Citation

Chen Xia, Dai Ting, Yin Meng-Ying, Xia Xing-Yuan, Xing Qiu-Ju, Tian Lei, Zou Jian-Ping. Enhanced anodic mass transfer enables interfacial Cl^• for efficient ammonia oxidation[J]. Chin. Chem. Lett., 2026, 37(3): 111445  

Xia Chen^a, Ting Dai^a, Meng-Ying Yin^a, Xing-Yuan Xia^a, Qiu-Ju Xing^a, Lei Tian^a,b,*, Jian-Ping Zou^a,*     

The rapid development of industrialization and urbanization has caused abundant amounts of ammonia (NH₄⁺/NH₃) in wastewater [1,2]. The discharge of ammonia nitrogen into natural water bodies can result in severe water contamination and accelerate the process of eutrophication, which has serious harm to the ecological environment [3]. Therefore, it is urgent to develop a green and efficient technology for the treatment of ammonia nitrogen in wastewater in light of the call for energy conservation and emission reduction [4-6].

Many efforts have been made to treat ammonia nitrogen by various technologies including air stripping [7,8], biological denitrification [9,10], and breakpoint chlorination [11,12]. Nevertheless, these methods have inherent limitations [13,14]. For instance, the air stripping exhibits poor removal efficiency for ammonia nitrogen with low concentration. Biological denitrification is easily disturbed by temperature and C/N ratio of wastewater and a long treatment period is required. Breakpoint chlorination is commonly observed in the chlorination of ammonia-containing wastewater treatments but its high cost and complex operation remain a problem. Recently, the electrocatalytic (EC) system based on the anodic chlorine evolution reaction (CER) has been identified as a highly sustainable approach for ammonia nitrogen removal due to its superior oxidation capacity / easy operation / regulation [15-17]. A wide variety of chlorine-containing active species can be produced in the EC system, which can efficiently and selectively oxidize ammonia into nitrogen (N₂) [18,19]. Among these chlorine-containing active species, chlorine radical (Cl^•) has a higher redox potential (2.55 V vs. SHE) than HClO/ClO^- (1.49/0.89 V vs. SHE), ClO^• (1.39 V vs. SHE), and Cl₂^•– (2.13 V vs. SHE) [20,21], thereby conferring a significant advantage in the removal of ammonia. Designed to produce Cl^• for water purification, a lot of advanced systems are developed, such as the photoelectrocatalytic system [22,23] and the EC system coupled with the advanced oxidation process [24]. However, due to the high reaction activity with Cl^- (8.00 × 10⁹ L mol^-1 s^-1), HClO/ClO^- (3.00/8.30 × 10⁹ L mol^-1 s^-1), and OH^- (1.80 × 10¹⁰ L mol^-1 s^-1), Cl^• is easily quenched into low reactive species [13,25]. The difficulty of controllable generation of Cl^• in catalytic systems for ammonia removal is extremely high. Therefore, it is urgent to develop novel systems to efficiently and controllably produce Cl^• for the treatment of ammonia nitrogen wastewater.

It is found from the Volmer step in CER that Cl^- can be oxidized into adsorbed Cl^• (Cl*) (Eq. 1) [26]. The adsorbed Cl^• will undergo self-quenching into chlorine gas (Cl₂) (Eqs. 2 and 3) [27,28]. If the adsorbed Cl^• at the anode interface can react with ammonia nitrogen and suppress their self-quenching, the Cl^•-mediated ammonia nitrogen mineralization process can be effectively and selectively achieved. However, in the CER-mediated ammonia nitrogen oxidation system, the active sites of the anode usually have a strong binding energy with Cl^- [16,29,30], which suppresses the adsorbed Cl^• overflowing from the interface for ammonia nitrogen oxidation. More seriously, the pH of ammonia nitrogen wastewater is generally neutral or weakly acidic, and ammonia mainly exists in the form of NH₄⁺ (pK_a(NH₄⁺/NH₃) = 9.25) [31]. There is a strong electrostatic repulsion between NH₄⁺ and the anode, which prevents the enrichment of NH₄⁺ on the anodic interface to react with Cl^•. These results make it difficult for Cl^• in the CER process to efficiently remove ammonia nitrogen, seriously affecting the treatment efficiency of ammonia nitrogen wastewater. Spinel oxides (AB₂O₄) with excellent electrochemical activity and adjustable electronic structure are the potential anode materials for the Cl^•-mediated ammonia nitrogen oxidation process [32-34]. In detail, the octahedral sites (B_Oh³⁺) with a high oxidation potential can effectively oxidize Cl^- to Cl^•, while the tetrahedral sites (A_Td²⁺) with a high energy level matching with NH₄⁺ may effectively enrich NH₄⁺. Modulating the elements of A_Td²⁺ can not only enhance the binding energy with NH₄⁺ but also promote Cl^• overflow from B_Oh³⁺. Therefore, the design and development of the novel AB₂O₄ anode to efficiently rich NH₄⁺ and produce Cl^• is of great significance for the removal of ammonia nitrogen wastewater.

Hence, this study introduces other ions such as Ni²⁺, Cu²⁺, or Zn²⁺ into the A_Td²⁺ of Co₃O₄ to induce the efficient enrichment of NH₄⁺ and the production of Cl^• at the anode, which realizes the Cl^•-mediated ammonia nitrogen oxidation process (Fig. 1a). Due to the high redox potential (E⁰ = 1.81 V) and rapid Co²⁺/Co³⁺ cycle, Cl^- can be efficiently oxidized into Cl^• at Co_Oh³⁺ [35,36]. Due to the high occupancy of 3d orbits, the introduction of Ni²⁺, Cu²⁺, or Zn²⁺ into the A_Td²⁺ of Co₃O₄ can couple the interactions of electron-repelling and π-donating in A_Td²⁺-O-Co_Oh³⁺ [37]. As a result, the electronic structure of Co_Oh³⁺ is regulated to weaken the binding energy with Cl^•, which can efficiently overflow from Co_Oh³⁺ sites for ammonia nitrogen oxidation. Besides, the binding energy and the matching degree of energy level between different A_Td²⁺ and NH₄⁺ are elaborately analyzed. Through batch experiments and electrochemical characterizations, the mass transfer of NH₄⁺ and production of Cl^• in the systems with different anodes are compared and their effects on ammonia nitrogen oxidation are investigated. Combined with theoretical calculations, the enhanced removal mechanism of ammonia nitrogen is discerned. This study opens a new avenue for the efficient mineralization treatment of actual ammonia-nitrogen wastewater and guides the design and synthesis of superior anodes for environmental remediation.

Figure 1

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Fig. 1. (a) Schematic mechanism of enhanced ammonia oxidation by CuxCo3-xO4 compared to conventional anode. (b) DFT-calculated LUMO and HOMO energy level differences between A2+ and NH4+ (A2+: Ni2+, Cu2+, Zn2+). (c) Adsorption energy between NH4+ and the ATd2+ site in AxCo3-xO4 electrodes. (d) Crystal structure of CuxCo3-xO4. (e) XPS spectra for Co 2p and (f) Raman spectroscopy of Co3O4 before and after modification with Cu. In-situ Raman spectroscopy of (g) NixCo3-xO4, (h) CuxCo3-xO4, and (i) ZnxCo3-xO4.

The energy level differences (ΔE) of HOMO/LUMO and the binding energy between NH₄⁺ and Ni_Td²⁺/Cu_Td²⁺/Zn_Td²⁺ are systematically investigated. As illustrated in Fig. 1b, ΔE between NH₄⁺ and Cu_Td²⁺/Zn_Td²⁺ is 1.66 and 1.19 eV, respectively, which is much lower than that of Ni_Td²⁺ (3.45 eV). A nearer energy level of HOMO/LUMO in two molecules is more conducive to forming more stable bonding orbitals [38-40]. Besides, the adsorption energy (E_ads) between the A_Td²⁺ sites of A_xCo_3-xO₄ (A_Td²⁺: Ni_Td²⁺, Cu_Td²⁺, or Zn_Td²⁺) and NH₄⁺ is further analyzed. As shown in Fig. 1c and Fig. S1 (Supporting information), the value of E_ads between NH₄⁺ and Zn_xCo_3-xO₄ is calculated to be merely −4.75 eV. Though a more stable bonding orbital between Zn_Td²⁺ and NH₄⁺, Zn_Td²⁺ is relatively inert and cannot effectively enrich NH₄⁺ because the 3d orbital of Zn²⁺ is fully filled by electrons (3d¹⁰) (Fig. S2 in Supporting information). By contrast, Cu_Td²⁺ has higher binding energy (−6.83 eV) with NH₄⁺ than Zn_Td²⁺ and Ni_Td²⁺ (−5.4 eV), which exhibits remarkable potential for enrichment of NH₄⁺. A_xCo_3-xO₄ with spinel structure was successfully synthesized by the one-pot hydrothermal method and it is verified that the tetrahedral sites in Co₃O₄ are substituted by Ni_Td²⁺, Cu_Td²⁺, or Zn_Td²⁺ (Figs. 1d-f, Figs. S3-S12, Table S1 in Supporting information). Moreover, the mass transfer direction of NH₄⁺ in the EC system was discerned over A_xCo_3-xO₄ anode. As shown in Fig. S13 (Supporting information), all anodes show no obvious adsorption performance for NH₄⁺ under no current applied condition. In the absence of an electric field, NH₄⁺ is uniformly distributed in the EC system and there is no concentration gradient between the anode and cathode regions. After the input of current (25 mA), the concentration of NH₄⁺ in the Pt cathode region decreases significantly from 5 mmol/L to approximately 3.2 mmol/L within 20 min, while that of the Cu_xCo_3-xO₄ anode region exhibits slow decreasing trends (4.3 mmol/L, 20 min). A distinct concentration gradient of NH₄⁺ is observed between the Cu_xCo_3-xO₄ anode and cathode regions, which is more significant than that in the EC system with Ni_xCo_3-xO₄, Zn_xCo_3-xO₄ or Co₃O₄ anode (Figs. S14-S16 in Supporting information). Due to the strong interaction between Cu_Td²⁺ and NH₄⁺, the dynamic mass transfer of NH₄⁺ from the cathode region to the anode region and from the solution to the anode interface occurs, resulting in the concentration of NH₄⁺ in the anode region much higher than that in the cathode region. This phenomenon demonstrates that Cu_xCo_3-xO₄ can overcome the electrostatic repulsion between NH₄⁺ and anode, which is conducive to effective reaction with active species at the anodic interface.

To further visualize the enrichment of NH₄⁺ at the anode interface, X-ray photoelectron spectroscopy (XPS) and in-situ Raman spectra were carried out. No obvious characteristic peak of NH₄⁺ is observed in the N 1s XPS of the Ni_xCo_3-xO₄, Zn_xCo_3-xO₄ and Co₃O₄ anodes before and after the input of current (Figs. S17-S19 in Supporting information). As a comparison, when Cu_xCo_3-xO₄ is the anode, the characteristic peak of N—H bond in NH₄⁺ emerges after the input of current, indicating NH₄⁺ can efficiently enrich at the Cu_xCo_3-xO₄ anode (Fig. S20 in Supporting information). In-situ Raman spectra provide more evidence that the introduction of Cu_Td²⁺ enhances the mass transfer of NH₄⁺ to the anode. As shown in Fig. 1h, the peaks around 1590 cm^-1 in-situ Raman spectra of the Cu_xCo_3-xO₄ system are assigned to the antisymmetric bending vibration of the HNH of NH₃ [41-43]. The intensity of these peaks first increases and then decreases during the reaction, indicating that NH₄⁺ can overcome the electrostatic interactions to enrich and be further oxidized on Cu_Td²⁺. The intensity of the A_1g peak first decreases and then increases during the reaction, which is attributed to the cycling of Co³⁺/Co²⁺ at the octahedral sites for the activation of Cl^-. The negligible peaks around 1590 cm^-1 in the in-situ Raman spectra of the Ni_xCo_3-xO₄, Zn_xCo_3-xO₄, and Co₃O₄ anodes illustrate the interactions between NH₄⁺ and the Ni_xCo_3-xO₄/Zn_xCo_3-xO₄/Co₃O₄ anodes are weak, leading to the inability of transferring NH₄⁺ to the anode under electrostatic repulsion (Figs. 1g and i and Fig. S21 in Supporting information). Based on the above results, it is confirmed that Cu_xCo_3-xO₄ can efficiently induce NH₄⁺ to overcome the electrostatic repulsion and realize its enrichment at the anode due to the strong interaction and moderate energy level matching between Cu_Td²⁺ and NH₄⁺, which facilitates rapid reaction with interfacial active species and thus efficient mineralization.

The removal performance of ammonia-nitrogen was investigated in the EC system with different anodes. As shown in Fig. 2a and Fig. S22 (Supporting information), 100% of NH₄⁺-N is removed in the EC system with Cu_xCo_3-xO₄ anode within 80 min at a k_obs of 4.4 × 10^–2 min^-1, which is more remarkable than Co₃O₄ (2.2 × 10^–2 min^-1) and commercial dimensionally stable anode (DSA) (0.7 × 10^–2 min^-1). It is worth noting that the introduction of Fe_Td²⁺, Mn_Td²⁺, Ni_Td²⁺, and Zn_Td²⁺ into the Co₃O₄ anode does not significantly improve the NH₄⁺-N removal performance. Since there is no special interaction between NH₄⁺ and A_xCo_3-xO₄ (A = Fe, Mn, Ni, Zn) anode, NH₄⁺ cannot enrich at the anode limited by electrostatic repulsion, resulting in that NH₄⁺-N cannot react in time with the anodic interface Cl^•. As a comparison, the Cu_xCo_3-xO₄ anode can efficiently enrich NH₄⁺ and achieve efficient mineralization of ammonia nitrogen by interfacial Cl^•. Besides, as demonstrated in Figs. 2b and c, the concentration of TN decreases simultaneously with the reduction of ammonia concentration and no undesirable products (such as NO₂^-, NO₃^-) are generated after the reaction. Although a small amount of chloramines (NH₂Cl, NHCl₂, and NCl₃) are produced during the reaction process, they are completely converted into N₂ as the reaction progresses. Moreover, five cycle experiments were performed to evaluate the durability of Cu_xCo_3-xO₄ electrodes. As shown in Fig. S23 (Supporting information), the degradation of NH₄⁺-N remains essentially unchanged after five consecutive runs. The morphology, structure and Co²⁺/Co³⁺ ratio of the Cu_xCo_3-xO₄ anode after five cycles are basically consistent with that before the reaction, and the leaching concentrations of cobalt ions are < 0.6 mg/L and those of copper ions are < 0.3 mg/L after each cycle (Figs. S24-S28 in Supporting information). These results confirm that the Cu_xCo_3-xO₄ anode has excellent stability. In addition, to further explore the feasibility of the practical application of the Cu_xCo_3-xO₄ anode, the cost analysis for ammonia nitrogen degradation is performed. As exhibited in Fig. 2d, Tables S2 and S3 (Supporting information), no matter the electrode cost or energy consumption, the Cu_xCo_3-xO₄ anode (0.44 ＄/piece, 15.1 kWh/kg_-N) is superior to DSA (1.47 ＄/piece, 31.3 kWh/kg_-N) and other ammonia oxidation reaction (AOR) anodes reported by the previous literature.

Figure 2

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Fig. 2. (a) Ammonia nitrogen removal performance in the EC system using different anodes. Concentrations of (b) different nitrogen species and (c) chloramine in the EC system with CuxCo3-xO4 anode. (d) Cost analysis of different systems for ammonia removal. Ammonia and total nitrogen removal performance of (e) CuxCo3-xO4 and (f) DSA anodes for treatment of practical wastewater. (g) Diagram showing the electrochemical reaction tank with multipairs of electrodes for landfill leachate treatment under continuous flow conditions. Time-dependent degradation rate of (h) NH4+-N and (i) total nitrogen for continuous treatment of landfill leachate by the CuxCo3-xO4 system with 3 pairs of electrodes.

To confirm the feasibility of Cu_xCo_3-xO₄ anode for the treatment of practical ammonia-nitrogen wastewater (pH 7.51), further experiments are constructed to evaluate its high activity. The main components of landfill leachate are presented in Table S4 (Supporting information) and the NH₄⁺-N concentration in the raw wastewater is 332.6 mg/L. As shown in Fig. 2e, the EC system assembled with Cu_xCo_3-xO₄ anode exhibits a significantly higher NH₄⁺-N removal and mineralization performance (99.1%, 70.7%) within 8 h than that with Co₃O₄ anode (85.9%, 60.1%) and DSA (55.8%, 40.0%) (Fig. 2f and Fig. S29 in Supporting information). More importantly, the scale-up experiment in a flow electrolyzer with Cu_xCo_3-xO₄ anode realizes 100% of NH₄⁺-N and 88.3% of total nitrogen (TN) removal during the four days continuous operation without an obvious decline in catalytic performance (Figs. 2g-i), which is also much superior to the system with DSA (NH₄⁺-N: 77.2%, TN: 66.1%) (Figs. S30 and S31 in Supporting information). Besides, the peak shapes and positions of the XRD diffraction peaks and Raman peaks of the Cu_xCo_3-xO₄ electrodes remained almost unchanged after the long-term reaction, indicating that the Cu_xCo_3-xO₄ electrodes have good stability (Figs. S32 and S33 in Supporting information). In conclusion, Cu_xCo_3-xO₄ demonstrates extraordinary activity for NH₄⁺-N mineralization and excellent feasibility for the purification of practical ammonia-nitrogen wastewater.

Electron paramagnetic resonance (EPR) technology and quenching experiments were performed to verify the active species during the NH₄⁺ mineralization process. An EPR signal with eleven-lines is observed in the EC system with Co₃O₄ and Cu_xCo_3-xO₄ anodes during the electrolysis of NaCl solution (Fig. 3a), corresponding to DMPO—OH^• and DMPO—Cl^• [44]. Compared with the Co₃O₄ anode, the higher signal intensity of DMPO—Cl^• on the Cu_xCo_3-xO₄ anode indicates the introduction of Cu_Td²⁺ facilitates the production of Cl^•. After the addition of NH₄⁺-N to the EC system with Cu_xCo_3-xO₄ anode, the DMPO—Cl^• signal disappears and the DMPO—OH^• signal is retained, confirming that NH₄⁺-N is susceptible to oxidation by Cl^• relative to OH^• (Fig. 3b). The role of Cl^• was further investigated using different quenchers. Hyposulphite (Na₂S₂O₃) is the quencher for OH^•, Cl^• and free chlorines; tertiary butanol (TBA) for OH^•, Cl^• and ClO^•; bicarbonate (HCO₃^-) for OH^•, Cl^• and Cl₂^•-; p-chlorobenzoic acid (PCBA) for OH^• (Table S5 in Supporting information) [45]. As shown in Figs. S34 and S35 (Supporting information), NH₄⁺-N is barely degraded using Na₂S₂O₃ as a quencher, confirming that direct oxidation of NH₄⁺-N on the Cu_xCo_3-xO₄ anode is difficult. PCBA shows a slight inhibitory effect on ammonia degradation, suggesting a limited role for OH^•. The degradation of NH₄⁺-N is significantly inhibited by the addition of HCO₃^- (49.2%, 80 min), suggesting that both Cl^• and Cl₂^•- are involved. According to previous studies, NH₄⁺-N has much higher reaction rates with Cl^• than Cl₂^•- (Table S6 in Supporting information), indicating Cl^• mediated radical oxidation plays a major role in NH₄⁺-N oxidation. The contribution of different active species to ammonia oxidation is evaluated and quantified based on the specific subtraction of ammonia oxidation rate constants for different quenchers (Fig. 3c). The detailed calculation of the contribution of each active species is shown in Fig. S36 (Supporting information). The Cl^• is mainly responsible (about 74.6%) for the overall ammonia oxidation. In addition, the concentration variation and current efficiency (η) of free chlorines (Cl₂, ClO^-, and HClO) in the different systems is quantified (Fig. S37 in Supporting information). Compared to DSA and Co₃O₄, The Cu_xCo_3-xO₄ system exhibits enhanced free chlorine production and higher current efficiency, further demonstrating that the introduction of Cu_Td²⁺ facilitates Cl^• generation.

Figure 3

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Fig. 3. (a) EPR spectra in CuxCo3-xO4/NaCl (brown line) and Co3O4/NaCl (blue line) systems. Conditions: [NaCl] = 60 mmol/L, constant current = 25 mA. (b) EPR spectra. Conditions: [NaCl] = 60 mmol/L, constant current = 25 mA. (c) Contributions of different active species in the CuxCo3-xO4 system. (d) LSV curves of CuxCo3-xO4 and Co3O4. Condition: [NaCl] = 60 mmol/L, scan rate: 5 mV/s. (e) Tafel slopes of CuxCo3-xO4 and Co3O4 anodes. Conditions: [NaCl] = 60 mmol/L. (f) Cyclic voltammetry (CV) of Co3O4 and CuxCo3-xO4 anodes. Conditions: [NaCl] = 60 mmol/L, scan rate: 5 mV/s. Bode phase diagrams of the operando EIS measurements on (g) Co3O4 and (h) CuxCo3-xO4 in the presence of NH4+.

The Cl^• generation and ammonia oxidation activities of the Cu_xCo_3-xO₄ anode are investigated by varied electrochemical characterizations. As shown in Fig. 3d, a higher current density is observed in the linear sweep voltammetry (LSV) curve with Cu_xCo_3-xO₄ anode than that with Co₃O₄ anode, and the current density is significantly increased after adding NH₄⁺. Although the introduction of Cu_Td²⁺ of Co₃O₄ does not significantly increase CER performance, the AOR activity is greatly promoted because NH₄⁺-N can efficiently react with the interfacial Cl^•. Besides, the Tafel slopes of the Cu_xCo_3-xO₄ anode are markedly lower than that of the Co₃O₄ anode in the absence and presence of NH₄⁺ (Fig. 3e), indicating that the Cu_xCo_3-xO₄ anode exhibits superior kinetics for Cl^• generation and AOR. Moreover, a redox peak is observed in the cyclic voltammogram (CV) curve around 1.5 V for both Co₃O₄ and Cu_xCo_3-xO₄ anodes (Fig. 3f), attributing to Co²⁺/Co³⁺. Specifically, only one anodic peak appeared at 1.65 V for the Cu_xCo_3-xO₄ anode, indicating that Cu_Td²⁺ can promote the cycle of Co²⁺/Co³⁺ to achieve efficient Cl^• generation. In addition, the electrochemical surface area (ECSA) of the electrodes is explored by the electrochemical double-layer capacitance (C_dl) to compare their intrinsic activities. Figs. S38 and S39 (Supporting information) illustrate that the C_dl value and intrinsic activity of the Cu_xCo_3-xO₄ anode are significantly higher than that of the Co₃O₄ anode, providing further evidence that the introduction of Cu²⁺ into the Cu_xCo_3-xO₄ increases the active sites for Cl^• production.

Electrocatalytic reaction kinetics and electrode/electrolyte interface properties are further investigated through electrochemical impedance spectroscopy (EIS). As shown in Figs. S40 and S41 (Supporting information), a transition peak at the 1.65 V potential is observed for both Co₃O₄ and Cu_xCo_3-xO₄ anodes. The transition peak is related to the unevenness charge contribution caused by the formation of oxidizing species on the electrode surface [46]. The transition peaks shift to higher frequencies and lower phase angles as the potential rises in CER processes. The frequency and phase angle of the Cu_xCo_3-xO₄ anode varies to a greater extent relative to Co₃O₄ with the increase of bias voltage, which indicates that the introduction of Cu_Td²⁺ enhances the Cl^• generation activity. Notably, an earlier and lower-frequency transition peak at the potential of 1.55 V is observed for the Cu_xCo_3-xO₄ anode after the addition of NH₄⁺ (Figs. 3g and h). These results indicate that the introduction of Cu²⁺ into the tetrahedral site of Co₃O₄ can effectively enrich NH₄⁺ and enhance the NH₄⁺-N oxidation performance.

By comparing the electronic structure of Co_Oh³⁺ sites and the adsorption capacity of reaction intermediates before and after Cu doping, the enhanced removal mechanism of ammonia nitrogen over the Cu_xCo_3-xO₄ anode is elaborately studied. As shown in Figs. 4a and b, the shift of the Co 2p XPS to the higher binding energies emerges after the introduction of Cu_Td²⁺ into Co₃O₄, indicating an increase in electron cloud density around Co(Ⅱ)/Co(Ⅲ) in Cu_xCo_3-xO₄ compared to Co₃O₄. Besides, the differential charge density and Bader charge of the different anodes are further conducted. As illustrated in Fig. 4c, the district around Cu_Td²⁺ is predominantly electronic dissipating (green), while the district around Co_Oh³⁺ is predominantly electronic accumulating (purple), indicating that electrons around Cu_Td²⁺ migrate towards Co_Oh³⁺. The Bader charge analysis shows that the introduction of Cu_Td²⁺ modulates the charge of the adjacent Co_Oh³⁺ from 1.22 to 1.24. As a result, the adsorption energy of Cl^- at the Co_Oh³⁺ sites is weakened from −0.15 eV to −0.10 eV (Fig. 4d), which could facilitate Cl^• overflowing from Co_Oh³⁺ for NH₄⁺-N oxidation. Finally, the Gibbs free energy of the Cl^• generation process is calculated by DFT to further understand the effect of Cu doping. During the Cl^• generation process, the rate-determining step (RDS) is the desorption of Cl* from Co_Oh³⁺, and its energy barrier is reduced from 2.57 eV to 2.52 eV by Cu doping (Fig. 4e). The results indicate that Cu doping facilitates the desorption of Cl* from Co_Oh³⁺ sites, thereby promoting the Cl^• for NH₄⁺-N oxidation.

Figure 4

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Fig. 4. XPS spectra of (a) Co 2p and (b) Cu 2p orbitals of Co3O4 with different Cu introductions. (c) Differential charge density diagram of CuxCo3-xO4. (d) Adsorption energy between Cl- and the structural Co(Ⅲ) in Co3O4 and CuxCo3-xO4 electrodes. (e) Gibbs energy diagrams for the Cl• generation reaction on Co3O4—Co(Ⅲ) and CuxCo3-xO4—Co(Ⅲ).

In conclusion, the enhanced removal mechanism of NH₄⁺-N over the Cu_xCo_3-xO₄ anode is explained as follows. The introduction of Cu_Td²⁺ into Co₃O₄ induces the enrichment of NH₄⁺ at the anode by utilizing the strong interaction between NH₄⁺ and Cu²⁺. Subsequently, the tetrahedral substitution of Cu²⁺ in Co₃O₄ can modulate the electronic structure of the Co_Oh³⁺ and promote the overflow of Cl^• from Co_Oh³⁺, thus achieving the efficient and selective mineralization of NH₄⁺-N by interfacial Cl^•.

In this study, we introduce Cu_Td²⁺ into the tetrahedral sites of Co₃O₄ to enhance the mass transfer of NH₄⁺ and weaken the binding energy of Cl^• at the Co_Oh³⁺ sites for the selective and efficient NH₄⁺-N mineralization induced by interfacial Cl^•. Compared to Ni_Td²⁺ and Zn_Td²⁺, Cu_Td²⁺ has moderate energy level matching and strong binding energy with NH₄⁺, which results in NH₄⁺ effectively overcoming electrostatic repulsion and enriching at the anode. Meanwhile, the electronic interactions of Cu_Td²⁺-O-Co_Oh³⁺ bonds increase the electron cloud density, weakening the binding energy of Cl^• at the Co_Oh³⁺ sites, which is conducive to the separation of Cl^• from the interface, and then reacting with NH₄⁺-N. As a result, NH₄⁺-N is efficiently mineralized into N₂ with low cost and energy consumption in the EC system assembled with Cu_xCo_3-xO₄ anode, superior to which is much that of the commercial DSA. Notably, the system exhibits excellent activity and durability in treating high concentrations of NH₄⁺-N from practical landfill leachate under continuous flow operation, confirming its excellent application potential. This paper provides a new idea for the efficient mineralization treatment of actual ammonia-nitrogen wastewater.

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.

Xia Chen: Writing – original draft, Software, Investigation, Formal analysis, Data curation. Ting Dai: Investigation. Meng-Ying Yin: Formal analysis. Xing-Yuan Xia: Software. Qiu-Ju Xing: Visualization, Supervision. Lei Tian: Writing – review & editing, Writing – original draft, Supervision, Methodology, Conceptualization. Jian-Ping Zou: Writing – review & editing, Project administration, Funding acquisition, Conceptualization.

This work was supported by the National Natural Science Foundation of China (Nos. 52300081, 52170082, 52470079, 51938007, and 52100186), the Postdoctoral Fellowship Program of China Postdoctoral Science Foudation (CPSF) (No. GZB20240175), the China Postdoctoral Science Foundation (No. 2024M750620), the Natural Science Foundation of Jiangxi Province (No. 20212ACB203008), and Key Laboratory of Jiangxi Province for Persistent Pollutants Prevention Control and Resource Reuse (No. 2023SSY02061). We are grateful for the financial support of the projects and research platform support provided by the laboratory.

Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.cclet.2025.111445.