Light-excited photoelectrons coupled with bio-photocatalysis enhanced the degradation efficiency of oxytetracycline
Graphical abstract
Introduction
Antibiotics are manufactured and prescribed to control bacterial infections in humans and agriculture. An estimated 200 000 tons of antibiotics are produced per year, tetracyclines being the most commonly used (Vaz et al., 2015). Oxytetracycline, one of the most widespread prophylactic antibiotics used in personal care products for its broad-spectrum activity and low cost, has been detected in soil, water, and even food products, in concentrations ranging from ppb to ppm (Ashfaq et al., 2017; Li et al., 2004). The polyaromatic ring-structure of oxytetracycline confers its chemical stability and relatively long half-life in the environment, which promotes the terrifying for antibiotics pollution (Yan et al., 2018).
A number of chemical oxidation methods (e.g., photolysis (Zhao et al., 2013), photocatalysis (Chen et al., 2016) and photo-Fenton oxidation (Pereira et al., 2014)) have been applied to study the removal of oxytetracycline in water. Although these processes are fast and robust, full mineralization is economically prohibitive and practically difficult (Li et al., 2011). To overcome these drawbacks, the intimately coupled photocatalysis and biodegradation (ICPB) process emerged and had been used widely (Ding and Zhao, 2017; Marsolek et al., 2008; Rittmann, 2017). In this process, photocatalysts and biofilms were coated onto the same carrier, wherein, the photocatalytic performance occurred on the outer surface, while biodegradation took place inside (Li et al., 2011; Marsolek et al., 2008; Wen et al., 2012). The biofilm was well sheltered from toxicants and oxidants due to the protection of the carrier. As the combined degradation process was repeated, compounds were effectively degraded.
Significant efforts have been made to promote the mineralization ability of the combined system, including the following effective strategies: building carriers that are less susceptible to deterioration (Wen et al., 2012), fabricating new types of photocatalysts (Zhang et al., 2016), increasing the loading rate of the photocatalyst to maintain photocatalytic efficiency (Li et al., 2012a), and providing extra electron donors to maintain a robust biofilm community (Xiong et al., 2018). So far, the combined degradation systems have been investigated for denitrification (Wen et al., 2012), dechlorination (Zhou et al., 2017), and degradation of dyes (Li et al., 2012b) and antibiotics (Xiong et al., 2017), revealing their potential for applications in practical wastewater treatment.
In this study, we provided a simple protocol to modify the ICPB process and enhance the degradation efficiency of oxytetracycline. As shown in Scheme 1, light excitation of photoelectrons coupled with bio-photocatalysis was developed as a synergistic degradation method. During the degradation process, a carrier with higher porosity than those utilized in previous works was used. Instead of protecting biofilms by placing them inside carriers, the photocatalysts and microorganisms were coated full of the carriers. The transfer of the light excited photoelectrons between photocatalysts and microorganisms occurred under visible light irradiation. Thus, photocatalytic degradation, microbial metabolic degradation, and the transfer of photoelectrons between photocatalysts and microbes as an assisted degradation method occurred simultaneously.
Many studies have illustrated the sterilization effect of photocatalysis. However, it has been demonstrated that microorganisms could use light excited electrons through the photocatalysis of semiconductors to stimulate growth, sustain cellular metabolism, regulate community structure, and contribute to environmental remediation (Duan et al., 2013; Lovley, 2011; Lu et al., 2012; Sakimoto et al., 2016). In addition, although microbes should have been protected inside the carriers in the previous works, few bacteria were still found on the outer surfaces of carriers (Li et al., 2012a; Zhou et al., 2015), indicating the survival of microbes and their resistance to negative influences. To demonstrate the feasibility of this protocol, the degradation efficiency, evolution of byproducts, the stability of degradation performance for oxytetracycline, and bio-transformation during the process were evaluated. The results confirmed that stimulating the transfer of photoelectrons between photocatalysts and microbes was a useful method to enhance the degradation efficiency of the bio-photocatalysis process.
Section snippets
Chemicals and reagents
The oxytetracycline (purity of 95.6%) and β-apo-oxytetracycline (purity of 95.6%) standards were sourced from Dr. Ehrenstorfer GmbH (Augsburg, Germany). The methanol, acetonitrile, and formic acid used were of high-performance liquid chromatography (HPLC) grade and were purchased from Merck KGaA (Darmstadt, Germany). The other chemicals and reagents were of analytical grade and purchased from Sinopharm Group Co. Ltd. (Shanghai, China). Ultrapure water was used throughout the experiment.
Carrier and coating of the photocatalyst
The
Enhanced removal of oxytetracycline by synergistic degradation
For the static test, the concentration change protocols for different mechanisms during short-term experiments (10 h) are shown in Fig. 1a. Light excitation of photoelectrons coupled with bio-photocatalysis was used as a synergistic degradation method. In addition, the influence of absorption, photolysis, biodegradation, and photocatalytic degradation of oxytetracycline had to be taken into consideration.
For the absorption performance, the final removal rates of the polyurethane carriers and Bi
Conclusions
In this study, a simple protocol was developed to enhance the bio-photocatalytic degradation efficiency of oxytetracycline by stimulating the transfer of photoelectrons between the photocatalysts and microbes using a higher porosity carrier (95%). The protocol did not involve the additional of supplementary electron donors for microbe metabolism or improving the loading rate of the photocatalysts. Although exposure to toxicants and oxidants, the negative influence on microbes could be weakened.
Acknowledgement
This work was supported by NSFC (21777155 and 41471260), the Xiamen Science and Technology Program (3502Z20172027) and the Fujian STS Project (2016T3032).
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