Ciprofloxacin (CIP) removal efficiency in aqueous solutions in the ultrasonic (US), K2S2O8, and US/K2S2O8 systems was investigated. The free radical generation and action ratio were studied based on variations of K2S2O8 concentration, ultrasonic power, pH, and the addition of isopropanol (ISP) or tert-butyl alcohol (TBA) in the US/K2S2O8 system. The results showed that under conditions of 20 mg·L−1 CIP concentration, 20 mmol·L−1 K2S2O8 concentration, an ultrasonic power of 360 W and pH = 7, CIP removal efficiency in the US/K2S2O8 system was 92.20% after 180 min. The reaction in the US/K2S2O8 system was explicitly divided into two stages: free radical generation and pollutants degradation. The ultrasonic and chain reaction facilitated enhanced generation of SO4−• and HO. The presence of K2S2O8 can promote HO generation and K2S2O8 concentration also exerted a significant effect on SO4−• generation, however, high concentrations were not beneficial to the reaction. Quenching reactions occurred under high concentrations of HO and SO4−•. During the initial stage of the reaction, HO played a more prominent role than SO4−•, however, the role of SO4−• gradually increased as the reaction proceeded and eventually surpassed HO.

  • The pollutants degradation process in the US/K2S2O8 system was explicitly divided into free radical generation stage and degradation stage.

  • The free radical generation mechanism and the interaction between HO and SO4−• were investigated in detail.

  • Evading the difficulty of HO and SO4−•in-situ determination, the ratio of HO, SO4−• and other actions were calculated, the correctness of the mechanism was proved.

AOPs

Advanced oxidation processes

HPLC

High performance liquid chromatography

PS

Persulfate

TBA

Tert-butyl alcohol

US

Ultrasonic

ISP

Isopropanol

CIP

Ciprofloxacin

In recent years, on account of its oxidation ability and good stability, advanced oxidation processes based on SO4−• are considered effective technologies for degrading refractory organic contaminants (Gao et al. 2020). SO4−• has a number of advantages, such as possessing a high oxidative potential (2.6–3.1 V), a long lifetime, and a wide operative pH range (Yen et al. 2011; Zhang et al. 2019), resulting in enhanced reaction with pollutants. As a source of SO4−•, persulfate (PS) can be activated by heat, ultraviolet, electromagnetic, transition metal ion, and ultrasonic (US) processes in order to generate SO4−• (Avetta et al. 2015). In addition, the stability of PS makes it convenient for transportation and preservation at room temperature (Li et al. 2019). At present, organic pollutant treatment by US/PS systems is considered a promising avenue for reasons of high efficiency and cleanliness (Chen & Huang 2015; Gao et al. 2018).

Extensive research has been conducted to determine the reaction mechanisms of pollutant degradation in US/PS systems. For example, Hao et al. (2014) indicated that SO4−• oxidation was responsible for ammonium perfluorooctanoate removal in the US/PS system, Nasseri et al. (2017) found that HO played a major role in the degradation of tetracycline in this system, and Monteagudo et al. (2018) studied the degradation of diclofenac in the US/PS system finding that, under the action of SO4−• and HO, diclofenac was oxidized. Darsinoua et al. (2015) reported that bisphenol A was efficiently decomposed by the combined effects of SO4−• and HO produced by ultrasound activation of PS. Chen & Huang (2019) used the US/PS system to treat dinitrotoluene, finding that SO4−• served as the main oxidant. The research described above has demonstrated the type and effect of free radicals and carried out specific analysis for different reactions, but the essential law of the degradation reaction of pollutants in the US/PS system could not be revealed.

According to the above analyses, in the US/PS system, the reaction follows of a series of processes including the generation of free radicals and the degradation of pollutants. Herein, this study defines these two stages as follows: (1) free radical generation: US effects in the aqueous solution can cause the rapid formation, growth, and implosive collapse of bubbles, resulting in extreme temperature and pressure rises, facilitating the formation of oxidizing species such as SO4−• and HO; (2) pollutants degradation: free radicals, such as SO4−•and HO, react with pollutants. It has been reported that the US/PS system includes the following equation: (1)–(10) (in Equations (1) and (2), ))) refers to ultrasonic waves) (House 1962; Wang et al. 2015; Monteagudo et al. 2018):
formula
(1)
formula
(2)
formula
(3)
formula
(4)
formula
(5)
formula
(6)
formula
(7)
formula
(8)
formula
(9)
formula
(10)

The above two-stage process is necessary for the complete degradation of pollutants in the US/PS system, since the follow-up reactions cannot occur without the generation of free radicals. The generation of free radicals in the first stage is affected by many factors; several studies have reported that the influence of parameters such as power, oxidant concentration, and pH on the degradation of pollutants mainly relies on the influence of free radicals (Darsinoua et al. 2015; Ferkous et al. 2017; Yousefi et al. 2019). However, the reaction rate in the second stage shows relative stability. A previous report (Ao et al. 2018) showed that the reaction rate constants of HO and SO4−• with CIP were k HO/CIP = 4.92 × 109 M−1 s−1 and k SO4−·/CIP = 1.2 × 109 M−1 s−1, respectively. Lei et al. (2019) also indicated that the efficiency of oxidation in the US/PS system is attributed primarily to the generation of free radicals. Additionally, multiple studies (Sharma et al. 2016; Wang et al. 2019b; Milh et al. 2020) have also shown that different intermediates were produced during the reaction of pollutants with HO and SO4−•, respectively. Consequently, free radical generation in the first stage is the decisive step during the degradation reaction, and only by understanding the generation mechanism of free radicals can we take effective control of the reaction. At present, the above two stages have not been divided and the mechanism of free radical generation was not realized explicitly.

In-situ determination of free radical concentration in the first stage is required to be researched. However, the generation of free radicals occurred in a dynamic environment and the free radicals showed a short survival time (Tang et al. 2018). There are no related reports on such methods, and quantitative efforts face great difficulties. The reaction mechanism can be shown through different reaction results, focusing on the design of the experiment and in-depth analyses of experimental phenomena in order to reveal free radical generation mechanisms. This is an effective way to overcome the difficult problem of the in-situ determination of free radicals.

The main objective of this work was to reveal free radical generation mechanism, in the first stage, during CIP degradation in the US/K2S2O8. To achieve this goal, the CIP aqueous solution was degraded in the US/K2S2O8 system and, comparative experiments to determine influencing factors such as power, K2S2O8 concentration, pH, and the addition of tert-butanol or isopropanol quenching agent were carried out. Furthermore, an in-depth analysis based on above experiment was conducted.

Chemicals and reagents

Potassium persulfate (K2S2O8, ≥99.5%) was purchased from Tianjin Damao Chemical Reagent Factory (Tianjin, China). Hydrogen peroxide (H2O2), sodium hydroxide (NaOH), sulfuric acid (H2SO4), tert-butyl alcohol (TBA), and isopropanol (ISP) of reagent grade were obtained from Tianjin Deen Chemical Reagent Factory (Tianjin, China). Methanol, triethylamine, and phosphoric acid of high performance liquid chromatography (HPLC) grade were purchased from Tedia Reagent Company (USA). CIP was obtained from Heilongjiang Kelun Pharmaceutical Co., Ltd (Heilongjiang, China) and its physical and chemical properties are shown in Table S1 (Supplementary data). All aqueous solutions in this study were prepared using ultrapure water (resistivity 18 MΩ·cm−1) obtained from a Millipore Milli-Q system.

Experimental procedure

CIP degradation experiments were conducted in a US cell disruptor (YMNL–1000Y, Yimanei, China). As shown in Figure S1 (Supplementary data), the device contains the following parts: (1) US generator, including control panel and display screen, and (2) transducer assembly, including transducer and horn, equipped with a Φ 6 mm titanium probe. The US generator and the transducer assembly were connected by cable, and the temperature probe was directly connected to the control panel.

The experimental steps were as follows. A 100 mL CIP aqueous solution with a mass concentration of 20 mg·L−1 was prepared and a set amount of K2S2O8 was added. The pH value was adjusted using 0.1 mol·L−1 H2SO4 and NaOH, and determined using a pH meter (PHS-3C, Leici, China). The CIP aqueous solution was transferred to the reaction vessel and the US cell disruptor was opened at the frequency of 25 kHz. The titanium needle was inserted 1.5–2 cm beneath the solution surface, and the titanium pulse (on/off) was set at 2 s/3 s. Under conditions of normal pressure and avoiding light, the US treatment was applied, and the temperature was monitored using a temperature probe. Samples of approximately 2 mL were collected every 30 min, and analysis was carried out immediately after sampling. Each group of experiments was repeated at least three times to acquire an average. CIP removal efficiency was calculated using Equation (11):
formula
(11)
where r is CIP removal efficiency (%), C0 and Ct are CIP mass concentration (mg·L−1) at time 0 and time t, respectively, and V is the volume of the CIP solution.

Analytical methods

CIP concentrations were detected using HPLC, Waters 2695, USA) equipped with a Waters-C18 column (150 mm × 4.6 mm, 5 μm); the column temperature was set at 30 °C, and the mobile phase comprised 25% methanol and 75% 0.25 mol·L−1 phosphoric acid aqueous solution, which was adjusted with triethylamine to pH 3.0. The mobile phase was passed through a 0.45 μm filter membrane before use, the flow rate was set to 1 ml·min−1, and the injection volume was 20 μL. The peak area was quantified using the external standard method with a maximum absorption wavelength of 278 nm for CIP and a retention time of 5.92 min.

H2O2 concentration was determined spectrophotometrically using the TiCl4 method (Laat & Gallard 1999).

Determination of active free radicals for CIP removal in the US/K2S2O8 system

In order to determine the active free radicals produced in the US/K2S2O8 system, the removal efficiencies of CIP in US, K2S2O8, and US/K2S2O8 were compared, and the effects of excessive TBA and ISP on CIP removal efficiency were investigated in the US/K2S2O8 system. The results are shown in Figure 1.

Figure 1

Comparison of the CIP removal efficiency of different systems. Reaction conditions: CIP concentration = 20 mg·L−1, frequency = 25 KHz, K2S2O8 concentration = 20 mmol·L−1, power = 360 W, pH = 7.

Figure 1

Comparison of the CIP removal efficiency of different systems. Reaction conditions: CIP concentration = 20 mg·L−1, frequency = 25 KHz, K2S2O8 concentration = 20 mmol·L−1, power = 360 W, pH = 7.

Close modal

As shown in Figure 1, both the US and K2S2O8 experimental groups exhibited limited ability for CIP degradation and their removal efficiencies over 180 min were only 14.63% and 3.70%, respectively. A completely different result was observed in the US/K2S2O8 experimental group, where CIP removal efficiency was up to 92.20%. The utilization efficiency of US energy alone was very low (Song et al. 2006), thus a low CIP removal efficiency was observed in the US experimental group. This was also the case for the K2S2O8 experimental group due to its stability at room temperature and slow chemical reaction rate (Matzek & Carter 2016). Thus, neither the US nor K2S2O8 system generated a sufficient amount of active free radicals for effective participation in the reaction.

An obvious increase in removal efficiency was observed in the control group, suggesting synergism between US and K2S2O8. It therefore became necessary to determine the nature of the active free radicals generated by this synergistic effect.

In this study, TBA and ISP were utilized as free radical quenchers to identify active free radicals in the degradation process. Previous research (House 1962; Liu et al. 2016; Qian et al. 2018) has shown that TBA, a commonly used HO quencher, can rapidly quench HO, whereas ISP is able to quickly quench HO and SO4−•.

It was noted that CIP removal efficiency in the TBA experimental group showed a trend of gradual acceleration with prolongation of the reaction time, reaching 63.31% in 180 min, i.e., 28.90% lower than the control group. Evidently, introducing excessive TBA as the HO quencher resulted in a decrease in CIP degradation efficiency, suggesting that a large amount of HO was produced in the US/K2S2O8 system. Additionally, CIP degradation efficiency was higher than that of the K2S2O8 system, indicating that free radicals other than HO were present.

The ISP experimental group exhibited a slow increase in CIP removal efficiency, reaching 23.94% in 180 min, i.e., 68.27% lower than that of the control group and 39.37% lower than that of the TBA experimental group; this can be explained by the elimination of both HO and SO4−•, implying that a large amount of SO4−• was produced in addition to HO.

The above experimental results demonstrated a synergy in the US/K2S2O8 system, suggesting that both HO and SO4−• were generated in high quantity during the CIP degradation process. This was more than that produced by either the US or K2S2O8 system in isolation.

HO generation mechanism in the US/K2S2O8 system

Influence of ultrasound on HO

Suslick (1990) determined that acoustic cavitation caused by US power could raise the temperature of the gas phase reaction zone to approximately 5,200 K, while the effective temperature of the liquid reaction zone reached about 1,900 K, and the partial pressure was 5.05 × 104 kPa, accompanied by a strong shock wave. The high temperatures and pressures generated during cavitation could decompose molecules, break chemical bonds, and generate free radicals. Under these conditions, H2O molecules were decomposed into free radicals, such as HO and H· (Equation (2)). Therefore, in order to test the effect of ultrasonic power on HO generation, the effect of TBA on CIP degradation was investigated by adding TBA to the US system.

As illustrated in Figure 2, the CIP removal efficiency for HO in the US experimental group was 14.63%, whereas that in the TBA experimental group was only 4.78%, indicating that ultrasonic power was an important cause of HO generation.

Figure 2

The removal efficiency of CIP on the US and US/TBA systems. Reaction conditions: CIP concentration = 20 mg·L−1, frequency = 25 KHz, power = 360 W, pH = 7.

Figure 2

The removal efficiency of CIP on the US and US/TBA systems. Reaction conditions: CIP concentration = 20 mg·L−1, frequency = 25 KHz, power = 360 W, pH = 7.

Close modal

Influence of chain reaction on HO generation

As described in the section on the influence of ultrasound on HO, US power was an important parameter in HO generation. However, it was reported that only around 10% of HO produced by US power enters into the liquid phase to participate in the reaction. Figure 1 also shows that CIP removal efficiency in the control group was much higher than that in the US experimental group, indicating that other HO generation mechanisms or other free radicals were present in the US/K2S2O8 system. Ultrasonic power between 90 W and 360 W was investigated for CIP degradation in the US/K2S2O8 system and the HO generation mechanism was further analyzed. The results of this analysis are shown in Figure 3.

Figure 3

(a) Relationship between the CIP removal efficiency and reaction time in the US/K2S2O8 system with powers of 90, 180, 270, and 360 W; (b) relationship between CIP removal efficiency and power at different reaction times. Reaction conditions: CIP concentration = 20 mg·L−1, frequency = 25 KHz, K2S2O8 concentration = 20 mmol·L−1, pH = 7.

Figure 3

(a) Relationship between the CIP removal efficiency and reaction time in the US/K2S2O8 system with powers of 90, 180, 270, and 360 W; (b) relationship between CIP removal efficiency and power at different reaction times. Reaction conditions: CIP concentration = 20 mg·L−1, frequency = 25 KHz, K2S2O8 concentration = 20 mmol·L−1, pH = 7.

Close modal

Figure 3(a) shows that the characteristics of CIP removal efficiency with time were significantly different under different power conditions. At powers of 90, 180, and 270 W, the rate of CIP removal efficiency increased gradually with the extension of reaction time, resulting in an upper concave curve. The higher the US power, the greater the increase in CIP removal efficiency. CIP removal efficiency in different US power experimental groups entered a plateau period after 150 min. After 180 min of reaction, CIP removal efficiency in the 90, 180, 270, and 360 W experimental groups were 86.71%, 88.46%, 90.71%, and 92.20%, respectively. Figure 3(b) shows the relationship between CIP removal efficiency and power change at different reaction times. The linear slopes at 60 and 90 min were very similar, however, this decreased after 90 min, as shown in Table S2, indicating that extended reaction times progressively reduced the effect of US power on CIP removal efficiency.

US power is thus clearly shown to promote the formation of HO. In a certain range, higher power induces greater HO generation (Zhang & Zheng 2009). A similar phenomenon was observed in the US/K2S2O8 system, such that the efficiency of CIP removal was elevated at higher power, suggesting that the presence of K2S2O8 in the US/K2S2O8 system had no negative impact on HO generation. After 150 min of reaction, no obvious differences were found between the various experimental groups: the impact of US power disappeared gradually, but a corresponding decline in removal efficiency did not occur, implying that, in addition to US power, other factors contributed to HO generation, or other free radicals were produced.

Free radicals produced during the activation of K2S2O8 can initiate a series of chain reactions, such that the degradation rate of pollutants depends on the occurrence and termination of chain reactions across multiple studies (Huang et al. 2002; Matzek & Carter 2016). In the US/K2S2O8 system, during the initial stages of the reaction, ultrasonic power caused hydropyrolysis, producing HO and H· (Equation (2)). Subsequently, HO and H· continued to react with K2S2O8 to form SO4−• (Equations (3) and (4)), which reacted with H2O or OH to form HO, as shown in Equation (5); this presented the characteristics of the chain reaction. In the above process, more SO4−• and HO would be produced in the US/K2S2O8 system (Monteagudo et al. 2018).

Under conditions of 90 W, 180 W, and 270 W, during the initial stages of the reaction, removal efficiency increased slowly and the number of free radicals was low. Due to the low power used, the CIP concentration was high, leading to the consumption of free radicals such that the number of free radicals was insufficient to initiate the chain reaction. As the reaction progressed, CIP concentrations decreased gradually and free radicals gradually accumulated, eventually triggering the chain reaction, at which point the system produced more free radicals to react with CIP. Therefore, the experimental phenomenon of a concave curve appeared before 150 min. When using a power of 360 W, the generation and consumption of HO and SO4−• were close to balanced, such that the linear experimental phenomenon appeared before 150 min. Considering these analyses, it can be inferred that the chain reaction in the US/K2S2O8 system also contributed to HO generation. Referring to Figure 1, after 180 min of reaction, the removal efficiency of CIP reached 92.20% in the US/K2S2O8 experimental group and only 14.63% when only the US experimental group was considered, indicating both that the chain reaction contributed greatly to the generation of free radicals and that the existence of K2S2O8 could promote HO generation.

Effect of quenching reaction on HO

Previous literature has indicated that excessive HO in the system causes quenching to produce H2O2 (Monteagudo et al. 2015) following reaction (6). Changes in H2O2 concentration over time were measured experimentally, as shown in Figure 4. The appearance of H2O2 in the system indicated the presence of an HO quenching reaction. After 90 min, H2O2 concentration in the system gradually increased, suggesting that the prevalence of this quenching reaction gradually increased. The influence of power after 90 min gradually weakened with increasing reaction time as shown in Figure 3(b), consistent with the law shown in Figure 4. After 150 min, H2O2 concentrations decreased, suggested that the quenching reaction was inhibited; this corresponded to the time at which the removal efficiency entered the plateau phase in Figure 3(a). This decrease in HO concentration was the fundamental reason for the weakening of the quenching reaction and decreased CIP removal efficiency entering the plateau phase.

Figure 4

Changes in H2O2 concentration during CIP degradation in the 360 W experimental group using the US/K2S2O8 system.

Figure 4

Changes in H2O2 concentration during CIP degradation in the 360 W experimental group using the US/K2S2O8 system.

Close modal

It was demonstrated by experimental results in sections influence of ultrasound on HO and on the chain reaction on HO generation and the effect of the quenching reaction on HO that US power and the presence of K2S2O8 can promote the formation of HO, while the synergism between them produces more HO through the chain reaction. The quenching reaction occurs at high HO concentrations.

SO4−• generation mechanism in US/K2S2O8

Figure 1 shows that SO4−• plays a greater role in the US/K2S2O8 system than in the K2S2O8 system. A previous study reported that US power can cause K2S2O8 to promote SO4−• (Wang et al. 2019a). The discussion in the section on the HO generation mechanism in the US/K2S2O8 system showed that both HO and SO4−• can be produced through the chain reaction. This raises the question of whether other factors also affect the generation of SO4−•. To explore SO4−• generation mechanisms, the effect of K2S2O8 concentrations of 5, 10, 15, 20, and 25 mmol·L−1 on the degradation of CIP in the US/K2S2O8 system was investigated. The results of this analysis are shown in Figure 5.

Figure 5

(a) Relationship between the CIP removal efficiency and reaction time in the US/K2S2O8 system with K2S2O8 concentrations of 5, 10, 15, 20, and 25 mmol·L−1; (b) relationship between CIP removal efficiency and the concentration of K2S2O8 after different reaction times. Reaction conditions: CIP concentration = 20 mg·L−1, frequency = 25 KHz, power = 360 W, pH = 7.

Figure 5

(a) Relationship between the CIP removal efficiency and reaction time in the US/K2S2O8 system with K2S2O8 concentrations of 5, 10, 15, 20, and 25 mmol·L−1; (b) relationship between CIP removal efficiency and the concentration of K2S2O8 after different reaction times. Reaction conditions: CIP concentration = 20 mg·L−1, frequency = 25 KHz, power = 360 W, pH = 7.

Close modal

Figure 5(a) shows that CIP removal efficiency varied over time under different K2S2O8 concentrations. A breaking point occurred at 90 min in the 5 mmol·L−1 K2S2O8 experimental group, at 60 and 30 min in the 10 mmol·L−1 K2S2O8 and 15 mmol·L−1 K2S2O8 experimental groups, respectively. Growth rates increased gradually with increasing reaction time after the breaking point. The increased removal efficiency in the 20 and 25 mmol·L−1 K2S2O8 experimental groups were more stable. CIP removal efficiency with different K2S2O8 concentrations but the same reaction time are compared in Figure 5(b), showing that the removal efficiency was highest when K2S2O8 concentrations were 20 mmol·L−1. At K2S2O8 concentrations lower than 20 mmol·L−1, CIP removal efficiency increased with increasing K2S2O8 concentration. However, a slight decrease in CIP degradation was observed when K2S2O8 concentrations were increased up to 25 mmol·L−1. At greater reaction times, the slope of the CIP removal curve gradually increased in the experimental group when K2S2O8 concentrations were lower than 20 mmol·L−1 (Table S3), indicating that K2S2O8 concentration positively influenced CIP removal efficiency.

In the 5, 10, and 15 mmol·L−1 K2S2O8 experimental groups, few free radicals were generated during the initial stage of the reaction. Nonetheless, the high concentration of CIP led to the consumption of free radicals, which were insufficiently present to facilitate the chain reaction. As the reaction progressed, CIP concentrations decreased and free radicals gradually accumulated, thus a chain reaction was initiated, causing the generation of more free radicals in the system and their reaction with CIP. Consequently, a breaking point appeared in the removal efficiency growth rate that, at higher K2S2O8 concentrations, appeared earlier. Under concentrations of 20 and 25 mmol·L−1 K2S2O8, the chain reaction was initiated during the early stage of the reaction, and the removal rate increased rapidly. The above experimental phenomena cannot be explained only by the chain reaction, but also require that K2S2O8 concentration had an important effect on the generation of SO4−•.

During the initial stage of the reaction, SO4−• generation reached its maximum at a K2S2O8 concentration of 20 mmol·L−1, whereas K2S2O8 concentrations exceeding 20 mmol·L−1 reduced CIP removal efficiency in the system due to the consumption of SO4−• by excess K2S2O8 (Wang et al. 2014), as shown in Equation (10).

The above experimental results showed that ultrasonic power and the chain reaction provided two pathways for the generation of SO4−• in the US/K2S2O8 system. K2S2O8 concentration had an important effect on SO4−• generation, whereas high K2S2O8 concentrations were not beneficial to the generation of SO4−•.

Interaction between SO4−• and HO in the US/K2S2O8 system

The above analyses show that ultrasonic power and the chain reaction are two main pathways by which SO4−• and HO are generated in the US/K2S2O8 system. The role of US power has been well illustrated, but additional experimental evidence is required to support the chain reaction. From the definition of the chain reaction, it can be inferred that the relationship between SO4−• and HO entails that they increase or decrease synchronously. Experimental results relating to this problem can provide support to infer the chain reaction.

To address the above question and understand how to weaken the generation of SO4−• and HO in the US/K2S2O8 system, changes in reaction effects were investigated and the mutual influence of SO4−• and HO was analyzed. Literature reports suggest that SO4−• is dominant under acidic conditions whereas HO predominated under basic conditions; however, both SO4−• and HO may predominate under neutral conditions (Liang et al. 2007). Therefore, the removal efficiency of CIP under various pH conditions (3, 5, 7, 9, and 11) was compared (Figure 6).

Figure 6

Relationship between CIP removal efficiency and reaction time in the US/K2S2O8 system at pH values of 3, 5, 7, 9, and 11. Reaction conditions: CIP concentration = 20 mg·L−1, frequency = 25 KHz, K2S2O8 concentration = 20 mmol·L−1, power = 360 W.

Figure 6

Relationship between CIP removal efficiency and reaction time in the US/K2S2O8 system at pH values of 3, 5, 7, 9, and 11. Reaction conditions: CIP concentration = 20 mg·L−1, frequency = 25 KHz, K2S2O8 concentration = 20 mmol·L−1, power = 360 W.

Close modal

Figure 6 shows that differences in CIP removal efficiency were noted under different pH conditions, with the highest CIP removal efficiency at pH 7.

Under neutral conditions, SO4−• and HO coexist, allowing the chain reaction to proceed smoothly. This results in a wide variety of free radicals in the system and the highest CIP removal efficiency. At pH 3 and 5, although SO4−• was dominant, the conditions of HO generation were poor and the chain reaction did not proceed smoothly, resulting in decreased CIP removal efficiency. Similar phenomena were found at pH 9 and 11, i.e., although HO was dominant, the formation conditions of SO4−• were poor, resulting in decreased CIP removal efficiency. Under both acidic and basic conditions, the chain reaction was affected.

Therefore, the above experimental results proved that, during the degradation of CIP in the US/K2S2O8 system, both SO4−• and HO increased simultaneously via the chain reaction, while a decrease in the concentration of certain active free radicals affected the reaction efficiency of the entire system.

Free radical generation mechanism

According to the above analysis, CIP degradation in the US/K2S2O8 system can be separated into the free radical generation stage and the reaction stage, as shown in Figure 7. The mechanism of free radical generation during the generation stage was as follows: (1) thermal dissociation of water caused by US cavitation leads to the generation of HO; (2) generation of local high temperature and high pressure and activation of K2S2O8 produces SO4−•; (3) rising water temperature caused by US power leading the activation of K2S2O8 and formation of SO4−•; (4) when HO and SO4−• accumulated, the chain reaction was initiated, HO stimulated K2S2O8 to form SO4−•, and SO4−• continued to react with H2O or OH to form HO; (5) the excessive accumulation of HO resulted in HO quenching to form H2O2; (6) SO4−• quenching formed S2O82− under conditions of excessive SO4−•; and (7) too much HO and SO4−• reacted to form HSO5.

Figure 7

Active free radical generation mechanism in the US/K2S2O8 system.

Figure 7

Active free radical generation mechanism in the US/K2S2O8 system.

Close modal

Verification of the effects of SO4−• and HO over time

The degradation of CIP in the US/K2S2O8 system involves complex reactions and the coexistence of multiple reactions. The main reaction type will vary depending upon reaction conditions. Both free radical generation and free radical reaction times are very short, thus only by correctly understanding free radical generation mechanisms can we predict the contribution of different free radicals in the degradation of CIP. Previous studies (Wei et al. 2017) have shown that the generation rate of HO in the US/K2S2O8 system was faster than that of SO4−•, but the survival time of SO4−• in aqueous solution (3 × 10−5–4 × 10−5 s) was longer than that of HO (1 × 10−9 s) (Zhang et al. 2019). Figure 3(b) shows that HO generation conditions were most favorable during the early stage of the reaction and gradually weakened in the later stages. Figure 5(b) shows instead that the formation conditions of SO4−• were weak in the early stage of the reaction but gradually increased during the later stages. According to the above experimental phenomena and analysis of the free radical generation mechanisms outlined in the section on the free radical generation mechanism, we suggest that the action of SO4−• and HO changed with time. During the initial stages of the reaction, the degradation of CIP was dominated by HO oxidation. As the reaction proceeded, HO accumulated, the chain reaction was initiated, S2O82− was excited and led to the formation of SO4−•, and the proportion of SO4−• oxidation reactions in the system gradually increased.

To test the above theoretical conjecture, the experimental results shown in Figure 1 were calculated, and the effects of SO4−• and HO were quantified. The quality difference of CIP removal between the control group and the TBA experimental group reflected the removal quality due to HO, whereas that between the TBA experimental group and the ISP experimental group reflected the removal quality due to SO4−•. CIP removal quality in the isopropanol experimental group reflected the removal quality due to other actions. Previous research has demonstrated that the degradation mechanism of the US/PS system mainly involved the synergistic action of pyrolysis and free radicals, with relatively little effect from O2− (Li et al. 2013). Here, the quenching experiment determined only the roles of SO4−• and HO. In order to facilitate this study, the role of free radicals other than SO4−• and HO, together with pyrolysis, were collectively referred to as other effects. The removal quality caused by the actions mentioned above and the residual quality were converted into a proportion of the initial addition quality, which indicated the proportion of SO4−• action, HO action, other action, and the unreacted part over different reaction times. This enabled the temperature of the system at different time points to be determined.

Figure 8 demonstrates that, after 180 min of reaction, the proportion of unremoved CIP was 7.80%, and the removed part (92.20%) was composed of HO action (28.90%), SO4−• action (39.37%) and other actions (23.93%). The action of HO on CIP degradation was rapid at first, but then slowed, whereas the SO4−• action on CIP degradation gradually increased over time. Before 90 min, the action of HO on CIP removal was higher than that of SO4−•. The actions of SO4−• and HO were equal at 90 min (both 21%). After 90 min, SO4−• occupied a higher proportion of CIP removal than HO. Our experimental results were consistent with the above conjecture.

Figure 8

Proportion of different actions in CIP degradation in the US/K2S2O8 system.

Figure 8

Proportion of different actions in CIP degradation in the US/K2S2O8 system.

Close modal

Previous research (Zou et al. 2014) has shown that the K2S2O8 peroxy bond was prone to breakage and to produce more SO4−• under high temperatures, whereas the dissolved oxygen in the system would decrease, resulting in a decrease in HO concentration (Yang et al. 2019). The above experimental results were in agreement with this change of temperature.

The experimental results shown in Figure 8 have verified the idea of changing roles of different free radicals in the reaction based the free radical generation mechanism described above, and have also demonstrated the proposed free radical generation mechanism.

Under the conditions of a CIP concentration of 20 mg·L−1, K2S2O8 concentration of 20 mmol·L−1, US power of 360 W, and pH 7, CIP removal efficiency was 92.20% after 180 min, i.e., better than that of either the US system (14.63%) or the K2S2O8 system (3.70%) when used alone. The synergistic effect in the US/K2S2O8 system facilitated greater generation of SO4−• and HO. The main mechanisms of free radical generation were US power and chain reaction. Cavitation effects caused by US power have been shown to promote the thermal dissociation of water to generate HO while activating K2S2O8 to produce SO4−•. With increasing water temperature, K2S2O8 continued to be activated to form SO4−•, leading to the accumulation of HO and SO4−• and eventually initiating a chain reaction in which HO stimulated K2S2O8 to form SO4−•; in turn, SO4−• continued to react with H2O or OH to form HO. During this process, HO and SO4−• increased or decreased simultaneously. The presence of K2S2O8 can promote the generation of HO, and the concentration of K2S2O8 also had a significant positive effect for SO4−•generation, although high concentrations were not beneficial. The quenching reaction occurs under high concentrations of HO and SO4−•. During the initial stages of the reaction, the role of HO was more important than that of SO4−•, whereas the role of SO4−• gradually increased as the reaction proceeded.

This work was supported by the National Natural Science Foundation of China (Grant no. 51908199), Key Scientific and Technological Project of Henan Province (Grant no. 172102310698).

All relevant data are included in the paper or its Supplementary Information.

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