Divergent performances of ZIF-67-derived Fe, Co, and FeCo catalysts in peroxymonosulfate activation: Mechanisms and application in ciprofloxacin degradation

Article

HuanxuanLi1✉

XuehengWei1

XingGao2

ChenLiu1

NingLi1

ChenXu1

JingangHuang1

XiuyanLiu1

ShaodanXu1✉Emailhxlee@hdu.edu.cnEmailxusd@hdu.edu.cn

1College of Materials and Environmental EngineeringHangzhou Dianzi University310018HangzhouChina

2China Aerospace Planning and Design Group Co., Ltd. Zhejiang Branch310000HangzhouChina

Huanxuan Li ^1*, Xueheng Wei ¹, Xing Gao ², Chen Liu ¹, Ning Li ¹, Chen Xu ¹, Jingang Huang ¹, Xiuyan Liu ¹, Shaodan Xu ^1*

¹ College of Materials and Environmental Engineering, Hangzhou Dianzi University, Hangzhou 310018, China.

² China Aerospace Planning and Design Group Co., Ltd. Zhejiang Branch, Hangzhou 310000, China

^*Corresponding authors. E-mail addresses: hxlee@hdu.edu.cn (H. Li), xusd@hdu.edu.cn (S. Xu).

Abstract

The degradation of antibiotics in wastewater treatment poses a major and persistent challenge. In this study, we report the use of zeolitic imidazolate framework-67 (ZIF-67), a metal–organic framework (MOF), as a precursor for synthesizing three distinct catalysts (ZIF-67(Fe)-800, ZIF-67(Co)-800, and ZIF-67(FeCo)-800) through pyrolysis. These catalysts were employed to activate peroxymonosulfate (PMS) for ciprofloxacin (CIP) degradation. Strikingly divergent catalytic performances were observed, with CIP degradation rate constants of 0.0027, 0.0731, and 0.0504 min^–1 for ZIF-67(Fe)-800, ZIF-67(Co)-800, and ZIF-67(FeCo)-800, respectively. Compared to PMS activation, these catalysts showed significantly reduced efficacy in activating peroxydisulfate and sodium percarbonate. Mechanistic investigations through electron paramagnetic resonance and quenching tests unambiguously identify hydroxyl radical (·OH), sulfate radical (SO₄^•−), and singlet oxygen (¹O₂) as the dominant reactive species in ZIF-67(Fe)-800/PMS and ZIF-67(Co)-800/PMS systems. The ZIF-67(FeCo)-800/PMS system exhibited distinct behavior, with SO₄^•− as the primary species accompanied by ·OH, ¹O₂, and superoxide radicals (O₂^•−). Notably, H₂PO₄⁻ and HCO₃⁻ exhibited strong inhibitory effects on CIP degradation, while SO₄²⁻ and CH₃COO⁻ leaded to modest rate enhancements. Our findings establish a rational framework for designing MOF-derived catalysts, demonstrating their superior efficacy in advanced oxidation processes for pharmaceutical pollutant removal.

Keywords:

Peroxymonosulfate (PMS)

ZIF-67 derived catalysts

ciprofloxacin (CIP)

catalytic mechanism

1. Introduction

Ciprofloxacin (CIP) is a broad-spectrum antibiotic commonly prescribed for treating a range of bacterial infections, such as urinary tract infections and specific forms of gastroenteritis [1]. However, CIP can lead to water pollution through several routes, primarily from pharmaceutical manufacturing, improper disposal, and the excretion of unmetabolized drugs by patients [2]. The presence of antibiotics in aquatic environments can foster the emergence of antibiotic-resistant bacteria, posing a significant public health threat and negatively affecting ecosystem health [3]. Therefore, the degradation of CIP in wastewater is a pressing environmental issue, particularly because of the rise of antibiotic resistance, which poses a substantial public health threat. In recent years, various methods have been developed and refined to tackle this challenge [4,5].

Numerous advanced water treatment technologies have been developed to address diverse contaminants, including adsorption, photocatalysis, and microwave catalysis. Adsorption is simple and cost-effective but transfers pollutants to solid waste, requiring disposal/regeneration [6]. Photocatalysis uses light (potentially solar) to mineralize pollutants, offering eco-friendly degradation; however, it faces challenges with catalyst efficiency, separation, light utilization in murky waters, and toxic by-product risks [7]. Microwave catalysis employs rapid heating and hotspots for tough contaminants but suffers from high energy/capital costs, limited water penetration, safety concerns, and scalability issues [8,9]. Peroxymonosulfate (PMS) is increasingly utilized in advanced oxidation processes (AOPs) for wastewater treatment because it can produce reactive oxygen species (ROS), such as sulfate radicals (SO₄^•−), hydroxyl radicals (·OH), and singlet oxygen (¹O₂). These ROS can effectively degrade a broad spectrum of contaminants under mild conditions [10,11]. The generation of ROS usually involves the activation of PMS or peroxydisulfate (PDS) through thermal, UV irradiation, and the use of transition metal catalysts like iron or cobalt. Compared to other activation methods, transition metal catalysts can significantly speed up the generation of ROS, maintain effectiveness across a wider pH range, and can be more economically viable in large-scale applications.

Metal–organic frameworks (MOFs) are crystalline porous materials comprising metal ions interconnected by organic ligands. Their unique advantages, including controllable pore size, tunable surface chemistry, low density, and high surface area, make them widely useful in fields such as adsorption, photocatalysis, and catalysis [12–15]. Furthermore, MOFs serve as effective PMS activators by providing uniformly distributed metal sites stabilized through organic ligand coordination [15]. Various strategies have been employed to enhance the efficiency of this oxidation process, including designing MOFs with high metal content (e.g., Co, Mn, Fe), tailoring the surface area of MOFs to improve PMS adsorption, and combining MOFs with other materials, such as carbon-based nanomaterials, to boost stability and reusability [16–18]. Zeolitic imidazolate framework-67 (ZIF-67) is a highly stable MOF renowned for its ability to maintain structural integrity across diverse environmental conditions. Its properties can be finely tuned by altering the ligand or metal center, enabling optimization for specific applications. This versatility has led to significant interest in its use, particularly in wastewater treatment. Qiao et al. [19] successfully prepared ZIF-67/wool fabric (ZW) and used it for adsorption of reactive dyes. HTNT@ZIF-67, prepared by combining ZIF-67 with hydrogen titanate nanotubes (HTNT), showed excellent removal rates for organic dyes and microplastics by adsorption and catalytic oxidation process in the presence of hydrogen peroxide (H₂O₂) [20]. ZIF-67@GO composites were prepared and tested as efficient adsorbents for removal of mercury (Hg²⁺) from aquatic environment [21]. Lin and Chang [22] synthesized ZIF-67 to activate PMS for the purification of wastewater containing Rhodamine B, demonstrating catalytic performance far superior to Co₃O₄. However, a significant challenge in utilizing MOFs for wastewater treatment is the secondary pollution caused by metal leaching, a consequence of their relatively poor stability. This issue greatly limits the practical engineering applications of MOFs in real-world scenarios [23–25]. The pyrolytic conversion of MOFs into carbon-based composites incorporating metallic nanoparticles, metal oxides, or hybrid configurations has emerged as a versatile synthetic strategy. Employing MOFs as sacrificial templates for carbonaceous material fabrication provides three critical advantages over conventional synthesis routes: (1) spatial confinement effects: the periodic arrangement of metal nodes and organic linkers in MOF precursors creates nanoscale compartments that suppress metal aggregation during thermal decomposition [26]. This structural confinement ensures homogeneous dispersion of catalytic active sites and preservation of quantum-sized effects in derived nanostructures. (2) Precursor-adaptable porosity: the inherent high pore volumes and tunable channel dimensions of MOFs enable infiltration of secondary precursors and in situ polymerization within framework cavities for tailored functionalization [15,27,28]. (3) Hierarchical pore engineering: while the microporous nature (≤ 2 nm) of pristine MOFs may limit mass transport, controlled pyrolysis induces structural transformations through ligand carbonization generating mesopores (2–50 nm), metallic nanoparticle exsolution creating macropores (> 50 nm), and resulting in tri-modal pore systems that enhance molecular diffusion [17,29]. This methodology synergistically combines the structural precision of MOF architectures with the functional versatility of carbon–metal composites, addressing key challenges in conventional catalyst design related to active site accessibility, structural stability, and mass transfer limitations.

In this study, ZIF-67(Co), ZIF-67(Fe), and ZIF-67(FeCo) were synthesized and used as precursors to produce various ZIF-67-derived catalysts, which were then applied to degrade organic pollutants using PMS, PDS, and sodium percarbonate (SPC). The morphology and structure of these catalysts were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and Raman spectroscopy. The influence of operational parameters such as catalyst dosage, PMS concentration, and the presence of co-existing inorganic anions on the degradation of CIP was systematically examined in the ZIF-67(FeCo)-800/PMS system. This research highlights the promising potential of MOF-derived catalysts in effectively removing antibiotics from water.

2. Materials and Methods

2.1. Reagents and materials

2-Methylimidazole, cobalt-nitrate hexahydrate (Co(NO₃)₂·6H₂O), ferrous chloride tetrahydrate (FeCl₂·4H₂O), ciprofloxacin (CIP, 98%), acid orange G (OG, 98%), potassium monopersulfate triple salt (KHSO₅·0.5KHSO₄·0.5K₂SO₄, PMS, purity > 47% KHSO₅ basis), cobalt iron oxide (CoFe₂O₄, < 100 nm, 99%), cobalt(II) oxide (CoO, reagent grade) reduced iron powder (Fe⁰, 98%, 150 µm), and ferrosoferric oxide (Fe₃O₄, 98%, 20 nm) were purchased from Macklin reagent Co., Ltd (Shanghai, China). 2,2,6,6-Tetramethyl-4-piperidinol (TEMP, 99%), 5,5-dimethyl-1-pyrroline N-oxide (DMPO, 97%), and iron oxide(II, III) (99.5%, 20 nm) were acquired from Shanghai Aladdin reagent Co., Ltd., China. Ethanol (EtOH, ≥ 99.7%, AR) were purchased from Shanghai Titan Technology Co., Ltd (Shanghai, China). Methanol (MeOH), tertiary butyl alcohol (TBA), furfuryl alcohol (FFA), and all other chemicals were purchased from Sinopharm Chemical Reagent Co., Ltd (Beijing, China). Unless otherwise specified, all chemicals and reagents used in this study were of commercially available analytical grade and used without further purification.

2.2. Synthesis of catalysts

2.2.1. Preparation of ZIF-67 precursors

The preparation method for ZIF-67(Co) was adapted from previous studies with slight modifications [20,22]. Specifically, 2.91 g (10 mmol) of Co(NO₃)₂·6H₂O and 6.56 g (80 mmol) of 2-methylimidazole were dissolved in 20 ml and 120 ml of ethanol, respectively, and stirred uniformly using a magnetic stirrer. Once both reagents were fully dissolved, the two solutions were combined and stirred for 12 h, yielding purple products. The products were then rinsed alternately with ethanol and deionized water, then dried overnight in a vacuum oven at 80°C. The synthesis of ZIF-67(Fe) and ZIF-67(FeCo) involved using FeCl₂·4H₂O and a combination of Co(NO₃)₂·6H₂O and FeCl₂·4H₂O, respectively, in place of Co(NO₃)₂·6H₂O, while keeping the total metal salt concentration at 10 mmol under the identical preparation conditions.

2.2.2. Preparation of ZIF-67 derived catalysts

The catalysts derived from ZIF-67 were produced by directly carbonizing the synthesized ZIF-67 precursors under a continuous flow of nitrogen gas, without any prior treatment, as illustrated in Fig. 1(a). The temperature was increased at a rate of 15°C per minute and maintained at 800°C for 2 h. The resulting black powders obtained from ZIF-67(Co), ZIF-67(Fe), and ZIF-67(FeCo) were labeled as ZIF-67(Co)-800, ZIF-67(Fe)-800, and ZIF-67(FeCo)-800, respectively.

2.3. Characterization

The structure and morphology of the as-prepared catalysts were determined by X-ray diffractometer (XRD, MinFlex600, Rigaku, Japan), scanning electron microscopy (SEM, ZEISS Sigma 300, Germany) and transmission electron microscopy (TEM, FEI Tecnai F30, USA). The surface chemistry and elemental composition of all samples were examined by X-ray photoelectron spectroscopy (XPS, Thermo Fisher Scientific K-Alpha, USA). The free radicals generated in ZIF-67(FeCo)-800/PMS system were determined by electron paramagnetic resonance (EPR) spectrometer (EMXplus, Bruker, Germany), the detail is presented in Text S1 (in Supporting Information). Raman spectra (100–4000 cm^–1) were obtained using a DXR Raman spectrometer (Thermo Fisher Scientific) with a 633 nm excitation source.

2.4. The catalytic performance of ZIF-67 derived catalysts

The performance of catalysts derived from ZIF-67 was assessed based on the degradation of CIP and acid orange G (OG) in the presence of PMS. The initial concentrations for CIP and OG were set at 10 mg·L^–1 and 0.2 mmol, respectively. All experiments were carried out in a constant-temperature (25°C) shaker (SHZ-C, Shanghai, China), operating at a rotational speed of 120 r·min^–1. For a standard experimental setup, a volume of 100 ml of either CIP or OG solution was introduced into a 250 ml beaker. To initiate the chemical reaction, a predetermined quantity of PMS along with the appropriate catalysts was added to the beaker. Periodic sampling was conducted at predetermined time intervals to monitor reaction progression, wherein 1.0 ml aliquots of the reaction mixture were immediately quenched with an equal volume of methanol (1.0 ml) to terminate catalytic activity. The quenched samples underwent membrane filtration (0.22 µm pore size) prior to quantitative analysis using a ultraviolet–visible (UV-Vis) spectrophotometer (Upper 752N, INESA Analytical Instrument Co.,Ltd, Shanghai, China) with characteristic absorbance measurements recorded at 277 nm (CIP) [30]. CIP concentration quantification was achieved through calibration against standardized reference curves established under identical analytical conditions.

3. Results and Discussion

3.1. Characterization of ZIF-67 derived catalysts

3.1.1. Crystallographic structure

The crystalline evolution of ZIF-67-derived catalysts was systematically investigated through XRD, with comparative analysis of the MOF precursor ZIF-67(Co) provided in Fig. S1. As depicted in Fig. 1(b), complete structural reconstruction was observed in pyrolyzed derivatives, confirming complete loss of crystallinity in the organic framework. This observation aligns well with the typical carbonization mechanism of MOF derivatives, wherein the coordinated framework undergoes thermal decomposition during pyrolysis, ultimately generating a hierarchically porous carbon architecture uniformly dispersed with transition metal-based nanoparticles (metallic/oxide phases). The confined growth of these nanocrystallites within the carbon matrix effectively restricts metal ion leaching by spatially isolating active species. Notably, the ZIF-67(Co)-800 pattern exhibited precise alignment with metallic cobalt (PDF#15–0806) and reference data [31], validating effective carbothermal reduction. Three prominent peaks at 44.2°, 51.5°, and 75.9° were unambiguously indexed to the (111), (200), and (220) planes of face-centered cubic (fcc) Co⁰, respectively [4,31,32]. Remarkably divergent behavior emerged in ZIF-67(FeCo)-800, manifesting dual-phase characteristics: (i) drastically diminished Co⁰ reflections (< 10% intensity vs. ZIF-67(Co)-800), impeding metallic phase formation, and (ii) nine emergent peaks at 18.3°, 30.1°, 35.4°, 37.1°, 43.1°, 53.4°, 56.9°, 62.5°, and 74.9°, exclusively matching the (111), (220), (311), (222), (400), (422), (511), (440), and (533) planes of spinel CoFe₂O₄ (PDF#22-1086) [29] and Fe₃O₄ (PDF#19–0602). Although the XRD diffraction peaks of Fe₃O₄ (PDF#19–0602) and CoFe₂O₄ (PDF#22-1086) were very close and difficult to distinguish, the subsequent XPS and ICP-OES analysis further confirmed that these diffraction peaks originated from both CoFe₂O₄ and Fe₃O₄. This iron-mediated phase reconstitution suggests competing reduction pathways: limited Co²⁺ to Co⁰ conversion versus dominant intermetallic coordination between Co²⁺ and Fe³⁺ to form CoFe₂O₄.

Meanwhile, the XRD pattern of ZIF-67(Fe)-800 exhibited characteristic diffraction peaks corresponding to both FeO (PDF#46-1312) and Fe₃O₄ (PDF#19–0602) phases. Specifically, the diffraction peaks observed at 36.3°, 42.2°, 61.2°, 73.1°, and 77.0° were indexed to the (111), (200), (220), (311), and (222) crystallographic planes of the FeO phase, respectively [32]. Concurrently, the characteristic peaks appearing at 18.3°, 30.1°, 35.4°, 37.1°, 43.1°, 53.4°, 56.9°, 62.5°, and 74.9° were assigned to the (111), (220), (311), (222), (400), (422), (511), (440), and (622) planes of the Fe₃O₄ phase [33].

Fig. 1

Schematic illustration of the synthesis procedure of ZIF-67 derived catalysts (a), XRD patterns (b), and Raman with a 633 nm laser line (c) of ZIF-67 derived catalysts.

The Raman spectra of the ZIF-67-derived catalysts were analyzed to identify their structural characteristics (Fig. 1(c)). The ZIF-67(Co)-800 spectrum exhibited characteristic vibrational signatures at at 1330 cm^–1 (D-band) and 1590 cm^–1 (G-band), diagnostic of defect-rich carbon domains and graphitic ordering, respectively [34]. Conversely, ZIF-67(Fe)-800 and ZIF-67(FeCo)-800 demonstrated significantly suppressed graphitization, as revealed by Raman spectroscopy showing a substantially diminished G-band intensity. This structural deviation aligned with their reduced carbonaceous matrix integrity, quantified via XPS analysis: C 1s atom content of 30.02% (ZIF-67(Fe)-800) and 35.75% (ZIF-67(FeCo)-800) were detected for the Fe-containing derivatives, respectively, versus 83.92% in the ZIF-67(Co)-800 (Table 1). This carbon structural dichotomy strongly corroborated the superior Co²⁺-dimethylimidazole coordination thermodynamics over Fe²⁺ counterparts. The low-frequency spectral deconvolution (200–1000 cm^–1) uncovered metal-oxide phase fingerprints [35,36]. Raman spectroscopy of ZIF-67(Co)-800 exhibited characteristic Co₃O₄ peaks at 194 cm^–1 (F_2g), 475 cm^–1 (E_g), 518/611 cm^–1 (F_2g), and 682 cm^–1 (A_1g) [37], indicative of surface Co⁰ oxidation to spinel oxide via atmospheric oxygen passivation. The resulting oxide phase remained below the XRD detection limit. ZIF-67(FeCo)-800 displayed hybrid spinel signatures at 607 cm^–1 (CoFe₂O₄ (Fe₃O₄) F_2g) and 663 cm^–1 (CoFe₂O₄ (Fe₃O₄) A_1g) [37,38], alongside α-Fe₂O₃ (hematite) modes at 223 cm^–1 (A_1g), 243/290 cm^–1 (E_g), and 408/493 cm^–1 (E_g) [39]. Similarly, ZIF-67(Fe)-800 showed distinct α-Fe₂O₃ bands at 223, 290, 408, and 493 cm^–1 in addition to Fe₃O₄ signatures, attributed to the oxidative transformation of Fe²⁺ (intermediate FeO species) to Fe³⁺. Surface oxidation pathways differed: Co-containing derivatives initiated via O₂ adsorption on metallic sites, whereas Fe-dominated systems favored bulk oxide formation due to iron’s higher oxygen affinity. The coexistence of XRD-detected Fe₃O₄ and Raman-identified α-Fe₂O₃ reflected spatially heterogeneous oxidation states within a core–shell architecture.

The chemical compositions of the prepared ZIF-67 derived catalysts were analyzed using XPS. As illustrated in Fig. 2(a), the C 1s spectra of these catalysts were fitted with three primary components at 284.8, 285.2, and 288.7 eV, which corresponded to sp² carbon, C–N/C–O bonds, and C = O bonds, respectively [27,40]. Figure 2(b) displayed the O 1s spectra for ZIF-67(Fe)-800, ZIF-67(Co)-800, and ZIF-67(FeCo)-800. These spectra were deconvoluted into three peaks, which were assigned to lattice oxygen (O_lattice) at 529.8 eV, defective oxygen (O_defective) at 530.5 eV, and adsorbed oxygen (O_absorbed) at 532 eV, respectively [32,40]. The small amount of lattice oxygen in ZIF-67(Co)-800 (Table 1) correlates with the XRD results that show no diffraction peaks for CoO in ZIF-67(Co)-800 (Fig. 1(b)).

Fig. 2

The XPS spectra of ZIF-67 derived catalysts for (a) C 1s, (b) O 1s, (c) Fe 2p, and (d) Co 2p.

The high-resolution Fe 2p XPS spectrum presented in Fig. 2(c) exhibited four distinct characteristic peaks. Through spectral deconvolution analysis, the doublet components at binding energies of 710.0 eV (2p_3/2) and 723.3 eV (2p_1/2) were assigned to Fe(II) species, while the corresponding peaks observed at 711.7 eV (2p_3/2) and 725.1 eV (2p_1/2) can be unambiguously identified as Fe(III) signatures, consistent with previously reported XPS data for iron oxide systems [41,42]. The high-resolution XPS analysis of ZIF-67(Co)-800 presented in Fig. 2(d) disclosed four deconvoluted components in the Co 2p_3/2 core-level spectrum at binding energies of 778.3 eV (assigned to metallic Co⁰), 780.2 eV (characteristic of Co(II) species), 782.7 eV (indicative of Co(III) oxidation state), and 786.5 eV (associated with shake-up satellite features), respectively [32,43]. In the higher binding energy regime, the corresponding Co 2p_1/2 spectrum demonstrated parallel peak components at 793.1, 795.8, 798.7, and 804.8 eV, which systematically corresponded to Co⁰, Co(II), Co(III), and their satellite structures, respectively, as corroborated by reference XPS studies of cobalt-containing compounds [31,32,35,44,45]. The presence of Co(II) and Co(III) is likely attributed to the bonding of Co ions with carbon or nitrogen [31]. Additionally, the oxidation of metallic Co⁰ on the surface of ZIF-67(Co)-800 also contributes to the presence of Co(II). Notably, the XPS analysis reveals a substantial attenuation of the Co⁰ characteristic peak intensity in ZIF-67(FeCo)-800 compared to its Fe-free counterpart ZIF-67(Co)-800. This suppression likely stems from iron incorporation-induced phase transformation, where partial metallic cobalt undergoes oxidation to form CoFe₂O₄ spinel structures. Furthermore, the intensified Co 2p_3/2 satellite features in ZIF-67(FeCo)-800 demonstrated significant electronic structure modification of cobalt species, consistent with previous reports on CoFe₂O₄ systems [44,45]. The enhanced satellite signatures can be rationalized through Fe-induced charge redistribution effects that amplify multiplet splitting and interfacial charge transfer phenomena. These spectroscopic observations correlate well with the XRD diffraction patterns, collectively corroborating the successful formation of CoFe₂O₄ during the controlled pyrolysis of ZIF-67(FeCo) precursors. Additionally, quantitative ICP-OES analysis of the Co/Fe molar ratio (1:6.6) (Table 1) suggests the co-existence of multiple iron-containing phases. While the some of iron participates in CoFe₂O₄ formation, the measured stoichiometric deviation implies the concurrent presence of Fe₃O₄ alongside residual metallic cobalt in the composite system. As shown in Table 1, ZIF-67(FeCo)-800 exhibits higher surface Co content (XPS data) despite its lower bulk Co content (ICP-OES data) compared to ZIF-67(Co)-800. Co surface enrichment is driven by three factors: 1) Thermodynamic segregation: Fe preferentially migrates into the carbon matrix due to its lower surface energy, while Co diffuses toward the surface to minimize interfacial energy during high-temperature carbonization (800°C); 2) Kirkendall effect: Differential interdiffusion rates of Fe and Co induce vacancy formation, promoting Co migration to the surface; 3) Oxidation state disparity: Fe rapidly oxidizes to Fe³⁺ and integrates into subsurface oxide/carbide phases, whereas Co²⁺/Co³⁺ species remain surface-proximate as XPS-detectable active sites. The Co-rich surface profoundly enhances PMS activation through increasing accessibility of active sites and electronic modulation by subsurface Fe.

Table 1
XPS and ICP-OES analysis for different as-prepared catalysts.
Samples	XPS for C atomic/%	XPS for O atomic/%	XPS for N atomic/%	XPS for Fe atomic/%	XPS for Co atomic/%	ICP-OES for Fe amount/%	ICP-OES for Co amount/%
ZIF-67(Fe)-800	30.02	48.77	0.99	20.22	–	68.34	–
ZIF-67(Co)-800	83.92	7.19	6.39	–	2.51	–	70.71
ZIF-67(FeCo)-800	35.75	45.14	0.84	14.44	3.84	60.85	9.18

3.1.2. Morphology characterization

The SEM images of the synthesized catalysts were analyzed to illustrate their structure and morphology. As shown in Fig. 3(a), ZIF-67(Co)-800 was shown to have a uniform hexagonal prism shape, with a consistent size of about 300 nm. In Fig. 3(c), ZIF-67(Fe)-800 displayed a nano-regular polyhedral structure with a smooth surface, though some agglomeration was observed. The nanoparticles were approximately 500 nm in size. EDS mapping analysis (Fig. 3(d)) confirmed the uniform distribution of C, O, N, and Fe within the ZIF-67(Fe)-800 catalyst. Compared to ZIF-67(Co)-800, the edges and corners of ZIF-67(Fe)-800 were less pronounced, but its surface appeared more smooth. For ZIF-67(FeCo)-800, the surface roughness was intermediate between that of ZIF-67(Fe)-800 and ZIF-67(Co)-800 (Fig. 3(e)). ZIF-67(FeCo)-800 exhibited a uniform polyhedral structure with an average particle size of 500 nm. The homogeneous spatial distribution of C, O, N, Co, and Fe elements, as evidenced by elemental mapping analysis (Fig. 3(f)), confirms the successful synthesis of bimetallic ZIF-67(FeCo). Notably, the reduced spectral resolution of Co in the ZIF-67(FeCo)-800 sample compared to its monometallic counterpart ZIF-67(Co)-800 can be attributed to the inherent limitations of SEM-EDS in distinguishing between Fe (Kα = 6.40 keV) and Co (Kα = 6.93 keV) X-ray emission lines, which exhibited overlapping energy signatures. However, ICP-OES quantification revealed a substantial reduction in cobalt mass content from 70.71% in ZIF-67(Co)-800 to 9.18% in ZIF-67(FeCo)-800 (Table 1), representing an 87% decrease in Co concentration. This dramatic compositional change, when coupled with XRD phase identification and XPS surface analysis, provides conclusive evidence for: 1) The coexistence of Fe and Co species within the ZIF-67(FeCo)-800 architecture, and 2) The dominant role of reduced metal content (rather than impurity interference) in diminishing Co-specific SEM-EDS signals. The complementary characterization approaches collectively validate the effective incorporation of both metallic elements while resolving potential ambiguities in elemental detection sensitivity.

TEM was utilized to further investigate the detailed morphology of ZIF-67(FeCo)-800. As illustrated in Fig. 3(g), ZIF-67(FeCo)-800 exhibited a uniform polyhedral structure, with an average particle size ranging from 200 to 500 nm. Additionally, the metal particle size, as detailed in the TEM image (Fig. 3(g)), ranged from 30 to 40 nm. High-resolution transmission electron microscopy (HRTEM) images of ZIF-67(FeCo)-800 revealed well-defined crystalline structures with clear lattice fringes, showing lattice spacings of 0.496, 0.291, and 0.243 nm (Fig. 3(h) and (i)). These spacings correspond to the reflection facets (111), (220), and (222) of CoFe₂O₄ (PDF#22-1086) and Fe₃O₄ (PDF#19–0602). This finding is consistent with the selected area electron diffraction (SAED) pattern of the crystal particles (Fig. 3(j)) and the other characterization data (including the XRD, Raman, XPS, ICP-OES, and EDS mapping).

Fig. 3

SEM images and the EDS mappings of ZIF-67(Co)-800 (a, b), ZIF-67(Fe)-800 (c, d), and ZIF-67(FeCo)-800 (e, f); TEM (g), HRTEM (h, i), and SAED (j) images of ZIF-67(FeCo)-800.

3.2. Catalytic performance

Fig. 4

(a) The comparison degradation of CIP by PMS in different systems; (b) CIP degradation rate constants of pseudo-first-order kinetics (k_obs) in the different systems; (c) The comparison degradation of OG by PMS in different systems; (d) The comparison degradation of CIP by PDS and SPC in different systems. Experiment condition: PMS = 1.0 mmol·L^–1; CIP = 10 mg·L^–1; OG = 0.2 mmol; catalysts = 50 mg·L^–1; initial pH unadjusted.

Figure 4(a) illustrates the degradation profiles of CIP across different systems. When either the catalysts or PMS was present alone in the CIP solutions, the concentration of CIP decreased negligibly, indicating that neither PMS nor the catalysts significantly oxidizes or adsorbs CIP on their own. In the ZIF-67(Co)-800/PMS system, nearly 100% of CIP was degraded within 150 minutes, whereas only 28.7% of CIP degradation rate was observed in the ZIF-67(Fe)-800/PMS system, suggesting that Fe₃O₄ has low activity for PMS activation. The ZIF-67(FeCo)-800/PMS system achieved a CIP degradation efficiency of 93.4%, slightly lower than that of the ZIF-67(Co)-800/PMS system.

According to the ICP-OES results (Table 1), the cobalt content in ZIF-67(Co)-800 was 70.71%, compared to only 9.18% in ZIF-67(FeCo)-800. This indicates that the effective active sites (Co) in the ZIF-67(Co)-800/PMS system, with a catalyst concentration of 50 mg·L^–1, amount to 35.36 mg·L^–1, which is 7.7 times higher than the 4.59 mg·L^–1 in the ZIF-67(FeCo)-800/PMS system. However, as shown in Fig. 4(b), the degradation rate constant for CIP in the ZIF-67(Co)-800/PMS system (0.0731 min^–1) was only 1.45 times higher than that in the ZIF-67(FeCo)-800/PMS system (0.0504 min^–1). The ZIF-67(Fe)-800/PMS system exhibited a much lower degradation rate constant of 0.0027 min^–1. These results suggest a synergistic effect between Fe and Co ions in ZIF-67(FeCo), which enhances PMS activation compared to the monometallic catalyst. It is noted that XPS analysis revealed distinct surface cobalt contents of 2.51% and 3.84% for ZIF-67(Co)-800 and ZIF-67(FeCo)-800, respectively (Table 1). This divergence from bulk compositions quantified by ICP-OES primarily stems from fundamental differences between the techniques: ICP-OES measures total elemental composition, whereas XPS probes the < 10 nm surface layer while simultaneously resolving oxidation states and elemental segregation. For example, the elevated cobalt content detected by XPS in ZIF-67(FeCo)-800 likely reflects surface enrichment of Co species during pyrolysis, undetectable by bulk-sensitive ICP-OES. These complementary techniques-surface-specific XPS versus bulk-representative ICP-OES—collectively demonstrate heterogeneous metal distribution and enable holistic understanding of material properties. Commercial nano-Fe₃O₄ (20 nm), micron-scale Fe⁰ (150 µm), and sub-100 nm CoFe₂O₄ demonstrated significantly inferior PMS activation capabilities compared to the ZIF-67(FeCo)-800 catalyst, achieving merely 16.4%, 15.6%, and 33.5% CIP removal efficiency respectively within 60 minutes (Fig. S2(a)). Notably, even the CoFe₂O₄-2 (Fe/Co = 2:1) material (Fig. S2(b)), synthesized via high-temperature calcination (800°C, 2 h, N₂ atmosphere) of Fe₃O₄ nanoparticles (50 nm) and cobalt (II) oxide precursors with controlled stoichiometry, exhibited similarly limited catalytic performance in PMS activation system (Fig. S2(a)). These comparative experiments conclusively highlight the superior catalytic activity of MOF-derived architectures over conventional metal oxide catalysts, suggesting their strong potential for advanced oxidation process applications.

To further evaluate the catalytic performance of the prepared ZIF-67-derived catalysts, OG degradation was performed under various systems (Fig. 4(c)). The results reveal consistent trends: neither PMS alone nor the catalysts alone yielded significant OG degradation, with the ZIF-67(Fe)-800/PMS system showing less than 10% removal. In contrast, complete OG degradation (100%) was achieved within 30 min in both the ZIF-67(Co)-800/PMS and ZIF-67(FeCo)-800/PMS systems. Furthermore, time-dependent UV-Vis spectra of both CIP and OG confirmed the excellent catalytic performance of ZIF-67(FeCo)-800 in PMS-driven wastewater treatment, as well as the accompanying decolorization of OG (Fig. S3). When evaluating the catalytic activity of the ZIF-67 derived catalysts for PDS and SPC in CIP degradation (Fig. 4(d)), all catalysts showed weaker performance compared to PMS. Specifically, only about 10%, 30%, and 45% of CIP were degraded in the ZIF-67(Fe)-800/PDS, ZIF-67(Co)-800/PDS, and ZIF-67(FeCo)-800/PDS systems, respectively. Furthermore, the CIP degradation rate remained below 15% in the SPC system across all ZIF-67 derived catalysts. Additionally, ZIF-67(Co)-800 and ZIF-67(FeCo)-800 exhibited superior or comparable PMS activation performance for contaminant removal when compared to other carbon-based catalysts, as shown in Table S1.

3.3. Reactive oxygen species for degradation of CIP

Chemical quenching experiments were performed to elucidate the role of ROS in the degradation of target pollutants, utilizing scavengers with known reaction rate constants for specific radicals [43,46]. To assess the contributions of ·OH and SO₄^•− to CIP degradation, methanol (MeOH; quenching ·OH with k = 9.7×10⁸ L·mol^–1·s^–1 and SO₄^•− with k = 1.1×10⁷ L·mol^–1·s^–1) and tert-butanol (TBA; quenching ·OH with k = (3.8–7.6)×10⁸ L·mol^–1·s^–1) were introduced into the reaction systems [23,43]. Furthermore, furfuryl alcohol (FFA), a scavenger of ¹O₂ (k = 1.2 × 10⁸ L·mol^–1·s^–1), ·OH (k = 1.5×10¹⁰ L·mol^–1·s^–1), and SO₄^•− (k=(4.1 ± 0.4)×10⁹ L·mol^–1·s^–1), was employed to evaluate the role of ¹O₂ [27,47,48]. Additionally, benzoquinone (BQ), which reacts with ·OH (k = 1.2×10⁹ L·mol^–1·s^–1), SO₄^•− (k = 7.8×10⁸ L·mol^–1·s^–1), superoxide radical (O₂^•−; k = 1.0×10⁹ L·mol^–1·s^–1), and ¹O₂ (k = 6.6×10⁷ L·mol^–1·s^–1), was used to confirm the contribution of O₂^•−. The relative contributions of ·OH, SO₄^•−, ¹O₂, and O₂^•− to CIP removal were quantified based on the observed degradation efficiencies using Eqs. (1)–(5).

Relative contribution of ⋅OH=

(2)

Relative contribution of SO₄^•− =

(3)

Relative contribution of ¹O₂ =

(4)

Relative contribution of O₂^•−=

(5)

where η is the CIP degradation rate, C_t is the concentration of CIP at time t, and C₀ is the concentration of CIP at time zero.

As shown in Fig. 5(a), 58.0% of CIP was degraded within 43 h in the ZIF-67(Fe)-800/PMS system without scavengers. The addition of MeOH, TBA, FFA, and BQ reduced the degradation efficiency by 16.4%, 8.4%, 48.3%, and 10.6%, respectively, confirming the involvement of ·OH, SO₄^•−, ¹O₂, and O₂^•−. Notably, the significant inhibition by FFA indicates that ¹O₂ is the primary contributor to CIP degradation in this system (Table 2). In the ZIF-67(Co)-800/PMS system, the degradation efficiency decreased from 91.6% (control) to 43.7%, 75.7%, 4.9%, and 57.8% upon addition of MeOH, TBA, FFA, and BQ, respectively, over 60 min (Fig. 5(b)). This corresponds to calculated contributions of ·OH, SO₄^•−, ¹O₂, and O₂^•− of 15.9%, 27.8%, 38.8%, and 17.9%, respectively (Table 2). For the ZIF-67(FeCo)-800/PMS system, the degradation rates declined from 83.5% (control) to 12.2%, 75.2%, 2.5%, and 59.9% with the addition of MeOH, TBA, FFA, and BQ, respectively. These results demonstrate that SO₄^•− plays the dominant role in CIP degradation within this system (Table 2).

Fig. 5

Effects of radical scavengers on CIP removal in the ZIF-67(Fe)-800/PMS system (a), ZIF-67(Co)-800/PMS (b), and ZIF-67(FeCo)-800/PMS (c) systems. Experiment condition: PMS = 1.0 mmol·L^–1, catalyst = 50 mg·L^–1, CIP = 10 mg·L^–1.

Systems	⋅OH	SO₄^•−	¹O₂	O₂^•−
ZIF-67(Fe)-800	8.4	8.0	31.9	2.2
ZIF-67(Co)-800	15.9	27.8	38.8	17.9
ZIF-67(FeCo)-800	8.3	63.0	9.7 7%	15.3

Electron paramagnetic resonance (EPR) spectroscopy was further employed to identify ROS generated during the ZIF-67(FeCo)/PMS degradation process. DMPO was used as the spin trap for ⋅OH, SO₄^•−, and O₂^•−, while TEMP trapped ¹O₂ [31,43]. As shown in Fig. 6, the ZIF-67(FeCo)-800 system alone exhibited no EPR signals, irrespective of the addition of DMPO or TEMP. In PMS solutions without catalyst, weak quartet signals characteristic of the DMPO–⋅OH adduct were detected (Fig. 6(a)). Similarly, when TEMP was used, characteristic triplet signals corresponding to the TEMP–¹O₂ adduct were observed (Fig. 6(c)). Upon combining PMS with ZIF-67(FeCo)-800, the intensities of both the DMPO–⋅OH and TEMP–¹O₂ signals increased significantly, confirming the generation of ⋅OH and ¹O₂. Additionally, distinct signals for the DMPO–SO₄^•− adduct were also detected in the ZIF-67(FeCo)-800/PMS system, verifying SO₄^•− formation. Notably, weak sextet signals attributed to the DMPO–O₂^•− adduct were also observed in the ZIF-67(FeCo)-800/PMS system (Fig. 6(b)), implying a limited contribution of O₂^•− to CIP degradation.

Fig. 6

EPR spectra of DMPO adducts (a, b) and TEMP adducts (c) in the ZIF-67(FeCo)-800/PMS system. Experiment condition: PMS = 1.0 mmol·L^–1, catalyst = 50 mg·L^–1, DMPO = TEMP = 0.2 mmol·L^–1.

3.4. The possible mechanism

Based on the comprehensive mechanistic analysis, SO₄^•− plays the predominant role in the molecular degradation of CIP. Figure 7 illustrates a proposed PMS activation mechanism mediated by ZIF-67(FeCo)-800, involving three critical pathways for ROS generation:

Radical initiation

Surface-adsorbed PMS undergoes electron transfer with metallic ≡ Co(0) and ≡ Co(II) sites (Eqs. (6), (7)), generating SO₄^•− while oxidizing the metal centers to ≡ Co(II) and ≡ Co(III). This redox transformation is supported by post-catalysis characterization. XRD analysis revealed the complete disappearance of metallic Co(0) diffraction peaks (Fig. S7), and XPS spectra showed the disappearance of the Co(0) photoelectron signal at ~ 778 eV (Fig. 8(b)).

Radical propagation: Subsequently, ≡Fe(II) and ≡ Fe(III) species react with additional PMS molecules to yield SO₄^•− and SO₅^•− radicals (Eqs. (8), (9)). These reactive intermediates undergo further conversion via three distinct pathways: (i) SO₅^•− disproportionation yielding ¹O₂ (Eq. (10)), (ii) ≡ Co(III)-mediated secondary SO₅^•− generation (Eq. (11)), and (iii) SO₄^•− hydroxylation to produce ·OH (Eq. (12)).

Redox cycling

Thermodynamic analysis based on standard reduction potentials (E⁰ (Co³⁺/Co²⁺) = 1.81 V vs. E⁰ (Fe³⁺/Fe²⁺) = 0.77 V) reveals spontaneous electron transfer between metal centers [49,50], enabling Fe(II)-mediated reduction of Co(III) (Eq. (13)). Furthermore, subsurface Fe donates electrons to surface Co via Fe–N–C bridges, lowering the Co(II)/Co(III) redox potential (E⁰). This facilitates faster PMS oxidation and Co(III) reduction (Eq. (11)). This synergistic ≡ Co(III)/≡Co(II) and ≡ Fe(III)/≡Fe(II) redox cycling sustains PMS activation and accounts for the catalyst's exceptional durability. The increased percentage of ≡ Fe(II) (Fig. 8(a)) confirms this inference. The enhanced Co oxidation in FeCo systems likely originates from localized electron transfer through Co–O–Fe linkages, facilitating Co(III)→Co(II) conversion without significantly altering Fe(II) electronic states, as confirmed by XPS. This mechanism is consistent with previous findings by Chen et al. [51] using CoFe₂O₄–graphene composites for norfloxacin degradation, validating the generalizability of bimetallic redox synergies in AOPs. Electrochemical characterization assessed conductivity and charge transfer kinetics. Cyclic voltammetry (CV) analysis (Fig. S8(a)) revealed ZIF-67(Co)-800 exhibits a more positive reduction potential and lower oxidation potential versus comparators, indicating enhanced electron transfer kinetics [52]—consistent with these materials' pollutant degradation efficiencies. Electrochemical impedance spectroscopy (EIS) (Fig. S8(b)) further demonstrated ZIF-67(FeCo)-800's substantially smaller semicircle diameter, signifying reduced charge transfer resistance, improved conductivity, and superior charge mobility. This facilitates more efficient PMS activation [53,54]. The collective evidence confirms the bimetallic catalyst minimizes interfacial resistance, enabling accelerated electron transfer and optimized PMS activation.

≡Co(0) + 2HSO₅⁻ → ≡Co(II) + 2OH⁻ + 2SO₄^•− (6)

≡Co(II) + HSO₅⁻ → ≡Co(III) + OH⁻ + SO₄^•− (7)

≡Fe(III) + HSO₅⁻ → ≡Fe(II) + H⁺ + SO₅^•− (8)

≡Fe(II) + HSO₅⁻ → ≡Fe(III) + OH⁻ + SO₄^•− (9)

2SO₅^•− + H₂O → 1.5¹O₂ + 2HSO₄⁻ (10)

≡Co(III) + HSO₅⁻ → ≡Co(II) + H⁺ + SO₅^•− (11)

SO₄^•− + H₂O → ·OH + HSO₄⁻ (12)

≡Co(III) + ≡ Fe(II) → ≡Co(II) + ≡ Fe(III), E^θ = 1.04 V vs NHE (13)

Fig. 7

The possible activation mechanism of PMS by ZIF-67(FeCo)-800.

Fig. 8

The XPS spectra of as-prepared and used ZIF-67(FeCo)-800 for (a) Fe 2p and (b) Co 2p.

3.5. Influence of parameters on the degradation of CIP

3.5.1. Effect of catalyst dosage

The impact of catalyst dosage on CIP degradation efficiency in the ZIF-67(FeCo)-800/PMS system is shown in Fig. 9(a). Increasing the catalyst concentration from 25 to 150 mg·L^–1 significantly enhanced CIP degradation, exhibiting a clear dose-dependent pattern. Specifically, raising the dosage from 25 to 50 mg·L^–1 improved CIP removal moderately from 84.5% to 88.2% after 120 min. Higher loadings (100 and 150 mg·L^–1) yielded substantially greater degradation efficiencies of 98.2% and 99.0%, respectively. This nonlinear response indicates optimal catalyst activity occurs within the 100–150 mg·L^–1 range under the tested conditions.

3.5.2. Effect of PMS concentration

The effect of PMS concentration on CIP degradation was investigated. As shown in Fig. 9(b), increasing the PMS concentration from 0.5 to 2.0 mmol·L^–1 significantly enhanced CIP degradation. Specifically, the degradation efficiency of CIP rose from 70.9% at 0.5 mmol·L^–1 PMS to 83.5% at 1.0 mmol·L^–1 PMS after 60 min. At a PMS concentration of 1.5 mmol·L^–1, 93.7% CIP degradation was achieved within 120 min. However, a further increase to 2.0 mmol·L^–1 PMS led to a less pronounced improvement in the degradation rate.

Fig. 9

Effects of (a) ZIF-67(FeCo)-800 dosage, (b) PMS concentration, and (c) anions on the CIP removal. (d) Recyclability of ZIF-67(FeCo)-800 for CIP removal at pH 7.0. Experiment condition: CIP = 10 mg·L^–1; PMS = 1.0 mmol·L^–1 (except b); PMS = 2.0 mmol·L^–1 (for d); ZIF-67(FeCo)-800 = 50 mg·L^–1 (except a), anions = 5.0 mmol·L^–1, T = 25 ℃, initial pH unadjusted.

3.5.3. Effect of anions

The composition of various water qualities is intricate and significantly influences chemical reactions, particularly due to the presence of diverse anions such as H₂PO₄⁻, SO₄²⁻, COOH⁻, and HCO₃⁻ which are commonly found in natural water environments [55]. Therefore, it is essential to understand how these prevalent anions affect the degradation of ciprofloxacin (CIP) within the ZIF-67(FeCo)-800/PMS system. As illustrated in Fig. 9(c), the anion H₂PO₄⁻, when present at a concentration of 5.0 mmol, demonstrates a marginal inhibitory effect on the degradation of CIP. In contrast, HCO₃⁻ shows a pronounced inhibitory impact, causing the degradation rate of CIP to diminish significantly from 88.2% to 27.4%. This substantial reduction is attributed to the interaction between the reactive radicals and HCO₃⁻ as outlined in Eqs. (14)–(17). Notably, the presence of 5.0 mmol·L^–1 SO₄²⁻ appears to enhance the degradation of CIP. Furthermore, during the initial 20 min, the addition of CH₃COO⁻ also promotes the degradation of CIP, corroborating findings from prior research [56].

H₂PO₄⁻ + ·OH ↔ H₂PO₄^• + OH⁻ (14)

H₂PO₄⁻ + SO₄^•− → H₂PO₄^• + SO₄²⁻ (15)

HCO₃⁻ + ·OH ↔ HCO₃^• + OH⁻ (16)

HCO₃⁻ + SO₄^•− ↔ HCO₃^• + SO₄²⁻ (17)

3.5.4. Reusability of ZIF-67(FeCo)-800

Catalyst reusability is critical for practical applications. The stability of ZIF-67(FeCo)-800 was evaluated over three consecutive catalytic cycles under identical conditions. After each cycle, the catalyst was recovered by filtration, rinsed with deionized water, and reused. Remarkably, CIP degradation efficiency remained at 97.8%, 92.1%, and 87.2% in the first, second, and third cycles, respectively, at an initial pH of 7.0 (Fig. 9(d)), demonstrating exceptional operational stability. Furthermore, the post-reaction XRD pattern of ZIF-67(FeCo)-800 showed remarkable consistency with that of the fresh catalyst, as evidenced by the overlapping diffraction peaks in Fig. S7, demonstrating the material's excellent structural stability. Moreover, cobalt ion leaching concentrations were 8.76 mg·L^–1 and 0.93 mg·L^–1 in the ZIF-67(Co)-800/PMS and ZIF-67(FeCo)-800 systems, respectively (Fig. S9). These results reveal that Fe stabilizes the carbon matrix, reducing Co leaching and enhancing structural stability, thereby ensuring sustained surface activity. Notably, leached metal ions contribute minimally to pollutant degradation (Fig. S10), demonstrating that heterogeneous catalysis dominates pollutant degradation.

4. Conclusions

In this study, ZIF-67 was used as a precursor to prepare ZIF-67(Fe)-800, ZIF-67(Co)-800, and ZIF-67(FeCo)-800 catalysts via pyrolysis. Characterization results obtained from XRD, XPS, SEM, and TEM revealed that the synthesized catalysts exhibited distinct morphologies, structures, and variations in catalytic performance when activating PMS, PDS, and SPC. Notably, the ZIF-67(Co)-800 catalyst, containing 70.71% cobalt, demonstrated the highest catalytic efficiency for PMS activation, achieving a CIP degradation rate constant of 0.0731 min^–1. In contrast, ZIF-67(FeCo)-800, with a lower cobalt content of 9.18%, exhibited a reduced CIP degradation rate constant of 0.0504 min^–1. While the cobalt content in ZIF-67(Co)-800 was 7.7 times greater than in ZIF-67(FeCo)-800, its CIP degradation rate constant was only 1.45 times higher. This underscores the crucial role of Fe–Co synergy in enhancing PMS activation efficiency. All three catalysts showed significantly weaker activity toward PDS and SPC activation compared to PMS. EPR and quenching tests confirmed the presence of ·OH, SO₄^•−, and ¹O₂ in the ZIF-67(Fe)-800/PMS and ZIF-67(Co)-800/PMS systems, with ¹O₂ being the primary contributor to CIP degradation. In the ZIF-67(FeCo)-800/PMS system, SO₄^•− played the dominant role in CIP degradation, with co-existing ·OH, ¹O₂, and O₂^•−. CIP degradation rates increased with higher PMS and catalyst concentrations. The addition of H₂PO₄⁻ and HCO₃⁻ inhibited CIP degradation, while SO₄^•− and CH₃COO⁻ showed slight promotion. This work provides new insights into MOF-derived catalysts and advances the understanding of AOPs for wastewater treatment.

Author Contribution

Huanxuan Li: Conceptualization, data analysis, and Funding acquisition. Xueheng Wei: Investigation, data curation. Xing Gao: Validation, visualization. Chen Liu: Developing mechanics modelling and analysis. Ning Li & Chen Xu: Catalyst preparation and data analysis. Jingang Huang: Resources, writing - review & editing. Shaodan Xu: Resources, conceptualization, supervision, Xiuyan Liu: Writing - review & editing, funding acquisition.

Acknowledgement

The authors would like to thank Qiannan Ma from Shiyanjia Lab (www. shiyanjia.com) for the XPS analysis and Ceshigo Research Service (www. ceshigo.com) for providing the Raman testing service.

Supplementary material

Supplementary materials associated with this article can be found, in the online version.

References[1] A.L. Batt, S. Kim, D.S. Aga, Comparison of the occurrence of antibiotics in four full-scale wastewater treatment plants with varying designs and operations, Chemosphere 68 (3) (2007) 428–435.[2] X.H. Li, C. Liu, Y.X. Chen, H.K. Huang, T.Z. Ren, Antibiotic residues in liquid manure from swine feedlot and their effects on nearby groundwater in regions of North China, Environ. Sci. Pollut. Res. Int. 25 (12) (2018) 11565–11575.[3] T. Pulingam, T. Parumasivam, A.M. Gazzali, A.M. Sulaiman, J.Y. Chee, M. Lakshmanan, C.F. Chin, K. Sudesh, Antimicrobial resistance: prevalence, economic burden, mechanisms of resistance and strategies to overcome, Eur. J. Pharm. Sci. 170 (2022) 106103.[4] Q.Z. Zhang, J. Tian, Y. Hu, S.L. Wu, D.Z. Chen, Co@C core-shell nanostructures anchored on carbon cloth for activation of peroxymonosulfate to degrade tetracycline, J. Environ. Chem. Eng. 11 (1) (2023) 109197.[5] Y. Huang, L.C. Nengzi, X.Y. Zhang, J.F. Gou, Y.J. Gao, G.X. Zhu, Q.F. Cheng, X.W. Cheng, Catalytic degradation of ciprofloxacin by magnetic CuS/Fe₂O₃/Mn₂O₃ nanocomposite activated peroxymonosulfate: influence factors, degradation pathways and reaction mechanism, Chem. Eng. J. 388 (2020) 124274.[6] M. Abbasi, O.S. Rizvi, G.W. Kajjumba, A. Javed, E. Khan, O. Tahir, R.S.Z. Saleem, T. Abbas, Modified biochar for lead removal from water: a critical review on synthesis, characterization, evaluation, and unveiling the adsorption mechanisms through molecular simulations, Rev. Environ. Contam. Toxicol. 263 (1) (2025) 8.

[7] H. K. Paumo, S. Dalhatou, L. Katata-Seru, B. P. Kamdem, J. O. Tijani, V. Vishwanathan, A. Kane,I. Bahadur, TiO₂ assisted photocatalysts for degradation of emerging organic pollutants in water and wastewater. J. Mol. Liq. 331 (2021) 115458.[8] Y. Wang, G. Sheng, S.Q. Xu, J.F. Wang, J.S. Yang, H. Jiang, X.D. Zhang, Microwave-assisted degradation of gatifloxacin using red mud-based perovskite catalyst: waste-treat-waste and enhanced catalytic mechanism, Surf. Interfaces 69 (2025) 106739.[9] J. Xu, G.H. Jiao, G. Sheng, N.P. Lin, B.J. Yang, J.F. Wang, H. Jiang, Y. Wang, X.D. Zhang, Microwave-assisted PMS activation Ti₃C₂/LaFe_0.5Cu_0.5O₃–P catalysts for efficient degradation of CIP: enhancement of catalytic performance by phosphoric acid etching, Sep. Purif. Technol. 354 (2025) 129357.[10] H.R. Zhang, R. Sun, T.Y. Zheng, X.L. Wang, Q.Y. Wang, L. Liu, H.Y. Yang, X. Xing, Peroxydisulfate activation by B–BC@Fe₃S₄ for norfloxacin degradation: radical and non-radical pathways, J. Water Process. Eng. 59 (2024) 105087.[11] G.J. Jiang, Y. Qi, L.Y. Qian, W. Li, Y.T. Gao, Y.C. Chen, Y.J. Cheng, X.J. Ying, S. Xie, S. Zhang, X.Y. Ye, Two birds with one stone: Co/Fe bi-MOFs@PAN nanofiber aerogel films for efficient degradation of organic contaminants and separation of emulsions, J. Water Process. Eng. 64 (2024) 105675.[12] W.Y. Huang, Z.Y. Zhang, J.W. Xu, H.P. Cui, K.X. Tang, D. Crawshaw, J.X. Wu, X.D. Zhang, L. Tang, N. Liu, Highly selective CO₂ conversion to CH₄ by a N-doped HTiNbO₅/NH₂-UiO-66 photocatalyst without a sacrificial electron donor, JACS Au 5 (3) (2025) 1184–1195.[13] J.N. Chen, Y.X. Wang, J.H. Huang, S.T. Ma, Y.Y. Zhang, F.K. Bi, X.D. Zhang, Electron beam irradiation modified UiO-66 supported Pt catalysts for low-temperature ethyl acetate catalytic degradation, Catalysts 15 (3) (2025) 220.[14] F.K. Bi, J.F. Wei, B. Gao, S.T. Ma, N. Liu, J.C. Xu, B.L. Liu, Y.D. Huang, X.D. Zhang, How the most neglected residual species in MOF-based catalysts involved in catalytic reactions to form toxic byproducts, Environ. Sci. Technol. 58 (44) (2024) 19797–19806.[15] Y.Q. Yang, B.B. Yang, X.Y. Gao, X.R. Dang, Z. Jin, X.D. Zhang, MOF-on-MOF composite material derived from ZIF-67 precursor activated by peroxymonosulfate for the removal of metronidazole, J. Water Process. Eng. 72 (2025) 107467.[16] H.X. Li, Z.X. Yang, S. Lu, L.Y. Su, C.H. Wang, J.G. Huang, J. Zhou, J.H. Tang, M.Z. Huang, Nano-porous bimetallic CuCo-MOF-74 with coordinatively unsaturated metal sites for peroxymonosulfate activation to eliminate organic pollutants: performance and mechanism, Chemosphere 273 (2021) 129643.[17] Y. Zhao, R. Gao, H. Wang, Y.P. Sun, X.H. Zhan, L. Chen, J.Y. Liu, H.X. Shi, ZIF-8-derived hollow carbon polyhedra with highly accessible single Mn–N₆ sites as peroxymonosulfate activators for efficient sulfamethoxazole degradation, Chem. Eng. J. 473 (2023) 145298.[18] Z.L. Wu, Y.P. Wang, Z.K. Xiong, Z.M. Ao, S.Y. Pu, G. Yao, B. Lai, Core-shell magnetic Fe₃O₄@Zn/Co-ZIFs to activate peroxymonosulfate for highly efficient degradation of carbamazepine, Appl. Catal. B Environ. 277 (2020) 119136.[19] X.R. Qiao, W.C. Gao, X.M. Liu, K.J. Fang, Q.J. Li, X. Lu, J.J. Si, M. Zhang, D.D. Liu, Preparation of zeolitic imidazolate framework-67/wool fabric and its adsorption capacity for reactive dyes, J. Environ. Manage. 321 (2022) 115972.[20] M. Zanaty, A.H. Zaki, S.I. El-Dek, H.N. Abdelhamid, Zeolitic imidazolate framework@hydrogen titanate nanotubes for efficient adsorption and catalytic oxidation of organic dyes and microplastics, J. Environ. Chem. Eng. 12 (3) (2024) 112547.[21] A.M. Fallatah, H.U.R. Shah, K. Ahmad, M. Ashfaq, A. Rauf, M. Muneer, M.M. Ibrahim, Z.M. El-Bahy, A. Shahzad, A. Babras, Rational synthesis and characterization of highly water stable MOF@GO composite for efficient removal of mercury (Hg²⁺) from water, Heliyon 8 (10) (2022) e10936.[22] K.A. Lin, H.A. Chang, Zeolitic imidazole framework-67 (ZIF-67) as a heterogeneous catalyst to activate peroxymonosulfate for degradation of Rhodamine B in water, J. Taiwan Inst. Chem. Eng. 53 (2015) 40–45.[23] X. Yan, Y.Y. Yao, C.M. Xiao, H. Zhang, J. Xie, S. Zhang, J.W. Qi, Z.G. Zhu, X.Y. Sun, J.S. Li, Shaping phenolic resin-coated ZIF-67 to millimeter-scale Co/N carbon beads for efficient peroxymonosulfate activation, Molecules 29 (17) (2024) 4059.[24] H.X. Li, Y.Z. Yao, J.L. Chen, C.H. Wang, J.G. Huang, J. Du, S.D. Xu, J.H. Tang, H.T. Zhao, M.Z. Huang, Heterogeneous activation of peroxymonosulfate by bimetallic MOFs for efficient degradation of phenanthrene: synthesis, performance, kinetics, and mechanisms, Sep. Purif. Technol. 259 (2021) 118217.[25] H.X. Li, Y.C. Lou, J.T. Zheng, L.Y. Su, S. Lu, C. Xu, J.G. Huang, Q.W. Zhou, J.H. Tang, M.Z. Huang, Synthesis and characterization of amino functionalized bimetallic metal–organic frameworks as peroxymonosulfate activator for water purification: performance and mechanistic studies, J. Environ. Chem. Eng. 10 (5) (2022) 108272.[26] Y. Miyah, N. El Messaoudi, M. Benjelloun, J. Georgin, D.S.P. Franco, Y. Acikbas, H.S. Kusuma, M. Sillanpää, MOF-derived magnetic nanocomposites as potential formulations for the efficient removal of organic pollutants from water via adsorption and advanced oxidation processes: a review, Mater. Today Sustain. 28 (2024) 100985.[27] T. Xiao, Y. Wang, J.Q. Wan, Y.W. Ma, Z.C. Yan, S.H. Huang, C. Zeng, Fe–N–C catalyst with Fe–N_x sites anchored nano carboncubes derived from Fe–Zn-MOFs activate peroxymonosulfate for high-effective degradation of ciprofloxacin: thermal activation and catalytic mechanism, J. Hazard. Mater. 424 (2022) 127380.[28] S.S. Gao, T.H. Li, X. Zhao, G.Z. Hu, X.Y. Cui, L.Y. Pan, T. Wågberg, Heterogeneous cobalt-based catalysts for boosting peroxymonosulfate activation: a review on cobalt-ion leaching inhibition strategies, Clean. Chem. Eng. 11 (2025) 100173.[29] D.L. Huang, G.X. Zhang, J. Yi, M. Cheng, C. Lai, P. Xu, C. Zhang, Y. Liu, C.Y. Zhou, W.J. Xue, R.Z. Wang, Z.H. Li, S. Chen, Progress and challenges of metal–organic frameworks-based materials for SR-AOPs applications in water treatment, Chemosphere 263 (2021) 127672.[30] M.M. Chen, H.Y. Niu, C.G. Niu, H. Guo, S. Liang, Y.Y. Yang, Metal-organic framework-derived CuCo/carbon as an efficient magnetic heterogeneous catalyst for persulfate activation and ciprofloxacin degradation, J. Hazard. Mater. 424 (Pt A) (2022) 127196.[31] C.Y. Xu, Q.Y. Liu, M. Wei, S.H. Guo, Y.P. Fang, Z.B. Ni, X.X. Yang, S.S. Zhang, R.L. Qiu, Co@CoO encapsulated with N-doped carbon nanotubes activated peroxymonosulfate for efficient purification of organic wastewater, Sep. Purif. Technol. 295 (2022) 121347.[32] H.X. Li, S. Lu, J.T. Zheng, N. Li, Y.C. Lou, J.H. Tang, J. Zhou, H.W. Zhang, M.Z. Huang, D. Wang, MOFs-derived hollow FeCo@C as peroxymonosulfate activator for degradation of organic pollutants: insight into the catalytic sites by experimental and theoretical study, Sep. Purif. Technol. 299 (2022) 121779.[33] J. Yan, L. Min, L. Zhu, M. N. Anjum, Z. Jing, H. Tang, Degradation of sulfamonomethoxine with Fe₃O₄ magnetic nanoparticles as heterogeneous activator of persulfate. J. Hazard. Mater. 186(2–3) (2011):1398–1404.[34] H.R. Tian, K.P. Cui, X. Chen, J. Liu, Q. Zhang, Size-matched hierarchical porous carbon materials anchoring single-atom Fe–N₄ sites for PMS activation: an in-depth study of key active species and catalytic mechanisms, J. Hazard. Mater. 461 (2024) 132647.[35] X.K. Zhang, Y.Y. Zhang, J.Q. Tian, Y.D. Guo, Z.K. Zhou, Z.Y. Liu, Z.W. Zhao, B. Liu, J. Li, Generating ¹O₂ and CoIV = O through efficient peroxymonosulfate activation by ZnCo₂O₄ nanosheets for pollutant control, Nano Res. 17 (9) (2024) 8025–8035.[36] C.W. Tang, C.B. Wang, S.H. Chien, Characterization of cobalt oxides studied by FT-IR, Raman, TPR and TG-MS, Thermochim. Acta 473 (1–2) (2008) 68–73.[37] V.G. Hadjiev, M.N. Iliev, I.V. Vergilov, The Raman spectra of Co₃O₄, J. Phys. C Solid State Phys. 21 (7) (1988) L199–L201.[38] H.X. Li, J.Q. Wan, Y.W. Ma, Y. Wang, Synthesis of novel core–shell Fe⁰@Fe₃O₄ as heterogeneous activator of persulfate for oxidation of dibutyl phthalate under neutral conditions, Chem. Eng. J. 301 (2016) 315–324.[39] R.J. Thibeau, C.W. Brown, R.H. Heidersbach, Raman spectra of possible corrosion products of iron, Appl. Spectrosc. 32 (6) (1978) 532–535.[40] H.Y. Chi, J.Q. Wan, Y.W. Ma, Y. Wang, M. Huang, X.T. Li, M.J. Pu, ZSM-5-(C@Fe) activated peroxymonosulfate for effectively degrading ciprofloxacin: in-depth analysis of degradation mode and degradation path, J. Hazard. Mater. 398 (2020) 123024.[41] H.X. Li, Y.Z. Yao, J. Zhang, J. Du, S.D. Xu, C.H. Wang, D. Zhang, J.H. Tang, H.T. Zhao, J. Zhou, Degradation of phenanthrene by peroxymonosulfate activated with bimetallic metal–organic frameworks: kinetics, mechanisms, and degradation products, Chem. Eng. J. 397 (2020) 125401.[42] Y. Zhuang, J.M. Liu, S.Y. Yuan, B. Ge, H. Du, C. Qu, H.Y. Chen, C.D. Wu, W.H. Li, Y. Zhang, Degradation of octane using an efficient and stable core-shell Fe₃O₄@C during Fenton processes: enhanced mass transfer, adsorption and catalysis, Appl. Surf. Sci. 515 (2020) 146083.[43] H.X. Li, L.Y. Su, J.T. Zheng, S. Lu, Z.X. Yang, C.H. Wang, S.D. Xu, Q.W. Zhou, J.H. Tang, M.Z. Huang, Y.Y. Zhang, MOFs derived carbon supporting CuCo nanospheres as efficient catalysts of peroxymonosulfate for rapid removal of organic pollutant, Chem. Eng. J. 451 (2023) 139114.[44] L.J. Xu, W. Chu, L. Gan, Environmental application of graphene-based CoFe₂O₄ as an activator of peroxymonosulfate for the degradation of a plasticizer, Chem. Eng. J. 263 (2015) 435–443.[45] C.Q. Tan, N.Y. Gao, D.F. Fu, J. Deng, L. Deng, Efficient degradation of paracetamol with nanoscaled magnetic CoFe₂O₄ and MnFe₂O₄ as a heterogeneous catalyst of peroxymonosulfate, Sep. Purif. Technol. 175 (2017) 47–57.[46] S. Liu, C. Liu, H.J. Zhang, W.Z. Zhang, W. Ding, H.L. Zheng, H. Li, Sulfite induced degradation of sulfamethoxazole by a silica stabilized ZIF-67(Co) catalyst via non-radical pathways: formation and role of high-valent Co(IV) and singlet oxygen, J. Hazard. Mater. 469 (2024) 133888.[47] C.J. Sui, Z.X. Nie, H. Liu, G. Boczkaj, W.Z. Liu, L.S. Kong, J.H. Zhan, Singlet oxygen-dominated peroxymonosulfate activation by layered crednerite for organic pollutants degradation in high salinity wastewater, J. Environ. Sci. (China) 135 (2024) 86–96.[48] Y.X. Huang, K.Y. Chen, S.X. Wang, S.Y. Zhao, L.Q. Yu, B.C. Huang, R.C. Jin, Synergizing electron transfer with singlet oxygen to expedite refractory contaminant mineralization in peroxymonosulfate based heterogeneous oxidation system, Appl. Catal. B Environ. 341 (2024) 123324.[49] X.J. Liu, J.B. Zhou, D. Liu, L. Li, W.B. Liu, S. Liu, C.J. Feng, Construction of Z-scheme CuFe₂O₄MnO₂ photocatalyst and activating peroxymonosulfate for phenol degradation: synergistic effect, degradation pathways, and mechanism, Environ. Res. 200 (2021) 111736.[50] S.N. Su, W.L. Guo, Y.Q. Leng, C.L. Yi, Z.M. Ma, Heterogeneous activation of oxone by Co_xFe_3−xO₄ nanocatalysts for degradation of rhodamine B, J. Hazard. Mater. 244–245 (2013) 736–742.[51] L.W. Chen, D.H. Ding, C. Liu, H. Cai, Y. Qu, S.J. Yang, Y. Gao, T.M. Cai, Degradation of norfloxacin by CoFe₂O₄-GO composite coupled with peroxymonosulfate: a comparative study and mechanistic consideration, Chem. Eng. J. 334 (2018) 273–284.[52] Y. Hu, D.Z. Chen, R. Zhang, Y. Ding, Z. Ren, M.S. Fu, X.K. Cao, G.S. Zeng, Singlet oxygen-dominated activation of peroxymonosulfate by passion fruit shell derived biochar for catalytic degradation of tetracycline through a non-radical oxidation pathway, J. Hazard. Mater. 419 (2021) 126495.[53] T. Yang, S.S. Fan, Y. Li, Q. Zhou, Fe–N/C single-atom catalysts with high density of Fe–N_x sites toward peroxymonosulfate activation for high-efficient oxidation of bisphenol A: electron-transfer mechanism, Chem. Eng. J. 419 (2021) 129590.[54] E.T. Yun, J.H. Lee, J. Kim, H.D. Park, J. Lee, Identifying the nonradical mechanism in the peroxymonosulfate activation process: singlet oxygenation versus mediated electron transfer, Environ. Sci. Technol. 52 (12) (2018) 7032–7042.[55] H.X. Li, J.Q. Wan, Y.W. Ma, Y. Wang, Z.Y. Guan, Role of inorganic ions and dissolved natural organic matters on persulfate oxidation of acid orange 7 with zero-valent iron, RSC Adv. 5 (121) (2015) 99935–99943.[56] H.X. Li, C. Xu, N. Li, S.D. Xu, C.H. Wang, J. Du, F. Li, Efficient peroxymonosulfate activation by MOFs derived cobalt–carbon composite for nonradical degradation of ciprofloxacin via Co(IV) = O, Chem. Eng. Sci. 296 (2024) 120275.

Electronic Supplementary Material

Below is the link to the electronic supplementary material

Supplementary Material 1

Yes