Carbon nanofibers supported Co/Ag bimetallic nanoparticles for heterogeneous activation of peroxymonosulfate and efficient oxidation of amoxicillin

https://doi.org/10.1016/j.jhazmat.2020.123290Get rights and content

Highlights

  • CNFs supported Co/Ag bimetallic catalyst was synthesized for PMS activation.

  • Co@CNFs-Ag showed efficient and stable catalytic activity for multiple runs.

  • Hydroxyl and sulfate radicals were identified and the latter played a major role.

  • Ag/Ag(I) could convert Co and Co(III) into Co(II) to improve catalytic efficiency.

  • Based on the identified intermediates, three degradation pathways were proposed.

Abstract

The carbon nanofibers supported Co/Ag bimetallic nanoparticles (Co@CNFs-Ag) were synthesized for heterogeneous activation of peroxymonosulfate and efficient oxidation of amoxicillin in this work. Co nanoparticles with a diameter of 20–30 nm were encapsulated in the carbon nanofibers to reduce the loss of Co during the preparation and catalysis processes. Ag nanoparticles (5–10 nm) were distributed on the surface of CNFs. Complete removal of amoxicillin could be achieved within 30 min by Co@CNFs-Ag activated peroxymonosulfate system. The high catalytic performance could be attributed to the large aspect ratio (> 10,000) of the carbon nanofibers and the mutual reaction of the Co/Ag bimetallic nanoparticles with peroxymonosulfate. The optimal mass ratio of oxidant and catalyst was 10 and the optimized pH was 7. Co@CNFs-Ag exhibited stable catalytic activity and minimal metal leakage over a period of 5 cycles. The activation energy of the system was 29.51 kJ/mol derived by the Arrhenius equation. Both hydroxyl and sulfate radicals contributed to amoxicillin degradation and the latter were key to the degradation. Finally, the reaction mechanism of bimetallic synergistic catalytic system and possible amoxicillin degradation pathways were elucidated. The results of this study provide novel insights for application of sulfate radical-based advanced oxidation processes in environmental remediation.

Introduction

Antibiotics are extensively utilized in human medicine, aquaculture, and livestock feeding to treat microbial infectious diseases (Chen et al., 2018a). Overuse of antibiotics and their subsequent discharge into wastewater result in widespread distribution of antibiotics in various natural matrices (Ashfaq et al., 2019; Leung et al., 2012). Worldwide researches indicate that considerable antibiotics (10−450 μg/L) were found in water bodies (Wang et al., 2020; Chen et al., 2020). Although there are no strict regulations on the maximum residual level of antibiotics allowed in water at the present stage, the EC50 values (concentration giving half of the maximum response) of some antibiotics were tested for non-target organisms. For example, the EC50 of amocicillin is less than 10 μg/L (Danner et al., 2019). Antibiotic pollution, specifically the generation of antibiotic resistant genes and bacteria, is detrimental to both ecological environment and human health (Kumari and Kumar, 2020; Chen et al., 2020). Antibiotics generally exhibit biological effects to exempt biological processes and the physical treatment methods are also inefficient to remove antibiotics (Wang and Wang, 2016). Most emerging wastewater treatment technologies, such as membrane filtration, electrochemical processes and UV irradiation, also have their shortcomings in treating antibiotic pollution, such as concentration polarization, the need of energy and complex instruments, respectively. (Emamjomeh et al., 2019; Ensano et al., 2019; Moarefian et al., 2014; Naghdali et al., 2020). Therefore, degradation of antibiotics is still challenging for practical wastewater treatment (Gothwal and Shashidhar, 2015).

The sulfate radical-based advanced oxidation processes (SR-AOPs) are eco-friendly and efficient for degrading or mineralizing recalcitrant pollutants (including antibiotics) due to fast reaction rates, strong oxidation ability (E0 = 2.5∼3.1 V), and long half-life of sulfate radicals (Ghanbari and Moradi, 2017; Hao et al., 2019; Wang and Zhuan, 2020; Zhang et al., 2015). According to the previous studies, SR-AOPs exhibit excellent performance to remove various antibiotics, such as β-lactams, 4-quinolones and macrolides antibiotics (Wang et al., 2019b; Chen et al., 2018b). However, direct reactions between peroxymonosulfate (PMS) and contaminants are slow and inefficient, so several approaches, including energy and transition metal catalysis, have been proposed to activate the PMS for subsequent generating of reactive species. Co(II) has been reported as the most effective catalyst for PMS activation; however, homogeneous catalytic systems can cause secondary water pollution (Anipsitaki and Dionysiou, 2003; Anipsitakis and Dionysiou, 2004; Wang et al., 2011, 2018a, 2018b). Therefore, intensive studies have focused on heterogeneous catalytic activation.

Bimetallic catalysis is also an effective way to reduce the leaching of ion compared with mono-metallic catalysis (Chen et al., 2018c; Wu et al., 2018). Co/Fe bimetallic catalysts were most widely used in SR-AOPs (Cai et al., 2015; Li et al., 2020). Nevertheless, Co/Fe systems indicate high magnetism and agglomeration, thus inevitably reducing catalytic activity (Cai et al., 2014; Duan et al., 2019; Yang et al., 2020). Therefore, non-magnetic metals are receiving increasing attention in this regard. In particular, Ag nanoparticles, which are characterized by abundant active sites as well as optimal thermal and electrical conductivities, are widely used in catalytic systems (Haneda and Towata, 2015; Wang et al., 2013; Zhang et al., 2011). Ag nanoparticles not only accelerate energy conversion and electron transfer but also effectively adjust the valence states of catalysts to achieve efficient catalysis (Veisi et al., 2019; Wang et al., 2018a). According to our literature survey, few reports have been devoted to the application of Co/Ag bimetallic nanoparticles catalyst, especially for SR-AOPs (Holewinski et al., 2014; Fernández José et al., 2005).

Moreover, immobilizing nanoparticles onto various support materials can improve dispersion and prevent the dissolution of ions, thus ensuring highly efficient catalysis performance at small dosages (Wang et al., 2019c; Shukla et al., 2010; Zhu et al., 2017). Carbon nanofibers (CNFs) can be utilized as supporters due to their large aspect ratio, high mechanical strength, corrosion resistance, optimal thermal and electron conductivities (Chinthaginjala et al., 2007; Thakur et al., 2010; Yu et al., 2008). The large aspect ratio provides adequate space for reaction, thus resulting in uniform distribution of the catalyst (Tsuji et al., 2015). CNFs supported catalysts have shown to exhibit long-term repeated use under extreme reaction conditions owing to the high mechanical strength and corrosion resistance of the CNFs (Van et al., 2014). Therefore, the CNFs could provide an ideal support structure for bare Co/Ag nanoparticles (Lin et al., 2018; Lubej et al., 2016; Saputra et al., 2012). It is worth to mention that electrospun CNFs using the incorporating approach were superior because the metal particles can be encapsulated in the CNF matrices, which leads to a more stable performance than other supporting catalysts (Zhang et al., 2018a).

In this study, a CNFs supported Co/Ag bimetallic nanoparticles (Co@CNFs-Ag) catalyst was synthesized by electrospinning, carbonization, and chemical reduction for heterogeneous activation of the PMS. Subsequently, the composites were characterized by multiple techniques. The performances of the catalysis were evaluated via amoxicillin degradation efficiencies because amoxicillin is a widely used antibiotic which exerts a significant environmental impact even at low concentrations. Moreover, reaction mechanisms were proposed based on quenching experiments, electron spin resonance spectroscopy (ESR) and X-ray photo-electron spectroscopy (XPS) analyses. Finally, amoxicillin degradation pathways were elucidated via degradation intermediate analysis. The study might improve the application of SR-AOPs in degradation of antibiotics and alleviate their risk to ecological systems.

Section snippets

Materials

All chemicals were analytical grade or higher and utilized without further purification. Amoxicillin, polyacrylonitrile (PAN, Mw 150,000), N,N-dimethylformamide (DMF), and 5,5-dimethy/L-pyrrolidine-N-oxide (DMPO) were purchased from Sigma-Aldrich (St. Louis, MO, USA). NaOH, HCl, Co(NO3)2, AgNO3, and NaBH4 were purchased from National Medicines Co., Ltd (Shanghai, China). High-performance liquid chromatography (HPLC)-grade tert butyl alcohol, acetonitrile, methanol, and formic acid were acquired

Characterization of catalysts

The morphologies and structures of NFs, Co@CNFs, and Co@CNFs-Ag were elucidated by the SEM and TEM images. As indicated in Fig. 2, all of them retained the original nanofiber structure with an average diameter of ∼150 nm and length spanning several millimeters. The results suggested that the large aspect ratio which more than 10,000 could efficiently enhance the dispersal and stabilization of the nanoparticles. The NFs were randomly distributed due to bending instability resulting from

Conclusions

In this study, Co@CNFs-Ag catalyst was used to activate PMS. The composites were successfully synthesized by electrospinning, carbonization, and chemical reduction. Co (20–30 nm) and Ag (5–10 nm) nanoparticles were immobilized onto the CNFs to avoid aggregation and the special composite structure could reduce ion leakage during the preparation and catalysis processes. Complete amoxicillin degradation was achieved in 60 min with an activation energy of 29.51 kJ/mol and the degradation efficiency

Declaration of Competing Interest

We claim no conflicts of interest with other people or organizations that can inappropriately influence our work.

Acknowledgements

This work was supported by the Beijing Natural Science Foundation (8202029), the National Key R & D Program of China (2018YFD0900805), the National Natural Science Foundation of China (Nos. U19A20107, 41831283 and 21307005), the Beijing Municipal Science and Technology Project (Z181100005518012), the 111 Project (B18006) and Beijing Advanced Innovation Program for Land Surface Science.

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