English

• Boron-doped diamond (BDD) is a most promising anode material in electrochemical oxidation wastewater treatment (EOWT) due to its wide potential window, high oxygen evolution potential, self-cleaning characteristic, and excellent corrosion resistance [1–8]. However, the widely application of large-scale BDD film is still a costly challenge because of their delicate preparation procedures and poor production efficiency [9–13]. Moreover, the BDD film is susceptible to peeling off from the substrate (e.g., Ti or Si) with different coefficient of thermal expansion when exposed to higher current densities during the treatment of real wastewater, which can lead to decreased performance and stability [14,15]. Magnetic BDD particles (called BDD particles in brief) are industrial waste materials that originate from the wear of diamond wire during the manufacturing process using wire cutting (Fig. S1 in Supporting information) [16–20]. These particles have certain potential as the alternatives of BDD film in the field of EOWT, not only fully utilize the economic benefits of such wastes, but also may alleviate the above problems of BDD film. Consequently, it is crucial to find a feasible strategy to assemble these particles as anode.

  Magnetically assembled electrode (MAE) uses the magnetic force generated by a magnet to attract the magnetic particles (auxiliary electrodes, AEs) on the surface of the two-dimensional main electrode (ME). The MAE is confidently named as 2.5-dimensional (2.5D) electrode, as it falls between the traditional two-dimensional (2D) and three-dimensional (3D) electrodes in structure [21–25]. Assembling BDD particles as new 2.5D electrode, i.e., 2.5D BDD electrode is expected to have the advantages of adequate active sites, good flexibility, high economic applicability, and good stability (or recyclability) (Fig. 1a), which is considered to be a suitable alternative of the aforementioned BDD film (i.e., 2D BDD electrode). On the other hand, BDD particles have incomparable oxidation power, which are expected to be more suitable to act as AEs in 2.5D electrode comparing with previously adopted AEs, such as Fe₃O₄/Sb-SnO₂ [26,27], Fe₃O₄/Pb₃O₄ [28], Fe₃O₄/polyaniline [29], and magnetic carbon nanotubes [30,31]. The development of new 2.5D BDD electrode is of great significance for the enrichment and enhancement of 2.5D electrode and its theory.

  Figure 1

  

  Figure 1.  Schematic diagrams of new 2.5D BDD electrode and results of material characterization: (a) Radar map of comparison between 2.5D BDD electrode and 2D BDD electrode; (b) Schematic diagrams of RuO₂-IrO₂/BDD(0.1 g) and RuO₂-IrO₂/BDD(0.3g); (c) Schematic diagram of the developed flow electrochemical reactor system in this study; (d) SEM image and XRD pattern of 2D Ti/RuO₂-IrO₂; (e) SEM images and XRD pattern of BDD particles; (f) Contact angles of water on three electrodes; (g) 3D ultra depth micrographs of three eletrodes.

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  In this study, Ti/RuO₂-IrO₂ electrode (Baoji Ti-Price Titanium Anode Technology Co., Ltd., China) was adopted as the partner of BDD particles (Shaanxi Metron Diamond Material Technology Co., Ltd., China) due to its modest oxidation power and excellent stability in EOWT. Different amount of BDD particles (i.e., 0 g/cm², 0.1 g/cm², and 0.3 g/cm²) were loaded on Ti/RuO₂-IrO₂ by a NdFeB magnet, and the electrodes were named as 2D Ti/RuO₂-IrO₂, RuO₂-IrO₂/BDD(0.1 g), and RuO₂-IrO₂/BDD(0.3g), respectively. As shown in Fig. 1b, RuO₂-IrO₂/BDD(0.1 g) has loose distribution of particles while the particles on RuO₂-IrO₂/BDD(0.3g) are tightly packed together and the ME is completely covered by these particles (electrode structure, morphology and electrochemical characterization test details are described in Supporting information). In EOWT experiments, electrode's abilities in breaking the key linkage, opening benzene ring, and mineralizing small molecule compound were tested by degrading 250 mL of synthetic wastewater including azo dye acid red G (ARG), p-benzoquinone (PBQ) and succinic acid (SA) (containing 100 ppm of target pollutant and 0.1 mol/L Na₂SO₄). A flow electrochemical reactor system (Fig. 1c) was developed to make the wastewater undergo forced convection. Different current densities (i.e., 20 mA/cm² and 50 mA/cm²) and different flow rates (i.e., 30, 100 and 250 mL/min) were investigated. In addition, degradation of four kinds of real wastewater namely 2,3-dichloropyridine wastewater, MVR wastewater, NaP wastewater and new energy wastewater were also carried out (Table S1 in Supporting information). Chemical oxygen demand (COD) was determined in the original solution as well as in the solution after different time treatments by using a COD rapid tester (Lian-Hua Tech. Co., Ltd., China. 5B-3C (V8)) according to the national standard of potassium dichromate method (HJ 828-2017) (details are described in Supporting information). Fourier spectrometer (Nicolet iS 10, Thermo Fisher, USA) was used to analyze the changes in the vibrational frequencies and intensities of chemical functional groups before and after real wastewater degradation. Multi-physics simulation (COMSOL Multiphysics 6.2) was carried out to confirm the structure-activity relationship of the new 2.5D BDD electrode. Finally, glycerol samples were tested using GC-MS to investigate the potential of the new 2.5D BDD electrode for electrosynthesis (test conditions are described in Supporting information).

  Fig. 1d verifies the morphology, composition and structure of 2D Ti/RuO₂-IrO₂, where it shows a rutile phase of Ru-Ir-Ti-O solid solution and a typical "crack-mud" morphology (the EDS element mapping of 2D Ti/RuO₂-IrO₂ electrode are shown in Fig. S2 in Supporting information). Fig. 1e demonstrates that the BDD particles used in this study has a typical polyhedral shape, a high crystallinity of diamond (diamond standard card (JCPDS NO. 06-0675)), and a 2.64% doping level of boron (the EDS element mapping and size distribution of BDD particles are shown in Supporting information, Figs. S3 and S4 in Supporting information). The results of the BET analysis for BDD particles are presented in Fig. S5 (Supporting information). Loading 0.1 g/cm² and 0.3 g/cm² BDD particles could elevate the electrode hydrophobicity by changing the water contact angle from 26.199° (Ti/RuO₂-IrO₂) to 96.932° and 114.941°, respectively (Fig. 1f). From Fig. 1g, it can also be seen that loading BDD particles significantly increases the surface roughness of the electrode, which benefits to the enlargement of surface area.

  The enlargement of surface area is confirmed by electrochemical characterization results (Fig. 2). From narrow range (0–0.3 V (vs. SCE)) obtained at different scan rates (Figs. 2a-c), the voltammetric charge (q*) and electrical double-layer capacitance (C_dl) could be obtained (Fig. S6 in Supporting information). The total voltammetric charge (q_T), outer voltammetric charge (q_o) and inner voltammetric charge (q_i) in Fig. 2d correspond to the total number of active sites, amount of outer active sites and amount of less accessible inner active sites, respectively. The q_i value of RuO₂-IrO₂/BDD(0.1 g) is 65.2% higher than that of the 2D Ti/RuO₂-IrO₂, indicating that BDD particles can introduce a large number of inner active sites. However, the q_i value of RuO₂-IrO₂/BDD(0.3g) is much lower than those of the other two electrodes, indicating that the excessive loading of AEs would cover the ME more significantly and compact the AEs layer. Furthermore, although the C_dl value of the 2D Ti/RuO₂-IrO₂ electrode is the highest (Fig. 2e), its charge density (the ratio of q_T and C_dl, referring to the number of active sites per unit area) is only 1.059 C/F, which is much lower than those of RuO₂-IrO₂/BDD(0.1 g) (1.855 C/F) and RuO₂-IrO₂/BDD(0.3g) (1.509 C/F). The normal CV curve (0~2.5 V (vs. SCE)) shown in Fig. 2f can reflect the effect of BDD AEs on the electrochemical activity of the electrode. The oxygen evolution potential (OEP) of the electrode do not increase obviously with the AEs loading, but the oxygen evolution activity (OER) of MAE decreases significantly. Figs. 2g-i reflect the Nyquist plots of the three electrodes at different potentials. Loading small amount of BDD AEs only changes the impedance slightly, but excessive loading of AEs has obvious negative effects in elevating the solution resistance (R_s) and charge transfer resistance (R_ct). In summary, loading BDD particles changes the composition and structure of the electrode, which indeed affects the electrochemical behaviors of the electrode and may play an important role in the further organic degradation.

  Figure 2

  

  Figure 2.  Electrochemical characterization results (in 0.5 mol/L Na₂SO₄ solution): (a) Narrow range CV curves of 2D Ti/RuO₂-IrO₂ at different scan rates; (b) Narrow range CV curves of RuO₂-IrO₂/BDD(0.1 g) at different scan rates; (c) Narrow range CV curves of RuO₂-IrO₂/BDD(0.3g) at different scan rates; (d) Voltammetric charges of three electrodes obtained from narrow range CV curves; (e) Double-layer capacitance (C_dl) obtained from narrow range CV curves; (f) Normal CV curves of three electrodes (scan rate: 0.01 V/s); (g) Nyquist plots obtained at equilibrium potential of 0 V (vs. SCE); (h) Nyquist plots obtained at equilibrium potential of 1.1 V (vs. SCE); (i) Nyquist plots obtained at equilibrium potential of 1.3 V (vs. SCE).

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  Fig. 3 illustrates EOWT results of three types of synthetic wastewaters and four types of real wastewaters. Fig. 3a shows the ARG removal rates under different current densities and flow rates. Under various circumstances, both RuO₂-IrO₂/BDD(0.1 g) and RuO₂-IrO₂/BDD(0.3g) have obvious enhancement in breaking the azo linkage comparing with 2D Ti/RuO₂-IrO₂, manifesting the superiority of BDD particles in OER side reaction inhibition. Specifically, loading more BDD particles is basically more effective. The PBQ degradation rate is also increased by loading BDD particles (Fig. 3b). For example, the degradation rate increases from original 74.25% to 84.18% and 95.12%, respectively, for RuO₂-IrO₂/BDD(0.1 g) and RuO₂-IrO₂/BDD(0.3g). Since ring opening relies strongly on the powerful oxygen reactive species, not the direct electron transfer (DET) [32], loading more BDD particles brings out more amounts of these species, leading to higher PBQ degradation rate.

  Figure 3

  

  Figure 3.  EOWT results of three types of synthetic wastewaters and four types of real wastewaters: (a) ARG removal rate versus time; (b) PBQ removal rate versus time; (c) COD removal rate of SA versus time; (d) COD variation of 2,3-dichloropyridine wastewater; (e) COD variation of MVR wastewater; (f) COD variation of NaP wastewater; (g) COD variation of new energy wastewater; (h) FTIR spectra of the remaining solid after drying the real wastewater samples (10 h treatment).

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  Interestingly, RuO₂-IrO₂/BDD(0.1 g) is more effective to degrade SA than 2D Ti/RuO₂-IrO₂, while RuO₂-IrO₂/BDD(0.3g) fails to degrade SA (Fig. 3c). The mineralization of SA depends on both ^•OH and DET [32], so RuO₂-IrO₂/BDD(0.1 g) is more capable of both ^•OH production and DET among all three electrodes. The strong DET feature of RuO₂-IrO₂/BDD(0.1 g) can also be proved by Fig. S7 (normal CV curves obtained in different solutions). Since 2D Ti/RuO₂-IrO₂ is widely known to has poor ^•OH production ability but good DET activity [33–35], loading appropriate amount of BDD particles (i.e., RuO₂-IrO₂/BDD(0.1 g)) may maximumly preserve the joint active sites on the interface between Ti/RuO₂-IrO₂ and BDD, which could harmonize DET with ^•OH production.

  For COD variation of real wastewaters, the effect of loading BDD particles depends on the type of real wastewater (Figs. 3d-g). In treating chlorine-containing real wastewaters (i.e., 2,3-dichloropyridine wastewater, MVR wastewater, and NaP wastewater), 2D Ti/RuO₂-IrO₂ with high chlorine evolution activity [4,36,37] is highly competitive in COD removal and color removal because the massive active chlorine species (Cl₂, HClO, etc.) with strong oxidation power will reinforce indirect oxidation and lead to rapid pollutant degradation. Even so, RuO₂-IrO₂/BDD(0.1 g) performs well in treating NaP wastewater, and RuO₂-IrO₂/BDD(0.3g) performs well in treating 2,3-dichloropyridine wastewater. New energy wastewater is colorless and does not contain chlorine ion. Loading more BDD particles leads to more efficient COD removal and less color change in new energy wastewater treatment, which means the intermediate products accumulation is well inhibited. In summary, comparing with the other two electrodes, RuO₂-IrO₂/BDD(0.1 g) shows a hybrid feature of Ti/RuO₂-IrO₂ and BDD, which is more able to adapt to the composition variation of wastewater [38].

  The effect of BDD particles on the pollutant degradation pathway could be speculated from the FTIR spectra of the remaining solid after drying the real wastewater samples (10 h treatment, Fig. 3h). In brief, the retention of C-H (~970 cm^-1, ~1385 cm^-1) or C=C (~1600 cm^-1, ~1625 cm^-1) is reduced and the generation of C-O (~1109 cm^-1, ~1132 cm^-1) is increased by the BDD particles. More importantly, the accumulation of chlorinated compounds (C-Cl, ~487 cm^-1) is inhibited more efficiently on RuO₂-IrO₂/BDD(0.1 g) and RuO₂-IrO₂/BDD(0.3g), reflecting the prospective application of the new 2.5D electrode in green EOWT.

  Based on all above results, the structure-activity relationship of the new 2.5D electrode could be proposed. Ti/RuO₂-IrO₂ may have a certain synergistic effect with BDD particles (Fig. 4a). Loading appropriate amount of BDD particles on Ti/RuO₂-IrO₂ not only introduces powerful active sites on BDD, but also retains a certain number of active sites on Ti/RuO₂-IrO₂, and further introduce more joint active sites on the interface between Ti/RuO₂-IrO₂ and BDD. The "2.5D" feature (hybrid feature) can be best reflected by the coexisting three types of active sites on three locations. The joint effort of DET, indirect oxidation by short-lived species (e.g., ^•OH) and indirection oxidation by long-lived species (e.g., active chlorine) on hybrid active sites would be benefit to mineralization of organics [39–48]. Excessive BDD particles would hinder the contact between the pollutant (or solution) and the underlying Ti/RuO₂-IrO₂ active sites (or joint active sites), making the electrode lose the "2.5D" feature and simply behave like a rough 2D BDD electrode. In addition, since the conductivity of BDD is limited, the contact resistance throughout the thick BDD layers would lead to insufficient polarization of BDD particles, which could be reflected by the potential distribution simulation result (Fig. 4b). Lower potential on outer BDD layer means the waste of power on these active sites in degrading refractory pollutant. However, it can still be confirmed that since Ti/RuO₂-IrO₂ is fully replaced by BDD, the OER side reaction is well inhibited, and the basic competence of the electrode in key linkage broken and ring opening of the pollutant is guaranteed.

  Figure 4

  

  Figure 4.  Composite figures that reflect the structure-activity of the 2.5D electrode: (a) Schematic diagram of four locations on 2.5D electrode in this study; (b) Simulation result of potential distribution on different electrodes under different current densities; (c) GC-MS bubble plots of original glycerol sample and samples obtained under different currents by different electrodes (each color of bubble represent an identified product, the bubble position corresponds to the retention time (RT) and the biggest mass-to-charge ratio (m/z), and the bubble area is proportional to the GC peak area); (d) The appearance of glycerol samples under varying BDD loads at distinct treatment times.

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  Thanks to the hybrid active sites, the new 2.5D BDD electrode also has potential in electrosynthesis. Taking glycerol oxidation for example (Figs. 4c and d), RuO₂-IrO₂/BDD(0.1 g) is good at producing glyceraldehyde (C₃H₆O₃) and glycerol ethers (C₄H₁₀O₄). In conclusion, the development of the new 2.5D BDD electrode is of great significance for the EOWT and potential electrosynthesis applications. This hybrid electrode is a potential promising candidate for the future development of electrochemistry. It unites both generalists (Ti/RuO₂-IrO₂ sites) and specialists (BDD sites) in a flexible and distinctive way. In addition, details of determination of reactive species (Table S2, Fig. S8 and S9 in Supporting information), toxicity evaluation (Fig. S10 and Table S3 in Supporting information), stability monitoring (Fig. S11 in Supporting information) and techno-economic analysis (TEA, Fig. S12 and Table S4 in Supporting information) are offered in the Supporting information section, which also demonstrate the superiority of the new 2.5D BDD electrode.

  In conclusion, the magnetically assembled electrode consisting of Ti/RuO₂-IrO₂ and BDD particles demonstrates outstanding electrocatalytic performance, with a distinct synergistic effect between the two components. The optimum amount of BDD particles (0.1 g/cm²) can preserve the joint active sites and sustain sufficient polarization of BDD particles, which is crucial for the degradation of pollutants and the generation of value-added products. Moreover, this study offers a valuable insight into the recycling of BDD particles.

  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.

  CRediT authorship contribution statement

  Dan Shao: Writing – review & editing, Writing – original draft, Validation, Supervision, Resources, Project administration, Methodology, Funding acquisition, Data curation, Conceptualization. Yujing Lyu: Writing – review & editing, Writing – original draft, Visualization, Software, Investigation, Formal analysis, Data curation. Chengyuan Liu: Resources. Hao Wang: Investigation. Ning Ma: Resources. Hao Xu: Supervision, Resources, Project administration, Funding acquisition. Wei Yan: Resources. Xiaohua Jia: Resources. Haojie Song: Supervision, Resources.

  Acknowledgments

  This study is financed by the National Natural Science Foundation of China (Nos. 21706153, 52270078) and Natural Science Basic Research Program of Shaanxi Province (Nos. 2018JQ2066, 2022JM-065).

  Supplementary materials

  Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.cclet.2024.110641.

  
  
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  Figure 1  Schematic diagrams of new 2.5D BDD electrode and results of material characterization: (a) Radar map of comparison between 2.5D BDD electrode and 2D BDD electrode; (b) Schematic diagrams of RuO₂-IrO₂/BDD(0.1 g) and RuO₂-IrO₂/BDD(0.3g); (c) Schematic diagram of the developed flow electrochemical reactor system in this study; (d) SEM image and XRD pattern of 2D Ti/RuO₂-IrO₂; (e) SEM images and XRD pattern of BDD particles; (f) Contact angles of water on three electrodes; (g) 3D ultra depth micrographs of three eletrodes.

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  Figure 2  Electrochemical characterization results (in 0.5 mol/L Na₂SO₄ solution): (a) Narrow range CV curves of 2D Ti/RuO₂-IrO₂ at different scan rates; (b) Narrow range CV curves of RuO₂-IrO₂/BDD(0.1 g) at different scan rates; (c) Narrow range CV curves of RuO₂-IrO₂/BDD(0.3g) at different scan rates; (d) Voltammetric charges of three electrodes obtained from narrow range CV curves; (e) Double-layer capacitance (C_dl) obtained from narrow range CV curves; (f) Normal CV curves of three electrodes (scan rate: 0.01 V/s); (g) Nyquist plots obtained at equilibrium potential of 0 V (vs. SCE); (h) Nyquist plots obtained at equilibrium potential of 1.1 V (vs. SCE); (i) Nyquist plots obtained at equilibrium potential of 1.3 V (vs. SCE).

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  Figure 3  EOWT results of three types of synthetic wastewaters and four types of real wastewaters: (a) ARG removal rate versus time; (b) PBQ removal rate versus time; (c) COD removal rate of SA versus time; (d) COD variation of 2,3-dichloropyridine wastewater; (e) COD variation of MVR wastewater; (f) COD variation of NaP wastewater; (g) COD variation of new energy wastewater; (h) FTIR spectra of the remaining solid after drying the real wastewater samples (10 h treatment).

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  Figure 4  Composite figures that reflect the structure-activity of the 2.5D electrode: (a) Schematic diagram of four locations on 2.5D electrode in this study; (b) Simulation result of potential distribution on different electrodes under different current densities; (c) GC-MS bubble plots of original glycerol sample and samples obtained under different currents by different electrodes (each color of bubble represent an identified product, the bubble position corresponds to the retention time (RT) and the biggest mass-to-charge ratio (m/z), and the bubble area is proportional to the GC peak area); (d) The appearance of glycerol samples under varying BDD loads at distinct treatment times.

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