Sulfate radicals induced from peroxymonosulfate by cobalt manganese oxides (CoxMn3−xO4) for Fenton-Like reaction in water

Accepted Manuscript Title: Sulfate radicals induced from peroxymonosulfate by cobalt manganese oxides (Cox Mn3−x O4 ) for Fenton-Like reaction in water Author: Yunjin Yao Yunmu Cai Guodong Wu Fengyu Wei Xingya Li Hao Chen Shaobin Wang PII: DOI: Reference:

S0304-3894(15)00296-4 http://dx.doi.org/doi:10.1016/j.jhazmat.2015.04.014 HAZMAT 16732

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Journal of Hazardous Materials

Received date: Revised date: Accepted date:

20-1-2015 5-4-2015 6-4-2015

Please cite this article as: Yunjin Yao, Yunmu Cai, Guodong Wu, Fengyu Wei, Xingya Li, Hao Chen, Shaobin Wang, Sulfate radicals induced from peroxymonosulfate by cobalt manganese oxides (CoxMn3-xO4) for Fenton-Like reaction in water, Journal of Hazardous Materials http://dx.doi.org/10.1016/j.jhazmat.2015.04.014 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Sulfate radicals induced from peroxymonosulfate by cobalt manganese oxides (CoxMn3-xO4) for Fenton-Like reaction in water Yunjin Yao a,c,d,*, Yunmu Cai a, Guodong Wu a, Fengyu Wei a, Xingya Li d, Hao Chen a, Shaobin Wangb,* a

Anhui Key Lab of Controllable Chemical Reaction & Material Chemical Engineering, School of

Chemistry and Chemical Engineering, Hefei University of Technology, Tunxi Road 193, Hefei 230009, China b

Department of Chemical Engineering, Curtin University, G.P.O. Box U1987, Perth, Western Australia 6845, Australia

c

State Key Laboratory of Materials-Oriented Chemical Engineering, Nanjing University of Technology, Nanjing 210009, China d

School of Chemistry and Materials Science, University of Science and Technology of China, Hefei, Anhui 230026, China

AUTHOR INFORMATION * To whom correspondence should be addressed: Name: Yunjin Yao; Phone: +86 551 62901458; Fax: +86 551 62901450; E-mail:[email protected] Name: Shaobin Wang; Phone: +61 8 9266 3776; Fax: +61 8 9266 2681; E-mail: [email protected]

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ABSTRACT: A series of CoxMn 3-xO4 particles as Fenton-like solid catalysts were synthesized, and their catalytic performance in oxidative degradation of organic dye compounds in water was investigated. The surface morphology and structure of the CoxMn3-xO4 catalysts were characterized by field emission scanning electron microscopy (FESEM), energy-dispersive X-ray spectroscopy (EDX), transmission electron microscopy (TEM), powder X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). The results showed that, as an oxide composite of Co and Mn elements, CoMn2O4 showed much stronger catalytic activity in peroxymonosulfate (PMS) oxidation than Co 3O4, Mn2O3, and their physical mixture. Typically, the uses of 0.02 g/dm3 CoMn2O4 and 0.2 g/dm3 PMS yielded a nearly complete removal of Rhodamine B (0.03 g/dm3) in 80 min at 25 °C. The efficiency of Rhodamine B decomposition increased with increasing temperature (15~ 55 ℃), but decreased with the increase of fulvic acid concentration (0 ~0.08 g/dm3). Furthermore, CoMn2O4 could maintain its catalytic activity in the repeated batch experiments. Moreover, HO and SO  radicals participating in the process were 4 evidenced using quenching experiments, and a rational mechanism was proposed. PMS oxidation with CoMn 2O4 is an efficient technique for remediation of organic contaminants in wastewater.

Keywords: Cobalt manganese oxides; Peroxymonosulfate; Fenton-like; organic pollutant; Sulfate radical;

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1. Introduction

In recent years, an innovative advanced oxidation technology (AOT) using peroxymonosulfate (PMS) to generate reactive oxygen species (ROS) such as sulfate radicals ( SO  4 ) to degrade or mineralize refractory organic pollutants have received increasing interest[1]. SO  4 has been known as a strong oneelectron oxidant for its high standard redox potential (E0 = 2.6 V vs NHE) that is similar to HO (E0 =  2.73 V vs NHE) [2]. Moreover, SO  4 is more efficient than HO to mineralize some refractory organic

pollutants for its selective oxidation capacity [3]. PMS as the major component of a commercial chemical Oxone (2KHSO5∙KHSO4∙K2SO4), is a water-soluble, environmentally friendly, safe to handle, strong and stable oxidizing agent, thus having a better potential application [4, 5]. The cost of Oxone ($2.2 per kg) is close to that of H2O 2 oxidant ($1.5 per kg) [6]. Since PMS itself is far more chemically stable in aqueous systems and has very low reactivity toward organic contaminants, activation processes are necessary for its application. PMS activation to produce SO 4 can be initiated under photochemical, thermal (also with metal catalysts) or chemical conditions, and SO 4 can rapidly attack organic pollutants with rate constants in the range of 107−10 10 M−1 s−1[2]. Among those, homogeneous catalytic oxidation by PMS coupled with transition metals shows efficient decontamination of organic pollutants because every single catalytic entity can act as a single active site[7]. However, the dissolved metal ions have been recognized as possible potential health hazards, which limit the scale-up of its application. Compared to homogeneous systems, heterogeneous Fenton-like systems for PMS activation have proven to be promising alternatives. Recently, bimetallic oxides (composite oxides of cobalt and another metal element) have attracted a great deal of research interest [8-10]. Su et al.[11] successfully prepared heterogeneous CoxFe3−xO4 nanocatalysts and found the intimate Fe-Co interactions are very critical for efficient heterogeneous activation of Oxone. Our preliminary study found bimetallic oxides CoFe2O4 and CoFe 2O4−graphene were effective to activate PMS for the degradation of phenol [12]. Similarly,

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Rhadf et al.[13] have observed the effective catalytic behavior of Mn3O4 nanocrystals on methylene blue degradation. The partial substitution of Co in Co3O4 with Mn (Cobalt manganese ternary oxides, CoxMn3-xO4, x=1, 2) is a widely used strategy for the reduction of the amount of the toxic Co element because manganese is 20 times cheaper than cobalt, more abundant in nature and more environmentally friendly[14]. Several methods have been developed to synthesize CoxMn3-xO4 in various applications, including catalysts [14, 15], supercapacitors, and lithium ion batteries [16-18]. Meanwhile, this special material has the advantages of high abundance, low cost, and low toxicity. Despite of the abovementioned advantages and unique structural characteristics, to our best knowledge, the use of CoxMn3xO4

particles to activate PMS has never been tested to date.

Therefore, the goal of the present work is, first of all, to provide a new Fenton-like catalyst, CoxMn3xO4,

 which exhibits supreme efficiency for yielding SO 4 and HO by PMS. The physical and chemical

characterizations of CoxMn3-xO4 were conducted and the efficiency of CoMn2O4/PMS oxidation was evaluated in views of the effects of oxidants (PMS, persulfate (PDS) and H2O2), reaction temperature, and fulvic acid (FA) concentration on the decolorization efficiency of organic dyes. Material stability, degradation mechanism, as well as the role of MnxCo3-xO4 were also studied.

2. Materials and Methods 2.1 Preparation of Co xMn3-xO4 catalysts. All of the reagents are of analytical purity and used without further purification. The preparation of CoxMn3-xO4 catalysts in this study is similar to the previous works [19, 20]. For example, 1 mmol of Co(OAc)2•4H2O and 2 mmol of Mn(OAc)2•4H2O were dissolved into 40 cm3 of ethylene glycol under magnetic stirring to form a clear solution, followed by the addition of 30 mmol of NH4HCO3 at room temperature. After being stirred vigorously for 30 min, the resultant mixture was transferred to a 50 cm3 Teflon-lined stainless steel autoclave for hydrothermal reaction at 200 ℃ for 20 h. The product of CoMn 2CO3 precursor was collected by centrifugation, washed with ethanol and water, and dried in an

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oven at 60 ℃. After calcining the collected precursors at 800 °C for 10 h in air, the grey-green composite of metal oxide CoMn2O4 was obtained. The different compositional CoxMn3-xO4 materials were prepared by adjusting Co(OAc)2•4H2O / Mn(OAc)2•4H2O reactant ratio in the first step. Co3O4 and Mn 2O3 were also prepared similarly by using Co(OAc)2•4H2O or Mn(OAc)2•4H2O as a precursor. All the products were stored in a desiccator for further experiments. The schematic representation for the synthesis of Co xMn3-xO4 catalysts is illustrated in Figure 1. [Figure 1]. 2.2 Catalyst characterization X-ray diffractometry (XRD) for the crystal structure of the products was carried out in a Rigaku D/max2500v/pc diffractometer with Cu Kα radiation. Fourier transform infrared spectroscopy (FT-IR) spectra were collected from 4000 to 400 cm−1 on a Perkin-Elmer Spectrum 100 instrument with samples pressed in KBr. The morphology and structure of the catalysts were characterized by a field emission scanning electron microscope (FESEM; Hitachi SU8020, Japan) and high-resolution transmission electron microscope (HRTEM, JEOL JEM-2100F). Energy dispersive X-ray (EDX), the selected area electron diffraction (SAED) and elemental mapping analysis were also performed during the HRTEM measurements. X-ray photoelectron spectroscopy (XPS) was conducted using an ESCALAB250 spectrometer equipped with a monochromatized Al Kα source. The charge effect was calibrated using the binding energy of C1s (284.6 eV) to reduce the sample charging effect.

2.3 Catalytic Experimental Procedure Experiments were conducted in a beaker with a magnetic stirrer, heating jacket and a condenser. Fuchsin basic (FB), Methylene blue (MB), Rhodamine B (RhB), Orange II (OII), Methyl violet (MV) and Basic green (BG) were selected as model pollutants for degradation experiments. A 500 cm3 reaction solution with desired initial concentrations of the organic dyes and PMS from Sigma-Aldrich was prepared with ultrapure water in all experiments. The mixture was magnetically stirred at a rotary

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speed of 500 rpm and desired temperature (15, 25, 35, 45, and 55 ℃). Specific concentration of CoxMn 3xO4

particles was introduced into the prepared solution to start the reaction. At different elapsed time (up

to 80 min), water samples were withdrawn at predetermined time intervals and immediately filtered (0.45 μm glass fiber filters) to remove essentially the catalyst solids. Then, 1.0 cm3 of methanol (MeOH), an effective quenching agent, was immediately introduced into 1.0 cm3 sample to quench residual sulfate radicals [6]. The concentrations of the residual organic dye were determined by monitoring decrease in absorbance at the wavelength of maximum adsorption with UV−vis spectroscopy. Total organic carbon (TOC) was measured using an element analyzer (vario TOC select, Elementar). Before measuring TOC, 10 cm3 sample were extracted, filtrated and quenched with 10 cm3 of 3 M sodium nitrite solution. The concentrations of leaching Co and Mn ions in the solutions were also determined with an atomic absorption spectroscopy (AAS, AA800, Perkin-Elmer). Humic substances (HS) (fulvic acid, FA ≥90%, CAS:1415-93-6) purchased from Aladdin Chemistry Co., Ltd. was selected as the model natural organic matter (NOM) to evaluate the effect of the NOM on organic dye removal because 70% of NOM is composed of HS. A few of other tests were carried out with different oxidants, H2O2 and PDS. To determine the active species during batch experiments, Tertbutyl alcohol (TBA) and MeOH were employed to distinguish HO and SO  4 radicals. Quenching experiments were performed by adding the quencher into the reaction solution before the addition of PMS. In this study, the Co xMn3-xO4 catalyst was gathered after completion of the reaction, washed, dried, and evaluated for its reusability. The experiments were carried out under the same reaction conditions with RhB (0.03 g/dm3), PMS (0.2 g/dm3) and the used catalysts (0.02 g/dm3) at 25 °C. All experiments were carried out in triplicate for a given condition. The reported data in this study were the means of the triplicate measurements. Standard deviations were calculated from the three independent measurements and found to be less than 5%.

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3. Results and Discussion 3.1 Catalyst Characterization Figure 2a compares XRD patterns of the synthesized Co xMn3-xO4 catalysts at different Co contents, together with Co 3O4, and Mn2O3. It can be seen that all the diffraction peaks for Co0.5Mn2.5O4 and CoMn 2O4 samples could be indexed to the tetragonal spinel (Co, Mn)3O4 (JCPDS no. 18-0408) [21]. Meanwhile, with the Co doping content increasing for Co2MnO4 sample, the diffraction patterns agree well with the standard diffraction patterns of cubic spinel phase Co2MnO4 (JCPDS no. 23-1237), exhibiting good crystallinity and purity [22]. The structure is formed as mixed valence oxide that adopts a cubic spinel structure in which manganese and cobalt are distributed over both octahedral and tetrahedral sites. Moreover, CoxMn 3−xO4 showed no cobalt oxide peaks, indicating that all the cobalt compounds were converted to cobalt manganese oxide. Therefore, the formation of bimetal spinel structure in the CoxMn 3−xO4 was inferred to result from strong interaction between Co and Mn elements at the molecular level (i.e. Co–Mn linkages) [11]. Additionally, poorly crystallized phase can be detected for Co3O4 (JCPDS no. 74-2120), Mn2O3 (JCPDS no. 71-0636) and CoMn 2O4 samples. The crystallite sizes calculated by the Scherrer’s equation of Co0.5Mn2.5O4, CoMn2O4, Co2MnO4, Co3O4, and Mn2O3 are 50.0, 76.2, 59.4, 45.5, and 61.5 nm, respectively. Figure 2b shows FT-IR spectra of the synthesized Co xMn3-xO4 catalysts with different contents of cobalt, together with Co 3O4, and Mn 2O3. As can be noted, the peaks around 3410 and 1630 cm−1 are attributed to the -OH stretching vibration as well as the deformation vibration of H2O resulted from moisture adsorption on the surface of particles. In the spectrum of Mn2O3, the broad band at 610 cm−1 is corresponded to the Mn−O group. The spectrum of Co3O4 shows peaks at 594 and 667 cm-1 that can be assigned to the vibrations of Co–O [23]. These peaks are shifted to low wavenumbers for CoxMn3-xO4, suggesting an increase in the strength of the Co–O bond in CoxMn 3-xO4 [21]. [Figure 2].

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FESEM, HRTEM, SAED, EDX, and elemental mapping further characterized the morphology of the as-synthesized CoMn2O4 catalyst. As shown in Figure 3a, the product is composed of regular microspheres with diameters of 0.25−0.35 μm. The microspheres are formed by the aggregation of nanoparticles with typical sizes ranging from 70 to 80 nm. Furthermore, TEM image (Figure 3b) also reveals that the microspheres are composed of numerous nanoparticles. A representative HRTEM image is shown in Figure 3e. The crystal lattice fringes with d-spacing of 4.78 and 3.03 Å can be assigned to the (111), (202) planes of spinel CoMn 2O4, respectively, which is consistent with the XRD results. The selected area electron diffraction (SAED) pattern and the corresponding two-dimensional fast Fourier transform (FFT) on the crystal particles are shown in Figure 3c and 3d, respectively, which is also consistent with the above results. The chemical composition of the products and the atomic Mn/Co ratio was determined by EDX analysis (the inset of Figure 3e), which corresponds to CoMn2O4. The TEM micrograph and the corresponding EDX mapping images clearly showed that Co, Mn and O elements are well uniformly distributed in the CoMn2O4 samples, indicating the formation of bimetal oxides but not simple mixtures of the corresponding two single-metal oxides(Figure 3f−i). [Figure 3]. 3.2 Catalytic evaluation. Control experiments were conducted to compare the eliminating efficiencies of organic dyes by various processes. RhB as an important representative of xanthene dyes was first selected for the degradation experiment due to its presence in the wastewater from several industries [24]. Figure 4a illustrates the efficiency of five metal oxides (i.e., CoxMn 3-xO4, Mn2O3, and Co3O4) to activate PMS for the degradation of RhB and the catalytic activity abided by the order of CoMn 2O4>Co 2MnO4> Co0.5Mn2.5O4>Co 3O4> Co3O4+ Mn2O3 >Mn2O3. In the presence of PMS alone, the oxidation efficiency was limited, and about 10% of RhB was degraded in 80 min. With CoMn2O4 only, no noticeable adsorption of RhB was observed. However, with addition of CoMn 2O4 and PMS simultaneously, fast and efficient degradation of RhB was achieved, and nearly 100% of RhB was degraded in 80 min,

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indicating that RhB was degraded in CoMn2O4/PMS but not adsorbed on the oxide particles. TOC measurement results indicated that about 10% of RhB could be oxidized to CO2 after 80 min of degradation over CoMn2O4/PMS system, revealing the intermediates (not analyzed in detail) are enriched in a variety of small-chain organic acids. In these systems, dissolved Co and Mn ions were detectable but at a quite low concentration (<0.1 mg/dm3), which suggests that the activation of oxidant (PMS) under these conditions is mainly heterogeneous taking place on the catalyst surface. UV-vis spectra changes of the RhB solution in the course of the degradation can be used to clarify the changes of structural characteristics of RhB during Fenton-like oxidation (Figure 4b). It is clearly seen that its maximum absorption peak at 554 nm in the visible light region diminished obviously along with the oxidation process, which was attributed to the decomposition of the conjugated xanthene ring in the RhB molecules [25]. This is reasonable because the N=N bond of the azo dye is a most active site for being attacked by radicals. No corresponding increase in absorption is observed in the ultraviolet region, which indicates the complete destruction of most aromatic structures by PMS catalyzed with CoMn2O4. This result is also consistent with the color change of the RhB solution upon increasing reaction time (the inset in Fig. 4b). The results also clearly show that the presence of Co in the Co xMn3-xO4 structure strongly influenced the RhB decomposition and the reaction rate first increased and then decreased with the increase of Co content, contributed to not only the higher activation rate of PMS caused by the doping of cobalt but also the intimate Co–Mn interactions [11]. CoMn2O4 is most active among the metal oxides tested for PMS activation. In contrast, Mn2O3 particles were used and induced 12% RhB degradation removal in 80 min, indicating Mn2O3 particles have low Fenton-like activity. Although Co3O4 particles were confirmed to be an efficient Fenton-like catalyst [26, 27], it only yielded a RhB removal of 23% in the presence of PMS, exhibiting lower catalytic activity than CoMn2O4 microparticles [28]. Furthermore, a physical mixture of Co 3O4 and Mn2O3 crystals with atomic ratio of Co: Mn = 1:2 was found to yield 15% RhB removal, which was much lower than CoMn2O4 particles, suggesting that there might be a synergy effect between Co and Mn species in CoMn2O4 microparticles.

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[Figure 4]. The performance of the CoMn 2O4/PMS coupled process was evaluated by degrading several other model pollutants under the similar conditions (Figure 5a). Experimental results confirmed the oxidative degradation of various organic dyes using CoMn2O4 solids in PMS activation and the decolorization efficiency followed the order: FB< MB< RhB< OII< MV< BG. The degradation rates involved in the removal of these six dyes differed from each other, which may be attributed to the difference in molecular structures [1, 29]. The removal efficiency of this process on multiple organic pollutants indicates the excellent catalytic performance of CoMn2O4 towards the oxidation of organic pollutants. The effect of solution temperature on the degradation of RhB by CoMn2O4/PMS process is shown in Figure 5b. Obviously, the catalytic activity significantly increased at elevated temperatures. For example, 55% RhB removal was achieved in 80 min at 15 °C, whereas complete degradation efficiency of RhB at 25, 35, 45, and 55 °C would be completed in 80, 40, 25, and 15 min, respectively. This might  be because SO  4 and HO radicals are formed by the thermal activation of PMS in aqueous solution

under thermally enhanced conditions. heat  HSO 5   SO 4  HO

(1)

Furthermore, high reaction temperatures led to a fast degradation rate, because RhB molecules at high temperatures can overcome the reaction activation energy easily [30]. It can be deduced that the temperature was conducive to the removal rate. Natural organic matter (NOM) is an important constituent in natural waters, which can either enhance or inhibit the rate of degradation of organic contaminants depending upon the concentration and type of NOM as well as other reaction conditions [31-33]. Thus, the presence of NOM has an impact on the efficiency of the CoMn2O4/PMS processes. HSs are macromolecular organic substances that account for a considerable fraction of NOM in surface waters and soils. In this study, we selected FA as a representative of organic matter to evaluate the effect of NOM on the RhB degradation efficiency. As shown in Figure 5c, RhB removal was obviously inhibited and the degradation efficiency at 80 min

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decreased from 100% to 48% by increasing the initial FA concentration from 0 to 0.08 g/dm3, indicating that FA competed with RhB for reactive radicals [3, 32]. Therefore, for practical in situ remediation using the PMS oxidant technique, the sulfate radicals consumed by NOM in groundwater should be considered in determinations of the PMS concentration. The effect of different oxidants such as PMS, PDS and H2O2 on the catalytic oxidation of RhB dye was also carried out (Figure 5d). The results show that the order of activity is PMS > PDS > H2O2. RhB removals of 100.0% for CoMn2O4/PMS, 4.0% on CoMn 2O4/PDS and 1.0% for CoMn2O4/H2O2 were achieved, which indicated that the unsymmetrical character of PMS made it easy to be activated [8]. This result was similar to the homogeneous catalytic oxidation in the literature [34], in which PMS showed significant reactivity with the majority of metal ions especially under neutral pH. [Figure 5]. Since the stability and reusability of the catalyst are critical in catalytic reactions, especially for practical industrial applications, the stability of CoMn2O4 catalyst was investigated by reusing the catalyst in five successive experiments under the same reaction conditions and the results are shown in Figure 6. It was found that RhB removal decreased only slightly from 100.0% to 87.0% after used for 5 cycles. No remarkable reduction in the catalytic activity was observed, indicating its good stability and long lifetime. Moreover, the good chemical stability of CoMn2O4 was further confirmed by XRD measurements (Figure 7a) on the used and fresh catalysts. Obviously, no considerable change in XRD profiles was observed after being used for 5 cycles, further revealing the durability of the catalyst. The FT-IR spectra of the catalyst before and after reuses are rather similar (Figure 7b). In addition, characteristic peaks of RhB and intermediate degradation species in the FT-IR spectrum of CoMn2O4 after reaction were not detected, meaning a negligible adsorption of these species on the surface of the catalyst and/or complete degradation of RhB [35]. To conclude, the excellent catalytic performance and long-term stability of the CoMn2O4 catalysts favors their practical applications. [Figure 6].

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[Figure 7]. To further understand the confinement and synergistic effects of CoMn 2O4 catalyst in Fenton-like reaction, XPS was used to evaluate the chemical composition of active species for the fresh and used CoMn 2O4. Figure 8a shows the elements Co, Mn, O, and adventitious C coexist in the obtained CoMn 2O4 sample. As indicated in Figure 8b, the Co 2p profiles of fresh CoMn 2O4 catalyst shows two main peaks at 780.5 and 795.8 eV with two additional satellite peaks at 781.8 and 797.0 eV, which correspond to Co 2p3/2 and Co 2p1/2, respectively. Two Co 2p3/2 peaks at 780.5 and 781.8 eV are ascribed to CoII and CoIII, respectively, whose atom ratio was 60.4:39.6. However, the Co 2p3/2 peak after catalytic oxidation process is composed of two peaks at 779.90 for CoII and 781.15 eV for CoIII, respectively, and their atom ratio was changed to 56.3:43.7. The Mn 2p spectrum of the fresh CoMn2O4 catalyst (Figure 8c) shows three Mn 2p3/2 peaks at 641.1, 642.1 and 643.4 eV assigned to MnII, MnIII and MnIV oxidation states, respectively, indicating the disordered CoMn 2O4 definitely coexisted as mixed phases [15, 36, 37]. Their atom ratio was 25.0:38.0: 37.0. After reaction, three oxidation states of Mn species were still present in the catalyst, but their atom ratio was changed to 32.5: 35.6: 31.9, indicating that valence of Mn on surface of the used catalyst had a noticeable change [38]. The electron couples of MnIII/MnII, MnIV/Mn III and CoIII/CoII are coexisting in the spinel CoMn2O4 structures, which provide a notable chemical activity. The high-resolution O 1s spectra of CoMn2O4 before and after oxidation are resolved into two individual peaks (Figure 8d). For the fresh CoMn2O4 catalyst, the O1s photoelectron spectra show two peaks at 530.3 and 531.4 eV, which can be assigned to the lattice oxide oxygen (O2−, denoted as OI) and adsorbed oxygen or surface hydroxyl species(denoted as OII) [35, 39]. It is reported that OI and OII are active oxygen species for the radical’s generation and oxidation decomposition [40]. After catalytic reaction, it was found out that the relative intensity of OI reduced from 71.5% to 60.6%, and the relative intensity of OII increased from 28.5% to 39.4%, indicating that both of them are involved during the degradation process. The increase of OII concentration might be ascribed to the formation of M-OH

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groups (M: Co or Mn) or O2- adsorbed on the CoMn2O4 surface, which may contribute to the enhancement of the Fenton-like process. The decrease of lattice oxygen in CoMn 2O4 may be oxidized by M3+ with its reduction to M2+. [Figure 8]. 3.3 Exploration of Catalytic Mechanism. In this study, radical quenching studies were further performed to identify the dominant reactive oxygen species (ROSs) in the heterogeneous CoMn 2O4/PMS reactions. It was previously reported that   three ROSs ( SO  4 , HO , and SO 5 ) can be generated for the degradation of organic pollutants by the

catalyst-mediated decomposition of PMS [41-43]. As it is well-known, MeOH is usually used to 9 scavenge both HO and SO  4 due to its high reactivity for both oxidation species ( k OH : 1.2-2.8 × 10

M−1s−1; kSO : 1.6-7.8 × 106 M−1s −1). In addition, TBA mainly is used as a quenching agent for HO 4

5 because of its high reactivity ( kOH : 3.8-7.6 × 108 M−1s−1), but not for SO  4 radicals ( kSO : 4-9.1 × 10 4

M−1s−1) [3, 8, 44]. Meanwhile, peroxomonosulfate radical ( SO  5 ) might also be formed, but would not contribute to RhB degradation for its lower redox potential(1.1 eV) [8]. Therefore, quenching tests were performed using TBA as a scavenger for HO and MeOH as a scavenger for HO and SO  4 . As shown in Figure 9a, when no quenching agent was added, about 100% RhB was degraded in 80 min. However, the addition of 1M TBA or MeOH resulted in decreasing RhB removal (in 80 min) to 89% and 80%, respectively. The inhibition was enhanced with the increased concentration, indicating HO and SO  radicals were involved in the oxidation process. MeOH inhibited RhB degradation more 4 significantly than TBA at the same concentration, suggesting that the main radical species generated  during the activation of PMS by CoMn2O4 were SO  was also 4 radicals. A small amount of HO

generated in the catalytic oxidation, which possibly comes from the reaction of sulfate radical with OH− in water. All of these results confirmed the involvement of HO and SO  4 radicals in the CoMn2O4/PMS oxidation process, which agrees with recent findings in the literature [32, 43, 44].

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The decoloration rate of RhB under N2 bubbling with the flow rate of 0.2 and 0.8 dm3·min-1 did not depressed but increased. The reason may be that N2 bubbling can accelerate the movement of the oxidant, dye molecules, intermediate compounds, etc., especially in favor of the transfer of molecules on the CoMn 2O4 surface. All the results suggest that the dissolved oxygen might not be involved in the CoMn 2O4 process. [Figure 9]. Based on the information above, main processes during the catalytic activation of PMS by CoMn2O4 are proposed as follows. First,  Co and  Mn ions act as Lewis sites and combine with dissociative adsorption of water molecules to generate  Co   OH and  Mn   OH [40, 45]. After addition of PMS,

 Co and  Mn on the catalysts would first react with PMS to generate HO (eqs (2)(3)). Moreover,  CoII   OH species on CoMn 2O4 surface activate PMS to generate surface-bound SO  4 (eq (4)) and some more  CoII   OH species can be produced from the formed  CoIII   OH species with the reaction of PMS (eq (5)). Similarly,  Mn II   OH and  Mn III   OH species on the catalyst surface can also react with PMS to produce surface-bound SO  (eqs (6),(8)) and then more of these species are 4 generated by the reactions between the formed  Mn III   OH and  Mn IV   OH species and PMS (eqs (7),(9)). The standard reduction potential of Co III / Co II is 1.81 V, while those of Mn III / Mn II and MnO2/Mn2O3 are 1.51 V and 0.15 V, respectively [11, 46]. Therefore, the reduction of Co III by MnII and MnIII is thermodynamically favorable (eqs (10)(11)), which means the generated CoII can be regenerated on the surface of CoMn2O4.  Co III /  Co II ,  Mn III /  Mn II ,  Mn III /  Co II and

 Mn IV /  Mn III are redox couples, behaving similarly to Fenton reaction according to the Harber– Weiss cycle [44, 47-50], which was evidenced by XPS. These may result in more active sites on the catalyst and contribute to the catalytic activity of CoMn2O4. In addition, SO 4 can react with water or

OH  to produce HO (eqs (12)(13)). The regeneration of the catalyst thus occurs cyclically until PMS is consumed completely [51-54]. Furthermore, RhB in aqueous solution was enriched continuously on

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 the surface of CoMn2O4 and broken down by SO  4 and HO (eq (14)). Herein, the possible mechanism

of RhB degradation by CoMn2O4/PMS system is described in Figure 9b.  Co II  HSO5  Co III  SO42   HO

(2)

 Mn II  HSO5  Mn III  SO 42  HO

(3)

  Co II   OH  HSO5  Co III   OH  SO  4  OH

(4)

 Co III   OH  HSO5  Co II   OH  SO5  H 

(5)

 Mn II   OH  HSO5  Mn III   OH  SO  4

(6)

 Mn III   OH  HSO5  Mn II   OH  SO5  H 

(7)

 Mn III   OH  HSO5  Mn IV   OH  SO  4

(8)

 Mn IV   OH  HSO5  Mn III   OH  SO5  H 

(9)

 Mn II  CoIII  Mn III  CoII

∆E=0.30V

(10)

 Mn III  CoIII  Mn IV  CoII

∆E=1.96V

(11)

  SO 4  H 2 O  HO  HSO 4

(12)

 2  SO 4  OH  SO 4  HO

(13)

SO  4 (or  OH)  Organic pollutants  ...many steps...  C O 2  H 2 O

(14)

4. Conclusion In this study, CoMn 2O4 catalyst was successfully synthesized and found to be efficient and environmentally benign for the heterogeneous activation of PMS, but showing almost no catalytic activity to PDS and H2O2. RhB degradation by CoMn2O4/PMS was enhanced with the increase of reaction temperature and inhibited with the increase of FA concentration. RhB removal decreased only slightly from 100.0% to 87.0% after 5 cycles, indicating the good activity and stability. The simultaneous presence of HO and SO in CoMn2O4/PMS system were identified by the radical 4 quenching studies. The XPS analysis and other experimental results indicated that the high catalytic

15

activity was attributed to the synergistic effect of  CoII /  Co III ,  Mn II /  Mn III and  Mn III /  Mn IV redox pairs. Based on this, a catalytic mechanism was proposed to understand the catalytic process. The novel catalyst would have potential applications in pollution control due to its good catalytic activity, chemical stability, and easy recycling.

ACKNOWLEDGMENTS The financial supports by the Anhui Provincial Natural Science Foundation (NO.1308085MB21), the National Natural Science Foundation of China (Grant 51372062), Technology Foundation for Selected Overseas Chinese Scholar of Anhui Province (NO. 2013AHST0415), the State Key Laboratory of Materials-Oriented Chemical Engineering (NO. KL13-12), the Fundamental Research Funds for the Central Universities (NO. 2012HGQC0010), and the Project Sponsored by the Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education Ministry are gratefully acknowledged. The partial support from the Australian Research Council for DP110103699 is also acknowledged. The authors declare no competing financial interest.

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G.P.

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22

Figure 1. Schematic illustration of the preparation method for Co xMn3-xO4 catalysts.

23

(a)

(b)

Figure 2. (a) XRD patterns and (b) FT-IR spectra of the synthesized Co xMn3-xO4, Co3O4, and Mn2O3 catalysts

24

(a)

(b)

250 nm

SU8020 5.0kV 7.5mm×100k SE(U)

(c)

(e) 111 202 113 d 202=3.03Å

(d) d111=4.78Å

(f)

(g)

(h)

(i)

Figure 3. Structural characterizations of the CoMn2O4 catalyst. (a) SEM, (b) TEM, (c) SEAD, (d) FFT ,(e) HRTEM image, (f) TEM image and the corresponding EDX mapping images for (g) O, (h) Mn and (i) Co.

25

(a)

(b)

Figure 4. (a) Comparison of the photocatalytic performance of samples under different conditions. (b) The UV–vis spectral changes of RhB in CoMn2O4 /PMS process. Inset: the structural formula of RhB and the photograph of the corresponding color change during different reaction times. Unless otherwise stated, the reaction conditions are based on: [RhB] = 0.03 g/dm3, [catalyst] = 0.02 g/dm3, T =25 °C, [PMS] =0.2 g/dm3, initial pH=6.29.

26

(a)

(c)

(b)

(d)

Figure 5. Factorial effects for the catalytic degradation of organic dyes by CoMn2O4 catalysts: (a) Different dyes, (b) Reaction temperature, (c) FA concentration, (d) Different oxidants. Unless otherwise stated, the reaction conditions are based on: [dye] = 0.03 g/dm3, [catalyst] = 0.02 g/dm3, T =25 °C, [PMS] = [H2O2] = [PDS] =0.2 g/dm3, initial pH=6.29.

27

Figure 6. Decolorization of RhB in multiple use of CoMn2O4 catalyst. Unless otherwise stated, the reaction conditions are based on: [RhB] = 0.03 g/dm3, [catalyst] = 0.02 g/dm3, T =25 °C, [PMS] =0.2 g/dm3, initial pH=6.29.

28

(a)

(b)

Figure 7. (a) XRD patterns and (b) FT-IR spectra of CoMn2O4 catalysts before and after reaction.

29

(a)

(c)

(b)

(d)

Figure 8. XPS spectra of the fresh and used CoMn2O4 catalysts. (a) Survey spectra, (b) Co 2p, (c) Mn 2p, and (d) O 1s.

30

(a) (b)

Figure 9. (a) Inhibition of TBA and MeOH on RhB degradation by CoMn2O4/PMS system. Unless otherwise stated, the reaction conditions are based on: [RhB] = 0.03 g/dm3, [catalyst] = 0.02 g/dm3, T =25 °C, [PMS] =0.2 g/dm3, initial pH=6.29. (b) Mechanisms for RhB degradation by CoMn2O4/PMS system.

31