Manganese doped magnetic cobalt ferrite nanoparticles for dye degradation via a novel heterogeneous chemical catalysis

Materials Chemistry and Physics 240 (2020) 122181

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Manganese doped magnetic cobalt ferrite nanoparticles for dye degradation via a novel heterogeneous chemical catalysis Ruyan Dou a, b, Hao Cheng b, c, Jianfeng Ma a, b, *, Sridhar Komarneni d, ** a

School of Environmental and Safety Engineering, Changzhou University, Jiangsu, 213164, China Guangxi Key Laboratory of Green Processing of Sugar Resources, College of Biological and Chemical Engineering, Guangxi University of Science and Technology, Guangxi, 545006, China c Province and Ministry Co-sponsored Collaborative Innovation Center of Sugarcane and Sugar Industry, Nanning, 530004, Guangxi, PR China d Department of Ecosystem Science and Management and Materials Research Institute, 204 Materials Research Laboratory, The Pennsylvania State University, University Park, PA, 16802, USA b

H I G H L I G H T S

� Composite cobalt ferrites doped with manganese degrade orange II (OII) pollutant. � Degradation of OII by Mn–Co ferrite was greatly enhanced in the presence of NaHSO3. � CoMn0.2Fe1.8O4 showed higher degrading ability and strong ferromagnetic property. � CoMn0.2Fe1.8O4 ferrite/NaHSO3 could greatly improve the catalyst separation. A R T I C L E I N F O

A B S T R A C T

Keywords: Ferrites Dye degradation Heterogeneous catalysis NaHSO3

Composite cobalt ferrites doped with manganese were successfully synthesized via sol-gel auto combustion method and were annealed at a temperature of 400 � C. The catalytic activity of a series of catalysts at different manganese contents was studied using the sodium bisulfite-assisted system for degradation of water-soluble organic pollutants such as orange II (OII). The obtained ferrite compositions were characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), vibrating sample magnetometer (VSM) and X-ray photoelectron spectrometer (XPS). The results showed that the degradation efficiency was greatly enhanced in the presence of NaHSO3. In addition, all the Mn-containing samples showed much higher capability to degrade OII than the pure CoFe2O4 under the same conditions. Among all the samples containing manganese, the CoMn0.2Fe1.8O4 sample showed higher degrading ability. The CoMn0.2Fe1.8O4 sample also showed strong ferromagnetic property with the saturation magnetization and remanence magnetization of 43.1 emu/g and 22.5 emu/g respectively, which is expected to provide a good magnetic separation performance.

1. Introduction Nowadays, with the rapid development of printing and dyeing in­ dustry, all kinds of dyes are being discharged into the environmental waters at random. Therefore, an effective and economic treatment of effluents has become a necessity step for clean production technology [1]. Conventional treatments like ion exchange, precipitation and electrochemical and biological methods, have their own limitations. To overcome their limitations, advanced oxidation processes (AOPs) [2–4] have become an emerging technology [5] for the degradation of

hazardous organic substances like dyes in industrial wastewater and this technology has become increasingly significant in recent years due to its speedy reaction rate and non-selective nature [6]. The so-called AOPs can be broadly defined as aqueous phase oxidation methods on the basis of the intermediacy of highly reactive species such as hydroxyl radicals (⋅OH) (predominantly but not exclu­ sively) destructing the target pollutants [7]. It also constitutes a series of similar but not identical chemical processes aimed at attacking organic pollutants in water, air, and soil [8]. In the past few decades, research and development concerning AOPs has been immense particularly

* Corresponding author. School of Environmental and Safety Engineering, Changzhou University, Jiangsu, 213164, China. ** Corresponding author. E-mail addresses: [email protected] (J. Ma), [email protected] (S. Komarneni). https://doi.org/10.1016/j.matchemphys.2019.122181 Received 27 March 2019; Received in revised form 5 September 2019; Accepted 12 September 2019 Available online 13 September 2019 0254-0584/© 2019 Elsevier B.V. All rights reserved.

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owing to two reasons, namely the diversity of technologies available and potential application prospects [7,9–13]. The application of AOPs using semiconductor materials as heterogeneous catalysts with the participa­ tion of strong oxidizing agents [e.g. O3, H2O2 etc.] has been widely studied by researchers due to the oxidation of a wide range of organic chemicals [14]. Semiconductor-assisted catalytic degradation of organic pollutants has attracted considerable attention because of its high effectiveness in complete chemical catalytic oxidation and mineralization of harmful organics. Metal oxides, nitrides, or sulfides (eg. TiO2 and MoS2) and the metal-free semiconductor (eg.C3N4) belong to the category of semi­ conductor catalysts. Among all the metal oxides, TiO2 has been one of the most popular semiconductor materials for several reasons, although the low utilization ratio of solar energy has limited its practical application. So far, transitional metal ions such as Co, Ni, Mn, and Fe etc, have been found to be useful for promoting catalytic oxidation reaction because of relatively high chemical activity as well as specificity of interaction [15]. They motivated researchers to make full use of tran­ sition metal oxide nanoparticles as efficient catalysts and combine them with strong oxidants (e.g. H2O2 and O3 etc.) to further improve degra­ dation efficiency. Over the past few years, great efforts have been devoted to the field of nanotechnology [16], especially about metal oxide nanoparticles. Transition metal oxide deserves attention from both scientific and technical perspectives. Furthermore, transition metal oxides are unique in that they can form more than one cationic oxidation state and are easily modified to improve catalytic performance. Spinel cobalt ferrite (CoFe2O4) is a kind of magnetic and electrical semi­ conductor material with high coercivity and moderate magnetization composed of earth-abundant, easily-available, and cost-effective cobalt and iron precursor elements. It also possesses some other unique char­ acteristics as follows: (1) narrow optical bandgap, (2) nontoxicity and natural abundance of elements, (3) good catalytic performance with large specific surface area and (4) its magnetic property can provide magnetic separation of solids from aqueous solution [17–21], the latter has emerged as one of the most promising options for application of catalysts in the field of environmental protection [22]. To date, there are a number of reports about modifications of CoFe2O4 or doping of CoFe2O4 with various metals and non-metals to accelerate its catalytic performance in the reduction of harmful sub­ stances [23–26]. Guo et al. [27] prepared magnetic S-modified CoFe2O4 nanoparticles with smaller particle size of about 38.6 nm and 49.8 m2/g of specific surface area, which showed dramatically increased catalytic ability compared with that of sample without doping of S. Li et al. [22] reported that porous cobalt ferrite microrods (CMR) displayed excellent adsorption capacity for MB after they were modified by embedding manganese oxides into their porous texture, indicating the important role of the in situ formed MnO2. Abundant hydroxyl groups on the surface of the newly formed manganese dioxide nanomaterials with a great many number of active sites could be useful for adsorption of or­ ganics. Goyal et al. [28] enhanced the performance of catalysts via successful incorporation of Al into the lattice of spinel nanoferrites (e.g. CoFe2O4 and NiFe2O4). It was acknowledged that the catalytic activity of spinel ferrites (M2þFe3þ 2O4) [29] is closely related to their struc­ ture. As catalysis is a surface phenomenon and the octahedral sites are exposed to the surface of the structure of spinel ferrites, the nature and mutual synergistic interactions of the metal ions located in octahedral sites are critical for the catalysis of spinel ferrites [28]. Doping transition metal ions in cobalt ferrite could change cationic distribution, which can affect catalytic activity by replacing existing ions with guest metal ions and such a process has been explored by some researchers [29,30]. Magnetic nanoscale catalysts could be easily separated from the aqueous medium by using a magnet with an external magnetic field [28, 31]and that’s why CoFe2O4 materials have a wide range of prospective applications in all fields. Furthermore, researchers found that CoFe2O4 possesses high electron transfer ability and stability and can form

heterojunctions when coupled with other semiconductors [32–35]. Therefore, a large number of studies have been reported regarding magnetic cobalt ferrite materials as carriers. Siadatnasab et al. [36] synthesized CuS/CoFe2O4 nanohybrid via loading CoFe2O4 (CFO) on the hydrothermally prepared CuS from copper (II) diethanol dithiocarba­ mate (Cu(DEDTC)2) complex. Li et al. [37] prepared magnetic CoFe2O4/AgBr composites by a facile precipitation–deposition method, which ensured that the composites could be easily separated from the reaction system. Gan et al. [18] found that synthesized Ag3PO4–CoFe2O4 nanocomposites could combine the photocatalytic activity of the Ag3PO4 and the magnetism of the CoFe2O4 to acquire a higher removal rate and magnetic separation of solid from water so as to avoid intro­ ducing secondary pollutant into the system [18]. Farhadi et al. [38] and He et al. [17] reported the preparation and the use of CoFe2O4@ZnS coreshell nanocomposite and CoFe2O4-rGO hybrids, respectively lead­ ing to a green and low-cost heterogeneous catalyst for degradation of organic contaminants in wastewater. Magnetic cobalt ferrite particles can be used directly as adsorbent or as magnetic carriers to remove contaminants in water. The coupling between the catalyst and magnetic material is a new approach to improve the catalytic activity of the catalyst. Many studies have shown that sulfate radicals (SO⋅- 4) possess a higher redox potential (E0 ¼ 2.5–3.1 V) than hydroxyl radicals (E0 ¼ 2.7 V), implying that they are able to degrade pollutants more thoroughly [39]. Sun et al. (2015) discovered a new advanced oxidation technology that involved bisulfite activated manganese oxidants (eg. KMnO4 and MnO2) to produce active manganese (III) species rather than hydroxyl and sulfate radicals to enhance the oxidation of organic con­ taminants. Sulfite (SO2 3) and bisulfite (HSO 3) species can be applied as the basis for a novel advanced oxidation process in environ­ mental protection [40]. It has been proved that manganese has the po­ tential of redox property and high catalytic activity [41–43]. Thus, it appears that active intermediate manganese (III) species produced from the manganese oxidant/HSO- 3 process are essential in order to play an important role in the oxidation of organic pollutants from wastewater [40,44–46]. In addition, Co2þ is considered to be one of the most effective ways to activate PMS/BS to produce SO⋅- 4, compared with other metal ions (Agþ, Ni2þ, Fe2þ, Fe3þ etc.) [47]. And Co2þ/PMS sys­ tem is even superior to traditional Fenton reaction. However, different from previous research, we put forward a novel system that could use bisulfite to activate catalyst to produce some strong oxidizing radicals not only �OH, SO- 4 but also Mn(III) strong oxidizing substance. To the best of the authors’ knowledge, there are no existing reports regarding chemical catalysis by incorporation of Mn into the lattice of cobalt fer­ rites to degrade OII dye wastewater in the presence of bisulfite [23]. This paper reports a successful doping of transition metal ion of manganese into cobalt ferrites (CFO) in different concentrations for degrading pernicious orange II dye. Catalysts were synthesized by solgel auto-combustion method and were combined with NaHSO3 as cocatalyst to form a new system, which was then used to decompose OII contaminants. A comparison of catalytic properties between pure and doped samples has been carried out. It was found that doped samples showed considerably improved chemical catalysis in the presence of NaHSO3 when compared to undoped CFO. Furthermore, the doped samples showed inconspicuous catalytic activity in the absence of NaHSO3. At the same time, the properties of the samples were charac­ terized and studied. Thus, the current work led to the economical and facile synthesis of manganese doped CFO samples, which are effective in decomposing harmful OII azo-dye in oxidant/HSO 3(chemocatalysis) process. 2. Experimental 2.1. Materials and reagents High purity (AR grade) of cobaltous nitrate [Co(NO3)2�6H2O], ferric 2

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nitrate [Fe(NO3)3�9H2O], manganese chloride [MnCl2�4H2O], citric acid monohydrate [C6H8O7�H2O] and ammonium hydroxide solution [NH4OH] were used for the synthesis of the ferrites with the formula of CoMnxFe2 xO4(x ¼ 0.0, 0.2, 0.4, 0.6, 0.8 and 1.0).

3. Results and discussion 3.1. XRD analysis The crystalline phases and crystallinity of the nanostructures were determined from XRD patterns. The powder XRD patterns of CoFe2O4, CoMn0.2Fe1.8O4, CoMn0.4Fe1.6O4, CoMn0.6Fe1.4O4, CoMn0.8Fe1.2O4 and CoMnFeO4 annealed at 400 � C for 2 h are shown in. Fig. 1. Analysis of XRD patterns of all the samples corroborates the formation of ferrites as they all have the same symmetry as that of CoFe2O4. As can be seen from the XRD patterns, all the diffraction peaks of cobalt ferrites with cubic spinel structure coincided with the standard JCPDS card of CoFe2O4 (No. 03–0864). It is obvious that all diffraction peaks indexed to CoFe2O4 in the range of 2θ from 5� to 80� can be seen at 18.3� , 30.1� , 35.5� , 36.7� , 43.2� , 53.6� , 57.1� and 62.7� . The intense peak at 35.5� was indexed to (311) plane. The absence of any additional peaks indicates that the synthesized samples have single-phase cubic spinel structure with little or no secondary phases. The ferrite compo­ sitions annealed at 400 � C with increasing manganese content showed broad XRD peaks, which could be attributed to the smaller crystal size [48]. The average crystallite size of catalysts can be estimated according to the diffraction peaks by using the Debye–Scherrer formula, D ¼ Kλ/βcosθ, in which D is the average crystalline size, β is the full width at half maximum (FWHM) of the diffraction peaks, λ is the wavelength of the radiation used, θ is the angle of diffraction and K is the Scherrer constant, 0.89 [45]. The average crystallite size of samples is measured and the values are listed in Table 1 [49].

2.2. Synthesis of ferrites A variety of preparation methods could be used for the synthesis of ferrite samples with the formula of CoMnxFe2 xO4(x ¼ 0.0, 0.2, 0.4, 0.6, 0.8 and 1.0). Here, the samples were synthesized by means of sol-gel auto-combustion route. The detailed procedure is as follows. Firstly, the precursor materials of Co(NO3)2�6H2O, Fe(NO3)3�9H2O, MnCl2�4H2O and C6H8O7�H2O were completely dissolved in deionized water separately in a minimum quantity of water. Next the amount of metal nitrates was calculated according to the molar ratio between them. The molar ratio of metal nitrates to citric acid used was 1:1. Each of the chemicals was dissolved separately and then mixed together. Then, a certain amount of ammonia water was added into the mixed solution drop by drop to adjust the pH to 5. At last, the mixed solution of pH 5 was continuously heated and stirred in a magnetically heated agitator until it became a gel. The obtained gel was continuously heated and stirred. The mixed gel was exothermic at high temperature and led to spontaneous combustion forming ferrite powders. These powders were calcined in a muffle furnace for 2 h at 400 � C for crystallization. 2.3. Analytical methods Characterization of the prepared samples was carried out by X-ray diffractometer (XRD), scanning electron microscope (SEM), vibrating sample magnetometer (VSM) and X-ray photoelectron spectrometer (XPS). The X-ray diffraction (XRD) patterns of samples were recorded on an X-ray diffractometer (Max-2550PC, Rigaku D) with Cu Kα radiation (40 kV, 300 mA) of 0.154 nm wavelength to confirm the crystalline nature of the materials. The scanning electron microscope (FEI Quanta650) was employed to observe the surface morphology of sam­ ples. The total surface areas of the as-prepared samples were determined with the method of Brunauer-Emmett-Teller (BET) using nitrogen adsorption-desorption isotherms on a Micromeritics ASAP 2010C analyzer. The elemental valence state of samples was analyzed by X-ray photoelectron spectrometer (Thermo K-Alphaþ) and the vibrating sample magnetometer (MPMS SQUID VSM) was used to analyze their magnetic property.

3.2. BET analysis The specific surface areas of the as-prepared catalysts were measured by the N2 adsorption-desorption method. The specific surface areas of the cobalt ferrite and Mn-doped counterpart are 5.3 and 12.4m2g 1, respectively. It is apparent that the surface area of Mn-doped counter­ part is larger than that of the cobalt ferrite, which is attributed to the introduction of the manganese oxide in the former. 3.3. SEM analysis The surface morphology of two samples were examined by scanning electron microscope (SEM). From the images shown in Fig. 2a, it could be seen that severe aggregation occurred in the pure cobalt ferrite, which is attributed to the strong magnetic property of the pure cobalt ferrite itself. However, the aggregation notably weakened with the presence of manganese in CoMn0.2Fe1.8O4 (Fig. 2b). This indicates that the presence of manganese effectively suppressed severe agglomeration of particles. The SEM pictures also revealed that the average grain diameter of the pure cobalt ferrite is much bigger than that of CoMn0.2Fe1.8O sample. Such dispersion of particles in CoMn0.2Fe1.8O4 is expected to increase the specific surface area of this sample for greater reactivity. It can be noticed from the figure that the ferrite particles possess somewhat sphere-like morphology [37].

2.4. Catalytic oxidation of OII The catalytic activity of the various materials was monitored by degradation of orange II (OII) dye in solution through chemical catalysis (with added NaHSO3). Each group of experiment was conducted in a beaker containing 100 mL OII dye solution at 25 � 2 � C under magnetic stirring (500 rpm). The concentration of OII dye solution was 60 mg/L. The reactions were initiated by simultaneously adding sodium bisulfite and the sample powders into the beaker containing OII dye whose concentration was 60 mg/L. Details of the process are given below: for the HSO- 3 process, 60 mg catalyst and 200 mg NaHSO3 were added into 100 mL of OII (60 mg/L) aqueous solution in the dark in a 100 mL beaker while stirring with a magnetic stirrer. Finally, 2 mL of mixture solution was withdrawn every 20min and filtered through a Millipore filter (pore size 0.22 μm) to remove the solid particles. After that, the resulting clear liquor was analyzed on a UV–vis spectrophotometer (UV-2450, Shimadzu, Japan) to record the concen­ tration changes of OII at a certain wavelength, λ ¼ 484 nm.

3.4. XPS analysis As the catalytic performance of the composite oxide is closely related to its valence state of elements, the surface chemical composition of CoMn0.2Fe1.8O4 sample was further analyzed by XPS measurement to test valence states of surface elements and components. Fig. 3 shows the XPS spectra of the CoMn0.2Fe1.8O4 sample. Three main peaks at binding energies around 710.7ev, 713.3ev, 724.0ev could be observed from Fig. 3a and they are assigned to Fe2p3/2 and Fe2p1/2 after deconvolu­ tion, confirming the existence of Fe3þ in the CoMn0.2Fe1.8O4 sample. Moreover, Fig. 3b displays the Mn2p XPS spectrum of the as-obtained sample. As can be seen, two predominant characteristic peaks at 642.0ev and 653.4ev belong to Mn2p3/2 and Mn2p1/2, respectively and 3

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(440)

(511)

(422)

(400)

CoFe2O4

(222)

(220)

(311)

Materials Chemistry and Physics 240 (2020) 122181

CoFe2O4 (JCPDS 03-0864)

Intensity(a.u.)

CoMn0.2Fe1.8O4

CoMn0.4Fe1.6O4 CoMn0.6Fe1.4O4 CoMn0.8Fe1.2O4 CoMnFeO4 20

30

40

50

60

2 Fig. 1. X-ray diffraction patterns of all samples annealed at 673K.

the values of coercivity decreased with the increase in Mn3þ concen­ tration. The remanence ratio or squareness ratio refers to the ratio of remnant magnetization (Mr) to saturation magnetization (MS). The remanence ratio of pure CoFe2O4 is a little bigger than that of CoMn0.2Fe1.8O4 sample, a characteristic parameter of ferromagnetic materials and an important parameter in their application. In conclusion, the result shows that both these samples show rela­ tively good magnetic properties. As for the CoMn0.2Fe1.8O4 sample, incorporation of Mn into the lattice of cobalt ferrites has not largely changed the magnetic property of the catalysts. Therefore, this sample could be used for magnetic separation of solid from wastewater after catalytic reaction [37,49].

Table 1 The average crystallite size of all samples annealed at 400 � C. x

2T (� )

FWHM (2� Th.)

D�2 (nm)

x

2T (� )

FWHM (2� Th.)

D�2 (nm)

0 0.2 0.4

35.5 35.4 35.4

0.240 0.352 0.318

35 25 27

0.6 0.8 1.0

35.4 35.5 35.6

0.541 0.743 0.726

17 13 13

these are attributed to Mn3þ species. Thus, the XPS data suggests the presence of manganese ions in the CoMn0.2Fe1.8O4 sample and their existence in the form of trivalent manganese. In the case of Co2p spectra (Fig. 3c), it is obvious that the two peaks of Co2p3/2 and the peak of Co2p1/2 are assigned to Co2þ at the binding energies of 780.0ev and 781.2ev and Co3þ at the binding energy of 797.1ev. The peak at 795.0ev is attributed to both Co2þ and Co3þ species affiliated to Co2p1/2 area [50,51].

3.6. Catalytic activity 3.6.1. Chemocatalytic performance testing The catalytic activity of samples with different manganese contents as heterogeneous Fenton-like catalysts was measured by determining the degradation of Orange II with the assistance of NaHSO3. The changes of the dye concentration with time were determined and presented in Fig. 5. It is apparent that the CoMn0.2Fe1.8O4 (x ¼ 0.2) sample presents the best degradation efficiency compared with all the other samples, especially when compared to the pure cobalt ferrite with no Mn. It is also clear from Fig. 5 that the degradation rates of pure cobalt ferrite and CoMn0.2Fe1.8O4 (x ¼ 0.2) are drastically different with the latter per­ forming the best. The reason for this performance of CoMn0.2Fe1.8O4 sample (Fig. 5) could be attributed to the presence of manganese ions, which are located in octahedral sites of the ferrite structure. All the samples containing Mn ions outperformed the cobalt ferrite without Mn ions (Fig. 5) suggesting a dominant role for Mn ions in determining the catalytic activity of the Mn containing ferrites. However, with the gradual increase of manganese content, the removal rate of OII dye wastewater decreased slightly instead of increasing continuously, which may be attributed to the conversion of some of the trivalent manganese ions to divalent manganese ions [40]. It appears that the Mn content of CoMn0.2Fe1.8O4 sample is ideal for the degradation efficiency of OII dye. In general, the removal rate of dye could be calculated by the � equation, C0C0Ct ) � 100% ¼ degradation, where C0 is the initial con­

3.5. Magnetism and magnetic separation properties of the samples The magnetic properties of the synthesized pure CoFe2O4 and CoMn0.2Fe1.8O4 samples were analyzed by room temperature Vibrating Sample Magnetometer (VSM) analysis. Typical magnetic hysteresis loops of samples, in the applied magnetic field sweeping from 20 kOe to þ20 kOe at room temperature, are presented in Fig. 4a and b. The determined magnetic parameters of the two samples are shown in Table 2 [49], which includes the saturation magnetization (MS), rema­ nence magnetization (Mr), coercivity (Hc), and squareness ratio (S ¼ Mr/Ms) [52]. As can be seen from the picture, synthesized samples display a ferromagnetic behavior. It can be seen that both MS (47.1 emu/g) and Mr (27.5 emu/g) of pure CoFe2O4 are larger than Ms (43.1 emu/g) and Mr (22.5 emu/g) of CoMn0.2Fe1.8O4 sample. Generally, Ms is related to the content of magnetic component [36]. Firstly, this decrease in all magnetic parameters may be attributed to the fact that the A-B super exchange interaction decreased due to the replacement of the Fe3þ ions in the octahedral B sites of the ferrite lattice by the lower magnetic Mn3þ ions [28]. Secondly, according to the results of XRD and SEM, since magnetic properties have a positive correlation with particle size, especially for the saturation magnetization (Ms), both the decrease of Ms and Mr might be also caused by the decreasing size [45,53]. In addition,

centration of the dye, and Ct is the concentration of dye at interval time, 4

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Materials Chemistry and Physics 240 (2020) 122181 21000

(a)

20000

Fe2p3/2

19000

3+

Fe

18000

Fe2p1/2

3+

Fe

17000

3+

Fe

Counts/s

16000 15000 14000 13000 12000 11000 10000 9000 8000 700 10000

705

710

720

725

730

735

740

Binding Energy (ev)

(b)

9500

Mn2p3/2 3+

Mn

9000

Counts/s

715

Mn2p1/2 3+

Mn

8500 8000 7500 7000 6500 632 45000

636

640

644

648

656

660

(c) Co2p3/2

42000 39000

2+

Co

Co2p1/2

36000

Counts/s

652

Binding Energy (ev)

33000

2+

2+

3+

Co /Co

Co

3+

Co

30000 27000

Fig. 2. SEM images of pure CoFe2O4 (a) and CoMn0.2Fe1.8O4 (b) sample.

24000

t. Various samples were tested separately together with sodium bisulfite for degrading the OII dye solution. The degradation efficiency of OII dye solution by CoMn0.2Fe1.8O4/HSO- 3 process was about 85.4% after 2 h of magnetic stirring while the removal rate of pure cobalt ferrite together with HSO- 3 process on degradation of OII dye solution was about 42.5% under the same reaction conditions, which proved that CoMn0.2Fe1.8O4 (x ¼ 0.2) was a more effective heterogeneous Fenton-like reaction catalyst. To determine the role of NaHSO3, some more experiments were carried out and the experimental results are shown in Fig. 6. When so­ dium bisulfite existed in the OII dye solution alone, the degradation efficiency approximately was 26% owing to the reduction potential of sodium bisulfite after 2 h of reaction. However, almost no change in concentration of OII dye solution occurred only in the presence of CoMn0.2Fe1.8O4 ferrite catalyst by itself. Thus, it was found that OII dye was degraded only in the presence of sodium bisulfite together with ferrite catalyst. The great improvement by the presence of NaHSO3 with CoMn0.2Fe1.8O4 may be attributed to the activation of sulfite by ferrite catalysts to produce strong oxidizing radicals during this process of chemical catalysis.

21000 770

775

780

785

790

795

800

805

810

Binding Energy (ev)

Fig. 3. XPS analysis of CoMn0.2Fe1.8O4 sample: (a) Fe spectra, (b) Mn spectra and (c) Co spectra.

3.6.2. Proposed chemical catalysis mechanism The mechanism of chemical catalysis property of ferrite catalysts was explored by reactions in the dark with the presence of 200 mg of sodium bisulfite. The decoloration of OII dye solution signified the existence of some active species with strong oxidation properties that participated in this chemical catalytic reaction resulting in degradation. Therefore, it is necessary to investigate and identify these active species in the reaction and decipher the synergistic mechanism of chemical catalysis in het­ erogeneous Fenton-like process. So as to identify the active species, some chemical reagents such as Tert-butyl alcohol (TBA), Ethanol (EtOH), and Pyrophosphoric acid (PP) were introduced into the reaction system. These are well-known as effective free radical scavengers of hydroxyl radical (�OH), sulfate radical (SO⋅- 4) and Mn (III) active 5

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Materials Chemistry and Physics 240 (2020) 122181

60

(a)

Magnetization (emu/g)

40

20

0

-20

-40

-60 -20000

-10000

0

10000

20000

10000

20000

Magnetic field (Oe) 60

(b)

Magnetization (emu/g)

40

20

0

-20

-40

-60 -20000

-10000

0

Magnetic field (Oe) Fig. 4. Magnetic hysteresis loops of (a) CoFe2O4 sample and (b) CoMn0.2Fe1.8O4 sample annealed at 673K.

(TBA), the degradation rate of dye solution clearly decreased compared with no scavengers in solution. This illustrated the existence and role of hydroxyl radicals (�OH) in the degradation of OII dye. However, when ethanol (EtOH) was introduced into this chemical catalysis process, the degradation was decreased and even lower than in the presence of TBA. This leads to the conclusion that a small amount of sulfate radicals existed, which inhibited the reaction, because ethanol could inhibit hydroxyl radicals and sulfate radicals at the same time. It is well known that the Mn (III) complex with pyrophosphoric acid has an absorption peak at 258 nm and therefore, it is a common method to study the role of Mn (III) in various processes. Pyrophosphoric acid (PP) was added into the reaction solution to test and verify the presence of Mn (III) active species. The absorbance was determined by UV–vis spectrophotometer (UV-2450, Shimadzu, Japan) at 258 nm. As can be seen from.

Table 2 Magnetic parameters of CoFe2O4 and CoMn0.2Fe1.8O4 samples. Sample

CoFe2O4 CoMn0.2Fe1.8O4

Saturation magnetization

Remanence

Coercivity

Squareness ratio

MS (emu/g)

Mr (emu/g)

Hc (kOe)

S ¼ Mr/Ms

47.1 43.1

27.5 22.5

3.5 2.5

0.58 0.52

species separately through which the chemical catalysis degradation mechanism of ferrite catalysts could be identified. The CoMn0.2Fe1.8O4 sample was chosen here as a representative of different CoMnxFe2-xO4 (x ¼ 0, 0.2, 0.4, 0.6, 0.8, 1.0) samples for further detailed study. Fig. 7 shows the effect of various free radical inhibitors on degradation of dye wastewater. It can be seen that with the addition of Tert-butyl alcohol 6

­

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Materials Chemistry and Physics 240 (2020) 122181

1.0

CoFe2O4

CoMn0.2Fe1.8O4

CoMn0.8Fe1.2O4

CoMnFeO4

CoMn0.4Fe1.6O4

0.8

CoMn0.6Fe1.4O4

C/C0

0.6

0.4

0.2

0.0 0

10

20

30

40

50

60

70

80

90

100

110

120

Reaction time(min) Fig. 5. The degradation curves of OII by different CoMnxFe2-xO4 (x ¼ 0, 0.2, 0.4, 0.6, 0.8, 1.0) samples in the presence of NaHSO3.

1.0 1.0

no scavenger 0.8

0.8

TBA

(C0-C)/C0 (%)

C/C0

0.6

0.4

0.2

CoMn0.2Fe1.8O4+NaHSO3

0.6

EtOH 0.4

PP

NaHSO3

CoMn0.2Fe1.8O4

0.0 0

10

20

30

40

50

60

70

80

90

100

110

0.2

120

Reaction time(min)

Fig. 6. The degradation curves of OII under different conditions: CoMn0.2 Fe1.8O4 alone, NaHSO3 alone, and the combination of CoMn0.2Fe1.8O4 and NaHSO3.

0.0

Reaction for 120 min Fig. 7. Degradation different conditions.

Fig. 7, the degradation efficiency of OII dye wastewater was greatly suppressed. Therefore, Mn (III) is an important reactive species as it played a key role in heterogeneous Fenton-like reactions. The chemo­ catalytic degradation efficiencies of dye over CoMn0.2Fe1.8O4 sample decreased in the following order: BQ＜TBA＜EtOH＜PP, implying that Mn (III) active radicals played an important role and were the main reactive oxygen species in this process. On the basis of the above results, the possible dye degradation synergistic mechanism is summarized as follows and the schematic illustration is shown in Fig. 8: Fe3þ þ HSO3 →Fe2þ þ ⋅SO3 þ Hþ

(1)

⋅SO3 þ O2 →⋅SO5

(2)

Co2þ þ ⋅SO5 þ H2 O→Co3þ þ ⋅SO4 þ 2OH

(3) 7

of

OII

by

CoMn0.2Fe1.8O4

sample

under

R. Dou et al.

Materials Chemistry and Physics 240 (2020) 122181

HSO3 þ MnO2 →Mn3þ þ OH þ SO24

(13)

2HSO3 þ O2 →2Hþ þ 2SO24

(14)

4. Conclusions In this work, several ferrites samples with different manganese contents were synthesized by sol-gel auto combustion and investigated for OII decomposition with the assistance of NaHSO3. The heteroge­ neous chemical catalysis system accelerated the production of the sul­ fate radicals and the Mn(III) species. The samples containing manganese showed higher degrading capability than that of the pure cobalt ferrite with the addition of NaHSO3. And the CoMn0.2Fe1.8O4/NaHSO3 system showed the optimum catalytic activity. Thus, it could be seen that Mn (III) species were activated by HSO- 3 and played an important role in the process of chemical catalysis. Furthermore, it was found that the asobtained CoFe2O4 nanoparticles exhibited a ferromagnetic behavior with the saturation magnetization and remnant magnetization of 47.1emu/g and 27.5emu/g, respectively at room temperature. Howev­ er, the saturation magnetization and remnant magnetization decreased with the addition of manganese, but the magnetism of CoMn0.2Fe1.8O4 sample has not decreased greatly, which indicated the incorporation of Mn element into the structure of cobalt ferrites and further confirmed by XRD patterns. On the basis of the present results, it can be concluded that ferrite samples could be facilely prepared and used as the hetero­ geneous catalysts activated by HSO- 3 in chemocatalysis reactions through the generation of sulfate radicals and trivalent manganese ions. The solid samples could be separated from solution conveniently with a magnet using an external magnetic field.

Fig. 8. Shematic illustration of synergistic reaction for the degradation of OII in CoMn0.2Fe1.8O4/HSO- 3 system.

Fe3þ þ e →Fe2þ

E∘ ¼ 0:77V

(4)

Co3þ þ e →Co2þ

E∘ ¼ 1:81V

(5)

Fe2þ þ Co3þ →Fe3þ þ Co2þ

E∘ ¼ 1:04V

(6)

MnðIIIÞ þ contaminant→Mn2þ þ products

(7)

Mn2þ þ ⋅SO5 þ H2 O→MnðIIIÞ þ ⋅SO4 þ 2OH

(8)

⋅SO4 þ OH →⋅OH þ SO24

(9)

3þ

The dissolved Fe ions from CoMn0.2Fe1.8O4 sample activated bisulfite (HSO- 3) to produce Fe2þ ions and sulfite radicals (⋅SO3 ) (Eq. (1)). As the reaction was exposed to open air, ⋅SO3 then reacted with oxygen to form ⋅SO5 (Eq.(2)). Meanwhile, Co2þ ions were oxidized to Co3þ by transferring electrons, accompanying with the generation of sulfate radicals (Eq.(3)) [51]. On the other hand, O2 in the solution would supplement the anoxic surface to maintain the activity of the catalyst. The balance between Co2þ and Co3þ was considered to be the key to the high efficiency of catalytic decontamination. According to the standard reduction potential (Eqs. (4) and (5)) [20], the catalysts con­ taining both Fe and Co are advantageous to the regeneration of CO2þ. Fe2þ is thermodynamically favorable to the reduction of Co3þ (Eq.(6)) [20]. It was found that manganese oxidants activated by HSO- 3 may lead to an extremely high rate of oxidation of organic pollutants during the catalytic process. However, results shown in Fig. 5 suggests that the removal rate of organic pollutants in OII dye solution over ferrite cata­ lysts is not proportional to the Mn contents because the degradation decreased slightly instead of continuously increasing with the gradual increase of manganese content. The reason for this phenomenon could be explained by Sun’s study (Sun et al., 2015). At first, the initial manganese oxides were activated by HSO- 3 to form reactive Mn (III) species. Then, the majority of Mn (III) species were converted to Mn2þ via reaction with organic pollutants (Eq.(7)). What’s more, the residual Mn (III) active species were exhausted by two pathways. One is consumed by the disproportionation (Eq.(11)) and another is used up by competing reactions with HSO- 3 (Eq.(12)). In addition, a part of HSO- 3 could react with MnO2 (Eq.(13)) or oxidized by oxygen (Eq.(14)) as the whole reaction process was exposed to air [40,42,43]. Fe2þ þ MnðIIIÞ→Fe3þ þ Mn2þ

(10)

2MnðIIIÞ þ 2H2 O→Mn2þ þ MnO2 þ 4Hþ

(11)

HSO3 þ 2MnðIIIÞ þ H2 O→2Mn2þ þ SO24 þ 3Hþ

(12)

Acknowledgements This work was supported by the National Natural Science Foundation of China (Grant No. 21477009), Natural Science Foundation of Jiangsu Province (SBK2016021419), “333 project” of Jiangsu Province and the Opening Project of Guangxi Key Laboratory of Green Processing of Sugar Resources (No. GXTZY201803). One of us (SK) was supported by the College of Agricultural Sciences under Station Research Project No. PEN04566. References [1] M.S. Khehra, H.S. Saini, D.K. Sharma, B.S. Chadha, S.S. Chimni, Biodegradation of azo dye C.I. Acid Red 88 by an anoxic–aerobic sequential bioreactor, Dyes Pigments 70 (2006) 1–7, https://doi.org/10.1016/j.dyepig.2004.12.021. [2] M. Muruganandham, Photochemical oxidation of reactive azo dye with UV–H2O2 process, Dyes Pigments 62 (2004) 269–275, https://doi.org/10.1016/j. dyepig.2003.12.006. [3] J. Chen, L. Zhu, UV-Fenton discolouration and mineralization of Orange II over hydroxyl-Fe-pillared bentonite, J. Photochem. Photobiol. A Chem. 188 (2007) 56–64, https://doi.org/10.1016/j.jphotochem.2006.11.018. [4] V.K. Gupta, R. Jain, A. Mittal, T.A. Saleh, A. Nayak, S. Agarwal, S. Sikarwar, Photocatalytic degradation of toxic dye amaranth on TiO(2)/UV in aqueous suspensions, Mater Sci Eng C Mater Biol Appl 32 (2012) 12–17, https://doi.org/10.1016/j. msec.2011.08.018. [5] A. Kalam, A.G. Al-Sehemi, M. Assiri, G. Du, T. Ahmad, I. Ahmad, M. Pannipara, Modified solvothermal synthesis of cobalt ferrite (CoFe2O4) magnetic nanoparticles photocatalysts for degradation of methylene blue with H2O2/visible light, Results Phys 8 (2018) 1046–1053, https://doi.org/10.1016/j.rinp.2018.01.045. [6] P. Oancea, V. Meltzer, Photo-Fenton process for the degradation of Tartrazine (E102) in aqueous medium, J. Taiwan Inst. Chem. Eng. 44 (2013) 990–994, https://doi.org/10.1016/j.jtice.2013.03.014. [7] C. Comninellis, A. Kapalka, S. Malato, S.A. Parsons, I. Poulios, D. Mantzavinos, Advanced oxidation processes for water treatment: advances and trends for R&D, J. Chem. Technol. Biotechnol. 83 (2008) 769–776, https://doi.org/10.1002/ jctb.1873. [8] R. Dewil, D. Mantzavinos, I. Poulios, M.A. Rodrigo, New perspectives for advanced oxidation processes, J. Environ. Manag. 195 (2017) 93–99, https://doi.org/ 10.1016/j.jenvman.2017.04.010. [9] S.A. Antony, A Novel One-Pot Combustion Synthesis and Opto-Magnetic Properties of Magnetically Separable Spinel, vol. 28, 2015, pp. 1405–1416, https://doi.org/ 10.1007/s10948-014-2864-x.

8

R. Dou et al.

Materials Chemistry and Physics 240 (2020) 122181

[10] T.A.S.A. Antony, Room temperature ferromagnetism of magnetically recyclable photocatalyst of Cu1 xMnxFe2O4-tio2(0.0�x�0.5), Nanocomposites 28 (2015) 1783–1795, https://doi.org/10.1007/s10948-014-2945-x. [11] G. Padmapriya, A. Manikandan, V. Krishnasamy, S. Kumar, S.A. Antony, Spinel NixZn1-xFe2O4(0.0�x�1.0) Nano-Photocatalysts : Synthesis , Characterization and Photocatalytic Degradation of Methylene Blue Dye, vol. 1119, 2016, pp. 39–47, https://doi.org/10.1016/j.molstruc.2016.04.049. [12] G. Mathubala, A. Manikandan, S.A. Antony, P. Ramar, Photocatalytic Degradation of Methylene Blue Dye and Magneto- Optical Studies of Magnetically Recyclable Spinel NixMn1-xFe2O4(x¼0.0-1.0)nanoparticles, vol. 1113, 2016, pp. 79–87, https://doi.org/10.1016/j.molstruc.2016.02.032. [13] D.M.A. Manikandan, E. Manikandan, S. Vadivel, M. Kumaravel, S. Hariganesh, Photocatalysis: present, past and future, Org. Pollut. Wastewater I. 24 (2018), https://doi.org/10.1016/j.cattod.2012.04.003. [14] D.B. Miklos, C. Remy, M. Jekel, K.G. Linden, J.E. Drewes, U. Hubner, Evaluation of advanced oxidation processes for water and wastewater treatment - a critical review, Water Res. 139 (2018) 118–131, https://doi.org/10.1016/j. watres.2018.03.042. [15] A. Akbari, M. Amini, A. Tarassoli, B. Eftekhari-Sis, N. Ghasemian, E. Jabbari, Transition metal oxide nanoparticles as efficient catalysts in oxidation reactions, Nano - Struct. Nano - Objects 14 (2018) 19–48, https://doi.org/10.1016/j. nanoso.2018.01.006. [16] N. Tsolekile, S. Parani, M.C. Matoetoe, S.P. Songca, O.S. Oluwafemi, Evolution of ternary I–III–VI QDs: synthesis, characterization and application, Nano - Struct. Nano - Objects 12 (2017) 46–56, https://doi.org/10.1016/j.nanoso.2017.08.012. [17] H.Y. He, J. Lu, Highly photocatalytic activities of magnetically separable reduced graphene oxide-CoFe2O4 hybrid nanostructures in dye photodegradation, Separ. Purif. Technol. 172 (2017) 374–381, https://doi.org/10.1016/j. seppur.2016.08.040. [18] L. Gan, L. Xu, K. Qian, Preparation of core-shell structured CoFe2O4 incorporated Ag3PO4 nanocomposites for photocatalytic degradation of organic dyes, Mater. Des. 109 (2016) 354–360, https://doi.org/10.1016/j.matdes.2016.07.043. [19] A. Hassani, G. Celikdag, P. Eghbali, M. Sevim, S. Karaca, O. Metin, Heterogeneous sono-Fenton-like process using magnetic cobalt ferrite-reduced graphene oxide (CoFe2O4-rGO) nanocomposite for the removal of organic dyes from aqueous solution, Ultrason. Sonochem. 40 (2018) 841–852, https://doi.org/10.1016/j. ultsonch.2017.08.026. [20] P. Hu, M. Long, Cobalt-catalyzed sulfate radical-based advanced oxidation: a review on heterogeneous catalysts and applications, Appl. Catal. B Environ. 181 (2016) 103–117, https://doi.org/10.1016/j.apcatb.2015.07.024. [21] D. Sun, Y. Han, S. Gao, X. Zhang, Surface modification of titania-coated cobalt ferrite magnetic photocatalyst by cold plasma, Surf. Coat. Technol. 228 (2013) S516–S519, https://doi.org/10.1016/j.surfcoat.2012.04.072. [22] S. Li, Z. Jia, Z. Li, J. Liu, Effective removal of methylene blue from aqueous solution by MnO2-modified porous CoFe2O4 microrods, Desalin. Water Treat. 57 (2015) 19301–19310, https://doi.org/10.1080/19443994.2015.1099473. [23] D. Das, D. Banerjee, M. Mondal, A. Shett, B. Das, N.S. Das, U.K. Ghorai, K. K. Chattopadhyay, Nickel doped graphitic carbon nitride nanosheets and its application for dye degradation by chemical catalysis, Mater. Res. Bull. 101 (2018) 291–304, https://doi.org/10.1016/j.materresbull.2018.02.004. [24] I.J.C. Lynda, M.D.A. Dinesh, A.M.S.K. Jaganathan, A. Baykal, S.A. Antony, Enhanced magneto-optical and photocatalytic properties of ferromagnetic Mg1 yNiyFe2O4(0.0�y�1.0) spinel, Nano-ferrites 31 (2018) 3637–3647, https:// doi.org/10.1007/s10948-018-4623-x. [25] A. Gholizadeh, Journal of Magnetism and Magnetic Materials A comparative study of the physical properties of Cu-Zn ferrites annealed under different atmospheres and temperatures : magnetic enhancement of Cu0.5Zn0.5Fe2O4 nanoparticles by a reducing atmosphere, J. Magn. Magn. Mater. 452 (2018) 389–397, https://doi. org/10.1016/j.jmmm.2017.12.109. [26] S. Jayasree, S.A. Antony, Magnetically Recyclable Spinel MnxNi1 xFe2O4 Morphological and Opto-Magnetic Properties, vol. 29, 2016, pp. 477–486, https:// doi.org/10.1007/s10948-015-3312-2. [27] X. Guo, H. Li, S. Zhao, Fast degradation of Acid Orange II by bicarbonate-activated hydrogen peroxide with a magnetic S-modified CoFe2O4 catalyst, J. Taiwan Inst. Chem. Eng. 55 (2015) 90–100, https://doi.org/10.1016/j.jtice.2015.03.039. [28] A. Goyal, S. Bansal, B. Chudasama, K.B. Tikoo, V. Kumar, S. Singhal, Augmenting the catalytic performance of spinel nanoferrites (CoFe2O4 and NiFe2O4) via incorporation of Al into the lattice, New J. Chem. 41 (2017) 8320–8332, https:// doi.org/10.1039/c7nj01486d. [29] M.N. Moura, R. V Barrada, J.R. Almeida, T.F.M. Moreira, M.A. Schettino, J.C. C. Freitas, S.A.D. Ferreira, M.F.F. Lelis, M. Freitas, Synthesis, characterization and photocatalytic properties of nanostructured CoFe2O4 recycled from spent Li-ion batteries, Chemosphere 182 (2017) 339–347, https://doi.org/10.1016/j. chemosphere.2017.05.036. [30] A. Sengupta, R. Rao, D. Bahadur, Zn2þ–Silica modified cobalt ferrite magnetic nanostructured composite for efficient adsorption of cationic pollutants from water, ACS Sustain. Chem. Eng. 5 (2017) 1280–1286, https://doi.org/10.1021/ acssuschemeng.6b01186. [31] J. Saffari, D. Ghanbari, N. Mir, K. Khandan-Barani, Sonochemical synthesis of CoFe2O4 nanoparticles and their application in magnetic polystyrene nanocomposites, J. Ind. Eng. Chem. 20 (2014) 4119–4123, https://doi.org/ 10.1016/j.jiec.2014.01.010.

[32] A.M.M. Durka, S.A. Antony, A novel synthesis , structural , morphological , and opto-magnetic characterizations of magnetically separable spinel CoxMn1 xFe2O4(0�x�1), Nano-catalysts 27 (2014) 2841–2857, https://doi.org/ 10.1007/s10948-014-2771-1. [33] E.H.A. Manikandan, P.K.S. Arul, A.B.R. Venkatraman, A Novel Synthesis of Zn2þdoped CoFe2O4 Spinel Nanoparticles : Structural , Morphological , Opto-magnetic and Catalytic Properties, Vol. 28, 2015, pp. 2539–2552, https://doi.org/10.1007/ s10948-015-3054-1. [34] A.M.M. Durka, S.A. Antony, Hibiscus rosa - sinensis leaf extracted green methods , magneto-optical and catalytic properties of spinel CuFe2O4 nano- and microstructures, J. Inorg. Organomet. Polym. Mater. 25 (2015) 1019–1031, https://doi.org/10.1007/s10904-015-0203-8. [35] A. Manikandan, R. Sridhar, S.A. Antony, S. Ramakrishna, A simple aloe vera plantextracted microwave and conventional combustion synthesis : morphological , optical , magnetic and catalytic properties of CoFe2O4 nanostructures, J. Mol. Struct. 1076 (2014) 188–200, https://doi.org/10.1016/j.molstruc.2014.07.054. [36] F. Siadatnasab, S. Farhadi, A. Khataee, Sonocatalytic performance of magnetically separable CuS/CoFe2O4 nanohybrid for efficient degradation of organic dyes, Ultrason. Sonochem. 44 (2018) 359–367, https://doi.org/10.1016/j. ultsonch.2018.02.051. [37] Z. Li, J. Ai, M. Ge, A facile approach assembled magnetic CoFe2O4/AgBr composite for dye degradation under visible light, J. Environ. Chem. Eng. 5 (2017) 1394–1403, https://doi.org/10.1016/j.jece.2017.02.024. [38] S. Farhadi, F. Siadatnasab, A. Khataee, Ultrasound-assisted degradation of organic dyes over magnetic CoFe2O4@ZnS core-shell nanocomposite, Ultrason. Sonochem. 37 (2017) 298–309, https://doi.org/10.1016/j.ultsonch.2017.01.019. [39] Y. Wang, L. Zhou, X. Duan, H. Sun, E.L. Tin, W. Jin, S. Wang, Photochemical degradation of phenol solutions on Co3O4 nanorods with sulfate radicals, Catal. Today 258 (2015) 576–584, https://doi.org/10.1016/j.cattod.2014.12.020. [40] B. Sun, X. Guan, J. Fang, P.G. Tratnyek, Activation of manganese oxidants with bisulfite for enhanced oxidation of organic contaminants: the involvement of Mn (III), Environ. Sci. Technol. 49 (2015) 12414–12421, https://doi.org/10.1021/acs. est.5b03111. [41] E. Hu, Y. Zhang, S. Wu, J. Wu, L. Liang, F. He, Role of dissolved Mn(III) in transformation of organic contaminants: non-oxidative versus oxidative mechanisms, Water Res. 111 (2017) 234–243, https://doi.org/10.1016/j. watres.2017.01.013. [42] B. Sun, H. Dong, D. He, D. Rao, X. Guan, Modeling the kinetics of contaminants oxidation and the generation of manganese(III) in the permanganate/bisulfite process, Environ. Sci. Technol. 50 (2016) 1473–1482, https://doi.org/10.1021/ acs.est.5b05207. [43] S. Rayati, Z. Sheybanifard, Manganese(III) porphyrin supported onto multi-walled carbon nanotubes for heterogeneous oxidation of synthetic textile dyes and 2,6dimethylphenol by tert-butyl hydroperoxide, Compt. Rendus Chem. 19 (2016) 371–380, https://doi.org/10.1016/j.crci.2015.12.001. [44] D. Huang, J. Ma, C. Fan, K. Wang, W. Zhao, M. Peng, S. Komarneni, Co-Mn-Fe complex oxide catalysts from layered double hydroxides for decomposition of methylene blue: role of Mn, Appl. Clay Sci. 152 (2018) 230–238, https://doi.org/ 10.1016/j.clay.2017.11.018. [45] T.-D. Dang, A.N. Banerjee, Q.-T. Tran, S. Roy, Fast degradation of dyes in water using manganese-oxide-coated diatomite for environmental remediation, J. Phys. Chem. Solids 98 (2016) 50–58, https://doi.org/10.1016/j.jpcs.2016.06.006. [46] G.M. Ucoski, G.S. Machado, G. de F. Silva, F.S. Nunes, F. Wypych, S. Nakagaki, Heterogeneous oxidation of the dye Brilliant Green with H2O2 catalyzed by supported manganese porphyrins, J. Mol. Catal. A Chem. 408 (2015) 123–131, https://doi.org/10.1016/j.molcata.2015.07.020. [47] A.D. Bokare, W. Choi, Review of iron-free Fenton-like systems for activating H2O2 in advanced oxidation processes, J. Hazard Mater. 275 (2014) 121–135, https:// doi.org/10.1016/j.jhazmat.2014.04.054. [48] S. Jauhar, S. Singhal, M. Dhiman, Manganese substituted cobalt ferrites as efficient catalysts for H2O2 assisted degradation of cationic and anionic dyes: their synthesis and characterization, Appl. Catal. Gen. 486 (2014) 210–218, https://doi.org/ 10.1016/j.apcata.2014.08.020. [49] B.J. Rani, M. Ravina, B. Saravanakumar, G. Ravi, V. Ganesh, S. Ravichandran, R. Yuvakkumar, Ferrimagnetism in cobalt ferrite (CoFe2O4) nanoparticles, Nano Struct. Nano - Objects 14 (2018) 84–91, https://doi.org/10.1016/j. nanoso.2018.01.012. [50] Y. Diao, Z. Yan, M. Guo, X. Wang, Magnetic multi-metal co-doped magnesium ferrite nanoparticles: an efficient visible light-assisted heterogeneous Fenton-like catalyst synthesized from saprolite laterite ore, J. Hazard Mater. 344 (2018) 829–838, https://doi.org/10.1016/j.jhazmat.2017.11.029. [51] C.H. Wu, J.T. Lin, K.A. Lin, Magnetic cobaltic nanoparticle-anchored carbon nanocomposite derived from cobalt-dipicolinic acid coordination polymer: an enhanced catalyst for environmental oxidative and reductive reactions, J. Colloid Interface Sci. 517 (2018) 124–133, https://doi.org/10.1016/j.jcis.2017.12.076. [52] L. Kumar, P. Kumar, V. Kuncser, S. Greculeasa, B. Sahoo, M. Kar, Strain induced magnetism and superexchange interaction in Cr substituted nanocrystalline cobalt ferrite, Mater. Chem. Phys. 211 (2018) 54–64, https://doi.org/10.1016/j. matchemphys.2018.02.008. [53] F. Huixia, C. Baiyi, Z. Deyi, Z. Jianqiang, T. Lin, Preparation and characterization of the cobalt ferrite nano-particles by reverse coprecipitation, J. Magn. Magn. Mater. 356 (2014) 68–72, https://doi.org/10.1016/j.jmmm.2013.12.033.

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