PVP-Assisted Synthesis of Fe-TiO₂ for Efficient Tetracycline Degradation via Peroxymonosulfate Activation

Abstract

Tetracycline (TC) is chemically stable and recalcitrant to natural degradation. Peroxymonosulfate (PMS)-based advanced oxidation processes offer an effective removal strategy, the efficacy of which relies on high-performance heterogeneous catalysts. Titanium dioxide (TiO₂) is an ideal material due to its stability and environmental compatibility, yet its practical application is hindered by inadequate PMS activation capacity, particle agglomeration, and difficult recovery. To address these limitations, a heterogeneous Fe/TiO₂ catalyst was constructed via Fe³⁺ doping, innovatively utilizing polyvinylpyrrolidone (PVP) as a structure-directing agent. PVP’s steric hindrance effectively suppressed nanoparticle agglomeration and enabled high dispersion of Fe active sites, simultaneously enhancing catalytic activity and stability. Under optimized conditions, the Fe/TiO₂/PMS system achieved 94.3% TC degradation, following pseudo-first-order kinetics and significantly outperforming pure TiO₂ used in this experimental system. Radical quenching verified sulfate radicals (SO₄•⁻) as the dominant species. The catalyst demonstrated excellent recyclability, retaining over 80% degradation efficiency after six cycles and enabling convenient magnetic separation. Moreover, in complex water matrices (tap water and seawater), it sustained high removal efficiency (>90% initially, >70% after six cycles), highlighting its superior anti-interference capability and practical potential. This work offers a strategic material design strategy for efficient and robust TC removal in challenging water environments.

1. Introduction

With the rapid development of the economy and the continuous deepening of industrialization, water pollution issues have become increasingly severe. Among them, the widespread presence of antibiotic contaminants in water bodies and their potential ecological risks have attracted extensive attention [1]. Tetracycline (TC), a typical broad-spectrum antibiotic, is extensively used in human medicine and livestock farming. However, its persistence and tendency to accumulate in aquatic environments pose a serious threat to ecological balance and public health [2]. Moreover, the accumulation of TC in organisms may induce adverse physiological effects, including nephrotoxicity, mutagenicity, and teratogenicity [3]. Consequently, the development of efficient TC treatment technologies holds significant importance.

Currently, techniques such as adsorption, biodegradation, membrane separation, and catalytic degradation have been employed for TC removal [4,5,6]. Among them, advanced oxidation processes (AOPs) are regarded as one of the most promising approaches for TC elimination due to their high efficiency, low cost, and mild reaction conditions [7]. In particular, the peroxymonosulfate (PMS)-activated AOP has garnered widespread attention owing to its exceptional degradation capability for emerging contaminants and broad pH adaptability [8]. Conventional PMS activation strategies primarily rely on homogeneous Fenton-like catalytic systems (e.g., using soluble iron salts). However, these systems face significant limitations: the continuous leaching of transition metal ions not only leads to catalyst deactivation but also causes secondary pollution, severely restricting their practical application [9]. To address these issues, the development of stable heterogeneous Fenton-like catalysts has become a research focus [10,11,12]. Among various heterogeneous catalysts, titanium dioxide (TiO₂) stands out due to its comprehensive advantages including non-toxicity, high chemical stability, abundance, and easily modifiable surface [13,14]. However, TiO₂-based materials generally suffer from limitations such as restricted PMS activation capability and difficult recovery [15].

To address these challenges, researchers have explored modification strategies including crystal phase control, elemental doping, and surface sensitization [16,17,18]. Among these, metal ion doping has been demonstrated as an effective approach to enhance the overall performance of TiO₂-based catalysts [19,20,21]. Fe³⁺-modified TiO₂ composite materials have attracted significant attention due to their high stability and tunable surface properties. The Fe³⁺ ion, with its similar radius to Ti⁴⁺, can be firmly incorporated into the TiO₂ lattice through lattice site substitution or surface anchoring [22]. For instance, Diamandescu et al. confirmed through extended X-ray absorption fine structure (EXAFS) analysis that Fe³⁺ can successfully substitute Ti⁴⁺ sites in the TiO₂ lattice, inducing local structural distortion characterized by an increase in Ti-O bond length and a decrease in Ti-Ti bond length. This structural alteration provides a microscopic basis for explaining the enhancement of its catalytic performance [23]. This process not only establishes efficient and stable heterogeneous active centers within the material but also introduces ferromagnetic components through successful Fe³⁺ doping, endowing the catalyst with responsiveness to external magnetic fields. The resulting magnetic properties enable rapid catalyst recovery through magnetic separation after reaction completion, eliminating dependence on traditional solid–liquid separation techniques such as centrifugation or filtration. This approach significantly enhances recovery efficiency and operational convenience. For example, Sun et al. [24] fabricated a magnetic Fe₂O₃/mesoporous black TiO₂ hollow-sphere heterojunction (M-Fe₂O₃/b-TiO₂) via wet impregnation and surface hydrogenation strategies, which exhibits broad-spectrum response and magnetic separation capability. Furthermore, the Fe active centers can directly and efficiently activate PMS by triggering homolytic cleavage of its O-O bond through interfacial electron transfer, continuously generating highly reactive oxygen species dominated by sulfate radicals (SO₄•⁻). Simultaneously, a unique synergistic mechanism may form between potential Ti³⁺ defects and Fe species in the material: Ti³⁺ acts as an endogenous electron donor, facilitating the reduction in surface Fe³⁺ to Fe²⁺, thereby significantly accelerating the Fe²⁺/Fe³⁺ interfacial catalytic cycle and enhancing both PMS activation efficiency and sustained radical generation capacity [25]. For instance, Xu et al. [26] activated the originally redox-inert TiO₂ by inducing a surface Fe²⁺/Fe³⁺ cycle through Fe doping in TiO₂. This modification not only confers exceptional Fenton-like activity to TiO₂ but also substantially addresses the technical challenges associated with the recovery and reuse of TiO₂ nanoparticles. Although the Fe³⁺ doping strategy can enhance the catalytic activity of TiO₂ and improve its recovery performance, such materials still face a critical challenge in practical Fenton-like applications: nanoparticles are prone to agglomeration during synthesis, leading to reduced active sites and diminished mass transfer efficiency, which consequently causes significant attenuation of catalytic and stability performance [27,28].

In response to the issues outlined above, several strategies have been explored in existing studies. For instance, constructing hollow or mesoporous structures to increase specific surface area or introducing carbon materials as dispersion carriers has been proposed. However, these approaches often involve complex synthesis procedures, and the introduction of additional components may lead to unfavorable interfacial charge recombination [29,30,31]. On the other hand, the use of conventional polymers or surfactants as dispersing agents represents an alternative approach. However, their insufficient stability under Fenton-like reaction conditions often leads to desorption from the catalyst surface, resulting in non-persistent dispersion effects [32,33]. It is noteworthy that even polyvinylpyrrolidone (PVP), a polymer capable of effectively suppressing random nanoparticle aggregation through coordination by its polar groups and steric hindrance from its hydrophobic chains, has been predominantly utilized in existing studies merely as a physical barrier [34]. This limited utilization hinders the full exploitation of its potential as a "structure-directing agent" for synergistically optimizing the overall material performance. Consequently, existing strategies often focus predominantly on addressing isolated challenges, such as agglomeration during synthesis or recovery difficulties, while lacking an integrated design capable of simultaneously optimizing activity, stability, and operational convenience. Crucially, the deep integration of the interfacial modulation capability of PVP with the magnetic and catalytic properties of Fe³⁺ doping remains a significant challenge. The fundamental goal of constructing a catalytic material that concurrently possesses high-density active sites, superior operational stability, and convenient magnetic separation is thus a critically underexplored scientific problem.

Based on the above analysis, we propose a PVP-assisted Fe³⁺ doping strategy to construct a high-performance Fe/TiO₂ catalyst via a facile one-step hydrothermal method. In this design, PVP molecules not only suppress particle agglomeration but also, through their coordination effect, guide and stabilize the highly dispersed state of Fe species while they are incorporated into the TiO₂ lattice, thereby establishing a robust Fe-O-Ti interface at the nanoscale. This synergistic design effectively inhibits nanoparticle aggregation, thereby enhancing the initial catalytic activity while laying a structural foundation for long-term cycling stability. Furthermore, the successfully incorporated Fe³⁺ serves as highly efficient active sites while simultaneously endowing the material with convenient magnetic separability. Performance tests demonstrate that the material maintains over 90% TC removal efficiency under complex water matrix conditions, along with favorable cycling stability. These results strongly validate the effectiveness of our proposed integrated design strategy, using PVP as a structure-directing agent, in simultaneously enhancing catalytic activity, stability, and operational convenience, offering a new perspective for the treatment of TC-containing wastewater in practical environments.

2. Results and Discussion

As illustrated in Scheme 1, a modified TiO₂ catalyst (Fe/TiO₂) was developed via a facile PVP-assisted hydrothermal method. The resulting catalyst effectively activated PMS through a synergistic action between TiO₂ and Fe species, generating highly oxidative radicals that subsequently attacked and degraded TC.

2.1. Characterization of Fe/TiO₂

The crystal structure and chemical environment of Fe/TiO₂ were systematically characterized by X-ray diffraction (XRD) and Fourier transform infrared (FT-IR) spectroscopy. The XRD pattern (Figure 1a) exhibits characteristic diffraction peaks at 25.3°, 37.8°, 48.0°, and 53.9°, which are indexed to the (101), (004), (200), and (105) crystal planes of anatase TiO₂ (JCPDS No. 21-1272) [35], respectively, indicating the formation of a well-crystallized anatase matrix. Moreover, no diffraction peaks corresponding to free iron oxide phases were detected, suggesting that the Fe³⁺ precursor did not form separate crystalline phases but was highly dispersed within the TiO₂ support. The chemical bonding states were further investigated by FT-IR spectroscopy (Figure 1b). The absorption band at 484.71 cm⁻¹ is attributed to the Ti-O-Ti skeletal vibration, confirming the preservation of the anatase structure as determined by XRD. The observed shoulder peak at 554.62 cm⁻¹, assigned to Fe-O bond vibration, directly demonstrates the successful incorporation of Fe species, forming stable configurations such as Fe-O-Fe or Ti-O-Fe, thereby constructing efficient heterogeneous active sites for PMS activation. Additionally, the appearance of the C-N stretching vibration peak at 1244 cm⁻¹ indicates the residual presence of PVP in the material. After the catalytic reaction, these key spectral features remained largely unchanged, demonstrating the excellent chemical bond stability of the catalyst under strong oxidizing conditions. N₂ adsorption–desorption analysis revealed fundamental differences in the pore characteristics between pristine TiO₂ (Figure 1c) and Fe/TiO₂ (Figure 1d). Fe/TiO₂ exhibited an order-of-magnitude enhancement in both specific surface area (237.2 m²/g) and total pore volume (0.275 cm³/g) compared to pristine TiO₂ (11.99 m²/g and 0.021 cm³/g, respectively), creating favorable conditions for high-density loading and full exposure of Fe active sites. The pore size distribution provided further structural insight: pristine TiO₂ showed a concentration of pores around 3.7 nm, indicative of tight nanoparticle packing, whereas Fe/TiO₂ displayed a dominant peak at 35.2 nm. This prominent shift confirms that PVP acted as a structure-directing agent, guiding the assembly of nanoparticles into a three-dimensional network with open mesoporous channels. Crucially, this mesoporous structure is highly advantageous over a microporous one, as it provides sufficiently large channels for the rapid diffusion of bulky PMS and TC molecules. Consequently, this optimized pore architecture significantly enhanced mass transfer of PMS and TC molecules, ensuring their efficient access to internal active sites [36].

To further investigate the morphological and structural characteristics of the catalysts, TiO₂ and Fe/TiO₂ were characterized by transmission electron microscopy (TEM). As shown in Figure 2a, pure TiO₂ exhibits a sheet-like morphology with poor crystallinity, as evidenced by vague lattice fringes. After Fe modification, Fe/TiO₂ (Figure 2b) maintained a uniformly dispersed nanosheet morphology without significant agglomeration, confirming that PVP effectively functioned as both a steric hindrance agent and a dispersant during synthesis. The high-resolution TEM image (Figure 2c) further reveals that the Fe/TiO₂ nanosheets possess clear and continuous lattice fringes with an interplanar spacing of approximately 0.35 nm, which corresponds well to the (101) plane of the anatase phase. This result is consistent with XRD analysis and indicates significantly enhanced crystallinity due to Fe doping and the hydrothermal process, providing a solid foundation for charge transfer and structural stability. Furthermore, the elemental mapping images (Figure 2d–f) clearly demonstrate a highly uniform distribution of Fe throughout the TiO₂ matrix with no observable local aggregation. This provides direct microscopic evidence that Fe species were successfully doped and uniformly anchored onto the TiO₂ surface.

X-ray photoelectron spectroscopy (XPS) analysis was conducted to further investigate the surface elemental composition and chemical states of the Fe/TiO₂ catalyst. The survey spectrum (Figure 3a) confirms the presence of Fe, Ti, and O elements. In the high-resolution Fe 2p spectrum (Figure 3b), the characteristic peaks observed at 711.2 eV and 709.8 eV are assigned to Fe³⁺ and Fe²⁺, respectively. The coexistence of the Fe³⁺/Fe²⁺ redox couple provides key sites for the electron transfer process in PMS activation during the Fenton-like reaction [37]. In the O 1s spectrum (Figure 3c), the peak located at 529.8 eV is attributed to contributions from both Fe-O and Ti-O bonds. The characteristic peak at 458.7 eV in the Ti 2p spectrum (Figure 3d) verifies the presence of Ti⁴⁺, corresponding to anatase TiO₂. Collectively, the XPS spectral analysis reveals strong interfacial interactions between the Fe and Ti species, which stabilize their stable and dispersed coexistence.

2.2. Optimization of Catalytic Condition

The optimal reaction conditions for TC degradation in the Fe/TiO₂/PMS system were determined by systematically investigating key parameters, including PMS concentration, initial TC concentration, and catalyst dosage. As the oxidant precursor, the PMS concentration directly governs the yield of active radicals in the system. As shown in Figure 4a, the degradation efficiency of TC initially increased and then decreased with increasing PMS concentration, reaching a maximum at 0.6 mM. In the low-concentration range (0.2–0.6 mM), the enhancement in degradation efficiency can be attributed to the increased activation of PMS molecules by the active sites on the Fe/TiO₂ catalyst surface, generating sufficient amounts of SO₄•⁻ and •OH radicals, thereby accelerating the oxidative decomposition of TC. However, when the PMS concentration exceeded 0.6 mM, the TC degradation efficiency decreased. This decline can be attributed to two primary reasons: first, excess PMS may scavenge SO₄•⁻ radicals in the system, consuming the highly reactive radical species; second, high PMS concentrations may lead to the occupation of catalyst active sites by its decomposition products or unreacted molecules, thereby reducing TC adsorption and inhibiting the reaction progress. Subsequently, the effect of initial TC concentration on the degradation performance was investigated. The effect of the initial TC concentration was also examined (Figure 4b). As expected, the degradation efficiency decreased with increasing TC concentration, which is consistent with expectations. This trend occurs because, at fixed catalyst and PMS loadings, the finite radical population becomes insufficient to degrade the higher quantity of TC molecules. Furthermore, TC and its degradation intermediates compete for the available radicals, ultimately leading to reduced apparent degradation efficiency.

Following the investigation of reactant concentrations, the role of catalyst dosage was evaluated. As the core component for PMS activation and radical generation, the catalyst dosage significantly influenced the process. As shown in Figure 4c, when the catalyst dosage increased from 0.1 g/L to 0.2 g/L, the degradation efficiency improved markedly and reached an optimum at 0.2 g/L, owing to the increased availability of active sites promoting PMS activation and sustained radical generation. However, a further increase to 0.4 g/L led to a decrease in efficiency, likely due to particle aggregation and increased mass transfer resistance at higher loadings, which reduced the accessibility of active sites. The reaction kinetics are shown in Figure 4d. The degradation of TC primarily occurred within the first 80 min, reaching 94.3%, after which the curve plateaued. This indicates that the catalytic oxidation reaction approached equilibrium by 80 min.

In summary, through systematic univariate optimization experiments, the optimal conditions for TC degradation were determined as follows: PMS concentration 0.6 mM, initial TC concentration 20 mg/L, Fe/TiO₂ dosage 0.2 g/L, and reaction time 80 min.

The reaction kinetics of the system and the enhancement effect of Fe modification on TiO₂ were further investigated. The degradation kinetics of TC under optimal conditions were fitted, indicating that the process follows a pseudo-first-order kinetic model (Figure 5a). This behavior demonstrates that the degradation rate of TC is proportional to its instantaneous concentration when the catalyst and PMS concentrations are held constant, a characteristic of typical heterogeneous catalytic oxidation processes. Furthermore, by comparing the catalytic performance of Fe/TiO₂ and pure TiO₂ under identical conditions, the significant effect of Fe modification was directly verified. As shown in Figure 5b, under natural light conditions, Fe/TiO₂ achieved a TC degradation rate three times that of pure TiO₂ used in this experimental system within the 80 min reaction period. This significant enhancement is primarily attributed to the successful introduction of Fe species, which creates highly dispersed active sites (e.g., Fe²⁺/Fe³⁺) on the TiO₂ surface. These sites are markedly more efficient than pure TiO₂ in activating PMS to generate highly oxidative SO₄•⁻ and •OH radicals.

2.3. Cycling Stability and Mechanism Investigation

The cycling stability and reaction mechanism are crucial for assessing the practical application potential of a catalyst and understanding its underlying chemical kinetics. The cycling performance of the Fe/TiO₂ catalyst was first evaluated. As shown in Figure 6a, the degradation rate of TC remained at approximately 80% after six consecutive cycles, indicating good structural stability and reusability. The slight decrease in the degradation rate could be attributed to the minor physical loss of the catalyst during each cycle. Furthermore, partial deactivation likely occurred because some active sites were occupied by intermediate products that were difficult to desorb during the reaction. To elucidate the dominant reaction mechanism, radical scavenging experiments were conducted. The inhibitory effects of different scavengers on the TC degradation rate showed significant differences (Figure 6b): Compared to the blank group, the reaction was strongly suppressed upon the addition of methanol, whereas the addition of TBA resulted in a relatively limited inhibitory effect. This contrast provides key evidence for identifying the dominant radical species. Methanol can efficiently quench both SO₄•⁻ and •OH, while TBA primarily quenches •OH, with its quenching rate constant for SO₄•⁻ being several orders of magnitude lower. Consequently, the experimental results clearly demonstrate that SO₄•⁻ plays a dominant role in this degradation system, while •OH plays a secondary role. Moreover, the addition of L-histidine and BQ only slightly inhibited the TC degradation process, indicating that singlet oxygen (¹O₂) and superoxide radicals (O₂•⁻) contributed minimally to the catalytic oxidation reaction. The possible degradation mechanism of TC in the Fe/TiO₂/PMS system is proposed to proceed via synergistic activation pathways. Specifically, Ti⁴⁺ sites form a surface complex with HSO₅•⁻, which undergoes interfacial electron transfer (likely from adsorbed TC molecules or the TiO₂ substrate itself) to generate SO₄•⁻ without altering the Ti oxidation state, which represents a non-radical activation route. Concurrently, the Fe species drive the radical-based activation through the Fe²⁺/Fe³⁺ redox cycle: Fe²⁺ reacts with HSO₅•⁻ to yield SO₄•⁻ and Fe³⁺, and the subsequent reduction of Fe³⁺ back to Fe²⁺ (facilitated by electron donors or TiO₂-mediated electron transfer) is crucial for sustaining the catalytic activity. Notably, the two pathways reinforce each other: the TiO₂ substrate not only contributes to direct PMS activation but also promotes the Fe³⁺/Fe²⁺ cycle, thereby collectively enhancing SO₄•⁻ production and enabling efficient TC mineralization.

2.4. TC Degradation in Real Water Samples

To evaluate the practical application potential of the catalyst, its performance and cycling stability for TC degradation were investigated in complex water matrices (tap water and seawater). As shown in Figure 7a,c, the TC degradation efficiency exceeded 90% in both real water matrices, with only a slight decrease compared to the ultrapure water system. This result indicates that the Fe/TiO₂/PMS system maintained excellent intrinsic catalytic activity despite the presence of common inorganic ions (e.g., Cl⁻, HCO₃⁻) in tap water or high salinity and complex organic matter in seawater. This robustness is likely due to the strong PMS activation capability of the stable Fe active sites on the catalyst surface, which were not readily completely poisoned by background constituents in the water. Furthermore, the degradation profiles over successive cycles (Figure 7a,c) and the corresponding histograms of degradation efficiency (Figure 7b,d) clearly show that after six consecutive cycles, the efficiency decreased to approximately 75% in tap water and 70% in seawater. This performance attenuation can be ascribed to competitive anions scavenging radicals, thereby reducing degradation efficiency and accelerating irreversible catalyst passivation. Additionally, natural organic matter (e.g., humic acid) or metal cations in seawater may form a surface coating on the catalyst, physically shielding active sites and hindering the diffusion and contact of PMS and TC. Nevertheless, the catalyst retained over 70% degradation efficiency after six cycles in these complex aqueous environments. This performance, superior to most reported results (Table S5), demonstrates its stable catalytic performance and potential for practical application.

3. Materials and Methods

3.1. Materials

Ferric chloride hexahydrate (FeCl₃·6H₂O) was purchased from Tianjin Opulent Chemical Co., Ltd., Shanghai, China. Other reagents including nano-titanium dioxide (TiO₂), urea (CH₄N₂O), polyvinylpyrrolidone ((C₆H₉NO)_n, PVP), potassium peroxymonosulfate (2KHSO₅·KHSO₄·K₂SO₄, PMS), tert-butanol (C₄H₁₀O, TBA), p-benzoquinone (C₆H₄O₂, BQ), and tetracycline (C₂₂H₂₄N₂O₈, TC) were obtained from Aladdin Chemical Reagent Co., Shanghai, China. All chemicals were used as received without further purification.

3.2. Characterization

The morphological features of TiO₂ and Fe/TiO₂ were investigated using transmission electron microscopy (TEM, Libra 200, Zeiss, Carl Zeiss AG, Jena, Germany). Crystal structure analysis was performed by X-ray powder diffraction (XRD, Rigaku, Tokyo, Japan) with 2θ scanning from 10° to 80° at a rate of 2°/min. The N₂ adsorption–desorption isotherms were measured using a Micromeritics ASAP 2020 surface area and porosity analyzer (Micromeritics, Norcross, GA, USA) to determine the specific surface area and pore structure. The elemental composition and surface chemical states of the materials were analyzed by X-ray photoelectron spectroscopy (XPS, Kratos AXIS ULTRA DLD, Kratos Analytical Ltd., Manchester, UK). Surface functional groups of the catalysts were characterized using Fourier transform infrared spectroscopy (FT-IR, JASCO FT-IR-420, JASCO Corporation, Tokyo, Japan). Additionally, ultraviolet-visible absorption spectra were recorded on a UV–Vis spectrophotometer (Purkinje General T700, Beijing Purkinje General Instrument Co., Ltd., Beijing, China).

3.3. Synthesis of Fe/TiO₂

1.0 g of nano-titanium dioxide (TiO₂) was dispersed in 50 mL of a solvent mixture of ethylene glycol and deionized water (1:1, v/v) and treated with ultrasonication to form a homogeneous suspension. Then, 0.56 g of ferric chloride hexahydrate (FeCl₃·6H₂O) was added to the suspension, and the mixture was stirred for 1 h to achieve sufficient adsorption of Fe³⁺ ions onto the TiO₂ support. Subsequently, 0.3 g of urea (CH₄N₂O) and 0.2 g of polyvinylpyrrolidone ((C₆H₉NO)_n, PVP) were added sequentially under continuous stirring, until a uniform mixture was obtained. The mixture was transferred into a Teflon-lined stainless-steel autoclave and reacted at 200 °C for 6 h. After the autoclave was naturally cooled to room temperature, the product was collected by centrifugation and washed three times with deionized water and twice with anhydrous ethanol. The solid was finally dried at 60 °C for 8 h under vacuum to obtain the powder sample.

3.4. Optimization of Catalytic Conditions

The effects of key operational parameters, including peroxymonosulfate (2KHSO₅·KHSO₄·K₂SO₄, PMS) concentration, tetracycline (C₂₂H₂₄N₂O₈, TC) initial concentration, catalyst dosage, and reaction time, on the degradation performance of Fe/TiO₂ were systematically investigated using a one-factor-at-a-time approach. The specific procedure was as follows: a predetermined mass of Fe/TiO₂ catalyst was placed in a beaker, and a TC solution of specified concentration was added. The mixture was sealed and then stirred at 500 rpm for 60 min in the dark to achieve adsorption–desorption equilibrium. Immediately after equilibrium, a sample was taken to measure the initial absorbance of TC at 355 nm. Subsequently, a PMS solution of given concentration was introduced to initiate the catalytic oxidation reaction. At predetermined time intervals, samples were continuously collected, and the variation in the characteristic absorption peak intensity of TC was monitored using a UV–Vis spectrophotometer. The degradation efficiency was calculated according to Equation (1):

$$Degradation (\%) = \left[\right(C_0 - C)/C_0] \times 100\%$$

(1)

where C₀ is the initial absorbance of the pollutant after adsorption equilibrium and before the catalytic oxidation reaction starts, and C is the absorbance of TC at a specific time during the catalytic reaction.

The parameter optimization procedure was as follows. First, with the catalyst dosage fixed at 10 mg and the initial TC concentration fixed at 40 mg/L, the influence of PMS concentration (0.2, 0.4, 0.6, 0.8, and 1 mM) was systematically investigated. Based on these optimization results, the optimal PMS concentration was applied to subsequently investigate the effects of the initial TC concentration (20, 40, 60, and 80 mg/L) and then the catalyst dosage (0.1, 0.2, 0.3, and 0.4 g/L) on the degradation performance. Finally, the reaction kinetics were evaluated over a time period of 15–120 min (Tables S1–S4).

3.5. Cycling Stability Test

The cycling stability of the Fe/TiO₂ catalyst was evaluated through six consecutive rounds of recovery and reuse. After each reaction cycle, the catalyst was recovered by magnetic separation, followed by ultrasonic cleaning sequentially with 20 mL of anhydrous ethanol and 20 mL of ultrapure water. This washing procedure was repeated three times. After vacuum drying at 35 °C for 8 h, the catalyst was reintroduced into the next catalytic reaction under optimal conditions. The stability was assessed by monitoring the variation in degradation efficiency across the cycles.

3.6. TC Degradation in Real Water Samples

To evaluate the applicability and anti-interference capability of the as-prepared Fe/TiO₂ in practical aquatic environments, its degradation behavior toward TC was systematically assessed in two complex aqueous matrices: tap water and seawater. All environmental water samples were filtered through 0.45 μm microporous membranes after collection to remove suspended particulate matter. Subsequently, TC solutions were prepared using these real water samples as the solvent, and degradation experiments were conducted under the optimized catalytic conditions. The performance was compared with that observed in an ultrapure water system.

4. Conclusions

This study demonstrates that PVP serves as a highly effective structure-directing agent for fabricating a high-performance Fe/TiO₂ catalyst via a facile one-step hydrothermal route. Beyond suppressing nanoparticle agglomeration, PVP guided the formation of a robust Fe–O–Ti interface, which was crucial for creating highly dispersed and stable Fe active sites. This synergistic design resulted in a catalyst that integrates exceptional PMS activation capability (achieving 94.3% TC degradation under optimum conditions), remarkable recyclability (>80% efficiency after 6 cycles), and convenient magnetic separation. The dominance of sulfate radicals in the system and its robust performance in complex water matrices underscore its potential for practical wastewater treatment.