The aim of this study was to determine the removal of ciprofloxacin (CIP) by the electro-persulfate (EC-PS) process using aluminum (Al) electrodes. The effects of variables including pH, contact time, PS concentration, initial CIP concentration and current density on the removal efficiency of CIP were studied. In order to determine the mechanisms of the EC-PS process, the radical scavenger tests, as well as energy dispersive spectroscopy (EDS) and Fourier transform infrared spectroscopy (FT-IR) were performed on the sludge. The results showed that the PS process alone had no effect on the CIP removal, and the EC process alone could remove 25% of CIP after 160 min. However, the EC-PS process under the optimum conditions: pH of 7, time of 40 min, current density of 2.75 mA/cm2, CIP concentration of 20 mg/L, and PS concentration of 0.84 mM removed 90% of CIP. The effect of the EC-PS process on the actual hospital wastewater was 81% in optimal conditions. The kinetic study also showed that the second-order kinetic model was the most consistent. The oxidation process during the initial contact was dominant in the EC-PS process and, over time, the EC process was dominant for CIP removal.
Over the last few decades, high levels of antibiotics have been used in many fields such as agriculture, aquaculture and animal husbandry. Pharmaceutical industries have been producing large volumes of effluents containing different kinds of antibiotics. If antibiotic-laden wastewaters are not treated effectively, surface water and underground water resources can be contaminated (Li et al. 2016; Frontistis et al. 2018). The presence of antibiotics in aquatic environments has raised concerns, because of not only the high potential for biocompatibility of these compounds, but also the increased antibiotic resistance of bacteria (Matzek & Carter 2017). Ciprofloxacin (CIP) is one of the third-generation fluoroquinolones, which has widely been used as an antibacterial agent in recent years. Its concentration measured near the pharmaceutical plants was very high, at about 50 mg/L (Li et al. 2018a). Fluoroquinolones may also cause physiologically teratogenic effects on plants and algae; they are also genotoxic and carcinogenic to organisms. Unfortunately, since these types of antibiotics are less biodegradable, conventional biological wastewater treatment processes are not able to successfully remove these compounds from wastewater (Cecconet et al. 2017; Ao et al. 2018; Li et al. 2018b). Therefore, the development of effective treatment methods is of great importance (Li et al. 2019). A newer form of advanced oxidation processes (AOPs) is based on the production of sulfate radical (SO̊4−). Sulfate radical-based AOPs have advantages compared to ̊OH-based AOPs, which include greater oxidation potential, better selectivity for target pollutants, longer half-life and activity at higher pH levels (Ao & Liu 2017; Ao et al. 2018; Malakootian & Heidari 2018; Malakootian et al. 2019). Persulfate (PS) can be activated to produce higher amounts of SO̊4− with higher standard redox potential (pH dependent) using different methods like thermal energy, UV, or microwave radiation and nanoparticles (Matzek & Carter 2016). Recently, electrochemistry has been used in several studies for the activation of PS to degrade the non-biodegradable and emerging compounds. This process is called electro-persulfate (EC-PS) and, recently, has attracted the attention of many researchers. Its mechanism is producing electrons on the cathode that react with S2O8, which, in turn, generates SO̊4−. Also, PS can be restored on the anode. In fact, a production cycle of SO̊4− and PS occurs, which enhances the activation of PS and decomposes pollutants in a shorter time (Chen & Huang 2015; Wacławek et al. 2015; Frontistis et al. 2018; Liu et al. 2018). In the past, various processes have been used to remove and mineralize ciprofloxacin, some of which include Fenton, electro-Fenton and electrochemical oxidation. In comparison with the above-mentioned processes, the EC-PS process has several benefits like much less contact time (6 hours for Fenton with similar efficiency) and less sludge production, lower treatment costs and less land requirement (Sekhar & Kumar 2014; Yahya et al. 2014; Shen et al. 2018). Thus, the aim of this study was to determine the removal of CIP by the EC-PS process using aluminum electrodes.
MATERIAL AND METHODS
CIP (purity ≥99%) was purchased from a pharmaceutical company inside Iran. Sodium persulfate (Na2S2O8), sodium hydroxide and hydrochloric acid were procured from Merck (Germany). All of the chemicals were of analytical grade. The stock and other solutions required for the experiments were prepared by deionized water.
Figure 1(a) shows a schematic diagram of the pilot used in this study. As can be seen, the experiments were performed in an 800-mL glass reactor containing 600 mL of the electrolyte solution. In the reactor, four electrodes were used, two of which were used as the cathode and the others were used as the anode. All electrodes were made of aluminum (99 wt%) with 25 mm × 150 mm × 1 mm in size. The electrodes were submerged in the solution and installed at a distance of 2 cm apart. The monopolar electrodes were connected in parallel and 50 mM sodium sulfate was added to the solution to support the electrolyte and ensure the electrical conductivity in the reactor. The distance of the electrodes from the floor and the walls were 10 mm. In order to mix the solution thoroughly, the reactor was placed on a magnetic stirrer. Direct current (DC) power supply was used to provide electricity in different quantities.
Effect of EC, PS and EC-PS
To investigate the effect of EC, PS, and EC-PS processes on CIP removal from aqueous solutions, a solution containing 20 mg/L of CIP was prepared. Then, the pH of the solution was adjusted to the desired level (H2SO4, 0.01 M and NaOH, 0.1 M). In the first step, the solution was examined using the PS process alone. In the second step, the solution was analyzed using the EC process. In the third step, PS was added to the solution and entered into the EC reactor. Kinetic study was also performed to evaluate the behavior of CIP removal by each process.
Effect of key parameters on EC-PS
The effective parameters in the EC-PS process include pH (3, 5, 7 and 9), PS concentration (0.42, 0.84, 1.26 and 1.68 mM), current density (0.9, 1.55, 2.75 and 4.25 mA/cm2) and CIP concentration (10, 20, 40 and 60 mg/L) were examined. In all the above steps, the main parameter was variable and other parameters were fixed. After determining the optimum conditions, the effect of the EC-PS process on hospital wastewater was studied. For this purpose, actual wastewater was prepared from Afzalipour Hospital (Kerman, Iran) and then its properties were measured.
Determination of radical scavengers and sludge characterizations
Due to the presence of both types of radicals, tert-butanol (TBA) and methanol were used to study the effect of each oxidizing species on the performance of the EC-PS process. Elements and compounds in the structure of the produced sludge from the EC-PS process were determined by using energy dispersive spectroscopy (EDS) (MIRA3 TESCAN-XMU) and Fourier transform infrared spectroscopy (FT-IR) (FT-IR 6300, Brucker).
RESULTS AND DISCUSSION
Comparison of EC, PS and EC-PS
The investigation of synergistic effects (SE) on EC, PS and EC-PS processes indicated that the SE value was 5.4. The values of the SE number illustrated that the removal efficiency of the EC-PS process was 5.4 times more efficient than that of the EC process alone, because the PS process had no effect on CIP removal. This is because of the fact that the efficiency of the combined EC-PS process was higher than those of other processes. It should be noted that both the PS anions and oxygen compete to be more absorbed on the aluminum cathode surface; this ultimately causes them to become sulfate radical and hydrogen peroxide. In the EC process alone, hydrogen peroxide is produced, but in the EC-PS process, sulfate radicals are produced. Thus, the CIP degradation efficiency through oxidation with the EC-PS process is higher than the oxidation of CIP with each of the EC and PS processes alone (Chen & Huang 2015; Ledjeri et al. 2017; Liu et al. 2018). In the study by Liu et al., the EC-PS process was used to remove tetracycline from aqueous solutions; they claimed that the removal efficiency of the method was much higher than any of the EC and PS processes alone (Liu et al. 2018), which is consistent with the results of our research.
Effect of initial pH on EC-PS and pH changes during the process
The effect of initial pH on the EC-PS process is shown in Figure 2(a). As indicated in the figure, the neutral pH of 7 had the highest CIP removal efficiency (90.2%) at contact time between 10 and 40 min. After 40 min, approximately all pH values had equal removal efficiency. In general, with increasing pH to the neutral value, the CIP removal efficiency increased. The only exception was the alkaline pH of 9, at which the CIP removal efficiency declined slightly. Initial pH is one of the most important factors affecting the mineralization of pollutants in the EC-PS process. Usually, in the EC-PS process the mineralization efficiency of pollutants enhances with declining pH to acidic conditions (Zhang et al. 2014; Wacławek et al. 2015; Matzek & Carter 2017). On the anode, MOx(̇OH) species is formed under acidic conditions. In addition, ̇OH radicals are formed in bulk solution (Bonyadinejad et al. 2015). On the other hand, with decreasing pH, the solubility of oxygen gas in the solution decreases. Reducing oxygen causes the absorption of PS anions on the aluminum cathode, which facilitates the formation of sulfate radicals. It should be pointed out that oxygen acts in competition with the PS anions and reduces the adsorption of PS anions on the electrode surface (Chen & Huang 2015). Figure 2(b) shows the trend diagram of pH changes during the EC-PS process. As can be seen, over time, all the initial pHs moved to the neutral pH of 7.5, and, after 30–40 min, the pH became similar in all solutions with different initial pH values. Regarding the pH changes and the optimal pH for the formation of sulfate and hydroxyl radicals, it is concluded that during the initial contact time both ̇OH and SO̊4− radicals are responsible for the CIP removal. On the other hand, based on pH changes during the process, at neutral pHs, the Al(OH)3 was produced faster than at other pHs and EC combined with oxidation for further CIP removal. However, the difference between CIP removal efficiency in acidic and neutral pH was very low, approximately 2–3% (Chung et al. 2012; Sun & Wang 2015). Due to reactions occurring in the EC-PS process, the SO̊4− radical is converted to the ̇OH radical. In addition, hydrogen peroxide (H2O2) is formed by S2O82 hydrolysis under acidic conditions, and subsequently hydrogen peroxide is produced from the reduction of the ̇OH radical in the cathode. Therefore, all oxidizers act simultaneously to remove contaminants, which increases the performance of the EC-PS process at initial contact times (Chung et al. 2012; Chen & Huang 2015; Wacławek et al. 2015).
In the electrocoagulation process, the anodic dissolution of electrodes also occurs, which produces metal hydroxide ions. These metal hydroxides cause the destabilization of pollutants through the formation of flocculants. In this study, Al3+ is combined with hydroxyl ions (OH) formed on the cathode and produces Al(OH)3 resulting in the destabilization and removal of CIP (Carreño et al. 2018). As a general rule, metal hydroxides are highly absorbent, and can absorb unstable pollutants and form flocculates. These fluctuations can be separated by sedimentation or flotation (Chung et al. 2012). Therefore, after the initial contact time, according to Figure 2(b), pH changed to 7.5; thus, the EC process plays the main role in CIP removal by forming Al(OH)3. Increasing acidic pH and decreasing alkaline pH and reaching pH of about 7.5 are also due to the formation of Al(OH)3; this observation is similar to the results of the study carried out by Cañizares et al., who investigated the electrochemical dissolution of aluminum electrodes in different conditions (Cañizares et al. 2005). In this study, at pH 9, the removal efficiency in the first 10 minutes of the EC-PS process was about 70%, which was a relatively high efficiency. Like acidic and neutral pHs, the high efficiency of CIP removal in alkaline pH and in the initial minutes can be due to the presence of sulfate and hydroxyl radicals. However, after the initial reaction time during which the pH of the solution is reduced, in addition to the oxidation process, the formation of metal hydroxides increases and coagulation may play a major role in CIP removal (Chen & Huang 2015; Liu et al. 2018). In general, the process efficiency in alkaline pH is less than at the optimal pH, which is neutral. The reason for this phenomenon is that the rate of aluminum dissolution during the EC-PS process at a neutral pH range is several times higher than at alkaline pH. Therefore, the formation of Al(OH)3 floces at neutral pH is higher than other pH ranges. Increasing the amount of flocculation also increases the removal of CIP from the solution. On the other hand, as mentioned above, at neutral pH, sulfate radicals are the dominant species, which have a higher standard redox potential than ̇OH radicals (Carreño et al. 2018).
Effect of PS concentration on EC-PS
As can be seen in Figure 3(a), the removal efficiency rates of CIP were 79.8, 88.12, 89.26 and 84%, respectively, for the PS concentrations of 0.42, 0.84, 1.26 and 1.68 mM at the optimum contact time of 40 min. The lowest cost must also be considered in selecting PS content (Chen & Huang 2015); in this study, 0.84 mM was found to be the optimum concentration of PS. In the study by Yang et al., who used the UV/persulfate process to remove CIP, the PS dose of 10 mM was used with approximately the same efficiency (Yang et al. 2019). Also, in the study by Ahmed et al., in which the UV/peroxymonosulfate process was used to remove CIP, the amount of PS used was 2.5 mM with a similar efficiency (Ahmed & Chiron 2013). Therefore, the amount of PS used in the present study was considered to be much lower than similar studies. Also, based on Figure 3(a), with increasing initial PS concentration, the efficiency increased. This phenomenon may be due to a significant increase in the number of sulfate radicals stemming from the cathode reduction of the PS anions (Chen & Huang 2015). But, at the highest PS concentration, equal to 1.68 mM, the efficiency of the process declined. The cause of this phenomenon is the side reaction between additional PS anions and sulfate radicals; in fact, the extra sulfate anion plays the role of scavenger for sulfate radicals (Sun & Wang 2015).
Effect of current density on EC-PS
Effect of initial CIP concentration on EC-PS
Figure 3(c) shows the results of the initial CIP concentration on the EC-PS process. As shown, with increasing initial CIP concentration, the removal efficiency decreased. The removal efficiencies for initial CIP concentrations of 10, 20 and 40 mg/L were 91.37, 89.38 and 82%, respectively. At an initial concentration of 10 mg/L and contact time of 30 minutes, CIP removal rates were obtained at 92.77%; at higher concentrations of antibiotics, lower removal efficiency was obtained at the initial contact time. This is because of the fact that, at low concentrations of pollutants, radicals and coagulants can easily react with a high percentage of contaminants and remove them from the aquatic environment. With an increasing amount of pollutant concentration in the environment, since the other parameters are constant, it is obvious that the CIP removal efficiency decreases (Chen & Huang 2015; Frontistis et al. 2018; Liu et al. 2018). Also, an increase in the concentration of pollutants in the reactor leads to the production of intermediate materials and coagulants, which can be consumed for removal of these materials, thereby reducing the process efficiency. The results of others researchers are consistent with the findings of this work (Lin et al. 2013; Chen & Huang 2015; Liu et al. 2018).
Effect of EC-PS process on actual wastewater
The actual sample taken from the Afzalipour Hospital and the properties were determined, which were as follows: pH: 7.6, BOD5: 38 mg/L, COD: 104 mg/L, TSS: 59 mg/L, NO3−:15 mg/L, PO43: 1.65 mg/L and CIP: 3.5 mg/L. The removal rate for this wastewater under the optimum conditions was equal to 81%. Reduction of the removal efficiency in the actual sample can be due to the interfering role of other pollutants.
Kinetic studies were used to determine the CIP removal behavior. The kinetic of the EC and EC-PS processes were determined using the equations presented in Table 1.
|Kinetic model .||Equation .||Linear form .|
|Kinetic model .||Equation .||Linear form .|
The results of the kinetic study have been given in Table 2 and Figure 4. As shown in the table, the results of the CIP removal followed the second-order kinetic model for EC and EC-PS processes. The highest R2 value is related to the second-order kinetic model for both processes.
|Kinetic model .||Constant .||Process type|
|PS process .||EC process .||EC-PS process .|
|Kinetic model .||Constant .||Process type|
|PS process .||EC process .||EC-PS process .|
Determination of radical scavengers
Radical scavenger experiments were performed using chemicals that selectively respond to the most important radicals. To study the contribution of oxidative species to the EC-PS process, tert-butanol (TBA) and methanol have been used in many studies. Specifically, the reported speed constants of these two radical scavengers are as follows: methanol is usually considered as a radical scavenger for SO̊4− and ̇OH with the rate constants of 3.2 × 106 and 9.7 × 108 M−1 s−1. Also, the rate constant of TBA for radical ̇OH is 418–900 times greater than that for SO̊4− radical (Nasseri et al. 2017; Zhang et al. 2017). As shown in Figure 4(d), the CIP removal efficiency via EC-PS without the presence of a radical scavenger was 83% at a contact time of 10 min. But, in the same conditions by adding TBA and methanol, the removal efficiency was reduced to 62.44 and 52.74%, respectively.
According to the obtained results, it is understood that in the EC-PS process using aluminum electrodes, contrary to our expectation, the ̊OH radical plays a major role in the EC-PS process and the SO̊4− radical has a lesser role in the early time. The reason for this phenomenon is that the SO̊4− radical is firstly converted to ̇OH radicals based on the reactions given in Equation (9) and ̇OH radicals play a major role in the CIP oxidation. A 30% reduction in the efficiency occurred after the addition of methanol due to its scavenger role for both SO̊4− and ̇OH radicals. The difference in efficiency reduction between the addition of methanol and TBA indicates the role of SO̊4− radicals, which is about 10% of the total efficiency (Nasseri et al. 2017; Zhang et al. 2017). After the initial contact time, the electrocoagulation process replaces the oxidation process, and because of the reactions that occur, Al3+ ions are combined with the hydroxyl ions (OH) (formed on the cathode) which leads to the formation of Al(OH)3. CIP molecules are attached to the Al(OH)3 flocs and physically removed from the reactor by sedimentation (Cañizares et al. 2005; Carreño et al. 2018).
Chemical structure of the sludge generated by the EC-PS process using FT-IR analysis
The KBr disc FT-IR spectra of the produced sludge from the EC-PS process at 500–4,000 cm−1 have been shown in Figure 5(a). According to the FT-IR spectrum of the sludge, the wide peak observed at 3,500 cm−1 is due to the overlapped OH and NH stretching modes with aliphatic C-H, C-H stretching at >3,000 cm−1, C = O stretching at 1,637 cm−1, C-H bending of methylene group at 1,404 cm−1, and C-O stretching at 1,130 cm−1. Absorption peaks around 433 cm−1 in FT-IR spectra are related to the metal-oxygen bond (Al-O) (Pavia et al. 2008). The FT-IR results show that the produced sludge contained the compounds derived from the CIP degradation. Also, the produced sludge from the EC-PS process contained aluminum oxide generated from the oxidation of the aluminum electrodes. The existence of the wide peak observed at 3,500 cm−1 is due to the OH illustrating the formation of the Al(OH)3 sludge.
Chemical structure of the sludge generated by the EC-PS process using EDS analysis
According to the results of the EDS analysis, as shown in Figure 5(b), it was observed that in the sludge from the EC-PS process, carbon, nitrogen, oxygen, fluorine, sodium, aluminum and sulfur contents were 3.47, 1.37, 53.64, 0.00, 0.25, 38.91 and 2.35%, respectively. These results indicate the presence of chemical elements resulting from the EC-PS process, as well as hydroxides and metallic oxides produced in the sludge. High amounts of aluminum and oxygen and the result of FT-IR analysis also confirm the formation of aluminum hydroxide (Al(OH)3), which is flocculated and physically removes the antibiotic from the wastewater. High levels of flocs also confirm that in the EC-PS process, the electrocoagulation process removes more CIP than the oxidation process. The oxidation process in its early stages leads to the removal of CIP and then the electrocoagulation process predominates.
Based on the results, the application of PS alone did not have any effect on the removal of the antibiotic. The EC process also had a 25% removal efficiency after 160 min. But the combined EC-PS process at the earliest times had the heightened effect on CIP removal from aqueous solutions. The CIP removal efficiency was 90% under the optimum conditions. Based on the results of the addition of radical scavengers in the EC-PS process, it is shown that, contrary to our expectation that the SO̊4− radicals would take the main role, the ̇OH radicals play a major role in this process, and SO̊4− radicals play a lesser role in the early stages of the process. High amounts of aluminum and oxygen, the result of the FT-IR analysis and the wide peak observed at 3,500 cm−1 illustrate the formation of the Al(OH)3 sludge.