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Article

CoFeNi-Layered Double Hydroxide Combined Activation of PMS and Ozone for the Degradation of Rhodamine B in Water

by
Xiaohan Zhu
,
Liang Song
and
Jia Miao
*
College of Civil Engineering, Zhejiang University of Technology, Hangzhou 310023, China
*
Author to whom correspondence should be addressed.
Separations 2025, 12(10), 276; https://doi.org/10.3390/separations12100276
Submission received: 28 August 2025 / Revised: 1 October 2025 / Accepted: 3 October 2025 / Published: 9 October 2025
(This article belongs to the Special Issue Photocatalytic Oxidation and Advanced Oxidation Processes for Sustainable Water Treatment)
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Abstract

The development of efficient and sustainable advanced oxidation processes (AOPs) for organic pollutant removal is of great significance for water purification. In this study, a CoFeNi-layered double hydroxide (CoFeNi-LDH) catalyst was synthesized and applied for the simultaneous activation of peroxymonosulfate (PMS) and ozone to degrade rhodamine B (RhB) in aqueous solution. The CoFeNi-LDH/PMS/ozone system achieved a remarkable RhB removal efficiency of 95.2 ± 1.2% within 8 min under neutral pH conditions. Systematic parametric studies revealed that synergistic interactions among CoFeNi-LDH, PMS, and ozone contributed to the generation of reactive oxygen species (ROS), primarily sulfate radicals (SO4•−) and singlet oxygen (1O2), as confirmed by EPR and quenching experiments. Density functional theory (DFT) calculations demonstrated that ozone enhanced PMS adsorption and activation at CoFeNi catalytic sites. The catalyst exhibited robust magnetic recyclability and structural stability after repeated use. This work highlights a synergistic catalytic strategy for PMS/ozone activation, offering an effective and environmentally friendly platform for dye wastewater remediation.

1. Introduction

Dye-containing wastewater represents a persistent environmental challenge due to its high chemical stability, complex composition, and resistance to biodegradation [1]. Rhodamine B (RhB) is a synthetic xanthene dye widely used in textiles, food additives, cosmetics, and biological staining [2]. RhB is frequently detected in industrial effluents and natural water bodies. An average concentration of 300 mg/L of synthetic dyes, including highly genotoxic and carcinogenic azo dyes, is present in the textile effluents [3]. As a recalcitrant organic contaminant, RhB poses serious ecological and health risks, including carcinogenicity, mutagenicity, and cytotoxicity [4]. Therefore, the development of effective and sustainable technologies to remove RhB from water is of great environmental significance.
Advanced oxidation processes (AOPs) are regarded as powerful technologies for degrading such persistent organic pollutants, relying on the in situ generation of reactive oxygen species (ROS) with strong oxidizing capabilities [5,6,7,8]. Among various AOPs, peroxymonosulfate (PMS)-based oxidation systems have garnered increasing attention due to the formation of sulfate radicals (SO4•−) and hydroxyl radicals (•OH), which can non-selectively degrade a broad spectrum of organic compounds [9,10]. PMS activation can be achieved through transition metal catalysts, UV irradiation, heat, or carbon materials, with transition metal-based heterogeneous catalysts being particularly attractive for their stability, recoverability, and catalytic efficiency [11,12,13]. Ozone is another widely used oxidant in water treatment, capable of directly oxidizing contaminants and generating •OH under alkaline or catalytic conditions [14]. However, the efficacy of ozonation alone is often limited by its relatively low radical yield and selectivity. To enhance oxidation efficiency, coupling ozone with other AOP components, such as PMS, has emerged as a promising strategy [15]. This combination can further introduce a variety of ROS including SO4•−, •OH, and the direct oxidation of O3 to fully exert the oxidative degradation ability. However, current research mostly focuses on the activation of single oxidants, while studies on PMS/ozone combined systems remain scarce. Most existing reports on combined PMS/ozone activation focus on the efficacy, such as Deniere et al., who studied the advantageous ratio of PMS and ozone [16], Guo et al., who focused on enhanced efficacy of metal ions on PMS/ozone [17], and How et al., who researched the improvement of the degradation kinetics of PMS/ozone system compared to the single system [18]. However, the underlying mechanism of heterogeneous activation for PMS/ozone system is still unclear, especially the behavior of PMS and ozone at the catalyst interface.
Layered double hydroxides (LDHs), consisting of positively charged metal hydroxide layers with exchangeable interlayer anions, have emerged as effective heterogeneous catalysts for PMS or ozone activation [19,20,21]. Their tunable metal compositions, abundant surface hydroxyl groups, and redox-active sites enable flexible design for advanced oxidation applications [22,23]. Zeng et al. reported that CoFeNi-LDH effectively activated PMS to remove RhB [24]. Although numerous studies have demonstrated the efficacy of LDHs in activating PMS or ozone individually, to date just a few research studies concentrated on the coupled system. Han et al. studied MgFe-LDH to activate PMS/ozone for the removal of RhB with good results (the RhB degradation rate was 0.34 min−1) [25]. However, they only focused on the efficiency of the coupled system without in-depth study of the intrinsic synergistic activated mechanism of ozone and PMS via LDH.
In this work, we developed a trimetallic CoFeNi-LDH catalyst using a facile coprecipitation method and investigated its application in the simultaneous activation of PMS and ozone for the degradation of RhB, a typical organic dye pollutant. The catalytic performance was evaluated under various environmental conditions, and mechanistic insights were gained through radical scavenging, EPR spectroscopy, and DFT calculations. The goal of this study is to elucidate the cooperative mechanism between PMS and ozone in the presence of CoFeNi-LDH and provide a sustainable strategy for the remediation of dye-contaminated water.

2. Experimental Methods

2.1. Chemicals

Potassium peroxymonosulfate (PMS, 2KHSO5·KHSO4·K2SO4, purity ≥ 47%, CAS: 70693-62-8), purchased from Aladdin Reagent Company (Shanghai, China); methanol (CH3OH), purchased from Sigma-Aldrich Reagent Company in the United States; rhodamine B (RhB), sodium thiosulfate (Na2S2O8), hexahydrate iron chloride (FeCl3·6H2O), hexahydrate cobalt chloride (CoCl2·6H2O), hexahydrate nickel chloride (NiCl2·6H2O), NaOH, HNO3, tert-butanol (TBA), benzoquinone (BQ), benzoic acid (BA), sodium azide (NaN3), 5,5-Dimethyl-1-pyrroline N-oxide (DMPO), 2,2,6,6-tetramethyl-4-piperidone (TEMP) were obtained from Sigma-Aldrich Chemical Co. Ltd., (Shanghai, China). All chemicals were of at least analytical grade and used without further purification. All water used in the experiment was double-distilled water.

2.2. Catalyst Synthesis

Prepare CoFeNi-LDH using the double-drop coprecipitation method [26]. Accurately weigh the three metal salts and dissolve them in 100 mL of water, named Solution A; 8 g of NaOH was dissolved in 100 mL of water (concentration 2 mol/L), named Solution B; Solutions A and B were slowly added dropwise to 150 mL of water under stirring, while maintaining a pH of 10.5 with 60 °C water bath. The resulting solution was crystallized at 60 °C for 24 h. Wash the precipitate until the supernatant is neutral, then dry the product in a 60 °C oven. The product was ground to obtain powder CoFeNi-LDH.

2.3. Characterization Methods

The crystalline structure of samples was recorded using X-ray diffraction (XRD, Philips PANalytical X’Pert PRO, Amsterdam, the Netherlands) with Cu Kα radiation. The surface morphology of samples was observed on a scanning electron microscope (SEM, HITACHI S-4800, Tokyo, Japan) and transmission electron microscopy (TEM, TF20, Joel 2100F, Peabody, MA, USA). The element composition was analyzed using an energy-dispersive X-ray spectrum (EDX) and elemental mapping in a Super-XTM system. X-ray photoelectron spectroscopy (XPS, ESCALAB MK II, Berkshire, UK) was used to analyze the elemental composition and chemical oxidation state. The magnetic properties of the material were tested by Vibrating Sample Magnetometer (VSM, LakeShore7404, O’Fallon, MO, USA). The electron paramagnetic resonance (EPR) signals of ROS spin trapped were carried out on a Bruker model ESR E500 spectrometer (UK) by spin-trap reagents of DMPO and TEMP.

2.4. Experimental Procedure

Weigh a certain amount of CoFeNi-LDH and add it to a series of 100 mL RhB solutions (concentration 20 mg/L) with a certain concentration of ozone. Then add a certain concentration of freshly prepared PMS to the solution and place it in a constant-temperature shaker. Samples were taken at different time points and were separated magnetically, and then 1 mL of the supernatant was placed in pre-prepared centrifuge tubes containing Na2S2O8 (concentration 0.5 mol/L, volume 1 mL), and the concentration of RhB was determined using UV spectrophotometry (U-3900, HITACHI, Japan) at λ = 550 nm. If necessary, the initial pH of the solution was adjusted using H2SO4 or NaOH solution. Triplicate batch tests were performed to ensure accuracy, and the average values with standard deviations were presented.
By systematically varying experimental conditions such as solution pH, CoFeNi-LDH dosage, PMS concentration, ozone concentration, and temperature in the aforementioned process, the degradation efficiency of CoFeNi-LDH/PMS/ozone system on RhB in water was systematically investigated.
For the quenching experiment, four quenching agents (tert-butanol, benzoquinone, benzoic acid, and sodium azide) were added to the initial RhB solution to quench different ROS and investigate the mechanism of RhB degradation in the system.

2.5. Analysis and Calculation Methods

The dissolved ozone was measured by indigo method [27]. Samples were collected in gas-tight glass syringes, kept at 0–4 °C, and analyzed immediately. Each sample (3–5 mL) was mixed with an equal volume of acidified indigotrisulfonate reagent (final indigo ≈ 5 × 10−5 M, pH 2.5), and absorbance at 600 nm was measured within 1 min against a reagent blank (UV–Vis spectrophotometer, 1.00 cm pathlength). Ozone concentration was calculated from the decrease in absorbance using a calibration curve prepared with iodometrically standardized ozone solutions; the method detection limit was ~0.002 mg L−1, and precision (RSD) was <3% at 0.10 mg L−1.
ΔA = Ablank − Asample
[O3] (mg/L) = ΔA600/0.42
The activation energy of the system was calculated via the arrhenius equation [28]:
lnkobs = lnA + (−Ea/RT)
where kobs (min−1) is the apparent rate constant, Ea (kJ/mol) is the activation energy for RhB degradation, T (K) is the thermodynamic temperature, R is the ideal gas constant (8.314 J/mol·K), and A is a constant.
The structure models of the complex structure of PMS or PMS/ozone on CoFeNi-LDH were optimized under the framework of density of functional theory (DFT) based on the Cambridge Sequential Total Energy Package known as CASTEP. The exchange correlation functional under the generalized gradient approximation (GGA) with norm-conserving pseudopotentials and Perdew–Burke–Ernzerhof (PBE) functional was adopted to describe the electron–electron interaction [29,30]. An energy cutoff of 340 eV was used and a k-point sampling set of 2 × 2 × 1 was tested to be converged. A force tolerance of 0.01 eV Å−1, energy tolerance of 1.0 × 10−5 eV/atom, and maximum displacement of 1.0 × 10−3 Å were considered. Each atom in the storage models is allowed to relax to the minimum in the enthalpy without any constraints. The vacuum space along the z direction is set to be 20 Å, which is enough to avoid interaction between the two neighboring images exchange correlation functional. Spin-polarized calculations were used in the geometry optimization stage. All DFT calculations above were performed using Material Studio 8.0 program.
The adsorption energy of the complex was calculated from the formula of Ead = E(A + B) − E(A) − E(B), where E(A) and E(B) are the free energy of isolated molecules, E(A + B) is the total energy of the complex structure.

3. Results and Discussion

3.1. Characterization

Figure 1a,b show the SEM image of CoFeNi-LDH. As can be seen from the figure, CoFeNi-LDH exhibits the typical layered structure of LDHs, with a rough surface. Further, the element mapping of the SEM images (Figure 1c–h) indicates that the elements Co, Fe, Ni, and O are uniformly distributed on the surface of the material. Figure 2a shows the XRD pattern of the CoFeNi-LDH. Clearly, the following diffraction peaks are observed at the diffraction angles 2θ of 11.4°, 22.4°, 34.6°, 35.8°, 60.6°, and 63.2°, corresponding to classical crystal planes (003), (006), (012), (311), (110), and (113) of LDHs [24]. The chemical composition of CoFeNi-LDH was examined via XPS analysis (Figure 2b). Five main peaks including Ni, Co, Fe, O, and C are found in the XPS spectrum also reflects the elemental composition of CoFeNi-LDH, and the atomic percentages are roughly consistent with the molar ratios of the various metal elements in the preparation process, indicating that the three metal elements precipitated uniformly. In addition, a largest proportion of O element (69.0%) mainly comes from the large number of hydroxyl groups on the surface of CoFeNi-LDH and the adsorbed H2O on the surface. Furthermore, the VSM test (Figure 2c) shows that its saturated magnetization is determined to be 0.72 emu/g, suggesting that CoFeNi-LDH is weak magnetic but has the advantage of being magnetically separable within 30 min [31]. Figure 2d shows the nitrogen adsorption–desorption curve of CoFeNi-LDH. The specific surface area is 167.7 m2/g with 6.6 nm average pore size. High specific surface area and small pore size are the remarkable characteristics of LDHs, which are beneficial for providing abundant sites for activation reactions. These characterization results prove that the synthesized materials have a typical structure and properties of LDHs.

3.2. Catalytic Performance Tests

Firstly, various systems (including PMS, ozone, PMS/ozone, CoFeNi-LDH, CoFeNi-LDH/ozone, CoFeNi-LDH/PMS, and CoFeNi-LDH/PMS/ozone) were tested for the degradation of RhB. As shown in Figure 3a, only PMS, ozone, or CoFeNi-LDH had limited removal of RhB (only 2.0%, 4.9%, and 16.2%, respectively). This indicates that the direct oxidation ability of PMS and the adsorption ability of CoFeNi-LDH are very weak, while the direct oxidation ability of ozone is stronger than that of PMS. In addition, 24.5% of RhB was removed by CoFeNi-LDH/ozone system, suggesting that ozone could be activated by CoFeNi-LDH. Further, PMS/ozone system that removed 40% RhB is higher than single PMS or ozone, indicating the synergistic effect of PMS and ozone. An amount of 58.3% RhB was removed by CoFeNi-LDH/PMS, showing the good activation ability of CoFeNi-LDH for PMS, which has been reported [24]. Obviously, when CoFeNi-LDH, PMS, and ozone added simultaneously, the color of solution fades quickly, and this system has the best degradation effect on RhB (95.2 ± 1.2%) within 8 min. The highly efficient activation and degradation effects of the CoFeNi-LDH/PMS/ozone system clearly demonstrate the synergistic and enhancing effects among the three components. The leaching metal ions were measured via ICP-MS. Co, Fe, and Ni ions are 0.52, 0.21, and 0.05 mg/L, respectively. Such low ion concentrations cannot induce an efficient homogeneous reaction. The efficient degradation of RhB in CoFeNi-LDH/PMS/ozone system mainly comes from heterogeneous reactions. In addition, the TOC removal in these different systems within 30 min were measured as depicted in Figure 3b. An amount of 70.5% TOC was removed in CoFeNi-LDH/PMS/ozone system, which is higher than the sum (62.3%) of the other three systems CoFeNi-LDH/PMS (40.2%), CoFeNi-LDH/ozone (13.6%), and PMS/ozone (8.5%). This is also strong evidence of the existence of synergetic effects in the system. Additionally, some other relative LDHs that were used to activate PMS or ozone for the degradation of RhB were compared with CoFeNi-LDH/PMS/ozone system in Table 1. Among these LDHs, the kobs of CoFeNi-LDH/PMS/ozone system is the highest proving the excellent performance of the system.
The pH value of the solution can affect the charged properties of the catalyst surface and the form of PMS in water, thereby affecting the degradation of pollutants [38]. Further, the removal of RhB by CoFeNi-LDH/PMS/ozone system at a pH range of 5.0 to 9.0 was investigated as presented in Figure 3c. Amounts of 54.3%, 74.4%, and 96.7% of RhB that were degraded by the system at pH values are 5.0, 7.0, and 9.0 within 4 min, respectively, and the reaction rate constants calculated using the first-order kinetic equation are 0.2, 0.38, and 0.78 min−1, respectively (Figure 3d). Some studies have shown that under slightly alkaline conditions, the abundant hydroxyl groups on the surface of the CoFeNi-LDH catalyst can chelate metal ions (Co2+ and Fe2+), preventing them to dissolve into the solution. The layer of complexes formed on the catalyst surface also facilitates the activation process of PMS [39,40]. Considering that the pH of environmental water is neutral, subsequent experiments were conducted with a solution pH of 7.0.
The dosage of the oxidizing agent directly impacts the ultimate degradation capacity of the degradation system for pollutants. To investigate the effect of PMS concentration on the degradation of RhB by the CoFeNi-LDH/PMS/ozone system, several different concentrations (0 mM, 0.25 mM, 0.5 mM, 1.0 mM, and 2.0 mM) of PMS were separately added into RhB solution (the concentration of 20 mg/L and the pH adjusted to 7.0) with CoFeNi-LDH dosage of 0.2 g/L and an ozone concentration of 5 mg/L. The experimental results are shown in Figure 3e. The degradation percent of RhB in the solution gradually increased with the increase in PMS concentration. When the PMS concentrations were 0, 0.25, 0.5, 1.0, and 2.0 mM, the degradation rates of RhB were 11.7%, 35.7%, 60.3%, 74.4%, and 81.9% within 4 min, respectively. This is because under conditions where the amount of catalyst CoFeNi-LDH and ozone concentration are constant, a higher PMS concentration in the solution results in a greater number of ROS generated within the same time frame. The abundance of ROS enables rapid degradation of RhB in the solution. However, when the PMS concentration exceeds 1.0 mM, further increasing its concentration does not significantly enhance the degradation efficiency of RhB. This is because excess HSO5− in the solution can convert highly oxidizing SO4•− and •OH into less oxidizing SO5•− through the following reaction (Equations (1) and (2)), which is not conducive to the degradation of pollutants in the solution [41,42]. The PMS concentration was fixed at 1.0 mM, and the effect of varying ozone concentrations (0 mg/L, 2.5 mg/L, 5.0 mg/L, and 7.5 mg/L) on the degradation efficiency of RhB in the solution was investigated (Figure 3f). As the ozone concentration increased from 0 to 7.5 mg/L, the degradation rate of RhB in the solution by the system rapidly increased from 39.2% to 84.9% within 4 min. Similar to the effect of PMS, when the ozone concentration is increased over 5.0 mg/L, the degradation efficiency of pollutants only had a limit increase. This is because excess ozone in the solution can convert SO4•− and •OH into SO5•− and HO2•, respectively (Equations (3) and (4)) [43,44]. Based on the above experimental results, subsequent experiments selected PMS and ozone concentrations of 1 mM and 5.0 mg/L, respectively.
HSO5− + SO4•− → SO42− + SO5•− + H+
HSO5− + •OH → SO5•− + H2O
O3 + SO4•− → SO5•− + O2
O3 + •OH → HO2• + O2
The dosage of the catalyst is another important factor influencing the performance of AOP system. To investigate the effect of CoFeNi-LDH dosage on the degradation efficiency of RhB, several different CoFeNi-LDH dosages were added to RhB solution (concentration of 20 mg/L) and the pH of the solution was adjusted to 7.0, while PMS and ozone concentrations of 1 mM and 5.0 mg/L were added. The experimental results are shown in Figure 3g. When the CoFeNi-LDH dosage were 0, 0.025 g/L, 0.05 g/L, 0.1 g/L, 0.2 g/L, and 0.3 g/L, the degradation rates of RhB in the solution were 10.0%, 55.2%, 58.6%, 78.2%, 94.5%, and 97.8%, respectively, within 8 min. Obviously, as the dosage of CoFeNi-LDH increases, more active sites were provided for the activation of PMS and ozone to produce ROS. Additionally, increasing the catalyst dosage facilitates the RhB onto the material surface, enabling oxidation reactions and the degradation of organic pollutants. Corresponding to this, the kobs values are increased as the dosage of CoFeNi-LDH increases (Figure 3h). However, the growth trend of kobs slows down when the dosage exceedes 0.2 g/L. Therefore, the subsequent experiments selected a CoFeNi-LDH dosage of 0.2 g/L.
Natural water bodies contain inorganic anions such as Cl−, NO3−, CO32−, HCO3−, and SO42−, with concentrations of approximately 1.5 × 10−3 mol/L [45]. According to literature reports, these coexisting ions could consume ROS in the catalytic system, thereby hindering the system’s degradation of the target pollutant [46,47]. In this study, 3 mM Cl−, NO3−, CO32−, HCO3−, and SO42− were used to investigate their effects on the degradation of RhB by the CoFeNi-LDH/PMS/ozone system, with the results shown in Figure 3i. Previous studies have shown that Cl− can compete with radicals in the system, thereby reducing the degradation rate of organic pollutants. It can also react with ROS to produce fewer active species (Cl•, Cl2•−, ClOH•−, and Cl2), thereby slowing down the degradation of pollutants [48,49]. In this study, the addition of Cl− to the reaction system significantly reduced the degradation rate of RhB in the solution. Besides, other anions (NO3−, CO32−, HCO3−, and SO42−) have only a slight negative effect on the removal of RhB, and when the reaction time is further extended to more than 15 min, the removal can reach 100%. Therefore, the CoFeNi-LDH/PMS/ozone system has the ability to resist ion interference.
To investigate the effect of temperature on the degradation of RhB by the catalytic system, three temperatures at 298 K, 308 K, and 318 K were carried out for the experiments. As the system temperature increased, the degradation rate of RhB gradually increased, indicating that the degradation of RhB by CoFeNi-LDH/PMS/ozone is a typical endothermic reaction. As shown in Figure 4a, the degradation rates of RhB by CoFeNi-LDH/PMS/O3 at 298 K, 308 K, and 318 K were 68.4%, 81.2%, and 94.1% within 4 min, respectively, and the corresponding kobs are 0.28, 0.42, and 0.72 min−1, respectively. The rate of a chemical reaction is closely related to the magnitude of its activation energy; the lower the activation energy, the faster the reaction rate. Therefore, reducing the activation energy effectively promotes the progression of the chemical reaction. In classical chemical kinetics, the apparent activation energy of a reaction can be calculated based on the influence of temperature on the reaction rate according to Arrhenius equation (as shown in Section 2.5). As shown in Figure 4b, the activation energy for RhB degradation in this system can be calculated from the slope of the fitted line as 33.33 kJ/mol [50]. This value is lower than the 64.14 kJ/mol, as reported by Channaeb et al. for the degradation of RhB [51]. These results indicate that RhB is more easily degraded in the CoFeNi-LDH/PMS/ozone system. Furthermore, studies have shown that when the activation energy is within the range of 10–13 kJ/mol, the reaction is primarily diffusion-controlled; however, the activation energy calculated in this study does not fall within this range. This experimental result indicates that the degradation of RhB in the CoFeNi-LDH/PMS/ozone system is controlled by intrinsic chemical reactions on the catalyst surface [52].
The reusability of a catalyst is the ultimate criterion for evaluating its practical application value. In this study, through multiple rounds of reusability experiments, the reusability of the CoFeNi-LDH catalyst in the CoFeNi-LDH/PMS/ozone process was evaluated, with RhB as the target pollutant. As shown in Figure 4c, RhB achieved a degradation rate of over 95% in all three experiments, indicating that the catalyst exhibits good reusability. Also, the degradation rates obtained from the first-order kinetic equation fitting for the three experiments were 0.36, 0.34, and 0.32 min−1, respectively, indicating that the catalytic activity of CoFeNi-LDH remains good (Figure 4d). In addition, the XRD pattern of CoFeNi-LDH before and after the reaction was measured as shown in Figure 4e, and the main characteristic peaks of CoFeNi-LDH remained well, implying the structure remains stable during the activation process. These results suggest that CoFeNi-LDH exhibits stable catalytic performance.

3.3. Reactive Species Identification

To identify the main ROS produced by CoFeNi-LDH/PMS/ozone system, EPR tests were performed using spin-trapping reagents of DMPO and TEMP to detect the generation of free radicals and singlet oxygen (1O2) accordingly. As depicted in Figure 5a, a set of increasing signals over time attributed to •OH and SO4•− were detected, showing that •OH and SO4•− were continuously produced in the system. In addition, the appearance of peaks belonging to •O2− (Figure 5b) and 1O2 (Figure 5c) proved the formation of •O2− and 1O2 in the system. Furthermore, quenching experiments were carried out to determine and compare the contribution of different ROS to the degradation of RhB among four systems including PMS/ozone, CoFeNi-LDH/ozone, CoFeNi-LDH/PMS, and CoFeNi-LDH/PMS/ozone. Firstly, the radical pathway was investigated with common scavengers of BA, TBA, BQ, and NaN3. BA could quench both SO4•− (k = 4.2 × 109 M−1 s−1) and •OH (k = 1.2 × 109 M−1 s−1) [53], while TBA was an effective scavenger of •OH (k = 6.0 × 108 M−1s−1) [54]. Additionally, BQ and NaN3 were commonly used to trap •O2− and 1O2 in the PMS activation system [55,56]. As shown in Figure 5d, the degradation rate of RhB in the CoFeNi-LDH/PMS/ozone system decreased after the addition of four quenching agents. The degradation rates of RhB in the systems with TBA and BA added decreased from 95.2% to 81% and 17%, respectively. Therefore, it was concluded that two types of free radicals, •OH and SO4•−, were generated in the system, and SO4•− played the primary role in the degradation of RhB. However, after adding BQ and NaN3 as quenching agents, the degradation rates of RhB decreased to 88% and 39%, respectively, indicating that the system also contains •O2 and 1O2. However, it should be noted that NaN3 not only reacted with 1O2 (k = 2 × 109 M−1 s−1) but also with •OH (k = 1.2 × 109 M−1 s−1) [57]. Comparing the result of NaN3 with TBA, it was reflected that 1O2 also played an important role for RhB degradation. Except for CoFeNi-LDH/ozone system (due to the low removal efficiency of RhB), a similar quencher inhibition trend to that observed in system CoFeNi-LDH/PMS/ozone was also found in the other two systems. This indicates that SO4− and 1O2 were also produced in PMS/ozone and CoFeNi-LDH/PMS system.

3.4. DFT Calculation

According to reports, oxygen on the surface of Coffeen-LDH is the main binding site for PMS molecules. However, oxygen atoms on the surface of CoFeNi-LDH are connected to various metal atoms (Co, Fe, and Ni), and differences in their chemical environments lead to differences in their adsorption capacity for PMS. Therefore, DFT calculations are used to obtain the adsorption energy of PMS on the LDH surface in order to find the optimal adsorption site. As shown in Figure 6a–d, four kinds of O sites that bond to CoFeNi, FeFeFe, FeFeCo, and FeFeNi were selected as binding sites to PMS. The negative adsorption energy indicates that the adsorption of PMS onto the CoFeNi-LDH surface is a spontaneous process. The adsorption energies of the four sites are −0.41 eV, −0.38 eV, −0.39 eV, and −0.37 eV, at CoFeNi, FeFeFe, FeFeCo, and FeFeNi sites, respectively. Among them, the adsorption energy of CoFeNi site is the strongest, indicating that the CoFeNi-doped sites are more conducive to the binding of PMS, thereby enhancing the PMS activation process. Furthermore, based on this complex structure, ozone was added around the PMS to compare the adsorption energy of PMS (Figure 6e). The results show that the adsorption energy increased to −0.49 eV, indicating that ozone enhanced PMS binding at CoFeNi site. This is important evidence, as it demonstrates that ozone promotes the activation process of PMS. In addition, as shown in the charge density diagram (Figure 6f), the Ni atoms exhibit a higher degree of electron loss compared with Fe and Co, indicating that Ni plays a significant role in modulating the electronic structure of the LDH.

3.5. Possible Catalytic Mechanisms

The chemical composition and valence states of the fresh and used CoFeNi-LDH was studied via XPS analysis to elucidate the catalytic mechanism in CoFeNi-LDH/PMS/ozone system. The XPS spectrum of Co 2P (Figure 7a) is divided to two peaks at 780.8 eV and 782.6 eV corresponding to Co(III) and Co(II), respectively [58]. The ratio of Co(III) increased from 49.5% to 60.7%, and Co(II) decreased from 50.5% to 39.3%. The increased valent of Co proved that Co element was involved in the activation process (Equations (5) and (6)). On the contrary, the XPS spectrum of Fe 2p (Figure 7b), the ratio of Fe(II) at 711.6 eV was increased from 38.4% to 49.3%, while Fe(III) at 714.8 eV was decreased from 61.6% to 50.7% [59], suggesting that Fe element could obtain electron during the activation process (Equation (10)). This is evidence that the electron could transport between Co and Fe and accelerate the activation process. In addition, O 1s XPS spectrum was also analyzed, as shown in Figure 7c. The peak at 530.6 eV belonged to the bond between O atoms and metals (OM), and the peak at 531.4 eV was attributed to hydroxyl groups (OOH) on the surface of CoFeNi-LDH [60]. The ratio of OM decreased from 65.3% to 31.7% and OOH increased from 34.7% to 68.3%. This may be due to the separation of SO4•− after the activation of surface-bound PMS, leaving hydroxyl groups. Since Ni is present in extremely low concentrations in CoFeNi-LDH, it is not a major component for the activation process. According to the XPS spectrum (Figure 7d), its peak position shows almost no change before and after the reaction, proving that it remains stable during the reaction and does not directly participate in the activation process.
On the basis of above results, possible PMS/ozone activation mechanism by CoFeNi-LDH for RhB degradation was proposed in Figure 8. Firstly, the adsorbing ozone molecular enhanced the adsorption of PMS molecular. Then, the redox cycles of Co and Fe mainly cause the activation of PMS and ozone, and the electron transfer between metals accelerates the activation process (Equations (5)–(10)). Various ROS including SO4•−, •OH, •O2−, and 1O2 were generated during the activation (Equations (11)–(13)), and SO4•− and 1O2 demined the RhB degradation.
Co(II) + HSO5− → Co(III) + SO4•− + OH−
Co(III) + HSO5− → Co(II) + SO5•− + H+
Fe(II) + HSO5− → Fe(III) + SO4•− + OH−
Fe(III) + HSO5− → Fe(II) + SO5•− + H+
SO4•− + H2O → SO42− + •OH + H+
Co(II) + Fe(III) → Co(III) + Fe(II)
2Co(III)/Fe(III) + 2SO5•− + H2O + HSO5− → 2Co(II)/Fe(II) + 3SO42− + 2•O2− + 3H+
Co(III)/Fe(III) + HSO5− + •O2− → Co(II)/Fe(II) + SO42− + 1O2 + OH−
•O2− + H2O → 1O2 + H2O2 + OH−

3.6. Possible Degradation Pathway of RhB

LCMS was used to measure the intermediate products of RhB, and nine main products of RhB were identified. The possible degradation pathway of RhB is shown in Figure 9. RhB (m/z = 443) proceeds via a sequential N-deethylation process, yielding intermediates P1 (m/z = 387) and P2 (m/z = 330), followed by the formation of P3 (m/z = 302) through stepwise removal of ethyl groups. Subsequent chromophore cleavage produces low molecular weight aromatic compounds including P4 (m/z = 182), P5 (m/z = 122), and P6 (m/z = 210). These aromatic fragments undergo further oxidative ring-opening reactions to generate small aliphatic acids such as P7 (m/z = 166), P8 (m/z = 146), and P9 (m/z = 118). Ultimately, these intermediates are mineralized into CO2, H2O, and other inorganic species. The acute toxicity assessments of RhB and its intermediate products were conducted using ECOSAR_V 1.11 software (Figure 9b), with fish, daphnid, and green algae selected as target organisms. The results showed that, with the exception of products 1, 2, 3, 4, and 6, which showed increased toxicity; the toxicity levels of the other products did not exceed that of RhB. Furthermore, the toxicity of the small molecule products was lower than that of RhB. This demonstrates that the system can effectively reduce the acute toxicity of RhB over the long term.

4. Conclusions

A CoFeNi-LDH catalyst was successfully synthesized and demonstrated outstanding performance in activating PMS and ozone simultaneously for efficient RhB degradation. The CoFeNi-LDH/PMS/ozone system exhibited superior degradation efficiency (95.2% within 8 min), excellent stability, and reusability under near-neutral pH conditions. Mechanistic studies confirmed that the synergistic activation of PMS and ozone produced multiple ROS, including SO4•−, •OH, •O2−, and 1O2, with SO4•− and 1O2 playing dominant roles. DFT calculations revealed that ozone facilitated PMS adsorption on CoFeNi sites, thereby enhancing activation efficiency. The system also showed strong resistance to interference from coexisting anions and maintained catalytic activity after repeated cycles. It should be noted, however, that this study used a single dye under relatively short reaction times and did not include real wastewater validation; future work will extend the system to diverse pollutants and practical water matrices. Overall, this work provides a promising and sustainable catalytic strategy for PMS/ozone-based AOPs and advances the application of multi-metal LDH materials in environmental remediation.

Author Contributions

Conceptualization, X.Z. and J.M.; methodology, X.Z.; software, X.Z. and L.S.; validation, X.Z., L.S. and J.M.; formal analysis, X.Z.; investigation, X.Z.; resources, J.M.; data curation, X.Z.; writing—original draft preparation, X.Z.; writing—review and editing, J.M.; visualization, X.Z.; supervision, J.M.; project administration, J.M.; funding acquisition, J.M. All authors have read and agreed to the published version of the manuscript.

Funding

This work was funded by Extracurricular Academic and Technological Foundation for College Students of Zhejiang University of Technology.

Data Availability Statement

Data is contained within the article.

Conflicts of Interest

The authors declare no conflicts of interest.

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Separations 12 00276 g001
Figure 1. (a) SEM images of (a,b) CoFeNi-LDH; (c–h) EDS elemental mapping images of CoFeNi-LDH: (c) overlay, (d) Fe, (e) Co, (f) Ni, (g) O, and (h) C.
Figure 1. (a) SEM images of (a,b) CoFeNi-LDH; (c–h) EDS elemental mapping images of CoFeNi-LDH: (c) overlay, (d) Fe, (e) Co, (f) Ni, (g) O, and (h) C.
Separations 12 00276 g001
Separations 12 00276 g002
Figure 2. (a) XRD pattern of CoFeNi-LDH; (b) the XPS spectrum of CoFeNi-LDH; (c) the magnetization curve of CoFeNi-LDH; (d) Nitrogen adsorption–desorption curve.
Figure 2. (a) XRD pattern of CoFeNi-LDH; (b) the XPS spectrum of CoFeNi-LDH; (c) the magnetization curve of CoFeNi-LDH; (d) Nitrogen adsorption–desorption curve.
Separations 12 00276 g002
Separations 12 00276 g003
Figure 3. (a) RhB degradation by different systems; (b) The TOC removal in different systems; (c) Effect of initial pH on RhB degradation by CoFeNi-LDH/PMS/ozone, and (d) corresponding kobs values; Effect of (e) the concentration of PMS, (f) the concentration of ozone, (g) the dosage of CoFeNi-LDH, and (h) the corresponding kobs values of the effect of the dosage of CoFeNi-LDH on RhB degradation; (i) different ions on RhB degradation by CoFeNi-LDH/PMS/ozone. ([RhB] = 20 mg/L, [catalyst] = 0.2 g/L, [PMS] = 1 mM, [ozone] = 5 mg/L, and [ions] = 3 mM).
Figure 3. (a) RhB degradation by different systems; (b) The TOC removal in different systems; (c) Effect of initial pH on RhB degradation by CoFeNi-LDH/PMS/ozone, and (d) corresponding kobs values; Effect of (e) the concentration of PMS, (f) the concentration of ozone, (g) the dosage of CoFeNi-LDH, and (h) the corresponding kobs values of the effect of the dosage of CoFeNi-LDH on RhB degradation; (i) different ions on RhB degradation by CoFeNi-LDH/PMS/ozone. ([RhB] = 20 mg/L, [catalyst] = 0.2 g/L, [PMS] = 1 mM, [ozone] = 5 mg/L, and [ions] = 3 mM).
Separations 12 00276 g003
Separations 12 00276 g004
Figure 4. (a) Effect of temperature on RhB degradation by CoFeNi-LDH/PMS/ozone and (b) the corresponding kinetic curves: ln(kobs) v.s. T−1; (c) the cycle experiments of CoFeNi-LDH/PMS/ozone for RhB degradation; and (d) the corresponding kinetic constant kobs; (e) XRD pattern of CoFeNi-LDH before and after the reaction. ([RhB] = 20 mg/L, [catalyst] = 0.2 g/L, [PMS] = 1 mM, and [ozone] = 5 mg/L).
Figure 4. (a) Effect of temperature on RhB degradation by CoFeNi-LDH/PMS/ozone and (b) the corresponding kinetic curves: ln(kobs) v.s. T−1; (c) the cycle experiments of CoFeNi-LDH/PMS/ozone for RhB degradation; and (d) the corresponding kinetic constant kobs; (e) XRD pattern of CoFeNi-LDH before and after the reaction. ([RhB] = 20 mg/L, [catalyst] = 0.2 g/L, [PMS] = 1 mM, and [ozone] = 5 mg/L).
Separations 12 00276 g004
Separations 12 00276 g005
Figure 5. EPR spectroscopy of (a) DMPO-•OH and DMPO-SO4•−, (b) DMPO-•O2 ([DMPO] = 100 mM), and (c) TEMP-1O2 ([TEMP] = 100 mM); RhB degradation with different quenchers by (d) CoFeNi-LDH/PMS/ozone, (e) PMS/ozone, (f) CoFeNi-LDH/ozone, and (g) CoFeNi-LDH/PMS. ([RhB] = 20 mg/L, [catalyst] = 0.2 g/L, [PMS] = 1 mM, [ozone] = 5 mg/L, [TBA] = 500 mM, [BA] = 500 mM, [BQ] = 10 mM, and [NaN3] = 10 mM).
Figure 5. EPR spectroscopy of (a) DMPO-•OH and DMPO-SO4•−, (b) DMPO-•O2 ([DMPO] = 100 mM), and (c) TEMP-1O2 ([TEMP] = 100 mM); RhB degradation with different quenchers by (d) CoFeNi-LDH/PMS/ozone, (e) PMS/ozone, (f) CoFeNi-LDH/ozone, and (g) CoFeNi-LDH/PMS. ([RhB] = 20 mg/L, [catalyst] = 0.2 g/L, [PMS] = 1 mM, [ozone] = 5 mg/L, [TBA] = 500 mM, [BA] = 500 mM, [BQ] = 10 mM, and [NaN3] = 10 mM).
Separations 12 00276 g005
Separations 12 00276 g006
Figure 6. Adsorption structure after geometric optimization: PMS adsorbed at (a) CoFeNi site, (b) FeFeFe site, (c) FeFeCo site, and (d) FeFeNi site of CoFeNi-LDH, and (e) PMS adsorbed at CoFeNi site of CoFeNi-LDH with O3. (f) The charge density map of CoFeNi-LDH (the yellow area represents electron depletion, and the blue area represents electron enrichment).
Figure 6. Adsorption structure after geometric optimization: PMS adsorbed at (a) CoFeNi site, (b) FeFeFe site, (c) FeFeCo site, and (d) FeFeNi site of CoFeNi-LDH, and (e) PMS adsorbed at CoFeNi site of CoFeNi-LDH with O3. (f) The charge density map of CoFeNi-LDH (the yellow area represents electron depletion, and the blue area represents electron enrichment).
Separations 12 00276 g006
Separations 12 00276 g007
Figure 7. The XPS spectrum of CoFeNi-LDH: (a) Co 2p, (b) Fe 2p, (c) O 1s, and (d) Ni 2P.
Figure 7. The XPS spectrum of CoFeNi-LDH: (a) Co 2p, (b) Fe 2p, (c) O 1s, and (d) Ni 2P.
Separations 12 00276 g007
Separations 12 00276 g008
Figure 8. Proposed mechanism of RhB degradation in CoFeNi-LDH/PMS/ozone system.
Figure 8. Proposed mechanism of RhB degradation in CoFeNi-LDH/PMS/ozone system.
Separations 12 00276 g008
Separations 12 00276 g009
Figure 9. (a) Proposed the possible degradation pathway of RhB in CoFeNi-LDH/PMS/ozone system; (b) The acute toxicity of RhB and its intermediate products.
Figure 9. (a) Proposed the possible degradation pathway of RhB in CoFeNi-LDH/PMS/ozone system; (b) The acute toxicity of RhB and its intermediate products.
Separations 12 00276 g009
Table 1. The performance comparison of CoFeNi-LDH with other relative LDHs.
Table 1. The performance comparison of CoFeNi-LDH with other relative LDHs.
Dosage
(g/L)		PMS
(mM)		OzoneRhB
(mg/L)		kobs
(min−1)		Ref.
ZnFe-LDH0.5-65 mg/h500.058[32]
MgFe-LDH0.5-65 mg/h500.054[32]
MgAl-LDH0.5-65 mg/h500.053[32]
NiFe–-LDH0.5-65 mg/h500.065[32]
NiAlLDH0.5-65 mg/h500.069[32]
CoCr-LDH0.022-500.079[33]
FeCo-LDH0.21-500.011[34]
CoAl-LDH0.10.3-800.1687[35]
FeAl-LDH0.10.66-100.41[36]
CoFeCu-LDH0.21-500.33[37]
MnFe-LDH0.31.30.1 L/min200.34[25]
CoFeNi-LDH0.215 mg/L200.42This work
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Zhu, X.; Song, L.; Miao, J. CoFeNi-Layered Double Hydroxide Combined Activation of PMS and Ozone for the Degradation of Rhodamine B in Water. Separations 2025, 12, 276. https://doi.org/10.3390/separations12100276

AMA Style

Zhu X, Song L, Miao J. CoFeNi-Layered Double Hydroxide Combined Activation of PMS and Ozone for the Degradation of Rhodamine B in Water. Separations. 2025; 12(10):276. https://doi.org/10.3390/separations12100276

Chicago/Turabian Style

Zhu, Xiaohan, Liang Song, and Jia Miao. 2025. "CoFeNi-Layered Double Hydroxide Combined Activation of PMS and Ozone for the Degradation of Rhodamine B in Water" Separations 12, no. 10: 276. https://doi.org/10.3390/separations12100276

APA Style

Zhu, X., Song, L., & Miao, J. (2025). CoFeNi-Layered Double Hydroxide Combined Activation of PMS and Ozone for the Degradation of Rhodamine B in Water. Separations, 12(10), 276. https://doi.org/10.3390/separations12100276

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