Efficient Degradation of Tetracycline via Cobalt Phosphonate-Activated Peroxymonosulfate: Mechanistic Insights and Catalytic Optimization

Abstract

The persistent contamination of aquatic systems by antibiotics, particularly tetracycline (TC), which induces antibiotic resistance genes and chronic toxicity to aquatic organisms, necessitates advanced oxidation processes. Herein, cobalt phosphonate (CoP) nanosheets with tailored Co/P ratios were synthesized to activate peroxymonosulfate (PMS) for TC degradation under visible light. Through a controlled-variable approach, the reaction parameters were systematically optimized. The refined CoP-3 system achieved 90.7% TC removal within 6 min, with the optimal degradation parameters determined as 0.1 g/L CoP-3 and 0.2 g/L PMS. Based on liquid chromatography–tandem mass spectrometry (LC-MS/MS) analysis, three degradation pathways were inferred. The toxicity of TC and its intermediates was assessed using quantitative structure–activity relationships (QSARs) via the Toxicity Estimation Software Tool (T.E.S.T). The results demonstrated reduced acute toxicity in intermediates compared to the parent compound. In practical applications, the catalyst achieved 64.7% and 80.8% TC removal rates in livestock wastewater and river water, respectively, while maintaining stable activity over four cycles. This demonstrates significant potential for engineering applications. The results were verified by free radical quenching experiments and were attributed to enhanced charge separation and an h⁺-dominated non-free radical pathway. This work provides a sustainable strategy for antibiotic remediation based on transition metal phosphonates.

1. Introduction

Tetracycline (TC) antibiotics pose escalating global threats to ecosystems and public health due to their widespread misuse and environmental persistence [1,2]. Recent data reveal that over 30,000 tons of TC class antibiotics are consumed annually worldwide, with agricultural runoff and pharmaceutical wastewater being primary contamination sources. Advanced monitoring studies detected alarming TC concentrations exceeding 1200 μg/L in hospital effluents and 85 mg/kg in manure-amended soils [3]. These residues drive antimicrobial resistance crises, with cutting-edge metagenomics identifying TC exposure as being responsible for 12–18 fold increases in resistant pathogens carrying tet (X) and other novel resistance genes [4,5]. The ecological impacts are equally severe; TC disrupts microbial communities at concentrations as low as 50 μg/L, impairing critical nutrient cycling functions [6]. Human health risks now include verified links to dysbiosis, heightened allergy susceptibility, and potential endocrine disruption. According to the Global Water Quality Monitoring Report, TC contamination has been detected in 76% of freshwater samples worldwide. Developing effective remediation strategies has become an urgent One Health priority.

Currently, the mainstream emerging antibiotic wastewater treatment technologies, both domestically and internationally, include coupled biological treatment methods [7], physical–chemical methods, membrane separation technology [8] and advanced oxidation technology [9]. The coupled biological treatment method has the advantages of reducing the use of chemical agents, avoiding secondary pollution, and having low operating costs. However, the presence of antibiotics inhibits microbial growth [10], thereby affecting the degradation efficiency of antibiotics in wastewater. Physical–chemical wastewater treatment methods do not rely on microbial metabolism and are suitable for treating high-concentration, difficult-to-degrade, or toxic wastewater. Membrane separation technology, as a key technique for treating antibiotic wastewater, efficiently removes antibiotic pollutants through mechanisms such as physical sieving, charge effects, and catalytic degradation. Sun et al. reported a strategy for preparing a series of inorganic–organic hybrid nanosheets based on pillar-supported aryl-embedded MXene nanosheets [11], where the membrane was formed through vacuum-assisted filtration. The prepared membrane demonstrated relatively high retention rates and stability when treating water containing antibiotics. However, the reusability of the membrane requires further discussion.

Advanced oxidation processes (AOPs) are a class of chemical oxidation methods that efficiently degrade organic pollutants by generating strong oxidizing radicals such as hydroxyl radicals (^•OH) and sulfate radicals (SO₄^•−). Although ^•OH is the primary radical, its non-selective nature and stringent pH requirements lead to more severe consumption when multiple pollutants are present in water, thereby reducing its effectiveness in degrading antibiotics. Advanced oxidation technologies based on SO₄^•− (SR-AOPs) are considered viable alternatives to traditional AOPs due to their high redox potential (2.5–3.1 V) [12,13], which is close to that of ^•OH (2.8 V), and their long half-life in the mineralization of organic pollutants. Additionally, SO₄^•− is more adaptable to a wide range of environmental pH values and exhibits greater selectivity, avoiding excessive degradation [14].

The integration of photocatalysis with peroxymonosulfate (PMS) activation represents a breakthrough in tetracycline antibiotic degradation, combining the advantages of both advanced oxidation processes [15,16,17,18]. This hybrid system generates multiple reactive oxygen species including ^•OH, SO₄^•−, holes (h⁺), and singlet oxygen through synergistic mechanisms, enabling mineralization of persistent tetracycline molecules under mild conditions [19,20]. Recent advances in catalyst design have significantly enhanced the efficiency of these systems, with cobalt-doped carbon nitride demonstrating a remarkable 99.2% tetracycline removal within 20 min by facilitating interfacial electron transfer between the photocatalyst and PMS [21]. Cobalt-based metal-organic frameworks have shown particular promise while maintaining excellent structural stability with minimal metal leaching [22]. Novel biomass-derived carbon materials with nitrogen and sulfur co-doping have exhibited exceptional performance, displaying reaction kinetics 5.3 times faster than conventional photocatalysis alone due to their unique defect-mediated activation pathways [23]. These innovative materials address the long-standing challenges of traditional water treatment methods by simultaneously improving light absorption, charge separation efficiency, and PMS utilization while demonstrating superior stability for practical applications [24,25].

Transition metal phosphonates (TMPs) have emerged as highly effective catalysts for pollutant degradation due to their unique electronic properties and exceptional stability. Recent studies demonstrate their remarkable performance in advanced oxidation processes, particularly for degrading persistent organic contaminants [26,27]. For instance, Ni2P-modified photocatalysts exhibit superior tetracycline degradation efficiency, achieving complete mineralization within 60 min by simultaneously enhancing charge separation and activating PMS [28]. Similarly, FeP nanoparticles demonstrate outstanding stability in acidic conditions while effectively decomposing various pharmaceuticals through radical and non-radical pathways [29]. These materials overcome key limitations of conventional catalysts, offering both high activity and long-term durability for water treatment applications. The synergistic effects between their metallic conductivity and catalytic sites enable efficient pollutant degradation while minimizing secondary contamination, making TMPs promising candidates for sustainable environmental remediation [30,31].

The main lines of this study are as follows: (1) characterization of the synthesized CoP-3 by SEM, TEM, EDS, XRD, XPS, and FT-IR spectra; (2) comparison of different cobalt–phosphorus feedstock molar ratios in terms of the TC degradation effect; (3) analysis of the effect of optimized experimental conditions (including TC concentration, PMS concentration, and catalyst dosage) on the degradation of CoP-3/PMS for comparison; (4) investigation of the interference effects of anions, organic compounds, and real water matrices to assess the practical viability of this degradation system; (5) identification of three degradation pathways through liquid chromatography–mass spectrometry (LC-MS) analysis, and toxicological simulations of the intermediate products; (6) comparison of the catalytic properties and the types of the main acting radicals by radical quenching experiments; and (7) evaluation of the application prospects of the materials from the perspectives of stability, reusability, and applicability.

In this study, five transition metal phosphonate catalysts were synthesized in different ratios using cobalt nitrate and phenylphosphonic acid as feedstock. Under visible-light conditions, the degradation of TC was evaluated through PMS activation, and the results showed that all five catalysts exhibited effective catalytic activity. The excellent performance is attributed to the optimized catalyst structure, which promotes efficient PMS activation and rapid generation of active species. Additionally, the catalysts demonstrated excellent cycling stability, maintaining activity across multiple reaction cycles. By systematically elucidating the synergistic interaction between TMPs and PMS activation, we established a new paradigm for efficient pollutant mineralization through radical and non-radical coupled pathways, providing fundamental mechanistic insights for the rational design of high-performance photocatalytic systems for environmental remediation. These findings significantly advance the current understanding of PMS-mediated photocatalysis, particularly highlighting the dual functions of TMPs as both an electron mediator and a catalytic active site. The demonstrated material durability and pH adaptability address key limitations in traditional advanced oxidation processes. Furthermore, the revealed structure–activity relationships lay the scientific foundation for developing next-generation catalysts through targeted atomic engineering.

2. Results and Discussion

2.1. Characterization of Catalysts

The morphology of CoP-X was examined using SEM and TEM. As illustrated in Figure 1a,b, the SEM images of CoP-3 reveal a leaf-like lamellar structure. According to literature reports, polar solvents influence the crystallization direction of complexes [32]. The observed morphology of the catalyst may be attributed to the use of DMF during the preparation process. Figure 1c,d display the TEM images of CoP-3, which confirm the lamellar morphology observed in the SEM images. Furthermore, energy-dispersive X-ray spectroscopy (EDS) analysis was conducted to examine the surface elemental composition of CoP-3. The results, presented in Figure 1e–j, demonstrate a uniform distribution of C, N, O, Co, and P elements across the catalyst.

The crystal structure of CoP-X was determined by XRD, as shown in Figure 2a. The results showed that all CoP-X samples exhibited good crystalline morphology. Despite the differences in the crystallinity of the catalysts with different ratios, all of them observed strong characteristic diffraction peaks at 2θ = 12.02, and the characteristic peaks of CoP appeared at 2θ ≈ 31.6° (200) and 36.3° (210), indicating the successful synthesis of the cobalt–phosphorus compounds [33,34,35,36]. Among them, the intensity of diffraction peaks of CoP-3 was significantly higher than that of other samples, indicating its better crystallinity.

To further identify the functional groups present in CoP-X, infrared absorption spectroscopy was performed, as illustrated in Figure 2b. The peak positions across all materials are generally consistent. A strong absorption peak at 3466 cm⁻¹ corresponds to the C-H bond in the benzene ring, while the peak at 1608 cm⁻¹ is attributed to the bending vibration mode of water molecules [37]. The absorption peak at 1437 cm⁻¹ represents skeletal vibrations of the benzene ring, and the peak at 1053 cm⁻¹ is characteristic of Co-O-P bonds [35]. Peaks between 900 and 1142 cm⁻¹ are associated with the bending modes of tetrahedral CPO₃ groups, and the band below 900 cm⁻¹ likely relates to Co-O stretching vibrations [38]. These results confirm that the functional groups present in CoP-X align with the expected structural composition.

Based on the infrared spectroscopy analysis, XPS was conducted to further investigate the bonding modes of elements in CoP-X, as shown in Figure 3a. The wide-spectrum XPS data confirm the presence of C (1s), O (1s), P (2p), N (1s), and Co (2p) in all CoP-X samples. From the high-resolution Co 2p spectrum (Figure 3b), the binding energy of Co in CoP-2 is observed at 780.8 eV and 796.7 eV, indicating that cobalt exists in the CoP catalyst as Co²⁺ [39,40,41,42]. Notably, the binding energy in CoP-2 decreases compared to that in CoP-1, suggesting a higher electron density in CoP-2. Conversely, compared to CoP-3, CoP-2 exhibits higher binding energy, indicating a lower electron density than CoP-3. The electron density differences influence the catalytic behavior: higher electron density facilitates the generation of SO₄^•− by donating electrons to PMS, whereas lower electron density promotes the attraction of electrons, favoring the formation of ¹O₂ [43]. The high-resolution spectrum of P 2p (Figure 3c) reveals that phosphorus predominantly exists as phosphorus–oxygen bonds [44,45], further confirming the chemical composition and crystal structure of the prepared samples. The XPS characterization results are consistent with those obtained from other techniques, providing insights into the valence states and charge transfer directions within the catalysts.

The optical properties of CoP-X were analyzed using UV-Vis DRS, as shown in Figure S1. CoP-X exhibits strong absorption in both the ultraviolet and visible regions, with notable peaks at approximately 275 nm and 540 nm, indicating high visible-light absorption efficiency across all samples.

The charge migration behavior in CoP-X catalysts was investigated through photoelectric current response experiments. As shown in Figure 4a, under visible-light irradiation, CoP-1 and CoP-3 exhibited significant photoelectric current responses during multiple on-off cycles. CoP-3 demonstrated a higher photoelectric current density than CoP-1, indicating superior separation and transfer capabilities of photogenerated electron-hole pairs in CoP-3 [46,47]. Furthermore, the photoelectric current remained stable under prolonged illumination, reflecting the continuous ability of CoP-3 to generate and transfer electrons and holes. The electrochemical impedance spectrum further corroborates the enhanced electron transfer capability of CoP-3. As shown in Figure 4b, the arc radius of CoP-3 is smaller than that of CoP-1, indicating lower electron transfer impedance and faster electron diffusion rates. The CoP-X catalysts effectively inhibit the recombination of photogenerated electron–hole pairs while promoting their separation and transfer, with CoP-3 demonstrating the most efficient performance among the samples [48].

2.2. Photocatalytic Activity of CoP Catalysts with Different Proportions

The photocatalytic activity of CoP-X catalysts with varying proportions of precursors under visible-light irradiation was evaluated using PMS for the degradation of TC as the target pollutant. After a 20 min dark reaction, the TC concentration decreased by 1.5%, indicating that the material has low adsorption performance. A quantity of 5.0 mg of PMS was introduced to the system, and the pollutant degradation rate was monitored, as shown in Figure 5a,b. The results reveal that, under the synergistic effect of PMS, all five catalysts significantly enhanced the TC degradation rate. Among these, the CoP-3 catalyst synthesized with 3 mmol phenylphosphonic acid (n[C₆H₇O₃P]:n = 3:2) exhibited the highest degradation efficiency, achieving a TC degradation rate of 65.5% within 6 min with a corresponding rate constant of 0.503 min⁻¹. These findings are consistent with the catalyst characterization results.

2.3. Catalytic Activity of CoP-3 Catalyst

The system was tested for the removal of TC under six conditions: visible light, PMS, and CoP-3 alone, and visible light + CoP-3, PMS + CoP-3 and visible light + CoP-3 + PMS. As shown in Figure 5c, d, the results indicate that the degradation rate in the presence of visible light, PMS, and CoP-3 alone is low and almost negligible. Under the conditions of visible light + CoP-3, PMS + CoP-3, and visible light + CoP-3 + PMS, the TC removal rates were 15.3%, 78.7% and 90.7%, respectively, which indicated that the material had certain light-responsive properties and substantially improved the degradation efficiency with the addition of PMS, and the combination of visible light and PMS improved the TC removal efficiency.

2.4. Optimization of Experimental Parameters in the CoP-3/PMS/Vis System

2.4.1. Influence of Catalyst Dosage

The role of catalyst dosage in TC degradation was investigated using a fixed TC concentration of 10.0 mg/L. Following a 20 min dark reaction, 5.0 mg of PMS was added, and the effects of varying catalyst dosages (2.5, 5.0, 10.0, 15.0, and 20.0 mg) were assessed, as shown in Figure 6a,b. The results demonstrate a strong correlation between catalyst dosage and degradation efficiency. For a dosage of 2.5 mg of CoP-3, the TC degradation rate reached 70.5% within 6 min, which slightly increased to 70.2% when the dosage was raised to 5.0 mg, with a rate constant of 1.148 min⁻¹. The higher doses (10.0–20.0 mg) all showed degradation rates of more than 60.0% within 6 min, which was slightly lower than the TC removal rate at a catalyst of 5.0 mg, probably due to the competition of activation sites of the catalyst for PMS molecules. The experimental results showed that the optimal catalyst dose of 5.0 mg (equivalent to a concentration of 0.1 g/L) achieved the best degradation performance under this experimental condition.

2.4.2. Influence of PMS Concentration

In order to investigate the effect of PMS concentration on TC degradation, the catalyst dose was fixed at 5.0 mg in the experiment, and the TC concentration was set at 10.0 mg/L. The PMS concentration was varied between 2.5 and 12.5 mg, and the results are shown in Figure 6c,d. The degradation rate was gradually increased with the increase in the PMS concentration, which indicated that the PMS provided free radicals for the whole reaction system [49]. At the PMS concentration of 0.20 g/L, the degradation effect did not change much and tended to stabilize around 90% with a rate constant of 1.917 min⁻¹. When the PMS concentration was further increased to 0.25 g/L, the degradation rate decreased slightly, which may be due to the generation of less reactive substances by the secondary reaction (e.g., SO₅^•−), as described in Reactions (1) and (2) [50]. In addition, the limited number of active sites on the catalyst surface restricted further acceleration of the degradation reaction. Therefore, the optimum PMS concentration for the system was determined to be 0.20 g/L.

HSO₅ + SO₄^•− → SO₅^•− + HSO₄

(1)

HSO₅ + ^•OH → SO₅^•− + H₂O

(2)

2.4.3. Influence of TC Concentration

The effect of varying TC concentrations (2.5, 5.0, 10.0, and 20.0 mg/L) on degradation efficiency was investigated under conditions of 5.0 mg catalyst and 10 mg PMS, as shown in Figure 6e,f. The degradation rates for TC concentrations of 2.5, 5.0, 10.0, and 20.0 mg/L were 89.8%, 85.9%, 65.5%, and 64.1%, respectively. After 6 min, the degradation rate for 5.0 mg/L TC reached 85.9%, indicating the highest degradation efficiency. Higher TC concentrations resulted in slower degradation rates, which could be attributed to the limited quantity of free radicals produced by the quantitative catalyst activation of PMS and the limited amount of TC that could be degraded [51]. However, the system exhibited effective TC degradation at all concentrations tested.

2.5. Response of the CoP-3/PMS/Vis Degradation System to External Disturbances

Exploring the effect of anions on the system, Figure 7a,b show that the solution with the addition of HCO₃⁻ was the most obviously inhibited, with a TC removal rate of only 13.3%, and CO₃²⁻, H₂PO₄⁻, and Cl⁻, in that order, had a decreasing effect on the removal rate of the system, with degradation rates of 69.7%, 76.5%, and 76.8%. At the same time, by adding HA to the system to simulate the effect of organic matter on the degradation process, it can be clearly seen that the degradation effect is inhibited, and the degradation rate compared with the blank experimental group decreased by 60.4%. The organic matter competes with the pollutants for the active radicals in the degradation process, resulting in a decrease in the TC degradation rate [52].

In this experiment, TC was added to four water samples (Xuanwu Lake, Chaohu Lake, Yangtze River, and animal husbandry wastewater) at a concentration of 10 mg/L (Figure 7c,d). Compared with the blank group, the system showed good catalytic degradation performance in Yangtze River and Chaohu Lake samples, and the removal rate of TC reached 79.9%. 79.9%, and 80.8%, which is attributed to the fact that these two water systems are natural water systems with less interference of substances. In contrast, the TC removal rate in Xuanwu Lake was slightly lower than in the previous two samples, because Xuanwu Lake is an artificial lake, and there is a certain concentration of N/P content interference. Livestock farming wastewater from suburban livestock and poultry breeding bases, which has a high content of organic matter and anions, is affected by various factors, and the removal rate of TC in it is 64.7%. The actual water samples’ interference experiments demonstrated that the degradation system has a certain degree of feasibility in practical applications, with good degradation effect, which is stable at more than 64%.

To investigate whether the degradation system can remove other pollutants, this study examined the degradation of three pollutants, oxytetracycline (OTC), ciprofloxacin (CIP), and Rhodamine B (RhB), by the CoP-3/PMS/visible-light system. The results are shown in the Figure 7e,f. During antibiotic degradation, CIP and OTC were partially degraded with removal rates of 45.4% and 53.5%, respectively, significantly lower than that of the tetracycline system. The lowest removal rate for CIP may be attributed to its nature as a fluoroquinolone antibiotic [53], which is notoriously difficult to degrade. In contrast, RhB achieved a removal rate of 95.6%, demonstrating excellent catalytic degradation performance.

2.6. Free Radical Analysis

The role of reactive radicals in PMS activation for TC degradation under visible-light irradiation was examined using quenching experiments. For free radical quenching experiments, ^•OH, superoxide radicals (O₂^•−), and h⁺ were quenched by the addition of IPA (200 mM), BQ (10 mM), and EDTA (10 mM), respectively [54,55,56].

Figure 8 shows that the degradation rate constant in the absence of quenchers was 0.989 min⁻¹, while the addition of EDTA as a quencher reduced the rate constant to 0.475 min⁻¹, with a degradation efficiency of about 38.5%. In contrast, the addition of BQ and IPA reduced degradation efficiencies to 47.2% and 60.0%. The results showed that EDTA had the greatest effect on the CoP-3/PMS system, inhibiting the TC degradation rate by 41.1%, indicating that h⁺ quenched by EDTA plays a dominant role in the CoP-3/PMS/Vis degradation system. It is inferred that a radical chain reaction exists in the system.

$$\mathrm{C}\mathrm{o}\left(\mathrm{II}\right)+{\mathrm{H}\mathrm{S}\mathrm{O}}_5^{-}\to \mathrm{C}\mathrm{o}\left(\mathrm{III}\right)+{\mathrm{S}\mathrm{O}}_4^{·-}+{\mathrm{O}\mathrm{H}}^{-}$$

(3)

$$\mathrm{C}\mathrm{o}\left(\mathrm{III}\right)+{\mathrm{H}\mathrm{S}\mathrm{O}}_5^{-}\to \mathrm{C}\mathrm{o}\left(\mathrm{II}\right)+{\mathrm{S}\mathrm{O}}_5^{·-}+{\mathrm{H}}^{+}$$

(4)

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

(5)

$$2{\mathrm{S}\mathrm{O}}_5^{·-}+{\mathrm{H}}_2\mathrm{O}{\to 1.5}^1{\mathrm{O}}_2+2{\mathrm{H}\mathrm{S}\mathrm{O}}_4^{-}$$

(6)

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

(7)

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

(8)

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

(9)

$${\mathrm{S}\mathrm{O}}_4^{·-}+{\mathrm{O}\mathrm{H}}^{-}\to {\mathrm{S}\mathrm{O}}_4^{2-}+·\mathrm{O}\mathrm{H}$$

(10)

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

(11)

$$·\mathrm{O}\mathrm{H}+{\mathrm{H}}_2{\mathrm{O}}_2\to {\mathrm{H}\mathrm{O}}_2^{·-}+{\mathrm{H}}_2\mathrm{O}$$

(12)

$${\mathrm{H}\mathrm{O}}_2^{·-}\to {\mathrm{H}}^{+}+{\mathrm{O}}_2^{·-}$$

(13)

$${\mathrm{S}\mathrm{O}}_4^{·-}/{\mathrm{O}}_2^{·-}/·\mathrm{O}\mathrm{H}{/}^1{\mathrm{O}}_2+\mathrm{C}\mathrm{I}\mathrm{P}\to \mathrm{I}\mathrm{n}\mathrm{t}\mathrm{e}\mathrm{r}\mathrm{m}\mathrm{e}\mathrm{d}\mathrm{i}\mathrm{a}\mathrm{t}\mathrm{e}\mathrm{s}+{\mathrm{C}\mathrm{O}}_2+{\mathrm{H}}_2\mathrm{O}$$

(14)

2.7. Transformation Pathways of TC Degradation

To elucidate potential intermediates generated during the degradation process of this system, we analyzed the intermediates in the degradation process by liquid chromatography–mass spectrometry (Figure S2). Three proposed pathways are illustrated in the Figure 9. In Pathway 1, TC was fragmented to P1 (m/z = 427) by the dehydration process, starting with ROS attacking the -N(CH₃)₂ group to generate P2 (m/z = 399). P2 was converted to P3 (m/z = 318) by deamination reaction, and P3 was degraded to P4 (m/z = 274) by combined demethylation [57]. In Pathway 2, subsequently, it was further oxidized to P5 (m/z = 359), before losing hydroxyl and opening the ring to form P6 (m/z = 245). According to the HOME and Fukui index, C29–C30 on the benzene ring was attacked via nucleophilic reaction and further opened, as well as hydroxylation, to produce P7 (m/z = 157) [58]. Furthermore, as displayed in Path 3, after demethylation, TC was gradually decomposed, and P9 (m/z = 397) was subsequently decomposed into P10 (m/z = 297) through deamination, demethylation, and dehydroxylation. Then, P6 (m/z  =  195) was generated from P10 (m/z  =  297) via dehydroxylation and the destruction of C–C bonds [59].

Finally, product m/z values of P8 (m/z = 136) and P12 (m/z = 74) were obtained via further cleavage of the ring along with hydroxylation [60,61].

The toxicity of the intermediates was simulated by T.E.S.T (5.1.2) toxicity analysis software in terms of developmental toxicity and biological toxicity (LC50 of fathead ninnow and Daphnia magna). The results are presented in Figure 10. The analyses showed that all the biological toxicity was reduced except for that of P1, P2, P4 and P5; in particular, the toxicity of P8/P10 was lower, and the liquid-plasmid results showed that P8 was also largely converted to P7. In the results, P8 was also converted to a large amount, and the degradation process was green and safe in terms of developmental toxicity [62].

In the LC50 simulation of two aquatic organisms, fathead ninnow and Daphnia magna, P12 showed low toxicity. A small amount of P12 generation was detected at six minutes of reaction time. However, the toxicity of P1/P2/P4/P5 was similar to that of TC, which did not significantly reduce the toxicity of TC; however, due to the short existence time, it was easily converted into other low-toxicity substances. It can be concluded that the degradation reaction process, which can reduce the toxicity of TC, has good ecological safety.

2.8. Catalyst Stability

The stability of the CoP-3 catalyst was evaluated through cyclic experiments under conditions of 5.0 mg catalyst, 10.0 mg PMS, and 10.0 mg/L TC, as shown in Figure 11. After four cycles, the TC degradation rate was maintained at about 70%, indicating stable catalyst activity without significant performance loss, thus confirming its high stability and favoring reproducibility in future practical applications [63,64].

2.9. Catalyst Performance Comparison

To investigate the superior catalytic efficiency of CoP-3 in this experiment, a comparison was made with relevant catalysts reported in previous literature, as shown in Table S1 [63,65,66,67,68]. It can be clearly observed that the degradation time in our system is shorter and the reaction rate constant is significantly higher than those in other catalytic systems, reached 0.989 min⁻¹. This system efficiently removes TC from wastewater while requiring a smaller amount of catalyst, offering significant cost advantages.

3. Materials and Methods

3.1. Chemicals and Reagents

The materials in this paper are shown in Supporting Information.

3.2. Synthesis of Catalysts

The preparation process is illustrated in Scheme 1. First, 2 mmol of cobalt nitrate was dissolved in 10 mL of N, N-Dimethylformamide (DMF) to prepare solution A. Separately, a specific amount of phenylphosphonic acid was dissolved in 15 mL of DMF to prepare solution B. After complete dissolution, solution A was slowly added to solution B under continuous stirring. The resulting mixture was stirred for 1 h using a magnetic stirrer, then transferred to a 50 mL reactor and heated in an oven at 160 °C for 12 h. After the reaction, the obtained samples were washed with ethanol, dried in an oven, and stored for subsequent use.

The amount of phenylphosphonic acid used was varied (1 mmol, 2 mmol, 3 mmol, 4 mmol, and 5 mmol) to synthesize different materials. The resulting catalysts were designated as CoP-X (X = 1, 2, 3, 4, or 5), where X represents the amount of phenylphosphonic acid employed in the synthesis process.

3.3. Characterization Techniques and Degradation Testing

Comprehensive details regarding the characterization apparatus and the TC degradation assessment are presented in the Supporting Information.

4. Conclusions

A series of CoP photocatalysts with tailored precursor ratios were synthesized and extensively characterized to establish structure–performance relationships. The optimized CoP-3 catalyst demonstrated exceptional photocatalytic activity, achieving rapid TC degradation (90.7% in 6 min) under visible light and the PMS system, attributed to its superior crystallinity, electronic structure, and efficient charge transfer capabilities. Systematic optimization of reaction parameters identified the ideal conditions for maximizing degradation efficiency: 0.1 g/L CoP-3 and 0.2 g/L PMS. Mechanistic investigations revealed that the generation of h⁺ and its synergy with active species significantly enhanced catalytic performance, offering new insights into PMS activation pathways. This study establishes a robust framework for designing advanced CoP-based photocatalysts, advancing their application in environmental remediation.