Ciprofloxacin transformation in aqueous environments: Mechanism, kinetics, and toxicity assessment during •OH-mediated oxidation

doi:10.1016/j.scitotenv.2019.134190

Science of The Total Environment

Volume 699, 10 January 2020, 134190

https://doi.org/10.1016/j.scitotenv.2019.134190 Get rights and content

Highlights

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Mechanism and fate of ^•OH-degradation of CIP are studied in water environments.
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The ^•OH preferentially attacks the piperazine ring by H-abstraction pathways.
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Reaction rates are more dependent on the concentration of ^•OH than temperature.
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The peroxy radicals prefer to form HO₂^• by direct concerted elimination mechanism.
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The hydroxyl-substituted CIP has a higher activity than its parent compound.

Abstract

The initial reactions of organics with ^•OH are important to understand their transformations and fates in advanced oxidation processes in aqueous phase. Herein, the kinetics and mechanism of ^•OH-initiated degradation of ciprofloxacin (CIP), an antibiotic of fluoroquinolone class, are obtained using density functional and computational kinetics methods. All feasible mechanisms are considered, including H-abstraction, ^•OH-addition, and sequential electron proton transfer. Results showed that the H-abstraction is the dominant reaction pathway, and the product radicals P7 single bond H, P9H, and P10H are the dominating intermediates. The aqueous phase rate coefficients for the ^•OH-triggered reaction of ciprofloxacin are calculated from 273 K to 323 K to examine the temperature dependent effect, and the theoretical value of 6.07 × 10⁹ M⁻¹ s⁻¹ at 298 K is close to the corresponding experimental data. Moreover, the intermediates P7 single bond H, P9H, and P10H could easily transform to several stable products in the presence of O₂, HO₂^•, and ^•OH. The peroxy radical, which is generated from the incorporation of H-abstraction product radicals (P7H, P9H, and P10H) with O₂, prefers to produce HO₂^• into the surrounding through direct concerted elimination rather than the indirect mechanism. In addition, the peroxy radical could react with HO₂^• via triplet and singlet routes, and the former is more favorable due to its smaller barrier compared with the latter. The hydroxyl-substituted CIP has higher activity than its parent compound in their reactions with ^•OH due to its lower barrier and faster rate. In addition, the -NHC(O)-containing compound IM3-P10-H-4 is harmful to aquatic fish and is the primary product in the ^•OH-rich environment according to the ecotoxicity assessment computations. This study can improve our comprehension on CIP transformation in complex water environments.

Graphical abstract

Introduction

Fluoroquinolones are ubiquitous in surface water, groundwater, and soil because of their widespread use and superior stability (Dodd et al., 2005). In natural waters and wastewaters, the concentrations of fluoroquinolones have increased from ng/L to mg/L (Johnson et al., 2015; Jia et al., 2012). As the third generation of antibiotic of fluoroquinolone class, ciprofloxacin (CIP) is a problematic pollutant because of its environmental risk growing with the use-pattern as early as 2000 (Halling-Sørensen et al., 2000). Nevertheless, this drug still has been widely applied to fight Gram-positive and -negative bacteria and improve the human health over the past decade (Hooper, 1998; Weber et al., 2011). However, CIP cannot be removed completely from the metabolism and thus will be discharged into water environment, making it toxic to some aquatic organisms (when the residual is present trace level) (Ebert et al., 2011) and affect the aquatic bacterial community composition (Novo et al., 2013). CIP has been continually detected in many rivers in China (Bu et al., 2013; Liu and Wong, 2013) and has been identified as one of the most representative emerging pollutants (Li et al., 2017). The CIP in water would produce antibiotic-resistant bacteria (Li et al., 2017) and then discount the efficacy of drugs, leading to increased risks to human health (Zhang et al., 2015). Therefore, aqueous phase CIP elimination is urgent and important.

Various methods, including advanced oxidation, adsorption, and biodegradation, have been proposed to experimentally remove CIP in water. Among these techniques, a wide range of advanced oxidation processes (AOPs) have been applied to improve CIP removal efficiency in natural waters (Feng et al., 2018; Guo et al., 2016; Jiang et al., 2016; Serna-Galvis et al., 2017; Thabaj et al., 2007). The reactivity of CIP toward ^•OH is higher than that of other radicals, such as SO₄^•-, O₃, H₂O₂, HClO, and ClO₂ in acidic medium (Feng et al., 2018). For example, the bimolecular reaction rate constant for CIP with ^•OH is ~10⁹ M⁻¹ s⁻¹ at room temperature, which is larger than that compared with the above free radicals (Thabaj et al., 2007). This finding suggests that the ^•OH-triggered reaction of CIP is the most vital factor in determining its fate in aquatic environment. Thus, the first step of CIP degradation induced by ^•OH is important to effectually advance AOP application in water treatment and control sequential reactions (An et al., 2014). With the large and increasing number of organic contaminants, theoretical computations are necessary to evaluate their environmental degradation behavior and pollution mechanism (Zhao et al., 2016; Dang et al., 2015). These methods produce substantial information on the active species or reaction intermediates that are involved in degradation reactions and crucial for the mechanism clarification but are difficult to detect experimentally (Qu et al., 2018; Qu et al., 2017; Luo et al., 2018). In addition, theoretical calculations successfully predicted the degradation mechanism of organic contaminants in water or atmosphere (Zhao et al., 2016; Dang et al., 2015; Qu et al., 2018; Qu et al., 2017; Luo et al., 2018). The calculated results are used to explain experimental findings and provide theoretical guidance for experimental studies. Here, the theoretical approach is adopted to study the molecular basis of environmental and chemical processes of CIP. To date, no computational study has been conducted on the degradation mechanisms and kinetics, lifetimes, and product distribution of CIP.

The transformation pathways, rate coefficients, and reaction region-selectivity during the ^•OH-degradation of CIP are researched through density functional theory. These factors are important for environmental protection. The method in this study was successfully applied to investigate the radical-initiated oxidation of organics. All possible degradation mechanisms of CIP, including hydrogen atom abstraction, ^•OH-addition, and sequential electron proton transfer, are considered. The branching ratios of all intermediates yielded during transformation are established over the temperature range from 273 K to 323 K by analyzing the product percentage and thermodynamics natures of all product radicals. The subsequent behaviors of the primary product radicals in the presence of O₂ and ^•OH are clarified. The toxicities of CIP degradation products are also estimated to assess CIP degradation risk.

Section snippets

Calculation methodology

Geometry optimization and frequency calculation for all reactants, products, and transition states are determined at the (U)M06-2×/6-31G(d,p) (Zhao and Truhlar, 2008) level of theory via Gaussian 09 program (Barone and Cossi, 1998). The density functional theory, such as M06-2×, is one of the most popular methods for electronic structure calculations, especially in radical and organic bimolecular reactions. The solvent effect employs a conductive polarizable continuum model (Frisch et al., 2009

Initial degradation mechanisms of CIP by ^•OH

Geometrical configurations and the initial ^•OH-decomposition channels of CIP, including H-abstraction, ^•OH-addition, and sequential electron proton transfer are displayed in Fig. 1a, Fig. 1b, Fig. 1c. Twelve H-abstraction pathways (Abs1–Abs12), ten ^•OH-addition pathways (Add1–Add10), and one sequential electron proton transfer pathway are initially found in the reactions of CIP with ^•OH.

From a thermodynamic point of view in Fig. 1a, all the H-abstraction channels are exothermic with negative

Conclusion

The evolution of ^•OH-mediated aqueous phase CIP oxidation is investigated by using quantum chemistry computations. The rate coefficients for the key reactions are computed on basis of thermodynamic data from (U)M06-2×/6–311++G(3df,3pd)//(U)M06-2×/6-31G(d,p) level of theory. The transformation of C– or N-central alkyl radical and the formation of –CH₂C(O)- and -NHC(O)-containing products are stimulated. Some important conclusions are generalized as follows:

(1)
Twelve H-abstraction pathways, ten ^•

Declaration of Competing Interest

The authors declare no competing financial interest.

Acknowledgements

This work is financially supported by doctoral research start-up fund of Shenyang Normal University (No. BS201842 or 054-91800161042), the basic scientific research project of universities in Liaoning province (No. LQN201907), the National Natural Science Foundation of China (No. 91545117, and 41775119), fund of key technology research and development project of Jilin province science and technology department (20190302130GX), and focus on research and development plan in Shandong province

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Ciprofloxacin transformation in aqueous environments: Mechanism, kinetics, and toxicity assessment during •OH-mediated oxidation

Highlights

Abstract

Graphical abstract

Introduction

Section snippets

Calculation methodology

Initial degradation mechanisms of CIP by •OH

Conclusion

Declaration of Competing Interest

Acknowledgements

App. Catal. B: Environ.

J. Hazard. Mater.

J. Colloid Sci.

Sci. Total Environ.

J. Hazard. Mater.

J. Hazard. Mater.

Water Res.

Chem. Eng. J.

Water Res.

Water Res.

Sci. Total Environ.

J. Hazard. Mater.

Environ. Int.

Water Res.

Water Res.

Water Res.

Chem. Eng. J.

Chem. Eng. J.

Polyhedron

J. Hazard. Mater.

Water Res.

Sci. Total Environ.

Kinetics and mechanism of •OH mediated degradation of dimethyl phthalate in aqueous solution: experimental and theoretical studies

Environ. Sci. Technol.

Ciprofloxacin transformation in aqueous environments: Mechanism, kinetics, and toxicity assessment during ^•OH-mediated oxidation

Initial degradation mechanisms of CIP by ^•OH