Reductant free green synthesis of magnetically recyclable MnFe2O4@SiO2-Ag coreshell nanocatalyst for the direct reduction of organic dye pollutants

The present paper describes in situ green immobilization of silver nanoparticles on MnFe2O4@SiO2 nanospheres using Epilobium parviflorum (EP) without using any other toxic chemicals and reducing or stabilizing agents. The morphology, composition, and magnetic properties of the resulting MnFe2O4@SiO2-Ag core-shell nanocatalyst were characterized by scanning electron microscope (SEM), transmission electron microscopy (TEM), thermogravimetric analysis (TGA), X-ray diffraction (XRD), vibrating sample magnetometer (VSM), and attenuated total reflectance-Fourier transform infrared spectroscopy (ATR-FTIR). The catalytic performance of the synthesized MnFe2O4@SiO2-Ag was employed on the organic pollutants dyes such as rhodamine B (RhB) and methylene blue (MB). The results revealed significant reduction performances for the MB (116.28 s−1 g−1) and RhB (27.12 s−1 g−1) over the existing literature. Furthermore, the MnFe2O4@SiO2-Ag exhibited high stability for the completion of the reduction of RhB between the reaction times of 13.1 (first) and 19.8 min (final) with the 100% decolorization efficiency even after several cycles with an excellent magnetic separation. Overall, this work demonstrates a simple and practical green synthetic route for the preparation of magnetic recyclable core-shell nanocatalyst that can be a good candidate for the treatment of organic contaminants in wastewater adhering to green chemistry principles for the environmental pollution concerns.


Introduction
Organic dye contaminants have become an acute concern and problem in the environment due to their release or discharge into the environment as arising intensive activities of different chemical industries, including food, textile, cosmetics, plastics, paint, and indeed domestic waste [1,2].Most of these waste dyestuffs or effluents are toxic, carcinogenic, and even mutagenic, as well as posing serious risks to living organisms, especially to human health [3][4][5][6].Although diverse techniques involving adsorption, precipitation, photocatalytic degradation, and advanced oxidation processes (AOPs) have been introduced to treat organic dye pollutants up to now, they could be most frequently timeconsuming, impractical, and expensive [7][8][9].For these reasons, there has been still a growing interest to develop methods or strategies for the removal of dye pollutants before their release from various industries into the environment.
Based on this purpose, metal nanoparticles with higher Fermi potential that enable them to catalyze electron transfer reaction with lowered reduction potential have attracted great interest in reducing organic dye pollutants [10,11].In particular, silver nanoparticles (AgNPs) among several noble metal-based catalysts containing gold, palladium, and platinum have gained significant research and application for a variety of catalytic reactions, some of which are reduction of organic compounds, selective oxidation, and NOx reduction, because of their unique properties, including low-cost, high optical, catalytic, and antibacterial properties [12,13].In this point, not only the use of reducing agents in the production of AgNPs might lead to environmental toxicity and biohazards but also because the industry promotes catalytic processes with ease operation, employ and recyclability, the use of green synthetic roots and environment in preparing a heterogeneous catalyst remains among the main research principles [14,15].For this reason, magnetic nanoparticles (MNPs) have received much interest in the heterogeneous catalyst as a useful support owing to their ease of separation from the reaction media using an external magnetic field compared to filtration and centrifugation processes, high dispersion, and recyclability [16][17][18].Therefore, MNPs can improve the separation and recovery of AgNPs from the reaction media.
Among different coating materials, silica as a protective shell can be facilitated to maintain the stability of MNPs and prevent their interaction with complex matrices with the desired stability [19,20].In addition, plant-mediated synthesis of nanoparticles has attracted great attention depending on its several advantages, comprising non-toxic, safe, cost effective, especially being environmentally friendly [21][22][23][24].Thus, the aforementioned environmental concerns can be overcome in the fast and economic production of magnetic core-shell nanoparticles with more stable properties via the immobilization of silver nanoparticles on silica coated MNPs by using plant extracts as reducing agents.
In our previous study, we have introduced Epilobium parviflorum (EP) extract as a novel reducing, stabilizing agent, and coating material for the preparation of Ag immobilized nanocatalyst using manganese ferrite magnetic core as an alternative to commonly used Fe3O4 core supports [25].Apart from this study, we addressed herein the green and successful preparation technique for the synthesis of highly stable MnFe2O4@SiO2-Ag core-shell magnetically recyclable nanocatalyst using EP extract for the first time.In this perspective, the current research has come to a focal point as the used EP extracts serve on the basis of the green synthesis of heterogeneous catalyst without using any additional chemicals, stabilizer, surfactant, toxic or extra reducing agents, and become inspiring for the future studied dealing with more environmental concerns.The MnFe2O4@SiO2-Ag has also been investigated as a useful catalyst in the reduction of some organic pollutant dyes.

Instrumentation
A Pan Analytical Empyrean diffractometer with a PixCell3D detector was used for the Powder X-ray diffraction pattern (XRD) measurements.Attenuated total reflectance-Fourier transform infrared spectroscopy (ATR-FTIR) spectra were collected between the wavelength range of 600-4000 cm -1 via PerkinElmer Spectrum 100 FT-IR Spectrometer.The water content of the samples was detected using TA Instrument (New Castle, DE) thermal analysis system with a heating program of 10 °C min -1 under air flow (100 mL min -1 ) by thermogravimetry.The morphological analyses were carried out using an electron microscope (SEM, ZEISS Sigma 300) integrated with energy-dispersive X-ray spectroscopy (EDS), and high contrast transmission electron microscope (TEM, Hitachi HT7700 with EXALENS).UV-Vis measurements were carried out via a Carry 60 UV-Vis spectrometer, (Agilent, USA) with a 1 cm quartz cell.

Synthesis of MnFe2O4 nanoparticles
A mixture of Iron (III) chloride hexahydrate (FeCl3.6H2O)(6.5 g) and manganese (II) chloride tetrahydrate (MnCl2.4H2O)(4.0 g), and 0.2 g polyvinylpyrrolidone (PVP) in 80 mL of de-ionized water (100 mL) was stirred vigorously for 3 h.Afterwards, 20 mL of 0.1M NH4OH solution was slowly added and irradiated under microwave for 20 min at 100 °C.After cooling the reaction mixture to room temperature, the black precipitate of MnFe2O4 nanoparticles were separated magnetically and washed three times with mixture of ethanol-deionized water [25].

Synthesis of MnFe2O4@SiO2 core-shell nanoparticles
Synthesis of MnFe2O4@SiO2 core-shell nanoparticles were simply adopted from the literature [20].In summary, 1.0 g of MnFe2O4 nanoparticles were added to a solution of 5.0 mL of NH4OH (25%) and 200.0 mL of ethanol and dispersed well.2.5 mL of tetraethyl orthosilicate was added over the resulting mixture dropwise while vigorously stirring.After stirring the mixture for 12 h at 40 °C, the obtained MnFe2O4@SiO2 nanoparticles were separated using an external magnet, washed several times with ethanol, and dried at room temperature.

Synthesis of MnFe2O4@SiO2 -Ag
The preparation of EP green tea extract was reported in our recent study [26].For further synthesis of the MnFe2O4@SiO2-Ag nanocatalyst, 50 mg of MnFe2O4@SiO2 was added over a stirring solution of 50.0 mL of AgNO3 (0.15 mM) and dispersed well.Afterwards, 8.0 mL of the EP extract was added while constant stirring at 50 °C for 60 min.After cooling the reaction mixture to room temperature, the precipitates were collected by using a niobium magnet and washed several times with distilled water and, later on, with three times with ethanol to get rid of impurities [20].

Catalytic activity of MnFe2O4@SiO2-Ag
The catalytic performance of MnFe2O4@SiO2-Ag was tested over the reduction reaction of RhB and MB by NaBH4.Prior to the catalytic reactions, in order to completely achieve adsorption-desorption equilibrium, the MnFe2O4@SiO2-Ag NPs (20 µL, 2.15 mg mL -1 ), de-ionized water (0.75 mL), and RhB (40 µL, 3.06 mM) were stirred for 30 min.After that, 2.25 mL portion of the 0.1 M NaBH4 was poured into this solution.By adopting the same procedure, the catalytic assays were completed by using MB (10 µL, 2.25 mM) and NaBH4 (2.25 mL, 0.1 M).UV-Vis measurements were recorded between the range of 350-700 nm and 500-750 nm for RhB ((λmax = 554 nm) and MB (λmax = 670 nm) to monitor the performed reaction until bleaching the color of the aqueous solutions of dyes.

Synthesis and characterization
In the nanocomposite catalyst design, MnFe2O4 was chosen as it has high saturation magnetization value and rough surface [17].Then, the MnFe2O4 surface was coated with SiO2 thin layer to prevent agglomeration and create a porous area [27].The facility of the EP green tea extract for the reduction and stabilization of metal nanoparticles as coating material with its rich content in terms of phenolic compound derivatives such as tannins, flavonoids, and phenolic acids has been demonstrated in our recent study [25].Considering this potential of EP extracts, herein, we employed them as efficient reducing agents for the immobilization of AgNPs on the protective SiO2 outer layer.Therefore, the resulting MnFe2O4@SiO2-Ag nanocomposite can be used as a low-cost, recyclable, environmentally friendly, and active catalyst platform.The experimental strategy for the preparation of MnFe2O4@SiO2-Ag was illustrated in Scheme 1.The MnFe2O4@SiO2-Ag was synthesized in two-step approach.In the first step, the silica layer was coated on the magnetic core nanoparticle, MnFe2O4.In the next step, silver ions was adsorbed and in situ reduced on the surface of the MnFe2O4@SiO2 core-shell nanospheres by means of EP green tea extract in aqueous solution without using any other organic solvent, stabilizing, or reducing agents.
The crystalline phase, morphology, and particle size of the as prepared MnFe2O4@SiO2-Ag samples were examined via X-ray diffraction (XRD), Scanning Electron Microscopy (SEM), and Transmission Electron Microscopy (TEM).440), which can be indexed to the JCPDS 17-465 [28,29].The SEM image of MnFe2O4 shows aggregates of well-defined spherical-like particles of sizes between 100-150 nm (Figure 1b).
The SEM micrographs of MnFe2O4@SiO2 and MnFe2O4@SiO2-Ag core-shell NPs are shown in Figures 2a-2d.In the current study, SiO2 thin layer was coated on the surface of MnFe2O4 magnetic core by hydrolysis of TEOS [30,31].The SEM image of MnFe2O4@SiO2 showed that the MnFe2O4 core was homogeneously and successfully coated with SiO2 layer.The detailed core-shell structure was further confirmed by high-resolution TEM image (Figure 2b, 2c) and EDS (Figure 2d).It can be seen from Figure 2e that an amorphous SiO2 layer with a thickness of ~20 nm was homogeneously distributed over the surface of MnFe2O4.Moreover, The EDS analysis of MnFe2O4@SiO2-Ag clearly displayed signals from Ag, Mn, Fe, and Si atoms (Figure 2d).In this point, our previous study confirms that AgNPs are formed as a result of the in situ upon oxidation of active phenolic functional groups and derivatives in the EP extract by Ag + ions at neutral pH value [25].The resulting AgNPs were observed in spherical shape with 15 nm of average particle size in TEM analysis (Figure 2e, 2f).These results suggest that the AgNPs could be formed in every layer of the SiO2 layer.Thus, porous outer shell coated on the magnetic support can create a platform for the acceleration of mass-energy transfer to active catalysts such as Ag, Au, Pd, etc. [32,33].Scheme 1. Systematic synthetic route for the production of MnFe2O4@SiO2-Ag nanocatalyst.XRD patterns of the MnFe2O4@SiO2-Ag contain peaks of both crystalline MnFe2O4 and AgNPs (Figure 3a).The sharp diffraction peaks at 2θ = 38.2°,44.3°, 64.5° and 76° can be indexed to the reflections of the (111), ( 200) and ( 220) crystalline planes of face-centered-cubic Ag (JCPDS card no.04-0783), respectively [25].In order to further confirm the composition and structure of the MnFe2O4@SiO2-Ag, thermal stability was investigated.A mass loss of MnFe2O4 is 5% up to 280 °C due to the volatilization of physically absorbed water and residual organic surfactant.As for MnFe2O4@SiO2-Ag, the mass loss is 1% higher than that of MnFe2O4 due to the decomposition of the thin layer of SiO2 [34] (Figure 3b).
The low recovery costs of catalysts are a significant factor in the development of sustainable catalyst systems [35][36][37][38][39][40][41].Therefore, magnetically supported catalyst systems are considered to be one of the most important platforms as they can be easily separated from the reaction media via the aid of an external magnet [28,42,43].Magnetic properties of the obtained catalysts were elucidated with vibrating sample magnetometer (VSM) analyzer between the range of -20000 Oe +20000 Oe at room temperature.The magnetization saturation values (Ms) of MnFe2O4 is 52.12 emu g −1 .However, the saturation magnetization of the silica-coated and Ag loaded MnFe2O4@SiO2-Ag NPs decreases as the silica shell thickness increases, and it has value of ∼33.51 emu g −1 with shell thickness of 20 nm, respectively (Figure 3c).ATR-FTIR spectroscopy was also used to monitor the SiO2 coating process of the MnFe2O4 surface and the Ag doping process with green synthesis [34,44].The ATR-FTIR spectrum of MnFe2O4@SiO2 exhibits a broad band in the region 3400 cm -1 and fewer intense band at 1650 cm -1 , which are due to O-H stretching and O-H deformation vibrations of coordinated water, respectively (Figure 3d) [45].These O-H bands also include Si-OH stretchings and vibrations of SiO2.The bands centered at 1090 cm -1 and 810 cm -1 are, respectively, assigned to the vibrations of Si-O-Si (asym) and the vibration of Si-O-Si (sym) [46].No significant change was observed in the ATR-FTIR spectra of MnFe2O4@SiO2-Ag on doping with AgNPs except minor intensity and position changes in the ∼750-1250 cm -1 region.These results show that Ag nanoparticles formed by reduction with green tea extract do not cause deformation on the SiO2 surface.

Catalytic properties of MnFe2O4@SiO2-Ag
Over the last decade, industrial effluents bearing organic dye pollutants and stemming from various activities such as textile, plastic, cosmetic and have come to a serious problem to be overcome [47].Due to their water solubility to some extend up to 10-200 mg/L, dye contaminants are regarded as one of the most important resources of the water pollution all over the world [48].In spite of numerous methods, involving precipitation, adsorption or biogenic treatment have been employed; the concerns still maintain due to their high cost, generation of inadmissible side products that might lead to damages on animal and human, comprising of liver, kidney, etc. [47], and requisition of possible high-energy demands, especially in massive treatments [49].Therefore, the complete removal of the organic pollutants from the industrial effluents by direct catalytic reductions has been occurring as a major environmentally friendly remedy [50].
Former studies have shown that AgNPs exhibited good catalytic activity and selectivity for various reactions [40,51,52].In the present study, the catalytic performance of the green synthesized MnFe2O4@SiO2-Ag nanocatalyst by using EP extract was evaluated in the model direct reduction reactions of MB and RhB by NaBH4, as they are good representative members of the hazardous organic pollutants [53,54].In addition, their decolorization processes can be easily monitored by naked eye and UV-Vis spectroscopy from the unique absorption bands at around 554 and 670 nm for RhB and MB, respectively [55,56].Thus, the practical investigation of the degradation of MB and RhB could be beneficial for the purification of dye effluents.As it can be observed from Figures 4a and 4b, conversions of dyes were completed in 7.39 min (MB) and 13.13 min (RhB) after addition of the MnFe2O4@SiO2-Ag nanocatalyst to the individual solutions, including the excess amount of NaBH4.The color bleaching of the aqueous solutions together with the leveling off the UV-Vis bands after gradual decreases were also indicated the completion of the reduction reactions successfully.These results confirmed the successful degradation of MB and RhB to their leuco forms [34,[57][58][59][60] by means of the redox reactions appearing on the surface of the electron relay systems (AgNPs) enabling the transfer of surface hydride ion electrons from BH4 -to the target acceptor dyes MB and RhB Possible reduction mechanism of the MB and RhB by MnFe2O4@SiO2-Ag was illustrated in Scheme 2. Taken into consideration the above results, it can be concluded that chromophore functional groups of C=N-and -N=N-present in MB and RhB have been successfully reduced to those of colorless C-N and N-N in the presence of immobilized AgNPs on the MnFe2O4@SiO2 surface [62,63].
In order to enlighten the catalytic role of the as synthesized nanocatalyst on diverse organic pollutants, rate constants for the MB and RhB reduction reactions were calculated and compared in Figure 4c.During the catalytic reduction studies, the concentration of the NaBH4 was used as excessively higher than the used dyes in order to obey the pseudo first-order kinetics described by ln (At /A0) = -kt, where k, t, At, and A0 correspond to apparent rate constant, reaction time, absorbances of dyes at time "t" and "0", respectively [64].The obtained results revealed that the MnFe2O4@SiO2-Ag exhibited higher catalytic towards MB (0.3 min -1 ) than RhB (0.07 min -1 ) (Figure 4c).To further get a better insight into the catalytic activity of the MnFe2O4@SiO2-Ag and show the facility of this work, normalized rate constants (knor=k/m, where the m is the catalyst mass) were calculated [65], and the performance of our catalyst was compared with the other catalyst systems in the literature.The results were summarized in Table .Compared with the other various metal-based catalyst systems, the catalytic activity of the green synthesized MnFe2O4@SiO2-Ag was distinctive and even satisfactory with the knor values of 116.28 s -1 g -1 and 27.13 s -1 g -1 for MB and RhB, respectively.Therefore, it could be inferred that the MnFe2O4@SiO2-Ag nanocatalyst can be utilized with a good potential for the reduction of dye contaminants in water and be promising to future studies with its environmentally friendly preparation process by EP extract without using extra reducing or stabilizing agent that might be toxic for the living organism and the environment.

Recyclability of the MnFe2O4@SiO2-Ag
The recyclability and stability of the catalyst are important factors to show the sustainability of the core-shell magnetic nanocatalysts prepared by using EP extract for the immobilization of AgNPs on the MnFe2O4@SiO2 surface.Thus, the recyclability tests were conducted on the model reduction reaction of RhB by NaBH4 in the presence of the MnFe2O4@SiO2-Ag, and the obtained results were presented in Figure 4d.For a routine cyclic test, an external niobium magnet was used to separate the used nanocatalyst from the reaction media after the catalytic degradation.Before starting the subsequent cycle, the recycled nanocatalyst was washed several times with water and subsequently three times with ethanol.After dried under vacuum, they were used for the next cycle.In each cycle, the same procedure was repeated.Figure 4d shows the recyclability test results.In the formation of these graphs, the maximum absorbance Scheme 2. Possible mechanism of the reduction of MB and RhB catalyzed by MnFe2O4@SiO2-Ag.
values of the RhB were used to calculate percent decolorization rates.This overlapped plot drawn from the decolorization rate % and time (min) proves that the MnFe2O4@SiO2-Ag maintains its catalytic activity through the five repeated cycles without any loss in its decolorization efficiency (100%), so that the possibility of leaching AgNPs from the MnFe2O4@SiO2-Ag nanocomposites were ignored.Nevertheless, it can be also seen from   .This increase could be attributed to loss of magnetic catalyst during the recovery process of the catalyst [65].Overall, the produced MnFe2O4@SiO2-Ag coreshell magnetic nanocatalyst were stable and sufficient enough for the reduction of RhB, and they could be good candidates and have a great potential for the removal of dye contaminants in water.

Conclusion
In the current study, we showed a green synthetic strategy for the immobilization of AgNPs on manganese ferrite nanoparticles coated with the protective silica layer by using EP extract without facilitating any other reducing or stabilizing agents.This approach presents significant advantages over the existing ones in terms of using mild reaction conditions, requiring no extra reducing agent or surfactant, organic solvent, and hazardous materials.Bio-based process used here does not generate environmentally hazardous waste.For this reason, the reaction product occurring in these processes do not frequently need purification.The prepared catalyst system in this study revealed sufficient catalytic activity for the removal of MB and RhB compared with the previous studies.Moreover, the superior magnetization characteristics of the MnFe2O4@SiO2-Ag led them to be used several times without losing a prominent catalytic activity in each successive cycle.Thus, the obtained overall results suggest that the MnFe2O4@SiO2-Ag core-shell magnetic nanocomposites could be highly efficient and stable catalytic systems for the treatment of organic or dye contaminants and numerous applications in heterogeneous catalysis considering the environmental pollution concerns.
Fig 4d that time required to complete the

Figure 4 .
Figure 4.The reduction of MB (a) and RhB (b) in aqueous solution using MnFe2O4@SiO2-Ag nanocatalyst.The comparison of the first-order kinetic plots of MB and RhB in the presence of MnFe2O4@SiO2-Ag (c).Recycling of the MnFe2O4@SiO2-Ag for the reduction of RhB by NaBH4 (d).

Table .
Comparison of the catalytic performances of MnFe2O4@SiO2-Ag with other catalyst system over MB and 4-NP reduction by NaBH4.