Degradation of aqueous cefotaxime in electro-oxidation — electro-Fenton —persulfate system with Ti/CNT/SnO2–Sb–Er anode and Ni@NCNT cathode
Introduction
Cefotaxime belongs to the category of β-lactam antibiotics and inhibits bacterial activity and growth through disrupting the synthesis of bacterial cell walls (Kummerer, 2009). Due to its broad-spectrum antimicrobial property and strong sterilization ability, the production as well as clinical and livestock consumption is presumed to be enormous. Even though the excise consumption data for cefotaxime was scarcely reported, it can be speculated that cephalosporins and penicillins together took over the second mostly used antimicrobial species in China (Ribeiro et al., 2018). In 2013, China released 53,800 tons of antibiotic into receiving environment through various sewage treatment facilities (Zhang et al., 2015). The feed concentration of cefotaxime was detected at 18.08 ± 4.02 μg L−1 and 0.55 ± 0.62 μg L−1 in the outlet of wastewater facility, which is located in a scaled pharmaceutical manufacturing park in Shijiazhuang city, China (Yu et al., 2016). In addition, 11% of 198 positive samples for cephalosporin antibiotics in the aquatic environment were demonstrated to contain cefotaxime, ranking the third after cephalexin and cefradine (Ribeiro et al., 2018). The lack of effective elimination of cephalosporins before wastewater discharge can subsequently lead to their wide occurrence in natural environments (Zhang et al., 2017; Wang et al., 2018). Furthermore, recent studies indicated that the phototransformation and incomplete hydrolysis of cephalosporin antibiotics in aqueous environment resulted in higher toxicity (Jiang et al., 2010; Wang and Lin, 2012; Zhang et al., 2017). Thus, highly effective treatment strategies in the abatement of cephalosporins are urgently needed.
Advanced oxidation processes (AOPs) have been developed for the removal of trace contaminants in aqueous environment (Watkinson et al., 2007; Liu et al., 2009; Wang and Wang, 2016). The main mechanism of AOPs (ozonation, Fenton oxidation, electrochemical oxidation et al.) is the production of strongly oxidizing species hydroxyl radical (OH). Meanwhile, AOPs of sulfate radicals have also attracted lots of attention and been conducted to decontaminate antibiotics such as carbamazepine and ibuprofen (Liu et al., 2018; Park et al., 2018).
The combination of electro-oxidation (EO) and electro-Fenton (EF) was regarded as a highly effective, environmental friendly and economical process (Martínez-Huitle et al., 2015). The continuous production of H2O2 and regeneration of Fe2+ at the cathode could decrease the addition of chemical substances, thus led to avoid the generation of secondary pollutants (He et al., 2014; Long et al., 2019). On the other hand, sulfate radicals (SO4−) can be generated from persulfate (S2O82−, PS) in the electrochemical process, as shown in Eq. (1). The SO4− has a competitive redox potential and much extended longevity (3–4 × 10−5 s) compared with OH (2 × 10−8 s), and is expected to be a more suitable alternative to treat antibiotics (Devi et al., 2016). Furthermore, it was found the S2O82− could be regenerated in the electrochemical system, thus lead to reduce the addition of PS chemicals (Eq. (2)). Previous studies have demonstrated the enhanced removal efficiency towards antibiotic ampicillin and sulfamethoxazole in the combined electrochemical oxidation — persulfate processes using boron-doped diamond anode and carbon anode, respectively (Frontistis et al., 2018; Song et al., 2018).
Therefore, the purpose of this study was to investigate the removal and mineralization of cefotaxime in an EO-EF-PS system (Govindan et al., 2014; Long et al., 2019). It is expected that the triple coupling system lead to an increased TOC removal rate because electrons and Fe2+ can simultaneously activate S2O82−, as shown in Eq. (3). In addition, previous studies demonstrated SO4− was more efficient at the elimination of aromatic ring or the double bond of the β-lactams which were the featured structure of cephalosporins (He et al., 2014). As known, SnO2–Sb electrode was considered to be one of the most promising dimensional stable anodes (DSA) electrode for application in EO because of its “non-active” surface, higher oxygen evolution potential (OEP), low cost and toxicity (Li et al., 2017; Moreira et al., 2017). However, the shortcomings of SnO2–Sb electrode were its insufficient electrochemical oxidation ability and short service life, which are both important for industrial application (Xu et al., 2017a). Some strategies have been recommended to enhance the performance of SnO2–Sb electrode, most focused on intermediate layers or catalytically active layers (Subba Rao and Venkatarangaiah, 2014; Xu et al., 2019). Based on these, Ti/CNT/SnO2–Sb–Er plate electrode was fabricated in this study to be utilized as anode. The CNT interlayer and addition of ErO2 may attribute to an enlarged specific surface area and higher oxygen evolution potentials. So the production capability of •OH on the anode can be further strengthened (Wang et al., 2010; Wu et al., 2017). Herein, a novel Ni encapsulated nitrogen doped carbon nanotubes (Ni@NCNT) was adopted as cathode (Kang et al., 2018). Because high content of nitrogen dopant is proved to greatly enhanced the catalyzing ability towards H2O2 production and PS activation (Li et al., 2018; Wu et al., 2018), the Ni@NCNT catalysts were prepared through a direct pyrolysis of a mixture of melamine and NiCl2·5H2O.
Altogether, the main goal of this study was to evaluate the removal of cefotaxime with novel Ti/CNT/SnO2–Sb–Er anode and Ni@NCNT cathode in EO-EF-PS system. The effects of main operating parameters (e.g., initial concentration of PS and Fe2+, current density, pollutant initial concentration, methanol and TBA) on EO-EF-PS system performance were optimized systematically. Last but not the least, a proposed degradation pathway of cefotaxime was presented.
Section snippets
Chemicals and materials
All chemicals and reagents were used without any further purification, some of which were analytical grade. SnCl4·5H2O, SbCl3 and Er(NO3)4 were obtained from Sigma-Aldrich’s. NiCl2·6H2O (≥98.0%), melamine (C3N6H6, ≥99.5%), Na2SO4 (≥98.0%), Na2S2O8 (≥98.0%), and Nafion (D520CS, 5%, Dupont) were obtained from Sinopharm Chemical Reagent Co. Ltd; FeSO4 (≥99.0%) and H2SO4 (98%) were purchased from Tianjin Fuchen Chemical Reagents Factory. Multi-walled carbon nanotubes (MWCNTs) were purchased from
Surface characterization and elemental analysis
The surface morphology represents the abundance of catalyzing active sites and structural stability. SEM images of Ti/SnO2–Sb, Ti/CNT/SnO2–Sb, Ti/SnO2–Sb–Er and Ti/CNT/SnO2–Sb–Er electrodes were shown in Fig. 1. It could be seen from Fig. 1a and b that the traditional Ti/SnO2–Sb electrode presented a compact and stratified microstructure with 0.5 μm particle diameter. Nearly half of the SnO2–Sb nanoparticles can reach this size level. As seen in Fig. 1c and d, the co-electrodeposition with Er(NO
Conclusions
Novel Ti/CNT/SnO2–Sb–Er anode electrode was prepared by modifying CNTs and ErO2 elements, and utilized to degrade cefotaxime. The materials were characterized by means of SEM and XPS. Linear voltammetry, cyclic voltammetry hydroxyl radical production test and accelerated lifetime test with Ti/CNT/SnO2–Sb–Er anode were performed, illustrating advanced catalyzing and stability features. Meanwhile, novel cathode material Ni@NCNT was tested by TEM, XPS and H2O2 production examination. Then
CRediT authorship contribution statement
Jiawei Lei: Data curation, Investigation, Writing - original draft. Pingzhou Duan: Data curation, Investigation, Writing - original draft. Weijun Liu: Data curation, Investigation, Formal analysis. Zhirong Sun: Methodology, Writing - review & editing. Xiang Hu: Supervision, Conceptualization, Methodology, Writing - review & editing.
Declaration of interests
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgement
We thank the National Natural Science Foundation of China (Project No. 51978030 and Project No. 51278022) for financial support.
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