Elsevier

Chemosphere

Volume 283, November 2021, 131251
Chemosphere

Characterization of enoxacin (ENO) during ClO2 disinfection in water distribution system: Kinetics, byproducts, toxicity evaluation and halogenated disinfection byproducts (DBPs) formation potential

https://doi.org/10.1016/j.chemosphere.2021.131251Get rights and content

Highlights

  • The destruction rate of ENO was higher in DI water than that in pilot-scale WDS.

  • ENO destruction was affected by pH, flow velocity and pipe materials.

  • Nine intermediates were identified in ENO destruction by ClO2.

  • ENO influence on halogenated disinfection byproducts formation can be ignored.

  • ENO destruction by ClO2 would increase the potential risk of water quality safety.

Abstract

Enoxacin (ENO) is widespread in water because it is commonly used as a human and veterinary antibiotic. However, little effort has been dedicated to revealing the transformation mechanisms of ENO destruction using ClO2, especially within a water distribution system (WDS). To address this knowledge gap, the kinetics, byproducts, toxicity, and formation potential of halogenated disinfection byproducts (DBPs) associated with ENO destruction using ClO2 in a pilot-scale PE pipe was explored for the first time. Statistical analyses showed that the destruction efficiency of ENO in the pilot-scale PE pipe was lower than that in deionized water (DI water), and the reactions in DI water followed the second-order kinetic model. Furthermore, pH has a significant effect on the destruction of ENO, and the removal ratio increased at a higher pH. Additionally, increasing the flow rate elevated the ENO removal efficiency; however, the influence of flow velocity was limited to ENO destruction. The ENO removal rates within the diverse pipes exhibited the following order: stainless steel pipe < PE pipe < ductile iron pipe. Nine possible intermediates were identified, and those that were formed by piperazine group cleavage represented the major primary byproducts of the entire destruction process. Additionally, the ENO destruction in a pilot-scale PE pipe had minimal influence on halogenated DBPs and chlorite formation. Finally, the toxicity evaluation illustrated that the presence of ENO increased the potential risk of water quality safety when treated with ClO2.

Introduction

Chlorination is the most commonly used disinfection technique for water treatment to prevent the spread of pathogenic microorganisms and bacteria (Gao et al., 2018; Zhang et al., 2019). However, a large amount of chlorine produces undesirable halogenated disinfection byproducts (DBPs), which are potentially carcinogenic, cytotoxic, genotoxic, and developmentally toxic (Richardson and Kimura, 2016; Sharma et al., 2014; Yang and Zhang, 2013; Wagner and Plewa, 2017). As an alternative disinfectant, chlorine dioxide (ClO2) is widely applied to reduce DBP formation (He et al., 2019; Hua and Reckhow, 2007; Shah et al., 2012; Han and Zhang, 2018; Han et al., 2021). ClO2 is sometimes utilized as a secondary disinfection approach to protect the water distribution system (WDS) from microbiological recontamination and fouling (Lv et al., 2021; Lu et al., 2012; Tao et al., 2018). The ClO2 concentrations in the finished water of the treatment plant are lower than 0.8 mg/L (USEPA, 1999). Therefore, ClO2 prevents the recontamination of microbes and reacts with organic matter to form DBPs when finished water is transferred to the WDS, and some of these DBPs can substantially affect human health (Volk et al., 2002).

Numerous studies have been carried out to evaluate the effect of ClO2 on disinfecting organic matter, particularly emerging contaminants that are not encompassed by legislation (Jia et al., 2018a; Navalon et al., 2008). However, most previous studies have been based on laboratory-scale experiments (e.g., ultrapure water, buffered water, or deionized water (DI water)), and the results deviate from those observed for actual WDS wherein changes in pH value, flow rate, or pipe material may affect the disinfection of ClO2 in WDS. Unfortunately, to the best of our knowledge, minimal investigations on ClO2 disinfection of emergent pollutants within the WDS have been carried out in published studies. Therefore, disinfection using ClO2 for removing fluoroquinolones (FQs, a typical type of emergent pollutant) in WDS, its related intermediates, and halogenated DBPs was first evaluated in this study.

As a class of synthetic broad-spectrum antibacterial pharmaceuticals, FQs are commonly used in many human and veterinary applications (Ling et al., 2017; Yassine et al., 2017). However, not all FQs can be metabolized adequately in human and animal bodies, and more than 70% of these compounds are excreted into municipal wastewater systems through feces and urine (Pan et al., 2021). Since FQs cannot be removed completely in conventional wastewater treatment processes, they are frequently measured within the water environment, with a range of concentrations in the ng/L-mg/L scale (Nasuhoglu et al., 2012; Speltini et al., 2010; Sturini et al., 2012; Yahya et al., 2014). FQs are prevalent in the water environment, which raises the possibility of bacterial drug resistance (Jia et al., 2018b; Johnson et al., 2015). The World Health Organization (WHO) reports that bacterial resistance to antibiotics poses a significant threat to human health (Pruden et al., 2013).

FQs are usually discovered in sources of drinking water (Annabi et al., 2016; Riaz et al., 2017; Yahya et al., 2014). When water containing FQs is handled using the traditional management system in water treatment plants, only approximately 15.5–73.6% of FQs are removed (Lia et al., 2018). Even though advanced treatment processes have been applied in some water treatment plants, FQs cannot be removed entirely. According to a study by Vieno, when ozonation was applied to water treatment plants, approximately 16% ciprofloxacin (CIP) was removed (Vieno et al., 2007). In addition, most of the FQs could be removed effectively when activated carbon filtration was used; however, the removal ratio of enrofloxacin was lower than 10% (Xu et al., 2015). This suggests that the FQs could not be eradicated from the finished water. A previous investigation reported that FQs concentrations in finished water ranged between 0 and 126.43 ng/L (Zhang et al., 2018). Therefore, the ClO2 contained within the finished water may react with the remaining FQs after it is delivered into the WDS. As a result, we must pay particular attention to the final FQ state during ClO2 disinfection to ensure drinking water safety.

Enoxacin (ENO) is a third-generation FQs, containing a naphthyridine ring in its structure (Sortino et al., 1998). Owing to its good absorption and low efficiency of adverse reactions (Liu et al., 2010), ENO is commonly used as a human and veterinary antibiotic to treat respiratory, urinary, skin, and gastrointestinal systems by inhibiting bacterial DNA-rotase in cells (Tong and Xiang, 2007). ENO cannot be adequately metabolized by humans and animals, resulting in its widespread disposal in water (Mabel et al., 2018; Tong and Xiang, 2007). In China, the concentration of ENO in water bodies can reach up to 448 μg/L (Bu et al., 2013). However, there is a lack of knowledge on the effectiveness of ClO2 in removing ENO, especially in WDS. Therefore, the removal ratio of ENO oxidation, formation of intermediates and halogenated DBPs, and potential toxicity risks associated with ENO destruction by ClO2 in WDS were first assessed in this study.

The specific aims of this study were to (i) examine ENO destruction in WDS as a function of varying ClO2 concentration, pH, flow velocity, and pipe material; (ii) explore the possible transformation products formed during the ENO destruction and identify the proposed reaction pathways; (iii) evaluate the formation of halogenated DBPs and chlorite during the oxidation of ENO by ClO2; and (iv) assess the toxicity variation during ENO destruction.

Section snippets

Reagents and materials

All chemicals were obtained from various manufacturers and were used directly without further purification. ENO (purity > 98%), 6 %–14% sodium hypochlorite (NaClO) solution, methyl tertiary butyl ether (MTBE), acetonitrile, and methanol of HPLC grade were purchased from Aladdin (Shanghai, China). Five halogenated DBP standards (three THMs, four HAAs, two HKs, one halogenated aldehyde, and nine HANs) with gas chromatographically pure mixture were provided by Cansyn (Toronto, Canada). The

ClO2 concentration

ENO destruction by ClO2 was investigated in DI water and a pilot-scale PE pipe at ClO2 concentrations ranging from 0.3 mg/L to 1.3 mg/L. Fig. 1 shows that the ENO degradation efficiency increased monotonically as the ClO2 concentration increased in both DI water and the pilot-scale PE pipe. When ClO2 concentration elevated in the range of 0.3–1.3 mg/L, ENO removal efficiency after 180 min increased from 38.0% to 64.3% in the pilot-scale PE pipe and from 45.7% to 68.4% in the DI water

Conclusions

This study elucidated the ENO destruction using ClO2 in both DI water and a pilot-scale PE pipe. Under the experimental conditions used in this experiment, the removal ratio of ENO by ClO2 in DI water was higher than that in the pilot-scale PE pipe, and the reactions between ENO and ClO2 in DI water followed the second-order kinetic model. The destruction of ENO using ClO2 showed a strong pH dependence as the destruction efficiency increased with a rise in pH. Additionally, pipe material and

Credit author statement

Guilin He: Methodology, Formal analysis, Writing – original draft, Funding acquisition. Tuqiao Zhang: Supervision, Funding acquisition, Conceptualization, Qingzhou Zhang: Investigation, Writing – review & editing, Feilong Dong, Methodology, Formal analysis, Yonglei Wang, Project administration, Supervision, Resources.

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.

Acknowledgments

This work has been funded by the Doctoral Research Fund of Shandong Jianzhu University (X20038Z0101); the China Postdoctoral Science Foundation (No. 2018M632465) and the Funds for International Cooperation and Exchange of the National Natural Science Foundation of China (51761145022).

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