Removal of Ciprofloxacin from Wastewater by Ultrasound/Electric Field/Sodium Persulfate (US/E/PS)

Abstract

Ciprofloxacin (CIP), as a common antibiotic used in human clinical and livestock farming, is discharged into natural water bodies and its concentration has increased in the last years. Its stable chemical structure is difficult to remove by conventional techniques. Residual ciprofloxacin in the environment has become an emerging micropollutant that promotes the generation of resistance genes of bacteria and endangers ecosystem balance and human health. Removal of ciprofloxacin from water by the system of ultrasound/electric field/sodium persulfate (US/E/PS) was investigated. Firstly, CIP degradation affects by different oxidation methods, such as ultrasonic oxidation, electro-oxidation, and persulfate oxidation, and their four combined oxidation methods (ultrasound-activated persulfate oxidation, electro-activated persulfate oxidation, ultrasound-enhanced electro-oxidation, and ultrasound-enhanced electro-activated persulfate oxidation), on the target contaminants were compared. Secondly, the influences of parameters on the CIP degradation by an ultrasound-enhanced electro-activation-persulfate reaction system were investigated. Thirdly, the possible free radical species in the ultrasound-enhanced electro-activation-sulfate reaction system were identified and the dominant free radical species in the system were analyzed. Finally, the samples of CIP in the US/E/PS system were tested by liquid mass spectrometry, and the possible intermediate products and degradation path were speculated. The results indicate that the US/E/PS system is of great potential application value in the removal of organic pollution and environmental purification.

1. Introduction

Wastewater from animal husbandry, aquaculture, and the pharmaceutical industry is the major source of antibiotics in the environment [1]. After being absorbed and metabolized by the human body or animal and plant bodies, antibiotic drugs are discharged out of the body with urine, feces, and the like in the form of mother body or metabolite, and then enter the sewage system and solid waste system [2,3]. Data show that nearly 80% of drugs used by livestock and aquaculture farmers enter the water environment or settle into the sediment with feed, urine, and feces [4]. Medical wastewater and pharmaceutical wastewater is characterized by high antibiotic content, high biological toxicity, and a large fluctuation range of water volume and concentration [5]. It is difficult to effectively remove antibiotics with traditional wastewater treatment processes. Undegraded drugs enter the natural environment with the treated wastewater or sludge [6,7]. Furthermore, improper disposal of expired antibiotic drugs is also an important reason for a large number of antibiotics entering the ecological environment [8,9].

Fluoroquinolones (FQs) belong to the third generation of quinolones and are new antibacterial drugs introduced in the 1970s [10]. They have the advantages of a wide antibacterial spectrum, little toxic and side effects, convenient use, and reasonable price, among which ofloxacin, ciprofloxacin (CIP), and enrofloxacin are the most widely used [11,12]. In recent years, a large number of oxidation technologies such as chlorine oxidation, ozone oxidation, ultraviolet light combined with hydrogen peroxide oxidation, photocatalytic oxidation, ultrasonic degradation, and permanganate oxidation have been studied and developed to treat FQs [13,14]. The concentration range of CIP detected in water is mainly at ng/L to μg/L. Unfortunately, the concentration of CIP detected gradually increased, and a CIP concentration range as high as mg/L has been found [15]. CIP shows strong chemical interference and has toxicity for microorganisms; it is also hard for CIP to be adsorbed and utilized by microbials. Therefore, activated carbon, chitosan synthesized adsorbent, or MOF materials are often used for CIP adsorption. However, the adsorption method cannot completely degrade CIP and make it innocuous. Therefore, advanced oxidation methods are proposed to degrade and remove it. It is reported in China’s sewage treatment engineering network that more than 5340 t of CIP was used in China in 2013. Its usage is even higher now, making CIP the second highest used antibiotic among all fluoroquinolones [16]. The frequent use of CIP causes some pathogenic bacteria to develop drug resistance, which will threaten human health if they survive in the environment for a long time. CIP can promote the generation of resistance genes (ARGs), and the spread and diffusion of resistance genes may accelerate the mass reproduction of resistant bacteria and form a potential threat to the microbial community structure, thus posing a secondary threat to human health and ecological environmental security [17,18]. In addition, CIP has an enrichment effect in the human body, which can interfere with normal hormone secretion, cause mental disorders, and destroy the normal metabolism of the human body. CIP also interferes with the human digestive system and can lead to gastric cancer [19,20]. Thus, CIP wastewater must be treated by highly efficient methods.

The advanced oxidation method is a process of oxidative degradation of target pollutants by generating hydroxyl radicals (•OH) [21,22]. Organic pollutants react with free radicals and then degrade into low-toxic or even non-toxic small molecular substances, or are directly mineralized into harmless inorganic substances such as carbon dioxide, water, inorganic mineral salts, etc. [23,24,25]. Compared with other oxidation technologies and other methods such as adsorption, biodegradation, flocculation, and membrane filtration, advanced oxidation technology has the following characteristics [26]: (1) strong oxidation capacity and fast reaction speed; (2) the reaction condition is mild and easy to control; (3) wide application range and no secondary pollution; (4) other treatment technology can be combined to reduce the treatment cost; (5) simple operation and easy equipment management. With the rapid development of the pharmaceutical industry and fine chemical industry, the problem of high concentration organic wastewater treatment and water micro-pollution has prompted the attention of environmental protection science and technology workers. However, it must be noted that advanced oxidation methods have limitations. Advanced oxidation process requires some pretreatment conditions to meet its operations. These pretreatment conditions include coagulation precipitation, turbidity removal, pH control, pollutant concentration control, temperature, and so on. Sometimes it is even necessary to use other water treatment methods to achieve better performance and efficiency. As a new star of water treatment technology that has arisen in the past 30 years, advanced oxidation technology has been noticed by workers in the field of water treatment research throughout the world [27]. The treatment of high-concentration organic wastewater and micro-polluted organic matter in the water body by advanced oxidation technology has also become a hot research topic [28,29].

Ultrasonic oxidation technology is a special application of ultrasonic chemical technology in purifying pollutants (especially difficult-to-degrade pollutants). [30]. Using ultrasonic cavitation to accelerate reaction or open up a new reaction channel, improve the yield of chemical reaction, or obtain new chemical reactants is a new frontier science. Its application research has attracted wide attention all over the world [31]. In addition, the advanced oxidation method, based on sulfate free radical (${\mathrm{S}\mathrm{O}}_4^{•-}$), is considered to be an effective organic wastewater treatment technology with high standard redox potential. As a strong oxidant, sodium persulfate (PS) can theoretically degrade most organic pollutants and has great utilization potential [32]. Using US and electric field-assisted activating PS technology has several evident advantages. On the one hand, US and electric field have a strong mechanical effect, which can enhance the mass transfer to speed up the oxidation reaction for good pollution degradation and removal effect [33]. On the other hand, the action of ultrasonic cavitation and electric field also activates PS to produces ${\mathrm{S}\mathrm{O}}_4^{•-}$, which can be used for the intensive treatment of organic pollutants [34]. Therefore, the combination of ultrasonic field, electric field, and PS is proposed for the efficient treatment of CIP wastewater.

Thus, it is proposed to combine US, electric field, and PS to treat CIP wastewater with high efficiency.

In this paper, ciprofloxacin was selected as the target contaminant and sodium persulfate (PS) was used as the oxidant. The reaction phenomena and laws of ultrasound-enhanced electro-activated persulfate removal of ciprofloxacin from deionized water were investigated by varying the parameters, such as ultrasonic power, electrode potential, reaction temperature, electrode potential, initial molar concentration ratio of PS to ciprofloxacin, and initial solution pH. Finally, the reaction mechanism was explained.

2. Experimental Materials and Methods

2.1. The Main Experimental Instruments and Equipment Are as Follows

High-performance liquid chromatograph, LC-15C, Shimadzu Company, Kyoto, Japan; electronic analytical balance, FA2204B, Shanghai Jingke Tianmei Scientific Instrument Co., Ltd., Shanghai, China; low-temperature constant temperature trough, THD-1015, Ningbo Tianheng Instrument Factory, Ningbo, China; ultrasonic cell grinder, JYP2-IIN, Ningbo Scientz Biotechnology Co., Ltd., Ningbo, China; DC power supply, GPR-6030D, Guwei Electronics (Shanghai) Co., Ltd., Shanghai, China; platinum electrode, 1 cm × 1 cm, Xuzhou Zhenghao Electronic Technology Co., Ltd., Xuzhou, China; CNC ultrasonic cleaner, KQ5200DB, Kunshan Ultrasonic Instrument Co., Ltd., Kunshan, China; ultraviolet-visible spectrophotometer, UV-3100, Shanghai Mapada Instrument Co., Ltd., Shanghai, China; electric blast dryer, DHG9075A, Shanghai Yiheng Scientific Instrument Co., Ltd., Shanghai, China; digital display pH meter, PHS-3C, Shanghai Yidian Scientific Instruments Co., Ltd., Shanghai, China; temperature-controlled magnetic stirrer, XMTD-702, Jintan Medical Instrument Factory; circulating water type multi-purpose vacuum pump, SHZ-D (III), Wuhan Keer Instrument and Equipment Co., Ltd., Wuhan, China; UPHW-II-90T, Chengdu Ultra Pure Technology Co., Ltd., Chengdu, China; two-dimensional liquid chromatography–ion trap mass spectrometry, 1100LC/MSD Trap, Agilent Company, Santa Clara, CA, USA.

2.2. Major Chemical Reagents and Pharmaceuticals

Ciprofloxacin, C₁₇H₁₈FN₃O₃, purity > 98.0%, Shanghai Aladdin Biochemical Technology Co., Ltd., Shanghai, China; norfloxacin, C₁₆H₁₈FN₃O₃, 98%, Shanghai Aladdin Biochemical Technology Co., Ltd.; acetonitrile, C₂H₃NHPLC, Sinopharma Group Chemical Reagent Co., Ltd. (Shanghai, China); methanol, CH₃OHHHPLC, Sinopharma Group Chemical Reagent Co., Ltd.; methanol, CH₃OHA. R, Sinopharma Group Chemical Reagent Co., Ltd.; tert-butanol, C₄H₁₀OA. R, Sinopharma Chemical Reagent Co., Ltd.; sodium hydroxide, NaOHA.R., Sinopharma Group Chemical Reagent Co., Ltd.; sulfuric acid, H₂SO₄ A. R, Sinopharma Group Chemical Reagent Co., Ltd.; hydrochloric acid, HClA.R., Sinopharma Group Chemical Reagent Co., Ltd.; sodium persulfate, Na₂S₂O₈A. R, Sinopharma Group Chemical Reagent Co., Ltd.

2.3. Experimental Methods

A double-layer glass jacket beaker with a volume of 250 mL and an inner diameter of about 6.5 cm was used as the reaction vessel, and the reaction solution was 200 mL. The pH value of the solution was adjusted by 1.0 mol/L NaOH and H₂SO₄ solution for coarse adjustment and 0.1 mol/L NaOH and H₂SO₄ solution for fine adjustment. After the reaction solution is prepared, the ultrasonic horn (located in the center of the reaction vessel, insert at liquid level, 3 cm from the bottom of the reactor), and a DC power supply electrode are used. The liquid level for the electrode is inserted at a symmetrical position on both sides of the horn, within 3 cm of the bottom of the reactor and 3 cm away from the positive and negative electrode. The react equipment for US/E/PS is sealed with a PVC membrane to prevent volatilization loss in the reaction process of the reaction solution. The circulation pump from the low-temperature constant temperature tank is employed for temperature control. When the temperature of the reaction system reaches the experimental set temperature, DC voltage and ultrasonic power are conducted to carry out the reaction. The experimental device is shown in Figure 1. The reaction time was set for 2 h, and the sampling time was set to 0 min, 15 min, 30 min, 45 min, 60 min, 90 min, and 120 min, respectively. In the experiment, 2 mL of ciprofloxacin (0.03 mmol/L) was sampled, and an appropriate amount of free radical inhibitor (methanol and tert-butanol) was added. After filtration by a 0.45 m filter membrane, the characteristic peak area was determined by high-performance liquid chromatography, and the concentration was determined by high-performance liquid chromatography. Finally, the samples under the parameters with good removal effect were analyzed by a high-performance liquid chromatograph, LC-15C, Shimadzu Company, Japan; Electronic Analytical Balance, FA2204B, Shanghai Jingke Tianmei Scientific Instrument Co., Ltd. with a XBridge-C18 (4.6 mm × 150 mm, 5 μm) reversed-phase column. The possible intermediates and degradation paths of the antibiotics were speculated with the ultrasound-enhanced electro-activated persulfate system. The experimental conditions are set at a pH of 5.0–11.0, initial molar concentration of ciprofloxacin was 0.03 mmol/L, initial molar concentration of PS was 1.5–30 mmol/L, ultrasonic power was 0–200 W, electrode potential was 0–9 V, temperature was 25–55 °C, and ultrasonic frequency was 20 kHz.

The samples were treated at standard atmospheric pressure and under light protection. The concentration of ciprofloxacin in the reaction solution was analyzed by HPLC at intervals of 30 min.

2.4. Solution Preparation and Measurement Method

2.4.1. Solution Preparation and Storage Method

To prepare and store 0.62 mmol/L (200 mg/L) of ciprofloxacin stock solution, 0.2000 g of the ciprofloxacin was accurately weighed, an appropriate amount of deionized water added, and an appropriate amount of hydrochloric acid was dropped to aid dissolution. Avoiding light and stirring with a magnetic stirrer for 8 h, the solution was transferred to a 1000 mL volumetric flask and the volume fixed with deionized water.

To prepare 100 mmol/L of the sodium persulfate solution, 11.9050 g of sodium persulfate was weighed accurately, dissolved in an appropriate amount of deionized water, and transferred to a 500 mL volumetric flask to fix the volume. The shelf life of sodium persulfate solution is about one week, and the expiration date of the agent should be ensured when it is used.

2.4.2. Analytical Method

An LC-15C high performance liquid chromatograph with an XBridge-C18 (4.6 mm × 150 mm, 5 μm) reversed-phase column is used. The liquid chromatographic test conditions for ciprofloxacin were as follows: the injection volume was 20 μL, the column temperature 30 °C, and the flow rate was 0.6 mL/min. The mobile phase was acetonitrile: 0.1% acetic acid = 20:80 (v/v). The detection wavelength was 278 nm, and the retention time of ciprofloxacin was 4.4 min under the liquid chromatographic conditions.

2.4.3. Index Calculation Formula

Removal Rate

The removal rate of ciprofloxacin is calculated by Equation (1):

$$\mathrm{Removal} \mathrm{rate}=\frac{C_0-C}{C_0}\times 100\%$$

(1)

where C₀ and C is the concentration of ciprofloxacin (mg/L) at the 0 and t moments, respectively.

First Order Reaction Rate Constant

For the reaction conforming to first-order reaction kinetics, the reaction rate constant can be expressed by Equation (2):

$$-\frac{dC}{dt}=kC$$

(2)

By integrating it, the Formula (3) is obtained:

$$\ln \left(\frac{C_0}{C_t}\right)=kt$$

(3)

where, C₀ is the initial substrate concentration, mmol/L; C_t is the substrate concentration at time t, mmol/L; t is reaction time, min; k is the rate constant, min⁻¹.

3. Results and Discussion

3.1. Validation of Ciprofloxacin Removal by Different Reaction Systems

Experimental conditions: the pH was 5.0, initial molar concentration of ciprofloxacin was 0.03 mmol/L, initial molar concentration of PS was 1.5 mmol/L, ultrasonic power was 100 W, electrode potential was 4 V, and temperature was 35 °C. Firstly, under this experimental condition, the removal effect of ciprofloxacin by different reaction systems was compared, such as blank, ultrasonic oxidation alone (US), electro-oxidation alone (E), persulfate oxidation alone (PS), ultrasound-activated persulfate oxidation (US/PS), electro-activated-persulfate oxidation (E/PS), ultrasound-enhanced electro-oxidation (US/E), and ultrasound-enhanced electro-activated persulfate oxidation (US/E/PS). The degradation effects of ciprofloxacin by the eight different reaction systems were carried out for 120 min. Results are shown in Figure 2 (C₀ was the initial molal concentration of substrate ciprofloxacin, and C_t was the molal concentration at reaction time t). Compared with ultrasonic oxidation alone and electro oxidation alone, the removal rate of ciprofloxacin was greatly increased after PS was added. The removal rate of ciprofloxacin by US/PS was 28.5% higher than that by ultrasonic oxidation, and the removal rate by E/PS was 27.1% higher than that under electro oxidation alone. This shows that the removal of ciprofloxacin by US and electro oxidation alone is very weak, and the removal of ciprofloxacin mainly depends on the oxidation of ${\mathrm{S}\mathrm{O}}_4^{•-}$. It is worth noting that the contribution of US/E to ciprofloxacin removal was better than that of ultrasonic oxidation and electro oxidation alone, which indicates that the combination of US and electro oxidation promotes the production of •OH and other active substances and has a strong synergistic effect [35]. The quasi-first-order reaction kinetics fitting curves of ciprofloxacin by different reaction systems were also investigated.

The removal effect of ciprofloxacin by the seven systems is shown in Figure 3. It can be seen from the figure that the system of US/E/PS shows the best degradation effect, and the pseudo-first-order reaction rate constant was 0.0046 min⁻¹, which is about 2.7 times that of persulfate oxidation alone (0.0017 min⁻¹). Under the experimental conditions, the removal rate of ciprofloxacin was 37.4%. The combination of US, E, and PS has a strong synergistic effect (the synergistic coefficient S of US/E/PS reaction system is equal to 2.10).

3.2. Effect of Ultrasonic Power on CIP Removal Efficiency

To investigate the effect of ultrasonic power on the removal of ciprofloxacin by ultrasonic enhanced electro-activated persulfate system, the experimental conditions were set as follows: ciprofloxacin’s initial molar concentration was 0.03 mmol/L, PS initial molar concentration was 1.5 mmol/L, the initial pH value was 5.0, the electrode potential was 4 V, the reaction temperature was 35 °C, and ultrasonic power was adjusted to 0 W, 50 W, 100 W, and 200 W. After reaction for 120 min, the experiment results are shown in Figure 4.

As can be seen from Figure 4, the introduction of ultrasound can enhance the removal effect of ciprofloxacin. The removal rate of ciprofloxacin reached 40.9% at 50 W, which was 13.2% higher than that of the reaction system without ultrasound (27.1%). It may be that the introduction of US strengthens the generation of ${\mathrm{S}\mathrm{O}}_4^{•-}$ by E/PS, and the local high-temperature and high-pressure environment formed at the same time directly removes ciprofloxacin from water [36,37]. The removal rate of ciprofloxacin hardly increased significantly with increasing ultrasonic power, which may be due to excessive ultrasonic power and rapid growth of cavitation bubbles. Excessive cavitation bubbles affect the effective contact between the active material and the substrate, reduce the reaction rate, and make the removal effect worse (the removal rates corresponding to 100 W and 200 W were 37.4% and 40.1%, respectively). When the ultrasonic power was at 100 W, the ciprofloxacin’s removal efficiency was relatively good, with low energy and economic consumption, compared with that of 200 W.

3.3. Effect of Electrode Potential on CIP Removal Efficiency

To investigate the effect of electrode potential on the removal of ciprofloxacin by US/E/PS, the experimental conditions were set as follows: ciprofloxacin’s initial molar concentration was 0.03 mmol/L, PS initial molar concentration was 1.5 mmol/L, initial pH was 5.0, ultrasonic power was 100 W, reaction temperature was 35 °C, and electrode potentials were adjusted to 0 V, 4 V, 6 V, and 9 V, respectively. The reaction was carried out for 120 min and the removal effect of ciprofloxacin was observed (Figure 5). When the electrode potentials were 0 V, 4 V, 6 V, and 9 V, the removal rates of ciprofloxacin were 28.5%, 37.4%, 33.5%, and 30.5%, respectively. It can be seen that electrifying the reaction system is helpful for ciprofloxacin removal, but with the increase in electrode potential, the removal effect of ciprofloxacin shows a downward trend [38]. The reason for the result is that, after activation, ${\mathrm{S}}_2{\mathrm{O}}_8^{2-}$ gains electrons on the surface of the cathode electrode to produce ${\mathrm{S}\mathrm{O}}_4^{•-}$, which increases the active material in the system and accelerates the removal [39]. However, when the electrode potential increases to a certain degree, the current efficiency decreases, the energy utilization decreases, and the removal effect of organic matter is weakened instead.

3.4. Effect of Reaction Temperature on CIP Removal Efficiency

To investigate the effect of temperature on the removal of ciprofloxacin by US/E/PS, the experimental conditions were set as follows. Ciprofloxacin’s initial molar concentration was 0.03 mmol/L, PS initial molar concentration was 1.50 mmol/L, initial pH was set to 5.0, ultrasonic power was 100 W, electrode potential was 4 V, and the reaction temperatures were adjusted to 25, 35, 45, and 55 °C. The removal of ciprofloxacin was observed within the reaction time of 120 min.

Figure 6 shows the trends of ciprofloxacin removal at different reaction temperatures. The removal rates of ciprofloxacin were 22.1%, 37.4%, 50.7%, and 68.6% at 25 °C, 35 °C, 45 °C, and 55 °C, respectively. The experimental results showed that the removal effect of ciprofloxacin was significantly enhanced with the increase in the reaction temperature. The removal rate at 55 °C was more than three times that at 25 °C. It is evident that temperature has a greater effect on the removal of ciprofloxacin by US/E/PS system. This may be due to the following reasons [40,41]: (1) the increase in reaction temperature enhances the effective collision between reactant molecules, resulting in a faster reaction rate; (2) higher reaction temperature reduces the dissolved oxygen content and weakens the competition of O₂ for ${\mathrm{S}}_2{\mathrm{O}}_8^{2-}$ to gain electrons; (3) the increase in reaction temperature added thermal activation to the ultrasonic activation and electrical activation of ${\mathrm{S}}_2{\mathrm{O}}_8^{2-}$, so that more ${\mathrm{S}\mathrm{O}}_4^{•-}$ is produced. In the real ciprofloxacin degradation process, the room temperature is near 35 °C, which is why it was chosen for the experiment.

The experimental results of ciprofloxacin at different temperatures are fitted with quasi-first-order kinetics. The fitting curve is shown in Figure 7a. According to the reaction rate, constant k obtained at different experimental temperatures (25 °C, 35 °C, 45 °C, 55 °C), the experimental results are plotted with ln (k) and 1/T, and Arrhenius equation fitting is carried out on the experimental results. As shown in Figure 7b, the apparent activation energy of ciprofloxacin’s chemical reaction in the US/E/PS system can reach 54.00 kJ/mol.

3.5. Effect of the Ratio of PS to Initial Molar Concentration of Ciprofloxacin on CIP Removal Efficiency

To investigate the effect of the initial concentration ratio of oxidant (PS) to substrate (ciprofloxacin, CIP) on the removal of CIP by the US/E/PS system, the experimental conditions were set as follows: the initial molar concentration of CIP was 0.03 mmol/L, the initial pH was set to 5.0, the ultrasonic power was 100 W, the electrode potential was 4 V, and the reaction temperature was 35 °C. The initial molar concentration ratios of PS and CIP were adjusted to 5:1, 100:1, 500:1, and 1000:1. The removal of CIP was observed after reaction for 120 min.

As shown in Figure 8, with the increase in the initial concentration’s molar ratio, the ciprofloxacin removal rate also increased accordingly. When the initial molar ratio of PS to ciprofloxacin was increased from 5:1 to 500:1, the removal rate of ciprofloxacin increased from 23.1% to 64.0%, and the quasi-first-order reaction rate constant increased from 0.0019 min⁻¹ to 0.0099 min⁻¹. When the ciprofloxacin concentration remains unchanged, increasing PS concentration can increase the yield and concentration of ${\mathrm{S}\mathrm{O}}_4^{•-}$ generated by PS activation, thus increasing the contact opportunity between active substance ${\mathrm{S}\mathrm{O}}_4^{•-}$ and ciprofloxacin and greatly speeding up the removal efficiency of ciprofloxacin [42]. Compared with 50:1, the ciprofloxacin removal rate increased by two times for 500:1, and PS waste was also evident, therefore, 50:1 is more suitable in US/E/PS.

3.6. Effect of Initial pH on CIP Removal Efficiency

To investigate the effect of initial pH value of the solution on the removal of ciprofloxacin by US/E/PS system, the experimental conditions were set as follows: the initial molar concentration of ciprofloxacin was 0.03 mmol/L, the initial molar concentration of PS was 1.5 mmol/L, the electrode potential was 4 V, the ultrasonic power was 100 W, and the reaction temperature was 35 °C. The pH of the system was adjusted to make the solution acidic, neutral, and alkaline (pH values were 5.0, 7.0, and 11.0, respectively). The trend of ciprofloxacin removal rate after reaction for 120 min is shown in Figure 9. When the solution was acidic (pH = 5.0), neutral (pH = 7.0), and alkaline (pH = 11.0), the removal rates of ciprofloxacin were 37.4%, 39.1%, and 42.3%, respectively. It can be seen that the removal effect of ciprofloxacin changed slightly as the solution changed from acid to neutral and then to alkaline. The three pKa values of ciprofloxacin are 5.10, 5.85, and 8.10, with three existential morphologies under different solution pH conditions. Since the main mechanism of oxidation of organic matter by ${\mathrm{S}\mathrm{O}}_4^{•-}$ is electron transfer reaction, the existence form of ciprofloxacin in the more alkaline solution was more conducive to the removal of ${\mathrm{S}\mathrm{O}}_4^{•-}$ [43,44], so the removal effect was better. The pH was set to 5.0 because the initial pH of the real ciprofloxacin wastewater is close to 5.0.

3.7. Effect of Free Radical Quenching Agents on CIP Removal Efficiency

The experimental conditions were set as follows: pH was 5.0, the ciprofloxacin’s initial molar concentration was 0.03 mmol/L, PS initial molar concentration was 1.5 mmol/L, the ultrasonic power was 100 W, the electrode potential was 4 V, and temperature was 35 °C. Figure 10 compares the changes in ciprofloxacin’s removal effect after 120 min of reaction for five cycles, such as: no radical quencher dosing, 7.5 mmol/L Methanol (Me) (Me:PS = 5:1), 150 mmol/L Me (Me:PS = 100:1) and 7.5 mmol/L tert-butanol (TBA) (TBA:PS = 5:1), 150 mmol/L Me (Me:PS = 100:1), and 7.5 mmol/L TBA (TBA:PS = 5:1), 150 mmol/L TBA (TBA:PS = 100:1). The apparent quasi-first-order reaction rate constants decreased from 0.0046 min⁻¹ to 0.0024 min⁻¹ (Me:PS = 5:1) and 0.0007 min⁻¹ (Me:PS = 100:1). Compared with the addition without quenching agent, the removal rate of ciprofloxacin decreased after TBA was added, which indicates that the removal of ciprofloxacin has the oxidation effect of •OH [45]. Compared with the addition of TBA, the removal rate of ciprofloxacin decreased more greatly after Me addition. It can be inferred that ${\mathrm{S}\mathrm{O}}_4^{•-}$ is the relatively dominant free radical in the ultrasonic enhanced persulfate oxidation system compared with •OH, and it also shows that the removal of ciprofloxacin is the result of the joint action of •OH and ${\mathrm{S}\mathrm{O}}_4^{•-}$ [46]. In addition, the EPR experiment directly detected ${\mathrm{S}\mathrm{O}}_4^{•-}$ and •OH in the US/E/PS again.

4. Mechanism Analysis

The experimental conditions were as follows: the pH was 5.0, ciprofloxacin’s initial molar concentration was 0.03 mmol/L, PS initial molar concentration was 1.5 mmol/L, ultrasonic power was 100 W, electrode potential was 4 V, and temperature was 45 °C. The samples of ciprofloxacin at 0 min, 60 min, 120 min, and 180 min were tested by liquid mass spectrometry (LC-MS), and the possible intermediates and degradation pathways were speculated. The HPLC spectrum of ciprofloxacin degradation products changed with time, as shown in Figure 11. According to the degradation products of ciprofloxacin, the speculated degradation path of ciprofloxacin by the US/E/PS system is shown in Figure 11. The retention time of ciprofloxacin under HPLC conditions in this experiment was t_R = 8.95 min. According to the distribution of products in HPLC chromatogram, it is preliminarily speculated that the main five products of degradation of ciprofloxacin by the US/E/PS system are detected.

Figure 11 displays that the peak value of t_R = 8.95 min decreased gradually with the reaction progress, indicating that ciprofloxacin was degraded, and product peaks with the retention times of 6.59 min, 7.49 min, and 9.60 min, respectively, appear. In Figure 12, the piperazine ring in the structure of ciprofloxacin was easy to cleave, and the product of 6.59 min (P.1) may be that the piperazine ring was attacked by ${\mathrm{S}\mathrm{O}}_4^{•-}$ and •OH, and the ring-opening of the piperazine ring removed C₂H₂O generation; ${\mathrm{S}\mathrm{O}}_4^{•-}$ and •OH were likely to completely dealkylate the piperazine ring after further action, and the product at 9.60 min (P.3) should be the product after completely dealkylating the piperazine ring. The substitution reaction may take place at two positions, and 7.49 min (P.2) may be the product of hydroxylation of the aromatic ring. Whether the quinolone structure in ciprofloxacin was cleaved or not is directly related to the concentration of ${\mathrm{S}\mathrm{O}}_4^{•-}$ and •OH participating in the reaction. It can be seen that US/E/PS can not only quickly degrade and remove CIP, but also produce intermediate products that are less toxic and have little inhibitory effect on microbial growth. The original chemical structure of CIP was completely destroyed, and low molecular weight and low toxicity CIP was generated. Therefore, US/E/PS can effectively degrade organic pollutants and is a safe and environmentally friendly technology with broad market application prospects.

5. Conclusions

In this paper, the removal of ciprofloxacin from water by US/E/PS system was investigated. The conclusions are as follows:

(1)

The reaction of ciprofloxacin by US/E/PS system is consistent with quasi-primary reaction kinetics, with a reaction rate constant of 0.0046 min⁻¹, which is about 2.7 times that of peroxynitrite oxidation alone (0.0017 min⁻¹). The combination of ultrasonic oxidation, electro-oxidation, and persulfate oxidation has a strong synergistic effect, with a synergistic coefficient of S = 2.10. Both ultrasonic oxidation and electro-oxidation can activate PS to continuously produce ${\mathrm{S}\mathrm{O}}_4^{•-}$ and •OH, which can effectively remove and degrade CIP.

(2)

Ultrasound can effectively increase the removal rate of ciprofloxacin. Excessive ultrasound power leads to excessive growth of cavitation bubbles, which affects the reaction and deteriorates the removal effect. The removal rates at 50 W, 100 W, and 200 W are 13.2%, 37.4%, and 36.9%, respectively.

(3)

It is helpful to remove ciprofloxacin by electrifying the reaction system, but the electrode potential increases excessively, the current efficiency decreases, and the removal effect of ciprofloxacin shows a downward trend. The removal rates of ciprofloxacin were 29.3%, 37.4%, 33.5%, and 30.5%, respectively, when the electrode potentials were 0 V, 4 V, 6 V, and 9 V.

(4)

The apparent activation energy of ciprofloxacin is 54.00 kJ/mol, and the removal efficiency of ciprofloxacin increases obviously with the increase in reaction temperature. The removal rate at 55 °C is more than three times that at 25 °C.

(5)

The removal efficiency of ciprofloxacin increases with the increase in the initial molar concentration ratio of persulfate to ciprofloxacin. The ratio of PS to ciprofloxacin increases 100 times from 5:1 to 500:1, and the removal efficiency of ciprofloxacin correspondingly increases from 23.1% to 64.0%, an increase of nearly three times. The quasi-stage reaction rate constant increased from 0.0019 min⁻¹ to 0.0099 min⁻¹, a 5.2-fold acceleration.

(6)

The pH of the solution has multiple effects on the removal of ciprofloxacin in the reaction system, such as the morphology of substrate, the type and activity of free radicals, etc. The removal rate is the highest when pH = 11.0. The removal rates of ciprofloxacin were 37.4%, 39.1%, and 42.3%, respectively, after reaction for 2 h at initial pH = 5.0, 7.0, and 11.0.

(7)

The sulfate radicals and hydroxyl radicals were detected in the US/E/PS system for the removal of ciprofloxacin from water, and the sulfate radicals were dominant. Therefore, strategies and methods to increase the number of sulfate radicals are beneficial to improve degradation and removal of CIP.

(8)

The piperazine ring in the structure of ciprofloxacin is easy to cleave, the piperazine ring (P.1) is attacked by ${\mathrm{S}\mathrm{O}}_4^{•-}$ and •OH, and the ring-opening of piperazine ring causes C₂H₂O generation. ${\mathrm{S}\mathrm{O}}_4^{•-}$ and •OH are likely to completely dealkylate the piperazine ring after further action, and the product (P.3) is generated. The product (P.2) comes from hydroxylation of the aromatic ring, and the substitution reaction may take place at two positions. Free radicals of ${\mathrm{S}\mathrm{O}}_4^{•-}$ and •OH are the most direct driving force for CIP decomposition, degradation, and removal.

Briefly, the results indicate that the US/E/PS system is of great potential application value in the removal of organic pollution and environmental purification.