Continuous Joule Heating for Scalable and General Fusion Ternary Metal Oxides with Triple-Active Fenton-Like Activity

Engineering ›› DOI: 10.1016/j.eng.2025.06.004

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Abstract

Multimetal oxide with asymmetric atomic sites offers potential solutions for Fenton-like reactions, while their pilot-scale synthesis remains challenging. Herein, we develop a continuous flash Joule heating method using a programmable logic controller with robotic arms to accomplish proof-of-concept of scalable production. The pilot-scale product (178.3 kg·h⁻¹·m⁻² electrode) of fusion ternary metal oxides was achieved for flow-through water treatment. Integrating multiple reaction electrodes with respective independent power further outlined an increased production path. Experiments and density functional theory calculations proved that fusion CuVFeO structure achieved the dual functionality of organics adsorption on Cu sites and peroxydisulfate activation on Fe sites. The synergistic reaction can be strengthened by V doping endowed with a d band center, leading to an increased Fe Bader charge. Therefore, triple site effects shorten the reaction distance between free radicals (SO₄^•− and •OH) and organics, enhancing free radicals’ utilization and production efficiency. CuVFe secures superior performance during long-term operations (1455 min) in a continuous flow-through device. flash Joule heating characterization determined multi transition metals (CuVFe, CoVFe, MgVFe) can be generally synthesized with a superior catalytic performance. Undoubtedly, continuous flash Joule heating sheds light on developing advanced oxidation materials for pilot-scale wastewater treatment.

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Flash Joule heating / Multimetal oxide / Fenton-like / Pilot-scale synthesis

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Xiangdong Zhu, Beibei Xiao, Fengbo Yu, Chao Jia, Liming Sun, Shicheng Zhang, Lianli Wang, Peixin Cui, Liang Wang, Xiaoguang Duan, Shaobin Wang, Yujun Wang. Continuous Joule Heating for Scalable and General Fusion Ternary Metal Oxides with Triple-Active Fenton-Like Activity. Engineering DOI:10.1016/j.eng.2025.06.004

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1. Introduction

Transition metal oxides, such as spinel oxides, have become the best option for enhancing reaction oxygen species (ROS) productivity and utilization efficiency due to the structurally tunable [[1], [2], [3], [4]]. For example, Fe–O–Mn superexchange interaction and the asymmetric tripe-atomic site (metal₁–oxygen–metal₂) contribute to the activation of the oxide agent due to the intermediate oxygen atom as a bridge for facilitating electron transfer [2,5]. In addition, multi-active sites have also been developed as different function sites, such as adsorption for organic pollutants and activation agents for oxygen [[6], [7], [8]]. However, multimetal catalysts, such as ternary metals or beyond, were usually prepared in conventional technology with a long-duration process (-hours), and the characteristics of materials are inferior due to phase separation and aggregation [9,10]. Meanwhile, such fancy materials cannot be scalable due to their low yield and tedious processes.

Current-induced flash Joule heating (FJH) technique has recently been conspicuous due to the distinct advantages for synthesizing various advanced materials [9,11,12]. Therefore, rapid heating and cooling in FJH reactions can promote the fusion of different metals, which is beneficial for enhancing the synergistic action of catalytic reactions [13,14]. However, the scalability of this technology still presents certain challenges [15]. Many researchers try to scale up reactors in a batch type [15]. However, this approach is inefficient for material production and usually leads to declined material performance due to the decreased power density. According to the equation (

p = \frac{1}{2 m} C {(U}_{2}^{2} - {U_{1}}^{2})

, U2 and U1 represent the voltages before and after the reaction), power density (p) decreased with increased mass (m) at constant voltage [16,17]. As the mass of the precursor increases, the large-capacity capacitor box will increase multiplicatively to achieve a specific energy density. Consequently, increased current and unutilized energy can damage system circuits and components of FJH devices, which may pose potential safety hazards [18].

Recently, the roll-to-roll manufacturing for scalable production of materials would be realizable but remains envisaging [15,19]. From the batch type to continuing Joule heating production, how to automatically execute, the reactor size, the capacitor box capacity, and the selection of precursors would be significant for the material performances and smooth operation. Therefore, we developed a continuing Joule heating production with a suitable amplification reactor (bore size of quartz tube is 10 mm) by a programmable logic controller (PLC) platform with a robotic arm for achieving the proof-of-concept of scalable production [20]. Only upon completion of the proof-of-concept, the integration of multiple reaction electrodes corresponding to respective independent power in one device can be further effectively designed for scalable production.

In Fenton-like reaction, multivalent metals (especially for transition metals) are significant catalytic elements for activating oxidants to produce ROS, such as Fe, Cu, Co, Mn, Mo, V elements, and so forth [21]. Considering the abundance of Earth’s elemental resources, cost-effectiveness, and environmental compatibility, this study selected Cu, V, and Fe elements as the focus of investigation. The metal-responding mechanism was elucidated by synthesizing various metal materials to achieve customized on-demand in advanced oxidation applications. The synthesis–structure–properties relationships, synergistic mechanism, and stability of performance and structure were also studied. Meanwhile, a continuous water treatment application was further developed to evaluate the practical applicability of the method.

2. Experimental section

2.1. Synthesis of ternary metal material by continuous FJH method

Compared to the batch-type FJH method, all processes were controlled by a PLC platform in the continuous FJH synthesis. To replace manual operations for achieving continuous synthesis, the following critical challenges must be addressed: ①How to accurately put the power sample into the react position; ②how to tightly compress the powder sample to achieve a closed circuit; ③how to keep vacuum during the reacted process. Therefore, two robotic arm workstations were designed with an imputation algorithm based on locking the coordinates to accurately grasp the sample. In addition, we employed vertical placement and spring squeezing to achieve precise sample compression and circuit closure. Meanwhile, a vacuum chamber is utilized as the reactor, achieving a seal through rubber rings. Finally, the automation program was written using CANopen Builder 6.06software for PLC. Therefore, based on the PLC platform, integrating multiple modules, precisely coordinated with each other, can achieve all the steps to enhance production efficiency. Briefly, the precursor (0.2 g) was put into a quartz tube, the copper mesh was placed on both ends of the quartz tube, and one end of the quartz tube was placed on a copper electrode in the loading position. After completing the aforementioned steps, the continuous FJH equipment can be operated according to the designated procedure. We found the optimal synthetic condition was set at 250 V for 30 ms for ternary metal material (Cu, V, and Fe elements). To evaluate the stability of continuous FJH synthesis, we selected five batches of synthesized samples and assessed the performance and structural changes of these five batches of materials separately.

2.2. Materials synthesis by FJH for illuminating structure–property relationship

Hydrochar powder (0.5 g; detailed information was provided in Appendix A) with unitary, binary, and ternary metals (cupric acetate, vanadium chlorine, and hydroxydiacetyl iron powder, the total molar quantities were 2.8 mmol) were mixed with uniformity in deionized water by oscillation and then dried. Different metals from the binary and ternary materials were added with equivalent molar quantities. Finally, carbon black at 10 wt% was mixed with dried solids as different precursors for FJH treatment. The precursor (80 mg) was put into a quartz tube and compressed with copper electrodes to minimize the sample resistance as far as possible. Then, a mild vacuum (more than 38 kPa) was kept in the reaction chamber to avoid sample oxidation. Finally, different metal/carbon materials (Cu, V, Fe, CuV, CuFe, VFe, and CuVFe) were synthesized by the FJH method. The operation voltage and reaction time were 250 V and 30 ms three times.

2.3. Material Characterizations

X-ray absorption spectra (XAS), including X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS), were analyzed for the different metal/carbon materials. The K-edge analyses of Cu, V, and Fe were performed with Si (111) crystal monochromators at the BL14W1 beamlines at the Shanghai Synchrotron Radiation Facility (SSRF; China). Detailed information can be seen in Appendix A.

Density functional theory (DFT) calculation evaluated the adsorption configurations of S₂O₈, SO₄, and HSO₄ on the graphene-supported Fe₂CuO₄ (222) and V-doped Fe₂CuO₄ (222) since the (222) facets have been identified by high resolution transmission electron microscope (HRTEM; FEI Tecnai F20, USA). The mentioned heterostructures were constructed by graphene with a supercell (5 × 5) and Fe₂CuO₄ (222) with a supercell (1 × 1). In this case, the lattice mismatching is 3.8%, and graphene is wrinkled under compression. More detailed information can be seen in the supplementary information.

3. Results and Discussion

3.1. Proof-of-concept of scalable production by continuous FJH system.

A continuous Joule heating system was constructed for multimetal oxide materials (CuVFe as representative of the ternary metal oxides) to facilitate pilot-scale application. The diagrammatic sketch and practical view of the continuous Joule heating device for scale-up preparation are shown in Figs. S1 and S2 in Appendix A. All the processes were controlled by a PLC platform (Fig. 1(a)). Two robotic arm workstations for grasping samples significantly improved the production efficiency (Movie S1 in Appendix A). This method can produce material up to 178.3 kg·h⁻¹·m⁻² electrode (Table S1 in Appendix A). According to the material demand, the next-generation device’s production rate can be further regulated via installment parallel reactors and independent power supply in a single apparatus (Fig. 1(b)). A reactor corresponding to an independent power for each reaction electrode can overcome the issues of reduced power density and interference between different reactors.

The reacted voltage and current indicated a similar variation trend (Fig. 1(c)). Meanwhile, all the maximum currents were about 260 A, indicating similar current-induced Joule heat and temperatures for breaking chemical bonds of precursors to produce active components. As anticipated, all the obtained materials exhibit an excellent and stable removal efficiency (above 90%) and are slightly different in the kinetics due to the difference in metal contents (Fig. 1(d) and Table S2 in Appendix A). The significant CuVFeO fusion structure (PDF No.75-1519 for Fe₂VO₄ and PDF No.01-077-0427 for CuFe₂O₄) was successfully synthesized (Fig. 1(e) and Fig. S3 in Appendix A). Meanwhile, Raman indicated the existing structure of few-layer graphene due to the ultra-high temperature and rapid cooling rate (Fig. S4 in Appendix A). Materials prepared by continuous Joule heating also have a similar surface area and pore size distribution (Fig. S5 and Table S3 in Appendix A). Therefore, it can be proved that the synthetic processes of continuous Joule heating for catalytic properties and multimetal oxide structure were stable in scale-up applications.

3.2. Synergistic activity of FJH-derived multimetal material.

After the proof-of-concept of scalable production, structure-property relationships were revealed to elucidate why ternary metal/carbon materials were used for catalytic reaction (Fig. 2(a)). Chloramphenicol (CAP) removal efficiency was found to be as follows: CuVFe or VFe > CuV or V > CuFe or Fe > Cu. CuVFe and VFe materials have excellent removal efficiency, especially regarding the kinetic rate constant (Fig. 2(b) and Fig. S6 in Appendix A). In addition, only Cu was inefficient for CAP removal according to the high metal Cu contents (Table S4 in Appendix A). CuVFe contains lower Fe and V contents than those of VFe, but has a similar CAP removal efficiency after adding the Cu element. Therefore, CuVFe material has a favorable structure for improving the CAP removal efficiency. We selected Cu (unary), CuV (binary), and CuVFe (ternary) materials as the main objects. The adsorption efficiencies of V, Cu, CuV, and CuVFe materials were 4.9%, 22.2%, 8.8%, and 9.5%, respectively (Fig. S7(a) and (b) in Appendix A). The Brunauer–Emmett–Teller (BET) surface areas of V, Cu, CuV, and CuVFe materials were 15.5, 18.5, 27.4, and 30.1 m²·g⁻¹, respectively (Fig. S7(c) and Table S5 in Appendix A). N₂ adsorption isotherm exhibits a typical IV isotherm with H3 hysteresis loop, suggesting the presence of mesopores [22,23]. The CAP adsorption was uncorrelated with the pore structure of materials (Fig. S7(d) in Appendix A). Therefore, Cu components in Cu-containing material would facilitate CAP adsorption, while CuV and CuVFe would decompose CAP via peroxydisulfate (PDS) activation [24,25].

3.3. FJH-derived fusion ternary metal oxide structure.

High active components can be induced by the current-derived ultra-high temperature (-3800 K within milliseconds; Figs. S8 and S9 in Appendix A and Fig. 2(c)) [26]. Regarding the morphology and microstructure, scanning electron microscope (SEM; Sigma 300, Germany) and transmission electron microscopy (TEM; FEI Tecnai F20, USA) show many sphere metal particles on the surface of the carbon substrate (Fig. S10 in Appendix A) [27]. The small metal particles were uniformly distributed with an average size of 46.2 nm for Cu, 33.0 nm for CuV, and 40.8 nm for CuVFe due to the rapid cooling (Fig. S11 in Appendix A) [28,29]. In addition, the metal and oxygen were distributed within the same particle characterized by TEM with an energy dispersive spectrometer (EDS), which indicated the existing fusion CuVO and CuVFeO structure (Fig. S12 in Appendix A). Meanwhile, the fusion CuVO and CuVFeO structures were further validated by the high-resolution TEM with EDS and ⁵⁷Fe Mössbauer spectroscopy (Figs. 2(d), 3(a), and 3(b) and Table S6 in Appendix A) [9,28,30,31].

Meanwhile, X-ray diffractometer (XRD; Smartlab, Japan) and selected area electron diffraction (SAED) results also indicated a binary metal oxide as CuVO (Cu_0.4V₂O₅, PDF No.46-0361) in CuV, ternary metal oxide as CuVFeO (PDF No.75-1519 for Fe₂VO₄ and PDF No.01-077-0427 for CuFe₂O₄) (Figs. S13 and S14 in Appendix A) [29]. It can be inferred that ultra-high temperatures can break the chemical bond of precursors to produce metallic oxide and promote the liquefaction of the metallic oxide, which can be solidified to form a multimetal oxide structure [29,32,33]. Meanwhile, Cu⁰ and Cu₂O in Cu material, VC in CuV material, and partial VOx in CuVFe material were produced by rapid thermal decomposition and carbothermal reduction reactions [[34], [35], [36]].

X-ray photoelectron spectroscopy (XPS; Thermo Kalpha, USA) results also indicated that multimetal oxide was formed with increasing metallic elements (Fig. S15 and Table S7 in Appendix A) [[37], [38], [39]]. The reason was that the weaker oxidation resistance of metal and entropy-increasing contribute to synthesizing multimetal oxide [40,41]. X-ray absorption near-edge structure (XANES) spectra and fitting valence results further indicated the values of the chemical valence of Cu element in CuV and CuVFe were similar and situated between Cu foil and Cu₂O, suggesting the existing Cu–O and Cu–Cu bonds [42]. In the same way, the values of the chemical valence of V and Fe elements were close to the values of V₂O₅ and Fe₂O₃ (Fig. S16 in Appendix A). Cu–O and Cu–Cu bonds were also verified by Fourier-transformed k³-weighted extended X-ray absorption fine structure (EXAFS) spectra, while the V–V and Fe–Fe bonds did not exist due to the radial distance shifting (Fig. S17 in Appendix A) [43,44]. Fitting results of EXAFS indicated that Cu–O, V–O/C, Cu–O–V, and Cu–Cu bonds existed in CuV, while the coordination number of Cu–Cu was lower than that of Cu foil because of the fusion CuVO structure (Fig. 3(c) and Tables S8–S10 in Appendix A). Similarly, the presence of metal–O and metal–O–metal bonds was confirmed, aligning with the XRD results attributed to the metal fusion processes occurring under ultra-high temperatures [9,39]. The wavelet transforms (WT) results were also consistent with those of the XANES and EXAFS analyses (Fig. S18 in Appendix A) [45]. All the above results indicated again that fusion CuVO and CuVFeO structures were formed in the FJH processes.

A high degree of graphitization for enhancing electron transfer was further confirmed by the weak D peaks for defect and strong G and 2D peaks for graphitization and layers (Fig. 3(d) and Fig. S19 in Appendix A) [11]. In addition, multilayer graphene can be shocked by the high current to form the few-layer graphene verified by the I_2D/I_G values, and I represnt the peak area (Table S11 in Appendix A) [46]. The linear sweep voltammetry and electrochemical impedance spectroscopy curves further proved the greater electron transfer ability of CuVFe than that of CuV and Cu due to higher catalytic ability (Fig. S20 in Appendix A and Fig. 3(e)). Meanwhile, adding PDS in CuVFe system can trigger a higher current due to PDS activation (Fig. 3(f)). In contrast, the current decreased upon adding CAP due to the rapid consumption of PDS-evolved reactive species. Therefore, FJH processes contributed to forming the fusion structure embedded in layered graphene, facilitating fast PDS activation and efficient removal of organic pollutants [2].

3.4. Synergistic mechanism of fusion ternary metal oxides.

Electron paramagnetic resonance (EPR; Bruker EMXplus, Germany) illustrated a sequentially increased intensity of the •OH peak in Cu, CuV, and CuVFe, while the intensity of ¹O₂ peak was similar (Fig. 4(a) and Fig. S21(a) in Appendix A). Results indicated that ¹O₂ was produced by the self-decomposition of PDS, and only PDS demonstrated a minor contribution of CAP removal. The CAP removal experiments in D₂O system also indicated that ¹O₂ was not the active species for CAP removal (Fig. S21(b) in Appendix A). Therefore, •OH may have dominantly contributed to CAP removal (Fig. 4(b)) [47]. The relationship between PDS activation and CAP removal efficiency further indicated that PDS consumption is independent of CAP addition, distinct from the nonradical electron transfer pathway, suggesting that CuVFe activated PDS to produce free radicals such as •OH (Fig. S21(c) in Appendix A and Fig. 4(c)). Quenching experiments illustrated that the •OH and SO₄^•− were the main active species for oxidation of CAP in the material surface (Fig. S22 in Appendix A). The absence of SO₄^•− in EPR may be that SO₄^•− existed in the material surface due to the strong adsorption ability and the slower capture efficiency of 5,5-dimethyl-1-pyrroline N-oxide (DMPO) for SO₄^•−(k_{SO4•−, DMPO} = 2.7 × 10⁻² (mol·L)⁻¹·s⁻¹ [48]) than that for •OH [48,49]. Meanwhile, the rapid hydrolysis rate of highly unstable DMPO^•−SO₄⁻ into DMPO^•−OH via nucleophilic substitution [50]. The relative contribution of •OH was increased in CuV and CuVFe (Fig. 4(d)) [6,51]. In addition, the CAP removal rate by commercial VO_x and VC was lower than that of CuV and CuVFe (Fig. S23 in Appendix A). Therefore, it can be inferred that fusion structures (CuVFeO) were the dominant active metal sites for organic pollutant removal. The ratio of Cu^0/+/Cu²⁺ and Fe²⁺/Fe³⁺ were changed in the catalytic process, while the ratio of V³⁺/V⁵⁺ was similar (Fig. S24 and Table S12 in Appendix A). It can be inferred that Fe site provided the electrons to PDS, while Cu site acquired the electrons from the organic pollutant during the catalytic process by the CuVFe material. V site may improve the coordination environment of Cu and Fe and contribute to activation efficiency. The Cu component was beneficial for CAP adsorption, as verified by the adsorption tests (Fig. S5(a)).

Furthermore, we used the same CuVFe precursor to prepare a CuVFe material by a conventional pyrolysis method. The CAP removal efficiency was inferior (43% removal efficiency) to that of the material prepared via FJH method (Fig. S25(a) in Appendix A). XRD illustrated that Fe₂VO₄ and Cu⁰ were produced, while the fusion CuVFeO did not appear at a limited temperature for hours due to the low compatibility of Cu, V, and Fe (Fig. S25(b) in Appendix A). Meanwhile, a low degree of graphitization and aggregation existed (Fig. S25(c) in Appendix A), which led to a lower current than that of CuVFe prepared by FJH (Fig. S25(d) in Appendix A). According to the EPR, the CAP removal rate decreased due to the weak intensity of •OH (Fig. S26 in Appendix A). Therefore, the synergistic action in the fusion structure was achieved, which can improve the utilization and production efficiency of radicals.

3.5. Density functional theory (DFT) calculations.

Furthermore, we theoretically unraveled the importance of fusion CuVFeO structure in the PDS activation. To simplify the complexity of the structure, we referred to the XRD result of CuVFe material. The structures of Fe₂CuO₄/graphene, Fe₂CuO₄-V1/graphene, and Fe₂CuO₄-V2/graphene were established (Figs. S27–S29 in Appendix A), and Fe₂CuO₄-V1 and Fe₂CuO₄-V2 represented the substitution of V in different Fe atoms (V atom is in-frame in top view). The adsorption energy values of PDS on Fe₂CuO₄/graphene, Fe₂CuO₄-V1/graphene, and Fe₂CuO₄-V2/graphene systems were −7.77, −7.87, and −8.15 eV, respectively, indicating the substitution of V enhances the adsorption ability toward PDS activation [25]. The high adsorption ability also indicated strong adsorption for SO₄^•−, which is consistent with the absence of SO₄^•− in EPR. Such deduction is further supported by the electron transfer from the catalytic material to PDS as well as the increased O-O bond length (Table S13 and Fig. S30 in Appendix A) [51,52]. Similarly, after the V substitution, the adsorption energy for SO₄ and HSO₄ was increased, which indicated that SO₄ adsorbed in material easily abstracts hydrogen from the solution system for the conversion from SO₄^•− to •OH [51,53].

We also evaluated the energy barrier of O–O bond breakage and found the lower energy barrier in the CuVFeO fusion structure. The energy barriers in PDS/Fe₂CuO₄-V1/graphene and PDS/Fe₂CuO₄-V2/graphene systems were 0.33 and 0.38 eV, respectively, while that of PDS/Fe₂CuO₄/graphene was 0.75 eV (Fig. S31(a) in Appendix A). Theoretically, a reaction with a reaction barrier less than 0.75 eV is expected to easily occur at room temperature [54]. It unveils the good activation ability of CuVFeO fusion structure toward PDS. Furthermore, the reduced reaction barriers supported the promotion effect by the V substitution toward O–O breakage (Fig. 4(e) and Fig. S31(b) in Appendix A). The improvement stems from the higher d band as well as the different electronegativity by the V dopant relative to the Fe element, which is evidenced by the upshifted d band center (Fig. S32 in Appendix A) and increased Bader charge at the Fe site (Fig. S33 in Appendix A). Thus, the Fe site, with a higher electron density, promotes its interactions with the peroxyl oxygen in PDS due to the strong interaction between V–O–Fe atoms (Fig. 4(f) and Fig. S33). Therefore, DFT calculations suggested that the fusion CuVFeO structure was important for activating PDS to produce the SO₄^•− and •OH. The catalytic mechanism can be proposed based on experimental results and density functional theory (DFT) calculations (Fig. 4(g)). Triple site effects can shorten the reaction distance between free radicals and organics. Meanwhile, asymmetric atomic sites (metal–O–metal) were beneficial for electron transfer to enhance free radicals’ utilization and activation efficiency [2,6].

The possible by-products from CAP degradation were analyzed. Results indicated five by-products were produced (Table S14 in Appendix A). With the reaction time increasing, the peak area of CAP was decreased, while that of other intermediate products was increased, suggesting the CAP degradation. These intermediate products of small molecular components were produced by oxidation, dehydrogenation, cleavage of the amide group, and hydroxylation [25]. Importantly, these tiny molecular components were further mineralized into inorganic ions (such as CO₂, NO₃⁻ and Cl⁻). Therefore, the proposed degradation pathway is shown in Fig. S34 in Appendix A.

In addition, different organic pollutants, including CAP, florfenicol (FF), sulfamethoxazole (SMX), and bisphenol A (BPA) were efficiently removed by the produced CuVFe (Fig. S35(a) in Appendix A). The removal efficiency of CuVFe material was also higher than other metal-based materials prepared by other methods (Fig. 5(a) and Table S15 in Appendix A). CuVFe material also has a universality for CAP removal in different pH and cyclic CAP degradation of CuVFe further indicated CAP removal was superior (over 95%) between cycle 2 (Figs. S35(b) and (c) in Appendix A). In the third cycle, the CAP removal rate rapidly decreased due to the substantial loss of active metal. Thus, FJH-derived CuVFe material has excellent removal efficiency for various organic pollutants and different pH systems, which is crucial for wastewater treatment due to the complex wastewater systems. In addition, FJH-derived material from different ternary metal/carbon precursors always presented superior CAP degradation efficiency (Fig. 5(b)), suggesting that FJH, as a state-of-the-art technology, held great scalable and general promise for mass production of nanostructured catalysts (CuVFe, CoVFe, MgVFe) for wastewater treatment.

Meanwhile, a wastewater treatment device and diagrammatic sketch of the scale-up treatment in the future are shown in Fig. S36 in Appendix A and Fig. 5(c). The CAP in lake water was treated, and the removal rate was achieved continuously at more than 90% for 1455 min with a stable flow rate (Fig. 5(d)). The material prepared by FJH method has a superb performance and stability for water pollutant removal. Meanwhile, the excellent synthetic stability, scalability, and removal efficiency illustrate that continuous Joule heating has significant practical application promise. Compared to the conventional methods such as pyrolysis and hydrothermal carbonization, FJH method enables instantaneous heating to 3400 ^oC, with the reaction completing within milliseconds (Fig. S37 in Appendix A). Ultra-high temperatures can break the chemical bond of precursors promote the liquefaction of the multimetal oxide, and further form a multimetal oxide within milliseconds. And FJH method also has a great potential application due to the higher product efficiency and lower energy consumption (Tables S16 and S17 in Appendix A).

4. Conclusions

Proof-of-concept of scalable and general production was successfully achieved using a PLC-based continuous FJH method with a reactor corresponding to an independent power. In addition, the ultra-high temperature and rapid cooling rate can overcome their compatibility issues among multiple metal components for synthesizing fusion CuVFeO structures. CuVFe with a fusion structure can achieve synergistic PDS activation and pollutant removal coordinated by the triple catalytic sites. Fe catalyzes PDS activation and Cu serves as the pollutant adsorption site. DFT calculation theoretically confirmed that the substitution of V in different Fe atoms, Fe₂CuO₄-V1 and Fe₂CuO₄-V2 have a lower energy barrier for breaking the peroxyl O–O bond and higher adsorption energy for PDS with more electron transfer to evolve radical species. More importantly, the continuous-flowing systems integrating mass catalyst production and modification, along with catalytic wastewater treatment, exhibit high purification efficiency and stability, showing great potential for practical application.

CRediT authorship contribution statement

Xiangdong Zhu: Writing – original draft, Validation, Project administration, Methodology, Formal analysis, Conceptualization. Beibei Xiao: Software, Resources. Fengbo Yu: Writing – original draft, Validation, Project administration, Methodology, Formal analysis, Conceptualization. Chao Jia: Investigation, Formal analysis. Liming Sun: Investigation, Formal analysis. Shicheng Zhang: Writing – review & editing. Lianli Wang: Software, Resources. Peixin Cui: Writing – review & editing. Liang Wang: Software, Resources. Xiaoguang Duan: Writing – review & editing. Shaobin Wang: Writing – review & editing. Yujun Wang: Writing – review & editing, Supervision, Funding acquisition.

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 was supported by the National Natural Science Foundation of China (42225701). We especially appreciate the XAFS measurements provided by the Shanghai Synchrotron Radiation Facility.

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