Bamboo charcoal fiber bundles loaded MOF-derived magnetic Co/CoO porous polyhedron for efficiently catalytic degradation of tetracyclines hydrochloride

With the development of modern society, environmental pollution has attracted more and more attention from society (Daghrir & Drogui 2013; Hu et al. 2019; Zhao et al. 2022). In recent decades, tetracycline hydrochloride (TCH) has been extensively utilized to deal with animal and human diseases, or as an additive to promote the rapid growth of poultries and aquatic animals or plants (Yang et al. 2018; Zhang et al. 2022c). It was reported that TCH has the following characteristics: high solubility, stable chemical structure, and non-biodegradable (Daghrir & Drogui 2013; Xu et al. 2022). High concentrations of TCH were detected in different water systems such as wastewater, groundwater, and surface water (Xu et al. 2020). However, the dissolution and metabolism of TCH in the environment cause a serious threat to the human body.

Until now, many technologies have been developed to effectively control and degrade TCH pollutants, including adsorption, biodegradation, catalytic oxidation exchange, membrane filtration, ozonation, and electrolysis (Lv et al. 2020). Generally speaking, advanced oxidation processes (AOPs) (Peng et al. 2021; Cai et al. 2022) have the advantages of simple and convenient operation, environment-friendly, pollution-free, and a high catalytic degradation efficiency. Oxidatively active species (reactive oxygen species, ROS) can be produced in AOPs to effectively destroy the stable chemical structure of organic pollutants and prompt them to mineralize into CO₂ and H₂O (Duan et al. 2020).

The peroxymonosulfate-based AOPs (PMS-AOPs) have gained wide attention due to the asymmetric structure of PMS that is more easily activated to generate ROS. Additionally, the transportation and storage of PMS are more convenient, and there are various ways to activate PMS, such as thermal activation, ultrasound-assisted activation, and ultraviolet radiation (Song et al. 2023). As the process of external energy supply involves many uncontrollable factors, its operation is complicated and inconvenient, and the potential for widespread usage is restricted. Transition metals and ions, including Ni, Mn, Fe, Co, Ag, Co2+, Ni2+, Fe3+, Mn2+, V3+, and Fe2+ (Hu et al. 2019; Kang et al. 2019; An & Xiao 2020), especially Co/Co2, with particular emphasis on Co/Co2+, have the potential to activate PMS and produce highly active substances. The utilization of transition metals has emerged as an alternate option for proficiently eliminating and breaking down contaminants through the activation of PMS.

In recent years, MOF/PMS-AOPs have attracted an increasing amount of interest. Metal-organic frameworks (MOFs) with three-dimensional pore structures (Wang et al. 2021c; Xue et al. 2022; Zhang et al. 2022b; Cao et al. 2023) are a kind of coordination polymers that developed rapidly in the last two decades. The huge specific surface area and regular rhombic dodecahedral crystal shape of Co-based ZIFs (ZIF-67) (commonly used MOF material) are undoubtedly a good choice for catalyst materials (An et al. 2018), and numerous carbon-based materials have been utilized as support templates to evenly distribute ZIF metal oxides to improve their stability and reusability (Xu et al. 2022). However, the poor stability of ZIF-67, easy overflowing of cobalt, and inconvenient separation from the solution will affect the repetitive applicability of catalysts based on the MOF and cause recontamination. In order to conquer these drawbacks, nowadays, high-temperature treatments have been applied for improvement (Long et al. 2022). It is reported that carbon and metal nanoparticles were derived from MOFs with organic linkers by a pyrolysis process. Metal nanoparticles could cause carbon graphitization and be controlled in graphite carbon (Yang et al. 2021). The unstable carbon atom causes the orderly transformation from a disorder layer structure to a graphite crystal structure and provides a large number of ion-operative pathways after the pyrolysis process (Yang et al. 2021).

Carbon precursors are generally used as catalysts (carbon nanotubes, graphene (oxide), and fullerene); however, the synthetic process is expensive and difficult for scale-usage (Xie et al. 2022), and thus the selection of suitable carbon-based carrier materials as support template has become a hot concern. Biomass fibers show the merit of widespread and relatively uncomplicated compositions, which are utilized as raw materials of carbon to acquire biochar fiber with a huge surface area and chemical stability (Ye et al. 2020). However, bamboo fiber bundles (BFBs) are obtained by a top-down method that is a simpler and more cost-effective method, and BFBs are acquired without further assembly (Jakob et al. 2022; Lamaming et al. 2022). In addition, fiber bundles are derived from bamboo with microfibril bundles, hollow parenchymal cell framework structures, and the graphitization of BFB can confine metal ions or metal nanoparticles within the graphite carbon. Additionally, BFB have unique microfibril bundles with hollow parenchymal cell framework structures as favorable embedding areas of active sites for the catalytic degradation of contaminants. Besides, carbon-based catalysts tend to mediate a non-radical pathway of singlet oxygen oxidation (⁠⁠). Additionally, the metal ions obtained by pyrolysis reduction are magnetic, which are easy to recover and reuse. However, there are few studies (Zhao et al. 2019; Wang et al. 2021b; Liao et al. 2022) on bamboo microfibril bundles as carriers for transition metal directly for the catalytic degradation of TCH.

As is well known, there is a mass of harmful viruses and bacteria in the surrounding water. Harmful bacteria and viruses would multiply in large numbers, bringing enormous damage to the health of human beings, and also deteriorates the water environment. Therefore, antibacterial property plays an important role in water treatment. General biochar itself does not kill microorganisms in large quantities and is not effective for long periods (Zhao et al. 2021); however, bamboo charcoal fiber bundles have particular bacteriostatic and Co ion also has bactericidal effects (Dai & He 2019). The Co ion penetrates into the bacterial cell wall and inhibits the enzyme activity, leading to the death of bacteria. The combination of bamboo charcoal fiber bundles and Co doubtlessly can be a novel antibacterial material.

In this study, BFBs were selected as the carbon material substrate for coating of ZIF-67 on bamboo fiber bundles (BFB@ZIF-67) via in-situ growth, and then MOF-derived magnetic Co/CoO nanoparticles with porous polyhedron (BCFB@PCo/CoO) were obtained after pyrolysis of ZIF-67 on BFB. The porous polyhedron structure of the ZIF-67 without agglomeration of nanoparticles was maintained after pyrolysis. Crystalline phase, morphology, pore structure, and composition are characterized. In addition, catalytic degradation of the BCFB@PCo/CoO catalysts, cycling stability, and antibacterial properties of the BCFB@PCo/CoO composite were investigated. The degradation mechanism of TCH in the presence of BCFB@PCo/CoO/PMS was discussed. This work provided a novel BCFB@PCo/CoO composite material to activate PMS to effectively promote the catalytic degradation of TCH for sewage purification. The above-mentioned results are important for novel bamboo fiber-based catalysts and extend the utilization of biomass.

All the materials, reagents, and chemicals were of analytical reagent (AR) grade and used directly without further purification. In this work, bamboo was chosen from Yibin of Sichuan, China. CH₃COOH, NaOH, MeOH, NaClO₂, EtOH, Na₂SO₄, NaCl, NaH₂PO₄, Na₂CO₃, peroxymonosulfate (PMS, 2KHSO₅·KHSO₄·K₂SO₄, ≥99.5%, AR), C₄H₆N₂ (98%), Co(NO₃)₂·6H₂O(99%), tert-butyl alcohol (TBA, C₄H₁₀O, AR), 1,4-benzoquinone (p-BQ, C₆H₄O₂, AR), L-histidine (C₆H₉N₃O₂, AR), tetracyclines hydrochloride (TCH), 5,5-dimethyl-1-pyrroline N-oxide (DMPO), 2,2,6,6-tetramethyl-4-piperidine (TEMP), and high-purity nitrogen (N₂, ≥99.99%) were used in the experiment.

In brief, BFBs were obtained by removing hemicellulose and lignin from natural bamboo (NB). Bamboo bulks (5 mm5 mm20 mm) were treated with 3.5 wt% NaOH solution at 90 °C for 12 h to remove hemicellulose and then washed with deionized (DI) water and immersed in 1 wt% NaClO₂ solution at 85 °C for 12 h to remove lignin. Meanwhile, CH₃COOH was used to adjust the pH value (approximately 4.5) and this process was operated twice. The bamboo bulks were then rinsed in DI water to remove residues. Bamboo bulk templates were obtained by freeze-drying bamboo bulk. Finally, the bamboo bulks were twisted into BFB.

1.312 g of (2-MI) and 1.164 g of Co (NO₃)₂ 6H₂O were placed in methanol and named as solution A and solution B, respectively. Later, solution A was poured into the beaker containing solution B, and the mixture was stirred at 120 rpm for 10 min. The purpose of this step was to facilitate better coordination of Co²⁺ ions with the 2-MI (2-mercaptobenzimidazole) organic ligand. 0.3 g of BFB were then added to the mixture and stirred for 5 min. Finally, ZIF-67 was grown in-situ on the BFB after being kept at room temperature for 24 h. Subsequently, BFB@ZIF-67 samples were washed several times in the methanol solution to remove residues. BFB@ZIF-67 was then formed after freeze-drying. Finally, a certain amount of BFB@ZIF-67 was heated under a nitrogen atmosphere at 800 °C and the final product BCFB@PCo/CoO was obtained.

The morphology of BFB, BFB@ZIF-67, and BCFB@PCo/CoO was tested by scanning electron microscopy (SEM). X-ray diffraction (XRD) was utilized to study the crystal structure. The chemical state of BCFB@PCo/CoO was investigated by X-ray photoelectron spectroscopy (XPS). Element distribution was characterized by energy-dispersive spectra (EDS). Special surface area and pore size were determined on a Brunner–Emmet–Teller analyzer (Gemini VII 2390, USA). The chemical structure of BCFB@PCo/CoO was confirmed by Fourier transform infrared spectroscopy (FTIR). The absorbency of TCH was determined using a UV–vis spectrophotometer at 357 nm wavelength. Radicals and non-radicals in the reaction system were detected for the identification of activated species and mechanisms were involved by electron paramagnetic resonance (EPR, BRUKER, EMX-PLUS). The magnetic property of the sample was investigated by a vibrating sample magnetometer (VSM) (LakeShore7404). Analysis of TCH intermediates was carried out by a liquid chromatograph tandem mass spectrometer (Agilent Technologies, 6545 Triple Quad LC-MS).

The catalytic degradation process of BCFB@PCo/CoO for TCH was taken in a beaker. The pH value was regulated with HCl and NaOH. The catalyst was added to the TCH solution and stirred for blending, and then 0.4 g/L of PMS was added. Absorbance before and after degradation was measured at 357 nm by a UV–vis spectrophotometer.

The crystallinity of BFB, BFB@ZIF-67, and BCFB@PCo/CoO is presented in Figure 3(a). Three obvious crystallization peaks of BFB at 2θ = 16.0°, 22.49°, and 35.0° are ascribed to typical cellulose I peaks (Johar et al. 2012; Cui et al. 2021). Additionally, several new peaks at 2θ < 26.0° appear in the XRD patterns of BFB@ZIF-67, which agree well with (002) and (112) crystal faces of ZIF-67, indicating that ZIF-67 are loaded on BFB successfully (Hou et al. 2020). Peak at 26.0° belongs to the (002) plane of amorphous carbon (Gu et al. 2022) in the XRD pattern of BCFB@PCo/CoO, indicating that graphitic carbon is obtained after carbonization. The phenomenon can be explained by the presence of a carbon source and the added Co can be used as a catalyst to generate graphitized carbon. For BCFB@PCo/CoO, the diffraction peak of ZIF-67 disappears and the peak of metallic Co is observed because ZIF-67 decomposes after calcination at high temperature. The diffraction peaks at 44.21° and 51.52° (JCPDS No.15-0806) belong to diffraction peaks at (111) and (200) of metal elemental Co (Hua et al. 2022; Wang et al. 2022a). Obviously, two peaks at 36.50° and 42.39° correspond to (111) and (200) crystal planes of CoO because Co²⁺ partly turns to Co on the surface of the catalyst after calcination at high temperature due to redox oxidation. In other words, the calcination of ZIF-67 inevitably dislodges its organic skeleton, thus releasing volatile gases such as CO₂ and H₂O, which reduce Co²⁺ to Co, indicating that metallic Co and CoO concurrently exist in BCFB@PCo/CoO (Xu et al. 2022). The Co nanoparticles and Co²⁺ ions could combine with the neighboring O atoms to form Co–O bonds and generate more catalytic active sites.

Chemical structures of BFB, BFB@ZIF-67,and BCFB@PCo/CoO are illustrated in Figure 3(b). For BFB, several peaks at 1,670–1,760 cm⁻¹ are assigned to the stretching vibration of C = C/C = O. Absorption bands located at approximately 675–870 cm⁻¹ are attributed to C–H bending vibration modes. The vibration band of the catalyst at 3,429 cm⁻¹ indicates the existence of the –OH, and the absorption bands of BFB@ZIF-67 and BCFB@PCo/CoO are located at approximately 573–667 cm⁻¹, which is attributed to Co–O vibration modes. The result indicates the existence of a Co²⁺ characteristic peak (Liu et al. 2020a) after carbonization. Subsequently, BCFB@PCo/CoO shows good catalytic activity of PMS activation for target pollutant degradation.

XPS spectra further confirm the chemical valence of the BCFB@PCo/CoO composite. The spectrum survey XPS (Figure 3(c)) shows that the sharp peaks at 298.12, 545.12, and 781.41 eV can be attributed to the C 1s, O 1s, and Co 2p of the BCFB@PCo/CoO catalyst. Three main peaks of the Co2p spectra at 780.25, 783.61/797.30, 781.73/795.13, and 787.02/803.50 eV are assigned to Co⁰(Co 2p_3/2), Co²⁺ (Co 2p_3/2 and Co 2p_1/2), Co³⁺ (Co 2p_3/2 and Co 2p_1/2), and satellite peaks (Figure 3(d)), respectively (Li et al. 2022a; Xu et al. 2022; Zhang et al. 2022c). The XPS peak of C 1s is presented in Figure 3(e). Three peaks at 284.79, 285.62, and 288.60 eV can be attributed to C–C/C = C, C–N, and C–O/C = O, respectively (Li et al. 2022b; Thanh-Binh et al. 2022). C = O sites contribute to generation. Three main peaks in the O 1s spectrum at 533.30, 532.83, and 531.96 eV are assigned to absorbing oxygen (O ads), Co–O and –OH of BCFB@PCo/CoO as shown in Figure 3(f), respectively. Co and Co–O (Co²⁺) hold a rapid reaction rate with PMS due to the unique properties in the process of degradation (Jin et al. 2023).

The specific surface area and the distribution of pore size of BCFB@PCo/CoO are important for catalytic performance. Parameters of the BET surface area and the average pore size distribution of BFB, BFB@ZIF-67 and BCFB@PCo/CoO are shown in Table 1. The specific surfaces of the BFB and BFB@ZIF-67 are 3.68 and 66.01 m²/g, respectively. However, the specific surface of BCFB@PCo/CoO arrives at 302.41 m²/g, which is greatly higher than those of BFB and BFB@ZIF-67 because the Co nanoparticles and CoO nanoparticles are dispersed on the surface of bamboo charcoal fiber bundles. A larger specific surface area is conducive to the enrichment of active sites, which means that BCFB@PCo/CoO can provide active sites for PMS activation (Zhang et al. 2022a). In addition, according to the average pore diameter, the mesopores of BFB could provide a great deal of sites for loading ZIF-67. Additionally, the pores of BFB@ZIF-67 and BCFB@PCo/CoO are mainly composed of mesopores. The mesoporous structure of BCFB@PCo/CoO shows a higher specific surface area and more catalytic active sites; therefore, Co and CoO nanoparticles are generally distributed on the fiber bundles, which is conducive to the catalytic degradation of pollutants.

The effect of BCFB@PCo/CoO dosages on TCH removal is presented in Figure 4(c). It is obvious that the removal efficiency increases significantly with the increase in the BCFB@PCo/CoO catalyst dosage. Only 97% of TCH can be removed after PMS is added by 0.1 g/L of BCFB@PCo/CoO within 15 min, but TCH can be almost completely removed when 0.2 g/L of BCFB@PCo/CoO is used and the removal efficiency reaches 99.94% after PMS is added within 15 min. Additionally, the k_obs value increases from 0.04127 to 0.3681 min⁻¹ as shown in Figure 4(d). The result can be interpreted as that the catalyst provides a mass of surface active sites in the whole system with the increase in BCFB@PCo/CoO as the reaction proceeds. In addition, removal efficiency increases with the increase in the amount of the BCFB@PCo/CoO dosage because a mass of radicals and non-radicals are produced by activating PMS, which leads to the increase in the TCH removal efficiency. However, the catalytic degradation performance slightly decreases by overproducing oxidizing radicals caused by the excessive addition of the BCFB@PCo/CoO catalyst, which promotes interaction among the radicals (Equations (4)–(6)). 0.2 g/L of BCFB@PCo/CoO catalyst achieves the desired catalytic degradation; therefore, 0.2 g/L BCFB@PCo/CoO is used in the subsequent test.

The effect of temperature on TCH degradation is shown in Figure 5(c). Obviously, the degradation efficiency of TCH increases under the catalytic action of the catalyst with the change of temperature from 5 to 25 °C and slightly decreases from 25 to 45 °C. These results indicate that the temperature plays an important role in PMS activation to catalyze organic pollutants. The possible reason may be attributed to the high temperature. PMS can quickly contact the catalyst surface to promote electron transfer in radical products, and pollutant molecules are simultaneously affected to move faster at high temperatures (Li et al. 2023b). However, the catalytic degradation performance slightly decreases by overproducing oxidizing radicals caused by higher temperatures, which promotes interaction among the radicals.

The degradation efficiency of BCFB@PCo/CoO/PMS for real water was also investigated to evaluate the potential of BCFB@PCo/CoO. Tap water, river water, rainwater, and well water were used as the solvents of the TCH solution to investigate the influent of different water quality on TCH degradation as shown in Figure 5(d). The BCFB@PCo/CoO/PMS system still could maintain a high removal capacity toward TCH in tap water, river water, rainwater, and well water with degradation degrees of 96, 91.8, 93.7, and 92.5%, respectively. It is found that the degradation efficiency for TCH in river water decreases distinctly, which might be related to the presence of high-concentration ions and reducing matters in the river water. The results further imply that BCFB@PCo/CoO/PMS possesses the general applicability for practical water treatment.

Natural organic matter (NOM) almost exists in wastewater everywhere (Jin et al. 2023). As an example, humic acid (HA) is chosen to study how NOM affects AOPs. The competing degradation of HA and TCH with the produced radicals and non-radicals may be the reason for the decrease in TCH degradation efficiency from 99.94 to 42.15% when the concentration of HA is 20 mM as shown in Figure 5(e). The presence of HA would dramatically increase the consumption of the generated ROS, inhibiting the breakdown of the TCH contaminants.

In addition, the TCH removal rate in the BCFB@PCo/CoO/PMS system was still higher than 78% after five recycles, as shown in Supplementary material, Figure S4a. The results demonstrate that the BCFB@PCo/CoO/PMS system has an excellent reusability and can become a good alternative novel product. The magnetic characterization of the BCFB@PCo/CoO composite was determined using VSM at room temperature as shown in Supplementary material, Figure S4b. The saturation magnetization strength (Ms) of BCFB@PCo/CoO is 4.09 emu/g, which allows for fast and easy separation of BCFB@PCo/CoO from aqueous solutions under an applied magnetic field. Supplementary material, Figure S3b shows that BCFB@PCo/CoO can be attracted by the magnet, and thus, the BCFB@PCo/CoO composite shows good potential in removing pollutants from aqueous solutions due to the property of easy separation.

A composite of MOF-derived magnetic Co/CoO nanoparticles with porous polyhedron was successfully embedded in bamboo charcoal microfibril bundles (BCFB@PCo/CoO) with hollow parenchymal cell frameworks derived from bamboo charcoal fiber bundles. The result shows that the specific surface area of the obtained BCFB@PCo/CoO is 302.41 m²/g. The degradation efficiency of BCFB@PCo/CoO/PMS for TCH could reach 99.94% within 15 min. Additionally, BCFB@PCo/CoO/PMS is less affected by environmental temperature, co-existing anions (except Cl⁻), humic acid in water, and different practical natural water, and shows a high efficiency in a broad pH range of 1.0–9.0. Results of quenching and EPR technology demonstrate that ⁠, ·OH, ⁠, and are involved in PMS activation and both radical and non-radical pathways presented a superior performance of the BCFB@PCo/CoO/PMS system in TCH degradation. Co and CoO nanoparticles were still well loaded on bamboo charcoal fiber bundles after the catalytic reaction. The possible degradation pathways were explored by liquid chromatography–mass spectrometry (LC–MS) analysis including the cycles of Co⁰ → Co²⁺⇋Co³⁺. The high degradation efficiency was maintained well after five recycles. The proposed BCFB@PCo/CoO composite demonstrates high degradation efficiency toward TCH, good regeneration capability, good antibacterial properties, and low secondary contamination toward the environment and it could be effectively recycled and reused by magnetic separation. Based on the intermediates measured by HPLC-MS, three degradation pathways of TCH were proposed. Therefore, the BCFB@PCo/CoO will be a potential catalyst for the purpose of purification of tetracycline-containing domestic/industrial/medical wastewater.

All authors contributed to the study's conception and design. Material preparation, data collection, and analysis were performed by L.D., C.C., M.Y., and S.J. The first draft of the manuscript was written by L.D. The investigation and methodology by J.L. The conceptualization, project administration, funding acquisition, supervision, writing review, and editing by R.G. All authors commented on previous versions of the manuscript and all authors read and approved the final manuscript.

This work was supported by the Science and Technology Planning Project of Sichuan Province (No. 2020YFN0150), Cooperation Project between Sichuan University and Yibin City (2020CDYB-5), the Opening Project of Jiangsu Engineering Research Center of Textile Dyeing and Printing for Energy Conservation, Discharge Reduction and Cleaner Production (ERC) (SDGC2223), and Yibin unveiling and commanding project (00308055A1282).

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.