Synthesis of a MnO2/Fe3O4/diatomite nanocomposite as an efficient heterogeneous Fenton-like catalyst for methylene blue degradation

Heterogeneous Fenton-like catalysts with the activation of peroxymonosulfate (PMS), which offer the advantages of fast reaction rate, wide functional pH range and cost efficiency, have attracted great interest in wastewater treatment. In this study, a novel magnetic MnO2/Fe3O4/diatomite nanocomposite is synthesized and then used as heterogeneous Fenton-like catalyst to degrade the organic pollutant methylene blue (MB) with the activation of PMS. The characterization results show that the Fe3O4 nanoparticles and nanoflower-like MnO2 are evenly distributed layer-by-layer on the surface of diatomite, which can be readily magnetically separated from the solution. The as-prepared catalyst, compared with other Fenton-like catalysts, shows a superb MB degradation rate of nearly 100% in 45 min in the pH range of 4 to 8 and temperature range of 25 to 55 °C. Moreover, the nanocomposite shows a good mineralization rate of about 60% in 60 min and great recyclability with a recycle efficiency of 86.78% after five runs for MB. The probable mechanism of this catalytic system is also proposed as a synergistic effect between MnO2 and Fe3O4.


Introduction
Organic contaminants are widely distributed in water and soil due to the excessive emissions of industrial processes, which causes a great threat to the ecosystem as well as to human health [1][2][3]. Most of the organic pollutants are toxic and can not be degraded spontaneously, thus various methods focusing on the removal of organic pollutant, including adsorption, photocatalysis and advanced oxidation processes, have been extensively studied over the past decades [2,4,5].
Among these methods, advanced oxidation processes (AOP) are considered as the most promising method because of the high removal efficiency and wide application scopes [6,7]. Ironbased homogeneous and heterogeneous Fenton or Fenton-like catalysts with the activation of H 2 O 2 can effectively generate hydroxyl radicals (•OH, the main reactive species for the degradation of organic contaminants) and show the advantages of a fast reaction rate and being cost-efficient [8]. However, the practical application is limited through issues such as the narrow acidic pH range, non-selective oxidation, H 2 O 2 storage and sludge removal [9,10]. To overcome the limitation, the heterogeneous Fenton-like system of MnO 2 -peroxymonosulfate (MnO 2 -PMS) is proposed. As alternatives to H 2 O 2 and hydroxyl radicals (•OH), peroxymonosulfate (PMS) and the sulfate radical (•SO 4 − ) exhibit a wider functional pH range, convenience in storage and higher oxidation selectivity. Also, compared to iron-based catalysts, MnO 2 shows good catalytic performance because of the Mn(III)/Mn(IV) redox loop and much less sludge formation due to the neutral functional pH value [11,12].
Although, the MnO 2 -PMS system shows good prospect in the treatment of organic pollutants, modifications focusing on the catalytic performances and the recycling of the catalyst still need to be optimized. Previous researches have proved that structure and morphology of catalysts with the same chemical composition can significantly affect the catalytic activity of the catalysts [9,13]. Nano-scaled metal catalysts tend to agglomerate and are difficult to be dispersed because of the large surface energy, thus active sites on the surface of catalysts are covered, which causes huge impact on the practical catalytic performance. In order to solve the problem, loading the catalysts on appropriate carriers to form a core-shell composite is the most effective strategy [14]. Through this strategy, catalysts with controllable particle size, better dispersion and decreased agglomeration are easy to be synthesized and are expected to significantly increase the contact between active sites and contaminants. In addition, with the support of carriers, the prepared composites possess a much higher specific surface area, which further enhances the adsorption and the consecutive degradation performance of the catalysts.
Diatomite, a natural porous mineral originating from the fossilization of diatom shells, is composed of amorphous silica. Hence, diatomite offers a high specific surface area, a mesoporous structure and superior physicochemical stabilities. Due to the above advantages, together with the abundant reserves, low price and eco-friendly nature, diatomite is a promising support material for catalyst nanoparticles in practical applications [15][16][17]. In addition to stability, separability is an extremely crucial factor for the recycling of catalysts. Magnetic separation is very attractive in the field of wastewater treatment as it provides a convenient and cost-effective way for catalyst collection. Fe 3 O 4 is a very popular magnetic material being systematically researched in various aspects from drug delivery to catalyst separation [18]. As a typical heterogeneous Fentonlike catalyst in H 2 O 2 activation systems, it is reported that when coupling with MnO 2 , the Fe 3 O 4 -MnO 2 pair presents a synergistic effect, which can significantly enhance the catalytic performance compared to the single-component catalysts [10]. In addition, the synergistic effect of the composite is deeply influenced by the contact area between the two phases, thus the core-shell structure of catalysts can dramatically magnify the contact area and further strengthen the synergistic effect from the structural aspect.
Herein, we propose a novel nanocomposite of magnetic MnO 2 / Fe 3 O 4 /diatomite as an efficient heterogeneous Fenton-like catalyst for PMS activation. In this study, the catalytic activity of the nanocomposite is evaluated systematically by decomposing methylene blue (MB) as the target pollutant, because MB is a typical organic dye with well-known toxicity and a threat to water environments [19]. The structural characterization and recyclability of the nanocomposite are also investigated.  Figure S1, Supporting Information File 1) and triethylene glycol (TREG) were obtained from Sinopharm Chemical Reagent Company (Shanghai, China). HCl and NaOH were purchased from Tianjin Guangfu Fine Chemical Institute (Tianjin, China). All chemicals used in the work were of analytical grade and used without any further purification. Raw diatomite was obtained from Linjiang City, Jilin Province, China. Deionized water was used throughout this study.

Synthesis of catalyst
Raw diatomite was purified by acid-washing in 2 M HCl solution at 75 °C for 4 h according to [20]. The Fe 3 O 4 /diatomite composite was prepared through thermal decomposition and in situ loading [3]. Typically, 0.4 g of purified diatomite was first put into 60 mL TREG under magnetic stirring for 30 min. Then 600 mg of Fe(acac) 3

Characterization methods
The X-ray powder diffraction (XRD) patterns of the samples were measured with a Bruker AXS D8 advance X-ray powder diffractometer utilizing a Cu Kα source (λ = 0.15418 nm). The functional groups of materials were characterized by Fourier transform infrared spectroscopy (FTIR) using a Nicolet Nexus 670 spectrometer. The morphology of samples was observed with a TESCAN MIRA3 LMU scanning electron microscope (SEM) and a JEOL JEM-1200EX transmission electron microscope (TEM). The X-ray photoelectron spectroscopy (XPS) measurements were carried out in an ultra-high vacuum VG ESCALAB250Xi electron spectrometer. The magnetic properties of the samples were performed by a Lakeshore 7404 vibrating sample magnetometer (VSM). The BET measurements of the samples were collected by a Micromeritics ASAP 2020 surface area and porosimetry system with N 2 at 77 K. The total organic carbon (TOC) of the catalytic regeneration system was measured with a Shimadzu TOC-L analyzer.

Evaluation of catalytic performance
The catalytic MB removal experiments were carried out in a 250 mL flask at constant temperature in a water bath. Typically, the catalyst was mixed with 200 mL of MB solution (10 mg/L) and stirred for 30 min to achieve adsorption equilibrium. Then, PMS (0.06 g) was added to the solution to activate the catalytic reaction. The initial pH values of the solution were adjusted by dilute NaOH and HCl solutions (1 mol/L). The water samples were taken with a syringe with filter membrane (0.45 μm) at predetermined times and analyzed by a UV-2600 spectrophotometer at the absorption wavelength of 664 nm. The effects of reaction temperature, pH value and PMS dosage on the catalytic reaction were also researched under same experimental conditions. Each point in all plots is the average value from three replicate experiments.

Recyclability test
The used catalysts after MB removal were magnetically collected for the next cycle of catalytic reaction, the experimental procedures and parameters were exactly same as those of the above removal experiments. The recycle efficiency (E R , %) was evaluated by comparing the removal rate performances of fresh and used catalyst.

Results and Discussion
Structural and morphological characterizations Figure 1 shows the XRD patterns of purified diatomite,       [27].  diatomite. Meanwhile, the plate-like morphology of diatomite is well preserved after the two-step loading process, revealing the negligible influence of the hydrothermal process on the structure of diatomite. In addition, Figure 3f and Figure 3g present SEM images of the MnO 2 /Fe 3 O 4 /diatomite composite and the corresponding energy-dispersive X-ray (EDX) spectrum and elemental mappings for O, Si, Fe and Mn elements, respectively. It can be found that all the elements are evenly distributed on the diatomite, and the calculated atomic fractions of Fe and Mn are 3.63% and 10.96%, respectively. All these results confirm the successful loading of of iron oxide and manganese oxide in the two-step procedure.
To further characterize the morphologies and structures of Fe 3 O 4 /diatomite and MnO 2 /Fe 3 O 4 /diatomite, transmission electron microscopy (TEM) and high-resolution transmission electron microscope (HRTEM) analyses were also performed. As shown in Figure 4a, the Fe 3 O 4 /diatomite exhibits a morphology similar to that seen in the SEM images, a layer of nanoparticles with diameter of about 10 nm are highly dispersed on the surface of diatomite, which demonstrates a porous polycrystalline structure composed of many interconnected nanoparticles [28].
The marked lattice fringe spacing of 0.28 nm in the HRTEM images (inset) is corresponding to the (331) planes of cubic magnetite [29]. Figure 4b shows the TEM images of MnO 2 / Fe 3 O 4 /diatomite, the nanoparticles on the surface are fully covered by a layer of rough 3D structured material. As seen in the magnified image (Figure 4c), a flower-like or urchin-like structure of the outer MnO 2 shell can be easily observed. The crystal structure of the outer shell is analyzed by using HRTEM, as shown in Figure 4d. As a whole, the chaotic and unclear lattice fringes in the image illustrate the poor crystallinity of MnO 2 , which is consistent with the XRD result. In some small parts of the area, however, typical spacings of 0.21 and 0.26 nm are measured, which corresponds to the (202) and (301) planes of MnO 2 [30]. The morphology of pure MnO 2 particles was also researched ( Figure      served. These are the typical characteristics of the Fe 3 O 4 structure [32]. The Mn 2p region (Figure 6c) exhibits two individual peaks at 653.9 and 642.2 eV, attributed to the Mn 2p 1/2 and Mn 2p 3/2 binding energies, respectively. As a result, the spin energy separation of Mn 2p peaks can be calculated as 11.7 eV, which is well in agreement with reports for MnO 2 [33]. In Figure 6d, the O 1s scan can be fitted into three symmetric peaks located at 532.39, 530.49 and 529.54 eV. Among them, the peaks at 532.9 and 530.49 eV are assigned to the oxygen in SiO 2 and Fe 3 O 4 , and the peak located at 529.54 eV is ascribed to lattice oxygen (Mn-O-Mn bond) in MnO 2 [34]. Therefore, the surface chemical composition results obtained from the XPS analysis ensure the formation of Fe 3 O 4 and MnO 2 , which further confirms the observation from the previous structural and morphological characterization. Figure 7 shows the N 2 adsorption-desorption isotherms and corresponding pore-size distributions of the samples. All three samples show a type-IV isotherm with type-H3 hysteresis loops (at about 0.50-0.99), which demonstrates a mesoporous structure of diatomite and as-prepared composites [35].

Catalytic activity
To demonstrate the catalytic activity of the as-synthesized catalysts toward PMS activation, MB was selected as the target contaminant for degradation. Figure 8 shows  Figure 8a, all catalyst systems display negligible adsorption removal in the initial adsorption process, and less than 10% MB was removed using the different systems. Among all materials, the MnO 2 /Fe 3 O 4 / diatomite exhibits the best adsorption efficiency compared to others, which may be ascribed to its higher surface area and pore structure as shown in the BET results. In the following so-called catalysis reaction without PMS, low removal efficiencies are observed in different mono-catalyst systems over the course of 60 min, revealing that the MB can not be degraded by these Fenton-like catalysts without the activation of PMS.  The functional pH range is of vital importance for the practical application of various catalytic systems, because it affects surface charge, functional groups and relative adsorption behavior of the dispersed catalysts. The application of homogeneous and heterogeneous iron-based Fenton or Fenton-like catalysts is limited to some extent by the narrow acidic pH range of the system itself. MnO 2 works in a much wider functional pH range compared to other catalysts [36]. In this study, the influence of the pH value on the MB degradation performance of MnO 2 / Fe 3 O 4 /diatomite was investigated and the results are shown in Figure 9a. The degradation rates of MB under different pH conditions are largely the same, which demonstrates that MnO 2 / Fe 3 O 4 /diatomite is a pH-independent catalyst possessing a wide functional pH range for the activation of PMS. This phenomenon can be explained by the stability of PMS under different pH conditions in aqueous solution [37].
The influence of the reaction temperature on the degradation efficiency of the MnO 2 /Fe 3 O 4 /diatomite-PMS system is shown in Figure 9b. MB is totally degraded after 45, 17, 20 and 15 min at temperatures of 25, 35, 45 and 55 °C, respectively. Generally, a higher temperature will accelerate the removal rate of MB, indicating an endothermic nature of this heterogeneous Fentonlike process. The enitial concentration of PMS plays an essential role in the degradation process. Figure 9c shows the MB degradation results as a function of the initial PMS concentration in the MnO 2 /Fe 3 O 4 /diatomite-PMS system. The time required to completely remove MB decreases from 65 to 20 min along the concentration of PMS increasing from 0.15 to 0.60 g/L, which is probably due to the higher PMS concentration promoting the generation of active radicals (•SO 4 − ). Figure 9d shows the TOC reduction as a function of the reaction time. Less than 10% of TOC is removed in the initial adsorption process. Then, the quantity of TOC decreases significantly in the subsequent 15

Recyclability of the catalyst
The recyclability of the MnO 2 /Fe 3 O 4 /diatomite catalyst was examined by cyclically reusing the material under the same experimental conditions and the results are shown in Figure 10. The catalyst maintains a high recyclability of 86.78% after five cycles. The slight reduction of recyclability is mainly ascribed to the mass loss in long-term tests [38,39]. The outstanding recyclability of the catalysts can be explained by two aspects: (i) The core-shell structure of MnO 2 /Fe 3 O 4 /diatomite ensures the physical stability under the mechanical stirring. (ii) The  catalyst in this heterogeneous Fenton-like system is easier to stabilize at the moderate catalytic conditions (without heating or acid soaking).

Probable reaction mechanism
Based on the core-shell structure of the MnO 2 /Fe 3 O 4 /diatomite, a plausible catalytic mechanism is proposed and schematically illustrated in Figure 11, which considers the different shells leading to different catalytic reactions during the MB degradation. The combined catalytic system is divided into two sections, namely, the outer MnO 2 -PMS system and the inner (1) (3)  /diatomite-PMS catalytic system was almost pH-independent over a wide range. Benefiting from the core-shell struc-ture and the neutral experimental conditions, the systems showed an excellent recyclability of 86.78% after five use cycles. A plausible mechanism of the catalytic reaction for the activation of PMS was proposed takign into account the high specific area of the core-shell nanocomposite and the synergistic effect between MnO 2 and Fe 3 O 4 . All these results reveal the MnO 2 /Fe 3 O 4 /diatomite composite is an effective, environmentally friendly and inexpensive Fenton-like catalyst for the removal of organic pollutants.

Supporting Information
Supporting Information File 1 Additional experimental data.