Magnetically recyclable spherical Cu₂O@Fe₃O₄ S-scheme heterojunction for efficient tetracycline removal via visible light-activated peroxydisulfate

Abstract

The overuse of antibiotics in treating bacterial infections is a significant threat to the environment and human health. The utilization of visible-light-assisted peroxydisulfate (PDS) activation for eliminating organic pollutants is a promising approach. This study uses a straightforward hydrothermal method to prepare magnetically recyclable spherical Cu₂O@Fe₃O₄. The efficacy of this material in removing antibiotic pollutants was assessed using simulated wastewater containing tetracycline (TC). TC removal was achieved by activating PDS with Cu₂O@Fe₃O₄ as the visible light photocatalyst. Experimental findings revealed that under specific conditions—a pH of 9, a Cu₂O@Fe₃O₄ concentration of 60 mg L⁻¹, and a PDS concentration of 25 mg L⁻¹—the removal rate of TC reached 97.67% after 30 min of irradiation. Moreover, Cu₂O@Fe₃O₄ exhibited excellent recyclability, maintaining a removal rate of 93.33% after five recycling rounds. X-ray diffraction characterization of the Cu₂O@Fe₃O₄ composite before and after cycling confirmed its robust stability and reusability. In situ X-ray photoelectron spectroscopy analysis showed that electrons migrated from Fe₃O₄ to Cu₂O during the photocatalytic reaction, indicating the formation of an S-type heterojunction in Cu₂O@Fe₃O₄. Free radical trapping experiments demonstrated the active involvement of ·OH, ·O₂⁻, SO₄˙⁻ and h⁺ radicals in TC removal.

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Introduction

The initial oversight of antibiotics' environmental toxicity owing to their remarkable therapeutic effects led to their excessive use. Over time, this misuse has resulted in serious environmental issues. Tetracycline (TC), extensively employed in medical and livestock sectors, is a significant environmental and health threat owing to its overconsumption.¹ Its accumulation in ecosystems and subsequent impact on microbial communities exacerbates the proliferation of antibiotic resistance.² Moreover, TC jeopardizes the safety of potable and irrigation water, disrupting human intestinal flora and posing health risks to various organisms. These adverse effects have raised significant concerns about TC contamination, requiring urgent attention as a public health issue.³ Consequently, there is a pressing need to develop efficient and cost-effective methods for TC degradation.

Various methods exist for removing antibiotics from wastewater, including adsorption treatment, biological treatment, and membrane processes.^4,5 Advanced oxidation processes (AOPs) are a promising technology for the partial or complete mineralization of pollutants through highly active free radicals such as hydroxyl radicals (·OH), sulfate radicals (SO₄˙⁻), and superoxide radicals (·O₂⁻).⁶ Among these, the peroxydisulfate activation process generates SO₄˙⁻, which has a higher standard redox potential (E₀ = 2.5–3.1 V) compared to ·OH (E₀ = 1.89–2.72 V), thereby exhibiting strong oxidation capability.⁷ Photo-assisted persulfate activation (PPA) is gaining significant attention owing to its environmental friendliness, sustainability, and excellent chemical stability. PPA is widely employed to degrade antibiotics in aquatic environments.

Photocatalysis is a novel and environmentally friendly advanced oxidation technology.⁸ For photocatalysts to be highly effective, they must have abundant active sites, broad spectral absorption, rapid separation and migration rates of photogenerated electrons and holes, and robust oxidation and reduction capabilities.⁹ Among these factors, the construction of heterogeneous junctions by combining semiconductors is recognized as one of the most promising approaches for achieving photocatalytic electron–hole separation through the electric field formed by the heterojunction.¹⁰ Heterogeneous photocatalysts exhibit distinct mechanisms for photogenerated carrier transfer, stemming from variations in preparation methods, energy bands, morphologies, and structures. Based on the diverse mechanisms of photogenerated carrier transfer, heterogeneous photocatalysts are generally categorized into I-, II-, Schottky-, Z-, and S-types.¹¹ S-Type heterogeneous photocatalysts garner significant attention owing to their unique advantages.^12,13 In S-type heterogeneous photocatalytic systems, incorporating internal built-in electric fields (IEFs) effectively regulates the direction of carrier migration, enhancing the oxidation and reduction capabilities and extending the lifetimes of photogenerated holes and electrons. Thus, establishing S-type heterogeneous junctions is an effective strategy for enhancing photocatalytic efficiency.

Thus far, cuprous oxide (Cu₂O) has exhibited remarkable photocatalytic performance, boasting a band gap width aligning with the visible light wavelength range, rendering it directly excitable by solar light.¹⁴ However, its instability arises from a tendency to undergo an irreversible redox disproportionation reaction.¹⁵ On the other hand, magnetite (Fe₃O₄) demonstrates photocatalytic capabilities and possesses considerable magnetic properties, facilitating easy separation and recovery from the reaction system, resulting in its widespread adoption in photocatalysis.¹⁶ Khaled Benabbas et al. Employed a hydrothermal approach to fabricate a magnetically separable Fe₃O₄/CuO composite core–shell heterostructure photocatalyst. Fe₃O₄/CuO displayed remarkable optical characteristics, including a low band gap value and superior light absorption capabilities. Evaluation of its photocatalytic activity in DR89 removal under visible light irradiation showcased the superior performance of the Fe₃O₄/CuO composite, surpassing pure CuO and Fe₃O₄ by 11 and 8 times, respectively. Similarly, Alani et al. used the hydrothermal method to produce a visible-light-driven bio-templated magnetic copper oxide (CuO/C/Fe₃O₄) photocatalyst. They found that CuO/C/Fe₃O₄ outperformed spherical polyhedral Fe₃O₄ NPs in catalytic activity. Visible light exposure of the CuO/C/Fe₃O₄ photocatalyst increased the generation of hydroxyl radicals (·OH), superoxide anion radicals (·O₂⁻), and the redox Fe³⁺/Fe²⁺ pair, thereby enhancing photodegradation performance.

In this study, we synthesized a magnetic Cu₂O@Fe₃O₄ composite material via a thermal precipitation method. In situ X-ray photoelectron spectroscopy analysis showed that electrons migrated from Fe₃O₄ to Cu₂O during the photocatalytic reaction, indicating the formation of an S-type heterojunction in Cu₂O@Fe₃O₄. A comprehensive structural analysis of Cu₂O@Fe₃O₄ was conducted utilizing scanning electron microscopy, X-ray diffraction techniques, and energy-dispersive X-ray spectroscopy. Additionally, we confirmed its magnetic nature and strong magnetic properties. Furthermore, we elucidated the optical absorption spectrum of Cu₂O@Fe₃O₄ through UV-visible diffuse reflection spectroscopy. To assess the practical utility of the composite material, we employed a TC solution to simulate antibiotic wastewater. We applied the Cu₂O@Fe₃O₄ composite material with sodium persulfate for efficient degradation. Under illumination, photogenerated electrons facilitate the in situ redox cycling of Cu(II)/Cu(I) in Cu₂O@Fe₃O₄, significantly enhancing the activation efficiency of PDS and yielding more sulfate radicals (SO₄˙⁻). Through this experimentation, we investigated the degradation mechanism and influential factors associated with the composite material, thus experimentally validating its potential for application in antibiotic wastewater treatment.

Materials and methods

Reagents

Ferrous sulfate heptahydrate (FeSO₄·7H₂O), hydrochloric acid (HCl), sodium molybdate dihydrate (Na₂MoO₄·2H₂O), disodium ethylenediaminetetraacetate (EDTA-2Na), sodium hydroxide (NaOH), isopropanol (C₃H₈O, IPA), copper nitrate trihydrate (Cu(NO₃)₂·3H₂O), L-ascorbic acid (ASA), ethylene glycol (C₂H₆O₂), ethanol (C₂H₅OH), tetracycline (TC), sodium persulfate (Na₂S₂O₈) were purchased from China Pharmaceutical Group. In addition, all solutions were prepared using distilled water.

Synthesis of Fe₃O₄

The hydrothermal method was utilized to synthesize Fe₃O₄ by introducing ferrous sulfate (FeSO₄) as a starter material. Briefy described, 0.278 g (1 mmol) ferrous sulfate heptahydrate (FeSO₄·7H₂O) and 0.242 g (1 mmol) sodium molybdate dihydrate (Na₂MoO₄·2H₂O) were dissolved in a 15 mL distilled water, respectively. After complete dissolution, the FeSO₄ solution was transferred into the Na₂MoO₄ solution, and pH to 9 was adjusted with NaOH solution. Subsequently, the mixture was transferred into a 50 mL Tefonlined autoclave, sealed, and heated for 24 h at 180 °C followed by cooling naturally. After that, the formed precipitate was recovered by applying a magnetic field with an external magnet in order to be washed with distilled water and ethanol; once washed, the prepared catalyst was put to dry at 60 °C for 12 h and the obtained product was labelled as Fe₃O₄.

Synthesis of Cu₂O@Fe₃O₄ composite

0.2416 g (1 mmol) of copper nitrate trihydrate (Cu(NO₃)₂·3H₂O) were dissolved in 8.5 mL of ethylene glycol (C₂H₆O₂) and 17 mL of ethanol (C₂H₅OH); after being stirred for 30 min; add 25 mg of iron oxide in the medium. Subsequently, the samples were placed in an ultrasonic cleaner for 30 minutes. After that, the mixture was transferred into a 50 mL Tefonlined autoclave, sealed, and heated for 5 h at 160 °C followed by cooling naturally. After that, it was washed several times with distilled water and ethanol and finally was put to dry at 60 °C for 24 h and the obtained product was labelled as Cu₂O@Fe₃O₄.

Characterizations

The powder X-ray diffraction (XRD) measurements were performed using a D8 Advance Bruker X-ray diffractometer, utilizing Cu Kα radiation (λ = 0.5418 nm) under conditions of 56 kV and 182 mA. The diffraction data were collected over a range of 15° to 80° (2θ) at room temperature. To observe morphology and perform element mapping analysis, a scanning electron microscope (SEM-JSM-6360LV) equipped with energy dispersive X-ray spectroscopy (EDX-S-3400N) was utilized. The chemical composition and valence states of the samples were analyzed using X-ray Photoelectron Spectroscopy (XPS-Thermo ESCALAB 250), with further investigation into the charge transfer pathway. Additionally, the spectra were calibrated relative to the C1s peak at 284.8 eV. The magnetic characteristics of the samples were investigated utilizing the Quantum Design-PPMS system. The optical adsorption capacity of the catalysts was assessed using the Hitachi U4100 UV spectrometer, operating within the spectral range of 190–900 nm. Transient photocurrent and electrochemical impedance were investigated using an electrochemical workstation (CHI 852C, Germany). Photoluminescence (PL) spectra were obtained using a HORIBA spectrometer, with a monochromator slit width of 5.0 nm.

Photocatalytic activity investigations

The performance of the Cu₂O@Fe₃O₄/PDS/Vis system was evaluated by assessing the removal rate of tetracycline (TC) from aqueous solutions. Experiments were conducted using TC concentrations ranging from 10 to 40 mg L⁻¹ in a total volume of 200 mL. Reactions were carried out in a 250 mL reaction vessel equipped with a metal halide lamp as the visible light source. To establish suitable reaction conditions, the pH of the system was adjusted between 3.0 and 11.0 using HCl and NaOH. The catalyst, Cu₂O@Fe₃O₄, was dosed within the range of 0.02 to 0.1 g L⁻¹ and mixed into the 200 mL TC solution. Subsequently, a specific dosage of PDS, ranging from 0.015 to 0.045 g L⁻¹, was added to activate the photocatalytic process. Samples were collected every five minutes and filtered through a 0.22 μm filter membrane to obtain a clear liquid suitable for analysis. The concentration of TC was determined using a UV-Vis spectrophotometer (MAPADA, UV-1800PC) at a wavelength of 355 nm. The removal efficiency of TC was calculated using the following equation:
where C₀ represents the initial concentration of TC, and C_t represents the concentration of TC at a given time.

Results and discussion

Characteristics of Cu₂O@Fe₃O₄

The X-ray diffraction (XRD) patterns of Fe₃O₄ and Cu₂O@Fe₃O₄ materials are shown in Fig. 1(a) and (b). Fig. 1(a) displays distinct characteristic peaks, with observed peaks at 2θ values of 18.3°, 30.1°, 35.4°, 37.1°, 43.1°, 53.4°, 56.9°, 62.5°, 65.7°, 70.9°, 74°, 75°, and 78.9°, precisely matching the peak positions and intensities of Fe₃O₄ (PDF#76-1849). These peaks correspond to the crystallographic planes (111), (220), (311), (222), (400), (331), (422), (511), (440), (531), (620), (533), and (444) of Fe₃O₄, revealing a face-centered cubic crystal structure with space group Fd3̄m (No. 227). Similarly, Fig. 1(b) exhibits characteristic peaks, with observed peaks at 2θ values of 18.3°, 30.1°, 35.4°, 37.1°, 43.1°, 53.4°, 56.9°, 62.5°, 65.7°, 70.9°, 74°, 75°, and 78.9° (marked as ‘◆’), aligning perfectly with Fe₃O₄ (PDF#99-0073). These peaks correspond to crystallographic planes (111), (220), (311), (222), (400), (331), (422), (511), (440), (531), (620), (533), and (444), indicating a face-centered cubic crystal structure. Moreover, peaks marked as “●” at 2θ values of 29.6°, 36.4°, 42.3°, 52.5°, 61.3°, 69.6°, 73.5°, and 77.4° are confidently assigned to diffraction peaks of cubic-phase Cu₂O nanoparticles, corresponding to crystal planes (110), (111), (200), (211), (220), (310), (311), and (222) of Cu₂O (PDF#99-0041), with a face-centered cubic crystal structure and space group Fd3̄m (No. 224). Based on XRD data analysis, it can be preliminarily concluded that the synthesized sample corresponds to the desired Cu₂O@Fe₃O₄ composite material, confirming the successful implementation of the employed methodology.

Fig. 1 (a) XRD patterns of Fe₃O₄ and (b) Cu₂O@Fe₃O₄.

For further analysis of the chemical composition and states of Cu₂O@Fe₃O₄, X-ray photoelectron spectroscopy (XPS) was employed, with results presented in Fig. 2. Fig. 2(a) provides the XPS spectra, where distinct peaks for O1s, Fe2p, Cu2p, CuLM2, and other species are observed. In Fig. 2(b), the XPS spectrum of iron in the composite material is depicted, revealing binding energies of Fe2p_1/2 and Fe2p_2/3 orbitals at 722.93 and 709.23 eV, respectively, consistent with literature data for Fe₃O₄.¹⁷ Fig. 2(c) presents the energy spectrum of Cu2p in Cu₂O@Fe₃O₄, showing Cu2p_3/2 and Cu2p_2/3 orbitals at 931.5 and 951.33 eV, respectively, with a 19.83 eV difference. The absence of significant satellite peaks between them suggests the presence of Cu₂O in the prepared samples.¹⁸ As depicted in Fig. 2(d), the composite sample exhibits two distinct peaks in the 1s orbital of oxygen, with binding energies of 528.92 and 530.19 eV, corresponding to lattice oxygen and oxygen molecules adsorbed on Cu₂O.¹⁹ These XPS results further corroborate the predominance of Fe₃O₄ and Cu₂O in the processed samples.

Fig. 2 (a) XPS patterns of Cu₂O@Fe₃O₄ survey, (b) Fe2p, (c) Cu2p, (d) O1s.

Low-magnification scanning electron microscopy (SEM) images of the Cu₂O@Fe₃O₄ nanoparticles are depicted in Fig. 3(a), showcasing spherical particles with a uniform size distribution, predominantly ranging from 1000 to 1800 nm. Further high-magnification observations (Fig. 3(b) and (c)) show that the composite particles comprise numerous smaller nanoparticles, resembling the reported Cu₂O shell structure. The fabricated composite's surface composition and element distribution were quantitatively analyzed using energy-dispersive X-ray spectroscopy (EDS) techniques. Fig. 3(c)–(e) clearly show the uniform distribution of Cu, O, and Fe elements. Fig. 3(f) provides the atomic content of each element, indicating that Cu, O, and Fe constitute 17.81%, 13.12%, and 4.34%, respectively. Based on preliminary estimation, the Fe₃O₄ to Cu₂O ratio is determined to be 1 : 3. The SEM-EDS spectrum further confirms the successful synthesis of composites incorporating Fe₃O₄ and Cu₂O.

Fig. 3 (a)–(c) SEM images of Cu₂O@Fe₃O₄; (d) SEM images; (e) EDS full spectrum; (f)–(h) single element distribution map: (f) Cu; (g) Fe; (h) O; (i) the atomic content of Cu, O, and Fe.

Through vibrating-sample magnetometer analysis, magnetic measurements and studies were conducted on the Cu₂O@Fe₃O₄ composite material. As depicted in Fig. 4, the composite material exhibits distinctive magnetic hysteresis loop characteristics. With the gradual increase of magnetic field strength, the material's magnetization increases, saturating at a magnetic field strength of approximately 1500 Oe. Analysis of Fig. 4 reveals the composite material's paramagnetic properties, with a residual magnetization of 2.7 emu g⁻¹, a coercivity measurement of 95.7 Oe, and a saturation magnetization of 10.4 emu g⁻¹. These characteristics underscore the composite material's high magnetic responsiveness and potential for separation and recovery, facilitating material recycling and reuse.

Fig. 4 The hysteresis loop of the Cu₂O@Fe₃O₄.

UV-Vis spectroscopy experiments were used to investigate the optical response of Cu₂O@Fe₃O₄ composite materials. As Fig. 5(a) shows, Cu₂O@Fe₃O₄ composite materials exhibited outstanding optical response. The composite materials demonstrated superior light absorption in the spectral range from 300 to 500 nm compared to pure Fe₃O₄. Employing the Kubelka–Munk relaxation function and UV-Vis spectroscopy, the energy gap of Cu₂O@Fe₃O₄ was estimated to be 1.74 eV, which is lower than that of Fe₃O₄ (2.12 eV). Literature reports suggest that a reduction in the energy gap can expand the light absorption range, consequently enhancing photon absorption and the generation of photogenerated carriers. This ultimately results in improved utilization efficiency of visible light during photocatalytic processes.²⁰

Fig. 5 (a) UV-Vis diffuse reflectance spectra of Cu₂O@Fe₃O₄ and Fe₃O₄, (b) (αhν)² and hν curves of Fe₃O₄@Cu₂O samples, (c) (αhν)² and hν curves of Fe₃O₄ samples.

We employed photoluminescence spectra (PL), intermittent transient photocurrent response spectra, and electrochemical impedance spectroscopy (EIS) for a comprehensive understanding of photo-electrochemical carrier separation. Fig. 6(a) clearly illustrates that the photoluminescence spectra (PL) intensity of Cu₂O@Fe₃O₄ is significantly lower compared to Fe₃O₄, indicating effective separation of photo-induced electrons and holes in the composite material.²¹ The photocurrent response of Cu₂O@Fe₃O₄, as shown in Fig. 6(b), is nearly four times higher than that of Fe₃O₄. This intensified photocurrent enhances the separation of electrons and holes. Fig. 6(c) compares the EIS of Cu₂O@Fe₃O₄ and Fe₃O₄, revealing that Cu₂O@Fe₃O₄ exhibits a smaller arc radius than Fe₃O₄, facilitating the transfer of photogenerated carriers. The abovementioned results further improve the successful construction of Cu₂O@Fe₃O₄ to avoid the combination of photo-induced electrons and holes. Therefore, this further confirms that Cu₂O@Fe₃O₄ possesses superior photocatalytic performance compared to pure Fe₃O₄. The degradation effect of tetracycline was studied by Cu₂O@Fe₃O₄ and Cu₂O under light, and the results are shown in Fig. S2.† Found that Cu₂O@Fe₃O₄ was superior to Cu₂O in its degradation activity. Please watch the ESI† for detailed data.

Fig. 6 (a) Photoluminescence spectra, (b) intermittent transient photocurrent response spectra, and (c) electrochemical impedance.

Effect of operational factors

Various reaction systems were established to investigate the removal rate of TC under different conditions. According to the calculation results illustrated in Fig. 7(a), it is evident that the removal rate under visible light conditions significantly surpasses that under dark conditions. In the presence of visible light, PDS is activated, leading to the degradation of TC (62.33%). Furthermore, Cu₂O@Fe₃O₄ exhibits activation under visible light (81.86%), while Cu₂O@Fe₃O₄/PDS achieves the highest removal rate (97.67%) because both Cu₂O@Fe₃O₄ and PDS are activated synergistically.

Fig. 7 (a) Comparison of the different reaction systems; (b) effect of Cu₂O@Fe₃O₄ dosage on TC removal performance; (c) effect of pH on TC removal performance; (d) effect of PDS dosage on TC removal performance; (e) effect of the initial TC concentration on TC removal performance; (f) recyclability and stability of the catalyst.

The impact of Cu₂O@Fe₃O₄ dosage (20, 40, 60, 80, and 100 mg L⁻¹) on the removal efficiency of TC was thoroughly examined. As depicted in Fig. 7(b), with the gradual increase in Cu₂O@Fe₃O₄ dosage from 20 to 60 mg L⁻¹, the removal rate of TC increases from 89.3% to 97.6%. However, upon further increasing the dosage to 80 mg L⁻¹, the removal rate declines, dropping to 92.7%. Subsequently, at a dosage of 100 mg L⁻¹, the removal rate rebounds to 93.9%. These findings suggest that the optimal Cu₂O@Fe₃O₄ dosage for photocatalysis is 60 mg L⁻¹. At lower concentrations, increasing the catalyst dosage enhances the removal efficiency of TC. However, when the dosage surpasses a certain threshold (e.g., from 60 to 100 mg L⁻¹), the removal rate of TC decreases owing to light scattering, which reduces light absorption within the reaction system.

The initial pH of the solution plays a crucial role in TC removal. Thus, we investigated the impact of various pH values (5, 7, 9, 11, and 13) on the removal rate of TC under conditions of TC concentration at 20 mg L⁻¹ and PDS concentration at 25 mg L⁻¹. The outcomes of these experiments are presented in Fig. 7(c). Increasing the solution's initial pH from 3 to 9 led to a slight increase in the rate of pollutant elimination, albeit not significantly. However, when the pH increased from 9 to 11, the removal efficiency of TC by Cu₂O@Fe₃O₄ composites considerably decreased from 97.6% to 43.1%. This decline indicates a decrease in the removal effectiveness of Cu₂O@Fe₃O₄ composites at higher pH values. Such an effect may be attributed to the decelerated rate of chemical reactions or the onset of competing reactions under high pH conditions, thereby impeding the degradation process to some extent.

The impact of PDS dosage on TC removal was investigated, with the corresponding data presented in Fig. 7(d). The experiment was conducted at room temperature, utilizing a Cu₂O@Fe₃O₄ dosage of 60 mg L⁻¹ and a pH of 9. After 30 min, it was observed that within the range of 15–25 mmol L⁻¹, the removal rate of TC exhibited an upward trend with increasing PDS concentration. This phenomenon can be attributed to the activation of PDS by photogenerated electrons or other free radical groups on the surface of Cu₂O@Fe₃O₄, thereby enhancing TC removal capability. However, beyond a PDS concentration of 25 mg L⁻¹, the removal rate of TC sharply declines. This decline may be attributed to potential competition that interferes with TC removal.

Various concentrations of TC (10, 20, 30, and 40 mg L⁻¹) were examined to evaluate their influence on the removal rate under different initial TC concentrations. Fig. 7(e) illustrates that, at room temperature, with a Cu₂O@Fe₃O₄ dosage of 60 mg L⁻¹, a solution pH of 9, and a PDS dosage of 25 mg L⁻¹, the initial TC concentration affects its removal rate. As the initial TC concentration increases from 10 to 20 mg L⁻¹, the removal rate gradually increases from 96.5% to 97.6%. However, upon further increasing the initial TC concentration to 90 mg L⁻¹, the removal rate declines to 20 mg L⁻¹. The increased TC concentration makes the wastewater deficient in sufficient radical oxidation, significantly reducing the removal rate.

To assess the stability of Cu₂O@Fe₃O₄, five cyclic reaction experiments were conducted under optimal conditions (room temperature, initial pH 9, TC dosage of 20 mg L⁻¹, Cu₂O@Fe₃O₄ composite catalyst dosage of 60 mg L⁻¹, and PDS dosage of 25 mg L⁻¹). The reacted Cu₂O@Fe₃O₄ composite material was collected using a magnet, washed, dried, and reused for subsequent cycles. As depicted in Fig. 7(f), the TC removal rates for the five cyclic experiments were 97.6%, 97.2%, 97%, 95.8%, and 93.3%, respectively. These results indicate a gradual decrease in the TC removal efficiency of the Cu₂O@Fe₃O₄ composite catalyst with increasing cycles. From the first to the fifth cycle, the TC removal rate consistently declined, suggesting a possible loss or deactivation of the catalyst's activity. This decline could be attributed to the blockage or damage of active sites on the catalyst surface, leading to reduced catalytic performance.

Complex XRD contrast after five cycling reactions

The Cu₂O@Fe₃O₄ composite underwent five cycles, with XRD performed before and after each cycle. Fig. 8 compares the XRD patterns of the composite before and after five reaction cycles. It was noted that the main diffraction peaks of the composite remained largely unchanged before and after cycling, indicating sustained stability in the crystal structure of the composite throughout the cycling process. However, there was a noticeable decrease in the intensity of each peak, suggesting that the cycling reactions might have reduced the material's phase purity.

Fig. 8 XRD patterns of Cu₂O@Fe₃O₄ contrast after five cycling reactions.

In situ X-ray photoelectron spectroscopy analysis

Fig. 9(a) compares the XPS spectra before and after illumination, revealing distinct peaks for O1p, Fe2p, Cu2p, CuLM2, and other species. Fig. 9(b)–(e) depict the XPS spectra of Cu₂O@Fe₃O₄ before and after illumination, demonstrating shifts in the peaks corresponding to O1s, Fe2p, Cu2p, and CuLM2. Upon illumination, the binding energy of Fe2p_1/2 and Fe2p_2/3 increased by 0.08 and 0.37 eV, respectively, indicating a decrease in the electron density of Fe₃O₄. Conversely, the binding energy of Cu2p_2/3 and Cu2p_3/2 decreased by 0.11 and 0.14 eV, respectively, indicating an increase in the electron density of Cu₂O.²² These observations suggest the transfer of electrons from Fe₃O₄ to Cu₂O.

Fig. 9 (a) In situ XPS full spectrum image; (b) O1s spectrum image; (c) Fe2p spectrum image; (d) CuLM2 spectrum image; (e) Cu2p spectrum image.

Free radical capture experiments

Previous studies have identified hydroxyl radical (·OH), superoxide radical (·O₂⁻), holes (h⁺), and sulfate radical (SO₄˙⁻) as the radicals present in the AOP system. To identify the specific free radicals responsible for oxidizing TC in wastewater during our experiments, scavengers including acetylsalicylic acid (ASA), isopropanol (IPA), disodium ethylenediaminetetraacetate (EDTA-2Na), and ethanol (EtOH) were introduced to target ·O₂⁻, ·OH, h⁺, and SO₄˙⁻, respectively. The results of these experiments are depicted in Fig. 10. Upon the addition of EDTA-2Na, ASA, IPA, and EtOH, the removal rate of TC decreased to 34.88%, 37.98%, 59.84%, or 86.82%, respectively. Hence, there is clear evidence that h⁺, ·O₂⁻, SO₄˙⁻ and ·OH play pivotal roles in the photocatalytic activation of sodium persulfate for TC removal.

Fig. 10 Degradation of TC by Cu₂O@Fe₃O₄ composite catalyst in different free radical scavengers.

Mechanistic analysis

According to the literature, Cu₂O and Fe₃O₄ exhibit distinct energy levels for their conduction and valence bands. Cu₂O features a conduction band at −1.2 eV and a valence band at 0.8 eV,²³ whereas Fe₃O₄ showcases a conduction band at 0.48 eV and a valence band at 2.08 eV.²⁴ When Cu₂O and Fe₃O₄ come into close contact under illumination, significant chemical interactions and charge redistribution at the interface facilitate electron transfer from Cu₂O to Fe₃O₄ (Fig. 11). This process generates an internal electric field (IEF) between the positively charged Cu₂O and negatively charged Fe₃O₄, accompanied by band bending. The combined effect of IEF and band bending excites Cu₂O and Fe₃O₄, prompting photogenerated electrons to migrate from the conduction band of Fe₃O₄ to the valence band of Cu₂O, where they recombine with holes. This photocatalytic mechanism follows an S-type mechanism. As a result, rapid recombination of weak charge carriers occurs, effectively preserving the potent h⁺ and e⁻ in the VB of Fe₃O₄ and e⁻ in the VB of Fe₃O₄ and CB of Cu₂O.^25,26

Fig. 11 Cu₂O@Fe₃O₄ mechanism of removal of TC by photocatalytic activation of PDS.

Under illumination, PDS undergoes activation, generating sulfate radicals (SO₄˙⁻) directly, and can also be further activated by acquiring free electrons. During this process, photogenerated electrons facilitate the in situ redox cycling of Cu(II)/Cu(I) in Cu₂O@Fe₃O₄, significantly enhancing the activation efficiency of PDS and yielding more sulfate radicals (SO₄˙⁻). These radicals may partially convert to ·OH and ·O₂⁻ during the reaction, subsequently combining with water molecules or dissolved oxygen to form species with potent oxidizing ability.

Furthermore, when free photo-excited electrons react with dissolved oxygen (DO) molecules, ·OH and ·O₂⁻ radicals are also generated. These radicals play a crucial role in degrading organic pollutants. Simultaneously, photogenerated holes directly participate in the decomposition of TC, accelerating the separation of electrons and holes, thereby enhancing the overall photocatalytic efficiency.

Conclusion

This study reports the preparation of a magnetic S-type heterojunction photocatalyst, Cu₂O@Fe₃O₄, via a simple hydrothermal method. Characterization through XRD, SEM, and EDS confirmed the successful synthesis of the Cu₂O@Fe₃O₄ composite material. Magnetic measurements revealed significant magnetic hysteresis behavior in Cu₂O@Fe₃O₄, indicating strong magnetic responsiveness and facilitating effective separation, recycling, and reuse of the composite material. UV-Vis spectroscopy experiments demonstrated superior light absorption performance of Cu₂O@Fe₃O₄ compared to pure Fe₃O₄ in the 300–500 nm spectral range. This composite material's estimated band gap was 1.74 eV, lower than the 2.12 eV of Fe₃O₄. This reduction in band gap widened the light absorption range, enhancing light absorption and the generation of photogenerated charge carriers, thereby improving the photocatalytic activity under visible light irradiation. The construction of the S-type heterojunction enabled efficient charge separation and optimal photo-redox power, leading to enhanced generation of active substances and improved photocatalytic performance of Cu₂O@Fe₃O₄.

The Cu₂O@Fe₃O₄ sample demonstrates remarkable photocatalytic efficiency in the PDS activation system. Under the conditions of pH 9, composite material concentration of 60 mg L⁻¹, and PDS concentration of 25 mg L⁻¹, 97.67% of TC is removed within 30 min. The degradation efficiency remains consistent even after five rounds of recycling, showcasing the excellent recyclability of Cu₂O@Fe₃O₄. Free radical trapping experiments revealed that the active radical species responsible for TC removal include h⁺, ·O₂⁻, SO₄˙⁻ and ·OH.

Data availability

The authors confirm that the data supporting the findings of this study are available within the article.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This research was funded by the Basic Scientific Research Youth Program (JYTQN2023373) of the Education Department of Liaoning Province and Muyuan Laboratory Research Program (14116022402, 14116022401).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4ra04174g

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