Elsevier

Chemosphere

Volume 286, Part 1, January 2022, 131559
Chemosphere

Influence of chemical speciation on enrofloxacin degradation by UV irradiation: Kinetics, mechanism and disinfection by-products formation

https://doi.org/10.1016/j.chemosphere.2021.131559Get rights and content

Highlights

  • The degradation of ENR by UV irradiation was the faster at pH 7.

  • Effects of NOM and common anions on degradation rate of ENR were evaluated.

  • 1O2 dominated ENR degradation under neutral and alkalinity condition.

  • The DBPs formation during UV pretreatment and post chlorination in different pH were discussed.

Abstract

Fluoroquinolones (FQs) were frequently detected in aqueous environment. The UV irradiation have been reported as an efficient method for FQs degradation. This study investigated the influence of chemical speciation on enrofloxacin (ENR) photolysis process by UV irradiation. The results showed that chemical speciation of ENR significantly affected the photodegradation kinetics, and the highest degradation rate was observed in the zwitterion form. Presence of natural organic matter (NOM) and inorganic anions had different degrees of influences on ENR photodegradation for three chemical speciation of ENR. The contribution of 1O2 on ENR degradation in neutral and alkalinity condition was significantly higher than that in acidic condition. The cation and zwitterion of ENR was beneficial to the formation of trichloromethane (TCM) and haloacetonitrile (HAN) during the chlorination alone. Compared with the chlorination of ENR, the UV pretreatment respectively caused 4.06-fold and 3.14-fold decrease in TCM formation at acidic and neutral reaction condition during subsequent chlorination. Also the decrease in HAN formation at neutral and alkalinity condition was found after UV treatment followed by chlorination. The UV pretreatment caused higher yield of HAN in the subsequent chlorination at acidic condition than that at neutral and alkalinity condition. Through the UV pretreatment at neutral condition, the generated concentration of halonitromethane (HNM) reached the maximum value during the subsequent chlorination. Potential toxic risk analysis showed the toxicity decreased in zwitterion form of ENR, while toxicity increased in cationic and anionic form after UV irradiation pretreatment.

Introduction

Fluoroquinolones (FQs) are a class of broad-spectrum antibiotics, and which are widely used to treat respiratory diseases and promote the growth of food-producing animals (Cullmann et al., 1993; Wammer et al., 2013; Geng et al., 2020). Due to its high consumption in humans and animals, large quantities of FQs have been discharged to aquatic environment from hospital wastewater, fish or animal farms and drug manufacturing plants (Van Doorslaer et al., 2014; Ding et al., 2020). It was reported that FQs had been detected at the influent of wastewater treatment plant (WWTP) with the level ranging from ng L−1 to μg L−1, and a higher concentration (31 mg L−1) was also founded at effluent of pharmaceutical plant (Watkinson et al., 2009; Sciscenko et al., 2020). Because of the long environmental persistence and structural stability, The removal of FQs via conventional treatment processes (coagulation, sedimentation, filtration) and biodegradation is limited (Michael et al., 2013). It has been reported that these antibiotics have potential adverse effects on humans and aquatic organisms, some researcher found FQs could accelerate the production of resistance strains and inhibit the growth of M. rosenbergii (Davide. Calamari, 2003; Zhang et al., 2019a). Therefore, it is necessary to search an efficient treatment process to remove FQs.

Compared with conventional physical-chemical and biotic treatment processes, photolysis (e.g. Ultraviolet irradiation) had some advantages including high inactivation efficiency of pathogenic microorganisms, few yield of disinfection byproducts, low cost of operation and small space-occupancy (Geng et al., 2020). It is preferentially considered as a green and efficient process to remove FQs (Sciscenko et al., 2020). Several previous studies have explored the degradation of FQs by sunlight irradiation, and pointed that FQs containing the chromophores can efficiently absorb the wavelength range (<780 nm) (Babic et al., 2013). Ge et al. found the FQs (gatifloxacin and balofloxacin) could be effectively degraded by sunlight and its half-lives was ranging from 14.5 to 169 min (Ge et al., 2015). Wang et al. reported that ordered mesoporous g-C3N4 could effectively degrade FQs through simulated sunlight irradiation (Wang et al., 2018). However, the degradation efficiency of FQs significantly decayed by sunlight alone in complex aquatic environment. Photolytic materials were expensive, and could not be applied on a large-scale. In sharp contrast to the low energy and long wavelength of sunlight, ultraviolet (UV) was an energetic short-wavelength light. Thus, more energy could be absorbed by FQs for photodegradation. To date, the research on the removal of FQs by UV light is limited.

Previous studies have proved that the photochemical degradation reaction of FQs had three types: direct, indirect and self-sensitized photolysis (Ge et al., 2015). Direct photolysis could be attributed to the cleavages, substitutions and rearrangements of the FQs, which followed a pseudo first-order kinetic model (Serna-Galvis et al., 2017). To the best of our knowledge, the change of pH values resulted in the formation of different dissociated species of FQs (cationic, zwitterionic, anionic and neutral form) (Zhang et al., 2019b), thus influencing the degradation of FQs by direct photolysis to some certain degree. The cation and anion of ENR (enrofloxacin) could be generated by the protonation of external amino group on the piperazine ring and deprotonation of carboxylic acid group, respectively. Zwitterion of ENR could be produced through the deprotonation of the carboxylic acid and protonation of amino group. Lizondo et al. thought that the proportion of neutral form was low and could be ignored. The different dissociated speciation of ENR was exhibited in Fig S1 (M6nica Lizondo, 1997). Usually, the change of structural prosperity for ENR will inevitably cause the changing of molar absorptivity (εFQs) and quantum yield (ΦFQs). Previous study had reported that the highest molar absorptivity (εFQs) will occur at neutral, followed by basic and then acidic reaction condition (Liang et al., 2015). Snowberger et al. also found the highest molar absorptivity that occurred at neutral, followed by basic and then acidic condition (Snowberger et al., 2016). Geng at al. Reported the pseudo first-order rate constant decreased with the increase of FQs concentration, which implied the self-sensitized photolysis also had an important contribution on the degradation of FQs (Geng et al., 2020) besides the direct photolysis. Moreover, the self-sensitized photolysis of FQs was attributed to the generation of hydroxyl radical (•OH) and singlet oxygen (1O2) in the solution through energy transfer process, which had been demonstrated to contribute to the photodegradation of FQs (Sturini et al., 2012; Zhang et al., 2019b). However, the indirect photolysis for FQs need to have the synthetic water matrices, namely the indirect photolysis of FQs tend to appear in natural water which contains other co-existing species (Guo et al., 2017; Ge et al., 2018; Deng et al., 2019). That is to say, the direct and self-sensitized photolysis is superior to indirect photolysis in terms of requirement of water matrix.

Recently, the UV irradiation based advanced oxidation has been regarded as a critical drinking water pre-treatment process, and subsequent disinfectants could react with organic pollutants to form disinfection by-products (DBPs) (Lu et al., 2018). The DBPs formation is related with the physicochemical property of contaminant. Shah et al. pointed that halonitriles and haloamides will generate through decarboxylation and aldehyde pathway (Shah and Mitch, 2012). Also UV irradiation could change the structure of precursors, and thus enhanced the formation of carbonaceous DBPs (C-DBPs) and nitrogenous DBPs (N-DBPs) (Li et al., 2020b). Recently, the formation of DBPs has focused on the synergistic UV treatment processes, such as UV/persulfate, UV/chlorine, UV/H2O2 (Dotson et al., 2010; Hua et al., 2019; Sun et al., 2019). However, the effect of UV pre-treatment alone on the DBPs formation during ENR chlorination was less investigated.

In this object, ENR was selected as the target contaminant to investigate and compare the degradation efficiency of ENR under different pH. The contribution of direct and self-sensitized photolysis for ENR degradation was investigated. The common anions and natural organic matters (such as fumic acid (FA), chloride (Cl), nitrate (NO3), bromide (Br) and iodide (I)) were also examined. In addition, the DBPs formation under different pH and potential toxicity risk of DBPs were investigated respectively.

Section snippets

Chemical and reagents

All reagents were at least analytical grade, unless specifically described. Enrofloxacin (ENR), furfural alcohol (FFA), Formic acid (FA), sodium bromide (NaBr), ascorbic acid, sodium phosphate dibasic (Na2HPO4) and sodium phosphate monobasic (NaH2PO4) were obtained from Shanghai Macklin Biochemical Co., Ltd. Potassium iodide (KI), hydrochloric acid (HCl), sodium hydroxide (NaOH), sodium chloride (NaCl) and potassium nitrate (KNO3) were purchased from Sinopharm Chemical Reagent Co., Ltd.

Compared the ENR degradation under different pH condition

The degradation of ENR fitted well with the pseudo-first-order kinetic, and the degradation rate (kobs,ENR) was related to the variation of pH (Fig S2). As pH increased from 3.0 to 11.0, the degradation rate of ENR was 2.29 × 10−3 min−1, 5.06 × 10−3 min−1, 3.28 × 10−2 min−1, 2.62 × 10−2 min−1, 2.16 × 10−2 min−1, respectively (Fig. 1). The highest degradation rate was found at pH 7.0, which was 10.0 times higher than the observed degradation rate at pH 3.0. This phenomenon was consistent with

Conclusion

The ENR photodegradation fitted well with the pseudo-first order kinetic model. The highest degradation rate was observed at zwitterion form of ENR, followed by anionic and cationic forms. In acidic condition, the presence of Br and NO3 could enhance the ENR degradation in varying degrees. For the neutral pH, the presence of FA could obviously inhibit the degradation of ENR while the I could slightly accelerate ENR degradation. As the ENR existed in the deprotonated form, the presence of NO3

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 was financially supported by China Postdoctoral Science Foundation (2018M641983) and the Shanghai Sailing Plan for the Young Scientific Talents (19YF1419900). We also thank the reviewers and editors for their valuable advice to improve this manuscript.

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