Introduction

Organic peroxides are used as initiators in the production of high-pressure polyethylene, and with the wide application of polyethylene, at the same time, the demand for organic peroxides is increasing. The production process of organic peroxides produces a large amount of wastewater, and the wastewater contains specific pollutants such as tert-butanol and tert-butyl hydroperoxide, which makes the wastewater irritating odor, refractory to degradation, high toxicity, and low biodegradability. However, conventional treatment techniques are difficult to deal with, and pre-treatment of organic peroxide production wastewater is required to remove some of the difficult-to-degrade compounds in the wastewater and improve the biodegradability of the wastewater. Common pretreatment methods include advanced oxidation (Sun et al. 2022; Feng et al. 2023; Borba et al. 2022), hydrolysis-acidification (Zhang et al. 2021), coagulation-precipitation (Zhou et al. 2020), etc. However, these pretreatment methods require functional materials or additional processes, which lead to an increase in the cost of wastewater treatment, so proposing economically feasible pretreatment methods for this type of difficult-to-biochemical-degradable wastewater is the focus of research (Ma et al. 2019).

Microelectrolysis is an effective pretreatment technology for the treatment of highly concentrated and difficult to degrade wastewater (Lin et al. 2019). IC-ME technology takes the treated wastewater as the electrolyte and adds iron filings and activated carbon to form a large number of tiny galvanic cells, and the main chemical reactions between iron and carbon are expressed as follows in Eqs. (1)–(4) (Li et al. 2021).

$$\mathrm{Anode}:\mathrm{ Fe}-2{\mathrm{e}}^{-}\to {\mathrm{Fe}}^{2+} {\mathrm{E}}^{0}({\mathrm{Fe}}^{2+}/\mathrm{Fe})=-0.44\mathrm{ V}$$
(1)
$$\mathrm{Cathode}: {2\mathrm{H}}^{+}+{2\mathrm{e}}^{-}\to 2[\mathrm{H}]\to {\mathrm{H}}_{2} {\mathrm{E}}^{0}({2\mathrm{H}}^{+}/{\mathrm{H}}_{2})=0.00\mathrm{ V}$$
(2)

Under acid aeration conditions:

$${\mathrm{O}}_{2}+{4\mathrm{H}}^{+}+{4\mathrm{e}}^{-}\to 2\mathrm{O}\uparrow \cdot +4[\mathrm{H}]\to {2\mathrm{H}}_{2}\mathrm{O }{\mathrm{E}}^{0}(\mathrm{O}/{\mathrm{H}}_{2}\mathrm{O})=1.23\mathrm{ V}$$
(3)

Under alkaline aeration conditions:

$${\mathrm{O}}_{2}+{2\mathrm{H}}_{2}\mathrm{O}+{4\mathrm{e}}^{-}\to {4\mathrm{OH}}^{-} {\mathrm{E}}^{0}({\mathrm{O}}_{2}/{\mathrm{OH}}^{-})=0.4\mathrm{ V}$$
(4)

The Fe2+ and [H] produced by the primary cell reaction have a reducing effect and can undergo redox reactions with pollutants in the wastewater. Their electrochemical oxidation can be used to remove or transform difficult organic compounds and improve the biodegradability of wastewater (Chen et al. 2019). Meanwhile, primary cells can effectively degrade or remove organic pollutants through adsorption, coagulation, and coprecipitation (Yang et al. 2017). IC-ME is widely used and has good effect on the treatment of pharmaceutical wastewater dye wastewater (Jia et al. 2020), waste leachate (Fu et al. 2021), and electroplating wastewater (Gao et al. 2023).Ma et al. (2019) used a homemade microelectrolysis recirculation system to pretreat industrial park wastewater, and the COD removal of industrial wastewater was 51% under the conditions of 25% filler-wastewater ratio, reflux rate of 16 L/h, HRT of 24 h, and aeration volume of 60 L/h. Zhang et al. (2019) used a peroxodisulfate-coupled IC-ME system to pretreat waste leachate, and the experimental results showed that the COD removal reached 62.91% under the optimal conditions of initial pH 7, Fe–C ratio of 3, and persulfate dosage of 85 mM. Yang et al. (2016) used Fe/Al/C microelectrolysis to pretreat oil refinery wastewater, and the experimental results showed that most of the organic matter in the refinery wastewater was removed, the biodegradability was enhanced, and the biotoxicity was reduced. Zhuang et al. (2019) used IC-ME coupled with Fenton oxidation to pretreat bamboo heat treatment wastewater, COD removal after pretreatment reached 60.29%, BOD5/COD (B/C)increased from 0.037 to 0.32, and wastewater biodegradability was improved. Dinçer et al. (2021) used Fenton oxidation to treat organic peroxide production wastewater, with a COD removal rate of 72.8%, a total organic carbon (TOC) removal of 58.0%, and photo-Fenton oxidation under the same conditions for 5 h resulted in COD removal of 78.8% and TOC removal of 59.2%. However, the study on the treatment of organic peroxide production wastewater using IC-ME has not been reported.

In this experiment, the IC-ME method was used to pretreat the wastewater of organic peroxide production. The objectives of this study were as follows: (1) to explore the effects of initial pH, iron filings dosage, iron-carbon mass ratio, and reaction time on the treatment effect of wastewater pollutants during the IC-ME to pretreat organic peroxide production wastewater. (2) The reaction conditions of IC-ME pretreatment wastewater were optimized by the response surface method. (3) The mechanism of IC-ME degradation of pollutants in organic peroxide production wastewater was explored by three-dimensional excitation-emission matrix fluorescence spectroscopy (3D-EEM), ultraviolet and visible spectrophotometry (UV–Vis), and gas chromatography mass spectrometry (GC–MS).

Materials and methods

Materials

The test water was taken from a chemical plant, and its water quality characteristics are shown in Table 1.

Table 1 Main water quality parameters of organic peroxide production wastewater
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The iron filings are taken from a mechanical processing plant, and the columnar activated carbon is made of a product of an environmental protection company in Gongyi (diameter of about 2 ~ 3 mm, length of about 3 ~ 5 mm), and pretreatment is required before use. The iron filings were washed with 10% NaOH alkali to remove the oil on the surface, then activated with 2% dilute sulfuric acid and rinsed with distilled water for use; the activated carbon was soaked with test wastewater for 2 days before use to eliminate the influence of adsorption of the activated carbon itself on the determination results, then rinsed and air-dried for use (Ma et al. 2019).

Equipment and operation

As shown in Fig. 1, the degradation test was carried out using a 250-mL conical flask, the initial pH of the wastewater was adjusted with NaOH (0.1mol/L) and dilute H2SO4 (0.1mol/L), then pretreated activated charcoal and iron filings were added and placed in an oscillator to carry out the reaction, and at the end of the reaction, the COD and UV254 of the wastewater were determined.

Fig. 1
figure 1

Experimental setup

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Analytical methods

The COD was determined by potassium dichromate method (Jiao et al. 2021), UV254 by UV spectrophotometry using HCAH DR5000 UV spectrophotometer; BOD5 was determined by dilution inoculation method using HACH HQ-10 dissolved oxygen meter and SPX-250B-Z incubator; pH was determined by pHS-25 acidity meter. A fluorescence spectrophotometer (F-7100, Hitachi) was used to determine the three-dimensional fluorescence spectra.

Experimental methods

  1. 1.

    Screening design of influencing factors

    The initial pH of the wastewater, the dosage of iron filings (g/L), the mass ratio of iron to carbon, and the reaction time (min) were used as the influencing factors to investigate the effects of the above factors on the removal efficiency of pollutants.

  2. 2.

    Response surface experimental design

    Response surface method (RSM) combines mathematical and statistical methods to model and optimize the effects of a few independent variables, and has been widely and effectively used in modeling and optimization of various engineering problems(Al-Sabur 2021).Based on the results of single-factor experiments, response surface analysis was used to investigate the effects of controllable variables on COD removal efficiency, and the initial pH, iron filings dosage, iron-carbon ratio and reaction time were selected as independent variables, with COD removal efficiency as the response value. The Box-Behnken design(BBD) model was used for a 4-factor, 3-level design scheme, as shown in Table 2, to investigate the relationship between the factors and response values, and regression fitting was performed to establish a mathematical model of the process of IC-ME for the treatment of organic peroxide production wastewater (Abdulgader et al. 2020).

Table 2 Response surface design factor levels and coding
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Results and discussion

Study on the influencing factors of pretreatment of organic peroxide production wastewater

Effect of initial pH

The raw water COD was 15636.4 mg/L and the UV254 was 2.881. The reaction time was 120 min, the dosage of iron filings was controlled to be 30 g/L, m(Fe)/m(C) = 1:1, to investigate the influence of the initial pH of different wastewaters on the effect of IC-ME. As shown in Fig. 2, the removal efficiency of COD and UV254 in IC-ME showed a trend of increasing first and then decreasing, and the removal efficiency of COD and UV254 reached the maximum value of 34.5% and 79.06%, respectively, at pH = 3. The lower the pH, the larger the potential difference of the iron carbon primary cell, and the reduced [H] and Fe2+ produced, which facilitated the redox reaction with organic matter and improved the removal efficiency of pollutants (Hu et al. 2022). However, too low a pH will accelerate the corrosion of iron and too much H+ will compete for the electrons generated at the anode in the primary cell reaction and polarization will occur, which will reduce the removal efficiency of pollutants (Che et al. 2017). When pH > 3, high pH is favorable to the formation of iron salt flocs, but as a result, the electrode reaction will become weaker and the dissolution of iron will be slower, causing a decrease in the removal efficiency of pollutants (Xu et al. 2016). Meanwhile, COD removal decreased with increasing pH because the redox potential of the IC-ME reaction was higher under acidic conditions than under alkaline conditions (Han et al. 2023).

Fig. 2
figure 2

Effect of initial pH on pollutant removal

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Effect of iron filings dosage

The COD of the raw water was 15,795.0 mg/L and the UV254 was 2.879. The initial pH = 3, m(Fe)/m(C) = 1:1, and the reaction time was 120 min to investigate the effect of different iron filings on the treatment effect of IC-ME. As shown in Fig. 3, as the dosage of iron filings increased from 10 to 60 g/L, the removal efficiency of pollutants showed a trend of slowly increasing and then slowly decreasing. When the dosage of iron filings was 40 g/L, the COD removal efficiency and UV254 removal efficiency reached the maximum value of 37.08% and 80.07%, respectively. It is because with the increase of iron filings, more primary cells are formed and the pollutant removal efficiency increases, but too much iron filings will consume H+ and reduce the production of reduced [H] in the system, resulting in poor pollutant removal effect(Li et al. 2022).

Fig. 3
figure 3

Effect of iron filings on pollutant removal

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When the dosage of iron filings was 30 g/L, the wastewater COD removal efficiency could reach 34.88%, which is only 2.2% lower than the dosage of 40 g/L. Considering the economy in actual wastewater treatment and reducing the wastewater treatment cost, the dosage of iron filings is selected to be 30 g/L, which not only has good treatment effect, but also reduces the cost of wastewater treatment.

Effect of iron-carbon ratio

The COD of raw water was 15,523.7 mg/L and UV254 was 2.509. The initial pH was controlled to be 3, the dosage of iron filings was 30 g/L, and the reaction time was 120 min, to investigate the effects of different iron-carbon ratios on the treatment effect of IC-ME. As shown in Fig. 4, with the change of iron-carbon ratio from 3:1 to 1:4, the pollutant removal efficiency showed a trend of increasing and then decreasing. m(Fe)/m(C) = 1:1, the COD removal efficiency and UV254 removal efficiency reached the maximum value of 34.07% and 80.47%, respectively. The main reason for this is the variation in the mass ratio of iron to carbon and the lack of iron or carbon in the microelectrolysis process, which forms insufficient primary cells and decreases the pollutant removal efficiency (Han et al. 2019). Therefore, when the same amount of iron and carbon is used, it is possible that at this time the largest number of tiny primary cells are formed and the pollutant removal efficiency is maximum.

Fig. 4
figure 4

Effect of iron-carbon ratio on pollutant removal

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Effect of reaction time

The COD of the raw water was 15,523.7 mg/L, and the UV254 was 2.509. The initial pH = 3, m(Fe)/m(C) = 1:1, and the dosage of iron filings was 30 g/L. The effect of reaction time on the effect of IC-ME was investigated. As can be seen from Fig. 5, in the early stage of the reaction, with the extension of the reaction time, the IC-ME redox reaction in the reaction system continued to occur, and the contaminant removal rate gradually increased. The pollutant removal efficiency gradually increased with the prolongation of reaction time. When the reaction proceeded to 120 min, the COD removal efficiency reached the maximum value of 34.03%, at this time the UV254 removal efficiency was 78.29%, after which the pollutant removal efficiency no longer increased. The reason is that the H+ in the system at the beginning of the reaction is high, the Fe2+ and Fe3+ content increases during the reaction process, the electrode degrades the organic matter by primary cell reaction; at the later stage of the reaction, the H+ content in the system decreases, the iron carbon electrode is wrapped by Fe(OH)2, Fe(OH)3, and organic matter, the efficiency of the primary cell reaction decreases, and the COD removal efficiency no longer increases (Wang et al. 2016).

Fig. 5
figure 5

Effect of reaction time on pollutant removal

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Response surface optimization

The results of the response surface test design are shown in Table 3.

Table 3 Response surface experimental design results
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The data were analyzed using Design Expert 12.0 statistical software, and the four factors of initial pH (A), iron filings dosage (B), iron-carbon ratio (C), and reaction time (D) and their interactions on the COD removal efficiency of the target response were obtained (Fard et al. 2021), and the second-order regression equation was used for fitting, and the regression equation for COD removal efficiency was:

$$\mathrm{Y}=35.64+0.45\mathrm{A}+0.21\mathrm{B}+0.078\mathrm{C}+0.14\mathrm{D}+0.070\mathrm{AB}+0.28\mathrm{AC}+0.18\mathrm{AD}+0.27\mathrm{BC}-0.16\mathrm{CD}-4.05{\mathrm{A}}^{2}-1.68{\mathrm{B}}^{2}-0.32{\mathrm{C}}^{2}-0.460{\mathrm{D}}^{2}$$
(5)

Analysis of COD removal rate RSM fitting results

Analysis of variance for the regression equation

As shown in Table 4, the regression equation model ANOVA was analyzed to test the significance of the model. Among them, larger F values and smaller p values can indicate the significant level of the correlation coefficient (Popović et al. 2019). The F value of this COD removal efficiency model was 39.42 with p less than 0.0001, which indicates a good significance of the model. Based on the magnitude of F value (Okolo et al. 2021), it can be concluded that the magnitude of the effect of four factors on COD removal efficiency is A > B > D > C, i.e., initial pH > iron filings dosage > reaction time > iron-carbon ratio. And the misfit term p value = 0.1059 > 0.05, which is not significant, indicates that the residuals are generated by random errors and the model is a good simulation of the experiment with less errors generated by the experiment (Srimoke et al. 2022). The R2 value of the RSM model for COD removal is 0.9753 close to 1.0, indicating a good correlation of the model. The correction coefficient of determination (Adj R2) = 0.9505 indicates that the model explains 95.05% of the variation in response values. The signal-to-noise ratio (Adeq Precision) value is greater than 4, which is within a reasonable range (Sadaf et al. 2022). The difference between Adj R2 and Pred R2 of this model = 0.9505–0.8474 < 0.2, which indicates that the model can significantly represent the variation of each influencing factor during the experiment (Keshmiri-Naqab and Taghavijeloudar 2023); the coefficient of variation (C.V.) value is 1.53% < 4%, which indicates that the test consists of a high reproducibility (Estrada-Vazquez et al. 2019). In conclusion, the quality of this fit is good.

Table 4 Analysis of variance results
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Response surface analysis

The initial pH, iron filings dosage, iron-carbon mass ratio, and reaction time and the effects of interaction between various factors on COD removal rate were investigated. The response surface and contour can visually show the influence of the interaction of influencing factors on the removal of organic pollutants by microelectrolysis reaction. From Fig. 6a, b, and d, it can be seen that the slope of the surface is large, indicating that the initial pH, iron-carbon dosage, and iron-carbon mass ratio of the solution have a great influence on the test results, which is consistent with the results of the analysis of variance of the regression equation. Figure 6e and f show that the slope of the response surface is small, indicating that the reaction time has little influence on the experimental results. It can be seen from Fig. 6 that the contour plots of each plot are oval, indicating that the interaction between the factors is significant and has a greater impact on the removal effect of pollutants(Fseha et al. 2023).

Fig. 6
figure 6

Response surface diagram of the influence of different factors on COD removal efficiency

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Model verification and optimization

Under the conditions of initial pH of 3.1, iron filing dosage of 30.5 g/L, iron-carbon mass ratio of 1.01, and reaction time of 122.8 min, the predicted COD removal of organic peroxide production wastewater by IC-ME was 35.67%, and three sets of parallel tests were conducted under optimal operation conditions, and the obtained COD removal efficiencies were 35.17%, 35.57%, and 35.35%, the experimental values are close to the predicted values, indicating a good fit of the prediction model (Fseha et al. 2023). Moreover, the B/C of the raw water was 0.113, and the B/C of the wastewater pretreated with IC-ME technology was 0.173 under the optimal process treatment conditions, which improved the biodegradability of the wastewater. The model can more accurately reflect the effects of various factors on the removal of organic peroxide production wastewater by IC-ME and has some practical value.

Kinetics of IC-ME for COD degradation

Under the conditions of initial pH of 3.1, iron filings dosage of 30.5 g/L, and iron-to-carbon mass ratio of 1.01, the kinetic tests of COD degradation by IC-ME were carried out under different reaction times (15 min, 30 min, 45 min, 60 min, 75 min, 90 min, 105 min, and 120 min), and the composite function and time relationship of COD concentration were analyzed. The common kinetic models are as follows (Maher et al. 2019):

$$\mathrm{Zero\;order\;kinetics}: {\mathrm{C}}_{\mathrm{t}}-{\mathrm{C}}_{0} = {\mathrm{K}}_{0}\mathrm{t}$$
(6)
$$\mathrm{First\;order\;kinetics}:\mathrm{ ln}({\mathrm{C}}_{\mathrm{t}}-{\mathrm{C}}_{0}) = {\mathrm{K}}_{1}\mathrm{t}$$
(7)
$$\mathrm{Second}-\mathrm{order\;kinetics}: (1/{\mathrm{C}}_{\mathrm{t}}-1/{\mathrm{C}}_{0}) = {\mathrm{K}}_{2}\mathrm{t}$$
(8)

where Ct is the COD concentration (mg/L) at the moment of t (min), C0 is the initial COD concentration (mg/L). K0, K1, and K2 are the rate constants for the zero-, one-, and two-stage reactions, respectively.

The kinetics of COD removal by IC-ME at kinetic reaction levels of 0, 1, and 2 are shown in Fig. 7A, B, and C, respectively. The results of rate constant (K) and regression coefficient (R2) are shown in Table 5, which indicates that the COD removal is best suited to the two-stage kinetic model, and the kinetics of COD removal is two-stage kinetics with a K1 value of 0.0208 min−1, and an R2 value of 0.9818 (Shokri and Nasernejad 2023). The kinetic correlation coefficient of the second-stage reaction for IC-ME was higher than that of the other reaction stages, indicating that the second-stage reaction was a better fit for the kinetic equation of COD degradation by IC-ME (Niu et al. 2023).

Fig. 7
figure 7

COD removal kinetics: zero-order (A), first-order (B), and second-order (C) kinetic models

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Table 5 Kinetic parameters of iron-carbon microelectrolysis for removal of COD
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Mechanistic study on the treatment of organic peroxide production wastewater by IC-ME

3D-EEM analysis

The 3D-EEM of organic peroxide production wastewater pretreated by IC-ME are shown in Fig. 8. The color of the fluorescence peaks in the 3D-EEM reflects the degree of contamination of the water, and the darker the color indicates the more serious contamination of the water by organic contamination. According to the FRI analysis method, region I (Ex/Em = 200 ~ 250 nm/280 ~ 330 nm) is tyrosine-based proteins; region II (Ex/Em = 200 ~ 250 nm/330 ~ 380 nm) is tryptophan-based proteins (Zhou et al. 2013). As shown in the figure, two obvious fluorescence peaks appeared in the 3D-EEM of the influent water: the Ex/Em = 270/310, which belonged to tyrosine-like, and the Ex/Em = 215/300 for aromatic protein-like substances.1 Compared with the fluorescence graph of the raw water, weak fluorescence peaks appeared in the 3D fluorescence spectrogram of the effluent water, and the obvious peaks were weakened, indicating that IC-ME pretreatment had a good treatment effect on macromolecular organic compounds such as tyrosine proteins and aromatic protein-like substances in wastewater.

Fig. 8
figure 8

3D-EEM of influent and effluent water

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UV–Vis analysis

UV–visible light (wavelength range 190 ~ 450 nm) was used to scan the raw water as well as the wastewater pretreated by IC-ME, and the UV–Vis are shown in Fig. 8.

From Fig. 9, it can be seen that the organic peroxide production wastewater has obvious absorption peaks between the wavelengths of 190–300 nm, which may be caused by unsaturated cyclic aromatic hydrocarbon compounds such as benzene rings and heterocycles in the organic peroxide production wastewater. After the treatment with IC-ME, the absorbance of the wastewater was reduced between the wavelengths of 220 and 290 nm, indicating that the unsaturated cyclic aromatic hydrocarbon compounds such as benzene ring and heterocyclic were destroyed and the structure tended to be simpler (Hou et al. 2022). After wavelength 300 nm, the absorption peaks did not change, indicating that the pollutants in the wastewater were degraded (Lu et al. 2021).

Fig. 9
figure 9

UV–Vis spectrogram

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GC–MS analysis

Changes in organic species and content of organic peroxide production wastewater before and after IC-ME treatment were analyzed using GC–MS, as shown in Fig. 10. By comparison, the number of chromatographic peaks in the effluent was significantly reduced and there was a significant decrease in height (Zhou et al. 2020). When the retention time was between 0 and 5 min, most of the pollutants in the wastewater before and after treatment were alkanes and furans, and the relative content was reduced after IC-ME treatment, and the types of pollutants did not change (Ulhaq et al. 2021); when the retention time was between 5 and 19 min, the most organic species were mainly organic acids, ketones, alcohols, and ester compounds, and after treatment, there were new acids and ketones substances, in this interval, the relative content of pollutants is significantly reduced; in the retention time between 19 and 26 min, the wastewater before treatment of high boiling point, large molecules of organic substances are mainly acids, esters, anilines, benzene rings, and other substances, and the relative content of the substances in this interval is greatly reduced after treatment, indicating that IC-ME can open the ring and break the chain of large molecules, degraded into small molecules, thereby improve the biochemistry of wastewater (Zhou et al. 2020). This is the same as the results of 3D-EEM and UV–Vis analysis.

Fig. 10
figure 10

Gas-mass spectra of influent and efflue

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nt water.

Conclusion

  1. 1.

    The optimal conditions for the pretreatment of organic peroxide production wastewater by IC-ME were obtained by using single-factor test and response surface optimization test: the initial pH was 3.1, the dosage of iron filings was 30.5 g/L, the mass ratio of iron and carbon was 1.01, and the reaction time was 122.8 min. Under these conditions, the COD removal efficiency of IC-ME in the degradation of organic peroxide production wastewater was 35.67% and the biodegradability of wastewater increased from 0.113 to 0.173. The kinetic test showed that the process of IC-ME for degrading COD of organic peroxide production wastewater was in accordance with the secondary reaction.

  2. 2.

    The three-dimensional fluorescence spectra, UV–Vis, and GC–MS analyses of the wastewater before and after the treatment of the IC-ME process showed that the IC-ME decomposed the large organic molecules, such as tyrosine-like proteins and aromatic proteins, through the opening of the ring chain breakage, and transformed them into the small organic pollutants that were easy to be biodegradable, so as to increase the biodegradability of the wastewater.

  3. 3.

    The pretreatment of organic peroxide production wastewater by IC-ME s technology is of practical value by optimizing the process conditions and providing reference for actual wastewater treatment.