1 Introduction

Azo dyes (–N = N–), accounting for about 80% of organic dyes, are the most widely used synthetic dyes in the printing and dyeing processes of the textile, paper, and leather industries. The wastewaters produced by these industries are a major source of environmental pollutions (Ziarani et al., 2018). Azo dye wastewater is characterized by strong toxicity, high-salt content, carcinogenicity, and mutagenicity. Moreover, azo dyes are difficult to degrade due to their complex aromatic structures (Selvam et al., 2003; Wang et al., 2018a, b). When untreated azo dye wastewater is discharged directly into natural water bodies, it will cause severe harm to the environment and possibly lead to human health. Traditional physical and chemical treatment technologies include adsorption, membrane separation, and oxidation. Adsorption and membrane separation can efficiently remove azo dyes, but they are merely physical processes unable to chemically degrade azo dyes to reduce toxicity (Kirubanandam et al., 2019). These physical technologies also have other disadvantages. Absorption is not sustainable without the regeneration and recycling of adsorbents (Collivignarelli et al., 2019). Membrane separation still faces the challenges such as membrane fouling, concentrate disposal, and frequent membrane cleaning (Malaviya & Singh, 2011). Among these techniques, AOPs can take advantage of powerful ∙OH radicals to chemically degrade azo dyes finally into inorganic compounds (Miklos et al., 2018; Oturan & Aaron, 2014). Consequently, AOPs have attracted significant attention from several researchers.

Peroxymonosulfate (PMS) has recently received increasing attention in the wastewater treatment process. PMS can be activated to form the strong oxidant and SO4−∙ (Wang & Wang, 2019). Moreover, SO4−∙ has a high redox potential (E0 = 2.5–3.1 V) and a long half-life, favoring long contact with target contaminants (Nihemaiti et al., 2018; Duan et al., 2018; Yang et al., 2011). These properties may enhance the degradation of target contaminants. Current research indicates that SO4−∙ can be activated by treating PMS with ultraviolet light irradiation (Jiang et al., 2018; Verma et al., 2016; Zhang et al., 2019), heating (Chen et al., 2018a, b; Liang & Bruell, 2008), or transition metal ions (Huang et al., 2018; Lin et al., 2017). However, activating PMS or PS by heating and UV irradiation is costly and requires high energy. Meanwhile, transition metal ions are readily available and low-cost, but their application is limited by lower recovery and the possible risk of secondary pollution. Therefore, new activation methods of PMS and PS are the direction of current AOP technology research.

Chloride ion (Cl−) is the main inorganic component in printing and dyeing wastewaters. The concentration of inorganic salts such as sodium chloride (NaCl) in such wastewaters can reach 5–100 g/L (Chen et al., 2007; Muthukumar & Selvakumar, 2004). Fang et al. found that halogen ions, such as Cl− and bromide (Br−), react rapidly with SO4−∙ to form less active halide ion radicals, reducing the degradation efficiency of organic matter (Fang et al., 2016). Additionally, halogen ions may be involved in the degradation pathway of organic matter, forming toxic halogenated organic compounds (Fang et al., 2016; Wang et al., 2018a, b). Some studies have shown a strong impact of Cl− on AOPs. Wang et al. reported the effect of Cl− on the bleaching of azo dyes in the Co2+/PMS system. At low concentrations in the reaction (0.05–50 mM), Cl− significantly inhibited the dye decolorization (Wang et al., 2011). This inhibitory effect decreased at high concentrations (> 50 mM), and Cl− accelerated the contaminant degradation at very high concentrations. Thus, it is essential to investigate the role of the wastewater matrix (containing elements, such as Cl−) in the PMS-activation process for dye-wastewater treatment.

AOPs dominated by non-radical ROS are strongly resistant to substrates such as anions in water. As the AOP degradation process is not affected by these substrates, AOPs have attracted an increasing share of attention (Chen et al., 2018a, b; Tao et al., 2012, 2014). The ROS produced by PMS activation are dominated by singlet oxygens (1O2) (Long et al., 2019; Luo et al., 2019). Many studies (Duan et al., 2016; Liang et al., 2017) have demonstrated that chemicals containing the ketone group can catalyze the production of 1O2 by PMS, thereby activating the degradation process of organic pollutants (Zhou et al., 2015). 1O2 is derived not from oxygen in the system but from peroxygen-bond cleavage of PS. Cyclohexanone is an important chemical material with many desirable characteristics, such as high solubility, and low volatility. It is widely used in electronics, medicine, and other fields. It is the main intermediate in preparing caprolactam and adipic acid. As a percipient mechanism, cyclohexanone-activated PMS maybe follows a similar reaction path as ketone-activated PMS. In this work, the reactions between PMS and cyclohexanone were investigated for the first time.

After understanding that Cl− has an important effect on PMS activation and AOPs can degrade azo dye wastewater, an integrated novel cyclohexanone/Cl−/PMS process is expected to degrade refractory organic contaminants. Acid orange 7 (AO7) is selected as a model pharmaceutical in this study. This study shows that cyclohexanone/Cl− can synergistically activate PMS to accelerate dye degradation. The main influencing factors of cyclohexanone/Cl−/PMS system were investigated by different reaction conditions. The mechanism of AO7 degradation in the cyclohexanone/Cl−/PMS system was analyzed using the electron paramagnetic resonance method in a quenching experiment.

2 Material and Methods

2.1 Reagents

Cyclohexanone was purchased from Aladdin Reagent (Shanghai) Co., Ltd., and AO7 (chemical structure shown in Fig. 1) was purchased from Sinopharm Chemical Co., Ltd (China). Persulfate (KHSO5∙0.5KHSO4∙0.5K2SO4, PMS) was purchased from Sigma–Aldrich. Analytically pure sodium nitrite (NaNO2), sulfamethoxazole (SMX), Bisphenol A (BPA), sodium chloride (NaCl), sodium tetraborate (Na2B4O7∙10H2O), boric acid (H3BO3), methanol (CH3OH, MeOH), tert-butanol (C4H10O, TBA), furfuryl alcohol (C5H6O2, FFA), sodium hydroxide (NaOH), sulfuric acid (H2SO4), potassium iodide (KI), and sodium hydrogen carbonate (NaHCO3) were purchased from Sinopharm Chemical Reagent Co., Ltd. The experimental water was ultrapure water.

Fig. 1
figure 1

Molecular structure of acid orange 7

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2.2 Degradation Experiment

AO7 was selected as the contaminant for evaluating the pollutant degradation ability of the cyclohexanone/Cl−/PMS system. A certain concentration of the PMS solution was injected into a 100-mL conical flask, followed by a certain amount of ultrapure water. The initial concentration of PMS in the reaction system was kept at 1 mmol/L. After adjusting the pH with dilute H2SO4 or NaOH, cyclohexanone, and Cl− at certain concentrations were added to the solution. Finally, a certain amount of AO7 was added, bringing the total solution volume to 100 mL. The reaction was initiated by shaking at 303 K. Samples were collected by a pipette (brand) at regular intervals, and PMS was rapidly quenched by adding excess NaNO2, terminating the reaction for subsequent determination.

2.3 Analytical Method

The sample abundances were measured in a UV–vis spectrophotometer (Mapada UV-1600 (PC)) at 484 nm, the maximum absorption wavelength of AO7, and were substituted into the standard curve to obtain the corresponding sample concentrations. The UV–vis spectra of AO7 were also scanned in a spectrophotometer from 250 to 700 nm. The residual PMS concentration was determined by iodimetry (Ball et al., 1967; Chen et al., 2016; Güpta, 1961). After allowing the color reaction of PMS and KI for 15 min, the absorbance at 352 nm was measured in the UV–vis spectrophotometer.

The concentration of SMX and BPA are quantitatively analyzed with HPLC (Agilent-1260, Agilent ZORBAX SB-C18 (4.6 × 250 mm 5-Micron)). Different concentration gradients are prepared and the peak area is measured under the set liquid sample method. The injection volume is 20μL, the column temperature is 25 °C, and the mobile phase flow rate is 1 mL/min, and then draw the relationship curve which is the standard curve of pollutants between peak area and concentration. The remaining concentration of subsequent samples is obtained by substituting the measured peak area into the standard curve.

The 1O2 signal was detected by electron paramagnetic resonance (EPR, JEOL-FA200 spectrometer) using TEMP as the spin-trapping agent (Luo et al., 2019; Sang et al., 2011; Wang et al., 2017). The instrumental parameters of EPR were set as follows: microwave frequency = 9454.46 MHz, modulation frequency = 100.0 kHz, modulation width = 5 × 1 mT, receiver gain = 100 × 1, time constant = 0.03 s, conversion time = 0.03 s, sweep time = 1 min.

The total organic carbon (TOC) of the reaction system is measured with a TOC analyzer. The concentration of all substances in the reaction system was kept constant, and a 200 mL system was used for the reaction. At intervals, take 20 mL of the reaction solution in a 25 mL colorimetric tube and immediately add 5 mL of Na2S2O3 quencher with a concentration of 200 mmol/L, and perform determination after the reaction is over.

3 Results and Discussion

3.1 Degradation of AO7 under Different Initial Conditions

Figure 2 shows the degradation rate of AO7 in the four different reaction systems: single PMS, cyclohexanone/PMS, Cl−/PMS, and cyclohexanone/Cl−/PMS. In the absence of chlorine and cyclohexanone (single PMS), the AO7 degradation rate was only 34.0% in 25 min. At pH of 9.0, PMS alone partially decolorized AO7; however, the dye degradation was greatly accelerated by adding either 1 mM cyclohexanone or 10 mM Cl−, indicating that both cyclohexanone and Cl− can activate PMS and effectively oxidize organic contaminants. When cyclohexanone and Cl− were added to the PMS system, the AO7 degradation rates were 80.3% and 42.8% at 25 min, respectively, indicating that cyclohexanone more effectively activates PMS than Cl−. In the cyclohexanone/Cl−/PMS system, AO7 was completely decolorized at 25 min, and the degradation rate was 100%. Yang et al. (2018a, b). investigated the effect of Cl− on contaminant degradation in an alkali-activated PMS system. They found that at low concentrations (< 5 mM), Cl− exerted no significant effect on the contaminant degradation. However, at high Cl− concentrations (> 50 mM), the pollutant degradation was compromised by the alkalinity. The inhibition became more severe as the alkalinity increased, inconsistent with our observations in the cyclohexanone/Cl−/PMS system.

Fig. 2
figure 2

Effect of initial conditions on the degradation of AO7. pH = 9.0, T = 303 K, [AO7] = 20 mg·L−1, [PMS] = 1 mM, [Cl−] = 10 mM, [cyclohexanone] = 1 mM

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3.2 Plausible Mechanism of AO7 Degradation in the Cyclohexanone/Cl−/PMS System

To confirm the oxide species that decolorized AO7 in the cyclohexanone/Cl−/PMS system, we added different quenchers (MeOH, TBA, FFA, or NH4+) to the reaction system. It is worth noting that MeOH can scavenge ∙OH(k∙OH = 1.9 × 109 M−1 s−1) and SO4−∙ (kSO4− ∙ = 1.6 × 107 M−1 s−1), TBA can scavenge only ∙OH (k∙OH = 6.0 × 108 M−1 s−1) (Luo et al., 2019; Sang et al., 2011), FFA can scavenge 1O2(k1O2 = 1.2 × 108 M−1 s−1) (Cheng et al., 2017), and NH4+ can scavenge HClO (Xiao-Yi et al., 2013). The MeOH, TBA, and FFA concentrations were fixed at 100 mM, and the NH4+ concentration was increased from 10 to 100 mM. Figure 3 shows the effects of different quenchers on the AO7 degradation. MeOH and TBA quenchers exerted almost no inhibitory effect on the AO7 decolorization, suggesting that neither ∙OH nor SO4−∙ were produced in the reaction system. However, the NH4+ quencher significantly inhibited pollutant degradation. As shown in Fig. 3, the inhibitory effect of NH4+ on the AO7 degradation was independent of NH4+ concentration, confirming that HClO produced in the reaction was completely quenched by the NH4+ concentration. This result can be explained by the reaction of NH4+ with HOCl, which forms chloramine with weak oxidizing ability (Eqs. (1)–(3)). However, the AO7 degradation was not completely inhibited, indicating that other oxide species were produced in the reaction. When FFA was added to the system, the reaction was almost completely inhibited. Several quenchers of 1O2 are reported to rapidly consume PMS, significantly decreasing the degradation efficiency (Yang et al., 2018a, b). Therefore, this result cannot prove that 1O2 was produced in the reaction system.

Fig. 3
figure 3

Effects of MeOH, TBA, FFA, and NH + 4 on AO7 degradation in the cyclohexanone/Cl−/PMS system. pH = 9.0, T = 303 K, [AO7] = 20 mg·L−1, [PMS] = 1 mM, [cyclohexanone] = 1 mM

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$${\mathrm{NH}}_{3} +\mathrm{ HOCl }\to \mathrm{ N}{\mathrm{H}}_{2}\mathrm{Cl }+ {\mathrm{H}}_{2}\mathrm{O}$$
(1)
$${\mathrm{NH}}_{2}\mathrm{Cl }+\mathrm{ HOCl }\to \mathrm{ NH}{\mathrm{Cl}}_{2} + {\mathrm{H}}_{2}\mathrm{O}$$
(2)
$${\mathrm{NHCl}}_{2} +\mathrm{ HOCl }\to \mathrm{ N}{\mathrm{Cl}}_{3} + {\mathrm{H}}_{2}\mathrm{O}$$
(3)

Next, the type of oxide species produced in the reaction was confirmed in the EPR analysis. Montgomery (Montgomery, 2002) was the first to discover that ketones can significantly catalyze PMS to produce oxygen under alkaline conditions. Lange and Brauer (1996) speculated based on previous studies that the decomposition of ketone-activated PMS should produce 1O2 instead of conventional oxygen. Through near-infrared emission spectroscopy analysis, they gave the most direct evidence that acetone activates the PMS system to produce 1O2. Zhou et al. (2015). the degradation of SMX by benzoquinone-activated PMS. A free-radical quenching study showed that no free radicals were generated in the system, and the staple oxide species was 1O2. Since the benzoquinone structure contains two carbonyl groups and two olefin functional groups, it can be regarded as a special ketone organic compound. As a percipient mechanism, they proposed that quinone-activated PMS follows a similar reaction path as ketone-activated PMS. And using the kinetic model, by calculating the value of KDI to reflect the reaction rate of quinone/ketone organics catalyzed by PMS, the KDI of cyclohexanone is (5.6 ± 0.1) × 104 M−2 s−1. It shows that cyclohexanone can catalyze PMS to produce 1O2 under alkaline conditions. On this basis, we propose a percipient reaction mechanism in which 1O2 (as a strong electrophile) is oxidized to TEMPO combined with TEMP. In this system, the EPR spectrum of TEMPO displays a characteristic signal of three equal-intensity lines (hyperfine splitting constant αN = 1.69 mT). A similar signal, with three equal-intensity lines for TEMPO, is displayed in the EPR spectrum of the present system (αN = 1.709 mT, g = 2.005) (Eq. (3) and Fig. 4), demonstrating direct evidence for 1O2 in the cyclohexanone/Cl−/PMS system.

$$G = 0.07145 \times \gamma /H,$$
(4)

where γ is the microwave frequency (MHz), and H is the magnetic field (mT). Therefore, the oxide species HOCl and 1O2 accelerated the AO7 degradation in the cyclohexanone/Cl−/PMS system according to the free-radical quenching experiment and EPR analysis.

Fig. 4
figure 4

EPR spectrum of oxide species produced in the cyclohexanone/Cl−/PMS system. pH = 9.0, T = 303 K, [AO7] = 20 mg·L−1, [PMS] = 1 mM, [Cl−] = 10 mM, [cyclohexanone] = 1 mM, [TEMP] = 10 mM

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3.3 Effect of Cyclohexanone Dosage

Figure 5a shows the degradation efficiency of AO7 at different cyclohexanone dosages. The catalyst plays an essential role by synergizing with PMS. Increasing the catalyst concentration creates more reactive sites; thus, resulting in more active species. At cyclohexanone dosages of 0, 0.1, and 1 mM, the AO7 degradation rates were 52.9%, 72.6%, and 100% within 25 min, respectively. AO7 was decolorized after only 10 min in 10-mM cyclohexanone. Figure 5b plots the rate constant of AO7 degradation as a function of cyclohexanone dosage. Clearly, increasing the cyclohexanone dosage increased the performance of the rate constant k of AO7 degradation.

Fig. 5
figure 5

a Effect of cyclohexanone dosage on AO7 degradation in the cyclohexanone/Cl−/PMS system. pH = 9.0, T = 303 K, [AO7] = 20 mg·L−1, [PMS] = 1 mM, [Cl−] = 10 mM. b The rate constant of AO7 degradation as a function of cyclohexanone dosage. pH = 9.0, T = 303 K, [AO7] = 20 mg·L−1, [PMS] = 1 mM, [Cl−] = 10 mM

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This result implies that cyclohexanone catalyzed the reaction between PMS and Cl− to produce more active chlorine. However, excess cyclohexanone activated PMS to produce 1O2, which is highly selective. Increasing the cyclohexanone dosage can increase the number of active catalyst sites; thus, explaining the increased degradation rate. The cyclohexanone dosage should be selected within the appropriate range for the economic and degradation cycle. Therefore, in subsequent experiments, we set the cyclohexanone concentration to 1 mM.

3.4 Effect of PMS Concentration

Figure 6 shows the effect of PMS concentration on the AO7 degradation. As the PMS concentration increased from 0 to 1 mM, the degradation rate of AO7 increased from 0 to 100% in 25 min. However, further increasing the PMS concentration decreased the degradation rate. At a PMS concentration of 5 mM, the AO7 degradation rate was 66.9% in 25 min.

Fig. 6
figure 6

Effect of PMS concentration on AO7 degradation. pH = 9.0, T = 303 K, [AO7] = 20 mg·L−1, [cyclohexanone] = 1 mM, [Cl−] = 10 mM

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To further evaluate the effect of PMS concentration on the AO7 degradation, the data were fitted.

to a quasi-first-order kinetic model as follows:

$$C={C}_{0} \mathrm{exp}(-kt)$$
(5)

Here, t represents the reaction time; C0 and C represent the AO7 concentration at time t = 0 and t, respectively; k represents the apparent reaction rate constant. Table 1 shows the apparent reaction rate constants at different PMS concentrations. The results confirm that the rate constant k increased with increasing PMS concentration up to a certain level. When the PMS was excessively concentrated, k was reduced to some extent, consistent with the degradation rate of AO7. This behavior may be explained by the increased number of active species of the reaction as the oxidant concentration increased, which would increase the reaction and degradation rates of AO7. However, at excessive oxidant concentrations, the reaction was inhibited by the self-scavenging of the excessive oxidant (Guan et al., 2018).

Table 1 Pseudo-first-order rate constants (k) of AO7 degradation at different PMS concentrations
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3.5 Effect of Cl− Concentration

Cl− is the main component of dye wastewater. Since Cl− oxidizes to HOCl/Cl2 in the presence of PMS (Javier & Solís, 2018), it is essential to study the potential impact of Cl−. Chen et al. (2016), Xiao-Yi et al. (2013). reported a dual effect of Cl− on pollutant degradation in a carbon nanotube-activated PMS system. Accordingly, the Cl− concentration must play a crucial role in AO7 decolorization. To investigate this role, we changed the Cl− concentration in the reaction system while keeping the remaining parameters constant. Figure 7 shows that the Cl− concentration played a positive role in the cyclohexanone/Cl−/PMS system. The addition of only 1-mM Cl− increased the decolorization rate of AO7 by about 18%. The degradation time was further shortened at higher Cl− concentrations. For example, in the presence of 50-mM Cl−, AO7 was completely decolorized after 15 min. However, when the Cl− concentration increased to 100 mM, the decolorization efficiency of AO7 was not significantly accelerated above that of 50-mM Cl−.

Fig. 7
figure 7

Effect of Cl− concentration on AO7 degradation. pH = 9.0, T = 303 K, [AO7] = 20 mg·L−1, [cyclohexanone] = 1 mM, [PMS] = 1 mM

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We surmise that the residual active species in the system could not function effectively when the target pollutant was completely degraded by the active species generated in the reaction system. Therefore, continuously increasing the Cl− concentration did not significantly change the degradation effect of the contaminants. It appears that increasing the Cl− concentration accelerated the AO7 degradation rate by promoting the catalytic process. In the activated PMS process, Cl− could be oxidized and transformed to Cl2∙− and other chlorine radical species. Also, Cl− could participate in PMS decomposition via a non-radical mechanism, generating HClO and Cl2 (Chan & Chu, 2009; Chen et al., 2016; Yuan et al., 2011), whereas HClO could rapidly decolorize the azo dyes (Yuan et al., 2011; Wang et al., 2011). It shows that the synergistic effect of Cl− on the cyclohexanone/PMS system depends on the kind of target pollutant. The main active substance in the cyclohexanone/Cl−/PMS system is 1O2. Therefore, the accelerating effect of Cl− at higher concentrations might be resulted from the generation of highly reactive HClO (Chen et al., 2016). In contrast, other AOP reports (Wang et al., 2011; Xiao-Yi et al., 2013) identified cyclohexanone (instead of Cl−) as the activator of PMS. Therefore, Cl− might promote the cyclohexane activation of PMS while activating PMS, doubly promoting the AO7 decolorization.

3.6 Possible Mechanism for PMS Activation by Base

pH is an important influencer of PMS activation. Homogeneous AOPs that activate PMS by transition metal ions perform better in acidic than alkaline conditions. Additionally, nonhomogeneous AOPs, which degrade contaminants to varying degrees over a wide range of pH values, always perform better in acidic or neutral solutions (Huang et al., 2017a, b). It can be seen from Fig. 2 that the degradation rate of AO7 was 34.0% within 25 min in a single PMS reaction system when the initial pH at 9.0, indicating that alkaline can activate PMS and degrade organic pollutants effectively. To study the effect of pH on the AO7 decolorization in the cyclohexanone/Cl−/PMS system, pH was varied as 5.0, 6.0, 7.0, 8.0, and 9.0, keeping other parameters constant. The results are shown in Fig. 8. Increasing the pH from 5.0 to 9.0 gradually increased the AO7 degradation rate. At a pH of 5.0, 0.01% of the AO7 was removed in 25 min, whereas at a pH of 9.0, the AO7 removal rate improved to 99.6% in 25 min.

Fig. 8
figure 8

Effect of initial pH on the AO7 degradation. T = 298 K, [AO7] = 20 mg·L−1, [PMS] = 1 mM, [cyclohexanone] = 1 mM, [Cl−] = 10 mM

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It is well-known that PMS is unstable and readily decomposes under alkaline conditions. Sulfate radical and hydroxyl radical are usually considered to be the reactive oxygen species generated in activated PMS processes. Section 3.2 suggests that neither ∙OH nor SO4-∙ was produced in the reaction system. In a previous report (Chengdu et al., 2016), AO7 was effectively decolorized in the base/PMS system, generating 1O2 and O2−∙ as the dominant species. Studies have shown that PMS, as a peroxide, can produce singlet oxygen (1O2) and sulfate radicals by slowly self-decompose under alkaline conditions (Eqs. (6)) (Zhou et al., 2015). We now investigate the residual PMS concentration in the cyclohexanone/Cl−/PMS system. Figure 9 plots the temporal PMS-concentration changes in the cyclohexanone/Cl−/PMS system under different pH conditions in the absence of pollutants. Figure 8 shows that when the pH is less than 7.0, PMS hardly decomposes; With the gradual increase of pH, the decomposition rate of PMS increases. It indicates that in this system, as the pH increases, the residual concentration of PMS decreases. Under acidic pH conditions, PMS decomposition was completely inhibited due to the lack of dye degradation at pH 5.0, 6.0, and 7.0. At higher pH, the decomposition rate of PMS increased in synchrony with an increasing rate of dye degradation. This result implicates PMS decomposition as a limiting factor in the AO7 degradation rate in the cyclohexanone/Cl−/PMS system. The pH of printing and dyeing wastewater fluctuates widely from 6 to 10. The AO7 decolorization was greatly accelerated at alkaline pH in the cyclohexanone/Cl−/PMS system, suggesting that our proposed method can exploit the high alkalinity of dye wastewater in practical applications.

Fig. 9
figure 9

Residual PMS concentration. T = 303 K, [AO7] = 20 mg·L−1, [PMS] = 1 mM, [cyclohexanone] = 1 Mm, [Cl−] = 10 mM

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$${{\mathrm{HSO}}_{5}}^{-} + {{\mathrm{SO}}_{5}}^{2-}\to {{\mathrm{HSO}}_{4}}^{-} + {{\mathrm{SO}}_{4}}^{2-} +{}^{1}{\mathrm{O}}_{2}$$
(6)

3.7 Effect of Temperature

In this section, we investigated the effect of temperature on the AO7 degradation, considering the wide temperature fluctuations induced by outflowing printing and dyeing wastewater. The reaction temperature is a fundamental factor in activation reactions. Figure 10 shows that increasing the temperature increased the AO7 degradation rate. The dye was completely decolorized within 10 min at 333 K, suggesting that in practical engineering applications, the dye-wastewater treatment will be most efficient in summer or when the dye effluent is released at high temperatures. It appears that under high-temperature conditions, cyclohexanone/Cl− can rapidly activate PMS to produce active species that degrade contaminants. However, the dye molecules can easily overcome their reaction activation energy at high temperatures. The apparent reaction rate constants were 0.0897, 0.1903, 0.2571, and 0.5753 min−1 t 293 K, 303 K, 318 K, and 333 K, respectively. From the Arrhenius relationship between the logarithmic reaction rate constant and inverse temperature, the apparent activation energy of the AO7 decomposition reaction can be calculated as follows:

Fig. 10
figure 10

Effect of temperature on AO7 degradation in the cyclohexanone/Cl−/PMS system. pH = 9.0, [AO7] = 20 mg·L−1, [PMS] = 1 mM, [cyclohexanone] = 1 mM, [Cl−] = 10 mM

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$$k = {Ae}^{-Ea/RT}.$$
(7)

Here, k is the rate constant; R is the molar gas constant (8.314 × 10−3 kJ∙mol−1 K−1); T is the thermodynamic temperature; Ea is the apparent activation energy (kJ∙mol−1); A is the form factor. The data fitting was strongly linear (R2 > 0.97), yielding an activation energy Ea of 35.736 kJ∙mol−1.

3.8 Effect of Humic Acid Concentration

Many natural organic substances are found in water. Several studies (Mahdi-Ahmed & Chiron, 2014; Zhang et al., 2019) have shown that the dissolved organic matter in natural water significantly affects the aqueous degradation of pollutants. Humic acid (HA) is an organic matter caused by the decomposition and transformation of microorganisms and a series of geochemical processes. Thus, HAs are widely distributed in soil and water environments. In this experiment, we investigated the effect of HA concentration on AO7 degradation in the cyclohexanone/Cl−/PMS system, maintaining other reagent concentrations and reaction conditions constant. Figure 11 shows that the AO7 degradation rate was insignificantly affected by the HA concentration (after 25 min, 100%, 99.3%, 99.9%, 100.0%, and 99.7% of AO7 degraded at HA concentrations of 0, 5, 10, 15, and 20 mg/L, respectively). This result may be due to the high selectivity of the oxide species. Since the oxide species weakly interact with HA, the AO7 decolorization will be independent of HA concentrations. However, Huang et al. (2017a, b) reported that HA significantly inhibits the removal of di-(2-ethylhexyl) phthalate by UV/PMS. Ji et al. (2016) investigated the effect of HA on the degradation of BPA by Co2+/PMS. They concluded that HA reduced degradation efficiency, contradicting our conclusions. The above studies showed that the pollutant degradation was mediated by free radicals generated in the system. Therefore, HA and target pollutants in the system might compete for reaction with free radicals. In Sect. 3.2, HOCl and 1O2 were identified as active species in the system. Since these species are more selective than radicals, the pollutant degradation in the present system was unaffected by HA.

Fig. 11
figure 11

Effect of HA concentration on AO7 degradation in the cyclohexanone/Cl−/PMS system. pH = 9.0, T = 303 K, [AO7] = 20 mg·L−1, [PMS] = 1 mM, [cyclohexanone] = 1 mM, [Cl−] = 10 mM

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3.9 Effect of Water Quality

To investigate the impact of the cyclohexanone/Cl−/PMS system in actual water bodies, we sampled the effluent from FuXing wastewater treatment plant (WTP) and its surface water (SW) and investigated the effect of the water matrix on the AO7 removal. Table 2 presents the pH, total organic carbon (TOC), chemical oxygen demand (COD), and total nitrogen (TN) contents of the two samples. Figure 12 compares the progress of the AO7 removals from deionized water (DI), WTP, and SW in the cyclohexanone/Cl−/PMS system. Furthermore, 99.4%, 99.6%, and 95% of AO7 were removed in DI, WTP, and SW reaction waters, respectively, after 25 min. WTP effluent exerted no significant effect on the AO7 removal, whereas SW exerted a slight inhibitory effect. These results clarify that the AOP was relatively insensitive to the water matrix.

Table 2 Attributes of two water matrix samples
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Fig. 12
figure 12

Effect of water matrix on AO7 degradation in the cyclohexanone/Cl−/PMS system. pH = 9.0, T = 303 K, [AO7] = 20 mg·L−1, [PMS] = 1 mM, [cyclohexanone] = 1 mM, [Cl−] = 10 mM

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3.10 Analysis of AO7 Degradation Process

To clarify the changes of molecular and structural characteristics of AO7 in the cyclohexanone/Cl−/PMS system, the UV–visible spectra were observed. Figure 13 shows the UV–visible spectra of the residual AO7 at the reaction time points of AO7 degradation in the cyclohexanone/Cl−/PMS system. The azo dye AO7 consists of an azo linkage, a benzene ring, and naphthalene ring, which exhibit different absorbance peaks. The maximum absorption wavelength for AO7 is 484 nm, which is linked to the orange color and can be attributed to the azo linkage. The other two bands located at 229 and 310 nm are related to the naphthalene and benzene rings, respectively. Two characteristic absorption peaks appear in the spectral scanning of AO7 at 484 and 310 nm. When the reaction system was activated at a pH of 9.0, the intensities of these AO7 peaks continuously decreased and the AO7 spectrum was nearly flat between 400 and 700 nm as the reaction proceeded, confirming that the azo bond linking the azo dye to the naphthalene ring structure was gradually destroyed by the reactive specie. In order to evaluate the mineralization ability of cyclohexanone/Cl−/PMS system, we detected the removal of TOC. Figure 14 shows the change of TOC during the degradation of AO7. The TOC removal rate at the end of the reaction is 29%. The results show that the cyclohexanone/Cl−/PMS system not only has a good decolorization and degradation effect but also has a certain mineralization ability. Combined with spectral scanning, it is speculated that the azo bond and naphthalene ring of AO7 are oxidized to form aromatic compounds with the benzene ring as the main body, and some intermediate products can be further degraded into small molecular organics and finally mineralized into CO2 and H2O. The presence of Cl− has inhibitory effects on AO7 mineralization. This may be attributed to (1) the consumption of the oxidant PMS and SO4−∙ by Cl−, resulting in less reactive inorganic radicals; and (2) generation of refractory organohalogen compounds and AOX measurements (Yuan et al., 2011). It indicated that Cl− promoted the decolorization effect, but inhibited the mineralization of AO7. Thus, further mineralization of organic pollutants in the cyclohexanone/Cl−/PMS system still needs to be studied.

Fig. 13
figure 13

UV–visible spectra of AO7 degradation. pH = 9.0, T = 303 K, [AO7] = 20 mg·L−1, [PMS] = 1 mM, [cyclohexanone] = 1 mM, [Cl−] = 10 mM

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Fig. 14
figure 14

The change of TOC during the degradation of AO7. pH = 9.0, T = 303 K, [AO7] = 20 mg·L−1, [PMS] = 1 mM, [cyclohexanone] = 1 mM, [Cl−] = 10 mM

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4 Conclusions

This study proposes an effective method for treating organic contaminants in salty wastewater. The proposed PMS activation successfully decolorizes azo dyes by synergistic activation of PMS with cyclohexanone and Cl−. Increasing the PMS concentration (0–1 mM), cyclohexanone dosage, Cl− concentration, reaction temperature, and pH accelerated the AO7 degradation in the cyclohexanone/Cl−/PMS system. In radical quenching experiments and EPR analysis, 1O2 and HOCl (instead of radicals in the catalytic oxidation process) were revealed as the main active species in the reaction system. The effects of HA concentration and water matrix on contaminant removal were also investigated. The cyclohexanone/Cl−/PMS system showed a high degree of selectivity. Additionally, the degradation rate of AO7 was unaffected by the background matrix and natural organic matter (NOM). This result indicates that non-free-radical AOPs can overcome the radical inhibition, water anions, and NOM that lower the pollution removal rate. Therefore, the proposed method for degrading organic pollutants in alkaline saline wastewater has potential application value.