy, Magnetic single-domain particles exhibit a critical size for superparamagnetism. When the particle size is smaller than this critical size, the particles become magnetically disordered and superparamagnetic, resulting in a Mössbauer spectrum with either a single peak or double peaks. On the other hand, when the particle size exceeds the critical size, the particles become magnetically ordered and exhibit either ferromagnetism or ferrimagnetism. This corresponds to the Mössbauer spectrum showing the splitting of the six-line spectrum. Combining SEM results, it is evident that the Fe₃O₄ particles synthesized in this study possess a single domain structure, since their critical size is approximately 128 nm. After microwave treatment, the size of the Fe₃O₄-Microwave particles undergoes an increase, resulting in the splitting of the Mössbauer spectral lines into six-line spectrum. Especially after heat treatment, the crystallinity of Fe₃O₄-Calcination is notably improved, and the particle size increases significantly. The Mössbauer spectral lines exhibit a clearly divided six-line spectrum, consisting of two sets of magnetically split six-line spectrum.

  Additionally, the determination of Fe ion occupation in tetrahedral and octahedral cavities within Fe₃O₄ crystals was carried out through the analysis of spectral lines associated with the A and B sites. When the Fe₃O₄-Pristine is examined in conjunction with the ⁵⁷Fe Mössbauer spectral fitting parameters listed in Table S3 (Supporting information), it primarily consists of four sets of spectral fits: 8^*Fe³⁺ (tetrahedron), 8^*Fe³⁺ (octahedron), 5^*Fe²⁺, and 3^*Fe²⁺. The respective contents of these sets are approximately 74.3%, 2.2%, 2.0%, and 17.21%, respectively. Thus, the Fe²⁺/Fe³⁺ ratio is calculated to be 0.4792. The Fe²⁺/Fe³⁺ ratios in Fe₃O₄ after microwave and calcined heat treatments were 0.5220 and 0.5197, respectively. These values closely matched the results obtained from Rietveld refinement and XPS analysis, providing further evidence that microwave treatment can achieve the same effect as high-temperature heat treatment.

  Fe₃O₄ with varying crystal chemical compositions, prepared under varied conditions, is anticipated to serve as catalysts to enhance the degradation of contaminants through microwave-assisted Fenton-catalysis. Fig. 6a illustrates the impact of Fe₃O₄ catalysts with varying crystal chemical compositions, prepared under different conditions, on the microwave-assisted Fenton-catalysed degradation of RhB. It is clear that the Fe₃O₄-Microwave catalyst, subjected to microwave treatment, exhibits the highest effectiveness in degrading RhB. Primarily on the previous analyses, it is clear that the Fe²⁺/Fe³⁺ content of microwave-treated Fe₃O₄-Microwave is the highest. The Fe²⁺/Fe³⁺ content and degradation effect of heat-treated Fe₃O₄-Calcination is medium, while the degradation effect of Fe₃O₄-Pristine is the poorest. This suggests that the Fe²⁺/Fe³⁺ content is the crucial factor influencing microwave-assisted catalytic degradation. The catalytic efficiency of Fe₃O₄ was significantly augmented when the concentrations of both Fe²⁺ and Fe³⁺ in the catalyst were suitable. The reaction kinetic constants of Fe₃O₄-Microwave were enhanced by 5.6 and 2.4 times when compared to Fe₃O₄-Pristine and Fe₃O₄-Calcination (Fig. 6b), accordingly. Among the catalysts, Fe₃O₄-Microwave with the highest Fe²⁺/Fe³⁺ ratio of 2.23 exhibited the most effective co-catalytic activity. Therefore, in the MW-H₂O₂-Fe₃O₄ system, Fe³⁺ acts as the active site for microwave catalysis. Increasing its content promotes the generation of more active species and enhances microwave absorption, thereby maximising the hot spot effect and non-thermal effect of microwaves. Both are necessary for the microwave-assisted Fenton-catalysed degradation of RhB in this system.

  Figure 6

  

  Figure 6.  (a) Effect and (b) degradation rate of Fe₃O₄ with different crystal chemical components prepared under different conditions on microwave-assisted Fenton catalytic degradation of RhB.

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  Given that MW-H₂O₂-Fe₃O₄-Microwave system exhibits superior degradation performance in microwave-assisted Fenton-catalysed degradation of RhB. To expedite the degradation of RhB, we investigated the elements that affect RhB degradation in the MW-H₂O₂-Fe₃O₄-Microwave system by manipulating variables such as catalyst dosage, H₂O₂ dosage, initial RhB concentration, and microwave power.

  The amount of catalyst used has the most direct effect on the degradation of RhB through microwave-assisted Fenton-catalytic process. Hence, this study examined the impact of Fe₃O₄-Microwave dosage on the degradation of RhB by the MW-H₂O₂-Fe₃O₄-Microwave system. The order of RhB removal efficiency, as evident from Fig. 7a, is as follows: Fe₃O₄-Microwave (2.0 g/L) > Fe₃O₄-Microwave (1.0 g/L) > Fe₃O₄-Microwave (0.4 g/L) > Fe₃O₄-Microwave (0.2 g/L), and there is a positive correlation between the catalyst amount and the efficiency of RhB degradation. Increasing the catalyst dosage from 2.0 g/L to 2.0 g/L resulted in an increase in the degradation of RhB from 77.8% to 99.4%. It can be attributed to the direct impact of catalyst dosage on the catalytic active sites, which enhances the Fenton reaction and leads to the generation of more hydroxyl radicals with potent oxidative properties. Consequently, the degradation efficiency of RhB is accelerated. Given the expenses associated with catalyst preparation and the focus on environmentally friendly practises, a dose of 1.0 g/L of Fe₃O₄-Microwave catalysts was chosen for a series of subsequent experimental investigations.

  Figure 7

  

  Figure 7.  (a) Effect of catalyst dose, (b) H₂O₂ dose, (c) initial RhB concentration and (d) microwave power on the degradation of RhB by MW-H₂O₂-Fe₃O₄-Microwave system.

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  The dose of H₂O₂ is a crucial component that influences the degrading effect of RhB in the direct MW-H₂O₂-Fe₃O₄-Microwave system. The impact of various amounts of H₂O₂ on the elimination of RhB may be shown in Fig. 7b. The data presented in clearly demonstrate that the order of RhB removal efficiency is as follows: H₂O₂ (0.8 mol/L) > H₂O₂ (0.6 mol/L) > H₂O₂ (0.4 mol/L) > H₂O₂ (0.2 mol/L) > H₂O₂ (0.1 mol/L), and it is evident that the dosage of H₂O₂ is directly proportional to the degradation efficiency of RhB. The increase in H₂O₂ content from 0.1 mol/L to 0.8 mol/L reflected in an increase in the degrading effect of RhB from 82.1% to 98.5%. The absence of H₂O₂ also results in the absence of potent oxidising hydroxyl radicals generated by Fenton activation. This phenomenon exhibits a strong resemblance to the findings of prior investigations [55–57]. In comparison to the conventional Fenton reaction, this system demonstrates the capability to efficiently degrade RhB while necessitating significantly less H₂O₂. Considering the expenses associated with subsequent experimental studies, an H₂O₂ concentration of 0.4 mol/L was chosen.

  The results of microwave-induced RhB degradation at different RhB initial concentrations are shown in Fig. 7c. The Fe₃O₄-Microwave exhibited degradation efficiencies of 90%, 97.5%, 98.5%, and 99.9% for RhB at starting concentrations of 5, 10, 20, and 40 mg/L, respectively. Moreover, it was discovered that the augmentation in the initial concentration of RhB not only had a more notable impact on the efficiency of degradation, but also on the pace of degradation. The primary reason for this is the low concentration of catalyst in the system. As the concentration of RhB pollutant increases, a significant number of RhB molecules attach to the catalyst's surface and incompletely degraded intermediates hinder further degradation, thus leading to a decrease in the degradation efficiency and degradation rate of RhB. The initial concentration of RhB was chosen to be 10 mg/L in a series of subsequent experimental studies.

  The degrading efficiency of RhB in the MW-H₂O₂-Fe₃O₄-Microwave system (Fig. 7d) was significantly influenced by the microwave power. The degrading reaction of 50 mL of RhB with a concentration of 10 mg/L under microwave irradiation primarily took place within the initial 2 min. By increasing the microwave power from 200 W to 800 W, the degrading efficiency of TC improved from 96.4% to 97.3% within the initial 2 min period. The degradation efficiency and removal rate exhibited a substantial increase within a brief duration, whereas the degradation efficiency and microwave power demonstrated a simultaneous increase. At a reaction time of 10 min, the degradation efficiencies were essentially equal across the power range of 200–800 W. So, augmenting the microwave power can significantly reduce the reaction time and enhance the degrading efficiency. This is because the increase of microwave power, on the one hand, enhances the microwave catalytic activity of Fe₃O₄-Microwave to a certain extent, and its "hot spot" and non-thermal effects can promote the degradation of RhB. On the other hand, the microwave effectively facilitates the generation of more powerful oxidative hydroxyl radicals in the Fenton system. The combined effect of these two factors significantly enhances the efficiency of RhB degradation. The subsequent series of experiments were selected as the microwave power of 500 W.

  The role of the synergistic reaction in RhB degradation was confirmed through catalytic tests conducted in several reaction systems. The degradation of RhB under different experimental circumstances is illustrated in Fig. 8. The figure demonstrates that lacking microwave irradiation, the catalyst Fe₃O₄-Microwave alone (Str.-Fe₃O₄-Microwave) and H₂O₂ alone (Str.-H₂O₂) only managed to degrade RhB by 7.8% and 7.4% respectively, and RhB degradation was minimal in these cases. The degradation efficiency of RhB in the H₂O₂-Fe₃O₄-Microwave system was 85.6%. This high efficiency can be attributed to the ability of the transition metal ion Fe²⁺ to generate reactive chemicals (^•OH) when combined with hydrogen peroxide, while the catalytic adsorption of RhB had a minor role in the degradation process. The introduction of microwave irradiation resulted in a degradation efficiency of 3.6% for the MW-Fe₃O₄-Microwave system. Observably, the microwave radiation catalyst generates a "hot spot" that leads to a localised rise in temperature on the catalyst surface, but only a portion of the RhB is degraded as a result. The MW-H₂O₂ system exhibited a degradation efficiency of 77.6% for RhB, indicating that microwaves can induce the production of hydroxyl radicals from hydrogen peroxide to facilitate the breakdown of RhB. Thus, the use of microwaves greatly augmented the efficacy of H₂O₂ in eliminating contaminants, and the inclusion of Fe₃O₄-Microwave further intensified the decontamination potential of H₂O₂. The Fe₃O₄-Microwave catalyst exhibited exceptional performance, achieving a degrading efficiency of up to 98.5% for RhB. The kinetic constants for the degradation of RhB in the MW-H₂O₂-Fe₃O₄-Microwave system were found to be 11.2 and 16.5 times greater than those for the Str.-H₂O₂-Fe₃O₄-Microwave and MW-H₂O₂ systems, respectively (Fig. 8b). Hence, the utilisation of catalyst and microwave may efficiently facilitate the degradation of RhB through Fenton reaction, and can further optimise the process by adjusting the crystal chemical composition.

  Figure 8

  

  Figure 8.  (a) Degradation of RhB by Fe₃O₄-Microwave in different reaction systems and (b) the corresponding secondary kinetic constants.

  DownLoad: Full-Size Img PowerPoint

  The examination of a catalyst's structure and performance is crucial in determining its reusability, which in turn defines the catalyst's universality in practical wastewater treatment. Hence, we investigated the effectiveness of RhB degradation using Fe₃O₄-Microwave catalysts in the MW-H₂O₂-Fe₃O₄ system during five cycles. Following each experiment, the solution that had undergone a reaction was allowed to settle and then subjected to centrifugation. The resulting catalyst was subsequently dried in an oven and utilised for the subsequent cycle of the experiment (Fig. S2 in Supporting information). The degrading efficiency of RhB showed a progressive decline as the number of cycles of Fe₃O₄-Microwave catalyst increased. Upon initial use, the Fe₃O₄-Microwave catalyst exhibited a degrading efficiency of 98.5% for RhB in solution. Even after undergoing five cycles of use, the Fe₃O₄-Microwave catalyst maintained a high level of catalytic activity. The degradation efficiency of RhB was measured at 95%, providing further evidence of the catalyst's stability and capacity to be reused in the MW-H₂O₂-Fe₃O₄ system.

  The mineralization of organic contaminants is crucial for the degradation process of microwave-assisted Fenton catalysis. The degree of mineralisation of organic contaminants was determined using UV–visible spectrophotometry. From Figs. 9a-c, it is evident that the decrease in absorbance of Fe₃O₄-Microwave after the degradation of RhB is more pronounced compared to Fe₃O₄-Pristine and Fe₃O₄-Calcination. This suggests that RhB and its intermediates are gradually breaking down. In contrast, the intensity of the peaks at the corresponding wavelengths is noticeably higher in the other two systems. The results are further supported by the measurements of TOCs and total carbon throughout the entire degradation process. Based on the data presented in Fig. 9d, it is evident that the levels of total carbon and total organic carbon in the three systems experienced a rapid decline at 2 min. Subsequently, there was only a slight variation as time progressed. Yet, in the Fe₃O₄-Microwave system, the levels of total carbon and total organic carbon decreased more significantly over time. This suggests that the system effectively achieved the mineralization of RhB. Furthermore, the two values were closely aligned, indicating that the carbon in the solution was predominantly mineralized in the form of inorganic carbon (CO₂). When thinking about the chemical composition of RhB and the ion chromatography results (Fig. 9e), it is clear that the Fe₃O₄-Microwave system exhibited a greater concentration of Cl⁻ in the solution following the degradation of RhB, in comparison to the Fe₃O₄-Pristine and Fe₃O₄-Calcination systems. This indicates that the catalyst effectively enhances the Fenton reaction in conjunction with microwave energy. Also, the mineralized nitrogen primarily exists in the form of NO₃⁻ and NH₄⁺. And when comparing the leaching concentration of Fe in the solution after five degradation cycles to the original RhB solution, it was observed that the concentration ranged from 0.146 mg/L to 0.667 mg/L at the first instance (Fig. 9f). This concentration level complies with the wastewater discharge standard. The small amount of Fe detected in the solution may originate from the residue in the centrifuged catalyst, indicating that the degraded solution does not cause any secondary pollution to the environment. Thus, the Fe₃O₄-Microwave catalyst has remarkable structural stability in the degradation environment, making it suitable for the microwave-assisted Fenton process to mineralize RhB.

  Figure 9

  

  Figure 9.  UV–vis absorption spectra of catalytic degradation of RhB in different reaction systems: (a) Fe₃O₄-Pristine, (b) Fe₃O₄-Microwave, (c) Fe₃O₄-Calcination, (d) TC and TOC in catalytic degradation of RhB in different reaction systems. (e) Ionic concentration, (f) leaching concentration of Fe after catalytic degradation of RhB in the MW-H₂O₂-Fe₃O₄-Microwave system.

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  An investigation of the process by which degradation occurs is beneficial for advancing the research and design of catalysts, as well as for proposing degradation pathways. which in turn, allows for the optimisation of the experimental design for microwave-assisted Fenton-catalysed degradation of RhB. The active species in the MW-H₂O₂-Fe₃O₄ system for the degradation of RhB were identified using EPR tests. The results of the tests, using DMPO as a trapping agent for ^•OH and O₂^·- and TEMPO as a trapping agent for h⁺ and e⁻, are demonstrated in Fig. S3 (Supporting information). Upon subjecting the solution to microwave radiation for a duration of 10 min, Fig. S3a revealed a quadruple peak intensity ratio of 1:2:2:1 for DMPO-^•OH, confirming the presence of ^•OH in the system. Further, the spectrograms for DMPO-O₂^•− exhibited a quadruple peak intensity ratio of 1:1:1:1, indicating the presence of O₂^•− in the system. Moreover, based on the triple peak intensity ratio of 1:1:1 found for TEMPO-h⁺ and TEMPO-e⁻ in Figs. S3c and d, respectively, it may be inferred that the system contains both h⁺ and e⁻. The EPR results revealed that the Fe₃O₄ reaction system, prepared under various conditions, generated different numbers of active species, with the order being ^•OH > h⁺ > e⁻ > O₂^•−. The presence of ^•OH played a significant role in the rapid degradation of RhB. Furthermore, Fe₃O₄-Microwave exhibited a more substantial production of active species (particularly ^•OH) compared to Fe₃O₄-Pristine and Fe₃O₄-Calcination. This learning aligns with previous experimental results, which indicated that a better Fe²⁺/Fe³⁺ ratio is favourable for the degradation of organic pollutants.

  In extra to the outcomes given in Fig. S3, the MW-H₂O₂-Fe₃O₄ system detected a greater plenty of ^•OH characteristic peaks. The reason for this can be attributed to the phenomenon that, when subjected to microwaves, the catalyst generates electron-hole pairs (Eq. 1). The holes then react with OH⁻ ions produced through water ionisation, resulting in the formation of OH (Eqs. 2 and 3). Fe₃O₄-Microwave containing Fe²⁺ active site also to generate the active species OH by Fenton reaction (Eq. 4). The microwave can also directly stimulate hydrogen peroxide to produce the active species ^•OH (Eq. 5), which is the primary factor responsible for the breakdown of RhB in the MW-H₂O₂ system. Beyond that, electrons will undergo a reaction with dissolved O₂ in water, which resulted in the generation of the O₂^·− (Eq. 6), and this radical can also participate in the breakdown process of RhB. Thus, while the degradation of RhB through microwave-assisted Fenton catalysis is a multifaceted process that encompasses several reactive species and is influenced by factors such as microwave-induced thermal and non-thermal effects (Eq. 7).

  (1)

  (2)

  (3)

  (4)

  (5)

  (6)

  (7)

  As mentioned above, Fe₃O₄ exhibits a proclivity to undergo an increase in Fe²⁺ following microwave or calcination heat treatment in an argon atmosphere, while Fe³⁺ experiences the converse alteration. The presence of dual catalytic active sites in the catalyst is essential for the process of microwave-assisted Fenton. Hence, an investigation was conducted to determine the specific functions of these active sites in microwave catalysis and Fenton catalysis, and the findings are presented in Fig. 10. Firstly, Fe₃O₄ belongs to a category of materials known as magnetically lossy materials in which the electronic spin magnetic moment is a key factor affecting the magnetic loss, which is closely connected to the unpaired electrons of the ions. Several studies have demonstrated that the electronic spin magnetic moment plays a crucial role in the microwave loss mechanism, which in turn has a substantial impact on the degradation of organic matter catalysed by microwaves. The electronic configuration of Fe³⁺ at the octahedral or tetrahedral position in Fe₃O₄ is t_2g²e_g³ [58]. Similarly, the electronic configuration of Fe²⁺ at the octahedral position is t_2g²e_g⁴. Thus, the number of unpaired electrons in Fe³⁺ and Fe²⁺ is 5 and 4, respectively. These unpaired electrons result in corresponding electronic spin magnetic moments of 5 µB and 4 µB. The Rietveld refinement analysis revealed that the treatment resulted in a reduction in the Fe³⁺ content in the catalyst. Additionally, the unit cell spin magnetic moments for Fe₃O₄-Pristine, Fe₃O₄-Microwave, and Fe₃O₄-Calcination were calculated to be 111.12 µB, 111.10 µB, and 111.01 µB, respectively (Fig. 10a). Thus, the existence of Fe³⁺ as the active site of the catalyst in microwave-assisted Fenton-catalysed degradation of dyes can significantly improve performance by producing "hot spot" and non-thermal effects. In Fig. 10b, the microwave reflectances of Fe₃O₄-Pristine, Fe₃O₄-Microwave, and Fe₃O₄-Calcination were −25.85, −24.86, and 22.73 dB, respectively. Generally speaking, a measurement of −10 dB indicates that the sample's microwave absorption may achieve 90%, while a measurement of −20 dB indicates that the sample's microwave absorption can achieve 99%. Based on the aforementioned findings, it is evident that a decrease in the Fe³⁺ content in the catalyst leads to a gradual decrease in the electronic spin magnetic moment and microwave reflectivity. This observation aligns with the excellent microwave absorption activity of Fe₃O₄-Pristine.

  Figure 10

  

  Figure 10.  The content of Fe³⁺ in Fe₃O₄ with different crystal chemical components prepared under different conditions and the corresponding: (a) Electron spin magnetic moments, (b) microwave reflection loss and microwave absorption. (c) Effect of Fe₃O₄ with different crystal chemical components prepared under different conditions on Fenton catalytic degradation of RhB.

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  By eliminating the possibility of dyes being adsorbed physically by the samples, Fig. 10c demonstrates the pattern of Fenton-catalysed degradation of RhB by Fe₃O₄, which was prepared under various conditions. The figure reveals that the redox reaction primarily takes place within the initial 2 min, and the degradation of RhB gradually intensifies as the Fe³⁺ content decreases. likewise, the reaction rate constants were computed and compared using pseudo-secondary kinetics, which were applied to the catalytic degradation of RhB by Fe₃O₄ Fenton produced at various conditions. The results revealed a positive relationship between the rate constants and the variation in Fe²⁺ content. Thus, the presence of Fe²⁺ as the catalytic active site in the Fenton redox system, along with its active species generated during the Fenton redox process, can significantly improve the efficiency of dye degradation. Out all the options, the Fe₃O₄-Microwave, which was treated with a microwave, demonstrated the most effective degrading performance. In contrast, the Fe₃O₄-Pristine and Fe₃O₄-Calcination showed unsatisfactory results due to the absence of Fe²⁺ and Fe³⁺ respectively.

  Microwave-assisted Fenton-catalysed degradation of RhB is a complex process. To gain a comprehensive comprehension of the intermediates generated during the degradation of RhB, the solutions were subjected to reactions for durations of 2, 5, and 10 min in the Fe₃O₄-Pristine, Fe₃O₄-Microwave, and Fe₃O₄-Calcination degradation systems. Subsequently, the solutions were identified and analysed using liquid chromatography-mass spectrometry (LC-MS). To further analyse the resulting intermediates, the intermediate structures were acquired using mass spectrometry analysis of samples that were eluted in a high liquid state for 0.9 min and 21.9 min, which were evaluated using positive ion mode (EIS+). The degradation of RhB occurred in three different systems: Fe₃O₄-Pristine, Fe₃O₄-Microwave, and Fe₃O₄-Calcination. The degradation process lasted for 2, 5, and 10 min in each system, respectively. The mass spectra corresponding to these degradation processes can be found in Fig. S4 (Supporting information). The precise molecular weight of the product can be determined by analysing the quasi-molecular ion [M + H]⁻. After 21.9 min of elution, the mass spectra of the initial RhB solution revealed a quasi-molecular weight of m/z = 443.2269 as the molecular weight of RhB. With an increase in reaction time, RhB underwent steady degradation, resulting in the formation of several intermediates, as indicated by the mass spectra.

  The application of liquid phase-mass spectrometry enabled the identification of the intermediates generated by the degradation of RhB using microwave-assisted Fenton catalysis, and to speculate on the RhB degradation mechanism and degradation pathway during the degradation process. A total of eleven intermediates were identified using LC-MS, and the hypothetical degradation pathway of RhB is illustrated in Fig. S5 (Supporting information). It illustrates that the degradation of RhB may be primarily categorised into four distinct steps [59,60]: (1) The process of N-deethylation. The RhB molecule (m/z = 443.2269) undergoes progressive assault by hydroxyl radicals, resulting in deethylation and the formation of N,N‑diethyl-N-ethylrhodamine (R1, m/z = 414.1937), as well as aminorhodamine (R2, m/z = 254.9364), (R3, m/z = 238.9589), (R4, m/z = 222.9212). (2) Cleavage of conjugated chromophore. The chromophore of the demethylated intermediate was destroyed and cleaved to the products R5 (m/z = 152.9975), R6 (m/z = 150.0278), R7 (m/z = 149.9978) and R8 (144.0278). (3) Ring opening. The benzene ring opens into small molecular compounds such as 1, 1, 4, 4-butanetetrol (R9, m/z = 122.0331) and 4-methylpentanoic acid (R10, m/z = 124.9788). (4) Mineralisation. Small molecule compounds are eventually further mineralised to CO₂ and H₂O.

  In this work, we have synthesised nanosized microspheres of Fe₃O₄ by a hydrothermal approach. This involved modifying the chemical composition of Fe₃O₄ crystals utilising microwave shock and high temperature calcination. An investigation was conducted to examine the impact of catalyst dosage, H₂O₂ dosage, RhB starting concentration, and microwave power on the degradation of RhB. Under the specified experimental settings of Fe₃O₄-Microwave = 1.0 g/L, H₂O₂ = 0.4 mol/L, RhB = 10 mg/L, MW power = 500 W, the findings demonstrated a removal rate of 98.5% for RhB after 10 min of treatment. It shows that the catalyst has excellent degradation performance. After a treatment duration of 10 min, the results indicated that the elimination efficiency of RhB was 98.5%. The cycle life experiments confirmed the exceptional structural stability and reusability of the Fe₃O₄-Microwave catalyst. The tests demonstrate that Fe³⁺ and Fe²⁺ can serve as the microwave catalytic activity centre and Fenton catalytic activity centre, respectively, thereby significantly enhancing the degradation efficiency. Compared with Fe₃O₄-Pristine and Fe₃O₄-Calcination systems, the reaction rate of Fe₃O₄-Microwave system was increased by 5.6 times and 2.4 times, respectively. Analysis of the degradation mechanism showed that hydroxyl radicals (^•OH) play an important role as the main active species in the whole degradation of the system. Microwave synergistic Fe₃O₄-Microwave-assisted Fenton degradation of RhB was more complete and could lead to the mineralisation of RhB.

  Declaration of competing interest

  The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

  Acknowledgments

  This work is supported by Beijing Natural Science Foundation (No. 2232062) and the Fundamental Research Funds for the Central Universities (No. 2652022006).

  Supplementary materials

  Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.cclet.2024.109771.

  
  
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  Figure 1  XRD patterns of Fe₃O₄ prepared by different methods.

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  Figure 2  (a, d, g) Scanning electron microscope images and (b, c, e, f, h, i) transmission electron microscope images of three Fe₃O₄ before degradation reaction.

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  Figure 3  N₂ adsorption-desorption curves of Fe₃O₄ prepared under different conditions: (a) Fe₃O₄-Pristine, (b) Fe₃O₄-Microwave, (c) Fe₃O₄-Calcination. Corresponding pore size distribution curves of Fe₃O₄ prepared under different conditions: (d) Fe₃O₄-Pristine, (e) Fe₃O₄-Microwave, (f) Fe₃O₄-Calcination.

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  Figure 4  XPS patterns of Fe₃O₄ prepared under different conditions: (a) Fe 2p and (b) O 1s for Fe₃O₄-Pristine. (c) Fe 2p and (d) O 1s for Fe₃O₄-Microwave. (e) Fe 2p and (f) O 1s for Fe₃O₄-Calcination.

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  Figure 5  ⁵⁷Fe Mössbauer spectra of Fe₃O₄ prepared under different conditions: (a) Fe₃O₄-Pristine, (b) Fe₃O₄-Microwave, (c) Fe₃O₄-Calcination, and (d) the Fe²⁺/Fe³⁺ ratio obtained from the fitting.

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  Figure 6  (a) Effect and (b) degradation rate of Fe₃O₄ with different crystal chemical components prepared under different conditions on microwave-assisted Fenton catalytic degradation of RhB.

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  Figure 7  (a) Effect of catalyst dose, (b) H₂O₂ dose, (c) initial RhB concentration and (d) microwave power on the degradation of RhB by MW-H₂O₂-Fe₃O₄-Microwave system.

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  Figure 8  (a) Degradation of RhB by Fe₃O₄-Microwave in different reaction systems and (b) the corresponding secondary kinetic constants.

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  Figure 9  UV–vis absorption spectra of catalytic degradation of RhB in different reaction systems: (a) Fe₃O₄-Pristine, (b) Fe₃O₄-Microwave, (c) Fe₃O₄-Calcination, (d) TC and TOC in catalytic degradation of RhB in different reaction systems. (e) Ionic concentration, (f) leaching concentration of Fe after catalytic degradation of RhB in the MW-H₂O₂-Fe₃O₄-Microwave system.

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  Figure 10  The content of Fe³⁺ in Fe₃O₄ with different crystal chemical components prepared under different conditions and the corresponding: (a) Electron spin magnetic moments, (b) microwave reflection loss and microwave absorption. (c) Effect of Fe₃O₄ with different crystal chemical components prepared under different conditions on Fenton catalytic degradation of RhB.

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