Fe–Mn Oxide Composite Activated Peroxydisulfate Processes for Degradation of p-Chloroaniline: The Effectiveness and the Mechanism

Abstract

The chemical co-precipitation method was used to prepare magnetically separable Fe–Mn oxide composites, and the degradation of p-chloroaniline (PCA) using MnFe₂O₄ activated peroxydisulfate (PDS). The MnFe₂O₄ catalyst exhibited highly catalytic activity in the experiments. XRD, FTIR, SEM and TEM were used to characterize the catalytic materials. MnFe₂O₄ calcined at 500 °C was more suitable as a catalytic material for PCA degradation. The elevated reaction temperature was beneficial to the degradation of PCA in neutral pH solution. The reaction mechanism of the MnFe₂O₄ catalyzed oxidative degradation of PCA by PDS was investigated by free radical quenching experiments and XPS analysis. The results showed that sulfate radicals (SO₄^•−), hydroxyl radicals (•OH) and singlet oxygen (¹O₂) may all be participated in the degradation of PCA. XPS spectra showed that the electron gain and loss of Mn²⁺ and Fe³⁺ was the main cause of free radical generation. The possible intermediates in the degradation of PCA were determined by HPLC-MS, and possible degradation pathways for the degradation of PCA by the MnFe₂O₄/PDS system were proposed.

1. Introduction

In the past decade, the emission of inorganic and organic contaminants from various industries has led to the degradation of ecosystems [1]. Dyes [2], pesticides [3] and antibiotics [4] have received more attention. Among them, p-chloroaniline (PCA) is often used in industry, mainly as an intermediate for the synthesis of azo dyes and chromophenols but also as an intermediate in pharmaceuticals (e.g., chlordiazepoxide, fenadine) and pesticides and as a colorant in the production of color cinema films [5].However, PCA is highly irritating and can enter the human body through the skin, seriously harming the immune, nervous and endocrine systems [6]. Therefore, the direct discharge of wastewater containing PCA can cause environmental pollution, and the study of an effective method for the treatment of PCA-containing wastewater is essential. Ali et al. [3] found the adsorption of fenuron pesticide adsorption on multi-walled carbon nanotubes and their removal in water. Kenawy et al. [2] developed a nano-composite material and tested their adsorption capacity for dyes in aqueous environments. However, adsorption does not realize the mineralization of organic substance [7]. The choice of advanced oxidation technology allows for the simultaneous decomposition and mineralization of organic matter.

Advanced oxidation technology based on sulfate radical has received extensive attention [8,9]. Sulfate radicals (SO₄^•−) are generated by advanced oxidation technology based on persulfate (PS). Its standard oxidation potential is 2.5–3.1 V. Persulfates include permonosulphate (PMS) and peroxydisulfate (PDS). The standard redox potential is higher than that of •OH. The •OH standard oxidation potential is 1.9–2.7 V [10]. The oxidation principle is also similar to •OH, which is stable for a longer time. Advanced oxidation is performed with highly reactive electron-catalyzed PS, which allows for rapid decomposition of organic matter until final mineralization [11].

Among the many activation methods, transition metal activation is of great interest because it is less energy intensive, cheaper and more reusable than other activation methods (e.g., photo-activation, thermal activation, etc.). Transition metal activated PS are generally divided into two main categories, one is the activation of PS by transition metal ions, such as Ce²⁺, Cu²⁺, Ag⁺, etc., called homogeneous activation. The other is the activation of PS by solid metals, metal oxides (zero-valent iron, etc.), called non-homogeneous activation. Non-homogeneous catalysts rely on their surface coordination of metal ions to activate PS and provide more active sites [12,13]. Li et al. [14] found that CuO/PS system showed good removal effect on ofloxacin and cefadroxil in water with 92% and 80% removal, respectively. Superoxide radical (O₂^•−) and SO₄^•− are the main active substances to remove them.

In the last few years, spinel ferrites (e.g., CuFe₂O₄, CoFe₂O₄, MnFe₂O₄, etc.) have been widely considered by researchers. Compared to other transition metals, iron is widely used because it is cheap and easy to obtain, less polluting to the environment and effectively activates PS [15,16]. In addition, because spinel ferrite is easily separated from the reaction solution, it can be re-used in the catalytic process [17]. Therefore, it is significant to focus on the PS catalyzed by spinel ferrites. [18]. Considering the market price of metals and heavy metal leaching, which causes secondary pollution to the environment, MnFe₂O₄ was introduced into the catalytic PS system as a catalyst, and the results showed good catalytic performance. Deng et al. [19] also found that MnFe₂O₄ activated PMS could degrade Orange II in water.

Herein, the chemical co-precipitation method was used to prepare magnetically separable Fe–Mn oxide composites. MnFe₂O₄ catalyst coupled with PDS can actively degrade PCA over a wide pH and temperature range. Furthermore, the effect of free radical generation on PCA degradation in MnFe₂O₄/PDS system was investigated. Finally, based on the identification of the intermediates, the pathway of degrading PCA was explored. The mechanism of PCA degradation by MnFe₂O₄ activated PDS was explained.

2. Materials and Methods

2.1. Reagents and Materials

All chemicals in this work were analytical grade. P-chloroaniline (C₆H₆ClN), anhydrous methanol (MeOH), and tertiarybutyl alcohol (TBA) were supplied by Maclean Biochemical Technology Co, Shanghai, China. Potassium persulfate (K₂S₂O₈) was purchased from CNW Technology, Germany. N-(1-naphthyl) ethylenediamine hydrochloride, manganese sulphate monohydrate (MnSO₄•H₂O) and ferric chloride hexahydrate (FeCl₃•6H₂O) were supplied by Comio Chemical Reagent Co, Tianjin, China. Ethylenediamine tetra-acetic acid (EDTA-2Na) was purchased from Solaibao Technology Co., Beijing, China. Sodium azide (NaN₃) was provided by Windship Chemical Reagent Technology Co., Tianjin, China.

2.2. Preparation of MnFe₂O₄

The MnFe₂O₄ used in the experiment was prepared by chemical co-precipitation. The preparation process and properties of the catalyst (MnFe₂O₄) were carried out based on the previous experimental results [20]. Firstly, MnSO₄•H₂O and FeCl₃•6H₂O were dissolved in deionized water (Mn²⁺:Fe³⁺ = 1:2, molar ratio). Then, NaOH solution was added to make the pH value of the mixed solution reach 11. Finally, it was filtered, and calcined in a muffle furnace.

2.3. Degradation Experiments

For catalyst (MnFe₂O₄) performance, 20 mg/L PCA was adjusted to the needed pH value with 0.1 mol/L sulfuric acid or sodium hydroxide. After that, 50 mL of 20 mg/L PCA and a certain amount of MnFe₂O₄ were added to each of the six conical flasks. A constant temperature shaker (HNY-2102C, honor, Zhengzhou, China) will be used to hold these conical flasks. After being placed for 30 min to reach adsorption equilibrium, a certain amount of PDS was added and shaken continuously. After 30, 60, 90, 120, 180 and 240 min of reaction, the conical bottles were removed and the concentration of the filtered solution was measured. For the determination of total organic carbon (TOC), the reaction solution was taken at a fixed point in time, filtered and added 1 to 2 drops of sulphuric acid. Then, the treated solution was put into the TOC analyzer (Trace Elemental Instruments, XPERT, Netherlands) for measurement and quenching experiments in which a quantity of quencher was added before a reaction. The experimental steps of HPLC-MS are as follows: at 0 h and 4 h of the reaction, the filtered PCA solution was taken, and extracted three times with methylene chloride. The organic phase was concentrated to near dryness by means of a rotary evaporator and mixed with 5 mL of ultrapure water [19]. Finally, the organic phase was filtered through the organic phase filter head for the determination of HPLC-MS.

2.4. Analysis Methods

The pH values were detected by pH meter (PHS-3C, Shanghai Electronic Scientific Instruments Co., Shanghai, China). The concentration of PCA was analyzed by N-(1-naphthyl) ethylenediamine azo spectrophotometric method using UV-VIS spectrometer (N5000, Shanghai Youke Co., Shanghai, China) at a wavelength of 545nm [21]. The HPLC-MS (W2489-QDa, Waters, Milford, MA, USA) with a reversed-phase C-18 column (4.7 × 250 mm) was used to measurement of intermediate products. The acetonitrile/ultrapure water (V/V = 55/45) was used as mobile phase; the column temperature was 40 degrees Celsius; the flow rate was 0.50 mL/min; the fragment ion scanning range was 50–1050 amu; and the mass spectrometer was subjected to electrospray ionization under the 600 V fragmentation voltage. X-ray diffraction (XRD, Dmax 2500V, Bruker Co, Billerica, MA, USA) with Cu Kα radiation (λ = 0.15406 nm) was used to analyze the crystallinity of the synthesized products between 10° and 80°. The different functional groups of MnFe₂O₄ were detected by Fourier-transform infrared (FTIR, Nicolet 380, Thermo Electric Corporation, Boston, MA, USA). X-ray photoelectron spectroscopy (XPS, Thermo Scientific ESCALAB 250XI, Shimadzu Corporation, Kyoto, Japan) was used to analyze chemical composition, element content and valence state of material surface. Scanning electron microscope (SEM, JSM-6700F, JEOL, Tokyo, Japan) and transmission electron microscope (TEM, JEM-200CX, JEOL, Tokyo, Japan) were used to observe the microscopic morphology and structure of catalytic materials.

3. Results and Discussion

3.1. Characterization of MnFe₂O₄

Relevant studies showed that the calcination temperature may affect the structure of the material itself, thus affecting its catalytic activity [22]. As shown in Figure 1a, the X-ray diffraction patterns of MnFe₂O₄ catalytic materials at various calcination temperatures. The samples with same composition correspond well to the characteristic diffraction peaks of cubic spinel-type MnFe₂O₄ (JCPDS 38–0430, a = b = 8.519 Å, c = 8.54 Å, α = β = γ = 90°) at 200–500 °C, indicating that they have a semicrystal structure. The characteristic peaks at 2θ = 18°, 29.6°, 34.8°, 42.4°, 52.6°, 56.0°and 61.5°were corresponded to (111), (202), (311), (400), (422), (333) and (440) crystal faces of MnFe₂O₄, respectively. The same XRD pattern of MnFe₂O₄ was reported by Deng et al. [19]. Therefore, the chemical co-precipitation method used in the experiments was effective in preparing pure MnFe₂O₄ material. However, the XRD results of the sample at 600 °C showed that other material components may be presented besides MnFe₂O₄, because its characteristic peaks can well correspond to Fe₂O₃ (JCPDS 33–0664, a = b = 5.0356 Å, c = 13.7489 Å, α = β = 90°, γ = 120°) and Mn2O3 (JCPDS 24–0508, a = 9.4161 = Å, b = 9.4237Å, c = 9.4051 Å, α = β = γ = 90°). The diffraction peaks near 2θ = 23.1°, 38.2° and 55.1°, which belonged to the characteristic peaks of (211), (400) and (044) planes of Mn₂O₃, respectively. The characteristic peaks near 2θ = 24.1°, 33.1°, 35.6°, 40.8°, 49.4°, 54.0°, 57.5°, 62.4°and 63.9°were corresponded to (012), (104), (110), (113), (024), (116), (018), (214) and (300) planes of Fe₂O₃, respectively.

In order to analyze and determine the changes of surface functional groups of MnFe₂O₄ before and after the reaction, FTIR spectroscopy was used to analyze MnFe₂O₄, and the results were presented in Figure 1b. Due to the stretching vibration of hydroxyl and carboxyl groups on the catalyst surface, the larger absorption peak appeared at 3421 cm⁻¹. The characteristic peaks also appear at 2926 cm⁻¹ and 2854 cm⁻¹, which may be related to the C-H extension vibration [23]. Through the bending vibration of water molecules on the catalyst surface, the characteristic peak at 1635 cm⁻¹ was generated [24]. In addition, the characteristic peaks at 471 cm⁻¹ and 576 cm⁻¹, which were resulted in the vibration of Mn-O and Fe-O chemical bonds in the MnFe₂O₄ catalyst. The low wave number 471 cm⁻¹ was related with octahedral coordination of Mn²⁺. The high wave number 576 cm⁻¹ was assigned to the tetrahedral coordination of Fe³⁺ [25]. Finally, it was worth noticing that the FTIR of the MnFe₂O₄ before and after the reaction were basically the same. The prepared MnFe₂O₄ was a spherical structure with a diameter of 0.1 μm~0.5 μm as derived from TEM images and SEM images (Figure S1).

3.2. Degradation Experiments in Different Systems

As exhibited in Figure 2a, it was observed that the effect of adsorption of PCA can be ignored when only MnFe₂O₄ and PDS were present. In the MnFe₂O₄/PDS system, the PCA removal rate reached 92% at 240 min. In addition, the degradation efficiency of PCA in the PDS/Fe³⁺ and PDS/Mn²⁺ systems was 5.45% and 13.30% in 240 min, respectively. The results showed that PDS can be activated by MnFe₂O₄. Similarly, Deng et al. [18] found that when PMS was activated by Fe³⁺ or Mn²⁺ to degrade BPA, the removal rates were less than 10%. It was much more than the 90% removal by heterogeneous activation systems (MnFe₂O₄/PMS). Figure 2b shows Fe₂O₃, Mn₂O₃ and Fe₃O₄ have lower removal rates than MnFe₂O₄ under the same conditions. The presence of Mn₂O₃ and Fe₂O₃ affect the degradation of PCA. The substitution of spinel tetrahedral positions was confirmed by comparing the MnFe₂O₄ and Fe₃O₄ activities.

In this study, the catalytic capability of MnFe₂O₄ was investigated at various temperatures of calcination and the results were displayed in Figure 3. This calcination temperature of 200 °C to 500 °C had little effect on the formation of its compositions. After 240 min of reaction, the removal of PAC was 90.12%, 85.57%, 87.45%, 92.16% and 80.81% with MnFe₂O₄ calcined at 200–600 °C, respectively. In addition, the data fitting indicated that the responses followed the quasi-first-order dynamic constants (k_obs) calculated at 0.012, 0.010, 0.010, 0.013 and 0.009 min⁻¹, respectively (Figure S2). These findings showed that the removal rate decreased when the calcination temperature reached 600 °C. The phenomenon may be due to decomposition of the MnFe₂O₄ material to other oxidizing substances (e.g., Fe₂O₃ and Mn₂O₃) at 600 °C [26,27]. As shown in Figure 2b, Fe₂O₃ and Mn₂O₃ did not effectively remove PCA. Thus, the degradation rate of PCA was reduced.

3.3. Effect of Initial pH and Reaction Temperature

The pH of PCA solution has a very significant effect on the degradation rate. Formation of free radicals and surface charge of catalysts were influenced by the pH value [28]. The effect of different pH on the degradation of PCA was shown in Figure 4a. The maximum yield of PCA decomposition was achieved at pH 7, K_obs was 0.013 min⁻¹ at pH 7 (Figure S3). There were no significant differences in the degradation efficiencies at pH 3, 5, 9. However, when the pH value becomes 11, the efficiency of PCA was reduced by 43.88% compared with that of pH 7.

There are three possible reasons for these results. First of all, as the initial pH of PCA increased from 3 to 11, the leaching concentrations of Fe³⁺ were 0.12, 0.18, 0.14, 0.16 and 0.52 mg/L after 4 h. Meanwhile, the leaching concentrations of Mn²⁺ were 25.82, 19.73, 15.43, 12.25 and 1.71 mg/L. It was inferred that the leaching of Mn²⁺ facilitates the decomposition of PCA, and it also showed that initial pH affects the reaction of MnFe₂O₄ and other substances [29]. Secondly, at pH 11, the SO₄^•− and •OH were consumed by reaction with ${\mathrm{OH}}^{-}$ (Equations (1)–(4)), which strongly prevented the degradation of PCA [30]. For another cause, •OH and SO₄^•− have a shorter lifetime in alkaline solutions and therefore cannot adequately capture the bulk phase of PCA [31]. Finally, the charge of PCA and the surface charge of MnFe₂O₄ are also a crucial factor influencing the degradation of PCA in solutions. The acidity coefficient value (pK_a) of PCA was reported to be 4.15. It meant that PCA was primarily present in solution as a form of cationic at pH 3 and existed as neutral or anion form at pH 5–11 [18]. The point of zer charge (pH_pzc) of MnFe₂O₄ was determined to be 4.71 (Figure S4). It indicated that the catalyst surface is positively charged when pH < pH_pzc. When pH > pH_pzc, the catalyst surface is oppositely charged as compared to pH < pH_pzc [32].The pH changes of the solution during reaction were monitored and displayed in Table S1. These findings suggest that the system pH environment was maintained at 4 or below after 30 min of reaction for solutions with initial pH values of 3 to 9. When the pH_pzc of MnFe₂O₄ is higher than the pH of the solution, it has a positive surface charge. This condition is favorable for the production of more SO₄^•−. When the pH value increases to 11, the electrostatic gravity effect between MnFe₂O₄ (pH > pH_pzc, negative charges) and PDS (anion) disappears, and the yield of oxidizing active substances decreased, which resulted in a decline in PCA removal yield.

$${\mathrm{SO}}_4{}^{-}+{\mathrm{H}}_2\mathrm{O}\to {\mathrm{HSO}}_4{}^{-}+\mathrm{OH}$$

(1)

$${\mathrm{SO}}_4{}^{-}+{\mathrm{OH}}^{-}\to {\mathrm{SO}}_4{}^{2-}+\mathrm{OH}$$

(2)

$$\mathrm{OH}+\mathrm{OH}\to {\mathrm{H}}_2{\mathrm{O}}_2$$

(3)

$${\mathrm{S}}_2{\mathrm{O}}_8{}^{2-}+{\mathrm{H}}_2{\mathrm{O}}_2\to 2{\mathrm{H}}^{+}+2{\mathrm{SO}}_4{}^{2-}+{\mathrm{O}}_2$$

(4)

This study also investigated the degradation of PCA by the MnFe₂O₄/PDS system at different reaction temperatures (Figure 4b). As shown, the PCA removal rate gradually increased when the temperature increased. After 240 min of reaction, the removal rate increased by 28.7% with a temperature increase of 20 °C. Therefore, temperature has an important effect in the reaction of PCA removal by catalyst activated PDS. K_obs were 0.0041 min⁻¹ at 15 °C and 0.0091 min⁻¹ at 35 °C, with significantly higher removal efficiency (Figure S5). PCA can be effectively degraded at room temperature. In addition, the Arrhenius equation was used to estimate the relationship between reaction rate and temperature (Equation (5)).

$${\mathrm{lnK}}_{\mathrm{obs}}=\mathrm{lnA}-{\mathrm{E}}_{\mathrm{a}}/\mathrm{RT}$$

(5)

where E_a was the activation energy (kJ/mol), K_obs was measured quasi-first-order dynamic constant, R was the universal gas constant (8.314 J/mol·k), A was Arrhenius constant and T was the temperature (K). In accordance with Equation (5), the E_a of PCA removal by MnFe₂O₄ was calculated to be 29.70 kJ/mol [20]. Therefore, what occurs in the MnFe₂O₄/PDS system is mainly a chemical reaction.

3.4. Quenching Experiments of PCA Degradation by MnFe₂O₄/PDS System

The reasons for PCA degradation in the MnFe₂O₄/PDS system were investigated by quenching experiments, and the main active species were examined. In quenching experiments, a probe scavenger was used to remove free radicals [33]. Several studies reported that there are two main reactive species (e.g., •OH and SO₄^•−) (Equations (1), (2) and (6)) [28]. MeOH has a high heat of reaction for both SO₄^•− and •OH (k_•OH = 9.7 × 10⁸ M⁻¹s⁻¹, k_SO4^•− = 3.2 × 10⁶ M⁻¹s⁻¹). Moreover, the kinetic rate of the reaction of TBA with •OH (k_•OH = (3.8–7.6) × 10⁸ M⁻¹s⁻¹) is faster, compared to SO₄^•− (k_SO4^•− = (4.0–9.1) × 10⁵ M⁻¹s⁻¹) [34,35].

Figure 5a,b shows the degradation of PCA in the MnFe₂O₄/PDS system when TBA, MeOH and PDS were added into solution at the ratios of 200, 500 and 1000. It could be observed that the PCA removal rate decreased significantly after the addition of both TBA and MeOH. In this work, TBA (64.58%) and MeOH (62.18%) had similar effects on the degradation of PCA. Their K_obs also decreased from 1.255 × 10⁻² min⁻¹ to 0.506 × 10⁻² min⁻¹ and 0.459 × 10⁻² min⁻¹, respectively (Figures S6 and S7).These findings indicated that both •OH and SO₄^•− maybe involved in PCA degradation. In addition to SO₄^•− and •OH as the main free radicals, non-radical (¹O₂) can also be produced at the same time (Equations (7) and (8)) [36]. As shown in Figure 5c, NaN₃ was used to quench ¹O₂ [37]. ¹O₂ played the significance role in the system. After adding NaN₃, the percentage of degradation decreased from 92.16% to 60.80%. The K_obs decreased from 1.255 × 10⁻² min⁻¹ to 0.421 × 10⁻² min⁻¹ (Figure S8).

$${\mathrm{S}}_2{\mathrm{O}}_8{}^{2-}+{\mathrm{MnFe}}_2{\mathrm{O}}_4\to 2{\mathrm{SO}}_4{}^{-}$$

(6)

$${\mathrm{S}}_2{\mathrm{O}}_8{}^{2-}+40{\mathrm{H}}^{-}\to 2{\mathrm{SO}}_4{}^{2-}+{}_{}{}^1\mathrm{O}{}_2+2{\mathrm{H}}_2\mathrm{O}$$

(7)

$${\mathrm{O}}_2{}^{-}+2{\mathrm{H}}_2\mathrm{O}\to {}_{}{}^1\mathrm{O}{}_2+2{\mathrm{H}}_2{\mathrm{O}}_2+2{\mathrm{H}}^{+}$$

(8)

3.5. Total Organic Carbon (TOC) Removal Efficiencies in MnFe₂O₄/PDS System

A number of studies have shown that it is extremely difficult to obtain the full mineralization of PCA by chemical treatment methods [38]. A high removal rate of organic matter does not mean that the organic matter is broken down into CO₂ and H₂O, which may also be present as other small molecules [39]. The TOC was measured under different PDS dosages. As displayed in Figure 6, when 0.4 mM of PDS was added into PCA solution, the removal rate of TOC was 23.50% in 240 min. However, the TOC removal rate was 35.07%, when 2.4 mM of PDS was added into PCA solution. The removal and mineralization rates of PCA were significantly increased. The results indicated that PCA could be effectively mineralized in the MnFe₂O₄/PDS system. The PCA was not completely mineralized to H₂O and CO₂ [40]. It is possible that inorganic or organic small molecule products of oxidation were developed in solution.

3.6. Mechanism of PCA Degradation by MnFe₂O₄/PDS Systems

To further investigate the degradation mechanism of PCA in the MnFe₂O₄/PDS system, the products formed after the oxidative degradation of PCA were identified by HPLC-MS. The HPLC-MS is a concatenated technique using liquid chromatography as the separation system and mass spectrometry as the detection system. HPLC-MS combines chromatography and mass spectrometry to obtain more quantitative detection results. [41]. It is capable of providing relative molecular mass and structural information for the quantitative analysis of different contaminants [2,42].

Several degradation intermediates of PCA in the MnFe₂O₄/PDS system were detected using HPLC-MS measurements. The result obtained was similar with previous studies [43]. The peak area of PCA decreased significantly after 4 h, accompanied by the generation of new peaks, indicating the generation of new intermediates. The mass spectrometry analysis in Figure 7 showed that two major intermediate products may be formed during PCA degradation. The corrected retention time for Peak1 (P₁) was 12.77 min and m/z was 158.08. The corrected retention time for Peak2 (P₂) was 20.63 min and m/z was 267.08. Compared with PCA, the peak areas of P₁ and P₂ were reduced by 87% and 28%, respectively. By comparison of the mass to nucleus ratio and its molecular composition, it was assumed that P₁ and P₂ may be chloronitrobenzene and 5-chloro-2-(4-chloro-phenyldiazene) phenol, respectively.

Combined with previous studies [43,44,45], two possible degradation paths of PCA in the MnFe₂O₄/PDS system were obtained and are shown in Figure 8. In pathway I, deaminization reaction had played an essential part in the process of PCA degradation. This deaminization process would generate B, Then, P1 (chloronitrobenzene) was formed under the attack of •OH. In pathway II, the benzene ring on PCA can form phenolic compounds in the presence of SO₄^•−, •OH and ¹O₂, such as 2-amino-5-chlorophenol, which reacts with B to form substance P₂ (5-chloro-2-(4-chlorophenyldiazene) phenol). PCA and its intermediate oxidation products ring cleavage reaction occurred in the presence of reactive species and were converted by the oxidation of SO₄^•− and •OH to minor organic molecules.

It is well known that although the main structures of the compounds are the same, different substituents have important effects on the properties of the substances. Structural features of the compound determine the solubility, molecular arrangement, space structure, intermolecular attraction and repulsion of organic compound. This affects the degree to which organic matter is catalytically degraded. The compound in this study was PCA. According to the functional group structure analysis of the PCA, when there is already an amino substituent, the benzene ring contains a second substituent, and the second substituent is a halogen group. The halogen group has a passivation on the benzene ring. Theoretically, this organic is more difficult to degrade than those substances with a single substituent (e.g., phenol, nitrobenzene, chlorobenzene, etc.). Therefore, it is of great significance to study the degradation of different substituent compounds.

This reaction process was a non-homogeneous process. The PCA was adsorbed onto the catalyst surface. The oxidation reaction occurred on the surface of the catalyst. The oxidation during the catalytic activity may lead to changes in the surface composition and chemical state of MnFe₂O₄ [46]. Therefore, XPS was used to characterize the surface chemical properties of MnFe₂O₄ before and after degradation experiment [47]. The results showed that elements C, O, Fe and Mn exist in MnFe₂O₄ (Figure 9a).

As seen from Figure 9b, fresh sample of Fe 2p, the binding energies (BE) of the Fe²⁺ peaks were 710.1 eV and 723.2 eV, which were accorded with Fe 2p_3/2 and Fe 2p_1/2, respectively. Fe³⁺ peaks displayed two obvious peaks locating at BE of 712 eV and 725.3 eV, which were assigned to the Fe 2p_3/2 and Fe 2p_1/2 level [46]. In addition, 718.2 eV and 732.4 eV were ascribed to shake-up satellite peaks [26]. The above results demonstrated the existence of Fe²⁺ and Fe³⁺ in the MnFe₂O₄. The content of Fe²⁺ was detected to be 32% and the content of Fe³⁺ was 68%. For MnFe₂O₄ after use, the positions of the characteristic peak did not change. However, the total peak area decreased slightly after degradation, indicating that oxidation reaction occurred [28]. In accordance with the areas of two characteristic binding energy peaks, the contents of Fe²⁺ was 34% and the content of Fe³⁺ was 66% in the MnFe₂O₄ sample after activating PDS.

Similarly, in Figure 9c, the MnFe₂O₄ possessed Mn 2p peaks located at 641.5 eV and 653.2 eV, which correspond the Mn 2p_3/2 and Mn 2p_1/2 [48]. Figure 9c also displayed two distinct satellite peaks that were obtained from the Mn 2p spectrum, which was the signal of the Mn²⁺ characteristic peak [18]. After degradation experiments, the Mn element in the catalyst stayed in the Mn²⁺ state with no obvious changes.

In the end, peak fitting was performed for O 1s in MnFe₂O₄ samples before and after the reaction. Figure 9d showed the three peaks of O 1s spectrum of fresh samples were distributed at 529.7 eV, 531.8 eV and 533.4 eV, which were from surface lattice oxygen (Olatt), surface adsorbed oxygen (Oads) and adsorbed water (Owat), respectively [49]. In addition, three peaks were detected at 529.8 eV, 531.8 eV and 533.4 eV, respectively, after O 1s envelop decomposition of the used samples. The corresponding area ratio of adsorbed H₂O increased from 16.47% to 25.98%. Compared with Olatt and Oads, the Owat possessed higher mobility.

In summary, this mechanism of the oxidative degradation of PCA by the MnFe₂O₄/PDS system was postulated, and the possible reaction processes are shown in Figure 10. First, the iron ions have higher catalytic activity to PDS in the octahedral sites of the MnFe₂O₄ spinel structure. Second, the active site undergoes a valence change at the solid–liquid interface, with iron and manganese ions as electron donors and S₂O₈²⁻ as electron acceptors to produce SO₄^•−, partially converting S₂O₈²⁻ to ¹O₂ and producing •OH. Finally, PCA reacts with various oxidation active substances [50].

4. Conclusions

In this work, MnFe₂O₄ was successfully synthesized by the chemical co-precipitation method. The catalyst was characterized by XPS, XRD, FTIR, TEM and SEM techniques. The efficiency of this catalyst in activating PDS for the degradation of PCA was investigated. The experiments showed that the decomposition of PCA by the MnFe₂O₄/PDS system depended on different reaction conditions (initial pH, reaction temperature, calcination temperature of catalyst). According to the quenching experiments, SO₄^•−, •OH and ¹O₂ issued from the MnFe₂O₄/PDS system were primary reactive oxygen species in the solution. Furthermore, the addition of more oxidant facilitated the mineralization of PCA. The PCA degradation intermediates were identified with HPLC-MS, and based on the results, the possible transformation pathways for PCA degradation was proposed. It is concluded that PCA can be degraded by both free radicals and non-free radicals generated in the MnFe₂O₄/PDS system.