Photoelectrocatalytic coupling system synergistically removal of antibiotics and antibiotic resistant bacteria from aquatic environment
Graphical Abstract
Introduction
In recent years, due to the abuse of antibiotics, antibiotic resistance has attracted widespread attention all around the world (Li et al., 2020). Chloramphenicol (CAP), as a broad-spectrum antibiotic with excellent antibacterial properties, is widely used to inhibit both Gram-positive, Gram-negative bacteria and other microorganisms in many countries and fields. CAP antibiotic resistant bacteria and CAP antibiotic resistance genes have become one of the major threats to ecological health (Chen et al., 2019, Balbi, 2004). Moreover, the rapid propagation of antibiotic resistant bacteria (ARB) (Ibáñez et al., 2013) and the vertical (cell division), horizontal (combination, transduction and natural transformation) gene transfer of antibiotic resistance genes (ARGs) allows the antibiotic resistance to spread widely among different hosts, which indirectly harm human health (Dodd, 2012). The aquatic environment, especially in municipal wastewater, as the ultimate recipient of the antibiotics and the main transmission medium of ARB and ARGs, was heavily polluted (Biancullo et al., 2019). Although a variety of wastewater treatment methods have been employed to remove the contaminants, the concentration of ARB and ARGs in the effluent is still at high level.
Nowadays, the shortage of water resources has made wastewater recycling (reclaimed water) increasingly common to alleviate water shortages. However, extensive research has shown that there are 1000 times more pathogenic bacteria in reclaimed water than in drinking water (Deng et al., 2019), moreover, a large number of ARGs were detected in the reclaimed water (Chaplin et al., 2012, Chen and Zhang, 2013). When reclaimed water was used in many activities, such as garden irrigating, car washing, landscape water replenishing, etc, humans are easily exposed to antibiotics, ARB, and ARGs through inhalation, dermal absorption, and ingestion (Wang et al., 2014), providing a pathway for the spread of antibiotic resistance to humans. Therefore, the safety of reclaimed water must be taken into consideration, and efficient and rapid approaches to degrade contaminants, ARB and ARGs in reclaimed water are in urgent demand (Iakovides et al., 2019).
The traditional methods for treating reclaimed water include biological treatment (activated sludge process, membrane bioreactor process), advanced treatment technologies (membrane filtration, activated carbon adsorption, reverse osmosis) and advanced oxidation processes (AOPs) (such as ozone treatment, chlorination treatment, electrocatalysis, photocatalysis, photo-Fenton reaction, UV/H2O2) (Girardi et al., 2011, Gerrity et al., 2011, Paul et al., 2010). Among them, instead of effectively removing ARB and ARGs, the biological treatment method can increase the concentrations of ARB and ARGs due to the high levels of activated sludge (Ezzariai et al., 2018, Krzeminski et al., 2019, Li et al., 2017). Besides, chlorination disinfection, membrane treatment, adsorption treatment are of no effect on removing ARB and ARGs and preventing the spread of antibiotic resistance (Sadiq and Rodriguez, 2004, Munir et al., 2011, Zheng et al., 2018).
The photocatalytic technology has been comprehensively researched due to its strong oxidation capability, fast reaction rate, and wide range of action (Varnagiris et al., 2020). However, the low separation efficiency of electron-hole (e--h+), the fast recombination speed and the limited light width hinder the application of photocatalytic technology (Sun et al., 2016). Photoelectrocatalytic (PEC) process combined photocatalytic with electrocatalysis process, could use potential bias to transfer photo-generated electrons from the conduction band of the photoanode to the external circuit and then to the reference electrode. Hence, the degradation efficiency and capacity in the PEC process were greatly improved, and ARB, ARGs were effectively removed compared to photocatalytic and electrocatalysis processes alone (Jiang et al., 2017). Although the PEC process owned higher oxidative degradation ability than other processes, it may be difficult to remove the contaminants that have strong oxidation resistance, while these contaminants were generally easier to be reductively degraded. The chemical degradation of CAP was more difficult than many other types of antibiotics because it involved reductive dechlorination and oxidative mineralization (Yu et al., 2019).
Therefore, this study used a proton exchange membrane (PEM) to divide the PEC system into an anode chamber and a cathode chamber, and used TNTs-Ag/SnO2-Sb as the anode, plus simulated natural light and current. The anode chamber had a strong oxidation ability, which removed most contaminants, ARB and ARGs. Ti-Pd/SnO2-Sb was used as the cathode. Among them, palladium (Pd)-based catalysis had emerged as a promising water treatment strategy, because the supported-Pd and Pd-based bimetallic catalysis activated dihydrogen (H2) and catalyze reduction transformation of contaminants in the aquatic environment (Deng et al., 2019). Especially under the action of an external current, the cathode chamber had a strong reducing ability, which removed contaminants that were difficult to degrade in the anode chamber. This system had strong oxidizing and reducing ability, which could solve the problem of ARB and ARGs pollution in reclaimed water, and remove most of the pollutants.
In this study, TNTs-Ag/SnO2-Sb anode and Ti-Pd/SnO2-Sb cathode were synthesized and characterized, and a novel type of PEC system was constructed. The efficiency of removing CAP, CAP resistant bacteria, and resistant genes at the same time was studied in PEC system. The influences of current density and electrolyte concentration on the removal of CAP and ARB in PEC system were investigated. The redox performance of the PEC system was verified by adding persulfate. The gel electrophoresis images of PCR amplification products of ARGs were used to evaluate the ability of different systems to damage DNA. Escherichia coli without ARGs was used to explore the ecotoxicity of treated CAP solution by PEC. The possible degradation pathways of CAP in the PEC system were also proposed based on the degradation products (DPs) identified by liquid chromatography-tandem mass spectrometry (LC-MS/MS).
Section snippets
Chemicals and reagents
CAP(> 99%), ethylene glycol, ammonium fluoride, SnCl4•5H2O, SbCl3, citric acid, AgNO3, NaClO4, ethylenediamine, terephthalic acid (TPA), ethanol, H2O2, PMS (KHSO5•0.5KHSO4•0.5K2SO4, KHSO5 ≥ 42%), PDS (K2S2O8), ChamQ SYBR Color qPCR Master Mix (2X) and 30% hydrogen peroxide were purchased from Sigma Aldrich (Shanghai, China). Chromatographic grade acetonitrile, methanol and methylene chloride were all purchased from Merck (Shanghai, China). The chloramphenicol-resistant E. coli (Rosetta (DE3)
Characterization of electrodes
The surface morphology and structure of electrodes were observed by SEM as shown in Fig. 1. The excellent performance of the electrode needed to have an active and stable electrode surface, and the radiation in the reactor needed to be uniform over the whole surface of the catalyst (Zhang et al., 2009). The prepared titanium dioxide nanotube arrays were neatly arranged and had uniform tube diameters of about 80 nm (Fig. 1 (a1)). The SEM image (Fig. 1 (a2)) of TNTs-Ag/SnO2-Sb indicated that
Conclusion
In this study, a novel PEC system was constructed using TNTs-Ag/SnO2-Sb anode and Ti-Pd/SnO2-Sb cathode. The PEC system had an excellent ability to remove contaminants and sterilize bacteria compared to photocatalytic and electrocatalysis systems. And it also could synergistically remove antibiotics, ARB and ARGs with strong oxidation ability in the anode. The cathode with excellent reducing performance could further degrade contaminants, which has great oxidation resistance. This PEC system
Declaration of Competing Interest
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.
Acknowledgements
This research was sponsored by the National Natural Science Foundation of China (grants 51878321, 21866017, and 41761092) and the Applied Basic Research Foundation of Yunnan Province (grant 2018FA007).
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