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Original article
11 (
6
); 924-934
doi:
10.1016/j.arabjc.2018.02.002

Metal doped manganese oxide octahedral molecular sieve catalysts for degradation of diclofenac in the presence of peroxymonosulfate

, ,
 State Key Laboratory of Pollution Control and Resource Reuse, College of Environmental Science and Engineering, Tongji University, China

⁎Corresponding authors. shijun215@tongji.edu.cn (J. Shi)

Received: , Accepted: ,
© The Author(s) 2018
Disclaimer:
This article was originally published by Elsevier and was migrated to Scientific Scholar after the change of Publisher.

Peer review under responsibility of King Saud University.

Abstract

Manganese oxide octahedral molecular sieve (OMS-2) and a series of OMS-2 doped with Co, Cu, and Ce were prepared via a solvent-free method, and tested in heterogeneous activation of peroxymonosulfate (PMS) for diclofenac (DCF) degradation in aqueous solutions. X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), scanning electron microscopy (SEM), N2 adsorption/desorption isotherms, Inductively Coupled Plasma Optical Emission Spectrometry (ICP-OES) and X-ray photoelectron spectroscopy (XPS) were used to characterize the properties of those materials. It was found that (Co + Ce)-OMS-2 seemed to be most efficient and had the best adsorption capacity, which may due to most abundant lattice oxygen and more macropore than other materials caused by cerium ions enter into the channel. Results from XPS suggested that the highly catalytic efficiency possible involved the activation of PMS to sulfate and hydroxyl radical meditated by the redox pair of Mn(IV)/Mn(III) and Co(III)/Co(II) in catalysts.

Keywords

Manganese oxide octahedral molecular sieve
Diclofenac
Peroxymonosulfate
1

1 Introduction

Diclofenac (DCF) is a non-steroidal anti-inflammatory drug (NSAID) derived from phenylacetic acid, commonly used as its sodium salt for medical application (Ziylan and Ince (2011)). It has the highest acute aquatic toxicity within the class of NSAIDs, and microorganisms that usually comprise of lotic biofilms are inhibited by concentrations around 100 μg/L, which indicated that conventional wastewater treatment processes cannot degrade DCF effectively (Fent et al., 2006; Paje et al., 2002). To effectively remove DCF, many novel DCF treatment methods such as Fenton reactions under UV-light irradiation, ozone oxidation, gamma irradiation, sonolysis and electrochemical incineration have been proposed (Pérez-Estrada et al. (2005); Coelho et al., 2009; Homlok et al., 2011; Hartmann et al., 2008; Brillas et al. (2010)). Advanced oxidation processes based on sulfate radical (SO4−•) (SR-AOPs) have become a promising alternative owing to some merits compared with those •OH-generating methods, such as higher oxidation potential (2.5–3.1 V vs. NHE), more selectively via electron transfer with organic compounds that contain unsaturated bonds or aromatic π electrons, and longer half-life period (30–40 μs) which enables SO4−• to have more stable mass transfer(Antoniou et al. (2010a); Olmez-Hanci and Arslan-Alaton, 2013). SO4−• can be produced by UV-based photolysis, heat, metal ion and metallic oxide activation of peroxymonosulfate (PMS) or persulfate (PS) (Hu and Long, 2016; Tan et al., 2012a; Tan et al., 2012b; Tan et al., 2014; Tan et al., 2013). In consideration of practical utility, heterogeneous catalysis with metal oxides attracts more and more attention recently.

Synthetic cryptomelane, known as octahedral molecular sieve (OMS-2), has been reported as an efficient catalyst in many oxidation reactions (Kumar et al., 2009; Schurz et al., 2009). The material exhibits prominent characteristics like mixed-valence manganese (Mn3+ and Mn4+), a composition of KMn8O16, a structure consisting of 4.6 Å × 4.6 Å tunnels due to a 2 × 2 arrangement of edge-shared MnO6 octahedra and easy release of lattice oxygen (Gandhe et al., 2007; Suib, 2007). Because the presence of mixed-valence manganese species in OMS-2, it has been proved to be a powerful catalyst for activation of PMS (Luo et al., 2015). For further improvement of catalytic activity, OMS-2 is commonly doped with foreign metal ions (e.g. Cu2+, Co2+, Sn4+, W6+, Ce3+, Ag+) using methods such as reflux, hydrothermal treatment and impregnation method (Ahmed et al., 2013; Calvert et al., 2008; Meng et al., 2016; Zacar et al., 2013; Sun et al., 2012; Zhang et al. (2016a)). For metal doped OMS-2, there is defects caused by metal ions in the framework or in the tunnels and hence increases number of active sites (Pahalagedara et al., 2014). It also has been reported that the structure, morphology, and lattice parameters of OMS-2 can be turned through doping metal ions (Pahalagedara et al., 2014; Shen et al., 2011; Sun et al., 2013). In this work, a series of OMS-2 and metal doped OMS-2 (Me-OMS-2) successfully synthesized for activation of PMS as catalysts by a solvent free method, followed by extensive characterization. Finally, the mechanism for DCF degradation was discussed based on experimental results.

2

2 Materials and methods

2.1

2.1 Materials

Potassium permanganate (KMnO4), copper acetate (Cu(Ac)2·H2O), tert-butyl alcohol (TBA) and ethanol (EtOH) were obtained from Sinopharm Chemical Reagent Corp, P. R. China. Manganese acetate (Mn(Ac)2·4H2O), cobalt acetate (Co(Ac)2·4H2O), cerium acetate (Ce(Ac)3·xH2O), Oxone (PMS, KHSO5·0.5KHSO4·0.5K2SO4) and diclofenac sodium (DCF) were purchased from Sigma Chemical Reagent Co. USA. All chemicals were used as received prepared without further purification, and all aqueous solution were prepared with deionized water.

2.2

2.2 Catalyst preparation

A solvent free method was used to prepare OMS-2 (Ding et al., 2005). Typically, 22.05 g (90 mM) of Mn(Ac)2·4H2O and 9.48 g (60 mM) of KMnO4 powders were mixed and ground homogenously in a mortar. The mixture was maintained at 80 °C for 4 h. The resulting black product was thoroughly washed with deionized water several times to remove any ions which may remain in the product, and finally dried at 80 °C in air overnight. Me-OMS-2 (Me = Co, Cu, Ce) catalysts were prepared by mixing KMnO4:Mn(Ac)2·4H2O:metal acetate in a molar ratio of 2:3:0.5, and the same procedure described above was followed, (Co + Ce)-OMS-2 were prepared by mixing KMnO4:Mn(Ac)2·4H2O:Co(Ac)2·4H2O:Ce(Ac)3·xH2Oin a molar ratio of 2:3:0.5:0.05 (Iyer et al., 2010; Samad et al., 2017).

2.3

2.3 Catalyst characterization

The crystalline structure of the catalysts was determined by a powder X-ray diffraction scanning (XRD, Brüker D8 advance diffractometer) using Cu Kα radiation. The beam voltage and beam current were set at 40 kV and 40 mA. The data were recorded at a scan rate of 10° (2θ) min−1 in the 10–80° range. Fourier transform infrared (FT-IR) spectra were recorded on a Nicolet 5700 spectrometer. The sample was mixed with solid KBr, then ground into powder and dried before pressed into a KBr wafer. The morphologies of the catalysts were studied using a Hitachi S4800field emission scanning electron microscope (FE-SEM) at a high voltage of 15 kV. A physisorption analyzer (Quantachrome PoreMaster-33) was used to measure the surface area and pore structure of the catalysts by N2 adsorption-desorption at 77 K. The surface area was determined by Multi-Point Brunauer-Emmett-Teller (BET) method at a Thickness interval of 0.50 Å. The pore volume and the pore diameter were determined by the Barrett-Joyner-Halenda (BJH) equation from the adsorption isotherm. Elemental analysis was conducted using an Agilent 720ESinductively coupled plasma emission spectrometer (ICP-OES), and all samples were dissolved using strong acid solution before being tested. X-ray photoelectron spectroscopy (XPS) results of the catalysts were recorded on a K-Alpha+ (Thermo Scientific) spectrometer, and the C 1s signal of contaminant carbon (BE = 284.6 eV) was taken as a reference for BE calibration. The intermediate products were determined by a Thermo Fisher Scientific TSQ Quantum.

2.4

2.4 Catalytic degradation experiment

Catalytic degradation experiments were carried out in 500mLsebc bottles with 250 mL aqueous solution. The sebc bottles containing aqueous solution were agitated in an air shaker at 250 rpm and 298 ± 1 K under normal laboratory light. In the beginning, catalysts and diclofenac sodium stock solutions were added into deionized water. And then, the obtained mixture was shaken for 30 min to achieve the adsorption/desorption equilibrium between catalysts and contaminants before the Oxone addition. The experiment was conducted at acidic medium (pH = 5.4 ± 0.2, no adjustment). About 2 mL solution sampled with sterile syringe at different interval time and filtered with 0.22 μm filter, then about 0.5 mL filtered solution took immediately by a pipette into a high-performance liquid chromatograph (HPLC) vial filled with 0.5 mL methanol(Oxone quencher) in advance. To gain better insight into the reaction mechanism, catalysts were separated by a decompress filter after standing at normal temperature for 24 h in the reaction solution, washed with deionized water several times to remove any ions which may remain in the product, dried at 80 °C in air overnight and then tested with XPS method. Response surface methodology (RSM) was applied to investigate the effects of three affecting factors including temperature, catalysts dosage and Oxone dosage. The Design-Expert software was used for regression analysis of the data and to estimate the coefficients of the regression equation.

2.5

2.5 Analysis

The concentration of DCF was measured by a reversed-phase high-performance liquid chromatography system (Aglient 1200). The analytical column was a C18 column (Gemini 5 μm, 150 mm × 4.6 mm, from Phenomenex). The injection volume was 50 μL. The column temperature was 30 °C. The mobile phase was a mixture of 80% HPLC-grade methanol and 20% MilliQ-water (containing 0.1% formic acid) at a constant flow rate of 1.0 mL/min. And the detection wavelength was set at 276 nm (Zhang et al. (2016b)). The intermediate products were analyzed using a LC-MS, which included an Aglient HPLC coupled with a triple-stage quadrupole massspectrometer (Thermo Fisher Scientific TSQ Quantum, U.S.). The analytical column was a C18 column (Athena, 2.1 × 100 mm, from CNW). The injection volume of the samples was 10 μL. The mobile phase was A: 98% H2O (0.1% formic acid) + 2% CH3OH, and B: 2%H2O (0.1% formic acid) + 98% CH3OH. Gradient elution was 0% of B to 100% by a linear increase during 12 min, and held constantfor an additional 6 min. The mass spectra data was obtained in the negative ion mode by scanning from m/z from 70 to 350 (Zhang et al., 2017; Shamsudin et al., 2017; Nordin et al., 2017).

3

3 Results and discussion

3.1

3.1 Characterization results

The XRD patterns of OMS-2 and Me-OMS-2 materials were in good agreement with the standard tetragonal structure of cryptomelane-type manganese oxide (JCPDS 29-1020) as shown in Fig. 1(A). No additional peaks assignable to segregated crystalline phases of Co, Cu or Ce oxides were observed, indicating that the Me-OMS-2 materials were phase pure and the doped metal species were well dispersed or incorporated into the structure (Ma et al., 2017). There was no obvious change of the Me-OMS-2 pattern with the doping of Co and Cu. However, the diffraction peaks of Ce-OMS-2 were broadened and decreased in intensity in comparison to the OMS-2 catalyst, implying decrease in both grain size and crystallinity of the material (Polverejan et al. (2004)). The presence of amorphous metal oxide in Me-OMS-2 can be detected by infrared spectroscopy, for it was more sensitive to impurity phase in the structure compared with the X-ray diffraction technique (King’ondu et al. (2011); Azizan et al., 2017). Fig. 1(B) displays the FT-IR spectra of OMS-2 and Me-OMS-2 materials. The peaks at about 714, 582, 523 and 467 cm−1 can be attributed to the vibrations of the MnO6 octahedral framework, which were the characteristic peaks of cryptomelane-type manganese oxide (Li et al., 2015). The 714 and 467 cm−1 band of Me-OMS-2 slightly increases in intensity compared with OMS-2 materials. The 582 cm−1 band obviously increases with the doping of Co, but it visibly decreases in intensity with the doping of Cu and Ce. Meanwhile with the doping of Ce, the 523 cm−1 band disappears nearly. The variation in the FT-IR spectra indicates that there was entrance of cobalt, copper or cerium into the framework of OMS-2, resulting in the new structure (Xie et al., 2016; De'nan et al., 2017).

(A) XRD and (B) FT-IR spectra of: (a) OMS-2, (b) Co-OMS-2, (c) Cu-OMS-2, (d) Ce-OMS-2, (e) (Co + Ce)-OMS-2.
Fig. 1 (A) XRD and (B) FT-IR spectra of: (a) OMS-2, (b) Co-OMS-2, (c) Cu-OMS-2, (d) Ce-OMS-2, (e) (Co + Ce)-OMS-2.

SEM images for OMS-2 and Me-OMS-2 materials synthesized in this study were shown in Fig. 2. OMS-2 prepared by solvent free method has nanorod morphology with a particle width of about 10 nm. The length of these rods vary from 40 to 100 nm. The surface morphology of OMS-2 remains unchanged after the addition of Co or Cu. However, upon the addition of cerium, there was a clear change of OMS-2 morphology. The length of Ce-OMS-2 and (Co + Ce)-OMS-2 rods decreases to less than 40 nm, and clusters were detected much more obviously. This was consistent with the information obtained from XRD patterns (Rahmat et al., 2017).

SEM images of: (A) OMS-2, (B) Co-OMS-2, (C) Cu-OMS-2, (D) Ce-OMS-2, (E) (Co + Ce)-OMS-2.
Fig. 2 SEM images of: (A) OMS-2, (B) Co-OMS-2, (C) Cu-OMS-2, (D) Ce-OMS-2, (E) (Co + Ce)-OMS-2.

N2 adsorption-desorption measurements were performed to investigate the BET specific surface area and the pore size of OMS-2 and Me-OMS-2 materials. Fig. 3 shows N2 adsorption-desorption isotherms and corresponding BJH pore-size distribution curves for the OMS-2 and Me-OMS-2 materials. The isotherm pattern of OMS-2, Co-OMS-2 and Cu-OMS-2 show a characteristic type IV isotherm pattern with a hysteresis loop of type H1 in the IUPAC classification, indicative of a solid mesoporous structure, while Ce-OMS-2 and (Co + Ce)-OMS-2 show type II isotherm pattern with a hysteresis loop of type H3, which was usually associated with the adsorption on aggregates of particles with a layered morphology (Xie et al., 2016). The BJH plot shows a narrow pore size distribution in the range of 3–30 nm for catalysts except Ce-OMS-2 and (Co + Ce)-OMS-2 which have more pores that were larger than 30 nm compared with others.

N2 adsorption–desorption isotherms for at 77 K for: (A) OMS-2, (B) Co-OMS-2, (C) Cu-OMS-2, (D) Ce-OMS-2, (E) (Co + Ce)-OMS-2, the insert shows the pore size distribution plot calculated by the BJH method.
Fig. 3 N2 adsorption–desorption isotherms for at 77 K for: (A) OMS-2, (B) Co-OMS-2, (C) Cu-OMS-2, (D) Ce-OMS-2, (E) (Co + Ce)-OMS-2, the insert shows the pore size distribution plot calculated by the BJH method.

In order to confirm the position of the added metals in the structure, the compositions of the OMS-2 and Me-OMS-2 materials were analyzed by ICP-OES and results were shown in Table 1. For Co-OMS-2 and Cu-OMS-2, the concentrations of K ions and Mn ions were all lower than those in the OMS-2 material, which means that the substitution of both Mn ions in the framework and K ions in the tunnel sites of OMS-2 by the added Co or Cu ions take place. Because the Mn ions mole fraction of Co-OMS-2 was lower than that of Cu-OMS-2, there were more Co ions in the framework of OMS-2 than Cu ions. However, the introduction of Ce ions results in decline of K ions but Mn ions stay constant, indicating the partial replace of K ions in the tunnels by Ce ions and insignificant substitution of Mn ions in the framework. Considering the crystal radii of Co2+(0.79 Å), Co3+(0.685 Å), Cu+(0.91 Å), Cu2+(0.87 Å), which were significantly lower than that of K+ (1.65 Å) but closerto that of Mn3+(0.785 Å) and Mn4+(0.67 Å), it was much easier for Co or Cu ions to substitute for Mn ions than K ions (Pahalagedara et al., 2014; Shannon, 1976; Hassan and Ismail, 2017). While, the crystal radii of Ce3+(1.15 Å) and Ce4+(1.01 Å) were higher than that of Co or Cu ions, which makes Ce ions easier to substitute for K ions and more difficult to substitute for Mn ions than Cu and Ce ions (Guo et al., 2010). Because the crystal of Co3+ was very close to that of Mn4+, CoO6 units in the OMS-2 framework can almost be the same as MnO6 units, which makes it quite easy to substitute for Mn ions.

Table 1 The mole fraction of K, Mn and added metals determined by ICP-OES.
Catalysts n(K)/n(K + Mn+Me) (%) n(Mn)/n(K + Mn + Me) (%) n(Me)/n(K + Mn + Me) (%)
OMS-2 33.52 66.48 –
Co-OMS-2 29.59 56.14 14.27
Cu-OMS-2 25.42 59.79 14.79
Ce-OMS-2 21.16 66.67 12.17
(Co + Ce)-OMS-2 27.56 59.47 11.57(Co), 1.40(Ce)

XPS was carried out to identify the surface elemental composition and the oxidation state of catalysts. The symmetrical O1s spectra could be deconvoluted to two peaks, OI(529.5 eV) and OII(531.2 eV) in Fig. 4, which can be assigned to the surface lattice oxygen and surface adsorbed oxygen, respectively (Tang et al., 2010; Rahman et al., 2017). The OII/OI ratio decrease in sequence of Ce-OMS-2(0.261) > OMS-2(0.173) > Cu-OMS-2(0.169) > Co-OMS-2(0.136) > (Co + Ce)-OMS-2(0.098). Some researchers reported that surface lattice oxygen and surface hydroxyl groups are active oxygen species which plays a critical role in oxidation reaction (Galtayries et al. (1998); Ren et al., 2015; Hanedar et al., 2017). Obviously, (Co + Ce)-OMS-2 is the richest in terms of surface lattice oxygen among the propared catalysts (see Fig. 5).

O 1 s XPS spectra of catalysts prepared.
Fig. 4 O 1 s XPS spectra of catalysts prepared.
(A) DCF degradation in different PMS oxidation systems, (B) The plots of ln(C/C0) versus reaction time which represent the fit using a pseudo-first-order reaction rate, [Oxone]=0.2 g/L, [catalyst]=0.1 g/L, [DCF]0=0.01 g/L, T = 25 °C.
Fig. 5 (A) DCF degradation in different PMS oxidation systems, (B) The plots of ln(C/C0) versus reaction time which represent the fit using a pseudo-first-order reaction rate, [Oxone]=0.2 g/L, [catalyst]=0.1 g/L, [DCF]0=0.01 g/L, T = 25 °C.

3.2

3.2 Catalytic performance

The conversion of DCF concentration was shown in Fig. 6. It was found from Fig. 6(A) that all catalysts had favorable catalytic performances, and the sequence of the degradation rate in PMS solution was (Co + Ce)-OMS-2 > Co-OMS-2 > OMS-2 > Cu-OMS-2 > Ce-OMS-2. As was shown that DCF was degraded quickly, and more than 88.4% of it was destructed within 17.5 min at Oxone and (Co + Ce)-OMS-2 doses of 0.2 g/L and 0.1 g/L, respectively, while only about 15.5% of DCF was destructed without any catalysts in 30 min. As shown in Fig. 6(B), the degradation of DCF followed the pseudo-first-order kinetics.

Response surface curve for the degradation rate of DCF.
Fig. 6 Response surface curve for the degradation rate of DCF.

And after 30 min of oscillation in this experiment, only Ce-OMS-2 and (Co + Ce)-OMS-2 absorbed nearly 7% of DCF in water, while others barely absorbed DCF. This result was in accordance with the previous material characterization. Ce-OMS-2 had the lowest grain size and crystallinity and more macropore than other materials, which may due to lots of Ce ions enter into the channel.

3.3

3.3 Effect of temperature, catalysts dosage and Oxone dosage

Fig. 7 shows the 3d response surface curve of the pseudo-first-order reaction rate as function of temperature, (Co + Ce)-OMS-2 catalyst dosage and Oxone dosage. It was observed that the increase of both temperature and catalysts dosage can promote the degradation rate dramatically, while the increase of Oxone dosage can only promote the rate quickly at the beginning which may due to the scavenging effect caused byextra PMS(Tan et al., 2014; Teng and Zhou, 2017). According to the model, the reaction rate, 0.53 min-1, can be optimal at 34.6 °C, 0.18 g/L catalyst dosage and 236.5 mg/L Oxone dosage.

DCF degradation with or without two free radical scavengers, [Oxone] = 0.2 g/L, [catalyst] = 0.1 g/L, [DCF]0 = 0.01 g/L, T = 25 °C.
Fig. 7 DCF degradation with or without two free radical scavengers, [Oxone] = 0.2 g/L, [catalyst] = 0.1 g/L, [DCF]0 = 0.01 g/L, T = 25 °C.

The rationality of model is tested by means of analysis of variance (ANOVA) and results are shown in Table 2 and 3. As shown in the table, the model F-value is 2047.67 which implies that model is significant, and the F-value of lack of fit is 4.33 which shows that the model is feasible and authentic. The model p-value is less than 0.05 which indicates that the model terms are significant at 95% confidence level (Deng et al., 2013; Basheer et al., 2017). The predicted model is obtained by the following quadratic polynomial function: k = 0.058 - 1.888 A - 1.992 B - 1.338 C + 0.057 AB + 1.000 E - 5 AC + 3.750 E - 3 BC - 8.500 A 2 + 11.352 B 2 - 4.155 C 2

Table 2 Experimental results of the applied factorial design for DCF degradation.
Experiment A:Temperature (°C) B: Catalysts dosage (g/L) C: Oxone dosage (mg/L) Actual value (min−1) Predicted value (min−1)
1 15 0.02 200 0.008 0.011
2 35 0.02 200 0.024 0.027
3 15 0.18 200 0.315 0.310
4 35 0.18 200 0.513 0.510
5 15 0.10 50 0.043 0.045
6 35 0.10 50 0.121 0.120
7 15 0.10 350 0.115 0.110
8 35 0.10 350 0.253 0.250
9 25 0.02 50 0.012 0.007
10 25 0.18 50 0.308 0.310
11 25 0.02 350 0.015 0.014
12 25 0.18 350 0.491 0.500
13 25 0.10 350 0.144 0.140
14 25 0.10 200 0.141 0.140
15 25 0.10 200 0.142 0.140
16 25 0.10 200 0.141 0.140
17 25 0.10 200 0.148 0.140
Table 3 ANOVA for Response Surface Quadratic Model.
Source Sum of Squares df Mean Square F Value p-value Prob > F
Model 0.39 9 0.043 2047.67 <0.0001 Significant
A 0.023 1 0.023 1094.64 <0.0001
B 0.31 1 0.31 14555.45 <0.0001
C 0.019 1 0.019 900.46 <0.0001
AB 8.281E−3 1 8.281E−3 392.20 <0.0001
AC 9.000E−4 1 9.000E−3 42.63 0.0003
BC 8.100E−3 1 8.100E−3 383.63 <0.0001
A2 3.042E−6 1 3.042E−6 0.14 0.7155
B2 0.022 1 0.022 1052.52 <0.0001
C2 3.681E−4 1 3.681E−4 17.43 0.0042
Residual 1.478E−4 7 2.111E−5
Lack of Fit 1.130E−4 3 3.767E−5 4.33 0.0954 Not significant
Pure Error 3.480E−5 4 8.700E−6
Cor Total 0.39 16

3.4

3.4 Activation mechanism of PMS on catalysts

SO4−• and SO5−• are main active free radicals produced by PMS coupled with metal ions, and •OH can be generated by reaction of SO4−• and H2O (Ding et al., 2013). SO5−• was usually considered not responsible for the degradation of organic pollutants due to its weak oxidizing ability (E (SO5−•/SO42−) = 1.1 V) (Ding et al., 2013; Pagano et al., 2012). In order to identify the main radicals in the system, two free radical scavengers, tert-butyl alcohol (TBA) and ethanol (EtOH) were used. TBA reacts rapidly with •OH and much slower with SO4−•, while EtOH can react quickly with these two kinds of free radicals (Fang et al., 2013; Tan et al., 2017; Ismail and Hanafiah, 2017). As shown in Fig. 7, the rate of reaction was significantly reduced in the presence of 100 mM TBA which indicated that • was involved in the degradation of DCF. When the same concentration of EtOH was added, a more significant reduction of reaction rate was observed, which proved that SO4−• played a more important role in the reaction system.

XPS of (Co + Ce)-OMS-2 before and after reaction was carried out to research the activation mechanism of PMS on catalysts. As shown in Fig. 8, the oxidation state of Mn, Co and Ce ions in the catalysts were characterized by analyzing Mn 2p, Co 2p and Ce 3d spectra. The obtained binding energy value of Mn 2p were identified by curve fitting as ∼641.6 eV and ∼642.2 eV for Mn 2p3/2, and ∼653.4 eV and ∼653.8 eV for Mn 2p1/2, which corresponds to Mn3+ and Mn4+.The Co 2p spectrum exhibits two mahor pearks, the obtained binding energy value were identified by curve fitting as ∼779.7 eV and ∼780.2 eV for Co 2p3/2, and ∼794.8 eV and ∼795.3 eV for Co 2p1/2, which corresponds to Co2+ in tetrahedral coordination and Co3+ in octahedral coordination in Co-OMS-2 (Tholkappiyan and Vishista, 2015). For Ce 3d spectra, six peaks at 916.7, 907.5, 900.8, 898.4, 889 and 882.3 eV can be identified, which are characteristic of the Ce4+ 3d final state, while four peaks at 903.6, 895, 885.7 and 880.5 eV corresponding to the Ce3+ 3d final state(Zhiqiang et al. (2010); Zou et al., 2009). As calculated from the ratio of the integrated areas of these peaks, Mn(III)/Mn(IV) decreased from 1.761 to 1.225, and Co(II)/Co(III) decreased from 1.790 to 1.106. While, Ce(III)/Ce(IV) increased from 0.496 to 0.749, and OII/OI increased from 0.098 to 0.299. In literature, the proposed activation process of PMS by manganese catalysts was involved the generation of SO5−• radicals through the reduction of high-valent state of ions and the formation of SO4−• radicals by oxidation of the generated low-valent state of ions (Ren et al., 2015; Saputra et al., 2013). So, the mechanism of activation of PMS by (Co + Ce)-OMS-2 catalyst was proposed as Eqs. (1) to (4).

(1)
Me(low valence) + HSO5− → Mn(high valence) + OH− + SO4−•
(2)
Mn(high valence) + HSO5− → Mn(low valence) + SO5−• + H+
(3)
SO4−• + H2O → •OH + SO42− + H+
(4)
SO5−• + O2− → SO4−• + O2
XPS spectra of (Co + Ce)-OMS-2 before and after reaction: (A) Mn 2p, (B) O 1 s, (C) Co 2p, (D) Ce 3d.
Fig. 8 XPS spectra of (Co + Ce)-OMS-2 before and after reaction: (A) Mn 2p, (B) O 1 s, (C) Co 2p, (D) Ce 3d.

3.5

3.5 Degradation pathway

LC-MS was used to monitor the generation of intermediate products. The mass DCF spectrum produced a molecular ion at m/z 295, and the major intermediate products observed were at m/z 282, 258, 241, 214, 210, 193 and 180. Considering the more selective of SO4−• than •, P1 or P2 can be generated at the first step by substitution or hydrolysis of sulfate derivative into hydroxyl derivative or dehalogenation, and then, P2 can be oxidized in two different ways to become P3 or P4 (Zhang et al., 2017; Deng et al., 2013; Antoniou et al. (2010b)). P3 can turn into P5 by dehalogenation, and P4 can be oxidized further into P6 or P7. The generated intermediates went through C—N cleavage, further dechlorination and ring cleavage, and all of them would eventually convert to inorganic compounds. The DCF degradation pathway was illustrated in Fig. 9.

The degradation pathway of DCF, [Oxone] = 0.2 g/L, [catalyst] = 0.1 g/L, [DCF]0 = 0.01 g/L, T = 25 °C.
Fig. 9 The degradation pathway of DCF, [Oxone] = 0.2 g/L, [catalyst] = 0.1 g/L, [DCF]0 = 0.01 g/L, T = 25 °C.

4

4 Conclusion

A series of OMS-2 doped with Co, Cu, and Ce were prepared via a solvent-free method and tested in heterogeneous activation of PMS for DCF degradation in aqueous solutions. With Co doped, the material seemed to be most efficient and had the most abundant lattice oxygen which played a critical role in oxidation reaction. While, with Ce doped, the material had the best adsorption capacity which may due to more macropore than other materials caused by cerium ions enter into the channel. So, (Co + Ce)-OMS-2 exhibited best catalytic ability among catalysts prepared in this study.

Acknowledgement

This study was supported by State Key Laboratory of Pollution Control and Resource ReuseFoundation (PCRRY16001), and Major Science and Technology Program for Water Pollution Control and Treatment (2017ZX07501001), China. We are grateful to the Key Laboratory of Yangtze River Water Environment, Ministry of Education, for providing facilities for the experiments and the analysis.

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