Abstract

The Fe-Mn oxides composite prepared by a chemical co-precipitation method was used as a heterogenous peroxydisulfate catalyst for the decomposition of aniline. This study investigated the mechanism of aniline degradation by PDS activated with catalyst. Reactive species resulting in the degradation of aniline was investigated via radical quenching experiments with different scavengers, including methanol, tert-butyl alcohol, EDTA and sodium azide. Based on the experiments made here, it is speculated that the predominant reactive species responsible for the degradation of aniline may be holes and singlet molecular oxygen rather than SO4·− and ·OH radicals. The degradation of compounds in catalyst/peroxydisulfate system was put forward. The three possible intermediates were speculated by high performance liquid chromatography-mass spectrometry, and two possible degradation pathways were proposed.

HIGHLIGHTS

  • Magnetic Fe-Mn particles based on spinel-type ferrite MnFe2O4 were synthesized.

  • Diagrammatic sketch of aniline degradation mechanism by peroxydisulfate in the presence of MnFe2O4 system was demonstrated.

  • Three main intermediate products and two possible paths were studied.

INTRODUCTION

As one of the most widely used sewage treatment technologies, advanced oxidation processes (AOPs) are applied in the oxidation of non-biodegradable, recalcitrant and toxic organic pollutants on account of the generation of highly reactive species under ambient conditions in recent years (Hodges et al. 2018). In previous studies, traditional AOPs based on the generation of hydroxyl radical (·OH), e.g. Fenton reaction (Fe2+/H2O2) (Verma & Haritash 2019), electron-Fenton (Yong et al. 2017), UV/H2O2 (Benito et al. 2017), O3/H2O2 (Can & Cakir 2010), have received popular attention owing to producing the strong oxidant ·OH (oxidation potential is 1.8–2.7 V vs. normal hydrogen electrode (NHE)) in mild reaction conditions. However, several weaknesses, such as narrow and limited pHs (2–4) under optimal conditions and the chemical instability and high-cost of hydrogen peroxide, were gradually discovered in the continuous research of the AOPs, leading to tremendously placed restrictions on its practical application (Duran et al. 2018; Huang et al. 2020).

In recent years, AOPs based on the generation of sulfate radical (SO4·–) have been the focus of numerous studies, and peroxymonosulfate (PMS, HSO5) and peroxydisulfate (PDS, S2O82−) have been proposed as candidates for such a purpose (Wacławek et al. 2017). Compared with ·OH, sulfate radical (SO4·–) not only carries forward the same advantages due to the higher reactive oxidation (2.5–3.1 V vs. NHE) (Zhu et al. 2019), but also solve the shortcomings resulting from its stability and wider working pH range (Hu & Long 2016). SO4·– is generated by activating the PDS and PMS through various methods such as heat (Ji et al. 2015), UV light (Khan et al. 2013), alkaline (Lominchar et al. 2018), metal ions (Wang et al. 2019a), and metal oxide (Du et al. 2017; Zhang et al. 2018) though the cleavage of their O–O bonds. They are both very stable in the solid state, but PDS is a cheaper oxidant than PMS (considered as oxone salt (2KHSO5·KHSO4·K2SO4) and H2O2 and have a higher oxidation potential than PMS (Zhou et al. 2019). Consequently, PDS has shown to be a promising application prospect for the decomposition of organic pollutants.

Among the catalytic methods in AOPs based on PDS, metal ion and metal oxide catalysis are gaining greater attention and have wider application rather than sonocatalysis, photocatalyst is and thermocatalysis because of the lower energy consumption and low price. Metal oxide can be reused and regenerated in the catalytic process, which demonstrates that metal oxide catalysis can further reduce the total cost compared with other methods (Zhou et al. 2019). In addition, the separation between catalysts and liquid in the waste water treated by the method is easy to be implemented in subsequent processes. Therefore, it is highly significant to research metal oxide catalysis in AOPs based on PDS.

Spinel-type ferrite oxides, mainly including CuFe2O4, MgFe2O4, NiFe2O4, CoFe2O4 and MnFe2O4 etc., have attracted attention due to their chemical stability and photochemical properties (Junlabhut et al. 2018). Recently, spinel-type ferrite oxides were used as the catalytic agent in AOPs. Wang et al. (2019b) report that 67% of levofloxacin was degraded within 1 h with the reaction condition of 0.6 g/L magnetic nickel ferrite/carbon sphere composite and 1.8 g/L PDS, which is described as a potentially useful catalyst in AOPs. Pan et al. (2017) reported that UV-filter benzophenone-3 in aqueous solution is removed in 6 h under the condition of [benzophenone-3]0: [PDS]0 = 1:1,000, catalyst load = 500 mg L–1. Generally, spinel-type ferrite oxides have gained more attention in catalytic oxidation.

Although SO4· is the main production of most persulfate processes, there are some investigations which indicate that partial persulfate activation processes involve a non-radical mechanism. Some investigations have found a non-radical mechanism for the degradation of organic contaminants. Zhang et al. (2014) reported that copper oxide (CuO) could efficiently activate PDS without producing SO4·. Meanwhile, Lee et al. (2015) reported that the persulfates bind onto the surface of CNTs, forming reactive complexes that are immediately decomposed upon reaction with organic compounds. Wu et al. (2018) discovered that the reaction of non-radical mechanism was observed in the degradation of aniline by rice straw biochar RSBC/PDS system and the holes were responsible for the reaction process. However, the research about the mechanism of non-radicals has not been greatly reported in current reports.

Single oxygen (1O2), which is one of the typical nonradical reactive oxygen species, has been widely used in the degradation of organic pollutants, particularly in photocatalytic degradation processes (Lee et al. 2011). The 1O2 in the process of degradation has strong selectivity to organic pollutants owing to its electrophilic nature (Cheng et al. 2017). Some studies reported that the activation process of PDS can produce 1O2 in some systems. Ma et al. (2018) reported that PDS is activated by nitrogen-doped mesoporous carbon to degrade the phenol through 1O2 and electron transfer. Shi et al. (2020) reported that 1O2 was one of the reactive oxidative species involved in the degradation of sulfamethoxazole through activation of persulfate by EDTA-2 K-derived nitrogen-doped porous carbons.

In this study, we investigated the activation mechanism of PDS by MnFe2O4 for the degradation of aniline. Radical scavengers identified the main reactive species responsible for aniline decomposed. Additionally, degradation mechanisms were proposed based on experimental results from spectroscopy measurement.

MATERIALS AND METHODS

Reagents and materials

All chemicals used in this study were at least of analytical grade and used as received without further purification, except as noted. Aniline was provided by Sigma-Aldrich Chemical Co., Ltd (Shanghai, China). Potassium persulfate was provided by ANPEL Laboratory Technologies Reagent Inc. (Shanghai, China). N-(1-naphthyl) ethylenediamine dihydrochloride, iron (III) chloride hexahydrate (FeCl3·6H2O) and manganese sulfate monohydrate (FeSO4·6H2O) were supplied by Kermel Chemical Reagent (Tianjin, China). Methanol anhydrous (MeOH) and tert-Butanol (t-BuOH), serving the scavengers of radicals, were obtained from Macklin Biochemical Co., Ltd (Shanghai, China). Ethylenediaminetetraacetic acid disodium salt (EDTA-2Na) used as the hole scavenger and sodium azide (NaN3) used as singlet molecular oxygen (1O2) scavenger, was bought from Guangfu Technology Development Co., Ltd (Chengdu, China). Deionized water was used throughout the experiments.

Synthesis of MnFe2O4

The MnFe2O4 catalyst was synthesized using the chemical coprecipitation method (Liu et al. 2020). First, 5.07 g MnSO4·H2O and 16.5 g FeCl3·6H2O were dissolved into 300 mL deionized water (Mn2+:Fe3+ = 1:2 at a molar ratio) with continuous mechanical stirring at 62°. Secondly, 8 M NaOH solution was added slowly to the mixture solution until the pH value of the solution reached approximately 11. Then, stirring was stopped and the mixture solution was maintained at 75° for 30 min. When cooling down to room temperature, the mixture solution was filtered to obtain the suspension which was then washed with deionized water and ethanol at least three times. Finally, the composite was calcined at 300° for 4 h in the muffle furnace and ground to powder for further use.

Characterization of MnFe2O4

The morphological representation and internal structure of MnFe2O4 was collected on a field emission scanning electron microscope (FE-SEM, JSM-6700F) equipped with energy-dispersive X-ray spectrometer and transmission electron microscopy (TEM, JEM-200CX, JEOL, Japan). Powder X-ray diffraction spectrum was recorded at 0.02°/s and operated at 40 kV and 15 mA current with Cu kα1 radiation (λ = 0.15406 nm) diffractometer (XRD, Dmax 2,500 V). The various functional groups of MnFe2O4 were detected by Fourier-transform infrared (FTIR, Nicolet380).

Degradation experiments

In a typical run, conical flasks containing 50 mL reaction solution were kept at a controlled temperature in an air bath shaker (HNY-2102C, Honour, China) to conduct the tests. The initial concentration of aniline was 20 g/L. After 30 min of adsorption equilibrium between aniline and MnFe2O4, the oxidation reaction was initiated by adding a desired amount of PDS. The pH in the solution was adjusted by H2SO4 or NaOH solution. After 0, 30, 60, 90, 120, 180 and 240 min, respectively, 1 mL of reaction solution was withdrawn and filtered through a 0.45 mm membrane into a liquid sampling bottle for further analysis.

Analytical methods

The concentration of aniline was analyzed with the colorimetric method using N-(1-naphthyl) ethylenediamine as a chromogenic agent. An N5000 UV-VIS spectrometer (Shanghai Youke, China) was used to measure the absorbance in a 1 cm cell at 545 nm. The intermediates were identified on a Waters W2489-QDa (USA) high performance liquid chromatography (HPLC) equipped with a Waters QDa mass spectrometer with a reversed-phase C18 column (4.7 × 250 mm). The mobile phase was also the mixture of acetonitrile-water (55:45, V:V) and the flow rate was set at 0.50 mL/min. The mass spectrometer was operated under electrospray positive ionization at a fragmentor voltage of 800 V with a mass scan range of 50–600 amu. The ionization source conditions were set at the drying gas flow at 600 °C. The metal valance state was identified by X-ray photoelectron spectroscopy (XPS, Al Kα, AXIS Supra spectrometer, Shimadzu, Japan).

RESULTS AND DISCUSSION

Characterization of catalysts

The XRD patterns were used to analyze the information of material composition, structure or morphology of atoms or molecules in materials. X-ray diffraction analysis is the main method to research the phase and crystal structure of materials. Thus, the X-ray diffraction of catalyst is shown in Figure 1 to determine the composition and crystallinity.

Figure 1

XRD spectrum of MnFe2O4.

Figure 1

XRD spectrum of MnFe2O4.

In the range of 2θ from 20 to 80°, it was noticed that there were several significant peaks in the X-ray diffraction of the material. The diffraction peaks at 2θ of 29.71, 34.98, 42.53, 56.20 and 61.66° mainly corresponded to the typical reflections of (220), (311), (400), (511) and (440) planes, respectively. The results mean that the material was indexed into the cubic spinel-type MnFe2O4 (JCPDS No. 38-0430). Wang et al. (2019b) also reported similar peaks in the XRD spectrum of MnFe2O4. Therefore, the pure MnFe2O4 was efficiently produced by using the chemical coprecipitation method in the experiment.

The transmission electron microscopy (TEM) and scanning electron microscope (SEM) images, where surface morphology and particle size was obtained, is demonstrated in Figure 2. It could be seen from TEM images that MnFe2O4 particles have spherical structures, while catalyst particles in micron scale with a diameter of 0.1–0.5 μm were confirmed by the SEM images.

Figure 2

SEM and TEM spectrum of MnFe2O4.

Figure 2

SEM and TEM spectrum of MnFe2O4.

The various functional groups of MnFe2O4 before and after reaction were confirmed in the FTIR spectra (Figure 3). The appearance of absorption peak at 3,424 cm–1 was due to the stretching vibrations of -OH moieties in carboxyl and hydroxyl groups (Yang et al. 2019). The corresponding functional groups with wave numbers of 2,853 and 2,923 cm–1 were associated to C-H stretching vibrations (Chakraborty et al. 2017). The peak at 1,644 cm–1 could be assigned to the bending vibrations of H2O molecules on the surface of MnFe2O4 (Zhu et al. 2019). Furthermore, there were two strongly characteristic peaks in the wave numbers of 471 and 576 cm–1 which is related to Mn-O and Fe-O vibrations in MnFe2O4 respectively. The lower wave number of 471 cm–1 was assigned to the octahedral coordination Mn2+, and the higher wave number of 576 cm–1 was related to the tetrahedral coordination Fe3+ (Kafshgari et al. 2017). It was observed that there were no differences in the FTIR spectra of the used and the virgin MnFe2O4. The results demonstrated the MnFe2O4 used as the catalyst was stable in the system MnFe2O4/PDS (Liu et al. 2020).

Figure 3

FT-IR spectra of MnFe2O4.

Figure 3

FT-IR spectra of MnFe2O4.

Figure 4

Effect of different systems on degradation of aniline by PDS catalyzed with MnFe2O4. Experimental conditions: aniline = 20 mg/L, PDS = 2.4 mM, MnFe2O4 = 1.3 g/L, Fe3+ = Mn2+ = 5.6 mM.

Figure 4

Effect of different systems on degradation of aniline by PDS catalyzed with MnFe2O4. Experimental conditions: aniline = 20 mg/L, PDS = 2.4 mM, MnFe2O4 = 1.3 g/L, Fe3+ = Mn2+ = 5.6 mM.

Degradation experiment in different systems

To confirm the MnFe2O4 accessed to activate PDS, Figure 4 shows the degradation of aniline by different systems. It can be seen that the maximum adsorption capacity of MnFe2O4 for aniline is 2.1 mg/L and aniline degradation by 2.4 mM PDS is only 7.1% at 240 min, which shows that they both had a negligible contribution to adsorb or oxidize the compound. Aniline removal efficiency in the MnFe2O4/PDS system was nearly 100% at 240 min. It is noteworthy that a pseudo-first-order degradation pattern (R2 > 0.90) was suitable for the aniline removal in PDS activation by MnFe2O4 and the fitting reaction rate constant (kapp) was 0.016. Besides, aniline degradation efficiencies in PDS/Fe3+ and PDS/Mn2+ system was 11 and 12% at 240 min, respectively, demonstrating that the heterogeneous activation was the dominant oxidation in the MnFe2O4/PDS system. These results demonstrate that MnFe2O4 could effectively activate PDS. Similarly, Deng et al. (2017) reported that the degradation of bisphenol A by homogeneous Mn2+/PMS, and Fe3+/PMS activation was less than 10% compared with the degradation of 90% by MnFe2O4/PMS system. Abroshan et al. (2018) reported that MnFe2O4 had a certain function in catalyzing PDS for the removal of levofloxacin in aqueous solution.

Effect of initial pH and reaction temperture

The influence of initial pH ranging from 3.0 to 11.0 on aniline degradation is illustrated in Figure 5. It is clear that the aniline degradation in the MnFe2O4/PDS system showed high efficiencies over a wide pH range of 3.0–9.0 and strongly alkaline pH could not benefit the degradation of aniline. Specially, when the pH value increased from 3.0 to 9.0, the minimal removal efficiencies of aniline achieved 95% within 4 h, and the corresponding kobs value was 0.0113 min–1. The aniline degradation had a significant reduction with the increasing of initial pH value to 11 and the relevant kobs was found to be 0.0019 min–1. The pHpzc of MnFe2O4, the pKa of aniline, and the pH value of solution should all be taken into consideration when discussing the influence of initial pH. It has been reported that the pKa value of aniline is 4.6, which means that aniline exists mainly in cationic forms in solution at pH 3 and mainly as neutral in a pH range of 5–11. The pHpzc of MnFe2O4 was determined to be 4.81. It demonstrated that the surface of the particle had a positive charge due to pH less than pHpzc and would be negatively charged owing to pH larger than pHpzc. To further determine the change of pH value in the system, variations of solution pH in these processes are also confirmed and it is clear that the values of pH from 30 to 240 min are controlled in a certain range from 4.50 to 3.50 when the initial pH was from 3.0 to 9.0 and the solution pH in 240 min tardily decreased to 10 when the initial pH was 11. Thus, as the initial pH value was under 10, the surface on MnFe2O4 would engender the electrostatic attraction force with PDS (anion) instead of the repulsion force with aniline. It could be speculated that the reaction between PDS and MnFe2O4 took precedence over the oxidative degradation of aniline in the PDS/MnFe2O4 system. As the initial pH value reached 11, the electrostatic attraction force between PDS and MnFe2O4 vanished (pH > pHZPC), leading to a decrease in the aniline degradation. Above all, it was likely that the MnFe2O4/PDS system had broad application prospects as part of wastewater treatment except in the strongly alkaline condition. What is more, to reach such prospects, the leaching of metal ion in this system is one of the questions of common interest. The leaching concentration of ferric ion after 4 h reaction was detected to be 0.14, 0.15, 0.1, 0.17 and 0.45 mg/L with the initial pH value increasing from 3.0 to 11.0 g/L, respectively. Meanwhile, the leaching concentration of manganese ion was 30.30,17.09, 13.40, 14.24 and 0.57 mg/L, respectively. The results show that further treatment in practical applications needs to be carried out for decreasing the concentration level of manganese in the solution.

Figure 5

Effect of solution initial pH on degradation of aniline by PDS catalyzed with MnFe2O4. Experimental conditions: initial pH = 5.00 ± 0.2 and T = 25°, initial concentration of aniline, PDS and MnFe2O4 were 20 mg/L, 2.4 mM, and 1.3 g/L, respectively.

Figure 5

Effect of solution initial pH on degradation of aniline by PDS catalyzed with MnFe2O4. Experimental conditions: initial pH = 5.00 ± 0.2 and T = 25°, initial concentration of aniline, PDS and MnFe2O4 were 20 mg/L, 2.4 mM, and 1.3 g/L, respectively.

The effect of reaction temperature in the MnFe2O4/PDS system was confirmed. Specially, the increasing of the corresponding kobs values is from 0.0055 min–1 at 10 °C to 0.018 min–1 at 30 °C. The results showed reaction temperature is an essential factor for this system, thus it is hard for it to be widely used in some lower temperature areas.

Mechanism of aniline degradation by MnFe2O4/PDS systems

In terms of previous reports, the predominant reactive species in PDS activation were SO4·– and ·OH (Nie et al. 2014). It was reported that MeOH could quench both SO4·– (kSO4·– = 1 × 109 M−1 s−1) and ·OH (k·OH = 1 × 107 M−1 s−1). t-BuOH showed a high reaction rate in quenching ·OH (k·OH = 5.2 × 108 M−1 s−1), yet it reacted slower with SO4·– (kSO4·– = 8.4 × 105 M−1 s−1) than MeOH (Guan et al. 2018). Thus, MeOH and t-BuOH were used as scavengers to distinguish the existence of SO4·– and ·OH in MnFe2O4/PDS systems separately. As Figure 6 shows, the degradation of aniline by the MnFe2O4/PDS system was examined in the presence of excess radical scavengers. Both the introduction of MeOH and t-BuOH in PDS/MnFe2O4 system had little influence on the degradation of aniline even if the scavenger was 1,000 times the molar ratio of aniline. This suggested that the mechanism of SO4·– generation probably has little influence on the aniline decomposition by the MnFe2O4/PDS system. Similarly, Shi et al. (2020) reported that the removal rate of sulfamethoxazole through the PDS activated by EDTA-2 K-derived nitrogen-doped porous carbons only reduced by 23% as the addition of MeOH which demonstrated that SO4·– and ·OH were not dominantly responsible in the reaction system. However, SO4·– and ·OH were still generated in the system.

Figure 6

Effect of different MeOH(a) and t-BuOH (b) on degradation of aniline by PDS catalyzed with MnFe2O4. Experimental conditions: initial concentration of aniline, PS and MnFe2O4 were 20 mg/L, 2.4 mM, and 0.9 g/L, respectively.

Figure 6

Effect of different MeOH(a) and t-BuOH (b) on degradation of aniline by PDS catalyzed with MnFe2O4. Experimental conditions: initial concentration of aniline, PS and MnFe2O4 were 20 mg/L, 2.4 mM, and 0.9 g/L, respectively.

To verify whether the main reactive species in MnFe2O4/PDS system responsible for the degradation of aniline was the hole which was similar to photogenerated active sites, we tested the radical trapping experiments, where EDTA was added into the reaction system as the hole radical scavenger (Figure 7(a)). Specifically, the removal efficiencies of aniline after 4 h significantly decreased from 90.1 to 47.0% with the concentrate of EDTA increasing from 0.01 to 1.00 g/L, respectively. This result showed the possible involvement of the hole in the degradation of the aniline. However, Wu et al. (2018) reported that the degradation of aniline in RSBC/PDS was completely inhibited when the addition of EDTA reached 1.00 g/L. This demonstrated that the MnFe2O4/PDS system may produce other reactive species except holes.

Figure 7

Effect of different EDTA(a) and NaN3 (b) concentration on degradation of aniline by PDS catalyzed with MnFe2O4. Experimental conditions: initial concentration of aniline, PS and MnFe2O4 were 20 mg/L, 2.4 mM, and 0.9 g/L, respectively.

Figure 7

Effect of different EDTA(a) and NaN3 (b) concentration on degradation of aniline by PDS catalyzed with MnFe2O4. Experimental conditions: initial concentration of aniline, PS and MnFe2O4 were 20 mg/L, 2.4 mM, and 0.9 g/L, respectively.

One of the main photogenerated reactive species was 1O2 (Zhang et al. 2020), which might have an effect on the facilitated degradation of aniline. To prove the hypothesis, NaN3, which was used as the scavengers of 1O2 to inhibit the oxidation process, was fed into the MnFe2O4/PDS system (Figure 7(b)). When the concentrate of NaN3 increased to 1.00 g/L, the degradation of the aniline in the MnFe2O4/PDS system was drastically decreased to over half of the control system. This indicated that the generation of 1O2 may be responsible for the degradation of aniline in the MnFe2O4/PDS system.

TOC removal efficiencies in MnFe2O4/PDS system

As shown in Figure 8, the removal efficiencies of TOC during the oxidation of aniline with different PDS concentrations were quantified. It can be seen that the TOC degradation degree of the PDS/MnFe2O4 system increased slowly with the reaction time. In particular, the TOC removal efficiencies after 4 h were 71, 59, 49 and 43% for 0.4, 0.8, 1.6 and 2.4 mM PDS, respectively. These results indicated that some organic oxidation products were formed and remained in the reaction solution.

Figure 8

Effect of different PDS concentrations on the removal efficiencies of TOC by PDS catalyzed with MnFe2O4. Experimental conditions: initial concentration of aniline and MnFe2O4 were 20 mg/L and 0.5 g/L, respectively.

Figure 8

Effect of different PDS concentrations on the removal efficiencies of TOC by PDS catalyzed with MnFe2O4. Experimental conditions: initial concentration of aniline and MnFe2O4 were 20 mg/L and 0.5 g/L, respectively.

Identification of intermediates and transformation pathways

To further explore the mechanism of aniline degradation in the MnFe2O4/PDS system, high-performance liquid chromatography-mass spectrometry (HPLC-MS) was introduced to identify the intermediates during the MnFe2O4/PDS system. The detected samples collected at the reaction time of 0 and 4 h were extracted with n-Hexane (the extraction ratio was 1:1) to analyze in HPLC-MS (Xie et al. 2012), respectively. The three main intermediates in the system are shown in Figure 9. All the peaks in the chromatogram were marked to respond with the analytical results in mass spectrometry.

Figure 9

HPLC typical chromatograms of intermediates (a–d) for aniline solutions collected after 0 and 4 h in MnFe2O4/PDS system.

Figure 9

HPLC typical chromatograms of intermediates (a–d) for aniline solutions collected after 0 and 4 h in MnFe2O4/PDS system.

As shown in mass spectrometry (Figure 10), the peak of a, b, c and d corresponded to m/z values of 94.07, 201.16, 124.01 and 184.04, respectively. The chemical formulas of products in the system was proposed by both the mass spectrometry and the retention time. It can be observed that there are three main byproducts which have the chromatographic peak areas with m/z 201.16, 124.01 and 184.04.

Figure 10

Proposed intermediates and mass spectra of the degradation products.

Figure 10

Proposed intermediates and mass spectra of the degradation products.

Based on the above discussion, two possible degradation pathways of aniline degradation in the system were proposed and demonstrated in Figure 11. In pathway I, through the hydroxylation of aniline, the aromatic ring was first attacked by ·OH and SO4·– and the deprotonation of aniline accomplished, leading to the formation of imino radical. Next, imino radical was further oxidized by holes and 1O2 into nitrobenzene. Then, a certain amount of nitrobenzene was possibly cleaved to small molecules such as the maleic acid and oxalic acid, and finally to CO2. The other may react with imino radical resulting in converting into 4–4′-diaminodiphenyl, which was oxidized to small molecular organics by the reactive species. In pathway II, the intermediate dianiline resulting from the bimolecular reaction occurred. Further, the reactive species ‘attacked’, inducing benzene ring opening of dianiline to phenylsuccinic acid and maleic acid.

Figure 11

Possible pathways of aniline mineralization by PDS/MnFe2O4.

Figure 11

Possible pathways of aniline mineralization by PDS/MnFe2O4.

Through the analysis of XPS, which was carried out to elucidate the charge of chemical valence, the element composition of Mn and Fe species is further confirmed, see Figure 12. The C 1s peak was located at 284.8 eV in order to calibrate this spectrum (Figure 12(a)), and it was detected that the main chemical components including Fe, Mn and O continuously existed in MnFe2O4 during the reaction. Figure 12(b) showed that the two peaks locating at binding energy of 710 and 724 eV corresponded to Fe 2p3/2 and Fe 2p1/2 in spectrum of virgin MnFe2O4 (Lai et al. 2020), respectively, which strongly confirmed that only Fe(III) species existed in MnFe2O4 rather than the Fe(II). As shown in Figure 12(c), it can also be observed that the peaks centered at 641.8 eV (Mn 2p3/2) and 653 eV (Mn 2p1/2) suggesting that Mn(II) was the entire state of Mn species (Deng et al. 2018). After the reaction process, the peaks of Fe 2p3/2 located at 712.7 eV and Mn 2p3/2 located at 645.7 eV corresponded to Fe(II) and Mn(III), and deconvolution of these two peaks represents that multivalent states of Fe(II)/Fe(III) and Mn(II)/Mn(III) coexisted on the surface. The proportion of Fe(II)/(Fe(III) + Fe(II)) and Mn(II)/(Mn(III) + Mn(II)) increased to 33.02 and 51.57%, which confirmed that the chemical transformations between these multivalent states occurred on the surface of specimen. Figure 12(d) presented the O 1s spectrum of used and virgin MnFe2O4, which both had three peaks, one locating at 529.9 eV indicating the form of surface lattice oxygen (Olatt) and others locating at 531.7 and 533.2 eV derived from surface adsorbed oxygen (Oads,) (Lai et al. 2021). Due to its higher mobility than Olatt, Oads could be integrated with Fe(III) and Mn(II) and then take part in the chain reaction in AOPs to the degradation of pollution (Kim & Shim 2010; Deng et al. 2017), and may cause the surface of MnFe2O4 to have more surface oxygen vacancies, which may further result in the generation of holes. Consequently, the proportion of Oads in MnFe2O4 may affect the catalyst activity. Specially, the MnFe2O4 corresponding area ratio of Oads/(Oads + Olatt) was increased by 13.98% after the reaction, which demonstrated that the catalytic activity of MnFe2O4 after use was promoted to a certain extent.

Figure 12

XPS spectra of the virgin and used MnFe2O4: (a) full-range scan, (b) Fe 2p core level, (c) Mn core level O, and (d) 1 s core level.

Figure 12

XPS spectra of the virgin and used MnFe2O4: (a) full-range scan, (b) Fe 2p core level, (c) Mn core level O, and (d) 1 s core level.

According to the results above, a schematic diagram of the possible oxidation mechanism of aniline by PDS/MnFe2O4 is shown in Figure 13. It speculated that the degradation of compounds in the MnFe2O4/PDS system mainly contained three results: (1) the adsorption between the S2O82– and the surface of MnFe2O4 occurred first; (2) then S2O82– was converted by MnFe2O4 which was used as the electron donor into the production of not only holes and 1O2 but also SO4·– and ·OH; (3) finally, reaction activation species reacted with compounds.

Figure 13

Schematic diagram of the possible oxidation mechanism of aniline by PDS/MnFe2O4.

Figure 13

Schematic diagram of the possible oxidation mechanism of aniline by PDS/MnFe2O4.

CONCLUSIONS

In this study, MnFe2O4 used as catalyst was synthesized by a chemical co-precipitation method and the heterogeneous catalytic oxidation of aniline with PDS was investigated; 97.98% of aniline was degraded in 4 h by MnFe2O4/PDS system. According to radical scavenging tests, both sulfate and hydroxyl radicals have little influence on the degradation of aniline in the MnFe2O4/PDS system. However, the degradation of aniline mainly depended on the reactive species of holes and 1O2. There were several intermediate productions, including 4–4′-diaminodiphenyl, dianiline and nitrobenzene for the decomposition of aniline in the MnFe2O4/PDS system. All these productions were further broken into small molecular substances and finally into CO2 and H2O.

ACKNOWLEDGEMENTS

This work was financially supported by the Science and Technology Department of Henan (161100310700), and Key Research and Extension Project of Henan Province (192102310225).

DATA AVAILABILITY STATEMENT

Data cannot be made publicly available; readers should contact the corresponding author for details.

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