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Article

Insight on the Ultrafast Water Treatment over NiFe-Layered Double Hydroxides via Electroactivation of Ferrate(VI): The Role of Spin State Regulation

by
Xinyu Gai
1,
Ningxuan Xue
1,
Pengxiang Qiu
1,*,
Yiyang Chen
1,
Da Teng
1,
Zhihui Zhang
2,
Fengling Liu
1,
Zhongyi Liu
3,* and
Zhaobing Guo
1,4
1
Jiangsu Key Laboratory of Atmospheric Environment Monitoring and Pollution Control (AEMPC), Collaborative Innovation Centre of Atmospheric Environment and Equipment Technology (CIC-AEET), School of Environmental Science and Engineering, Nanjing University of Information Science and Technology, Nanjing 210044, China
2
Jiangsu Key Laboratory of Advanced Catalytic Materials and Technology, Advanced Catalysis and Green Manufacturing Collaborative Innovation Center, Changzhou University, Changzhou 213164, China
3
Tianjin Key Laboratory of Structure and Performance for Functional Molecules, College of Chemistry, Tianjin Normal University, Tianjin 300387, China
4
Suqian University, Suqian 223800, China
*
Authors to whom correspondence should be addressed.
Water 2025, 17(9), 1369; https://doi.org/10.3390/w17091369
Submission received: 8 April 2025 / Revised: 27 April 2025 / Accepted: 29 April 2025 / Published: 1 May 2025
(This article belongs to the Special Issue Application of Advanced Oxidation Processes (AOPs) for Wastewater Treatment)
Browse Figures
Versions Notes

Abstract

:
Ferrate (Fe(VI)), an emerging green oxidant and disinfectant in water treatment, faces challenges due to its limited reaction efficiency stemming from a highly electron-deficient state. To address this, we designed NiFe-Layered Double Hydroxides (NiFe-LDHs) with different spin states to enhance electron transfer efficiency in Fe(VI)-mediated advanced oxidation processes (AOPs). We hypothesized that fine-tuning the spin state of NiFe-LDHs could optimize the balance between adsorption capabilities and electronic structure regulation. Our experiments revealed that intermediate-spin NiFeLDH-1, with a magnetic moment of 0.67 μB, exhibited the best catalytic performance, achieving 100% phenol removal. The NiFeLDH-x/Fe(VI) system demonstrated a significant synergistic enhancement in degradation efficiency. In addition, NiFeLDH-1 showed excellent performance in stability and continuous flow experiments. This study unveils a novel correlation between spin polarization and catalytic efficiency, offering insights into the optimization of electrocatalysts for Fe(VI)-mediated AOPs. The findings suggest that spin state modulation is a promising strategy to enhance the electrocatalytic activity and stability of non-noble metal catalysts.

1. Introduction

Ferrate (Fe(VI)), an emerging agent in water treatment, serves as a versatile green oxidant and disinfectant [1,2,3,4]. In Fe(VI)-mediated advanced oxidation processes (AOPs), pollutants are rapidly adsorbed and oxidized without producing secondary pollution [5,6,7]. It is well known that electron transfer is the rate-determining step in Fe(VI) treatment. However, the reaction efficiency of Fe(VI) is limited by its highly electron-deficient state [8,9]. As a result, the direct application of Fe(VI) in water treatment usually requires larger dosages and higher operating costs [2,10,11]. Therefore, improving the electron transfer efficiency of Fe(VI) is critical.
Electrocatalysis, known for its superior efficiency and economic feasibility, is widely used in electron-mediated reaction processes [12,13,14]. Developing advanced electrocatalysts with outstanding electron-conduction and catalytic stability can significantly enhance the efficacy of Fe(VI)-mediated AOPs. NiFe-Layered double hydroxides (NiFe-LDHs), a type of non-noble metal catalysts, are characterized by their exchangeable cations, anion-exchangeable layers, and tunable layered structures [15,16,17]. However, one of the primary drawbacks of NiFe-LDHs is their poor electrical conductivity, which hinders efficient electron transport and, thus, hampers their overall electrocatalytic activity [18,19]. Additionally, the weak adsorption of oxygen-containing intermediates on NiFe-LDHs affects their performance in certain catalytic reactions [20]. Furthermore, NiFe-LDHs exhibit low cycle stability, which diminishes their electrochemical performance [21,22]. Therefore, it is essential to explore effective modification methods to modify NiFe-LDHs.
Traditional modification methods of NiFe-LDHs, such as noble metal atoms doping, vacancy defect introduction, and heterostructure construction, face several challenges. For instance, doping with noble metal increases the cost of catalyst preparation, introducing vacancy defects compromises the structural stability [23], and constructing heterostructures makes it challenging to control the stability and synergistic effects at the interface [24,25,26]. These traditional modification methods are insufficient to meet the desired requirements. To address these challenges, modifying the spin state of NiFe-LDHs presents a promising strategy for enhancing both electrocatalytic activity and stability [27,28]. High-spin catalysts, characterized by a higher concentration of unpaired electrons, are particularly beneficial for reactions requiring multiple electron transfers [29,30], such as in Fe(VI)-mediated AOPs [31]. However, excessively high-spin states tend to lead to over-adsorption of intermediates [32,33], hindering the reaction. On the other hand, low-spin catalysts exhibit superior thermodynamic and chemical stability [34,35], though this configuration might diminish the reactivity at the active sites [36,37]. Therefore, it is crucial to finely tune the spin state of the catalyst to achieve an optimal balance between strong adsorption capabilities and effective regulation of the electronic structure.
To validate the aforementioned hypothesis, we prepared NiFe-LDHs with varying spin states (denoted as NiFeLDH-x) and conducted experiments to investigate the relationship between spin state and degradation efficiency in the NiFeLDH-x/Fe(VI) system. As expected, intermediate-spin NiFeLDH-1 (0.67 μB) exhibited the best catalytic performance with a 100% removal of phenol. This study unveils a novel correlation between spin polarization and catalytic efficiency, offering new insights into the optimization of electrocatalysts for Fe(VI)-mediated AOPs.

2. Experimental Section

2.1. Materials

Urea, nickel chloride hexahydrate (NiCl2·6H2O), hexahydrate ferric chloride (FeCl3·6H2O), ammonium fluoride (NH4F), phenol, sodium thiosulfate pentahydrate (Na2S2O3·5H2O), L-histidine, 4-hydroxy-TEMPO (TEMPOL), tertiary butanol (TBA), acetic acid, acetonitrile, and methyl phenyl sulfoxide oxide (PMSO2) were obtained from Aladdin Inc. (Shanghai, China). Potassium ferrate was purchased from Adamas Inc. (Shanghai, China). Boric acid (H3BO3) and sodium tetraborate (Na2B4O7·10H2O) were supported by Sinopharm Chemical Reagent Inc. (Shanghai, China). 2,2-azino-bis-(3-ethylbenzothiazoline-6-sulfonate) (ABTS) and methyl phenyl sulfoxide (PMSO) were obtained from Macklin Inc. (Shanghai, China).

2.2. Catalysts Preparation

Typically, a mixture containing NiCl2·6H2O (1.8 mmol), varying amounts of FeCl3 (0.6 mmol, 0.3 mmol, or 0.9 mmol), urea (16.6 mmol), and NH4F (12.8 mmol) was dissolved in 20 mL of deionized water. The solution was stirred continuously for 30 min. Subsequently, this solution was transferred to a 50 mL Teflon-lined stainless-steel autoclave, which was then placed in an oven and heated at 120 °C for 12 h. After the autoclave was naturally cooled to room temperature, the resulting electrocatalysts, labeled as NiFeLDH-1, NiFeLDH-2, and NiFeLDH-3, were washed three times with deionized water and ethanol to remove impurities. Finally, the washed electrocatalysts were dried in an oven at 60 °C.

2.3. Structural Characterization

X-ray diffraction (XRD, Rigaku Smart Lab, Rigaku Corporation, Tokyo, Japan) was employed to analyze the crystalline structure of the prepared catalysts, with a scan range of 10° to 80° using Cu Kα radiation (λ = 1.5418 Å). X-ray photoelectron spectroscopy (XPS, Thermofisher Escalab250 XI, Thermo Fisher Scientific, Waltham, MA, USA) was applied to investigate the chemical states of the surface elements. Scanning electron microscopy (SEM, ZEISS Gemini 300, Carl Zeiss AG, Oberkochen, Germany) and high-resolution transmission electron microscopy (HRTEM, FEI TALOS F200X, Thermo Fisher Scientific, Waltham, USA) were utilized to observe the microstructure and internal morphology of the catalysts. The magnetic properties of materials were measured using zero field-cooled (ZFC) and field-cooled (FC) techniques.

2.4. Electrochemical Measurements

The catalytic performance was assessed by an electrochemical workstation (Shanghai Chenhua Instrument Co., LTD., CHI660E, Shanghai, China) with a standard three-electrode system at room temperature. The three-electrode system consisted of an Ag/AgCl reference electrode, a platinum plate counter electrode, and the prepared catalysts as the working electrodes. The catalyst ink was prepared by dispersing 3 mg of catalyst into a solution containing 990 μL of ethanol and 10 μL of Nafion solution (5 wt%), followed by ultrasonic treatment. A 20 μL aliquot of the catalyst ink was then dropped onto a glass carbon electrode to form the working electrode. Additionally, cyclic voltammetry (CV), linear sweep voltammetry (LSV), Tafel slope measurements (Tafel), electrochemical active surface area (ECSA) determination, open circuit potential (OCP) measurements, and electrochemical impedance spectroscopy (EIS) were conducted to examine the electrochemical characteristics of the material.

3. Results and Discussion

To investigate the morphology and surface microstructures, SEM and HRTEM were employed. As revealed in Figure 1a–c, NiFeLDH-x exhibited vertical nanoplate structures, maintaining their inherent nanosheet morphology. The HRTEM images of NiFeLDH-x (Figure 1d–f) showed a typical lattice fringe of 0.25 nm, corresponding to the (012) plane in NiFeLDH-1. Correspondingly, lattice fringes of 0.24 nm and 0.27 nm were observed in NiFeLDH-2 and NiFeLDH-3, respectively. The variation in the Fe/Ni ratio significantly affected the crystal structure of NiFeLDH-x, primarily due to the larger ionic radius of iron ions, which resulted in an increase in interlayer spacing [38].
The structures of NiFeLDH-x were verified by XRD. As shown in Figure 2a, the well-defined diffraction peaks were assigned to the (003), (006), (012), (015), (018), (110), and (113) planes of NiFe-LDH (JCPDS# 80-1665). No additional impurity peaks were observed as the concentration of Fe3+ varied. The presence of Fe3+, which has a larger ionic radius, resulted in an increase in the interlayer spacing. Consequently, there was a shift in the diffraction peak positions towards lower angles as the iron content increased. This observation is consistent with the TEM results.
To explore the electronic structure changes of NiFeLDH-x, XPS analysis was performed. The XPS survey spectra confirmed the presence of Ni, Fe, and O in NiFeLDH-x (Figure 2b–d). In the high-resolution Ni 2p spectrum of NiFeLDH-1 (Figure 2b), the binding energies of Ni 2p3/2 and Ni 2p1/2 were located at 856.13 eV and 873.73 eV, respectively, which corresponded to Ni2+ [39]. These were accompanied by satellite peaks at 861.94 eV and 879.54 eV. The binding energies at 713.1 eV and 726.1 eV, corresponding to Fe 2p3/2 and Fe 2p1/2, were assigned to Fe3+ species [40], respectively (Figure 2c). In contrast, the Ni 2p3/2 and Ni 2p1/2 peaks of NiFeLDH-2 and NiFeLDH-3 both shifted to lower binding energies compared to NiFeLDH-1. Furthermore, the Fe 2p1/2 peaks of NiFeLDH-x shifted to higher binding energies with increasing Fe doping, which could be attributed to the higher electronegativity of Ni relative to Fe.
In the high-resolution O 1s spectra of NiFeLDH-x (Figure 2d), the peak labeled O1 at 531.2 eV corresponded to M-OH bonds [41], the peak O2 at 531.8 eV was attributed to surface oxygen defect/vacancy species (Ovac) [42], and the component O3 at 532.3 eV was associated with hydroxy species from surface-adsorbed water [43]. As the Fe content in NiFeLDH-x increased, the O1 peak shifted towards higher binding energies, indicating an enhanced strength of the bond between iron ions and oxygen.
To clarify the spin states of NiFeLDH-x, temperature-dependent magnetization measurements were performed under an applied field of 1 kOe during the ZFC processes. The magnetic susceptibilities, obtained from magnetization measurements (χ = M/H), conformed to the Curie–Weiss law (χ = C/(T − Θ), where C represents the Curie constant. The effective magnetic moment (μeff = μB (8C)0.5) was calculated by linearly fitting of χ−1 − −T (Figure 2e) [44]. NiFeLDH-3 exhibited the highest experimental μeff (5.15 μB), followed by NiFeLDH-1 (0.67 μB) and NiFeLDH-2 (0.38 μB). The order of the spin states corresponded to the μeff values, indicating that a higher μeff value was associated with a higher spin state.
The catalytic performances of NiFeLDH-x were evaluated based on their ability to electrocatalytically decompose phenol (Figure 2f). When Fe(VI) alone was added, the phenol degradation rate was 58%. With only NiFeLDH-x added, the phenol degradation was approximately 10% (Figure S1). However, when NiFeLDH-x/Fe(VI) was introduced, a significant synergistic enhancement in the degradation rate was observed. Among the prepared catalysts, NiFeLDH-1 demonstrated the highest catalystic activity, attaining a complete degradation rate of 100% under identical conditions. The relationship between the rate constant (Figure S2) and temperature was modeled using the Arrhenius equation (Figure 2g–i). The activation energy for the reaction with NiFeLDH-1 was calculated to be 30.6 kJ·mol−1, which was lower than that for NiFeLDH-2 (32.7 kJ·mol−1) and NiFeLDH-3 (37.1 kJ·mol−1), further demonstrating that NiFeLDH-1 exhibited superior catalytic performance.
Figure 3a was a volcano plot depicting the relationship between phenol degradation rates and spin states. To further delve into the degradation mechanism in the NiFeLDH-x/Fe(VI) system, it was indispensable to identify and analyze the primary reactive species involved in the process. Fe(V/IV), formed during the reduction of Fe(VI), was found to exhibit higher reactivity towards certain organic pollutants than Fe(VI). To precisely assess the role of Fe(V/IV) in pollutant elimination, PMSO was used to quench Fe(V/IV) [45,46]. Upon interaction with Fe(V/IV), PMSO facilitates an oxygen atom transfer (Figure 3b,c), resulting in the formation of PMSO2, a product that is not produced in free radical-mediated reactions. The impacts of PMSO on phenol degradation across different NiFeLDH-x catalysts were similar (Figure 3e), suggesting that the observed differences in catalytic performance were not attributable to the behavior of Fe(V/IV).
The radical scavenging experiments (Figure 3d) identified singlet oxygen (1O2•) as the predominant reactive species contributing to the degradation of phenol [47,48]. The occurrence of a higher spin state in NiFeLDH-x was found to be associated with enhanced paramagnetism and increased interlayer spacing, which facilitated the adsorption of oxygen-containing spin-magnetic intermediates. Trapping 1O2• had the least impact on NiFeLDH-3 (Figure 3e), implying that more singlet oxygen was adsorbed rather than reacting. Consequently, the presence of excessively high spin states may impede the reaction, thereby diminishing the catalytic efficacy. Therefore, the reaction mechanism is summarized in Figure 4.
To explore the effect of the spin-state on performance, the electrochemical behavior of NiFeLDH-x was examined under identical conditions. Polarization curves were recorded to evaluate the electrochemical performance of the NiFeLDH-x catalysts (Figure 3f–h). NiFeLDH-1 exhibited a lower oxidation onset potential and a higher current density, indicating faster electron transfer and enhanced electrocatalytic activity. To further elucidate the reaction kinetics of NiFeLDH-x, the Tafel slopes were calculated (Figure 3i). The Tafel slope of NiFeLDH-1 was 1.26 mV·dec−1, which was significantly lower than those of NiFeLDH-2 (3.24 mV·dec−1) and NiFeLDH-3 (3.36 mV·dec−1), suggesting more favorable reaction kinetics for NiFeLDH-1.
Furthermore, the semicircular arc observed in the Nyquist plot for NiFeLDH-1 was smaller than those for NiFeLDH-2 and NiFeLDH-3 (Figure 5a), suggesting reduced charge transfer resistance (Rct). This can be attributed to the optimal ratio of Ni2+ to Fe3+ in NiFeLDH-1 [49]. These observations implied that NiFeLDH-1 possessed superior electronic conductivity and charge transfer efficiency, both of which were essential for electrocatalytic applications. The ECSA of the catalysts was quantified at various CV scan rates within a non-faradaic potential region (Figure 5e). The ECSA values for NiFeLDH-1, NiFeLDH-2, and NiFeLDH-3 were 0.984, 0.614, and 0.504 mF·cm-2, respectively. NiFeLDH-1 exhibited the largest ECSA, signifying a greater reactive surface area and a higher number of active sites. In contrast, NiFeLDH-3 exhibited a reduced capacitive area, primarily resulting from the excessive Fe3+ doping. Under ambient conditions, NiFeLDH-x displayed minimal potential fluctuations, with the OCP remaining stable over extended testing periods. As shown in Figure 5f, the addition of Fe(VI) led to a rapid increase in the OCP of NiFeLDH-x. Notably, the OCP increase for NiFeLDH-2 and NiFeLDH-3 was less pronounced compared to NiFeLDH-1. Once the potential stabilized, the addition of phenol caused a decrease in the OCP, indicating that phenol served as an electron acceptor within the reaction system. This observation further suggested that NiFeLDH-1 possessed a higher capacity for substantial electron acceptance.
To optimize the experimental conditions and minimize energy consumption, a series of controlled experiments were conducted (Figure 6a). Notably, the degradation rate initially increased as the initial pH value shifted from 6 to 7, followed by a subsequent decrease as the pH value ranged from 7 to 9. This behavior could be attributed to the pH sensitivity of Fe(VI). Under acidic conditions, the oxidizing capacity of Fe(VI) was enhanced, but its stability was compromised, leading to easier decomposition. Conversely, under alkaline conditions, the stability of Fe(VI) was improved, but the oxidizing capacity of Fe(VI) was weakened. The experimental results indicated that a pH of 7 is the optimal condition for the degradation of phenol.
As illustrated in Figure 6a, the optimal experimental conditions were achieved at an applied potential of 0.8 V (vs. RHE), with a catalyst dosage of 10 mg and a phenol/Fe(VI) molar ratio of one-fifteenth. This approach achieved a balance between economic feasibility and environmental sustainability, optimizing phenol degradation while minimizing the use of chemicals.
Electrocatalytic durability is a critical performance indicator for electrocatalysts. Figure 6b demonstrated that the performance of NiFeLDH-1 remained stable even after 1000 CV cycles of continuous operation, with only a minimal decline in activity observed (Figure 6b). Similarly, the catalytic activity showed negligible changes after five experimental runs, confirming that NiFeLDH-1 retains robust catalytic activity over multiple cycles (Figure 6c).
To simulate actual water treatment, three continuous-flow cells were employed (Figure 6e). Phenol and Fe(VI) solutions were introduced separately into the initial flow cell and then flowed through the three cells. The experiments were conducted at room temperature, with electrocatalytic reactions initiated to simulate the electrochemical conditions encountered in practical water treatment processes. The experimental results indicated that phenol was essentially eliminated from the wastewater after treatment in the four flow cells, achieving a degradation efficiency of 100% (Figure 6d).

4. Conclusions

In summary, the synthesis and characterization analysis of NiFeLDH-x with varying spin states was successfully achieved. Notably, the intermediate spin NiFeLDH-1 exhibited the highest catalytic efficiency, as anticipated. When utilized in the electrocatalyst/Fe(VI) system, NiFeLDH-1 achieved 100% phenol degradation in the H-type electrolytic cell, and consistent 100% phenol degradation was also observed in the four flow cells. The superior electrocatalytic performance of NiFeLDH-1 was further confirmed by its lower oxidation onset potential, reduced Tafel slope, and enhanced ECSA. Moreover, NiFeLDH-1 maintained excellent stability under long-term and cyclic testing conditions. This research provided valuable insights into the role of spin polarization in enhancing electronic catalytic performance.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/w17091369/s1, Text S1: Catalyst performance; Figure S1: The degradation of phenol in NiFeLDH-1, Fe(VI) and NiFeLDH-1/Fe(VI) system; Figure S2: Pseudo-first-order kinetics o NiFeLDH-x/Fe(VI) system at 25 °C; Figure S3: The cyclic voltammetry (CV) of NiFeLDH-x; Figure S4: Effect of phenol/Fe(VI) on the NiFeLDH-1/Fe(VI) system; Figure S5: Effect of pH on the NiFeLDH-1/Fe(VI) system; Figure S6: Effect of catalyst dosage on the NiFeLDH-1/Fe(VI) system; Figure S7: Effect of potential on the NiFeLDH-1/Fe(VI) system.

Author Contributions

X.G.: writing—original draft. N.X.: investigation. P.Q.: conceptualization, methodology, validation, funding acquisition. Y.C.: investigation. D.T.: investigation. Z.Z.: investigation. F.L.: writing—review and editing. Z.L.: investigation. Z.G.: supervision, funding acquisition. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by the National Natural Science Foundation of China (Nos. 51908294, 42473024) and Suqian Talent Program Project (SQXY202429).

Data Availability Statement

All data generated or analyzed during this study are included in this published article and its Supplementary Information Files.

Conflicts of Interest

There are no conflicts of interest declared in this article.

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Water 17 01369 g001
Figure 1. SEM images of (a) NiFeLDH-1, (b) NiFeLDH-2, (c) NiFeLDH-3; low-magnification and high-magnification TEM images of (d) NiFeLDH-1, (e) NiFeLDH-2, (f) NiFeLDH-3.
Figure 1. SEM images of (a) NiFeLDH-1, (b) NiFeLDH-2, (c) NiFeLDH-3; low-magnification and high-magnification TEM images of (d) NiFeLDH-1, (e) NiFeLDH-2, (f) NiFeLDH-3.
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Figure 2. (a) XRD images of NiFeLDH-x, (b) high-resolution Ni 2p XPS spectrums, (c) high-resolution Fe 2p XPS spectrums, and (d) high-resolution O 1s XPS spectrums of NiFeLDH-x, (e) magnetization curves of NiFeLDH-x, (f) electrocatalytic decomposition of phenol by NiFeLDH-x, (g–i) temperature degradation kinetics of NiFeLDH-1, NiFeLDH-2, and NiFeLDH-3.
Figure 2. (a) XRD images of NiFeLDH-x, (b) high-resolution Ni 2p XPS spectrums, (c) high-resolution Fe 2p XPS spectrums, and (d) high-resolution O 1s XPS spectrums of NiFeLDH-x, (e) magnetization curves of NiFeLDH-x, (f) electrocatalytic decomposition of phenol by NiFeLDH-x, (g–i) temperature degradation kinetics of NiFeLDH-1, NiFeLDH-2, and NiFeLDH-3.
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Figure 3. (a) The correlation between phenol degradation and μeff; (b) phenol degradation with PMSO in NiFeLDH-x/Fe(VI) system; (c) PMSO consumption and PMSO2 production amount in NiFeLDH-x/Fe(VI) system; (d) the trapping experiments for identifying active species; (e) phenol degradation in the presence of PMSO or 1O2; (f–h) polarization curves recorded on NiFeLDH-x under various conditions; (i) the corresponding Tafel plots.
Figure 3. (a) The correlation between phenol degradation and μeff; (b) phenol degradation with PMSO in NiFeLDH-x/Fe(VI) system; (c) PMSO consumption and PMSO2 production amount in NiFeLDH-x/Fe(VI) system; (d) the trapping experiments for identifying active species; (e) phenol degradation in the presence of PMSO or 1O2; (f–h) polarization curves recorded on NiFeLDH-x under various conditions; (i) the corresponding Tafel plots.
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Figure 4. Schematic illustration of reaction mechanism for NiFeLDH-x/Fe(VI) system.
Figure 4. Schematic illustration of reaction mechanism for NiFeLDH-x/Fe(VI) system.
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Figure 5. (a) The EIS plots of NiFeLDH-x; cyclic voltammograms at different scanning rates of (b) NiFeLDH-1, (c) NiFeLDH-2, and (d) NiFeLDH-3; (e) the ECSA of NiFeLDH-x; (f) OCPT curves of NiFeLDH-x.
Figure 5. (a) The EIS plots of NiFeLDH-x; cyclic voltammograms at different scanning rates of (b) NiFeLDH-1, (c) NiFeLDH-2, and (d) NiFeLDH-3; (e) the ECSA of NiFeLDH-x; (f) OCPT curves of NiFeLDH-x.
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Figure 6. (a) Effect of potential, phenol/Fe(VI) molar ratio, catalyst dosage, and pH on the NiFeLDH-1/Fe(VI) system; (b) CV curves of NiFeLDH-1 before and after 1000 cycles; (c) cycling measurement of phenol degradation in the NiFeLDH-1/Fe(VI) system; (d) phenol removal efficiency at different flow rates in the continuous-flow NiFeLDH-1/Fe(VI) reactor system; (e) schematic of the continuous-flow NiFeLDH-1/Fe(VI) reactor system.
Figure 6. (a) Effect of potential, phenol/Fe(VI) molar ratio, catalyst dosage, and pH on the NiFeLDH-1/Fe(VI) system; (b) CV curves of NiFeLDH-1 before and after 1000 cycles; (c) cycling measurement of phenol degradation in the NiFeLDH-1/Fe(VI) system; (d) phenol removal efficiency at different flow rates in the continuous-flow NiFeLDH-1/Fe(VI) reactor system; (e) schematic of the continuous-flow NiFeLDH-1/Fe(VI) reactor system.
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MDPI and ACS Style

Gai, X.; Xue, N.; Qiu, P.; Chen, Y.; Teng, D.; Zhang, Z.; Liu, F.; Liu, Z.; Guo, Z. Insight on the Ultrafast Water Treatment over NiFe-Layered Double Hydroxides via Electroactivation of Ferrate(VI): The Role of Spin State Regulation. Water 2025, 17, 1369. https://doi.org/10.3390/w17091369

AMA Style

Gai X, Xue N, Qiu P, Chen Y, Teng D, Zhang Z, Liu F, Liu Z, Guo Z. Insight on the Ultrafast Water Treatment over NiFe-Layered Double Hydroxides via Electroactivation of Ferrate(VI): The Role of Spin State Regulation. Water. 2025; 17(9):1369. https://doi.org/10.3390/w17091369

Chicago/Turabian Style

Gai, Xinyu, Ningxuan Xue, Pengxiang Qiu, Yiyang Chen, Da Teng, Zhihui Zhang, Fengling Liu, Zhongyi Liu, and Zhaobing Guo. 2025. "Insight on the Ultrafast Water Treatment over NiFe-Layered Double Hydroxides via Electroactivation of Ferrate(VI): The Role of Spin State Regulation" Water 17, no. 9: 1369. https://doi.org/10.3390/w17091369

APA Style

Gai, X., Xue, N., Qiu, P., Chen, Y., Teng, D., Zhang, Z., Liu, F., Liu, Z., & Guo, Z. (2025). Insight on the Ultrafast Water Treatment over NiFe-Layered Double Hydroxides via Electroactivation of Ferrate(VI): The Role of Spin State Regulation. Water, 17(9), 1369. https://doi.org/10.3390/w17091369

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