English

• With the development of China’s economic level and the acceleration of the industrial process, the continuous improvement of people’s material living standards and consumption levels, more and more new pollutants, including persistent organic pollutants controlled by international conventions, antibiotics, endocrine disruptors and other substances continue to enter the environment [1-4]. When antibiotic pollutants enter the natural environment, they will not only inhibit some bacteria to affect the microecological balance, but also harm various organisms in the environment, and even induce superbacteria with resistance genes [5-13]. Moreover, the chemical composition and structure of these antibiotic pollutants are stable, stable and persistent, which brings great challenges to traditional water treatment technologies [14,15]. Therefore, it is of great significance to explore effective control and efficient removal of antibiotics in the environment. Among the various antibiotic pollutants, ofloxacin is a frequently studied target pollutant due to its widespread use in medicine and veterinary applications, leading to its common presence in wastewater and aquatic environments. Besides, it is resistant to conventional treatment methods, raising concerns about environmental persistence and the development of antibiotic-resistant bacteria.

  At present, the removal of antibiotic pollutants in water mainly includes physical and chemical methods (such as adsorption method, wet oxidation method, coagulation method) and biochemical technologies (precipitation method, activated sludge method, biofilm method, etc.) [16-20]. These methods cannot effectively remove various new pollutants remaining in water bodies, and usually require further treatment to meet discharge requirements [7,21,22]. Advanced oxidation technology (AOPs) refers to the complete and non-selective removal of organic macromolecules from water by the generation of free radicals with strong oxidation capacity, which oxidizes difficult to degrade into small molecules, and even completely mineralizes them into CO₂ and H₂O [23,24]. In recent years, the advanced oxidation technology of persulfate is a water treatment technology that activates persulfate into strong oxidizing sulfate radical through various activation methods, and degrades pollutants in water through the free radical [25–27]. At present, the main activation methods of persulfate are energy-based activation (thermal activation, photoactivation, ultrasonic activation, etc.) and homogeneous catalytic activation (alkali activation, quinone activation, Co, Fe and other metal ion activation) and heterogeneous catalytic activation (metal oxides, nano carbon materials) [26,28-30]. Nano-carbon materials mainly include fullerenes, graphene, carbon nanotubes, nano-diamond, etc. These materials have huge specific surface area, excellent electronic conductivity, and easy to modify the surface, which makes them possible to be excellent catalytic materials [31,32]. However, compared with metal catalysts, nano-carbon materials catalyzing persulfate (PMS or PDS) to degrade pollutants still have the following problems: (Ⅰ) The catalytic activity of nano-carbon materials is poor, and the stability is low. With the increase of catalytic times, the reactivity decreases rapidly, and the ability to resist the influence of complex water environment media is weak; (Ⅱ) Most commercial nano carbon materials have harsh synthesis conditions and small yield, which limits their large-scale use [31,32].

  By combining the advantages of metal catalysts and nano-carbon materials, the preparation of new metal carbon-based catalysts is one of the important means to promote the popularization of heterogeneous catalytic persulfate technology. It has been found that in the process of reaction between transition metals and various small molecular carbon sources (such as dicyandiamide, melamine, o-phenylenediamine, pyrocatechol) at high temperature, transition metals will form nanocrystal structures, while small molecular carbon sources can be combined with transition metal nanocrystals through gas phase to form metal-carbon composite catalysts [5,33-35]. This kind of composite catalyst with special structure can effectively prevent metal dissolution in the catalytic process. In addition, the coated metal nanocrystals form a special metal-carrier structure with the carbon-based shell structure, which can promote the transfer of electrons from the metal nanocrystals to the carbon-based shell during the activated persulfate process, and realize the cooperative activation of the metal nanocrystals and the carbon-based shell. However, the effect of different small molecules as carbon sources on catalyst structure is still less studied. In this paper, different small molecule carbon sources (melamine, pyrocatechol, o-phenylenediamine, and dicyandiamine) were used to regulate the external carbon-based structure of iron/nitrogen/carbon-based composites (Fe-N/C). The structural-activity relationship in the degradation of antibiotic contaminants by catalysts were investigated, so as to optimize the persulfate conversion pathway and degradation reaction of ofloxacin (OFL) in water.

  Scanning electron microscopy (SEM) was used to characterize the surface morphology and crystal morphology of the iron carbon-based small molecule carbon source composite catalyst. The SEM scanning diagrams of Fe-N-C derived from different small molecule carbon sources (pyrocatechol, o-phenylenediamine, and dicyandiamine) were shown in Fig. 1. As shown in Figs. 1a-d, the surface morphology of the catalyst prepared based on melamine and dicyandiamide showed a large number of slender fibrous structures, which were interlaced to form a complex network structure. In these fibrous structures, some fragments or granular material can also be observed, which may be remnants of incomplete reactions or byproducts of catalysts [36,37]. The diameter of each fiber is relatively small (250 nm) and the surface is smooth, showing high structural integrity. There is a clear gap between the fibers, indicating that the fibers are not tightly packed, and may form a large specific surface area, indicating that the catalyst may have good catalytic performance and high specific surface area. The surface morphology of the catalysts prepared by o-phenylenediamine and pyrocatechol is mainly composed of irregular lamellar structures (Figs. 1e-h). Compared with the-phenylenediamine-derived Fe-N/C, the surface of the catalyst prepared by pyrocatechol is rough and uneven, with more granular structure and more pores and crack. In summary, there are significant differences in the morphology of the catalysts prepared by these four different precursor systems. The Fe-N/C catalysts prepared by pyrocatechol and o-phenylenediamine mainly showed a sheet structure, while the Fe-N/C catalysts prepared by melamine and dicyandiamine showed a cumulated fibrous structure. These structural characteristics will directly affect the catalytic performances and application scenarios of as-prepared catalysts.

  Figure 1

  

  Figure 1.  SEM images of Fe-N/C by using the melamine as a precursor at different scales (a) 5 µm, (b) 500 nm. SEM images of Fe-N/C by using the dicyandiamide as a precursor at different scales (c) 5 µm, (d) 500 nm. SEM images of Fe-N/C by using the pyrocatechol as a precursor at different scales (e) 5 µm, (f) 500 nm. SEM images of Fe-N/C by using the o-phenylenediamine as a precursor at different scales (g) 5 µm, (h) 500 nm.

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  The phase composition and crystal structure of the Fe-N/C catalysts were characterized and determined by X-ray diffraction (XRD). The XRD patterns of different Fe-N/C catalysts prepared at versatile temperatures were shown in Fig. 2. At 700 ℃, the main diffraction peak for different samples occurs at about 27° (Fig. 2a). As the temperature increased to 800 ℃, the diffraction peak intensity and shape of the Fe-N/C catalysts changed significantly (Fig. 2b). The peaks of melamine and pyrocatechol-derived Fe-N/C became sharper and stronger indicating a more pronounced crystal structure, and the peaks of other samples also strengthened, showing higher crystallinity. However, at 900 ℃, the peak of melamine-derived Fe-N/C disappeared (Fig. 2c), possibly due to thermal decomposition destroying its crystal structure, forming an amorphous form or other amorphous phase. The Fe-N/C catalysts derived pyrocatechol and o-phenylenediamine showed similar diffraction peaks at 700, 800, and 900 ℃, but with variations in intensity and shape. These results show that different Fe-N/C catalysts exhibit different crystallization characteristics and thermal stability at different temperatures.

  Figure 2

  

  Figure 2.  XRD patterns of different catalysts prepared at versatile temperatures (a) 700 ℃, (b) 800 ℃, (c) 900 ℃.

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  The X-ray photoelectron spectroscopy (XPS) analysis of Fe-N/C catalysts prepared from different small carbon sources (800 ℃) showed that these catalysts were rich in carbon, nitrogen and iron species (Fig. S1 in Supporting information). The C 1s spectrum of the melamine-derived catalyst showed a high proportion of C—C bonds (61.33%). The proportion of C—O-C bonds was 18.46%, and this peak represents that there are some ether or ester structures in the Fe-N/C catalyst. The proportion of O—C = O bonds is 20.21%, and this peak indicated the presence of carboxyl or ester groups in the catalyst, which may be due to oxidation or the presence of such structures in the precursor (Fig. 3a). As for the N 1s spectrum of melamine-derived Fe-N/C, the contents of pyridine nitrogen (47.58%) and graphite nitrogen (39.54%) were dominated (Fig. 3b), which played an important role in improving the electronic conductivity and catalytic activity of the catalysts [32,38]. The mixture of metallic iron and different oxidation states of iron exists in Fe 2p spectrum of melamine-derived Fe-N/C (Fig. S2 in Supporting information), which showed the Fe exists mainly in Fe(Ⅲ) form on the surface of carbon catalysts. The C 1s, N 1s of pyrocatechol, o-phenylenediamine, and dicyandiamine-derived Fe-N/C catalysts were showed in the Figs. 3c-h. Similarly, the C 1s spectra of other catalysts exhibited the highest C—C bonds (54.26%−64.45%), and pyridine nitrogen and graphite nitrogen almost accounted for 60%−70% of the nitrogen content in these Fe-N/C catalysts (Figs. 3i-k). As a result, N-doped carbon substrates prepared by using different small molecule carbon sources was suitable for anchoring the iron species to form differential metal-supports complexes for activating the PMS to degrade pollutants [11,39,40].

  Figure 3

  

  Figure 3.  XPS C 1s (a) and N 1s (b) of melamine-derived Fe-N/C. XPS C 1s (c) and N 1s (d) of pyrocatechol-derived Fe-N/C. XPS C 1s (e) and N 1s (f) of o-phenylenediamine-derived Fe-N/C. XPS C 1s (g) and N 1s (h) of dicyandiamine-derived Fe-N/C. (i) Element components of different Fe-N/C catalysts. (j) Radar map representing the relationship between the Fe-N/C catalysts prepared from different small carbon sources and the components of nitrogen species. (k) Radar map representing the relationship between the Fe-N/C catalysts prepared from different small carbon sources and the components of carbon species.

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  The pyrolysis temperature in the tube furnace largely determines the morphology and spatial structure of the catalyst. High temperature can make holes for the catalyst to increase their defect sites, thus improving the Fenton-like performance of the catalyst [32]. However, too high pyrolysis temperature can also lead to the collapse of the spatial structure of the material or the material bonding, which has a negative impact on their catalytic performances [41,42]. The degradation performances of OFL via activating PMS by Fe-N/C catalysts prepared with different small molecular organics at different pyrolysis temperatures against were shown in Fig. 4 and Fig. S3 (Supporting information). The catalyst prepared with melamine achieved remarkable effect at 700 and 800 ℃ (Figs. 4a and b), but at 900 ℃, the catalyst prepared with melamine had a sharp decline in effect (Fig. 4c), and its degradation rate dropped from 0.042 min^-1 to 0.0017 min^-1 (Figs. 4d and e). In contrast, the pyrocatechol-derived catalysts prepared at different temperatures exhibited relatively stable catalytic performances with the K_obs data in the range of 0.018–0.024 min^-1. However, the removal efficiency of OFL by the pyrocatechol-derived Fe-N/C was relatively lower than that of dicyandiamide-derived Fe-N/C, which also exhibited good stability results at the three experimental temperatures. It can be seen that the pyrolysis temperatures of 700–800 ℃ can make the material form a mature and stable structure, while the excessively high temperature may cause the collapse of the material structure. This is consistent with the XRD diffraction pattern, and the C-peak of Fe-N/C prepared by melamine as the precursor system disappears at 900 ℃, indicating that the absence of carbon-based materials led to a significant deterioration in its degradation performance [11,43]. In addition, the dosages of different Fe-N/C catalysts on the OFL degradation performances were given in Figs. 4f and g, and Fig. S4 (Supporting information). The corresponding K_obs were shown in Fig. 4h. Results indicated that the degradation process would trend to equilibrium with the increasing dosage from 0.05 g/L to 0.2 g/L. As a result, an economic dosage (0.1 g/L) can be selected for subsequent experiments.

  Figure 4

  

  Figure 4.  Degradation efficiencies of OFL by a series of small organic molecules-derived Fe-N/C catalysts prepared at different pyrolysis temperatures (a) 700 ℃, (b) 800 ℃, (c) 900 ℃. (d) Comparison of K_obs in different small organic molecules-derived Fe-N/C catalysts/PMS systems. (e) Radar map representing the relationship between the K_obs and Fe-N/C catalysts prepared at different pyrolysis temperatures. OFL degradation performances by pyrocatechol-derived Fe-N/C (f) and dicyandiamine-derived Fe-N/C (g) in terms of different catalyst dosages. (h) Comparison of K_obs in Fe-N/C catalysts/PMS systems in terms of different catalyst dosages. Reaction condition: [PMS] = 0.5 mmol/L, [OFL] = 10 mg/L, pH 6.8.

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  The co-existence of anions in solution can significantly influence the degradation performance of catalysts in advanced oxidation technology through competitive reaction, surface active site coverage, redox potential change and pH buffering [41,44]. The adsorption and degradation effects of different catalysts (pyrocatechol-derived Fe-N/C, and dicyandiamine-derived Fe-N/C) under the interference of different anions were shown in Figs. 5a-d, Figs. S5 and S6 (Supporting information). SO₄^2- which to a certain extent inhibits the catalytic activation of PMS to degrade OFL, while Cl^- and H₂PO₄^- have no obvious inhibition on the whole degradation process. However, CO₃^2- greatly inhibited the process of catalyst degradation and adsorption of ofloxacin. CO₃^2- would affect the pH value in the solution through buffering and inhibit the generation of ^•OH radicals, resulting in the reduced adsorption and degradation efficiencies [26,45]. Therefore, implementing appropriate pretreatment strategies and optimization of reaction conditions can reduce the negative effects of anions and improve the efficiency of catalyst activated PMS degradation of OFL.

  Figure 5

  

  Figure 5.  OFL adsorption by pyrocatechol-derived Fe-N/C (a) and dicyandiamine-derived Fe-N/C (c). OFL degradation by pyrocatechol-derived Fe-N/C (b) and dicyandiamine-derived Fe-N/C (d). OFL degradation by pyrocatechol-derived Fe-N/C (e) and dicyandiamine-derived Fe-N/C (f) at different pH conditions. Reaction condition: [PMS] = 0.5 mmol/L, [OFL] = 10 mg/L, [Coexisting anion anions] = 10 mmol/L.

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  It was known that the solution pH can affect the dissociation form of OFL and the activation effect of PMS [46]. The adsorption and degradation rates of the two catalysts (pyrocatechol-derived Fe-N/C, and dicyandiamine-derived Fe-N/C) at different initial pH values were shown in Figs. 5e and f, and Figs. S7 and S8 (Supporting information). Under acidic conditions, high concentration of H⁺ ions may compete with the active site on the catalyst surface and inhibit the formation of sulfate radical (SO₄^•-) and hydroxyl radical (^•OH). Under alkaline conditions, too high concentration of OH^- ion may lead to excessive consumption of radicals or the formation of inactive by-products, reducing the degradation efficiency. Therefore, moderate pH conditions (such as pH 8) are the optimal choice for improving the degradation efficiency of OFL in both Fe-N/C+PMS systems. Relatively, the dicyandiamine-derived Fe-N/C exhibited the stronger anti-interference capacity in different pH conditions, which may be due to its fiber structure. The fiber structure can provide more buffer space between the catalyst/PMS/pollutants, thus counteracting the impact of pH changes on the degradation process [37,47-49]. In addition, under other pH conditions, the system also showed a good removal efficiency for OFL degradation, reflecting the strong pH adaptability of the catalytic systems.

  Density functional theory (DFT) calculation further identified the role of different species on the PMS adsorption (Figs. 6a-h), which was conducted with Gaussian 09 program [50]. The electrostatic potential (ESP) was calculated by an electronic wavefunction analysis software Multiwfn [51], which was then visualized by VMD software (isofurface contour = 0.002 e/bohr³) [52]. The ESP showed that the negative potential area distributed on the PMS molecule. The results of adsorption energy indicated that the C—O-C and FeN_x exhibited the stronger PMS binding energy (−1.93 eV). Other species such as O—C=C, graphitic N, pyridinic N, pyrrolic N also exhibited a certain extent of binging capacity towards PMS. As a result, the PMS molecules could be adsorbed onto the surface of catalyst by the co-adsorption of these species for subsequent radical generation [43]. This was confirmed by the EPR spectra, which could show the existence of radicals and ¹O₂ based on the DMPO and TEMP signals (Figs. 6i and j). As is shown in Fig. 6i, the PMS/catalyst system exhibit a characteristic 1:2:2:1 splitting pattern due to hyperfine interactions with the hydrogen nucleus. A sextet splitting pattern can also be observed, which could be attributed to the hyperfine interactions with the sulfur nucleus. However, the characteristic triplet of ¹O₂ was not observed when TEMP was added in the catalytic system (Fig. 6i). The results showed that the main active species could be the sulfate radical (SO₄^•-) and hydroxyl radical (^•OH) in the degradation system [53,54].

  Figure 6

  

  Figure 6.  Binding of PMS by different configurations (a) O—C = C, (b) C—O-C, (c) graphitic N, (d) pyridinic N, (e) pyrrolic N, (f) graphene, (g) FeNx. (h) Binding energies of PMS by different configurations. EPR spectra using DOMP (i) and TEMP (j) as probe.

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  In conclusion, the introduction of different small molecular carbon sources significantly affects the distribution of active sites and electronic structure on the catalyst surface, thereby regulating the generation and migration of free radicals. In addition, there are significant differences in the degradation efficiencies of catalysts prepared from different small molecular carbon sources, which relate to the electronic transfer capability of the metal support and the abundance of active sites. DFT and experimental results indicate that catalysts rich in C—O-C and FeNx exhibit better catalytic activity, which may be associated with higher adsorption energies. It has been identified that the main active species for the catalytic degradation of ofloxacin are sulfate radical (SO₄^•-) and hydroxyl radical (^•OH).

  Declaration of competing interest

  The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

  CRediT authorship contribution statement

  Siyuan You: Writing – original draft, Validation. Rui Li: Writing – original draft, Methodology, Formal analysis, Conceptualization. Haoyun Lu: Visualization, Resources. Lifei Hou: Data curation, Conceptualization. Xing Xu: Writing – review & editing, Visualization, Resources, Formal analysis. Yanan Shang: Writing – review & editing, Writing – original draft, Visualization, Validation, Supervision, Software, Resources, Project administration, Methodology, Funding acquisition.

  Acknowledgments

  The research work was supported by National Natural Science Foundation of China (Nos. 52170086, 52300056) and Natural Science Foundation of Shandong Province (Nos. ZR2021ME013, ZR202211280298). The authors also want to thank Conghua Qi from Shiyanjia Lab (www.shiyanjia.com).

  Supplementary materials

  Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.cclet.2025.110955.

  
  
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• 

  Figure 1  SEM images of Fe-N/C by using the melamine as a precursor at different scales (a) 5 µm, (b) 500 nm. SEM images of Fe-N/C by using the dicyandiamide as a precursor at different scales (c) 5 µm, (d) 500 nm. SEM images of Fe-N/C by using the pyrocatechol as a precursor at different scales (e) 5 µm, (f) 500 nm. SEM images of Fe-N/C by using the o-phenylenediamine as a precursor at different scales (g) 5 µm, (h) 500 nm.

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  Figure 2  XRD patterns of different catalysts prepared at versatile temperatures (a) 700 ℃, (b) 800 ℃, (c) 900 ℃.

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  Figure 3  XPS C 1s (a) and N 1s (b) of melamine-derived Fe-N/C. XPS C 1s (c) and N 1s (d) of pyrocatechol-derived Fe-N/C. XPS C 1s (e) and N 1s (f) of o-phenylenediamine-derived Fe-N/C. XPS C 1s (g) and N 1s (h) of dicyandiamine-derived Fe-N/C. (i) Element components of different Fe-N/C catalysts. (j) Radar map representing the relationship between the Fe-N/C catalysts prepared from different small carbon sources and the components of nitrogen species. (k) Radar map representing the relationship between the Fe-N/C catalysts prepared from different small carbon sources and the components of carbon species.

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  Figure 4  Degradation efficiencies of OFL by a series of small organic molecules-derived Fe-N/C catalysts prepared at different pyrolysis temperatures (a) 700 ℃, (b) 800 ℃, (c) 900 ℃. (d) Comparison of K_obs in different small organic molecules-derived Fe-N/C catalysts/PMS systems. (e) Radar map representing the relationship between the K_obs and Fe-N/C catalysts prepared at different pyrolysis temperatures. OFL degradation performances by pyrocatechol-derived Fe-N/C (f) and dicyandiamine-derived Fe-N/C (g) in terms of different catalyst dosages. (h) Comparison of K_obs in Fe-N/C catalysts/PMS systems in terms of different catalyst dosages. Reaction condition: [PMS] = 0.5 mmol/L, [OFL] = 10 mg/L, pH 6.8.

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  Figure 5  OFL adsorption by pyrocatechol-derived Fe-N/C (a) and dicyandiamine-derived Fe-N/C (c). OFL degradation by pyrocatechol-derived Fe-N/C (b) and dicyandiamine-derived Fe-N/C (d). OFL degradation by pyrocatechol-derived Fe-N/C (e) and dicyandiamine-derived Fe-N/C (f) at different pH conditions. Reaction condition: [PMS] = 0.5 mmol/L, [OFL] = 10 mg/L, [Coexisting anion anions] = 10 mmol/L.

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  Figure 6  Binding of PMS by different configurations (a) O—C = C, (b) C—O-C, (c) graphitic N, (d) pyridinic N, (e) pyrrolic N, (f) graphene, (g) FeNx. (h) Binding energies of PMS by different configurations. EPR spectra using DOMP (i) and TEMP (j) as probe.

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