Abstract

Antibiotics are known today as emerging contaminants due to potentially adverse effects on aquatic ecosystems and the health of humans and animals, even at very low concentrations. The present study was conducted to evaluate the efficiency of the UV/S2O8 process and affecting factors (pH, initial metronidazole (MNZ) concentration, initial persulfate concentration and reaction time) in removing antibiotic MNZ. The results obtained from the experiments showed that the UV/S2O8 process efficiency is higher in acidic pH values due to production of further radical SO4 and increases with extended contact time, but the efficiency of the process is reduced by increasing the concentration of MNZ. In assessing the effect of initial persulfate concentration on the process efficiency, MNZ removal efficiency was also increased by 99.5% after contact time of 35 min with increasing the initial persulfate concentration up to 1 g/L. However, the process efficiency was decreased at higher concentrations (2 mg/L) due to reaction of sulfate radicals with each other or with persulfate and its saturation. The kinetic data fitted the pseudo-first-order kinetic model (R2 > 99%). The findings of this study clearly demonstrated the high potential of the UV/S2O8 process in the degradation of MNZ.

INTRODUCTION

The shortage of drinking water supplies in recent decades has increased the focus on wastewater treatment in the current century (Alvarez-Torrellas et al. 2016; Bazrafshan et al. 2016). Although conventional wastewater treatment processes are somewhat effective in removing most contaminants, they are, however, not applicable in the removal of emerging contaminants such as pharmaceutical compounds, detergents and cosmetics. Presence of these substances in the effluent will be possible in the case of lack of special treatment and release into the wastewater streams (Sui et al. 2012; Michael et al. 2013). Increasing the use of drugs, and in particular antibiotics, results in the discharge of high volumes of waste containing pharmaceutical compounds into the environment and water resources (Daughton & Ternes 1999). Detection of antibiotics in sewage from different parts of the world, on the one hand, and the persistence of some of them in aquatic environments, on the other hand, increase the public concern and extend studies in this regard (Michael et al. 2013; Chen & Zhou 2014). Recent studies have shown the adverse effects of antibiotics on aquatic ecosystems and the potential for bacterial resistance against antibiotics; thus, they are known as emerging contaminants even at very low concentrations (Carrales-Alvarado et al. 2014; Mohammadi et al. 2015).

Currently, more than 250 antibiotics have been registered for medicinal purposes, accounting for over 15% of drug consumption, and, of course, increasing demand will lead to increased production in the future (De La Cruz et al. 2014; Ahmed et al. 2015). The main sources of antibiotic release to the environment are wastewater from urban wastewater treatment plants (disposal of 90% of consumed antibiotics through urine and stool), sewage from pharmaceutical factories, hospitals and treatment centers (Michael et al. 2013; Yu et al. 2016). Metronidazole (MNZ) is one of the most commonly produced nitroimidazole antibiotics in many countries. This antibiotic has high water solubility, is toxic and is potentially carcinogenic and mutagenic (Bendesky et al. 2002). It is used frequently to treat inflammations and infections of anaerobic bacteria and protozoa in medicine, and is added to chicken and fish feed as an antiparasitic agent in veterinary medicine (Bendesky et al. 2002; Fang et al. 2011; Kamani et al. 2017).

Therefore, this antibiotic is also found in wastewater from slaughterhouses, in addition to wastewater from pharmaceutical factories, and urban and hospital wastewaters, and can cause damage to humans and animals, if not eliminated before the discharge of wastewater into the environment (Ingerslev & Halling-Sorensen 2000; Alexy et al. 2004). Conventional wastewater treatment processes have no remarkable efficacy in the removal of antibiotics. Accordingly, it is necessary to employ novel methods and technologies to remove the antibiotics. Various methods have already been used to remove antibiotics, including absorption, membrane processes, electrochemical treatment and biodegradation. Some of the disadvantages of these methods include high cost of the process, ineffective removal of the pollutant with its phase transition, lack of efficiency for sewage containing high concentrations of antibiotics and the production of sludge and disposal-related problems (Guo et al. 2013; Jiang et al. 2013).

Advanced oxidation processes (AOPs) are chemical and non-selective processes that result in the decomposition of highly reactive species such as hydroxyl radicals and their attack on the chemical compounds present in the environment (Sarmah et al. 2006). In recent years, various studies have been done on the destruction of antibiotics using different AOPs. Persulfate ion is one of the most effective oxidants used in the treatment of organic pollutants, whose use is justifiable due to specific properties, including high oxidizing properties, high water solubility, easy storage at ambient temperature, easy transportation due to solid form of persulfate salts, lower cost than other oxidants, the safety of the manufactured byproducts and proper persistence (Xu & Li 2010; Xie et al. 2012). In addition, persulfate ion, if stimulated and activated, has the potential to produce sulfate radicals that have a significantly higher oxidation potential (equal to 2.6 V) compared to persulfate ions (Xu & Li 2010; Carrales-Alvarado et al. 2014).

Various methods like the use of metal catalysts, heat and UV radiation are recruited to activate the persulfate (Liu et al. 2013). Activation with metal catalysts increases the oxidation efficiency but the health effects of the remaining metal ions are important as a negative factor. Also, activation with heat does not have a significant effect on the oxidation potential (Deng et al. 2013). Meanwhile, the use of UV radiation for activation is completely environmentally friendly, in addition to high efficiency for the production of sulfate radicals, and does not leave any residue, as well as its operating cost being lower than activating with metal catalyst (Jaafarzadeh et al. 2016).

In the process of activating the persulfate by UV radiation, two radicals of sulfate are generated from each persulfate ion according to the following equation (Lin et al. 2013):  
formula
(1)
The produced sulfate radicals have a high ability to oxidize organic compounds and Gao et al. (2012) showed a 97% efficiency of the UV/S2O8 process in the removal of sulfamethazine. Lin & Wu (2014) found that the UV/S2O8 process is capable of decomposing antibiotic ciprofloxacin by 95% over a period of 30 min. This present study examines the process efficiency UV/S2O8 and the effective associated factors for the removal of MNZ.

MATERIALS AND METHODS

The current applied research was carried out experimentally and on a laboratory scale. MNZ (C6H9N3O3, 99%) and sodium persulfate (Na2S2O8, 99%) were used for testing, which were the products of Merck, Germany. Stock solutions of antibiotic and sodium persulfate were prepared by dissolving accurately weighed amounts of the respective chemicals in double distilled water followed by dilution to the required volume. Then, the required concentrations in each step were made by further dilution. The stock solution of antibiotic was prepared weekly and kept in darkness and at temperatures below 4 °C; 0.1 NaOH and HCl (Merck, Germany) were used to adjust the pH. Table 1 shows the physical and chemical properties and the molecular structure of MNZ.

Table 1

Physical and chemical properties of MNZ

Characteristic Metronidazole antibiotic 
Molecular structure  
Molecular formula C6H9N3O3 
Molecular weight (g/mol) 171.2 
Water solubility (g/L) 9.5 
pKa 2.55 
Melting point (°C) 159–163 
KH (mol/dm3.atm) 5.92 × 107 
VP (Pa) 4.07 × 107 
Characteristic Metronidazole antibiotic 
Molecular structure  
Molecular formula C6H9N3O3 
Molecular weight (g/mol) 171.2 
Water solubility (g/L) 9.5 
pKa 2.55 
Melting point (°C) 159–163 
KH (mol/dm3.atm) 5.92 × 107 
VP (Pa) 4.07 × 107 

pKa: acid dissociation constant at logarithmic scale; KH: Henry's law coefficient; VP: vapor pressure.

The AOP for removal of MNZ was performed through the sulfate radicals in a chamber (Figure 1). The used chamber is composed of two parts. The internal and main chamber consists of a glass container with a volume of one liter and has a UV lamp inside to perform the oxidation process. The outer compartment houses the main chamber and has a larger glass compartment. The flow of water is continuously maintained in order to reduce the heat generated by the UV lamp. Based on similar studies, the factors affecting the process include pH, antibiotic concentration, persulfate concentration and time. In this study, the effect of each factor was studied on the efficiency of the UV/S2O8 process (Lin et al. 2013; Lin & Wu 2014). All experiments were carried out at ambient temperature range and a magnetic stirrer was used to disperse the oxidizing agent properly. At the end of each stage, the MNZ concentration was measured by high performance liquid chromatography with a UV detector at a wavelength of 348 nm. The percentage MNZ removal efficiency was calculated using Equation (2):  
formula
(2)
where C0 and C are the initial and final concentrations of MNZ after the contact time, respectively, and E shows antibiotic removal efficiency (Farzadkia et al. 2014; Lin & Wu 2014).
Figure 1

Schematic diagram of cold-water chamber.

Figure 1

Schematic diagram of cold-water chamber.

RESULTS AND DISCUSSION

Effect of initial pH of the solution on MNZ removal efficiency

The production of active and reactive free radicals is the main mechanism in the AOP (Gogate & Pandit 2004). The pH of the solution affects the properties of the pollutant and the photocatalyst and thus can determine the dominant radicals type in the AOP and thereby affect the process efficiency (Daghrir et al. 2013).

In order to evaluate the effect of solution pH on the efficiency of the UV/S2O8 process in MNZ degradation, the experiments were carried out inside the chamber exposed to UV radiation emitted by a lamp with 8-watt radiant power. Figure 2 shows the percentage of MNZ removal with the initial concentration of 40 mg/L using 0.5 g/L of persulfate at the pH range of 3 to 11 at different times. As is known, the removal efficiency has a direct relationship with the prolonged contact time, and an inverse association with increasing pH (Lee et al. 2012). The highest MNZ removal efficiency was observed at pH of 3 and the contact time of 35 min and the lowest removal at pH of 11 and contact time of 5 min. This can be attributed to the dominant radical type at different pH values. The UV/S2O8 process produces sulfate radicals. At low pH values, the production level of sulfate radicals increases by acid catalysis. Therefore, the sulfate radicals will predominantly be in acidic conditions. By increasing pH and producing alkaline pH, the sulfate radicals may react with OH and produce hydroxyl radicals, which have less power to oxidize MNZ than the sulfate radicals (Equations (3)–(5)) (Lee et al. 2012; Norzaee et al. 2017).

Figure 2

Effect of solution pH on the efficiency of UV/S2O8 process in the removal of MNZ.

Figure 2

Effect of solution pH on the efficiency of UV/S2O8 process in the removal of MNZ.

The reaction between hydroxyl radicals and persulfate radicals at higher pH values results in the production of sulfate ions. This reaction causes the loss of free radicals and the reduction of the AOP efficiency. Similar results have been reported in the study of Lin et al. (2013). In this study, due to the slight difference in removal efficiency at pH values of 3 and 5, and with regard to the costs of neutralizing the final solution, the pH = 5 was selected as optimal pH (Gao et al. 2012; Lin et al. 2013; Lin & Wu 2014).  
formula
(3)
 
formula
(4)
 
formula
(5)

Effect of initial MNZ concentration on the efficiency of UV/S2O8 process

The concentration of pollutants is always considered as one of the important factors to have an effect on its removal efficiency (Pi et al. 2013). In this study, in order to evaluate the effect of initial MNZ concentration on the efficiency of removal by the UV/S2O8 process, the experiments were carried out at pH = 5, persulfate concentration of 0.5 g/L using UV radiation emitted from a 8-watt lamp at different times, and the MNZ concentrations of 5–100 mg/L. The results showed that the efficiency of UV/S2O8 process in the removal of MNZ was reduced by increasing the initial concentration of antibiotics. Figure 3(a) and 3(b) show respectively the surface and contour plots related to the effect of initial MNZ concentration on the removal efficiency. As is clear, 5 min after the onset of the process, the MNZ removal efficiency at the initial MNZ concentrations of 5, 10, 20, 40, 60, 80 and 100 mg/L was 82, 79, 77, 76.5, 74.1, 74 and 73.1%, respectively. At the end of the process (after contact time of 35 min), the removal efficiency increased to 99.99, 99.3, 98.3, 96.5, 92.9, 89.88 and 87.8%, respectively. Due to the constant concentration of persulfate used in all steps (0.5 g/L), the results suggest that the persulfate dosage was insufficient to remove the MNZ when increasing the antibiotic concentration. Additionally, the UV implies that penetration rate somehow experienced a change in state to that of a solution, which results in the reduction of free radicals produced, thereby reducing the efficiency of the AOP (Rasoulifard et al. 2012; Wu et al. 2012). Farzadkia et al. (2015) investigated the photocatalytic degradation of MNZ with illuminated TiO2 nanoparticles and determined that the efficiency of the process is reduced by increasing the concentration of MNZ (Wang et al. 2016). Similar results have been reported in the study of Lin et al. (2013) and Lin & Wu (2014).

Figure 3

Surface (a) and contour plots (b) related to the effect of initial MNZ concentration on the efficiency of UV/S2O8 process.

Figure 3

Surface (a) and contour plots (b) related to the effect of initial MNZ concentration on the efficiency of UV/S2O8 process.

Effect of Na2S2O8 concentration on MNZ removal efficiency

The type and concentration of the used oxidizing agent are the factors affecting the efficiency of the AOP (Pi et al. 2013). To evaluate the effect of initial persulfate concentration on the efficiency of the MNZ removal process, the experiments were carried out at pH = 5 and MNZ concentration of 40 mg/L at persulfate concentrations of 0.1–1 g/L at different times (Figure 4). According to Figure 4, it can be found that the MNZ removal efficiency has improved with increasing persulfate concentration, so that the removal efficiency goes up to more than 99% at a concentration of 1 g/L after 35 min of contact time, while in this contact time, the removal efficiency at a concentration of 0.1 g/L of persulfate is about 84%. However, by increasing the persulfate concentration to 2 g/L, although the MNZ removal efficiency is greater than the early reaction times, the overall removal efficiency has decreased after 35 min of contact time. The cause of reduced removal efficiency at high concentrations of persulfate can be searched for in the dual behavior of persulfate at different concentrations. Persulfate not only can produce sulfate radicals in the decomposition of pollutants, but also has the ability to produce hydroxyl radicals in direct reaction with water. Therefore, increasing the persulfate to a certain concentration results in high production of free radicals and thus improved MNZ removal efficiency. In high concentrations, the persulfate becomes an agent for absorption and consumption of free radicals (Equations (6) and (7)). In addition, the produced sulfate radicals can react with persulfate, as the following equation, resulting in the saturation of sulfate radicals and reduction of the process efficiency. Lin et al. (2013) have also reported similar results.  
formula
(6)
 
formula
(7)
Figure 4

Effect of initial concentration of Na2S2O8 on the efficiency of UV/S2O8 process.

Figure 4

Effect of initial concentration of Na2S2O8 on the efficiency of UV/S2O8 process.

Efficiency of the MNZ removal under different processes

In order to evaluate the efficiency of UV radiation and persulfate alone in removal of MNZ and to compare the efficiency of removal with the UV/S2O8 process, the experiments of MNZ removal were carried out at the optimal pH of 5 and the same conditions as the previous stage; the results are shown in Figure 5. According to Figure 5, it can be seen that the efficiency of MNZ removal using S2O8 has been negligible compared with UV/S2O8. In addition, the highest efficiency of MNZ removal with UV and S2O8 alone were 2.25 and 25.5% after contact time of 35 min, which are much lower than that of UV/S2O8 (92.75%). A comparison of the UV/S2O8 process for degradation of various organics is given in Table 2.

Table 2

Degradation of various pollutants using UV-activated persulfate

Pollutant pH Persulfate conc. (mmol/l) Power (W) Time (min) Removal efficiency (%) Reference 
Polyvinyl alcohol 10 97 Lin et al. (2013)  
Ciprofloxacin 7.1 30 95 Lin & Wu (2014)  
Microcystis aeruginosa 23 120 98.2 Wang et al. (2016)  
Phenol – 0.5 20 900 95 Avetta et al. (2015)  
Tetramethyl 50 15 130 100 Wang & Liang (2014)  
Atrazine 7.4 – – 50 Khan et al. (2014)  
Pollutant pH Persulfate conc. (mmol/l) Power (W) Time (min) Removal efficiency (%) Reference 
Polyvinyl alcohol 10 97 Lin et al. (2013)  
Ciprofloxacin 7.1 30 95 Lin & Wu (2014)  
Microcystis aeruginosa 23 120 98.2 Wang et al. (2016)  
Phenol – 0.5 20 900 95 Avetta et al. (2015)  
Tetramethyl 50 15 130 100 Wang & Liang (2014)  
Atrazine 7.4 – – 50 Khan et al. (2014)  
Figure 5

Efficiency of the MNZ removal under different conditions

Figure 5

Efficiency of the MNZ removal under different conditions

Kinetics of MNZ degradation by the UV/S2O8 process

According to studies, the pseudo-first-order kinetic model is used to describe the photocatalytic degradation of organic compounds. Farzadkia et al. (2015) and Elmolla & Chaudhuri (2010) also applied this model for the photocatalytic degradation of MNZ. The degradation of MNZ using the UV/S2O8 process is based on the pseudo-first-order kinetic model, as the following equation:  
formula
(8)
where and are the initial and final concentrations of MNZ after the contact time, respectively, and k is a constant of degradation value; k value is equal to the slope of the plot of versus time t. In this research, for studying the kinetic of the reaction, based on the results of the experiments in Figure 4, the plot of was drawn and the values of k and R2 were determined in different conditions.
The photocatalytic degradation of organic compounds is an electrically energy-dependent process. Costs related to energy consumption account for a large part of operating costs. Therefore, it is necessary to assess the amount of energy consumption in the AOP as one of the factors affecting its application on the massive scales. Considering that the photocatalytic degradation of MNZ follows the pseudo-first-order kinetic model in the present study, the electrical energy was evaluated by calculating electrical energy in each order. The following equation is used to calculate electrical energy per order (EE/O):  
formula
(9)
where P is the power of the UV lamp (kW), V is the volume of MNZ aqueous solution and k is the constant of MNZ degradation (Daneshvar et al. 2005; Lin & Wu 2014; Farzadkia et al. 2015).

Table 3 shows the values of k, R2 and reaction energy for MNZ degradation at different concentrations of persulfate. The high correlation coefficient values for all persulfate concentrations (R2 > 99%) showed that the data obtained from experiments were consistent with the pseudo-first-order kinetic model. Moreover, in this study, the lowest recorded value was 3.49kWh/m3.order for the concentration of 1 g Na2S2O8/L.

Table 3

Parameters of the pseudo-first-order kinetic model and electrical energy for MNZ degradation by the UV/S2O8 process

Na2S2O8 g/L E%
 
pH
 
k (min−1R2 EE/O (kWh/m3.order) 
5 min 35 min initial final 
0.1 67.50 84.25 5.0 3.8 0.0244 0.9915 12.57 
0.3 69.75 88.00 5.0 3.3 0.0308 0.9945 9.97 
0.5 74.25 97.00 5.0 2.9 0.0746 0.9910 4.11 
7.0 80.50 98.25 5.0 2.7 0.0787 0.9935 3.90 
1.0 89.75 99.50 5.0 2.4 0.0879 0.9126 3.49 
2.0 91.25 97.00 5.0 2.2 0.0351 0.9067 8.73 
Na2S2O8 g/L E%
 
pH
 
k (min−1R2 EE/O (kWh/m3.order) 
5 min 35 min initial final 
0.1 67.50 84.25 5.0 3.8 0.0244 0.9915 12.57 
0.3 69.75 88.00 5.0 3.3 0.0308 0.9945 9.97 
0.5 74.25 97.00 5.0 2.9 0.0746 0.9910 4.11 
7.0 80.50 98.25 5.0 2.7 0.0787 0.9935 3.90 
1.0 89.75 99.50 5.0 2.4 0.0879 0.9126 3.49 
2.0 91.25 97.00 5.0 2.2 0.0351 0.9067 8.73 

CONCLUSION

The present study was carried out to evaluate the efficiency of the UV/S2O8 process in the MNZ removal from aqueous solution. For this purpose, the effect of main factors in the process such as pH, initial MNZ concentration, initial persulfate concentration and contact time on the process efficiency was investigated. The findings of the empirical studies showed that the efficiency of MNZ degradation by persulfate is much higher when exposed to UV radiation compared to when using persulfate or UV alone for degradation. It was also found that the UV/S2O8 process efficiency was higher in acidic pH values due to further production of SO4 radicals and also showed an increasing trend by prolonging contact time, so that more than 99% of the antibiotic was removed from the solution at pH of 3–5, the MNZ concentration of 40 mg/L of and Na2S2O8 concentration of 1 g/L after 35 min. Therefore, it can be concluded that the UV/S2O8 process can be effective in removing MNZ from aqueous solution.

ACKNOWLEDGEMENTS

This article is derived from the MSC dissertation by Ms Parisa Tavassoli (Project No. 7290). All authors are grateful to the Zahedan University of Medical Sciences for the financial support of this study. Furthermore, all authors wish to thank Dr Hossein Kamani, Dr Ferdos Kord Mostafapour and Mr Davoud Balarak for their support during analysis of the experiments.

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