Degradation of Tetracycline Hydrochloride via Activation of Peroxymonosulfate by Magnetic Nickel–Cobalt Ferrite: Role of High-Valent Metal Species as Primary Reactive Agents

Abstract

Non-radical-based advanced oxidation processes, particularly those dominated by high-valent metals, have caught a great deal of attention because of their exceptional degradation selectivity and robust interference resistance. This study reports the synthesis of a novel ferrite, designated as Co_0.5Ni_0.5Fe₂O₄, through a solvothermal reaction, aimed at activating PMS for the removal of TCH from water. It was observed that calcination time played an important role in adjusting the particle size of the catalyst, which subsequently increased its surface area. This enlargement, in turn, led to an increase in active sites, ultimately enhancing the catalytic efficiency. Within the Co_0.5Ni_0.5Fe₂O₄/PMS system, high-valent metals $\mathrm{F}\mathrm{e}\left(\mathrm{I}\mathrm{V}\right)$ and $\mathrm{C}\mathrm{o}\left(\mathrm{I}\mathrm{V}\right)$ became prominent as the primary active species, with ${}^1\mathrm{O}_2$ serving as a secondary contributor. The activation mechanism of PMS was thoroughly analyzed and discussed. Co_0.5Ni_0.5Fe₂O₄ exhibited remarkable stability in complex reaction environments and during multiple recycling tests, maintaining a TCH removal efficiency exceeding 98%. This study not only increases awareness of the interaction between catalyst structure and performance but also provides a viable platform for high-valent metal-dominated ferrite catalysts.

1. Introduction

Antibiotics are indispensable in global healthcare for combating infectious diseases, finding widespread application in medicine, agriculture, and animal husbandry due to their efficiency, cost-effectiveness, and sustainability [1]. However, their incomplete absorption leads to the release of residues, such as tetracycline hydrochloride (TCH), into the environment through various pathways, including industrial wastewater, sewage, sludge, and runoff, posing significant aquatic pollution risks [2]. TCH contamination not only affects water quality but also poses a threat of inducing antibiotic resistance, which can propagate through the food chain, disrupting ecosystems [3]. Given the annual global antibiotic consumption exceeding 1 million tons [4], the diverse sources and pressing environmental issues associated with tetracycline antibiotics demand urgent attention.

Advanced oxidation processes (AOPs), particularly those driven by PMS have gained significant recognition for their efficacy in degrading a wide range of contaminants, attributed to their high oxidation potential and broad applicable pH range [5]. In these processes, PMS disrupts the internal peroxide bond via a mechanism involving the transfer of a single electron, generating ${\mathrm{S}\mathrm{O}}_4^{•-}$ and ${}^{•}\mathrm{O}\mathrm{H}$, which are pivotal for the degradation of persistent organic pollutants [6]. Catalysts play a crucial role in these degradation systems by facilitating the rapid activation of PMS to degrade aquatic pollutants. Current research focuses on enhancing the catalytic activity of these catalysts to accelerate the breakdown of PMS and the production of reactive species, leading to a significant improvement in the degradation efficiency of pollutants [7]. By adjusting catalyst preparation conditions and incorporating additional substances, researchers can modify the catalyst’s structure and surface properties, enhancing its stability and durability [8].

Recently, non-radical-dominated degradation pathways, particularly those mediated by high-valent metals, have garnered increasing attention due to their exceptional selectivity in degradation and robust resistance to interference [9]. The direct detection of $\mathrm{F}\mathrm{e}\left(\mathrm{I}\mathrm{V}\right)$ species by Pestovsky et al. in 2005 through XANES (X-ray Absorption Near Edge Structure) spectroscopy sparked a surge in reports on the function of $\mathrm{F}\mathrm{e}\left(\mathrm{I}\mathrm{V}\right)$ in AOPs [10,11]. Bao and colleagues achieved sustained generation of $\mathrm{F}\mathrm{e}\left(\mathrm{I}\mathrm{V}\right)=\mathrm{O}$ by establishing a neutral environmental condition, enabling the ZnFe-LDH/PMS system to exhibit long-term stability and selective degradation under challenging conditions, such as high salinity and the existence of natural organic matter [12]. Among other metals, Cobalt (Co) is recognized for its high efficiency in degrading organic pollutants [13], with $\mathrm{C}\mathrm{o}\left(\mathrm{I}\mathrm{I}\right)$ undergoing a transfer of two electrons to form $\mathrm{C}\mathrm{o}\left(\mathrm{I}\mathrm{V}\right)$, although the high-valent cobalt complex ($\mathrm{C}\mathrm{o}\left(\mathrm{I}\mathrm{V}\right){\mathrm{O}}^{2+}$) experiences strong electronic repulsion, facilitating its conversion back to $\mathrm{C}\mathrm{o}\left(\mathrm{I}\mathrm{I}\mathrm{I}\right)$ [14].

Spinel ferrites, noted for their high catalytic activity, robust stability, and low saturation magnetic moment, exhibit exceptional physicochemical properties and are widely used in water environment remediation [15]. Their molecular formula is typically $\mathrm{M}{\mathrm{F}\mathrm{e}}_2{\mathrm{O}}_4$, with cobalt ferrite ($\mathrm{C}\mathrm{o}{\mathrm{F}\mathrm{e}}_2{\mathrm{O}}_4$) and its composites showcasing exceptional catalytic activity. The existence of Fe promotes the formation of hydroxyl groups and $\mathrm{C}\mathrm{o}\left(\mathrm{I}\mathrm{I}\right)$ complexes on the catalyst surface, enhancing the activation efficiency of PMS [16]. Additionally, Fe acts as a reducer for ${\mathrm{C}\mathrm{o}}^{3+}$, promoting the regeneration of ${\mathrm{C}\mathrm{o}}^{2+}$ and improving the catalyst’s stability [17]. Notably, $\mathrm{C}\mathrm{o}{\mathrm{F}\mathrm{e}}_2{\mathrm{O}}_4$ possesses excellent ferromagnetic properties, enabling rapid separation and recovery via an external magnetic field [18]. Li et al. employed synthesized $\mathrm{C}\mathrm{o}{\mathrm{F}\mathrm{e}}_2{\mathrm{O}}_4$ to activate PMS for the degradation of sulfamethoxazole, atrazine, and benzoic acid, achieving degradation rates exceeding 99% within 30 min [19]. Doping transition metals into ferrite-based catalysts is an effective strategy to tailor their catalytic properties. Nguyen et al. used various proportions of Nd doping in $\mathrm{C}\mathrm{o}{\mathrm{F}\mathrm{e}}_2{\mathrm{O}}_4$ to facilitate the breakdown of methylene blue (MB) through catalysis, with $\mathrm{C}\mathrm{o}{\mathrm{N}\mathrm{d}}_{0.05}{\mathrm{F}\mathrm{e}}_{1.95}{\mathrm{O}}_4$ exhibiting the highest catalytic activity [20]. Nickel cobalt ferrite has recently garnered attention as an excellent microwave absorbing material, with Cheng and colleagues discovering that nickel incorporation significantly enhances its relative permittivity, attributed to modifications in the electronic structure, which augment the material’s spatial charge polarization capabilities [21]. An elevated permittivity could facilitate rapid electron transfer during catalytic reactions, particularly in PMS systems [22], facilitating the formation of higher-valent metal species and potentiating oxidative performance.

In this study, nickel–cobalt ferrite (Co_0.5Ni_0.5Fe₂O₄) was selected as the research subject for PMS activation. Magnetic Co_0.5Ni_0.5Fe₂O₄ was synthesized through a solvothermal reaction, and the impact of preparation conditions, particularly calcination time, on its catalytic performance was investigated. TCH was identified as the target contaminant to analyze the types of active species, activation mechanisms, and interference resistance within the Co_0.5Ni_0.5Fe₂O₄/PMS system.

2. Experimental

2.1. Materials

The chemicals utilized in this study were sourced from reputable suppliers. Cobalt nitrate hexahydrate (${\mathrm{C}\mathrm{o}\left({\mathrm{N}\mathrm{O}}_3\right)}_2·6{\mathrm{H}}_2\mathrm{O}$), nickel nitrate hexahydrate (${\mathrm{N}\mathrm{i}\left({\mathrm{N}\mathrm{O}}_3\right)}_2·6{\mathrm{H}}_2\mathrm{O}$), and ferric nitrate nonahydrate (${\mathrm{F}\mathrm{e}\left({\mathrm{N}\mathrm{O}}_3\right)}_3·9{\mathrm{H}}_2\mathrm{O}$), alongside potassium peroxymonosulfate (PMS), were sourced from Kermel Chemical Reagent Co., Ltd., Tianjin, China. Additionally, high-purity 2,2,6,6-tetramethylpiperidone (TEMP, 98%) and 5,5-dimethyl-1-pyrrolidine N-oxide (DMPO, 97%), as well as analytical-grade methanol (MeOH), tert-butyl alcohol (TBA), furfuryl alcohol (FFA), p-benzoquinone (PBQ), dimethyl sulfoxide (DMSO), and tetracycline hydrochloride (TCH), were all procured from Aladdin Industry Corporation, Shanghai, China. Deionized water was utilized as the solvent throughout the experimental procedures.

2.2. Synthesis of Co_0.5Ni_0.5Fe₂O₄ Catalysts

During the solvothermal synthesis of ferrites, the calcination duration can effectively modulate the particle size of the ferrites [23]. In this study, catalysts were synthesized through solvothermal reactions, and catalysts with varying particle sizes were synthesized by controlling the calcination time. In detail, 5 mmol of ${\mathrm{C}\mathrm{o}\left({\mathrm{N}\mathrm{O}}_3\right)}_2·6{\mathrm{H}}_2\mathrm{O}$ (1.46 g) and 5 mmol of ${\mathrm{N}\mathrm{i}\left({\mathrm{N}\mathrm{O}}_3\right)}_2·6{\mathrm{H}}_2\mathrm{O}$ (1.45 g) were introduced to 50 mL of deionized water. The combined solution was agitated for 30 min until completely dissolved, forming solution A. Subsequently, 20 mmol of ${\mathrm{F}\mathrm{e}\left({\mathrm{N}\mathrm{O}}_3\right)}_3·9{\mathrm{H}}_2\mathrm{O}$ (8.08 g) was added to an extra 50 mL of deionized water and agitated for 30 min until fully dissolved, forming solution B. Solutions A and B were then combined together and agitated for an additional 30 min to ensure complete uniformity. Next, 20 mL of a NaOH solution (6 mol·L⁻¹) was gradually mixed dropwise with the A/B mixture. Once the addition was complete, the mixed solution’s temperature was brought up to 90 °C and stirred for 1 h to allow the reaction to proceed. After the reaction, the solution was set aside to cool naturally. The mixed solution was then centrifuged at 6000 rpm and washed until it reached a neutral pH, yielding a wet powder. This wet powder was dried in a vacuum chamber at 60 °C for 12 h. Following drying, it was placed in a muffle furnace for calcination to obtain Co_0.5Ni_0.5Fe₂O₄ nanoparticles. The calcination conditions varied according to the desired particle size of Co_0.5Ni_0.5Fe₂O₄: for small particle size, calcination was conducted at 700 °C for 2 h; for medium particle size, at 700 °C for 6 h; and for large particle size, at 700 °C for 24 h.

2.3. Characterization of Co_0.5Ni_0.5Fe₂O₄ Catalysts

The surface features of Co_0.5Ni_0.5Fe₂O₄ were carefully analyzed with a scanning electron microscope (SEM, Hitachi S-4800, Science, Suzhou, China) operated at 20 kV, enabling clear visualization of its structural features. Additionally, transmission electron microscopy (TEM, Thermo Fisher Scientific, Waltham, MA, USA) was used to explore the microstructure and nanoscale features of the material further, providing detailed insights into its morphology. To understand the crystal structure, the X’Pert PRO diffractometer from PANalytical, Almelo, The Netherlands, was used for X-ray diffraction (XRD) analysis. The incident source for the diffractometer was Cu’s Kα radiation (λ = 0.15406 nm). The scanning step size was set to 0.02°, with a power of 40 kV × 50 mA. The data was collected over a scanning angle 2θ range of 5° to 90°, providing comprehensive information about the crystalline structure of the sample. The functional components and molecular interactions within the synthesis catalysts were characterized using Fourier Transform Infrared (FTIR, Thermo Fisher Scientific, Waltham, MA, USA) spectroscopy, conducted over a wide spectral range of 400 to 4000 cm⁻¹. An in-depth examination of the elemental composition and electronic configuration was performed via X-ray photoelectron spectroscopy (XPS), utilizing a PHI-5300 ESCA system with the C1s peak calibrated at 284.8 eV as a reference. Raman spectroscopic analysis was carried out using a LabRAM HR system equipped with a 514 nm laser, scanning within a range of 200 to 1200 cm⁻¹ to gain insights into molecular vibrations. The BET (Brunner–Emmet-Teller) technique was applied to measure the surface area and pore volume distribution of Co_0.5Ni_0.5Fe₂O₄ through nitrogen adsorption and desorption measurements conducted at −196 °C using a BELSORP-max gas adsorption analyzer (MicrotracBEL Corp., Osaka, Japan), with degassing conditions maintained at 200 °C for 600 min. Data analysis was performed using the BELSORP Data Analysis Software BELMaster—Ver 7.3.2.0 (MicrotracBEL Corp., Osaka, Japan).

2.4. Degradation Experiments

During all degradation experiments carried out, a 200 mL conical flask was utilized, maintaining a constant temperature of 25 °C and employing magnetic stirring. During a typical experimental run, 20 mg of catalyst was dispensed into 100 mL of the TCH. The resultant mixture was then stirred magnetically for 20 min at 20 °C in dark conditions to achieve an absorption–desorption equilibrium. Subsequently, 20 mg of PMS was added to the suspension. Immediately following this addition, the solution pH was promptly modified to the predetermined level using HCl (0.01 M) and NaOH (0.01 M) as necessary to initiate the reaction. The degradation process was then allowed to proceed for a duration of 60 min. At regular intervals throughout the reaction, samples were withdrawn from the solution, filtered through 0.22 µm membrane filters, and treated with a solution of ${\mathrm{N}\mathrm{a}}_2{\mathrm{S}}_2{\mathrm{O}}_3$ (0.1 M) to prevent further oxidation.

In this study, the concentrations of TCH in samples were assayed using a high-performance liquid chromatography (HPLC) system (Waters Corporation, Milford, MA, USA) equipped with a Waters 2489 UV detector on a Waters XBridge C18 column (4.6 × 250 mm, 5 μm). The mobile phase was comprised of 0.5% triethylamine in water solution, pH adjusted to 5.0 with phosphoric acid, mixed with acetonitrile. The chromatographic conditions included a flow rate set at 1.0 mL/min, a 20 μL injection with detection at 360 nm wavelength, and a constant column temperature of 30 °C was maintained.

Additionally, the samples were evaluated for stability. Five reuse experiments were conducted. Following each experiment, the collected catalyst was washed with deionized water and subsequently used in the next experimental phase.

3. Results and Discussion

3.1. Characterization of Catalyst

The morphology of the three synthetic particle size samples was detected using SEM and TEM, as shown in Figure 1. As illustrated in Figure 1a–c, with the extension of calcination time, the particle size of the catalyst significantly increases. From the SEM images, it can be observed that the catalyst exhibits a spherical-like nanoparticle morphology. Additionally, through the TEM images, it can be roughly measured that the small particle size (calcination time of 2 h) is around 40nm. Notably, the TEM images also clearly show that the particle sizes are relatively uniform and exhibit good dispersion. In addition, as shown in Figure 1d, in the SEM image of the catalyst after the catalytic degradation reaction, we can observe that the structure of the catalyst has not changed.

The sample’s specific surface area, pore volume, and porosity were analyzed using adsorption–desorption isotherms of N₂, as shown in Figure 2a,b. The adsorption capacity of the material in the low-pressure region is initially low but increases with rising system pressure. The adsorption isotherms obtained after calcination for different durations consistently show Type IV characteristics, including a hysteresis loop in the adsorption–desorption process. This loop results from capillary condensation within the material’s pores, which occurs when the pore diameter is sufficiently small [24]. As pressure increases, condensation begins at a critical pressure (P), which is inversely related to pore diameter [25]. The critical pressure increases with longer calcination times, suggesting an enhancement of average pore size.

The Barrett–Joyner–Halenda (BJH) analysis further supports this trend. After 2 h of calcination, the average pore diameter is 10.424 nm, featuring a surface area of 63.686 m²/g and a pore volume of 0.166 cm³/g. Notably, a distinct peak is observed in the smaller particle size range (2 h), corresponding to smaller pore diameters. After 6 h, the pore diameter increases to 15.523 nm, with a surface area of 41.723 m²/g and a pore volume of 0.1619 cm³/g. By 24 h, the average pore diameter reaches 19.155 nm, accompanied by a surface area of 25.875 m²/g and a pore volume of 0.1239 cm³/g. In this case, although no prominent peak is detected in the larger particle size ranges, the data demonstrate a predominant proportion of pores distributed across larger diameters. These results collectively confirm that longer calcination times lead to larger pore sizes, accompanied by a reduction in surface area and pore volume.

Further structural details were obtained through XRD, as displayed in Figure 2e. The XRD patterns reveal that the major diffraction peaks of Co_0.5Ni_0.5Fe₂O₄ nanoparticles exhibit 2θ values of 18.42°, 30.31°, 35.70°, 37.32°, 43.38°, 53.81°, 57.39°, and 63.02°, corresponding to the crystal planes of (111), (220), (311), (222), (400), (422), (511), and (440), respectively. Comparison of these peaks with the standard card (PDF#54-0964) indicates that no other characteristic peaks appear in the spectrum, perfectly aligning with the characteristic peaks of the standard spinel-type nickel–cobalt ferrite.

Figure 2c shows the Raman spectrum of the catalyst. Raman spectroscopy of the sample reveals intense and sharp Raman peaks, indicative of its high crystallinity. The spectrum prominently exhibits three characteristic peaks corresponding to the A_1g, E_g, and 3T_2g modes. The A_1g mode, identified at 689 cm⁻¹, results from the symmetric stretching vibrations within metal–oxygen bonds. This is correlated with the cationic bonding in Ni/Co/Fe-O. The E_g mode at 310 cm⁻¹ and the 3T_2g mode around 471 cm⁻¹ are ascribed to the bending vibrations within the metal–oxygen bonds (Fe/Co/Ni) [26], with the E_g mode representing a symmetric vibration and the 3T_2g mode representing an antisymmetric bending vibration [27]. These observations further corroborate the well-ordered crystallographic structure of the material.

To obtain information about the functional groups present on the catalyst surface, FT-IR spectroscopy was conducted, with the results displayed in Figure 2d. A distinctive absorption peak observed around 598 cm⁻¹ within the observed spectrum indicates the stretching vibration of metal–oxygen bonds (Ni/Co/Fe-O) located at tetrahedral sites. Furthermore, the band around 410 cm⁻¹ is attributed to the stretching vibration of metal–oxygen bonds (Ni/Co/Fe-O) situated at octahedral positions [28]. The spinel-type ferrite prepared via the solvothermal method exhibits a significant presence of -OH groups on its surface, which is consistent with the stretching vibration detected at 3425 cm⁻¹ within the observed spectrum. The frequency bands observed at 1621 and 1054 cm⁻¹ are caused by the stretching vibrations of H-O-H, indicating the absorption of water on the catalyst surface [29]. Additionally, a nitrate band was detected in the vicinity of 1337 cm⁻¹, which can be credited to the incomplete decomposition of nitrate throughout the calcination process of the catalyst [30]. FT-IR analysis was also conducted on the catalyst post-reaction. In the FT-IR spectrum shown in Figure 2d, the used catalyst exhibited two additional functional groups at 2936.1 cm⁻¹ and 1133.2 cm⁻¹ compared to the fresh material. These peaks are attributed to organic residues from the degradation of TCH, which may have adsorbed onto the catalyst surface during the reaction. No notable alterations were observed in the catalyst’s surface functional groups when comparing the spectra obtained before and after the reaction, indicating that the residues did not significantly alter the intrinsic properties of the catalyst. While these residues do not significantly affect the catalytic activity within the tested cycles (as evidenced by the following high stability over five cycles), we acknowledge that their accumulation could potentially lead to catalyst poisoning over extended use.

To further evaluate the structural stability of the catalyst, XRD analysis was performed on the used material. The results reveal that the spinel structure of Co₀.₅Ni₀.₅Fe₂O₄ remains largely intact, with no significant changes in the crystal phase compared to the freshly prepared catalyst. This confirms the high stability of the Co₀.₅Ni₀.₅Fe₂O₄ catalyst during the catalytic process, further supporting its high purity and excellent structural integrity. However, the presence of residual organic species on the surface, as detected by FT-IR, suggests that long-term exposure to high concentrations of TCH or other organic pollutants could gradually reduce the catalyst’s efficiency.

3.2. Catalytic PMS Activation for TCH Degradation

The evaluation aimed to assess the adsorption and catalytic capabilities of TCH on catalysts with three different particle sizes. The results indicated that the absorption of TCH by all prepared samples was virtually negligible, with less than 10% absorption observed within a 20-min period (Figure 3a).

As depicted in Figure 3a, following a duration of 60 min dedicated to the catalytic degradation of TCH, it is evident that catalysts with small and medium particle sizes exhibit degradation rates of 99.6%, significantly outperforming the large particle size catalysts, which achieved a degradation rate of 97.4%. Furthermore, the pseudo-first-order reaction kinetics constants (k_obs) for TCH removal were determined to be 0.0996 min⁻¹, 0.0956 min⁻¹, and 0.0627 min⁻¹ for small, medium, and large particle sizes, respectively. These findings point out the critical role of catalyst particle size in influencing the catalytic degradation rate, highlighting potential size-dependent efficiencies that could be optimized for enhanced performance in catalytic applications. Additionally, the observed trend suggests that smaller particles may offer superior reactivity, possibly as a result of increased area of the surface and accessibility of active sites [31], as evidenced by the comparisons made through previous SEM and BET analyses.

Furthermore, in the practical application of sewage treatment, catalyst stability and reproducibility take on a critical role. As illustrated in Figure 3b, upon repeating the use of the catalyst with small particle size five times, the degradation efficiency of TCH can surpass 98%. Notably, the catalyst recovery protocol involves centrifuging the catalyst followed by rinsing it with deionized water five times and drying it, achieving a catalyst recovery rate of 98% in each cycle. This high recovery rate, even when accounting for operational losses, demonstrates low metal leaching and underscores the catalyst’s exceptional reusability. This approach not only enhances the catalyst’s durability but also maintains its high performance over multiple cycles, thereby contributing to the efficiency and sustainability of the sewage treatment process.

3.3. Identification of Reactive Species

3.3.1. Free Radical Quenching Experiment

In order to fully comprehend the mechanisms underlying the degradation of TCH in the Co_0.5Ni_0.5Fe₂O₄/PMS system, experiments to quench were carried out to pinpoint the specific free radicals contributing to this degradation process. In such experiments, methanol (MeOH) served as a versatile scavenger, effectively targeting both hydroxyl radicals (${}^{•}\mathrm{O}\mathrm{H})$ and sulfate radicals (${\mathrm{S}\mathrm{O}}_4^{•-}$), as a result of its interaction with both species. Conversely, tert-butanol (TBA) was employed as a selective scavenger, primarily targeting ${}^{•}\mathrm{O}\mathrm{H}$ radicals, allowing for differentiation between the contributions of ${}^{•}\mathrm{O}\mathrm{H}$ and ${\mathrm{S}\mathrm{O}}_4^{•-}$ to TCH degradation. Furthermore, to comprehensively evaluate the function of added reactive oxygen species (ROS) within the Co_0.5Ni_0.5Fe₂O₄/PMS system, 4-benzoquinone (p-BQ) was served as a specific quenching agent for superoxide radicals (${\mathrm{O}}_2^{•-}$). This approach facilitates the assessment of the potential participation of ${\mathrm{O}}_2^{•-}$ involved in the degradation process of TCH. Beyond these conventional radicals, Fenton-like reactions often involve other active species, such as singlet oxygen (${}^1\mathrm{O}_2$) and high-valent metal species, which necessitate specific quenching strategies. To this end, furfuryl alcohol (FFA) was employed to scavenge ${}^1\mathrm{O}_2$, providing insights into its potential contribution to TCH degradation [32]. Additionally, dimethyl sulfoxide (DMSO) was used to quench high-valent metal species [33], thereby elucidating their function in the overall degradation process.

As illustrated in Figure 3c, upon exposure to quenching agents such as MeOH (500 mM), TBA (500 mM), and p-BQ (5 mM), the efficiency of degradation of TCH in the Co_0.5Ni_0.5Fe₂O₄/PMS system marginally declined to 95.3%, 99.5%, and 93.9%, respectively. This observation suggests that the underlying mechanism may not conform strictly to the conventional free radical oxidation pathway. Conversely, when FFA (5 mM) and DMSO (1 mM) were employed as quenching agents, the degradation rate of TCH underwent a substantial decrease, reaching 59.7% and 35.3%, respectively. These findings, coupled with the quenching effects observed, collectively indicate that the major active species involved in the degradation of TCH in this system are likely high-valent metals, suggesting a non-radical oxidation mechanism.

3.3.2. EPR Analysis

Electron paramagnetic resonance (EPR) spectroscopy was applied to provide additional evidence for the occurrence of reactive oxygen species (ROS) in the Co_0.5Ni_0.5Fe₂O₄/PMS system. In this analysis, TEMP (2,2,6,6-tetramethylpiperidine) and DMPO (5,5-dimethyl-1-pyrrolidine-N-oxide) served as specific traps for EPR detection [34]. Specifically, DMPO functions as a scavenger for ${}^{•}\mathrm{O}\mathrm{H}$, ${\mathrm{O}}_2^{•-}$, and ${\mathrm{S}\mathrm{O}}_4^{•-}$, whereas TEMP is designed to capture ${}^1\mathrm{O}_2$. As illustrated in Figure 3d,e, distinctive triplet peaks corresponding to TEMP-${}^1\mathrm{O}_2$ were observed, alongside weak characteristic peaks indicative of DMPO-${\mathrm{O}}_2^{•-}$. Notably, as presented in Figure 3f, no notable peaks were detected for ${}^{•}\mathrm{O}\mathrm{H}$ and ${\mathrm{S}\mathrm{O}}_4^{•-}$ radicals upon utilizing DMPO, which is attributed to the oxidation of metals in high oxidation states, including $\mathrm{F}\mathrm{e}\left(\mathrm{I}\mathrm{V}\right)$ and $\mathrm{C}\mathrm{o}\left(\mathrm{I}\mathrm{V}\right)$ converting DMPO to DMPOX [9]. To corroborate these findings, the catalyst was separated from the Co_0.5Ni_0.5Fe₂O₄/PMS system and subsequently tested with a capture agent. This additional test yielded no signals indicative of ${}^{•}\mathrm{O}\mathrm{H}$ or ${\mathrm{S}\mathrm{O}}_4^{•-}$ free radicals, confirming their absence within the catalytic system. Collectively, the EPR analysis results align consistently with the quenching experiments.

3.3.3. Evolution Mechanism of ${}^1\mathrm{O}_2$

Both in quenching experiments and EPR detection, we observed that ${}^1\mathrm{O}_2$ emerged as a pivotal reactive species. In this study, we delved into the formation mechanism of ${}^1\mathrm{O}_2$ within the Co_0.5Ni_0.5Fe₂O₄/PMS system. Prior research has delineated three primary pathways for the generation of ${}^1\mathrm{O}_2$ in AOPs based on PMS: the breakdown of PMS (Equations (1) and (2)) [35], ${\mathrm{O}}_2^{•-}$ (Equations (3) and (4)) [36] and lattice oxygens (${\mathrm{O}}^{\mathrm{*}}$) (Equation (5)) [37].

$${\mathrm{H}\mathrm{S}\mathrm{O}}_5^{-}+{\mathrm{S}\mathrm{O}}_5^{2-}\to {\mathrm{H}\mathrm{S}\mathrm{O}}_4^{-}+{\mathrm{S}\mathrm{O}}_4^{2-}+{}^1\mathrm{O}_2$$

(1)

$${\mathrm{H}\mathrm{S}\mathrm{O}}_5^{-}\to {\mathrm{S}\mathrm{O}}_5^{•-}+{\mathrm{H}}^{+}+{}^1\mathrm{O}_2$$

(2)

$${\mathrm{O}}_2^{•-}+2{\mathrm{H}}_2\mathrm{O}\to {}^1\mathrm{O}_2+{\mathrm{H}}_2{\mathrm{O}}_2+2{\mathrm{H}}^{+}$$

(3)

$$2{\mathrm{O}}_2^{•-}+2{\mathrm{H}}^{+}\to {}^1\mathrm{O}_2+{\mathrm{H}}_2{\mathrm{O}}_2$$

(4)

$${\mathrm{O}}^{\mathrm{*}}+{\mathrm{H}\mathrm{S}\mathrm{O}}_5^{-}\to {}^1\mathrm{O}_2+{\mathrm{H}}_2{\mathrm{O}}_2$$

(5)

In the quenching experiment, it was noted that p-BQ presented a non-significant inhibitory impact on ${\mathrm{O}}_2^{•-}$. Moreover, the EPR detection revealed a notably weak signal for DMPO-${\mathrm{O}}_2^{•-}$, indicating that ${}^1\mathrm{O}_2$ in the Co_0.5Ni_0.5Fe₂O₄/PMS system is not primarily generated through the intermediacy of ${\mathrm{O}}_2^{•-}$. Numerous studies have indicated that the breakdown process of PMS significantly contributes to the formation of ${}^1\mathrm{O}_2$ [35,38]. As shown in Figure 3d, it is evident that ${}^1\mathrm{O}_2$ is not produced under conditions where only PMS is present, due to the notably slow rate at which PMS decomposes. However, upon the addition of the Co_0.5Ni_0.5Fe₂O₄ catalyst, a distinct ${}^1\mathrm{O}_2$ characteristic peak becomes observable. Consequently, the reaction between the functional groups of Co_0.5Ni_0.5Fe₂O₄ and PMS accelerates the formation of ${}^1\mathrm{O}_2$, rather than merely relying on the spontaneous decomposition of PMS. Furthermore, lattice oxygen from transition metals may also contribute to the generation of ${}^1\mathrm{O}_2$ [37]. This is supported by subsequent XPS analysis, which reveals a decrease in lattice oxygen content from 74.34% to 73.56% (Figure 4e), indicating that the reduction in lattice oxygen may be a source of ${}^1\mathrm{O}_2$ production.

3.4. Reaction Mechanism for PMS Activation by Co_0.5Ni_0.5Fe₂O₄ Catalyst

XPS can be utilized for both quantitative and qualitative investigations into the chemical states and compositions of elements present on catalyst material surfaces [39]. Figure 4a depicts the elemental scan of the material, revealing the existence of five elements: Co, Fe, Ni, O, and C, irrespective of the reaction stage. Notably, the detected carbon may be due to the attachment of inorganic carbon to the catalyst surface. The elemental proportions before and after the reaction are as follows: prior to the reaction, the ratio of Co: Fe: Ni: O was 8.16: 28.99: 5.56: 57.29; whereas after the reaction, it shifted slightly to 8.20: 28.70: 5.92: 57.18.

Through comprehensive XPS characterization of the elemental compositions prior to and following the reaction, insights into the potential activation mechanism of PMS can be deduced. As presented in Figure 4b,c,e, the valence state variations of Fe, Co, and Ni during the reaction are examined. Prior to the reaction, the proportions of ${\mathrm{F}\mathrm{e}}^{2+}$ and ${\mathrm{F}\mathrm{e}}^{3+}$ were 59.23% and 40.77%, respectively. Post-reaction, a decrease in ${\mathrm{F}\mathrm{e}}^{2+}$ to 56.58% was observed, indicating a transformation likely involving the redox chemistry of iron ions during PMS activation. Similarly, ${\mathrm{N}\mathrm{i}}^{2+}$ exhibited a notable decline from 61.17% to 51.90% after the reaction, suggesting its active participation in the catalytic process. Notably, the percentage of ${\mathrm{C}\mathrm{o}}^{2+}$ remained relatively unchanged, transitioning from 36.52% before the reaction to 36.24% afterward. This remarkable stability can be attributed to the higher standard reduction potential of ${\mathrm{C}\mathrm{o}}^{2+}$ to ${\mathrm{C}\mathrm{o}}^{3+}$ in comparison to ${\mathrm{F}\mathrm{e}}^{2+}$ to ${\mathrm{F}\mathrm{e}}^{3+}$. Consequently, the thermodynamics favor the reduction of ${\mathrm{C}\mathrm{o}}^{3+}$ by ${\mathrm{F}\mathrm{e}}^{2+}$, facilitating a cyclic redox process between ${\mathrm{C}\mathrm{o}}^{2+}$ and ${\mathrm{C}\mathrm{o}}^{3+}$ [17]. This phenomenon underscores the potential synergistic effect between Fe and Co in promoting the activation of PMS, where ${\mathrm{F}\mathrm{e}}^{2+}$ serves as a reductant to maintain the cycle of ${\mathrm{C}\mathrm{o}}^{2+}$ to ${\mathrm{C}\mathrm{o}}^{3+}$, enhancing the overall catalytic efficiency. In the prior exploration of the mechanisms underlying the formation of ${}^1\mathrm{O}_2$, the generation of ${}^1\mathrm{O}_2$ was considered to be the interaction between the catalyst and PMS, rather than just the self-decomposition of PMS. Therefore, it is evident that metal cations participate in the activation of PMS to generate ${}^1\mathrm{O}_2$ (Equations (6) and (7)) [40,41].

In quenching experiments, DMSO exhibited a significant inhibitory effect on the catalytic reaction, indicating that the primary active species in the catalytic system are high-valent metal ions. Numerous studies have highlighted the pivotal role of $\mathrm{F}\mathrm{e}\left(\mathrm{I}\mathrm{V}\right)$ and $\mathrm{C}\mathrm{o}\left(\mathrm{I}\mathrm{V}\right)$ for the onset of PMS activation within metal-based PMS systems [42]. The formation process of $\mathrm{F}\mathrm{e}\left(\mathrm{I}\mathrm{V}\right)$ and $\mathrm{C}\mathrm{o}\left(\mathrm{I}\mathrm{V}\right)$ can be briefly described as the divalent metal ions donating two electrons, which are then accepted by PMS as the acceptor, resulting in the generation of high-valent metal species (Equations (8) and (9)). The PMS initially adsorbs onto the catalyst surface in the form of ${}_{}{}^{*}\mathrm{H}\mathrm{O}{\mathrm{S}\mathrm{O}}_4^{2+}$ and coordinates with the surface ${\mathrm{C}\mathrm{o}}^{2+}$/${\mathrm{F}\mathrm{e}}^{2+}$ to form a transient ${\mathrm{C}\mathrm{o}}^{2+}$/${\mathrm{F}\mathrm{e}}^{2+}\mathrm{O}\mathrm{O}{\mathrm{S}\mathrm{O}}_3^{3+}$ complex. Subsequently, the transient complex undergoes a dual-electron transfer process to generate $\mathrm{C}\mathrm{o}\left(\mathrm{I}\mathrm{V}\right)$ and $\mathrm{F}\mathrm{e}\left(\mathrm{I}\mathrm{V}\right)$ [34]. It is noteworthy that upon analyzing the changes in the XPS results for O 1s prior to and following the reaction, an increase in the proportion of surface hydroxyl groups located on the surface of the catalyst can be observed. The establishment of hydroxyl groups on the surface may be due to the physical adsorption of water on Ni/Fe sites [43], resulting in the formation of ${\mathrm{F}\mathrm{e}}^{2+}$-OH/${\mathrm{N}\mathrm{i}}^{2+}$-OH species via Equation (10). In addition, Co can also generate Co-*OH through the cleavage of surface-adsorbed PMS, leading to the formation of adsorbed hydroxyl radicals [44]. According to relevant literature, surface hydroxyl groups function as intermediary species that facilitate the electron transfer involving Fe sites and oxygen-oxygen bonds. Consequently, this facilitates the formation of $\mathrm{F}\mathrm{e}\left(\mathrm{I}\mathrm{V}\right)$ through a mechanism involving a process where two electrons are exchanged [45]. Additionally, the presence of Fe may lead to the elongation of Co-O bonds, enhancing their tendency to undergo cleavage and interact in chemical reactions, thus yielding a greater abundance of metal–PMS complexes and forming more abundant $\mathrm{F}\mathrm{e}\left(\mathrm{I}\mathrm{V}\right)$=O or $\mathrm{C}\mathrm{o}\left(\mathrm{I}\mathrm{V}\right)$=O species [46].

Notably, Ni functions as a bifunctional electron-transfer mediator in the catalytic system. On one hand, it facilitates the heterogeneous decomposition of PMS to generate ${}^1\mathrm{O}_2$. On the other hand, the surface-adsorbed hydroxyl groups on nickel act as electron-transfer bridges, significantly accelerating the transformation of divalent metal ions to high-valent states ($\mathrm{F}\mathrm{e}\left(\mathrm{I}\mathrm{V}\right)$/$\mathrm{C}\mathrm{o}\left(\mathrm{I}\mathrm{V}\right)$). Furthermore, Ni incorporation may modulate the local electronic structure to optimize Co-O bond strength, thereby promoting the formation of additional metal–PMS complexes.

$${\mathrm{F}\mathrm{e}}^{3+}/{\mathrm{C}\mathrm{o}}^{3+}/{\mathrm{N}\mathrm{i}}^{3+}+{\mathrm{H}\mathrm{S}\mathrm{O}}_5^{-}\to {\mathrm{F}\mathrm{e}}^{2+}/{\mathrm{C}\mathrm{o}}^{2+}/{\mathrm{N}\mathrm{i}}^{2+}+{\mathrm{S}\mathrm{O}}_5^{•-}+{\mathrm{H}}^{+}$$

(6)

$$2{\mathrm{S}\mathrm{O}}_5^{•-}\to {2\mathrm{S}\mathrm{O}}_4^{2-}+{}^1\mathrm{O}_2$$

(7)

$${\mathrm{F}\mathrm{e}}^{2+}+{\mathrm{H}\mathrm{S}\mathrm{O}}_5^{-}\to \mathrm{F}\mathrm{e}\left(\mathrm{I}\mathrm{V}\right){\mathrm{O}}^{2+}+{\mathrm{H}\mathrm{S}\mathrm{O}}_4^{-}$$

(8)

$${\mathrm{C}\mathrm{o}}^{2+}+{\mathrm{H}\mathrm{S}\mathrm{O}}_5^{-}\to \mathrm{C}\mathrm{o}\left(\mathrm{I}\mathrm{V}\right){\mathrm{O}}^{2+}+{\mathrm{H}\mathrm{S}\mathrm{O}}_4^{-}$$

(9)

$${\mathrm{F}\mathrm{e}}^{2+}/{\mathrm{N}\mathrm{i}}^{2+}+{\mathrm{H}}_2\mathrm{O}\to {\mathrm{F}\mathrm{e}}^{2+}-\mathrm{O}\mathrm{H}+{\mathrm{N}\mathrm{i}}^{2+}-\mathrm{O}\mathrm{H}$$

(10)

3.5. Influence of Crucial Elements

3.5.1. Influence of Catalyst and PMS Dosage

Figure 5a,b illustrate how catalyst and PMS dosages affect the degradation of TCH. As the catalyst dosage is increased from 0.05 g/L to 0.4 g/L, the k_obs value rises from 0.0337 min⁻¹ to 0.2279 min⁻¹. Evidently, a higher quantity of catalysts offers more reaction sites to facilitate PMS activation, leading to more efficient breakdown of TCH. Similarly, in the system of Co_0.5Ni_0.5Fe₂O₄-activated PMS, the removal of TCH exhibits a consistent trend with varying PMS dosages, where the degradation rate of TCH positively correlates with the concentration of PMS. Drawing from the activation mechanism analysis discussed earlier, it is postulated that at higher PMS dosages, the generation of high-valent metals and ROS is enhanced.

3.5.2. Influence of Initial Concentration of TCH

The influence of varying TCH concentrations at the start on its degradation efficiency is depicted in Figure 5c. Specifically, when the initial concentrations of TCH were set at 10 mg/L, 20 mg/L, and 40 mg/L, the corresponding calculated k_obs values were 0.1587 min⁻¹, 0.0996 min⁻¹, and 0.0611 min⁻¹, respectively. The rate at which TCH degrades in the system decreased with the increase in the initial TCH concentration. At lower TCH concentrations, the radicals generated within the system were sufficient to oxidize and degrade the contaminants. However, with the increase in the initial TCH concentration, more contaminants accumulated on the surface of the catalyst, impeding the reaction between PMS and the catalyst. This resulted in a reduction in the production of active species, which were insufficient to remove high concentrations of TCH. Furthermore, the concentration of degradation products generated in the system also increased with the rising TCH concentration [47]. These degradation products could potentially compete with the parent contaminant for the limited radicals available.

3.5.3. Influence of Initial pH

The solution’s initial pH is a crucial parameter in advanced oxidation processes, as it significantly influences the efficiency of PMS activation, along with the surface charge of solid catalysts [48]. In this study, we conducted an investigation across a wide pH range to evaluate its impact on the catalytic degradation system. Our findings revealed that, although the degradation rate of TCH remained high at 98% over a pH range between 3.0 and 9.0, the observed rate constants (k_obs) decreased as the pH increased. Under acidic conditions, the catalyst’s surface, which is positively charged facilitated the adsorption of ${\mathrm{H}\mathrm{S}\mathrm{O}}_5^{-}$ ions [43,49], which in turn encouraged the creation of high-valent metal species and enhanced the activation efficiency of PMS. Notably, under acidic conditions, $\mathrm{F}\mathrm{e}\left(\mathrm{I}\mathrm{V}\right)$ and $\mathrm{C}\mathrm{o}\left(\mathrm{I}\mathrm{V}\right)$ can exist stably [50,51]. Consequently, the highest PMS activation efficiency was observed within a narrow acidic pH range between 3.0 and 5.0. However, at a pH of 9.0, a large amount of ${\mathrm{H}\mathrm{S}\mathrm{O}}_5^{-}$ is converted into ${\mathrm{S}\mathrm{O}}_5^{2-}$ ions [52]. The presence of a significant proportion of ${\mathrm{S}\mathrm{O}}_5^{2-}$ ions led to increased electrostatic repulsion between the PMS and the catalyst surface with a negative charge. This repulsion further reduced the efficiency of TCH degradation.

3.5.4. Influence of Co-Existing Ions

In practical wastewater systems, numerous coexisting ions are typically present. This study examines the impact of four inorganic anions, namely chloride (${\mathrm{C}\mathrm{l}}^{-}$), carbonate (${\mathrm{C}\mathrm{O}}_3^{2-}$), nitrate (${\mathrm{N}\mathrm{O}}_3^{-}$), and sulfate (${\mathrm{S}\mathrm{O}}_4^{2-}$), on the rate at which TCH degrades in a catalytic system. To prevent the introduced inorganic anions from interfering with the pH of the catalytic system, the target inorganic anions were added during the adsorption process of the catalyst. After the adsorption experiments, the pH of the solution was readjusted to the predetermined initial level to ensure the stability and comparability of subsequent reaction conditions. At higher concentrations, ${\mathrm{C}\mathrm{l}}^{-}$ slightly inhibits the reaction system. This inhibition arises from ${\mathrm{C}\mathrm{l}}^{-}$ altering the behavior of the O-O bond cleavage in PMS, hindering the formation of Fe(IV), and consuming a significant amount of Co(IV) through oxygen transfer reactions, simultaneously generating the byproduct hypochlorous acid (HOCl) [53]. Although HOCl is an oxidant with weaker oxidative capacity compared to Co(IV), it can still oxidize TCH. At low concentrations, the presence of ${\mathrm{C}\mathrm{l}}^{-}$ generates a small amount of HOCl, thereby marginally enhancing TCH degradation. However, as ${\mathrm{C}\mathrm{l}}^{-}$ concentration increases, its inhibitory effects on peroxymonosulfate (PMS) activation, and the formation of high-valent metal species (e.g., Fe(IV) and Co(IV)) gradually become predominant. This leads to a reduction in the concentration of effective oxidants within the reaction system, ultimately suppressing TCH degradation efficiency. Consequently, the influence of ${\mathrm{C}\mathrm{l}}^{-}$ on the reaction system exhibits a concentration-dependent behavior: low concentrations slightly promote degradation, while high concentrations slightly inhibit it.

In contrast, ${\mathrm{C}\mathrm{O}}_3^{2-}$, ${\mathrm{N}\mathrm{O}}_3^{-}$, and ${\mathrm{S}\mathrm{O}}_4^{2-}$, which are oxygen-containing anions, demonstrate a promotional effect on the catalytic system. The presence of ${\mathrm{C}\mathrm{O}}_3^{2-}$ has been demonstrated to facilitate the transformation of ${\mathrm{C}\mathrm{o}}^{3+}$ to ${\mathrm{C}\mathrm{o}}^{2+}$ [54], which is advantageous for the recycling of cobalt, promotes the formation of $\mathrm{C}\mathrm{o}\left(\mathrm{I}\mathrm{V}\right)$, and enhances catalytic efficiency. Under the influence of PMS, ${\mathrm{N}\mathrm{O}}_3^{-}$ can be oxidized to ${\mathrm{N}\mathrm{O}}_3^{•}$, possessing a high redox potential [55]. Consequently, the presence of ${\mathrm{N}\mathrm{O}}_3^{-}$ slightly elevates the reaction rate. The addition of ${\mathrm{S}\mathrm{O}}_4^{2-}$ may inhibit the spontaneous decomposition of PMS, allowing a greater portion of PMS to be utilized for the oxidative generation of $\mathrm{F}\mathrm{e}\left(\mathrm{I}\mathrm{V}\right)$ and $\mathrm{C}\mathrm{o}\left(\mathrm{I}\mathrm{V}\right)$.

Overall, the catalyst demonstrates robust efficiency in degrading TCH under various reaction conditions and exhibits high resistance to interference from coexisting ions. Its capability to retain high performance when exposed to different anions underscores its applicability in practical settings for wastewater treatment.

3.6. Catalyst Performance Comparison

To further evaluate the catalytic performance of the catalyst, we synthesized two additional spinel ferrite catalysts, $\mathrm{C}\mathrm{o}{\mathrm{F}\mathrm{e}}_2{\mathrm{O}}_4$ and $\mathrm{N}\mathrm{i}{\mathrm{F}\mathrm{e}}_2{\mathrm{O}}_4$, using the solvothermal method, and compared their catalytic performance with that of the novel Co_0.5Ni_0.5Fe₂O₄ catalyst. The results are presented in Figure 6.

Under identical conditions, the reaction rate constant for Co_0.5Ni_0.5Fe₂O₄ was 0.0996 min⁻¹, with a TCH removal efficiency of 99.58%. In contrast, $\mathrm{N}\mathrm{i}{\mathrm{F}\mathrm{e}}_2{\mathrm{O}}_4$ exhibited a reaction rate constant of 0.0591 min⁻¹ and a TCH removal efficiency of 96.61%, while $\mathrm{C}\mathrm{o}{\mathrm{F}\mathrm{e}}_2{\mathrm{O}}_4$ showed a reaction rate constant of 0.0641 min⁻¹ and a TCH removal efficiency of 97.74%. These results clearly demonstrate that Ni doping into $\mathrm{C}\mathrm{o}{\mathrm{F}\mathrm{e}}_2{\mathrm{O}}_4$ significantly enhances its catalytic performance.

Previous studies have reported that in the $\mathrm{C}\mathrm{o}{\mathrm{F}\mathrm{e}}_2{\mathrm{O}}_4$/PMS system, the primary active species generated is ${}^1\mathrm{O}_2$ [40]. However, based on our experimental findings, we conclude that after Ni doping into $\mathrm{C}\mathrm{o}{\mathrm{F}\mathrm{e}}_2{\mathrm{O}}_4$, the dominant active species in the PMS system shifts to high-valent metals ($\mathrm{F}\mathrm{e}\left(\mathrm{I}\mathrm{V}\right)$ and $\mathrm{C}\mathrm{o}\left(\mathrm{I}\mathrm{V}\right)$). This transformation not only significantly improves the stability and environmental resistance of the catalyst but also enhances its ability to degrade electron-rich organic pollutants such as TCH. The synergistic effect between Co and Ni in the bimetallic spinel structure likely contributes to the generation and stabilization of these high-valent metal species, which are more effective in breaking down complex organic molecules.

4. Conclusions

In summary, magnetic Co_0.5Ni_0.5Fe₂O₄ was successfully synthesized via a solvothermal reaction, with the catalyst particle size being modulated by controlling the calcination duration. Subsequent experiments involving the activation of PMS for TCH degradation demonstrated that smaller particles exhibited faster degradation rates, as a result of their expanded surface area and greater availability of active sites. Quenching experiments and EPR analysis additionally confirmed that high-valent metals functioned as the main active species, with ${}^1\mathrm{O}_2$ playing a secondary role. Notably, Co_0.5Ni_0.5Fe₂O₄ maintained high catalytic activity across various reaction environments and retained its stability and reusable practicality after multiple cycles of use. This study introduces a ferrite capable of utilizing high-valent metals as the primary active species, thereby providing a novel platform for the development of advanced catalytic materials in environmental remediation and water treatment applications.