Study On the Degradation Mechanism of 2-Amino-4-Acetaminoanisole From Wastewater by Nano-Fe3O4 Catalyzed Heterogeneous Fenton System

Wide use of 2-Amino-4-Acetaminoanisole (AMA) as a coupling component in the synthesis of many commercial dyes leads to the generation of AMA dyed wastewater. Discharge of untreated AMA dyed wastewater could bring environmental concerns. The present study featured H 2 O 2 heterogeneous Fenton system to degrade 2-Amino-4-Acetaminoanisole from wastewater using nano-Fe 3 O 4 catalyst prepared via the co-precipitation method. Based on a single factor and taking the AMA removal rate as the response value, the Box-Benhnken (BBD) response surface method was used to investigate the individual effects of Fe 3 O 4 dosage, H 2 O 2 dosage, initial pH, and reaction time. For the interaction study, the experimental data were processed with Design-Expert 10.0 software to obtain a quadratic response surface model. The results showed that the order of the inuence of the four independent variables on the response value is as follows: nano-Fe 3 O 4 dosage > H 2 O 2 dosage > reaction time > pH. The obtained mathematical model exhibited a high degree of t with the maximum AMA removal eciency reaching to 100%. The optimal reaction conditions considered in this study are 1.70 g/L of Nano-Fe 3 O 4 dosage, 53.52 mmol/L of H 2 O 2 dosage, pH 5.14 and 388.97 mins as system reaction time. Furthermore, HPLC-MS was employed to analyze the degradation mechanism of AMA and the reaction intermediate products. Findings of this research provides fundamental theory and could guide subsequent practical AMA treatment during wastewater treatment.


Introduction
Aniline is an important pollutant in industrial wastewater ( (Wang D. et al. 2011). Aniline is also included in "14 key environmental pollutants blacklist" by the European Union (Bi and Huang 2013). Aniline compound is a highly toxic substance with hazardous properties belonging to category 6.1 toxic substances with carcinogenic, teratogenic, and mutagenic effects (Emtiazi et al. 2001). Improper contact with aniline compounds can cause hypoxia in the human tissues and damage the central nervous system, cardiovascular system, and other organs. It is even lethal in severe cases. Improper discharge of anilinecontaining wastewater could cause serious pollution to the surrounding environment (Xie et al. 2012). 2amino-4-acetaminoanisole (AMA) is one type of aniline, the main downstream products of which include Disperse Blue HGL, Disperse Blue 79, and other dyes (Sekar 1996). The process of producing such products results in a large number of by-products like 2-nitro-4-methoxyaniline and 3-nitro-4methoxyaniline. Meanwhile, the wastewater also contains a high level of sulfuric acids and acetic acid. In the light of the above, it is necessary to develop an e cient and economical method for the treatment of 2-amino-4-acetaminoanisole production wastewater.
The remediation of this type of wastewater faces great challenge (Sekar 1996). The commonly used treatment methods include adsorption ( (Cui et al. 2017;Marques et al. 2015). Some of these technologies have the disadvantages of low e ciency, secondary pollution, and high-cost concerns. As aniline and its derivatives are recognized as hard-to-degrade organic compounds, it is usually necessary to use a strong oxidant to assist degradation (Benito et al. 2017; Ku et al. 2010;Yan J. et al. 2011). Advanced oxidation processes (AOPs) can be used as an effective and feasible technology to remove organic pollutants, by generating active hydroxyl radicals (·OH) to achieve the purpose of degradation ( Yang et al. (Yang C. et al. 2021) prepared Fe (II)-nano-Fe 3 O 4 @PAC heterogeneous Fenton catalyst (MFC) by coprecipitation impregnation method, and reported that MFC is an effective catalyst in the Fenton process for aniline degradation. The degradation and mineralization rate of 5 mg / L aniline solution reached 91.2% and 75.77% within 30 min, respectively. In addition, Fe 2+ / Fe 3+ on the surface of MFC and Fe 2+ / Fe 3+ in the solution affected the degradation of aniline. Using SnO 2 -Sb 2 O 3 -pto / Ti anode, aniline was effectively degraded in alkaline medium with pH 11.0 by electrocatalytic oxidation (Li et al. 2003 (Xu et al. 2018). However, its application in the degradation of AMA has not been reported yet.
In the present study, a nano-Fe 3 O 4 catalyst was prepared based on the co-precipitation method, and the degradation performance of the catalyst in the H 2 O 2 heterogeneous Fenton system on AMA was studied.
Furthermore, the degradation mechanism of AMA was also analyzed in this study.

Preparation of Nano-Fe 3 O 4
Fe 3 O 4 nanoparticles were prepared by coprecipitation method. Brie y, a certain amount of FeSO 4 .7H 2 O and FeCl 3 .6H 2 O with the molar ratio of 3:4 was dissolved in 700 ml ultrapure water and the total iron concentration of 0.1 mol / L was generated. The solution was then transferred to a 1000 ml round bottom ask, and connected with a gas pipe, a pH meter, and a mechanical stirring paddle. The gas ow rate was controlled to 0.1 L / min and the dissolved oxygen in the water was removed for 10 min. The mechanical stirring speed was gradually increased to 2000 R / min. NaOH solution was also gradually added into the above mixed solution and the stirring speed was maintained. When pH reached 6.5, a black colloidal substance appeared, and NaOH solution continued to be added until pH reached 13.0. This state was kept and stirred for 30 min. The pH probe was then replaced with a thermometer with continued stirring. After heating to 80 ℃, the high temperature was kept for 30 min to crystallize the material. After the mixture was reduced to room temperature, the solid particles were separated by the magnetic eld and cleaned several times with ultrapure water. The pH of the supernatant was eventually dropped to 7. The magnetic particles were evenly dispersed in the anaerobic reagent bottle lled with ultrapure water. After each use of the catalyst, the high purity nitrogen was blown for 15 min to remove oxygen, and the nano-Fe 3 O 4 particles were stored in the anaerobic closed water environment.

Characterization of the catalyst
The phase analysis of the material was carried out by X 'PERT PRO X-ray diffractometer (Panako, Netherlands) using the Cu target radiation, 40kV as the test voltage, 40mA as the current, and the scanning range of 10°~90° (2theta). At different scales and magni cations, the morphologies of the samples were analyzed by SU8010/S4800 (Japan/Hitachi) high-resolution cold-eld emission scanning electron microscope (SEM) with a working voltage of 5kV and a resolution of 1.3nm. SEM analysis was accompanied with an energy dispersive X-ray spectroscopy (EDS) to determine the elemental composition of the catalyst.

AMA wastewater indicator monitoring
AMA was determined by Agilent 1260 high-performance liquid chromatography equipped with Zorbax Eclipse Plus C18 chromatographic column (column length 150 mm, inner diameter 4.6 mm, particle size 3.5 µm). The experimental conditions were as follows: column temperature: 40 O C; injection volume: 5µL;detection wavelength: 254 nm and ow rate: 1.2 mL/min. The Mobile phase was a mixture of methanol/ammonium acetate buffer (v/v: 33/67) with 1 g/L of ammonium acetate buffer. The wastewater studied in this project comes from the mother liquor water of a dyeing and chemical company in China. The wastewater production process is that para-anisole undergoes acetylation, nitri cation, and other reactions, and then undergoes reduction, precipitation, centrifugation, drying, and other processes to nally obtain 2-Amino-4-acetamido anisole. Due to the long production line, wastewater was generated in each process. Preliminary determination of the composition indicators of the wastewater was carried out, as shown in Table 1. (1) where ρ 0 is the initial mass concentration of AMA (µg/L); ρ t is the mass concentration of AMA after treatment, µg/L.  Figure 1 shows the XRD patterns of the as-prepared sample. Compared with the data in the standard card JCPDS(NO.65-3107), it can be seen that there are no other miscellaneous peaks. The pure phase magnetite Fe 3 O 4 with cubic inverse spinel structure is obtained, and the highest diffraction peak of the crystal plane is near 35.56° (2285). The as-synthesized sample has shown sharp diffraction peaks, high signal-to-noise ratio, and good crystallinity. Figure 2 depicts an SEM scan image and EDS analysis of the as-prepared sample. Figure 2

Feasibility Study
Parallel experiments were carried out in a Fenton revealed that increasing the dosage of H 2 O 2 can produce more ·OH radicals, which eventually enhance the removal rate of AMA. However, the addition of too much H 2 O 2 results in ·OH consumption, which generates less oxidizing HO 2 ·. HO 2 · can not only react with H 2 O 2 to prevent it from generating ·OH but also it can react with ·OH to prevent it from degrading AMA. Successful catalytic oxidation of AMA in the Fenton system was attained at 75 mmol/L, which could be considered as the optimum dosage of H 2 O 2 .

Effect of pH on Removal E ciency
To study the effect of pH alteration on the Fenton removal of the AMA, COD and chromaticity, 2 g/L of the nano-Fe 3 O 4 catalyst along with 75 mmol/L of H 2 O 2 solution was subjected into the system at the reaction temperature of 22 ℃. The reaction time was set to 360 mins. The pH range of 2-9 was considered in the experiment. Figure 4 (a), (b), and (c) depicts the experimental results.
With the pH rising from 2 to 9, removal of both AMA and COD initially increases and then decreases gradually whereas the chromaticity removal gradually declines. The maximum removal e ciency of AMA, COD and Chromaticity appeared at the pH of 5 with respective removal rates of 99.99%, 94.61% and 99.98%. The concentration of AMA and COD were 15.76 g/L and 258 mg/L, respectively; while the initial chromaticity was 8 pcu. The results revealed that Fe 3 O 4 nanospheres have good catalytic activity under weak acid conditions, but the catalytic activity is inhibited under alkaline conditions. Therefore, it can be stated that the best solution pH value for better degradation and removal of AMA in  (Fig. 5 (c)) while the removal rate is almost constant after 1.0 g/l catalyst loading. The removal e ciency of AMA and COD followed similar pattern. Both AMA and COD rst increased and then decreased gradually. The maximum removal e ciency for either AMA, COD or Chromaticity was achieved at 1.5g/L Fe 3 O 4 catalyst loading with removal rate of 99.99%, 98.90% and 99.98%, respectively. The concentration of AMA and COD were 10.33 g/L and 54 mg/L, respectively, while the chromaticity was 6 pcu. In line of the above, it can be stated that the best Fe 3 O 4 catalyst dosage for the degradation of AMA in the Fe 3 O 4 + H 2 O 2 -like Fenton system is 1.5 g/L.

Effect of Reaction Time on Removal E ciency
The effect of reaction time on the removal of AMA, COD and Chromaticity was studied considering various reaction times (60, 120, 180, 240, 300, 360, 420 and 480 mins). The values for temperature, nano-Fe 3 O 4 dosage, H 2 O 2 solution and pH were kept constant. Figure 6 (a), (b), and (c) depicts the experimental results.
As the reaction time increases from 60 min to 480 min, the removal rates for chromaticity, AMA and COD also increase. When the reaction time reaches 360 min, the removal e ciency does not increase signi cantly or even decreases to a certain extent. The removal rate for AMA, COD and Chromaticity reaches 99.99%, 98.50% and 99.85%, respectively. The respective concentration of AMA and COD were 10.46 g/L and 72 mg/L while the chromaticity was 57 pcu. At reaction time of 360 mins, the optimum AMA degradation rate was attained by Fe 3 O 4 + H 2 O 2 Fenton system.

In uence of Temperature on Removal E ciency
The effect of reaction temperature on Fenton removal of AMA, COD and Chromaticity was also studied by varying temperatures (0, 5, 10, 15, 20, 25, 30 and 35 ℃). The reaction time was set to 360min and other experimental parameters and conditions were kept constant. The results are shown in Fig. 7 (a), (b), and (c).
As the reaction temperature rises from 0 ℃ to 35 ℃, the chromaticity removal rate also rises rst and is nearly completely removed after reaching 10 ℃. The removal rate of AMA and COD rst increased and then decreased. The best removal rates were attained when the reaction temperature reached 25 ℃. The respective removal rate of COD and Chromaticity were 99.42% and 99.98%. At this reaction temperature, AMA is no longer detectable. COD concentration was detected as 28 mg/L whereas chromaticity was 8 pcu. The results indicated that the reaction temperature affects the activity of hydroxyl radicals. As the reaction temperature rises, the activity of free radicals can be enhanced. However, too high temperature could decompose hydrogen peroxide. Therefore, it can be deduced that the best reaction temperature for degradation and removal of AMA by Fe 3 O 4 + H 2 O 2 Fenton system is 25 ℃. See Table 2for the levels and codes of each factor. Taking the concentration (Y) of AMA as the response value, the experimental data are tted by polynomial regression analysis, and a typical four-factor quadratic polynomial model can be obtained.
The model is as follows: (2) Where β 0 is the constant term representing the center point correction coe cient; X i X j is the experimental factor; β i is the linear coe cient; β ij is the quadratic term coe cient; β ij is the interaction term coe cient; ε is the residual error of the constructed model.

2 Regression equation and variance analysis
Based on the principle of Box-Behnken central combination design, the experimental scheme was designed, and a total of 29 groups of experiments were carried out. The experimental design scheme and results are shown in Table 3. Using Design-Expert 10.0 software, the experimental data in Table 3 were tted by quadratic multinomial regression, and the following equation was obtained.   The signal-to-noise ratio is equal to 60.304 > 4, which indicates that there is enough signal and the value is within a reasonable range. Note: P < 0.01 means the model is "highly signi cant", P < 0.05 means the model is "signi cant", P > 0.05 means the model is "not signi cant", model determination coe cient R2 = 0.9972, correction determination coe cient R2adj = 0.9944, prediction determination coe cient R2pred = 0.9838, coe cient of variation CV = 6.85%, signal-to-noise ratio = 60.304.

Residual Analysis
Residual refers to the error between the predicted value and the actual value given by the model established by the software. Among them, the internalized residual is used to express the degree to which the standard deviation deviates from the actual and predicted response value, which is mainly shown graphically as to whether each data point is linearly distributed. Foreign student residuals are used to describe whether each group of data is an outlier relative to the tted equation. As shown in Fig. 8a, the experimental data points are evenly distributed on both sides of the tting straight line, and the data has no problem. This further con rms that the predicted value is close to the actual value. As shown in Figs. 8b and 8c, the data points are randomly distributed without any regularity, which proves the randomness of the experimental group. The residual values are all within the range of 3, indicating that there are no abnormal points in the tting process, and the experimental operation is reliable. Figure 8d shows a linear relationship between the actual value of AMA concentration and the predicted value. The experimental data points are basically distributed on a straight line, which highlights that there is a good linear relationship between the predicted value and the actual value of the model. Therefore, this implies the model has good accuracy and persuasion.

Factor effect analysis
To further understand the removal of AMA by experimental factors and the interaction between nano-Fe 3 O 4 dosage, H 2 O 2 dosage, pH, and reaction time, a quadratic response surface diagram was drawn using software, as shown in Fig. 9.
As portrayed in Fig. 9, when one factor is xed, the response value always increases rst and then decreases with the change of another factor. This explains that each factor has an optimal value within the experimental level range. Any two factors taken as the central values, the contour lines corresponding to the response surface are elliptical rather than circular, which con rms the obvious interaction between the four factors. The optimal values of the model are 1. With the change of reaction time from 30 min to 240 min, each substance was gradually degraded. Over time, various substances are eventually degraded into small molecular organic substances such as H 2 O and CO 2 . Only resorcinol with m/z of 112 was found in the nal mass spectrogram, and it can be inferred that the possible degradation routes of AMA are the three routes in Figure 11.

Conclusions
In this research, the nano-Fe 3 O 4 catalyst was successfully prepared by the co-precipitation method, and the catalytic degradation performance of the catalyst for 2-amino-4-acetylaminoanisole production wastewater in an H 2 O 2 heterogeneous Fenton-like system was studied. Optimized by response surface methodology, the results revealed that the Fenton Mass spectrogram of AMA degradation course in nano-Fe 3 O 4 + H 2 O 2 Fenton system was also investigated in this study. Two possible degradation pathways of AMA were deduced. One is the oxidation by ·O 2 − and ·OH radicals to remove acetamino functional groups, and then amino functional groups to resorcinol or Hydroquinone. The other pathway is to use ·OH radical to remove acetamino and methoxy functional groups. Finally, the resultant compounds in the two pathways can be further transformed chemically to H 2 O, CO 2 , and other small organic compounds through the destruction of the benzene ring. This study on the degradation mechanism of AMA can provide guidance for the same type of high concentration wastewater, and can also be further improved on the basis of this method, so as to achieve the purpose of popularization and application. Figure 1 XRD patterns of magnetic Fe3O4 nano-spheres  The in uence of pH on the removal rate of each index Figure 5 The in uence of Fe3O4 dosage on the removal e ciency of each index Figure 6 The in uence of reaction time on the removal rate of each index Figure 7