Abstract

The frequent detection of antibiotics in water bodies gives rise to concerns about their removal technology. In this study, the degradation kinetics and mechanisms of norfloxacin (NOR), a typical fluoroquinolone pharmaceutical, by the UV/peroxydisulfate (PDS) was investigated. NOR could be degraded effectively using this process, and the degradation rate increased with the increasing dosage of PDS but decreased with the increasing concentration of NOR. In real water, the degradation of NOR was slower than that in ultrapure water, which indicated that laboratory results cannot be directly used to predict the natural fate of antibiotics. Further experiments suggested that the degradation of NOR was the most fast under neutral condition, the existence of HA or FA inhibited the degradation of NOR, and the presence of inorganic ions (NO3, Cl, CO32− and HCO3) had no significant effect on degradation of NOR. Total organic carbon (TOC) removal rate (40%) indicated NOR was not completely mineralized, and six transformation products were identified, and possible degradation pathways of NOR had been proposed. It can be prospected that UV/PDS technology could be used for advanced treatment of wastewater containing fluoroquinolones.

INTRODUCTION

As a class of widely-used antibacterial drugs for both human and animals, fluoroquinolone antibiotics were most frequently identified in natural and wastewaters (Guo et al. 2017). Even in trace level, they are demonstrated to have potentially induced the generation of quinolone-resistant pathogens, threatening human health and ecological safety by gene transformation (Carabineiro et al. 2011). Therefore, in recent years, removal technology of fluoroquinolone has received increased attention (Matzek & Carter 2016; Waclawek et al. 2017; Ike et al. 2018).

Several technologies have been utilized to remove norfloxacin (NOR), such as adsorption (Wan et al. 2018), biodegradation (Peng et al. 2019), Fenton oxidation (Wang et al. 2018), ozone oxidation (Ling et al. 2018), photodegradation (Zhang et al. 2019) and photocatalysis (An et al. 2010; Yang et al. 2019) and so on. Recently, SO4−· generated by activation of peroxydisulfate (PDS) was reported to effectively mineralize the target pollutants. In comparison with ·OH (1.8–2.7 V), SO4−· has a higher redox potential (2.5–3.1 V) and higher selectivity to organic compounds (Xu et al. 2016). SO4−· can be formed by activation of PDS by several methods, such as ultrasound, alkaline, heat, electrochemical, UV, and so on. Among these, UV/PDS was rather promising due to lower energy consumption and no secondary pollution issue, e.g. heavy metals (Ou et al. 2017). A series of studies had shown that UV/PDS had higher removal efficiency than that of UV/H2O2, for example, bisphenol A (Kwon et al. 2015) sulfonamide antibiotics (Zhang et al. 2016), cylindrospermopsin (He et al. 2014), iodinated disinfection byproducts (Xiao et al. 2016), and so on. Recently, significant advances have been achieved in understanding the cost-effective UV/PDS degradation of antibiotics (Serna-Galvis et al. 2017; Sun et al. 2019). However, detailed information on the photodegradation of fluoroquinolones in aquatic environment is still lacking.

In this work, the degradation of NOR was investigated by UV/PDS in aqueous solution. NOR was selected as the target contaminant because it was extensively used in prescriptions for humans and it has been widely detected in surface water and wastewater (de Souza Santos et al. 2015). Hence, the objectives of this study are to: (1) establish the degradation kinetics of NOR by UV/PDS; (2) evaluate the effects of water matrices on the degradation of NOR by UV/PDS; (3) examine the mineralization degree; (4) identify the main products and elucidate the degradation mechanism.

METHODS

Reagents

NOR (C16H18FN3O3, guarantee reagent) was purchased from Hefei Bomei Biotechnology Co. Ltd, China. PDS, NaNO3, NaCl, Na2CO3, NaHCO3, H2SO4 and NaOH were all of guarantee reagents and were supplied by the Sinopharm Chemical reagent Co. Ltd, China. Humic acid (HA) was obtained from Sigma Aldrich, USA, and fulvic acid (FA) was obtained from Shanghai Future Industrial Co. Ltd, China. Formic acid and acetonitrile of a suitable grade for high performance liquid chromatography (HPLC) were separately obtained from Tianjin Guangfu Fine Chemical Research Institute, China and Fisher Chemicals, USA. All the reagents were used as received without further treatment. Ultrapure water, with a resistivity of at least 18.2 MΩ/cm, was produced by using a 1820A Ultrapure system (Molecular Water System Co. Ltd, China).

UV/PDS degradation reaction

The degradation reactor was self-made: it was composed of a 500 mL cylinder, an 11 W low-pressure Hg vapor lamp (Institution of Light Source, Beijing, China) emitting UV light at 254 nm, and quartz sleeves set between the cylinder and the lamp. To obtain homogeneous UV exposure, magnetic stirring was conducted during the reaction. As reported in our previous work (Xue et al. 2017), lamp power output was 2.12 mW cm−2, and the effective path length of reactor was 1.9 cm.

Mixed solution of NOR and PDS was transformed into the reactor, and after magnetic stirring for 2 min, the lamp was switched on. Two samples were withdrawn at certain time intervals and were analyzed immediately by HPLC. When required, HA, FA, NO3, Cl, CO32−, and HCO3 was separately added to the NOR solution before the reaction took place.

Analytical methods

HPLC (Agilent 1200 series, USA) was used to analyzed concentration of NOR. It was equipped with a variable wavelength detector (VWD) and a Symmetry-C18 column (4.6 × 150 mm, 5 µm). The mobile phase was 30% acetonitrile and 70% formic acid solution (0.2%, v/v), the flow rate was 0.2 mL/min, the detection wavelength was 278 nm, the column temperature was 30 °C, and the injection volume was 10 µL.

The total organic carbon (TOC) was analyzed by a TOC-Vcph analyzer (Shimadzu, Japan) and the pH value was determined by a PHSJ-5pH-meter (Shanghai Precision & Scientific Instrument Co. Ltd, China).

Ultra-high performance liquid chromatography-time-of-flight (UPLC-TOF) was used to analyze the degradation products of NOR by UV/PDS. An Acquity UPLC system (Waters Corporation, Milford, MA, USA) was equipped with a BEH C18 Column (2.1 × 50 mm, 1.7 µm) (Milford, MA, USA). The column temperature was 30 °C. The mobile phases were composed of solvent A (0.1% formic acid in water) and solvent B (acetonitrile). The solvent gradient was 0–2 min, 5–60% B; 2–6 min, 60–70% B; 6–9 min, 70–100% B. The flow rate of 0.3 mL/min. The injection volume was 10 µL.

TOF detection was performed on a Q-TOF SYNAPT G2 HDMS system (Waters MS Technologies, Manchester, UK) equipped with a dual electrospray ionization probe. The analysis conditions were as follows: the resolution mode was selected for positive ionization mode, source temperature was maintained at 110 °C, desolvation gas temperature was 300 °C, cone gas flow was 30 L/h, desolvation gas flow was 600 L/h, capillary voltage was 2.5 kV. Sample cone voltage was 35 V, and the extraction cone voltage was 5.0 V. Data were acquired between m/z 50 and 1,200 Da, with a scan time of 0.2 s, and were further processed with MassLynx 4.1 software (Waters).

Real water sampling and treatment method

The degradation reaction of NOR was also conducted in real water, i.e. raw water from the Nierji reservoir which was located in Nierji town, autonomous county of Molidawa Daur Nationality in Inner Mongolia Autonomous Region, China. This water was defined as surface water (SW). The water sample was filtered by a 0.45 µm membrane and was stored at 4 °C. Water quality parameters are shown in Table 1.

Table 1

Real water quality parameters

pH value 7.63 
DOC (mg·L−19.74 
A254 (cm−10.3036 
Alkalinity (HCO3, mmol·L−11.80 
pH value 7.63 
DOC (mg·L−19.74 
A254 (cm−10.3036 
Alkalinity (HCO3, mmol·L−11.80 

RESULTS AND DISCUSSION

Effect of PDS dose

The initial PDS dose is an important parameter in evaluating the practicability of UV/PDS technology, as the removal efficiency for target pollutants would be low when the PDS dose was low; however, a high PDS concentration may increase the operation cost. Figure 1 depicts that adding PDS efficiently accelerated the removal of NOR; the higher the dose of PDS, the faster the NOR degradation. This was because PDS could generate reactive ·SO4 and ·OH, under UV irradiation, ·SO4 and ·OH could decompose NOR.

Figure 1

Effect of initial concentration of PDS on the degradation of NOR in UV/PDS system, [NOR]0 = 5 µM.

Figure 1

Effect of initial concentration of PDS on the degradation of NOR in UV/PDS system, [NOR]0 = 5 µM.

It was noted that NOR could not be removed by PDS without UV activation (data not shown).

When the initial concentration of PDS was 100 µM, the degradation rate of NOR reached 93.46% at 9 min. Next, we chose 100 µM PDS as the optimal condition.

The degradation of NOR in the UV/PDS system obeyed the pseudo-first-order kinetics equation, as shown in Table 2. When the initial concentration of NOR and PDS was 5 µM and 100 µM, the degradation rate constant of NOR was 0.5510 min−1. Guo et al. (2017) reported that under the condition of 13 µM NOR and 500 µM PDS, the degradation rate constant of NOR was 0.5940 min−1, which was similar to ours.

Table 2

The kinetics parameters of degradation of NOR by UV/PDS

Concentration of PDS (µM)Equationk (min−1)R2
ln(Ct/C0) = −0.0657t + 0.0198 0.0657 0.9848 
ln(Ct/C0) = −0.0980t − 0.0285 0.0980 0.9859 
10 ln(Ct/C0) = −0.1280t − 0.0122 0.1280 0.9970 
25 ln(Ct/C0) = −0.2810t − 0.0087 0.2810 0.9855 
50 ln(Ct/C0) = −0.3602t − 0.0339 0.3602 0.9962 
100 ln(Ct/C0) = −0.5510t − 0.1869 0.5510 0.9651 
Concentration of PDS (µM)Equationk (min−1)R2
ln(Ct/C0) = −0.0657t + 0.0198 0.0657 0.9848 
ln(Ct/C0) = −0.0980t − 0.0285 0.0980 0.9859 
10 ln(Ct/C0) = −0.1280t − 0.0122 0.1280 0.9970 
25 ln(Ct/C0) = −0.2810t − 0.0087 0.2810 0.9855 
50 ln(Ct/C0) = −0.3602t − 0.0339 0.3602 0.9962 
100 ln(Ct/C0) = −0.5510t − 0.1869 0.5510 0.9651 

[NOR]0 = 5 µM.

Effect of initial concentration of NOR

Degradation of NOR with different initial concentrations at a fixed initial PDS concentration of 100 µM was evaluated. As shown in Figure 2, with an increase in the NOR concentration (5–50 µM), the removal efficiency of NOR decreased. This was because increasing the NOR concentration would reduce the amount of ·SO4 and ·OH available for the NOR. However, with the increasing of NOR initial concentration, the absolute degradation amount increased, especially at the later stage of the reaction. This can be attributed to stronger reaction impetus from higher initial concentration. To maintain a faster reaction rate, 5 µM NOR was selected for the subsequent experiments.

Figure 2

Effect of initial concentration of NOR on its degradation in UV/PDS system, [PDS]0 = 100 µM.

Figure 2

Effect of initial concentration of NOR on its degradation in UV/PDS system, [PDS]0 = 100 µM.

Effect of initial solution pH

Solution pH value was a key factor in the degradation of pollutants in UV/PDS process based on two reasons. Firstly, as a previous study reported that SO4· was the dominant radical at pH<9; while the fraction of ·OH increased gradually from pH 9 to 11 and ·OH became the main radical species when pH>11 (Liu et al. 2016). Secondly, NOR has three different forms at different pH ranges due to its two pKa values of 6.10–6.22 and 7.70–8.51, which may subsequently lead to its different reactivity towards radical species (Guo et al. 2017).

The original pH value for a mixed solution of NOR and PDS was 5.54, H2SO4 and NaOH were used to adjust the pH value. The effect of pH value on the degradation of NOR is shown in Figure 3. It can be seen that NOR has the highest degradation rate under a neutral condition (pH = 7.12). Both acidic and alkaline conditions inhibited the degradation of NOR, and the inhibition of alkaline condition was more significant. Guo et al. (Guo et al. 2017) also found both acidic (pH 3.0 and 5.0) and alkaline conditions (pH 11.0) inhibited the degradation of NOR compared with that under neutral condition (pH 7.0), which was in line with ours. However, their further study indicated that pH 9.0 was the optimal pH for NOR degradation by UV/PDS process. Considering the natural water system was near-neutral, we used pH 7.0 for further study.

Figure 3

Effect of initial pH on the degradation of NOR in UV/PDS system, [NOR]0 = 5 µM, [PDS]0 = 100 µM.

Figure 3

Effect of initial pH on the degradation of NOR in UV/PDS system, [NOR]0 = 5 µM, [PDS]0 = 100 µM.

Effect of water quality

The laboratory experiment conducted in ultrapure water (UW) may not necessarily be the same to that in natural water, so here we investigated the degradation of NOR in real water (SW), aiming to give reference to the practical application of UV/PDS technology. The results are presented in Figure 4. It was obvious that the degradation of NOR in the actual water by UV/PDS was prohibited compared with that of the ultrapure water. This was because the reaction mixture solution prepared by SW was weakly alkaline (pH = 7.63), while the alkaline condition did not facilitate the degradation of NOR (Figure 3). What is more, real water had a certain absorption of UV254; it could compete with PDS to absorb UV light, thus reducing the production of SO4· in the reaction system, and then the degradation of NOR was slowed down.

Figure 4

Effect of water quality on the degradation of NOR in UV/PDS system, [NOR]0 = 5 µM, [PDS]0 = 100 µM.

Figure 4

Effect of water quality on the degradation of NOR in UV/PDS system, [NOR]0 = 5 µM, [PDS]0 = 100 µM.

Effect of HA and FA

Water matrix components, such as natural organic matter (NOM) affect the effectiveness of the UV/PDS process through scavenging radicals and filtering UV light (Wang et al. 2016). Here, we chose HA and FA as the representatives of NOM and analyzed their effects on the NOR degradation in UV/PDS system, with the results being shown in Figures 5 and 6.

Figure 5

Effect of HA on the degradation of NOR in UV/PDS system, [NOR]0 = 5 µM, [PDS]0 = 100 µM.

Figure 5

Effect of HA on the degradation of NOR in UV/PDS system, [NOR]0 = 5 µM, [PDS]0 = 100 µM.

Figure 6

Effect of FA on the degradation of NOR in UV/PDS system, [NOR]0 = 5 µM, [PDS]0 = 100 µM.

Figure 6

Effect of FA on the degradation of NOR in UV/PDS system, [NOR]0 = 5 µM, [PDS]0 = 100 µM.

The results indicated that both HA and FA inhibited the degradation of NOR, with an increasing concentration of HA and FA the reduction effect increased and the NOR degradation rate declined. This was because HA and FA could absorb the UV light, thereby lowering the UV activation of PDS, further reducing the generation of ·SO4, and inhibiting the degradation of NOR. In addition, NOR could compete with HA and FA for ·OH and ·SO4, leading to the slower NOR degradation rate. By comparison, we knew that the effect of FA on degradation of NOR was comparable to that of HA.

Effect of inorganic anions

Similar to NOM, the inorganic anions affect the UV/PDS degradation process through scavenging ·OH and SO4−· to form new radicals (Fu et al. 2019). Therefore, we explored the effect of anions (NO3, Cl, CO32− and HCO3) on the degradation of NOR. The results are shown in Figure 7. It can be seen that Cl slightly increased the degradation of NOR, while other anions (NO3, CO32− and HCO3) weakly reduced the degradation of NOR. However, by comparison, we could see that both the promotion and inhibition effects were not obvious. After reaction for 5 min, the degradation rate of NOR all exceed 90% with and without anions.

Figure 7

Effect of anions on the degradation of NOR in UV/PDS system, [NOR]0 = 5 µM, [PDS]0 = 100 µM, [NO2]0 = [Cl]0 = [CO32−]0 = [HCO3]0 = 5 µM.

Figure 7

Effect of anions on the degradation of NOR in UV/PDS system, [NOR]0 = 5 µM, [PDS]0 = 100 µM, [NO2]0 = [Cl]0 = [CO32−]0 = [HCO3]0 = 5 µM.

Identification of intermediates and degradation pathways of NOR

During the degradation of NOR by UV/PDS ([NOR]0 = 5 µM, [PDS]0 = 100 µM), the degradation efficiency of NOR has reached 100% after irradiation for 5 min (Figure 1), while the removal rate of TOC was only 40% even after prolonged irradiation for 60 min, which suggested that NOR was indeed decomposed, but it was not completely mineralized.

During the degradation of NOR by UV/PDS ([NOR]0 = 5 µM, [PDS]0 = 100 µM), the pH value of the solution reduced from 5.54 to 5.07 after irradiation for 5 min, which indicated defluorination may occur and hydrofluoric acid was formed. To verify this deduction, we analyzed the degradation products of NOR.

To obtain NOR degradation products information, high concentration of NOR (50 µM), PDS (0.5 mM), and prolonged UV irradiation time (10 min) was applied. Finally, six degradation products of NOR were identified by UPLC-TOF, detailed information is shown in Table 3. It should be noted that the retention time for NOR (C16H18FN3O3) was 3.050 min, and its m/z was 320.

Table 3

NOR degradation products formed in UV/PDS process identified by UPLC-TOF

m/zRetention time (min)Molecular formulaDifference with NORDegradation site moiety
227 0.50 C10H11FN2O3 −6C 7H N Piperazinyl, quinolinic 
246 7.78 C13H15N3O2 −3C 3H F O Quinolinic 
274 9.64 C15H16FN3O3 −C 2H 2O Piperazinyl, quinolinic 
290 7.94 C14H15N3O4 −2C 3H F, +O Quinolinic 
318 9.78 C16H19N3O4 −F, +OH Quinolinic 
368 14.94 C16H18FN3O6 +3O Piperazinyl 
m/zRetention time (min)Molecular formulaDifference with NORDegradation site moiety
227 0.50 C10H11FN2O3 −6C 7H N Piperazinyl, quinolinic 
246 7.78 C13H15N3O2 −3C 3H F O Quinolinic 
274 9.64 C15H16FN3O3 −C 2H 2O Piperazinyl, quinolinic 
290 7.94 C14H15N3O4 −2C 3H F, +O Quinolinic 
318 9.78 C16H19N3O4 −F, +OH Quinolinic 
368 14.94 C16H18FN3O6 +3O Piperazinyl 

[NOR]0 = 50 µM, [PDS]0 = 0.5 mM, irradiation for 10 min.

Given the molecular structure of NOR and the nature of UV/PDS process, the degradation mechanisms of NOR were proposed, as shown in Figure 8. The degradation pathways for each products were detailed as follows.

  • (a)

    The -F on the quinolone ring was replaced by ·OH forming the product m/z 318, and the methyl group on the quinolone ring of m/z 318 was removed and generated the product m/z 290, and product m/z 246 was derived from the the elimination of carboxyl group from the quinolone ring of product m/z 290.

  • (b)

    ·OH attacked the piperazine ring of NOR forming the intermediate product m/z 336. Hubicka et al. (Hubicka et al. 2013) also obtained the product m/z 336 when analyzing the degradation product of NOR by oxidation of acidic potassium permanganate. The product m/z 336 was further subjected to a hydroxylation reaction to give the product m/z 368.

  • (c)

    The carboxyl group on the quinolone ring was removed and the quinolone ring was further decomposed, the piperazine ring undergoes an elimination reaction to form product m/z 227. Tang et al. (2016) also obtained the product m/z 227 in the Bi2WO6 photocatalysis degradation of NOR, which was in agreement with our findings.

  • (d)

    Decarboxylation of quinolone and dehydrogenation of piperazine ring resulted in the generation of product m/z 274.

Figure 8

The degradation pathways of NOR in UV/PDS system.

Figure 8

The degradation pathways of NOR in UV/PDS system.

From the above information, we could see that not only piperazinyl and quinolone moieties were decomposed, but defluorination process also occurred. While previous studies had reported that fluoroquinolones were prone to degradation in piperazinyl and quinolone moieties, but they were difficult to defluorinate (Pi et al. 2014). Our experiments confirmed that UV/PDS technology could be used for advanced treatment of antibiotic wastewater.

Guo et al. (2017) also found that NOR could be degradaded by UV/PDS through defluorination, hydroxyl substitution and decarboxylation and so on. However, the degradation products (m/z 251, 276, 294, 304, 310, 322, 350) of NOR reported by them were totally different from ours. Such difference may be derived from the different experimental conditions because pH value 9.0 was adopted and dissolved oxygen was removed from the mixed solution of NOR and PDS in their experiments.

CONCLUSION

UV/PDS could quickly degrade NOR, and the degradation rate increased with the increasing dosage of PDS but decreased with the increasing concentration of NOR.

In real water, the degradation of NOR was slower than that in ultrapure water, which indicated that laboratory results cannot be directly used to predict the natural fate of antibiotics. Further experiments suggested that the degradation of NOR was the most fast under neutral condition, the existence of HA or FA inhibited the degradation of NOR, while the presence of inorganic ions (NO3, Cl, CO32− and HCO3) had no significant effect on degradation of NOR.

TOC removal rate (40%) indicated NOR was not completely mineralized, and six transformation products were identified, and possible degradation pathways of NOR had been proposed. Defluorination process indicated that UV/PDS technology could be used for advanced treatment of wastewater containing fluoroquinolones.

ACKNOWLEDGEMENTS

This study was supported by the Open Project Program of Key Laboratory of Groundwater Resources and Environment (Jilin University), Ministry of Education, and the National Natural Science Foundation of China (Nos. 41722110 and 41571474).

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