1. Introduction
Tetracycline hydrochloride (TC) is a broad-spectrum antibiotic commonly used in human and veterinary medicine [1, 2]. It ranks as the second most frequently employed antibiotic for treating bacterial infections worldwide [3]. However, the extensive use of TC poses a significant environmental challenge [4]. Due to TC’s hydrophilic nature (log Kow = −1.25) [5], a substantial portion of TCs consumed by humans and livestock is excreted in feces and urine, leading to elevated TC levels in aquatic environments [6]. TC persists in various environmental compartments, including surface water, groundwater, drinking water, wastewater, sediment, and sludge, thereby posing potential risks to both animals and humans [7]. The potential ecological consequences of TC in the environment have garnered considerable attention.
In recent years, advanced oxidation processes (AOPs) utilizing hydrogen peroxide (H2O2) have gained widespread recognition for their efficacy in degrading antibiotics due to their rapid reaction kinetics and potent oxidative capacity [8]. H2O2, as a green oxidant, can generate hydroxyl radicals (·OH), hydrogen peroxide radicals (·O2H), superoxide radicals (·O2−), among others [9]. The high oxidation potential of these free radicals enables them to oxidize nearly all organic substances to mineralization without causing secondary contamination [10]. To enhance the degradation efficiency of contaminants using active free radicals, a trending approach is the discovery or synthesis of efficient catalysts that can generate reactive oxygen species for organic pollutant degradation. Studies have shown that natural mineral schorl can effectively activate H2O2 and exhibit a high removal rate of tetracycline hydrochloride [11]. The primary reactive species responsible for TC oxidation on schorl’s surface are ·OH, ·O2−, and HO2·. Additionally, Sang et al. [12] utilized natural pyrite (iron disulfide) and straw biochar (BC) to develop a novel heterogeneous Fenton catalyst for ciprofloxacin (CIP) degradation, achieving a 96.8% degradation rate within 20 minutes while minimizing waste and pollution. Consequently, natural minerals exhibit significant potential as heterogeneous catalysts for pollutant degradation, providing diverse metal elements and active surface sites that play a pivotal role in the activation process [13]. Selecting an appropriate low-cost natural material catalyst remains a key challenge.
Natural coal gangue (CG) is a byproduct frequently generated during coal mining or washing processes [14] and is common in coal-related industries. Coal gangue is a kind of coal-containing kaolin, the main components are Al2O3, SiO2, and contains different amounts of Fe2O3, CaO, MgO, Na2O, K2O, and trace rare elements. CG is rich in minerals and metallic elements, exhibiting chemical stability. However, the accumulation of CG during coal production leads to issues such as groundwater contamination, air pollution, and land occupation [15]. Consequently, exploring CG material recycling to achieve waste resource utilization is imperative [16]. Some studies have highlighted the potential of CG in catalytic pollutant oxidation. For instance, Li et al. employed Cu and Ag-loaded CG to catalyze the activation of persulfate (PS) for TC degradation. During the degradation process, CG served as a carrier for Cu/Ag/CG composite catalyst, promoting redox reactions between Cu/Cu2+ and accelerating the reaction rate [17]. Zhang et al. employed natural CG to activate PMS for TC removal, achieving an efficiency of approximately 80% within 60 minutes. The strong adsorption capability of CG’s surface hydroxyl and structural hydroxyl on PMS promoted the generation of active substances. These findings underscore the potential application value of natural CG as a catalyst. Nonetheless, further research is essential to advance the application of natural minerals as catalysts in wastewater treatment, owing to the presence of numerous impurities in natural minerals and their relatively low catalytic activity. In AOPs, the Fenton reaction of H2O2 is often pH-dependent [18], and the efficiency hinges on electron transport. Transition metals can activate H2O2 [19], improving electron transport and pollutant degradation. While Co2+ is known for its efficiency, it can cause secondary water pollution in homogeneous catalysis, and Co group can leach and agglomerate [20]. Therefore, the exploration of new Co-based nanocatalytic materials to address these issues is imperative.
Zeolitic imidazolate framework (ZIF) belongs to the subclass of metal-organic frameworks (MOF), typically synthesized using dimethylimidazole organic ligands and transition metal ions [21, 22]. ZIF-67, a Co-based heterogeneous catalyst, has been primarily employed for peroxymonosulfate (PMS) activation [23]. Zhang and Dai cultivated ZIF-67 in situ on grapefruit peel (PP). The resulting PP/ZIF-67 material underwent one-step pyrolysis to produce a derivative composite (Co3O4/C-PC), which was used to activate peroxymonosulfate (PMS) for CIP degradation [24]. The results demonstrated a CIP degradation efficiency of 93.3% in the Co3O4/PMS system-PC, even after multiple cycles of use. In summary, ZIF-67 exhibits substantial potential for PMS activation, yet its potential for loading onto CG to activate H2O2 for pollutant degradation remains unexplored. Therefore, by harnessing the strengths of CG and ZIF-67, we investigated the synthesis of a ZIF-67/CG composite catalyst and evaluated its performance in H2O2 activation.
The objectives of this study were as follows: (i) to prepare ZIF-67/CG composites and characterize their properties; (ii) to investigate the degradation characteristics of the composite in activating H2O2 for TC removal and study the impact of key parameters on TC removal efficiency; (iii) to explore the primary active species involved in TC degradation within the ZIF-67/CG-activated H2O2 system and analyze the potential TC degradation mechanism. This study aims to uncover the application potential of ZIF-67-loaded CG for the removal of TC from aqueous solutions.
2. Materials and Methods
2.1. Materials
The waste coal gangue used in this study was sourced from Xinhua District, Pingdingshan City. Tetracycline hydrochloride (C22H25ClN2O8, ≥98%) was procured from Aladdin Reagent Co. Ltd. (Shanghai, China), while H2O2 (30%) was obtained from Nanjing Reagent. 2-Methylimidazole (2-MeIm, ≥98%) was supplied by Tianjin Kemio Chemical Technology Co. Ltd. Cobalt (II) nitrate hexahydrate (Co(NO3)2·6H2O, ≥99%) was provided by Shanghai Maclean’s Biochemical Co. Ltd. Detailed information about other chemicals is available in Table S1.
2.2. Preparation of ZIF-67/CG
ZIF-67 nanoparticles were prepared following a previously reported method [9]. Initially, 0.164 g of 2-MeIm was dissolved in 50 mL of deionized water. Subsequently, 150 mg of CG particles were added to the beaker. The beaker was placed on a magnetic stirrer and stirred at 300 rpm at 30°C for 5 minutes. A specific quantity of Co(NO3)2·6H2O was introduced into the mixed solution and continuously stirred for 30 minutes. Following this, the mixture was subjected to vacuum extraction, and the filtered mixture was washed three times with deionized water. The material was then dried at 45°C in a vacuum drying oven for 12 hours to produce the ZIF-67/CG material. Prior to formal experimentation, the impact of different ZIF-67 loadings on CG for H2O2 catalysis was investigated, and the optimal ZIF-67 loading for composite preparation was determined (Text S1, Fig. S1).
2.3. Characterization of ZIF-67/CG
The surface morphology of ZIF-67/CG was analyzed using a Scanning Electron Microscope (SEM, Merlin compact, Zeiss, GER). Surface area analysis of the ZIF-67/CG samples was performed using a Brunauer Emmett Teller (N2-BET) analyzer (ASAP2460, Micromeritics Instrument Corp., USA). Fourier-transform infrared (FTIR) spectroscopy (Vertex80, Brooke, Germany) was used to determine the functional groups present in ZIF-67/CG. X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) were carried out to detect the surface elemental composition using equipment from Rigaku (Empyrean, Netherlands) and Thermo Scientific (Escalab 250Xi, USA), respectively.
2.4. Activated Degradation Procedure
For degradation experiments, 60 mg of ZIF-67/CG compound was added to a 200 mL conical flask. Subsequently, 100 mL of a 20 mg/L TC stock solution and 2 mM 30% H2O2 solution were added to the conical flask and mixed thoroughly. The conical flasks were then placed in a constant oscillator at 25°C and 120 rpm. Throughout the reaction, samples were extracted at predefined intervals (e.g., 10, 20, 30, 40, 50, 60 minutes), and the filtrate was passed through a 0.45 μm organic filter membrane. The withdrawn solution’s absorbance was immediately measured at a wavelength of 357 nm using an ultraviolet-visible spectrophotometer (UV759CRT, Shanghai, China).
To investigate the effect of pH on degradation, 0.1 mol/L H2SO4 and 0.1 mol/L NaOH were used to adjust the initial pH to levels of 3, 5, 6.2 (natural), 7, 9, 10, and 11, respectively. Additionally, the impact of temperature on degradation was studied at 20 °C, 25 °C, 35 °C, and 45 °C. The effects of catalyst dosage, H2O2 dosage, and inorganic anions on the TC degradation efficiency of ZIF-67/CG were also explored. The degradation efficiency of TC catalyzed by ZIF-67/CG-H2O2 was calculated using Eq. (1):
where C0 and Ct are the concentrations of TC at time zero (initial concentration) and t, respectively.
All degradation experiments for each experimental parameter were conducted in triplicate. After the first reaction, the used ZIF-67/CG composite was collected from the solution, washed repeatedly with deionized water, and dried at 45 °C in a vacuum drying tank for recycling performance experiments. Additionally, a portion of the solution after each reaction was filtered through a syringe filter head for ion leaching experiments, and the cobalt and iron contents were determined using atomic absorption spectrometry.
3. Results and Discussion
3.1. Characterization of ZIF-67/CG
The scanning electron microscopy (SEM) images in Figure 1a and b reveal distinctive structural features. Specifically, the carbon graphene (CG) material exhibits a flat and smooth structure, while the ZIF-67/CG composite demonstrates a well-dispersed and porous structure. Elemental mapping via energy-dispersive X-ray spectroscopy (EDS) confirms that cobalt (Co), nitrogen (N), and oxygen (O) are the primary constituents of the ZIF-67 shell, whereas iron (Fe), oxygen (O), and carbon (C) predominate in the CG structure (Fig. 1c–i).
Furthermore, the Brunauer-Emmett-Teller (BET) analysis, as shown in Fig. 2a, highlights distinctive characteristics of both the original CG and ZIF-67/CG. The N2 adsorption-desorption isotherm of ZIF-67/CG conforms to Type IV isotherms [25], likely attributed to the presence of nanoparticles, indicative of micropores and mesoporous structures within ZIF-67/CG, consistent with SEM observations. Specifically, the BET surface area of CG measures 6.40 m2/g, whereas ZIF-67/CG boasts a higher surface area of 10.96 m2/g, underscoring the existence of smaller particles on the CG surface. The increased BET surface area and porosity of ZIF-67/CG offer additional active sites for chemical reactions. X-ray diffraction (XRD) analysis, depicted in Fig. 2b, ZIF-67/CG peaks at 10.5°, 18.1°, 30.8°, 35.9°, 49.9°, 59.8° and 67.8°, fitting well to the characteristic peak of ZIF-67 (pdf # 43-0144), with peaks of 44.3°,51.3° and 75.7° at 2θ, corresponding to the (111), (200), (220) crystal surface of Co (JCPDS NO.15-0806), respectively. It reveals discernible diffraction peaks corresponding to ZIF-67 and cobalt (Co) on CG, corroborating the successful incorporation of ZIF-67 onto the CG matrix [26]. Fig. 2c presents Fourier-transform infrared spectroscopy (FTIR) data for the original CG and ZIF-67/CG. The CG spectrum exhibits a peak at 3436 cm−1, corresponding to the O-H vibration of hydrogen-bonded hydroxyl groups [27]. In contrast, the ZIF-67/CG spectrum displays a broader and more prominent peak, indicative of enhanced hydrogen bonding between ZIF-67 and CG. Furthermore, the CG peak at 1580 cm−1 represents the vibration of C=C [28], while the ZIF-67/CG peak at this wave number is broader and more pronounced, signifying increased C=C within ZIF-67, likely due to the presence of 2-MeIm in ZIF-67. The peak at 1432 cm−1 corresponds to the stretching vibration of C=N, with a decrease observed in the ZIF-67/CG spectrum, suggesting disruption of C=N bonds during the catalytic reaction involving H2O2. Finally, the ZIF-67/CG spectrum exhibits a peak at 426 cm−1, attributed to the stretch of Co-N [29]. Importantly, all characteristic peaks of ZIF-67 are present in the composite ZIF-67/CG, with no significant changes observed in the CG-specific peaks, underscoring the stability and suitability of CG as a carrier for ZIF-67.
The comprehensive X-ray photoelectron spectroscopy (XPS) examination of ZIF-67/CG (Fig. 3) uncovers a notable absence of cobalt (Co) peaks in the original CG. However, in the composite material ZIF-67/CG, a prominent Co peak is evident (Fig. 3a).
Further scrutiny of the Co 2p spectrum (Fig. 3b) reveals satellite peaks at 786.5 eV and 780.9 eV, corresponding to Co 2p3/2 and Co-N, signifying the predominant chemical state of Co in ZIF-67/CG as Co (II), thereby confirming the presence of ZIF-67 in the material [30]. Analysis of the O 1s spectrum (Fig. 3c) identifies peaks at 531.53 eV and 532.49 eV, representing Co-OH and C-O, respectively. Co-OH is a novel peak following ZIF-67 loading, formed by the exposure of divalent cobalt and coordinated deprotonated water [31–33]. ZIF-67 presence is further substantiated by the N 1s peak, as evidenced in Fig. 3d, where peaks at 398.8 eV, 400.2 eV, and 401.97 eV correspond to pyridine nitrogen, N-H, and graphite nitrogen, respectively. Notably, the N-H content of ZIF-67/CG increases from 34.7% to 59.4% (Table S2), primarily attributable to the imidazole ring. Additionally, the 403.64 eV peak in the N 1s spectrum arises from pyridine compounds, confirming the loading of nitrogen dimethyl imidazole onto CG. The Fe 2p spectrum illustrates the presence of Fe2+ (65.8%) and Fe3+ (34.2%) in raw CG (Table. S2), whereas the Fe 2p spectrum of ZIF-67/CG (Fig. 3f) displays a single peak, suggesting the formation of iron compounds during the loading reaction.
3.2. ZIF-67/CG Activated H2O2 on TC Degradation Performance
The degradation of TC was investigated using several different systems: a sole H2O2 system, a sole ZIF-67/CG system, and a ZIF-67/CG-activated H2O2 system. As depicted in Fig. 4a, the sole H2O2 system exhibited minimal TC degradation in the solution. In contrast, the sole ZIF-67/CG system achieved a 40.2% TC removal efficiency within 60 minutes. This indicates that the composite ZIF-67/CG alone played a certain role in TC degradation, possibly attributed to the presence of abundant ·OH groups on the CG surface and the interaction of Co elements within the ZIF-67/CG structure. However, when ZIF-67/CG was used to activate H2O2, the TC degradation efficiency of the composite system was significantly enhanced, reaching 82.8% within 60 minutes. Furthermore, after ZIF-67/CG activated H2O2 to degrade TC, the consumption of H2O2 reached 84.2% after 1 hour compared to the H2O2 system alone (Fig. S3). This result confirms that the ZIF-67/CG synthesized in this experiment had a superior activating effect on H2O2.
Meanwhile, the degradation efficiency of TC by CG alone was 22.66% within 120 minutes, while the CG-activated H2O2 system achieved 55.3% TC removal (Fig. S2). This confirms that the degree of H2O2 activation by CG loaded onto ZIF-67 and the subsequent TC degradation efficiency were higher than those of the CG-activated H2O2 system alone. Overall, the experimental results indicated that these composites possess excellent ability in activating H2O2 for TC degradation.
3.2.1. The effects of catalyst ZIF-67/CG dosage and dosage of oxidant H2O2
The effects of ZIF-67/CG dosage on TC removal was shown in Fig. 4b, the degradation efficiency increased with an increase of ZIF-67/CG dosage. When the dosage of ZIF-67/CG increased to 0.6 g/L, the degradation efficiency of TC was 82.8%, which maintained unchanged with further enhancing the ZIF-67/CG dosage, indicated that the degree of electron transport has reached the maximum. So, 0.6 g/L catalyst was selected for the subsequent experiments. In the certain range of ZIF-67/CG dosage, the more ZIF-67/CG was added, the more active sites were generated, in favor of producing more active radicals and non-radicals, thus H2O2 could be activated more quickly to remove TC.
Figure 4c showed the effect of H2O2 dosage on the TC efficiency of degradation. The results in Fig. 4c indicated that the degradation efficiency of TC was 70.45%, 75.47%, and 80.23% at H2O2 dosage of 0.5 mM, 1 mM, and 2 mM, this may be because H2O2 could be activated by the abundant active sites on ZIF-67/CG surface and generate more reactive species for TC degradation. When the dosage of H2O2 increased to 3 mM, the degradation efficiency of TC was 80.91% at 60 min. However, the H2O2 concentration was increased to 3 mM, the degradation efficiency of TC within 60min was slightly lower than 2 mM H2O2. It may be that the excessive H2O2 would quench the generated reactive species [34]. Therefore, 2 mM H2O2 was selected for the subsequent experiments. In conclusion, we found that synthetic ZIF-67/CG was an advantageous good catalyst for activating H2O2 for TC degradation.
3.2.2. The effects of pH and temperature
The impact of different pH values on TC degradation efficiency was comprehensively investigated, as illustrated in Fig. 4d. As for TC solution, the initial pH was determined to be 6.2, and the other solution pH were adjusted by adding certain amount of NaOH and H2SO4. The results revealed that ZIF-67/CG maintained high catalytic activity, and TC was effectively degraded in the initial solution pH range of 5.0–11.0, in which the TC degradation reaches to 80%. Generally, the increase of the solution pH could inhibit the production of ·OH, which would reduce the degradation efficiency of organic pollution [35]. However, under the alkaline conditions of PH in this study, other ROSs were speculated that were involved in the degradation of TC in ZIF-67/CG/H2O2 system, so that the degradation efficiency of TC was not reduced. What’s more, the TC degradation effectiveness dropped to only 62% at pH = 3. This discrepancy can be attributed to the quenching effect of H+ ions on the ·OH radicals at strongly acidic conditions (pH = 3), thereby weakening the oxidation capacity of the system, as depicted in Eq. (2) [36]. The TC degradation rate exhibited fluctuations under various pH conditions, likely due to the reactions between ·OH radicals and H+ and OH− ions. Nevertheless, the overall TC degradation efficiency remained relatively consistent [37]. These findings underscore the broad pH applicability of ZIF-67/CG in the activation of H2O2 for TC degradation.
In our present study, we systematically examined the influence of different temperatures (20, 25, 35, and 45 °C) on TC degradation. As depicted in Fig. 4e, the reaction temperature was increased from 25°C to 45°C, the degradation rate of TC was increased from 80.37% to 82.5%. The rate of TC degradation exhibited an upward trend with increasing temperature. This phenomenon can be attributed to the heightened molecular motion at higher temperatures, facilitating faster reactions that reach equilibrium within 1 hour.
3.2.3. The effect of inorganic anions
In real water, the presence of various inorganic anions may influence the reaction efficiency of activated H2O2 degradation TC process. In present study, Cl−, HCO3−, SO42−, NO3−and H2PO4− were selected for investigating the effect of natural inorganic ions on the reaction. It can be seen that H2PO4− has a significant inhibitory effect on TC degradation (Fig. 5a). When 10mM H2PO4− was added to the system, the degradation efficiency of TC was reduced to 65.2%, because the presence of H2PO4− could cause the consumption of active species (Eq. (3)).
Moreover, H2PO4− also was adsorbed to the composite to reduce the reaction active site. For HCO3−, the presence of HCO3− has a slight promoting effect on the reaction rate (Fig. 5b), because HCO3− acts as a buffer to balance the solution pH during the reaction to achieve a faster reaction rate. However, the HCO3− concentration was 10 mM, the promoting effect was less than 5 mM, and this phenomenon was because that HCO3− could react with ·OH to generate ·HCO3− with weaker oxidation ability (Eq. (4)). Besides, the existence of Cl− inhibited TC degradation (Fig. 5c), this phenomenon was because that Cl− could react with ·OH to generate ·HOCl− with weaker oxidation ability (Eqs. (5)–(7)) [38]. For NO3−and SO42−, NO3− had little effect on degradation reaction while SO42− had a slight inhibition on the reaction (Fig. 5d and 5e), it was possible that the reaction between SO42− and ·OH generated SO4·−, and SO4·− also reacted with ·OH to form HSO5−, which not only produced weaker free radical oxidation capacity than ·OH, but also consumed ·OH (Eqs. (8)–(9)) [39, 40].
3.3. Identification of Reactive Species
In order to investigate the active species involved in the reaction, we selected free radical scavengers, namely methyl alcohol (MeOH), p-benzoquinone (PBQ), tert-butyl alcohol (TBA), and furfuryl alcohol (FFA), to conduct quenching experiments. We combined these scavengers with the electron paramagnetic resonance (EPR) technique to identify the responsible reactive radicals during the oxidation process. TBA and MeOH were employed as quenching agents for ·OH, with rate constants kTBA−·OH = 6.0×108 M−1s−1 and kMeOH−·OH = 9.7×108 M−1s−1, respectively [2, 41].
As depicted in Fig. 6a–c, in the absence of radical scavengers, the degradation efficiency of TC reached 80.36% at equilibrium. However, upon the addition of 200 mM MeOH or TBA, the degradation efficiency of TC decreased to 76.58% and 78.83%. MeOH and TBA supplemented with 200 mM showed 3.78% and 1.53% lower solution degradation efficiency than without the quencher, respectively. This indicated that the ·OH quenching experiment did not play a major inhibitory role. It is speculated that TBA quenched the ·OH produced by H2O2 in solution without completely quenching the ·OH in the hole of coal gangue itself, and ·OH can still degrade TC in the subsequent reaction [16]. Quenching 1O2 with FFA (kFFA,1O2 = 1.2×108 M−1s−1), as shown in Fig. 6b, led to the inhibition of TC degradation. Specifically, when 50 mM FFA was introduced into the system, the TC degradation efficiency reduced to 70.1%. Further addition of 200 mM FFA resulted in a more substantial inhibition, with TC degradation efficiency decreasing to 55.5%. This decrease amounted to a reduction of 27.3% compared to the system without FFA quencher, and the Kobs was significantly reduced from 0.0280 min−1 to 0.0142 min−1 (Fig. 6e). It indicated the participation of 1O2 in the TC degradation process. When ·O2− was quenched using 1,4-PBQ (k1,4-PBQ,·O2− = 2.9×109 M−1s−1) [42], as shown in Fig. 6d, the addition of 0.2 mM 1,4-PBQ resulted in a TC degradation efficiency of 65.4%. Furthermore, the Kobs was obviously reduced from 0.0280 min−1 to 0.0163 min−1 (Fig. 6e). And when 0.5 mM 1,4-PBQ was added, the efficiency decreased to 55.5%. This finding further supports the notion that 1,4-PBQ inhibits TC degradation, emphasizing the significant role played by ·O2− in the catalytic degradation of TC. An Electron Spin Resonance (EPR) test was conducted in a ZIF-67/CG activated H2O2 system. As shown in Fig. 7, distinct characteristic signals corresponding to 1O2, ·O2−, and ·OH were observed. Based on these results, it can be reasonably speculated that pivotal roles are played by 1O2, ·O2−, and ·OH in the ZIF-67/CG activated H2O2 system for TC degradation.
3.4. ZIF-67/CG Activated H2O2 System for Degradation TC Mechanism
According to the FTIR infrared spectrum of ZIF-67/CG, the Co-N bond exhibited signs of damage in the ZIF-67/CG composite following catalytic degradation. This observation implies that the crystalline structure of ZIF-67 was somewhat disrupted during the catalytic degradation process (Fig. 2c). Furthermore, alterations in the levels of Co2+ and Co3+ were noted post-reaction, indicating the involvement of cobalt (Co) in the degradation process. Additionally, the C=C peak at 1580 cm−1 exhibited an increase in intensity and a shift in position, signifying the participation of C=C bonds in the reaction. The content of C=C increased in the composite material following the reaction (Table S3), attributed to the conversion occurring during the reaction between C=C and C-C.
Based on the aforementioned analysis, we demonstrate that ·OH groups on the composite’s surface directly engage in the activation of H2O2, generating superoxide radicals that facilitate the degradation of TC, as illustrated in Eqs. (10), (19), and (20). Furthermore, the specific surface area of the composite loaded with CG-loaded ZIF-67 increased, along with the number of active sites, which supported degradation. The composite material stimulates H2O2 through the transfer of surface charge, leading to the production of Co2+. Co3+ receives a negative charge from CG, producing Co2+. Furthermore, Co2+ and Co3+ undergo a conversion between variable valence states through reactions with H2O2, generating numerous active species that accelerate the reaction’s progress, as depicted in Eqs. (11)–(14).
Examining the elemental composition of ZIF-67 before and after the catalytic reaction, we observed changes in the content of C=C, C=N, C=O, and C-O, shifting from 39.2%, 27.8%, and 33.0% to 51.0%, 24.7%, and 24.3%, respectively (Table S3). These changes can be attributed to the simultaneous conversion of C=C and C-C with ·OOH and ·OH through the action of H2O2. ·OOH and ·OH, in turn, react to produce H2O and O2, leading to the generation of ·O2− (Eqs. (15)–(18)). In combination with quenching experiments and EPR analysis, we hypothesize that the active species involved in TC degradation primarily include ·OH, ·O2−, and 1O2, with ·OH and 1O2 playing prominent roles [43–45]. The composite ZIF-67/CG activated H2O2 system exhibits multiple degradation pathways, encompassing both free radical and non-radical mechanisms. Importantly, cobalt (Co) serves as a key player in electron transport during the degradation process, with electron transfer between Co2+ and Co3+ being the crucial factor for the rapid generation of reactive species [43]. This experiment successfully achieved the swift conversion between these two states, ultimately leading to the rapid degradation of TC.
3.4.1. Stability and reusability of ZIF-67/CG
The cyclic performance of ZIF-67/CG/H2O2 for TC degradation was investigated. As shown in Fig. 8a, the TC removal rates were determined as 80.37%, 69.96%, 55.22% in three consecutive cycles, respectively. Moreover, the quasi-first-order kinetics were used to fit the degradation situation in the three consecutive runs, as shown in Fig. 8c, the Kobs were 0.0221 min−1, 0.0195 min−1, 0.0138 min−1 in the three cycles, respectively. The composite material prepared in the reusable experiment had a certain amount of loss [46, 47], and the Co2+ leaching due to the weak crystallinity of ZIF-67, the degradation rate of TC will decrease with the increase of cycle runs. However, ZIF-67/CG still maintained its H2O2 activation capability even after three cycles. The leaching amounts of the Fe and Co ions in three cycles were shown in Fig. 8b, the concentrations of Co ions in the treated solutions within the three cycles were 2.333, 1.913, and 0.905 mg/L, respectively. What’s more, the concentrations of iron ions were 0.051, 0.062, and 0.073 mg/L, respectively.
4. Conclusions
In this study, we synthesized ZIF-67/CG catalysts, employing ZIF-67 and CG as precursors, with the aim of achieving highly efficient activation of H2O2. Specifically, we investigated the catalytic performance under various conditions. When the ZIF-67/CG catalyst was dosed at 0.6 g/L, H2O2 was added at a concentration of 2 mM, and the initial concentration of the target contaminant (TC) was 20 mg/L, the degradation efficiency of TC reached an impressive 82.8% within a 60-minute reaction time. The rapid generation of reactive species was attributed to electron transfer between Co2+ and Co3+ ions, which was identified as a pivotal step in the catalytic mechanism. Among the reactive species generated, hydroxyl radicals (·OH) and singlet oxygen (1O2) were found to be the primary active species responsible for the degradation of TC. Notably, the ·OH species present on the surface of the composite materials exhibited a pronounced degradation effect on TC. This study contributes valuable insights into the utilization of natural solid waste CG and Co-based heterogeneous catalysts in the field of catalytic activated oxidants, offering a foundational reference for further research and applications in this area. However, the concentration of Co2+ in the cyclic performance of composite materials and ion leaching experiment may be due to the weak crystallinity of ZIF-67 or the secondary contamination of Co2+ to water. Therefore, the method of ZIF-67 should be explored to enhance the stability and reuse of the catalyst. At the same time, there is a lack of evaluating the feasibility of catalyst in industrial practical application with actual wastewater, so the potential of the prepared catalyst in actual industrial wastewater should be further explored in the subsequent research work. In addition, on the basis of this experiment, the surface hydroxyl group of coal gangue should be fully utilized in the future to continue to explore more applications of coal gangue to see whether it has the potential to activate H2O2/PMS catalytic degradation of other pollutants, so as to evaluate whether the prepared composite material has wide applicability.
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