The removal ef ﬁ ciencies and mechanism of aniline degradation by peroxydisulfate activated with magnetic Fe-Mn oxides composite

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 SO 4· (cid:1) 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. of aniline by peroxydisulfate in the Three main intermediate products and two possible paths were studied.


INTRODUCTION
Cakir ), 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. ; Huang et al. ).
In recent years, AOPs based on the generation of sulfate radical (SO 4 ·-) have been the focus of numerous studies, and peroxymonosulfate (PMS, HSO 5 À ) and peroxydisulfate (PDS, S 2 O 8 2À ) have been proposed as candidates for such a purpose (Wacławek et al. ). Compared with ·OH, sulfate radical (SO 4 ·-) not only carries forward the same advantages due to the higher reactive oxidation (2.5-3.1 V vs. NHE) (Zhu et al. ), but also solve the shortcomings resulting from its stability and wider working pH range (Hu & Long ). SO 4 ·is generated by activating the PDS and PMS through various methods such as heat ( Ji et  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. ). 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.

Reagents and materials
All chemicals used in this study were at least of analytical grade and used as received without further purification, The morphological representation and internal structure of

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.
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.  (220), (311), (400), (511) and (440)

Degradation experiment in different systems
To confirm the MnFe 2 O 4 accessed to activate PDS, Figure 4 shows the degradation of aniline by different systems.

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. The effect of reaction temperature in the MnFe 2 O 4 /PDS system was confirmed. Specially, the increasing of the corresponding k obs 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 MnFe 2 O 4 /PDS systems
In terms of previous reports, the predominant reactive species in PDS activation were SO 4 ·and ·OH (Nie et al.
Thus, MeOH and t-BuOH were used as scavengers to distinguish the existence of SO 4 ·and ·OH in MnFe 2 O 4 /PDS systems separately. As Figure 6 shows and ·OH were not dominantly responsible in the reaction system. However, SO 4 ·and ·OH were still generated in the system.