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Article

Large-Scale Preparation of Granular Sludge@MOF-Derived Hierarchical Porous Carbon Catalysts for Advanced Oxidation Process: Preparation Process and Intrinsic Degradation Mechanism

by
Yu Liu
1,
Zhonglin Chen
1,
Chenyong Liu
1,
Wenhui Li
1,
Xiyue Yao
2,
Tian Tian
1,
Chenyu Zhao
1,
Shaoqun Tao
1,
Weichuan Qiao
1 and
Ming Zhang
1,*
1
Department of Environmental Engineering, College of Ecology and Environment, Nanjing Forestry University, Nanjing 210037, China
2
College of Geography and Environmental Sciences, Tianjin Normal University, Tianjin 300387, China
*
Author to whom correspondence should be addressed.
Catalysts 2025, 15(3), 264; https://doi.org/10.3390/catal15030264
Submission received: 29 December 2024 / Revised: 26 February 2025 / Accepted: 10 March 2025 / Published: 11 March 2025
(This article belongs to the Special Issue Catalysis in Pollution Degradation and Environmental Remediation)
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Abstract

:
Tetracycline (TTCH) is widely used but difficult to remove, which poses a threat to the health of the ecosystem, so it is urgent to take effective measures to remove it. Granular sludge plays an important role in biochemical treatment. Its rich functional groups and loose porous structure make it a potential catalyst carrier. This study utilized granular sludge as a precursor and modified it by loading a Zn/Co-zeolite imidazolate framework (ZIF-67/8). After carbonization, a granular sludge-supported MOF-derived porous carbon material (GSZC-800) with high catalytic activity is produced. The degradation rate of tetracycline reached a maximum of 90.8% after 40 min of reaction, and the optimal conditions were 20 ppm of initial pollutant concentration, 0.05 g/L of catalyst, and 0.10 g/L of peroxymonosulfate (PMS), which is superior to biomass-charcoal derived catalysts that have been reported in the literature. Through ion interference experiments, radical quenching experiments, electron transfer mechanism studies, and fluorescence spectroscopy analysis, it is suggested that this is a non-radical mechanism dominated by a single linear oxygen species. The catalyst GSZC-800 exhibits an ease of preparation and accessibility, has a wide range of applicable pH values, and effectively removes different pollutants. It has potential applications in treating actual wastewater and various pollutants. This study not only provides a new idea for the high-value utilization of granular sludge, but also provides an important theoretical and experimental basis for the development of efficient and stable non-homogeneous catalysts.

Graphical Abstract

1. Introduction

Granular sludge refers to the sludge particles that play a role in purifying sewage in UASB (Upflow Anaerobic Sludge Blanket) process. UASB is a commonly used wastewater treatment technology to convert organic pollutants into biogas by the action of anaerobic microorganisms. Aerobic granular sludge is a bioaggregate made up of microorganisms that aggregate. It is very regular in shape, generally round or oval, with a particle size of 0.1–3.0 mm, individually as large as 5 mm, a smooth surface, light yellow or orange in color, and rich in pores. Functional groups on bacteria, protozoa, and extracellular polymers in granular sludge include carboxyl groups, hydroxyl groups, and other components that can provide binding sites, and are easily adjusted to provide a variety of active centers. It is therefore a potential raw material for the preparation of sewage sludge peat [1,2,3,4]. Physical properties such as the pore structure and specific surface area of biomass charcoal affect its adsorption of pollutants [5]. Utilizing porous materials can promote both pollutant migration and degradation, and ROSs (Reactive Oxygen Species) are generated by activating H2O2 and peroxymonosulfate (PMS) [6]. The pre-treatment process to prepare carbon materials by directly using sludge from sewage plants as substrate is complicated and costly [7]. In contrast, granular sludge is formed naturally in anaerobic reactors, it is easy to utilize it directly and reduce the pre-treatment cost, so it is widely used in water treatment. Although sludge biochar has the disadvantage of a relatively low specific surface area [8,9], its surface area can be effectively increased through chemical activation, metal loading, and hydrothermal heating, and the pore structure can be improved to a certain extent [10,11,12,13].
Metal–organic framework (MOF) materials composed of metal ions and multifunctional organic ligands provide a good basis for the preparation of carbon-based materials due to their unique crystal structure, porous structure, and high carbon content. Their large specific surface area, high porosity, strong electrical conductivity and high stability make them show good potential in the fields of adsorption, catalysis, and energy conversion [14,15,16,17,18,19]. Li et al. [20] prepared a three-dimensional layered nitrogen-doped magnetic carbon (Co@N-C) through a one-step pyrolysis of ZIF-67 (Zeolitic Imidazolate Framework-67) and a urea precursor, which could effectively activate the PMS, the catalyst dosage at 0.15 g/L could completely remove BPA (Bisphenol A) within 5 min, and the leaching of Co was only 0.11 mg/L after 24 h. Bhadra et al. [21] pyrolyzed KOH-activated MAF-6 (Metal–Organic Framework-6) at 1000 °C to obtain a carbon material with a huge specific surface area (3123 m2/g), which showed good stability over a wide pH range and is a promising adsorbent for removing a wide range of pollutants from water. However, the weak coordination bonds of MOFs are unstable in water and are prone to hydrolysis, resulting in the collapse of the backbone structure, which affects the application performance of the materials [22]. In addition, the aggregation of MOF materials leads to the inability for good dispersion, which in turn masks the active sites and prevents full contact with the medium. Therefore, it is necessary to carry out post-treatment to improve the stability of MOFs. By incorporating other materials into the cavities of the MOFs, the resulting MOF composites can be endowed with the combined properties of the two components and can even acquire some new features such as hydrophobicity, water stability, mechanical properties, and so on [23,24,25,26,27]. MOF-derived materials exhibit extensive applications in the environmental field, characterized by their high specific surface area and high stability. However, MOF-derived materials often suffer from poor control over structural evolution and the loss of active sites [15]. In this study, the introduction of granular sludge not only provides a stable skeletal structure but also avoids potential structural collapse and the active site loss of MOF-derived materials at high temperatures through synergistic effects during the carbonization process.
Given the promising application prospects of biomass carbon-loaded MOFs and the successful preparation of catalysts by previous research groups, this study proposes a novel approach using granular sludge as a precursor. By loading zeolite imidazolate frameworks and subsequent carbonization, we successfully synthesized granular sludge-supported MOF-derived porous carbon materials (GSZC-800) with high catalytic activity, demonstrating their potential as highly efficient catalysts. Tetracycline is a stable and difficult-to-treat organic pollutant, which poses a significant risk to the health and stability of the ecosystem [28], so we chose to target tetracycline as the pollutant. GSZC-800 exhibits excellent catalytic performance owing to its four key advantages: (1) a high heteroatom doping content, (2) high specific surface area, (3) tunable pore structure and chemical properties, and (4) an optimized hierarchical porous structure [29,30,31,32]. In this study, we developed a novel granular sludge-loaded MOF-derived porous carbon material (GSZC-800) by carbonizing granular sludge modified with a zeolite-imidazole framework (ZIF-67/8). To demonstrate the superior catalytic properties of GSZC-800, we systematically investigated its ability to activate persulfate for pollutant degradation. The effects of various parameters including the carbonization temperature (700 °C, 800 °C, and 900 °C), catalyst dosage, PMS concentration, initial pollutant concentration, reaction temperature, pH, ionic interferences, and different water sources were thoroughly evaluated. Furthermore, we elucidated that the primary degradation mechanism is a non-radical pathway dominated by singlet oxygen (1O2), which significantly enhances the catalyst’s efficiency and stability.

2. Results and Discussion

2.1. Catalyst Characterization

The preparation process and possible degradation mechanism of GSZC-800 are illustrated in Figure 1a. The multilevel porous carbon catalyst was prepared by pyrolyzing ZIF-67/8 loaded on granular sludge. In the SEM images of GS (Figure 1b–e), it can be observed that the rough surface of sludge particles was covered with many tiny bumps and depressions on top. These features may be formed by microorganisms and organic or inorganic substances in the sludge. In addition, some pore structures and interwoven structures inside the granular sludge can be seen, which may facilitate the attachment and growth of microorganisms as well as the diffusion and degradation of pollutants. Figure 1f shows the microstructure of the granular sludge loaded with ZIF-67/8. Compared with the GS, the surface of the sludge particles in this image became rougher and uneven, which may be due to the attachment of ZIF-67/8 on the surface of the sludge particles. The surface of the granular sludge in the SEM image of GSZC-800 (Figure 1g) became smoother and denser. This may be a change in the surface morphology caused by the pyrolysis of the substances during the carbonization process. These carbonized particles of sludge may have higher stability and catalytic properties, which further enhance the catalytic degradation ability.
X-ray diffraction (XRD) can be used to further identify the crystallinity of the sample. In Figure 2a, GSZ exhibited dual characteristic peaks for both ZIF-67 and GS. Among them, the characteristic peaks located at 7.48°, 10.37°, 12.78°, and 18.10° can be attributed to the (011), (002), (112), and (222) lattice planes of ZIF-67, respectively [33]. The characteristic peaks at 26.68° and 29.71° can be attributed to the (510) and (440) lattice planes of ZIF-8, respectively [34]. The characteristic peaks at 36.18° and 45.98° for GSZ correspond to those of GS. Figure 2b presented the XRD patterns of the samples after pyrolysis, showing similar characteristic peaks among the three samples. The characteristic peaks at 44.18° and 50.96° for GSZC-800 can be assigned to the (111) and (200) crystal planes of ZIF-67/8C, respectively. Both GSZC-800 and GSC exhibited similar characteristic peaks at 32.28°. These results indicated the successful synthesis of GSZC-800.
Figure 2c shows the FTIR spectra of ZIF-67/8, GSZ, and GS for functional group studies. In the FTIR spectra of GS, the characteristic peaks at 1050 cm−1, 1538 cm−1, and 3279 cm−1 were generated by the stretching vibrations of C-O for alcohols, C=C for aromatics, and C-H for alkynes, respectively [35,36]. For ZIF-67/8, the cluster of absorption peaks at 1305 cm−1 corresponds to in-plane and out-of-plane bending vibrations of the imidazole ring of the material’s organic ligand dimethylimidazole [37], and C=N bonding on the 2-methylimidazole ring was observed at 1576 cm−1. After depositing ZIF-67/8 on GS to form the GSZ composite, the positions of the characteristic absorption peaks corresponding to GS in the IR spectra were almost the same and only slightly shifted, which indicated that the GS composition and structure were undamaged. In addition, vibrational peaks in the range of 749 cm−1 to 1050 cm−1 appeared for GSZ that matched well with the ZIF-67/8 samples, which further proved the formation of GSZ.
The adsorption degradation capacity of a catalyst is related to its specific surface area and pore size (Figure 2d,e). Figure 2d shows the nitrogen adsorption isotherms of the four samples, GSZC-700, GSZC-800, GSZC-900, and GSC-800. The N2 adsorption–desorption isotherms of these samples were classified as Type IV. Their SBET and pore volumes were 129.43 m2/g and 0.20 cm3 g−1, 150.61 m2/g and 0.14 cm3 g−1, 69.50 m2/g and 0.10 cm3 g−1, and 79.10 m2/g and 0.13 cm3 g−1, respectively (Figure 2e and Table 1). The results show that GSZC-800 has the largest specific surface area, mainly because at 700 °C, the organic matter in the granular sludge begins to decompose and form a preliminary porous structure, but the pore structure is not fully developed at this time, and the specific surface area is relatively low. As the temperature rises to 800 °C, the organic material is further decomposed, the pore structure is optimized, and the specific surface area is maximized. However, when the temperature continues to rise to 900 °C, the excessively high temperature causes the collapse of the pore structure and the aggregation of metal nanoparticles, thereby reducing the specific surface area and the active site of the catalyst. Therefore, GSZC-800 could provide more active sites, promoting the activation of PMS and the generation of single-linear oxygen. In addition, the synergistic effect of nitrogen-doped carbon structures and cobalt nanoparticles further enhanced the electron transfer ability and redox properties of the catalysts.
The gravimetric analysis (TGA) showed (Figure 2f) that the mass of ZIF-67/8 decreases sharply at 500 °C and continues to decrease up to 900 °C. This was due to the large number of organic ligands (e.g., 2-methylimidazole ligand) and metal ions (e.g., zinc and cobalt) in its structure, which tended to decompose at high temperatures, leading to the drastic decrease in mass. Compared with ZIF-67/8, GS and GSZ showed a slower decrease in mass percentage. The gradual decrease in the mass percentage of GS with increasing temperature indicated that GS may lose part of its mass at high temperatures due to pyrolysis or volatilization. The mass loss of GSZ was more significant from 100 °C to 700 °C, and the rate of mass loss tended to flatten out from 700 °C to 900 °C, which indicated that GSZ can be pyrolyzed and has good thermal stability under high-temperature conditions.
Figure S1 shows the Zeta Potential of GSZC-800. According to the measured data, the isoelectric points of the catalyst GSZC-800 were a pH = 5.00 and a pH = 8.80. The surface of the catalyst was negatively charged at 5.00 < pH < 8.80. The surface of the catalyst was positively charged at pH > 8.80. At pH > 8.80, the catalyst surface was positively charged. A 90.8% catalytic efficiency was achieved at a pH = 6, when the catalyst surface was negatively charged, which may be possible electrostatic attraction with positively charged tetracycline molecules to promote adsorption and degradation.

2.2. Catalytic Degradation Experiments

In this section, the catalytic performance of GSZC-800 in the system of activated persulfate degradation of tetracycline at different carbonization temperatures was investigated. The three temperatures selected were 700 °C, 800 °C, and 900 °C, respectively. As can be seen from Figure 3a, the degradation of tetracycline was firstly increased and then decreased with the increase in carbonization temperature, and the order of carbonization temperatures from low to high degradation rates was 700 °C, 900 °C, and 800 °C. Obviously, the GSZC-800 prepared at 800 °C showed the best catalytic activity, and the degradation rate of tetracycline could reach 90.8% after 30 min of reaction. In addition, after the addition of synthesized GSZC-800 to the solution system of activated persulfate degradation of tetracycline, the efficiency of this system in degrading tetracycline was significantly improved compared to the peroxymonosulfate system alone, with the degradation rate increasing from 16.9% to 90.8% after 30 min of reaction. The experiments showed that the addition of PMS alone did not have a good degradation effect, probably due to the fact that PMS is more stable and has a weak self-decomposition ability [32], and on the other hand, it indicates that GSZC-800 has an excellent ability to activate PMS. According to the experimental data, it can be seen that the degradation rate of tetracycline by GSZC-800 reaches 90.8% within 30 min, which is the largest degradation rate. Secondly, the specific surface area and pore structure of GSZC-800 are optimal at 800 °C. In addition, the active site distribution at 800 °C is optimal. Therefore, the following experiments are centered around selected GSZC-800 obtained by carbonization at 800 °C.
A review of the literature [38] revealed that the graphitization of carbon materials gradually increased with increasing pyrolysis temperature, which promoted the process of activating PMS. This was due to the fact that the electrical conductivity, stability, and catalytic activity of the carbon materials was enhanced accordingly with the increase in the degree of graphitization. However, on the other hand, higher pyrolysis temperatures exacerbated the aggregation of cobalt nanoparticles, leading to a decrease in the number of active centers and lowering the catalyst’s reactivity. Therefore, the pyrolysis temperature played a key role in balancing the degree of aggregation and graphitization of cobalt nanoparticles. Figure 3b showed that the kinetic constant of the reaction increases and then decreases with the increase in carbonization temperature. The kinetic constant reached a maximum of 0.019 min−1 at a carbonization temperature of 800 °C. The adsorption capacity of the catalyst was tested 10 min before the reaction, and the adsorption of tetracycline by the catalyst under the carbonization conditions of 700 °C, 800 °C, and 900 °C was 2.0%, 34.4%, and 29.1%, respectively. The best adsorption performance of the catalysts obtained at 800 °C may be due to the maximum specific surface area and the most suitable chemical composition, which also led to the best catalytic effect. The catalyst performance at 700 °C was poorer, which may be due to the higher density and smaller specific surface area of the catalyst obtained at this carbonization temperature, which could not react with the target pollutants in solution in full contact. And the catalyst performance at 900 °C decreased compared to that at 800 °C, probably because the structure of the granular sludge was partially damaged by the increase in temperature, and the pore collapse and cobalt particle aggregation led to the mutual masking of the active sites, which made the active sites of the catalyst decrease instead of increasing.
The experiments explored the effects of catalyst dosing, PMS dosing, pollutant concentration, temperature, and pH on the catalytic degradation process. Figure 3c shows the effect of catalyst dosing on the degradation efficiency of tetracycline. The results showed that the degradation rate of the tetracycline degradation process increased slightly with the increase in catalyst dosing, but the degradation rate tended to be the same after 30 min, and the degradation rates were 90.8%, 88.7%, and 89.7% at the dosages of 0.05, 0.10, and 0.20 g/L, respectively. The catalyst plays a role in activating PMS and providing an electron transfer path in the reaction. Although the increase in the amount of catalyst can provide more active sites to activate PMS and generate the radical SO4•−, the degradation rate of tetracycline is not significantly improved due to the annihilation reaction between radicals.
Figure 3d shows the effect of the PMS dosage on the degradation rate of tetracycline, from which it can be seen that the removal rate of tetracycline increased with the increase in PMS dosage, and the degradation rates were 80.6%, 90.8%, and 91.2% for a PMS of 0.05, 0.10, and 0.20 g/L, respectively. Comparison of the data revealed that the degradation rate increased by 10.2% from 0.05 to 0.10 g/L, while the degradation rate increased by only 0.4% from 0.10 to 0.20 g/L. This showed that although the degradation of tetracycline is improving with the increase in PMS dosage, this good effect was not infinite and the rate of improvement of the degradation efficiency decreases with the increase in PMS dosage; that is to say, when the dosage of PMS is increased to a certain value, the change in the degradation rate was no longer significant. This may be due to the limitation of tetracycline concentration and catalyst dosage. When the PMS dosage continued to increase, a large amount of SO4•− was generated, which caused a self-burst, but led to the removal rate becoming smaller. The increase in the dosage of PMS can lead to secondary pollution and pipeline corrosion caused by the residual sulfate ions, and the overall removal effect was considered, so 0.10 g/L was the most appropriate choice.
The effect of pollutant concentration on the degradation rate is shown in Figure 3e, which indicates that the catalytic efficiency decreases as the concentration of tetracycline increases. The best catalytic degradation was achieved at a tetracycline concentration of 5 ppm, with a removal rate of 91.2% after 40 min of reaction. Tetracycline concentrations of 20 ppm and 40 ppm showed removal rates of 90.8% and 77.7%, respectively. Therefore, in practical applications, multiple factors should be considered, including catalyst cost, oxidant cost, pollutant concentration, and secondary pollution [39].
The pH value has a certain impact on the degradation rate of tetracycline. The initial pH value of the solution was 6, and by adjusting the pH using NaOH and HCl solutions, the initial pH values of the solutions were set to 4, 7, and 10, respectively. Figure 3f indicates that acidic and weakly acidic conditions were more favorable for the catalytic degradation of tetracycline compared to neutral and alkaline conditions. After 40 min of reaction, the degradation rates under initial pH values of 4, 6, 7, and 10 were 88.9%, 90.8%, 88%, and 85.3%, respectively. The removal rate of tetracycline was relatively stable, suggesting that the synthesized GSZC-800 can be used within a wide range of pH values while maintaining good catalytic performance. However, weakly acidic conditions are most conducive to the catalytic degradation of tetracycline, while alkaline conditions have a certain negative impact on the catalytic degradation of tetracycline. Therefore, in other experiments, we chose not to adjust the initial pH value of the solution, which remained at 6.
In general, the degradation temperature had a significant effect on the reaction rate, and Figure S2a showed the reaction process of the system at three different temperatures, 298 K, 308 K, and 318 K. From Figure S2a, it can be seen that the tetracycline degradation rate was gradually accelerated with the increase in the reaction temperature, and the degradation first increased and then decreased, which indicated that the reaction of tetracycline degradation by activated PMS of GSZC-800 was an adsorptive reaction within a certain range of temperatures, and an exothermic reaction within a certain range of temperatures. After 40 min of reaction, the degradation rates at 298 K, 308 K, and 318 K were 87.1%, 88.5%, and 86.8%, respectively. Therefore, the optimal temperature of the reaction is 25–35 °C, which is suitable for the reaction at room temperature without heating. The reaction rates at the three temperatures were then fitted to obtain a reaction activation energy of 55.1 kJ/mol (Figure S2b). The rate constant k for the reaction was obtained by the first-order kinetic equation at the different temperatures:
l n C t C 0 = − k t
where C0 represents the initial concentration of the tetracycline solution and Ct represents the actual concentration. The activation energy, which is an indicator of the activation capacity of the reaction catalyst, was obtained from the Arrhenius equation by fitting different values of k for 298 K, 308 K, and 318 K. The activation energy of the reaction catalyst was obtained from the Arrhenius equation:
ln ⁡ k = l n k 0 − E a R 1 T
where T is the temperature of the reaction system (K); k0 represents the Arrhenius factor (g mg−1min−1); R is the universal gas constant (8.314 J mol−1K−1).

2.3. The Anions and Natural Organic Matter Effects

Ions and organic matter in nature can react with free radicals and reduce the catalytic efficiency of the degradation system. Therefore, it is necessary to evaluate the effect of anions and organic matter. In this section, the effects of different concentrations of Cl−, H2PO4−, HCO3−, and HA on the tetracycline degradation system in which GSZC-800 activates PMS is investigated. As shown in Figure 4, different concentrations of Cl−, H2PO4−, HCO3−, and HA all inhibited the degradation of tetracycline in the system, but the degree of inhibition varied. Cl− is widely present in natural water, and it is generally believed that the presence of Cl− will have an inhibitory effect on the degradation system. The results of Cl− interference experiments were shown in Figure 4a. With the increase in Cl− concentration from 5 mM to 10 mM, the inhibitory effect was enhanced, and the rate of tetracycline degradation was decreased from 90.8% without Cl− to 85.5% with the addition of 10 mM of Cl−. However, when the ion concentration was increased to 15 mM, the tetracycline degradation rate was 85.2% with no significant decrease. This experimental result suggests that Cl− may have reacted with ·OH and the reactive radicals in the system were thus reduced. It is inferred that ·OH, may be one of the free radicals produced by activated PMS that plays a major role in degrading tetracycline. This was explored by undertaking experiments with H2PO4− and HCO3− as interfering ions. As can be seen from Figure 4b, the addition of 5 mM of H2PO4− produced a significant inhibition in the degradation of tetracycline in the system, and the degradation rate decreased from 90.8% to 59.3%. However, as the H2PO4− addition continued to increase, the reduction in the tetracycline degradation rate became smaller. This may be due to the reaction of H2PO4− not only with ·OH, but also with SO4•−. It further suggested that ·OH and SO4•− may be the main substances produced in the system to degrade tetracycline. In addition, in Figure 4c, the HCO3− inhibition was greatest at 5 mM and the least at 1 mM, which may be related to the fact that the addition of HCO3− changes the pH of the system. The secondary reaction equations (Equations (3)–(6)) [38] can explain the inhibition mechanism. Figure 4d shows the results of the experiments using humic acid (HA) as a macromolecular interfering agent, and the degradation rate of tetracycline decreased from 90.8% to 78.3% with 10 mg/L of HA, which could be attributed to the fact that the generated ·OH and SO4•− would consume the HA, and therefore the dosage of PMS should be increased in the practical application. However, there was no significant difference between 20 mg/L and 10 mg/L of HA, indicating that the effect of HA on the degradation of tetracycline was stable within a certain concentration range.
SO4•− + HCO3− → HCO3·+ SO4•− k = 2.8 × 106 M−1s−1
·OH + HCO3− → CO32− + H2O   k = 8.5 × 106 M−1s−1
SO4•− + H2PO4− → HPO4·+ SO4•− k = 1.2 × 106 M−1s−1
·OH + H2PO4− → HPO4·+ OH− k = 1.5 × 105 M−1s−1

2.4. Degradation Mechanism

2.4.1. Radical Quenching Experiment Results

The results of the ionic interference experiments in the previous section tentatively suggested that · O H and SO4•− may be the main substances produced in the system to degrade tetracycline. To further investigate the degradation mechanism of the system, free radical quenching experiments were performed. Previous results showed [33] that alcohols containing α-hydrogen such as methanol (MeOH) are very reactive with hydroxyl and sulfate radicals, and alcohols without α-hydrogen such as tertiary butyl alcohol (TBA) are also reactive with hydroxyl radicals, and the rate of reaction between TBA and ·OH is much larger than that between methanol and SO4•−. Therefore, MeOH and TBA are often used to differentiate between ·OH and SO4•−. The molar ratios of the two additions to the system to the amount of PMS were both 1000. The experimental results are shown in Figure 5a, and the degradation curves of tetracycline showed that the addition of methanol and tert-butanol did not have a significant effect on the degradation. The degradation rate of tetracycline in the blank control group without a burst reagent was 90.8% after 40 min of reaction, 87.9% after the addition of methanol, and 91.2% after the addition of tert-butanol. This may be due to the fact that the addition of tert-butanol produced other substances that facilitated the existing reaction. L-Histidine is considered to be an effective bursting agent for 1O2, and in order to further characterize the active substances in the reaction, experiments were carried out using L-histidine as the bursting reagent. The degradation of tetracycline deteriorated significantly after 10 mM of L-histidine was added to the reaction system. After 40 min of reaction, the degradation rate of tetracycline was 50.7%, which decreased by 40.1% compared with the control. The results showed that the system mainly realized the degradation of tetracycline through the non-radical mechanism of generating 1O2, while the radical mechanism of generating ·OH and SO4•− was not the main degradation mechanism. 1O2 is electrophilic and can selectively degrade organic pollutants.

2.4.2. Results of Premixing Experiments

PMS and catalyst GSZC-800 were premixed for 5 min, 10 min, and 30 min, respectively, before tetracycline was added, and the results are shown in Figure 5b, which shows that the rate of tetracycline degradation decreased significantly after premixing. In addition, the longer the premixing time, the worse the degradation effect was. When unmixed, the degradation rate was 90.8%; the degradation rates after 5 min and 30 min of mixing were 78.8% and 74.4%, respectively. The premixing may change the nature and active sites of the catalysts, thus affecting the electron transfer efficiency. The results showed that the electron transfer pathway was not the main degradation mechanism in this system.

2.4.3. Two-Dimensional Fluorescence Analysis

According to the literature, hydroxylated benzoic acid can be verified for the presence of SO4•− [40]. In this study, we fixed the excitation wavelength, scanned a range of emission wavelengths Em, and used benzoic acid as a probe and GSZC-800 catalyst to activate the PMS to generate active radicals. Figure S3 showed that a weak emission peak can be observed at 408 nm, indicating a weak self-decomposition of PMS. The characteristic peak of benzoic acid remained weak after the addition of the catalyst, again demonstrating that the generation of SO4•− in the reaction system is limited. These results suggested that the non-radical-dominated advanced oxidation process is the main degradation mechanism in the system.

2.4.4. Results of Three-Dimensional Fluorescence Experiments

Three-dimensional fluorescence testing serves as an effective, sensitive, and versatile analytical method to further determine degradation effects. The reaction solution was taken out at reaction times of 2 min, 5 min, 10 min, and 20 min for 3D fluorescence detection. In Figure 6a, the initial fluorescence intensity in region III and region V reached 861 (Em = 444 nm, Ex = 245 nm) and 1100 (Em = 440 nm, Ex = 301 nm), respectively. After 5 min and 10 min of degradation, the fluorescence intensity gradually decreased in region III and region V, indicating that tetracycline was gradually degraded in the system. After 20 min of degradation, the fluorescence intensity was obviously weakened, as shown in Figure 6d; for the fluorescence intensity at Em = 444 nm, Ex = 245 nm was 489, which was a decrease of 372 compared with that at 2 min; at Em = 440 nm, Ex = 301 nm was 767, which was a decrease of 333 compared with that at 2 min. The above results showed that after 20 min of degradation, tetracycline was gradually degraded in the system. After 20 min, tetracycline was degraded significantly in the GSZC-800-activated PMS system.
In summary, Figure 7 shows the possible mechanism for the degradation of TTCH in the GSCZ-800/PMS system; the primary mechanism mainly involves non-radical pathways, especially the generation of single-linear oxygen (1O2). During the activation of PMS, the active sites on the surface of the GSZC-800 catalyst (e.g., cobalt- and nitrogen-doped carbon structures) are able to effectively activate PMS to generate single-linear oxygen. The singlet oxygen has strong oxidizing properties and can selectively attack the unsaturated bonds in the tetracycline molecule, leading to its degradation into small molecule compounds. In addition, the high specific surface area and multistage pore structure of the catalyst provided abundant active sites for the reaction, which promoted the adsorption and degradation of pollutants. The dominant role of single-linear oxygen in the degradation process was further confirmed by free radical burst experiments and fluorescence spectroscopy analysis, while the roles of ·OH and SO4•− were relatively weak.

2.5. Regeneration Performance and Actual Water Application Results

In this section, the reuse and regeneration performance of the catalyst was tested. As shown in Figure 8a, the tetracycline removal rate reached 90.8% when the catalyst was used for the first time, and the second degradation experiment was conducted after collecting the materials and drying them, and it was found that the tetracycline degradation rate was decreased, so the catalyst collected from the second time was regenerated by carbonization at 800 °C, and the carbonization process was the same as that of the synthesis of the catalyst. The third degradation experiment was conducted with the regenerated catalyst, and the results showed that the tetracycline degradation rate increased from 57.8% in the second experiment to 68.4%, which demonstrated the regenerable performance of the catalyst.
In order to understand the surface bonding pattern and chemical composition of the GSZC-800 catalysts before and after the degradation of tetracycline, their Co 2p and N 1s were analyzed by X-ray photoelectron spectroscopy (XPS) (Figure 8b–d). Figure 8c shows the high-resolution spectra of Co2p, which were fitted to give the Co(0), Co(II), Co(III), and satellite peaks. The peaks at 780.9 and 796.1 eV belong to Co2+, and those located at 779.6 and 795.1 eV are characteristic peaks of Co3+. The peaks at 793.5 and 778.7 eV represent the Co 2p1/2 and Co 2p3/2 of metallic cobalt, respectively [41]. The high-resolution spectra of Co 2p further indicate the successful loading of ZIF-67/8 on the catalyst. The Co peak at Co 2p1/2 was weakened in the used catalyst, indicating a slight leaching of metallic cobalt from the catalyst surface. This may be due to some chemical interactions during the catalytic reaction. Figure 8d shows the high-resolution spectra of N 1s, and the characteristic peaks located at 401.1, 400.2, and 398.3 eV can be found to correspond to graphitic, pyrrolic, and pyridine nitrogen, respectively. Among them, the graphitized nitrogen content is the highest, indicating the high degree of graphitization of the catalyst. This may be due to the role of the cobalt element in promoting graphitization during catalyst preparation. And the graphitic nitrogen was considered to be an intrinsic active site in the catalytic PMS activation process, which played a key role in the reaction of tetracycline degradation [20].
According to Figure S4a, it can be seen that the specific surface area of the catalyst after the reaction is larger than that before the reaction. It may be that during the reaction, certain by-products generated (such as nanoparticles or porous carbon) adhere to the surface of the catalyst, resulting in an increase in the specific surface area [42]. Figure S4b shows the infrared spectra of GSZC-800 before and after the reaction. The peaks at 1406 cm−1 are ascribed to C-H out-of-plane bending [43]. At 3642 cm−1 is the stretching vibration of free hydroxyl O-H. The characteristic peaks of the two samples are generally similar and slightly offset. After the catalytic reaction, the intensity of the peaks decreases, indicating that the substances are involved in the reaction.
Figure 9a,b demonstrated the application of the system of GSZC-800-activated PMS in real water bodies. The practical applications included tap water and river water (Purple Lake Creek), and the results still maintained good removal rates of 91.0% and 91.5%, respectively. The reaction kinetic constants in the tap water, purple lake stream water, and deionized water systems were 0.023, 0.021, and 0.019 min−1, respectively, which showed that the substances in the tap water and river water had almost no effect on the reaction system, and the removal effect of pollutants was more obvious, which may be due to the fact that the presence of certain particles in the actual water body has a facilitating effect on the system to activate the PMS [44], which indicates that the technique has the potential to be applied to treat real wastewater.
This section also explores the degradation effect of GSZC-800 obtained by carbonization at 800 °C on different pollutants in persulfate system. The three pollutants were tetracycline, methyl orange, and malachite green. From Figure 9c,d, it can be seen that the degradation effect of GSZC-800 on tetracycline was better than that of methyl orange, but not as good as that of malachite green. This may be related to the structural composition of different pollutants and their reaction with the catalyst. After 30 min of degradation, the removal rate of methyl orange was 78.1% and that of tetracycline was 90.8%. In the adsorption stage, the removal rate of malachite green reached 86.5%, and after 30 min of degradation, the removal rate was as high as 99.8%. This indicates that GSZC-800 has a good degradation effect on a variety of pollutants and has the potential to treat actual complex wastewater.
As shown in Figure S5, GSZC-800 is a versatile and highly efficient catalyst that can be used to degrade a wide range of water pollutants, including organic dyes and antibiotics. Its high degradation efficiency for AO7 (86.7%) and BPA (97.8%) suggests that it can achieve similarly high removal rates for other pollutants. However, the performance may be influenced by background conditions such as pH, ionic strength, and the presence of NOM. By optimizing these conditions, GSZC-800 can be effectively applied to treat various types of contaminated water, making it a promising material for environmental remediation.

2.6. Possible Degradation Pathways and Toxicology Research

To analyze the detailed degradation pathway, LC-MS was employed to further investigate the degradation products. Table S4 and Figure S6 present the MS spectra and possible degradation products. Figure 10a illustrates the three main pathways for degrading TTCH: (I) First, TCCH (m/z = 445) was fragmented into P1 (m/z = 357) by deamidation, demethylation, and dihydroxylation [45]. P1 was then converted to P2 (m/z = 274) after further dealkylation and ring cleavage [46]. P3 (m/z = 218) can be formed by a combination of demethylation, dehydroxylation, and ring cleavage. P3 can then be converted to P4 (m/z = 132) through decarbonylation and ring cleavage. Subsequently, P4 was dissociated by a series of functional groups promoted by further oxidation to the low molecular weight organic component P5 (m/z = 100) [47]. (II) TTCH forms the product P6 (m/z = 453) attributed to the electron reduction in demethylation. The intermediate P6 (m/z = 453) was then attacked by ROSs generated in the coupled system and converted to P7 (m/z = 340) via a ring-opening process [48]. (III): The TTCH molecule was first converted to the unstable intermediate P9 (m/z = 432) in the presence of the active substance, which was decomposed into small molecules (CO2 and H2O, etc.) by degreasing, ring opening, and dealkylation [49].
The Toxicity Evaluation Software Test (T.E.S.T.) Version 5.1 was used to evaluate the biotoxicity of TTCH and its degradation intermediates. The 50% lethal dose (LD50), mutagenicity, developmental toxicity (Devtox), bioconcentration factor (BCF), and mutagenicity in rats receiving the degradation intermediates orally are shown in Figure 10b. P3 and P11 both showed low mouse lethality risk, P4 showed the lowest developmental toxicity, P10 showed the lowest BCF, and P12 showed the lowest mutagenicity.
In addition, we thoroughly mixed the degraded solution, the undegraded tetracycline solution, and the catalyst solution into the E. coli medium so as to observe the effect of each solution on bacterial growth. Escherichia coli (E. coli) was cultured for toxicity experiments of degradation products, and tetracycline, GSZC-800, and degradation products were added to the E. coli medium, respectively, combined with a blank group for control culture. As can be seen from Figure 10c, after 24 h, the number of E. coli colonies in the four Petri dishes was significantly different. The plates of the blank group were dotted with well-grown single colonies, which appeared to be round and creamy-white, with neat colony edges and smooth surfaces. Tetracycline produced significant antimicrobial activity with no E. coli growth. Compared with the blank group, the number of colonies of degradation products and GSZC-800 was somewhat limited, but was significantly more than the tetracycline group, indicating that the toxicity of the degradation products was lower than that of tetracycline, and the catalyst GSZC-800 had low toxicity, which is in line with its environmental friendliness [44]. The above results indicated that the advanced oxidation process of activated PMS using GSZC-800 are non-toxic and safe.

3. Experiment

3.1. Materials

Material and chemicals were shown in the Supplementary Materials (Tables S1 and S2).

3.1.1. Synthesis of Granular Sludge Loading ZIF-67/8 (GSZ)

After freeze-drying the granular sludge at −80 °C for 8 h, 15.0 g of 2-methylimidazole was dissolved in 300 mL of deionized water, and 3.0 g of granular sludge was added as A. Measurements of 1.0 g of cobalt nitrate and 1.0 g of zinc nitrate were added into 100 mL of deionized water and 50 mL of ethanol to dissolve, and this was considered as B. The solution of A was magnetically stirred at 500 r/min for 20 min, and then the solution of B was added with stirring. The solution was stirred magnetically at 500 r/min for 22 h. At this time, the granular sludge was loaded with purple precipitate. The product was filtered out, frozen at −80 °C, and dried under a vacuum for 24 h to obtain the precursor GSZ.

3.1.2. Synthesis of Granular Sludge Loading ZIF-67/8 Biomass Porous Carbon Materials (GSZC-800)

The precursor of the GSZ material was placed into a tube furnace and heated to 700 °C, 800 °C, and 900 °C at a heating rate of 5 °C/min in a nitrogen atmosphere and kept for 2 h. When the sample was cooled down to room temperature, the black fine lumps obtained were GSZC-800.

3.2. Experiment Procedures

3.2.1. Catalytic Performance Experiment

Tetracycline was used as a target pollutant to evaluate the catalytic degradation performance of GSZC-800 by activating PMS. After adding 5 mg of catalyst to 100 mL of tetracycline solution (20 mg/L) with magnetic stirring at 600 rpm for 10 min, 10 mg of PMS was added to the system and degraded for 30 min. Samples (2 mL) were taken at selected time points and filtered through a 0.22 µm organic phase filter tip, and the concentration of tetracycline in solution was measured via high-performance liquid chromatography (HPLC). In the determination of tetracycline via liquid chromatography (HPLC), the mobile phase consisted of acetonitrile and a deionized aqueous solution (volume ratio of 35:80), and the flow rate was 0.3 mL/min. The chromatography was performed on a C18 reversed phase column (250 mm × 4.6 mm, 5 μm). The detection wavelength was 355 nm and the sample size was 20 μL. Through these conditions, the concentration of tetracycline can be effectively separated and quantitatively detected. The reaction system used in this study was a closed batch reactor, the reactor was a beaker with a capacity of 150 mL which was placed on a magnetic stirrer, and after the desired reaction time was reached, the magnetic stirrer was stopped and the sample was removed to terminate the reaction.

3.2.2. Anion and Natural Organic Matter Interference Experiments

Ion interference experiments allow the study of the resistance of experimental systems to interference. Three kinds of anions, namely, Cl−, H2PO4−, and HCO3−, are chosen as ionic interferences, and humic acid (HA) was chosen as a macromolecular interference (at 10 and 20 mg/L). In the experiments, the catalyst concentration was 0.05 g/L and the tetracycline concentration was 20 ppm.

3.2.3. Radical Quenching Experiment

During the synergistic activation of PMS by transition metals and carbonaceous materials, in general, strong oxidizing substances such as SO4•−, ·OH, and 1O2 are produced. In order to investigate the mechanism of tetracycline degradation by granular sludge-loaded ZIF-67/8, a radical quenching experiment was performed after the ionic interference experiment. The results of previous studies showed that α-hydrogen containing alcohols such as methanol (MeOH) react very easily with hydroxyl and sulfate radicals, while alcohols without α-hydrogen such as tertiary butyl alcohol (TBA) react easily with hydroxyl radicals, and the reaction rate of TBA with ·OH is much higher than that of methanol with SO4•− [50]. Therefore, MeOH and TBA are often used to distinguish between ·OH and SO4•−. In general, methanol can burst ·OH and SO4•−, TBA can burst ·OH, and L-histidine can burst 1O2. In this experiment, the concentration of the catalyst within the reaction system was 0.05 g/L, the concentration of tetracycline was 20 ppm, and the content of PMS was 0.10 g/L. The catalyst concentration of the reaction system was 0.05 g/L, the concentration of tetracycline was 20 ppm, and the content of PMS was 0.10 g/L. The reaction rate of the reaction system was 0.10 mg/L.

3.2.4. Fluorescence Spectral Analysis

Fluorescence spectroscopy uses a beam of light to excite the electrons in the molecules of certain compounds and cause them to emit light. The light passes through a monochromator into a detector to be detected and is used to measure and identify molecules or changes in molecules. In order to investigate the mechanism of tetracycline degradation by GSZC-800-activated persulfate, two-dimensional fluorescence spectroscopy was carried out after premixing experiments using benzoic acid as a fluorescent molecular probe.
Three-dimensional fluorescence (EEM, excitation–emission matrix) is a measurement that is becoming more widely used in the field of fluorescence spectroscopy. EEM is a 3D scan that yields a spectral map of excitation wavelength—emission wavelength—fluorescence intensity. When the type and concentration of fluorescent components in solution change, the fluorescence position and fluorescence intensity detected by three-dimensional excitation–emission matrix (3D-EEM) fluorescence spectroscopy change accordingly. Therefore, in order to investigate the changes in tetracycline during the degradation process, samples were taken at reaction times of 2 min, 5 min, 10 min, and 20 min, respectively, and the 3D-EEM was performed by an F-7000 FL Spectrophotometer, with an excitation wavelength in the range of 200.0–400.0 nm, and an emission wavelength in the range of 200.0–550.0 nm.

3.2.5. Experiments of Regeneration Performance and Application in Real Water Bodies

When the catalyst was used for the first time, the tetracycline removal rate reached 90.8%, and the material was collected and dried for the second degradation experiment, and the tetracycline degradation rate was found to decrease, so the catalyst collected for the second time was regenerated by carbonization at 800 °C, and the carbonization procedure was the same as that of the catalyst synthesis. The regenerated catalyst was used for the third degradation experiment. The deionized water used in the experiments was replaced with tap water and river water, and the two pollutants, methyl orange and malachite green, were degraded so as to explore the practical application capability of GSZC-800.

3.2.6. Biological Toxicity Test

Cultivation of E. coli was carried out for toxicity experiments of degradation products, tetracycline, GSZC-800, and degradation products were added to the E. coli medium, respectively, combined with a blank group for the control culture, and the number of colonies was observed at 0 h and 24 h of culture.

4. Conclusions

In this study, we proposed a granular sludge-loaded MOF-derived porous carbon material (GSZC-800) with high catalytic activity by carbonization using granular sludge as a precursor modified by a zeolite-imidazole-loaded skeleton. The modified graded porous carbon materials increased the active sites and improved the catalytic reaction activity. Different carbonization temperatures (700 °C, 800 °C, 900 °C), catalyst dosages, PMS dosages, initial pollutant concentrations, reaction temperatures, pHs, ionic interferences, and different geographical water sources affected the catalytic performance. The catalyst obtained by carbonization at 800 °C showed the highest degradation rate (90.8%) for tetracycline. In addition, tetracycline degradation became better with increasing catalyst and PMS dosage, and better with decreasing initial pollutant concentration. The range of the optimal reaction temperature was 25–35 °C, the optimal pH was weakly acidic. A variety of concentrations of Cl−, H2PO4−, HCO3−, and HA all initiated different degrees of inhibition on the degradation of tetracycline. After repeated use, the catalyst regeneration performance was good. The main degradation mechanism of the system was determined to be the generation of single-linear oxygen by free radical burst experiments, premixing experiments, and fluorescence spectroscopy analysis. Therefore, the material has a good pollutant removal effect as well as excellent prospects for practical applications.

Supplementary Materials

The following supporting information can be downloaded at: www.mdpi.com/article/10.3390/catal15030264/s1, Table S1: List of main chemical and reagents; Table S2: Experimental instrument and equipment; Table S3: The comparison of performance of different catalysts; Figure S1: Zeta Potential of GSZC-800; Figure S2: (a) effect of temperature on the catalytic degradation process; (b) kinetic analysis of tetracycline degradation at different temperatures; Figure S3: Two-dimensional fluorescence analysis of PMS/ TTCH/ GSZC-800 system; Figure S4: (a)The BET of the catalyst before and after reaction; (b) the FTIR of the catalyst before and after reaction; Figure S5: (a)Treatment effect of GSZC-800 on AO7; (b) treatment effect of GSZC-800 on BPA; Table S4: Several intermediates detected by LC-MS and the m/z values; Figure S6: The MS results of intermediates. The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/catal15030264/s1. Please see references [51,52,53,54,55,56].

Author Contributions

Methodology, Z.C. and C.L.; software, W.L., T.T. and C.Z.; formal analysis, X.Y.; investigation, W.Q.; data curation, S.T.; writing—original draft, Y.L. and M.Z. All authors have read and agreed to the published version of the manuscript.

Funding

This work was financially supported by the National Natural Science Foundation of China (52300048); the China Postdoctoral Science Foundation (2023M741627); and the Natural Science Research of the Jiangsu Higher Education Institution of China (22KJB610004).

Data Availability Statement

No data were used for the research described in the article.

Acknowledgments

The authors are grateful for continuing support from their respective departments.

Conflicts of Interest

The authors declare no conflict of interest.

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Catalysts 15 00264 g001
Figure 1. (a) Schematic of the preparation process; (b–e) the SEM and enlarged SEM images of the GS; (f,g) the SEM images of GSZ and GSZC-800.
Figure 1. (a) Schematic of the preparation process; (b–e) the SEM and enlarged SEM images of the GS; (f,g) the SEM images of GSZ and GSZC-800.
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Catalysts 15 00264 g002
Figure 2. (a) The XRD spectra of GSZ, ZIF-67/8, and the GS precursor; (b) after pyrolysis; (c) the FTIR spectra of GSZ, ZIF-67/8, and the GS; (d) BET after the pyrolysis of GSZC-700, 800, 900, GS-800; the (e) pore size distribution; (f) the TGA spectrum of GS, ZIF-67/8, and GSZ.
Figure 2. (a) The XRD spectra of GSZ, ZIF-67/8, and the GS precursor; (b) after pyrolysis; (c) the FTIR spectra of GSZ, ZIF-67/8, and the GS; (d) BET after the pyrolysis of GSZC-700, 800, 900, GS-800; the (e) pore size distribution; (f) the TGA spectrum of GS, ZIF-67/8, and GSZ.
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Catalysts 15 00264 g003
Figure 3. Effect of pyrolysis temperature on catalytic efficiency: (a) tetracycline degradation curve; (b) kinetic constant and adsorption capacity; (c) effect of catalyst dosing; (d) effect of PMS dosing; (e) effect of tetracycline (TTCH) concentration; (f) effect of pH.
Figure 3. Effect of pyrolysis temperature on catalytic efficiency: (a) tetracycline degradation curve; (b) kinetic constant and adsorption capacity; (c) effect of catalyst dosing; (d) effect of PMS dosing; (e) effect of tetracycline (TTCH) concentration; (f) effect of pH.
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Catalysts 15 00264 g004
Figure 4. The anions and natural organic matter effects: (a) Cl−; (b) H2PO4−; (c) HCO3−; (d) HA.
Figure 4. The anions and natural organic matter effects: (a) Cl−; (b) H2PO4−; (c) HCO3−; (d) HA.
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Catalysts 15 00264 g005
Figure 5. (a) The effect of scavengers on TTCH degradation in GSZC-800/PMS system for TBA, MeOH, and L-histidine; (b) the degradation efficiency of TTCH after mixing GSZC-800 and PMS.
Figure 5. (a) The effect of scavengers on TTCH degradation in GSZC-800/PMS system for TBA, MeOH, and L-histidine; (b) the degradation efficiency of TTCH after mixing GSZC-800 and PMS.
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Catalysts 15 00264 g006
Figure 6. Three-dimensional fluorescence spectra of intermediates: (a) t = 2 min; (b) t = 5 min; (c) t = 10 min; (d) t = 20 min, red lines divide EEM into five regions (I–V).
Figure 6. Three-dimensional fluorescence spectra of intermediates: (a) t = 2 min; (b) t = 5 min; (c) t = 10 min; (d) t = 20 min, red lines divide EEM into five regions (I–V).
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Catalysts 15 00264 g007
Figure 7. Possible reaction mechanisms for the removal of TTCH in the GSZC-800/PMS system.
Figure 7. Possible reaction mechanisms for the removal of TTCH in the GSZC-800/PMS system.
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Figure 8. (a) Catalyst reuse and regeneration performance test; (b–d) XPS spectra before and after GSZC-800 reaction.
Figure 8. (a) Catalyst reuse and regeneration performance test; (b–d) XPS spectra before and after GSZC-800 reaction.
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Catalysts 15 00264 g009
Figure 9. (a,b) Treatment effect of GSZC-800 on different actual water bodies; (c,d) treatment effect of GSZC-800 on different pollutants.
Figure 9. (a,b) Treatment effect of GSZC-800 on different actual water bodies; (c,d) treatment effect of GSZC-800 on different pollutants.
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Catalysts 15 00264 g010
Figure 10. (a) The possible degradation pathways of TTCH in the GSZC-800 system; (b) the oral rat LD50, development toxicity, bioconcentration factor, and mutagenicity of TTCH and the degradation byproducts; (c) digital images of the growth state of the E. coli blank, pure tetracycline, the catalyst GSZC-800, and intermediate products of tetracycline degradation.
Figure 10. (a) The possible degradation pathways of TTCH in the GSZC-800 system; (b) the oral rat LD50, development toxicity, bioconcentration factor, and mutagenicity of TTCH and the degradation byproducts; (c) digital images of the growth state of the E. coli blank, pure tetracycline, the catalyst GSZC-800, and intermediate products of tetracycline degradation.
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Table 1. The specific surface areas, pore volumes, and average pore sizes of the catalysts.
Table 1. The specific surface areas, pore volumes, and average pore sizes of the catalysts.
SampleSBET (m2/g)Vpore (cm3g−1)Pore Size (nm)
GSZC-700129.430.207.08
GSZC-800150.610.144.82
GSZC-90069.500.106.20
GSC-80079.100.137.30
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MDPI and ACS Style

Liu, Y.; Chen, Z.; Liu, C.; Li, W.; Yao, X.; Tian, T.; Zhao, C.; Tao, S.; Qiao, W.; Zhang, M. Large-Scale Preparation of Granular Sludge@MOF-Derived Hierarchical Porous Carbon Catalysts for Advanced Oxidation Process: Preparation Process and Intrinsic Degradation Mechanism. Catalysts 2025, 15, 264. https://doi.org/10.3390/catal15030264

AMA Style

Liu Y, Chen Z, Liu C, Li W, Yao X, Tian T, Zhao C, Tao S, Qiao W, Zhang M. Large-Scale Preparation of Granular Sludge@MOF-Derived Hierarchical Porous Carbon Catalysts for Advanced Oxidation Process: Preparation Process and Intrinsic Degradation Mechanism. Catalysts. 2025; 15(3):264. https://doi.org/10.3390/catal15030264

Chicago/Turabian Style

Liu, Yu, Zhonglin Chen, Chenyong Liu, Wenhui Li, Xiyue Yao, Tian Tian, Chenyu Zhao, Shaoqun Tao, Weichuan Qiao, and Ming Zhang. 2025. "Large-Scale Preparation of Granular Sludge@MOF-Derived Hierarchical Porous Carbon Catalysts for Advanced Oxidation Process: Preparation Process and Intrinsic Degradation Mechanism" Catalysts 15, no. 3: 264. https://doi.org/10.3390/catal15030264

APA Style

Liu, Y., Chen, Z., Liu, C., Li, W., Yao, X., Tian, T., Zhao, C., Tao, S., Qiao, W., & Zhang, M. (2025). Large-Scale Preparation of Granular Sludge@MOF-Derived Hierarchical Porous Carbon Catalysts for Advanced Oxidation Process: Preparation Process and Intrinsic Degradation Mechanism. Catalysts, 15(3), 264. https://doi.org/10.3390/catal15030264

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