A Novel Metal-Containing Mesoporous Silica Composite for the Decolorization of Rhodamine B: Effect of Metal Content on Structure and Performance

A series of novel MnxFey@SiO2 (x,y = 1–20%) nanocomposites were synthesized for the first time via the sol-gel/combustion method with different content of precursors (Mn and Fe acetate salts). The effect of precursor content and ratio on physicochemical properties were observed by various characterization methods. Moreover, Rhodamine B (RhB) was chosen as the target pollutant to test the performance of these nanocomposites under a photocatalytic Fenton-like reaction. The results showed that the nanocomposite morphology improved by increasing Fe and Mn content. In this study, interesting behavior was observed in BET results which were different from the fact that increasing metal content can decrease the surface area. This study revealed that one metal could be more critical in controlling the properties than another. Moreover, the precursor ratio appears to have a more tangible effect on the surface area than the effect of precursor content. Among all synthesized nanocomposites, Mn1Fe5@SiO2 showed the highest surface area of 654.95 m2/g. At optimum batch conditions (temp = 25 °C, catalyst dosage = 1 g L−1, H2O2 = 75 mmolL−1, and initial RhB concentration = 50 mg L−1), complete removal (simultaneous adsorption/degradation) occurred using Mn1Fe5@SiO2 at neutral pH. This study showed that the designed nanomaterial could be used as a dual functional adsorbent/photocatalyst in different environmental applications.


Introduction
Dyes are considered organic pollutants, which are known for their harmful effects on the environment and ecosystem. According to the annual worldwide dye production database, over 10,000 types (over 700,000 tones) of dyes are manufactured every year [1]. Dyes have considerable applications in several industries, including printing, pulp mill, textile, electroplating, and cosmetics, which cause severe water contamination. Most common carcinogenic products, such as benzidine, are present in organic dyes that must be removed before they discharge into wastewater and other water sources [2]. Nowadays, the removal of organic pollutants like dyes has become a concern for environmental scientists. Various treatment techniques have been established, such as coagulation or flocculation, ultra-filtration, reverse osmosis, and biodegradation. However, most of these conventional approaches have faced several drawbacks. Therefore, it is essential to find new methods to remove organic pollutants [3]. Rhodamine B(RhB) is an amphoteric dye employed in food and fabric dyeing industries and belongs to the xanthene dyes class. RhB is one of the most challenging organic pollutants to degrade and control. According to tron microscope (HR-TEM), Thermogravimetric analysis (TGA), N 2 adsorption-desorption isotherm, X-ray diffractometer (XRD), Fourier-transform infrared (FTIR), Ultraviolet-visible light-diffuse reflectance spectroscopy (UV-Vis DRS), X-ray photoelectron spectroscopic (XPS), and electron spin resonance (ESR) were carried out. The application of this material was tested by the degradation of Rhodamine B (RhB) under a photocatalytic Fenton-like reaction. The influence of various experimental parameters such as catalyst dosage, initial concentration of RhB, UV power, and H 2 O 2 concentration on the degradation process was also investigated.

Synthesis of Mn x Fe y S photocatalysts
A series of novel Mn x Fe y @SiO 2 (x,y = 1-20%) nanocomposites were synthesized for the first time via the sol-gel/combustion method with different content of precursors (Mn and Fe salts). The Mn x Fe y @SiO 2 nanocomposites were fabricated according to our previously published method with modifications in precursors and conditions [23,24]. Iron-Manganese containing amorphous silica (Mn x Fe y @SiO 2 ) was synthesized using the sol-gel/combustion technique at room temperature. Different percentage of Manganese(II) acetate and Iron(II) acetate was dissolved in 60 mL of CTAC solution and mixed for 15 min. After gaining a homogenous solution, 1 g of glycine was added to the mixture and stirred for 30 min. Next, 60 mL of TEOS solution was added to the solution and mixed for another 30 min. The solution was transferred to the incubator and stirred at 25 • C and 200 rpm for 24 h to form a gel. The gels were dried at 70 • C for 4 h in an oven. Finally, Mn x Fe y @SiO 2 was obtained by calcination at 550 • C for 6 h under air. It should be noted that depending on the precursor content, the gel showed different colors (Table 1).

Instruments
All the instruments used in this study are mentioned in Supplementary Data.

Experimental Procedure
The utilized photoreactor in this experiment consisted of one quartz tube in the middle and four UV-C lamps around it. The UV lamps were low-pressure mercury lamps with a power of 8 W and an intensity of 0.5 W/m 2 . A variable voltage transformer was used to adjust the UV light intensity of the UV lamps. In order to prevent the effect of heat in the removal experiment, the solution temperature was kept constant (25 • C) using cooling fans installed at the bottom of the photoreactor. It should be noted that it was designed in black to minimize the influence of other light sources in the experiments. The prepared Nanomaterials 2022, 12, 4108 4 of 17 nanocomposites were examined for their ability to degrade RhB under photocatalytic Fenton-like reactions. In a typical experiment, 1g L −1 of the nanocomposites was added to a 200 mL RhB solution with (50 mg L -1 ) concentration. Then, 75 mmol L -1 of 28% H 2 O 2 was poured into the previous solution. At desired time intervals, 4 mL of the RhB solution was taken to analyze the removal performance using UV-Vis spectroscopy (LAMBDA 365 UV/Vis Spectrophotometer, Perkin Elmer).

XRD:
The composition of the Mn x Fe y @SiO 2 nanocomposites was identified using the XRD technique ( Figure 1). The indicated peaks can be indexed to Mn 2 O 3 -Fe 2 O 3 @ SiO 2 . A broad diffraction peak at 2θ of 21.63 • represents amorphous silica [25]. The nanocomposites with lower Fe and Mn content (Mn 1 Fe 1 @SiO 2 , Mn 1 Fe 5 @SiO 2 , and Mn 5 Fe 1 @SiO 2 ) showed low-intensity peaks with a smaller number of peaks; while, for the samples with higher content of metal (Mn 5 Fe 5 @SiO 2 and Mn 20 Fe 20 @SiO 2 ), peaks were visible with higher intensities. Nine diffraction peaks at 2θ = 24.  [26]. Furthermore, in the sample with higher Mn content, Mn 3 O 4 was also observed [27].
The utilized photoreactor in this experiment consisted of one quartz tube in the middle and four UV-C lamps around it. The UV lamps were low-pressure mercury lamps with a power of 8 W and an intensity of 0.5 W/m 2 . A variable voltage transformer was used to adjust the UV light intensity of the UV lamps. In order to prevent the effect of heat in the removal experiment, the solution temperature was kept constant (25 °C) using cooling fans installed at the bottom of the photoreactor. It should be noted that it was designed in black to minimize the influence of other light sources in the experiments. The prepared nanocomposites were examined for their ability to degrade RhB under photocatalytic Fenton-like reactions. In a typical experiment, 1g L −1 of the nanocomposites was added to a 200 mL RhB solution with (50 mg L -1 ) concentration. Then, 75 mmol L -1 of 28% H2O2 was poured into the previous solution. At desired time intervals, 4 mL of the RhB solution was taken to analyze the removal performance using UV-Vis spectroscopy (LAMBDA 365 UV/Vis Spectrophotometer, Perkin Elmer).

XRD:
The composition of the MnxFey@SiO2 nanocomposites was identified using the XRD technique ( Figure 1). The indicated peaks can be indexed to Mn2O3-Fe2O3 @ SiO2. A broad diffraction peak at 2θ of 21.63° represents amorphous silica [25]. The nanocomposites with lower Fe and Mn content (Mn1Fe1@SiO2, Mn1Fe5@SiO2, and Mn5Fe1@SiO2) showed low-intensity peaks with a smaller number of peaks; while, for the samples with higher content of metal (Mn5Fe5@SiO2 and Mn20Fe20@SiO2), peaks were visible with higher intensities. Nine diffraction peaks at 2θ = 24.15°, 33 [26]. Furthermore, in the sample with higher Mn content, Mn3O4 was also observed [27].  Textural properties: The configuration and microstructure of the Mn x Fe y @SiO 2 nanocomposites were analyzed by field emission scanning electron microscopy (FE-SEM) and high-resolution transmission electron microscope (HR-TEM) images (Supplementary Figures S1 and S2). Sphericallike particles were observed in all the images, and by the increasing Fe and Mn concentrations, the spherical structure improved and became more uniform (Supplementary Figure S1). The HR-TEM image of nanocomposites has been illustrated in Supplementary Figure S2 The peak centered at 967 cm -1 confirms the presence of silanol groups on the silica surface or pores in all the nanocomposites. The small peak at 1641 cm -1 and the broad peak at 3433 cm -1 are due to the stretching vibration of OH groups in absorbed physical water and Si-OH, respectively. The Si-O band always appears around 461 cm -1 , which here is covered by stretching vibration of the metal-oxygen bond in the range of 469 cm -1~4 80 cm -1 [18,23]. Soubaihi R. et al., 2021 studied Pd/SiO 2 Catalyst characteristics, and similar peaks for SiO 2 have been reported [28]. lustrated in Supplementary Figure S2. The Fe2O3/Mn2O3 nanoparticles could be observed in the silica.
FTIR: The FTIR analysis was carried out to identify the functional groups on the surface of MnxFey@SiO2 nanocomposites ( Figure 2). Peaks at 464 cm -1 , 810 cm -1 , 967 cm −1 , 1086 cm -1 , 1641 cm -1 , and 3433 cm -1 are observed in the transmission spectra of all nanocomposites. The absorption band at 1086 cm -1 was related to the antisymmetric stretching of Si-O-Si. The symmetric mode of Si-O-Si appeared at 810 cm -1 . Moreover, the stretching vibration of Si-OH (967 cm -1 ) and bending of Si-O (464 cm -1 ) are also observed. The peak centered at 967 cm -1 confirms the presence of silanol groups on the silica surface or pores in all the nanocomposites. The small peak at 1641 cm -1 and the broad peak at 3433 cm -1 are due to the stretching vibration of OH groups in absorbed physical water and Si-OH, respectively. The Si-O band always appears around 461 cm -1 , which here is covered by stretching vibration of the metal-oxygen bond in the range of 469 cm -1~4 80 cm -1 [18,23]. Soubaihi R. et al., 2021 studied Pd/SiO2 Catalyst characteristics, and similar peaks for SiO2 have been reported [28].

TGA:
The thermal stability of the nanocomposites is shown in Figure 3. The samples were heated from 25 °C to 900 °C, and weight loss in the samples was reported. An insignificant weight loss was observed at <100 °C in all nanocomposites due to the adsorbed moisture on the nanoparticle surface. Moreover, the weight loss in higher temperatures was also insignificant (2-4%), showing that the prepared nanocomposites were thermally stable.

TGA:
The thermal stability of the nanocomposites is shown in Figure 3. The samples were heated from 25 • C to 900 • C, and weight loss in the samples was reported. An insignificant weight loss was observed at <100 • C in all nanocomposites due to the adsorbed moisture on the nanoparticle surface. Moreover, the weight loss in higher temperatures was also insignificant (2-4%), showing that the prepared nanocomposites were thermally stable.

BET:
Porosity characteristics of nanocomposites were tested by N 2 adsorption-desorption isotherm ( Figure 4). Based on IUPAC classification, synthesized nanocomposites show N 2 sorption isotherms of Langmuir type IV isotherm and are considered as a H2(b) type hysteresis loop, which verifies their mesoporous nature [29]. The specific surface areas (S BET ) were calculated using the Barrett-Joyner-Halenda method. The results showed that the sample showed a surface area in the range of 320.74 m 2 /g-654.95 m 2 /g. The fact that surface area decreased with an increasing metal fraction is reported in many studies [30], and the lowest surface area was expected to be M 20 Fe 20 @SiO 2 . However, in this study, among all the nanocomposites, the lowest surface area (S BET = 320.74 m 2 /g) belonging to the sample M 5 Fe 1 @SiO 2, which has a high Mn and low Fe content, and the highest specific surface area of 654.95 m 2 /g belonged to the sample with more Fe and less Mn content (M 1 Fe 5 @SiO 2 ). From these results, it can be concluded that increasing Mn has a more substantial effect compared to Fe in decreasing the surface area. Moreover, the ratio of Mn and Fe is more important than their content in nanocomposite.  XPS: the oxidation state of elements in the prepared nanocomposite was analyzed using XPS. The XPS spectra showed Si2p, O1s, Mn2p, and Fe2p, which demonstrated the presence of these elements in the prepared samples. In this study, the Fe and Mn content is low, and the related peaks were not visible in full spectra in Figure 5.
In the Si spectrum, a single peak appeared at binding energies (B.E.) of 103.62 eV belonging to the O-Si-O bond related to the Silica network ( Figure 6a). The Mn spectra showed two distinguishable peaks observed at B.E. of 641.57 eV for Mn2p 3/2 and 652.89 eV for Mn2p 1/2 , which corresponded to the Mn (III) oxidation state (Figure 6b) [31]. Hazarika K. et al. 2018 reported that the spin energy separation of the Mn 3+ oxidation state is 11.7 eV. Similar results have been observed in this study, and the spin energy separation of Mn 3+ was found to be 11.3 eV [32]. Similarly, at the Fe 2p spectrum, two peaks at 711.45 eV for Fe 2p 3/2 and 724.72 eV for Fe 2p 1/2 were observed, which were a fingerprint of Fe(III) in the nanocomposites (Figure 6c) [33]. The shakeup peaks observed at the higher binding energy near the main peaks were assigned to the satellite peaks of Fe(III) oxidation states, which are shown by blue arrows.   for Mn2p1/2, which corresponded to the Mn (III) oxidation state (Figure 6b) [31]. Hazarika K. et al. 2018 reported that the spin energy separation of the Mn 3+ oxidation state is 11.7 eV. Similar results have been observed in this study, and the spin energy separation of Mn 3+ was found to be 11.3 eV [32]. Similarly, at the Fe 2p spectrum, two peaks at 711.45 eV for Fe 2p3/2 and 724.72 eV for Fe 2p1/2 were observed, which were a fingerprint of Fe(III) in the nanocomposites (Figure 6c) [33].

ESR:
To further understand the elemental state of the nanocomposites, ESR analysis was conducted at room temperature (Figure 7). The high-intensity signal at 332 mT was assigned to be g = 1.98, which was attributed to the existence of the Fe 3+ ions in all the nanocomposites. The sextet was observed approximately at 306 mT, 314 mT, 323 mT, 344

ESR:
To further understand the elemental state of the nanocomposites, ESR analysis was conducted at room temperature (Figure 7). The high-intensity signal at 332 mT was assigned to be g = 1.98, which was attributed to the existence of the Fe 3+ ions in all the nanocomposites. The sextet was observed approximately at 306 mT, 314 mT, 323 mT, 344 mT, and 354 mT, which were attributed to g = 2.16, 2.1, 2.03, 1.91, and 1.86, respectively. Nanomaterials 2022, 12, x FOR PEER REVIEW mixture with Mn(III) (Mn2O3) [35,36]. These results are in accordance with the which showed the presence of mixed Mn valence (Mn2O3 and Mn3O4) in nano with high Mn fractions. It should be noted that in the XRD spectra of the nano with low Mn concentration, the Mn3O4 was not visible. However, ESR analy the structure with more detail and accuracy and proved that even nanocomp low Mn contents Mn3O4 exist.

UV-Vis DRS:
The optical properties and bandgap of synthesized nano were studied using UV-Vis DRS (Figure 8a,b). The spectrum showed that al composites could absorb the whole spectrum of UV, visible, and IR. The obse ple peaks were related to the absorption bands of Mn2O3 and Fe2O3 in the SiO2 Region 1 (250-400 nm) mainly results from the ligand to metal charge-tr sitions, and Region 2 (400-600 nm) is considered to be the result of the pai process ( 6 A1( 6 s) + 6 A1( 6 s) to 4 T1( 4 G) + 4 T1( 4 G)). Region (600-750 nm) is assigned to to 4 T2( 4 G) transition at about 650 nm, and Region 4 (750-900 nm) is the 6 A1( 6 s transition at about 830 nm. Moreover, the band in the 375-440 nm range is a the d-d transitions of Mn 3+ in Mn2O3 [37].

UV-Vis DRS:
The optical properties and bandgap of synthesized nanocomposites were studied using UV-Vis DRS (Figure 8a,b). The spectrum showed that all the nanocomposites could absorb the whole spectrum of UV, visible, and IR. The observed multiple peaks were related to the absorption bands of Mn 2 O 3 and Fe 2 O 3 in the SiO 2 .

Effect of Metal Content on Performance
All the synthesized nanocomposites were studied for their ability to degrade R dye under a photocatalytic Fenton-like process, and the results are shown in Figure 9. T graphs showed that Mn1Fe5@SiO2 had the highest removal performance (99.66%), a Mn20Fe20@SiO2 showed the lowest removal (54.89 %). These results can be attributed to large surface area of Mn1Fe5@SiO2, which can provide more active sites, increasing the moval efficiency. Based on these findings, experiments were continued usi Mn1Fe5@SiO2, and the influencing parameters, including RhB initial concentration, ca lyst dosage, H2O2 concentration, and UV intensity, have been investigated. Region 1 (250-400 nm) mainly results from the ligand to metal charge-transfer transitions, and Region 2 (400-600 nm) is considered to be the result of the pair excitation process ( 6 A 1 ( 6 s) + 6 A 1 ( 6 s) to 4 T 1 ( 4 G) + 4 T 1 ( 4 G)). Region (600-750 nm) is assigned to the 6 A 1 ( 6 s) to 4 T 2 ( 4 G) transition at about 650 nm, and Region 4 (750-900 nm) is the 6 A1( 6 s) to 4 T 1 ( 4 G) transition at about 830 nm. Moreover, the band in the 375-440 nm range is attributed to the d-d transitions of Mn 3+ in Mn 2 O 3 [37].
The optical bandgap can be calculated using the equation αhυ = A(hv − E g ) n , Where hν is photon energy, α is absorption coefficient, and n represents 1 /2 for a direct transition semiconductor and 2 for an indirect transition semiconductor [38]. A and E g represent the constant light frequency and bandgap energy, respectively. The bandgap energy was calculated from the plot of (αhν) 2 versus hν plots. The bandgap energy of Mn 1 Fe 1 @SiO 2 , Mn 1 Fe 5 @SiO 2 , Mn 5 Fe 1 @SiO 2 , Mn 5 Fe 5 @SiO 2 , and Mn 20 Fe 20 @SiO 2 was 1.62, 1.49, 1.78, 1.57, and 1.44 eV, respectively.

Effect of Metal Content on Performance
All the synthesized nanocomposites were studied for their ability to degrade RhB dye under a photocatalytic Fenton-like process, and the results are shown in Figure 9. The graphs showed that Mn 1 Fe 5 @SiO 2 had the highest removal performance (99.66%), and Mn 20 Fe 20 @SiO 2 showed the lowest removal (54.89 %). These results can be attributed to the large surface area of Mn 1 Fe 5 @SiO 2, which can provide more active sites, increasing the removal efficiency. Based on these findings, experiments were continued using Mn 1 Fe 5 @SiO 2 , and the influencing parameters, including RhB initial concentration, catalyst dosage, H 2 O 2 concentration, and UV intensity, have been investigated. dye under a photocatalytic Fenton-like process, and the results are shown in Figure 9. The graphs showed that Mn1Fe5@SiO2 had the highest removal performance (99.66%), and Mn20Fe20@SiO2 showed the lowest removal (54.89 %). These results can be attributed to the large surface area of Mn1Fe5@SiO2, which can provide more active sites, increasing the removal efficiency. Based on these findings, experiments were continued using Mn1Fe5@SiO2, and the influencing parameters, including RhB initial concentration, catalyst dosage, H2O2 concentration, and UV intensity, have been investigated.

Effect of Experimental Parameters
Catalyst dosage: Figure 10a illustrates the effect of catalyst dosage on the degradation of RhB using Mn 1 Fe 5 @SiO 2 . In order to optimize the catalyst dosage, experiments were performed with different dosages of Mn 1 Fe 5 @SiO 2 in the range of 0.25 to 1.5 g L -1 . The removal efficiency was 43.2% and 99.8% for the 0.25 g L -1 and 1. 5 g L -1 catalysts, respectively. Increasing the dosage of Mn 1 Fe 5 @SiO 2 can increase the presence of active sites and the hydroxyl radicals ( • OH) formation, which finally enhances the removal performance. When the catalyst increased from 1 g L -1 to 1.5 g L -1, , the change in degradation efficacy was insignificant, and a dosage of 1 g L -1 was chosen to continue the experiments.
Initial concentration of RhB: The effect of the initial concentration of RhB on removal efficiency of Mn 1 Fe 5 @SiO 2 was evaluated with different RhB concentrations (10-100 mg L -1 ). Low dye concentrations (10 mg L -1 and 25 mg L -1 ) could be removed completely in the first 30 min. However, removal of high concentrations of 50 mg L -1 and 100 mg L -1 could reach 98.2 % and 90.6%, respectively, after the solution was irradiated for 60 min (Figure 10b). It can be seen that when the initial concentration of the RhB increased, it resulted in a decrease in degradation performance, which can be attributed to two main reasons. First, as the initial RhB concentration increases, more RhB molecules attach to the surface of the catalyst, decreasing the accessibility of active sites and minimizing the generation of e − /h + pairs and hydroxyl radicals. Second, very high concentration of RhB inhibits UV/catalyst interactions because the dye can absorb the UV light and prevent it from being absorbed by the catalyst, which can also hinder the generation of electron/hole (e − /h + ) pairs [39,40]. In this work, 50 mg L −1 of RhB was chosen as the optimized parameter.

H 2 O 2 concentration (electron acceptor):
The efficiency of the photocatalytic reaction increases by preventing electron/hole (e − /h + ) recombination. Adding an electron acceptor such as H 2 O 2 can effectively decrease the recombination of e − /h + . The degradation of RhB over Mn 1 Fe 5 @SiO 2 was investigated using different concentrations of H 2 O 2 (5 mmol L −1-120 mmol L −1 ) (Figure 10c). With an increase in H 2 O 2 concentration from 5 mmol L −1 to 75 mmol L −1 , the removal efficiency increased from 65.68% to 100%. Increasing the H 2 O 2 concentration causes an increase in the removal performance because, under UV irradiation, the O-O bonds hydrogen peroxide could be broken to create hydroxyl radical ( • OH). Therefore, more H 2 O 2 concentrations can produce a higher number of ( • OH) radicals. However, precautions are needed because, when H 2 O 2 exceeds a certain level, it will act as a scavenger, decreasing dye degradation efficiency. In this study, by increas-ing the H 2 O 2 concentration to 120 mmol L −1 , the efficiency decreased to 88.2% [41,42]. Therefore, for subsequent experiments, 75 mmol L −1 of H 2 O 2 was used. UV power: The effect of UV power (8 W,16 W, 24 W, and 32 W) on RhB degradation was tested using Mn 1 Fe 5 @SiO 2 (Figure 10d). Results showed that the removal was 88.21% for 8w UV power, which reached 100% as the UV power increased to 32 W. For higher UV powers, more photons can be formed and absorbed by the surface of the catalyst, which means more electron-hole (e -/h + ) pairs can be generated. Consequently, the number of hydroxyl radicals accelerates and improves the removal performance. Moreover, in higher UV power, more hydroxyl radicals can be generated via H 2 O 2 photodissociation, which means a greater number of • OH radicals can be generated and attack the organic pollutants [43,44]. In this work, the power of 32 W was used as the optimized parameter.
pHzpc: The zero-point charge (pHzpc) value of photocatalysts was determined by pH drift method, as shown in Figure 11. The pHzpc value was found to be 5.7, 5.68, 6.05, 6.08, and 6.11 for Mn 1 Fe 1 @SiO 2 , Mn 1 Fe 5 @SiO 2, Mn 5 Fe 1 @SiO 2 , Mn 5 Fe 5 @SiO 2 , and Mn 20 Fe 20 @SiO 2 , respectively. The surface of nanocomposites was positively charged at pH below pHzpc, whereas it was negatively charged at pH above pHzpc. Since, in this experiment, the solution pH was 6.96 (neutral), the nanocomposite surface was negatively charged. nomaterials 2022, 12, x FOR PEER REVIEW Figure 11. Zeta potential of used nanocomposites in this study.

Degradation Performance in the Optimum Condition
Mn1Fe5@SiO2 was studied for the degradation of RhB dyes in various re ods. Figure 12 showed that, in the absence of the catalyst and using H2O2 or only 7.2∼15.7% of the dye could be degraded. Results indicated that 47.89 % adsorbed on Mn1Fe5@SiO2, indicating the catalyst capability in the adsorpti related to high surface area and more active sites. Moreover, based on Figure  Mn1Fe5@SiO2 is 5.68, and the pH of the RhB solution is 6.96; therefore, the su of Mn1Fe5@SiO2 is negative in pH higher than pHzpc. Thus, there will be force between positive charge RhB and negative charge nanocomposite, wh the removal of RhB due to simultaneous adsorption /degradation at neutral p removal of RhB was achieved under the heterogeneous photo-Fenton proces of this study have been compared with similar studies in Table 2.

Degradation Performance in the Optimum Condition
Mn 1 Fe 5 @SiO 2 was studied for the degradation of RhB dyes in various removal methods. Figure 12 showed that, in the absence of the catalyst and using H 2 O 2 or UV system, only 7.2∼15.7% of the dye could be degraded. Results indicated that 47.89% of RhB was adsorbed on Mn 1 Fe 5 @SiO 2 , indicating the catalyst capability in the adsorption, which is related to high surface area and more active sites. Moreover, based on Figure 11 pHzpc of Mn 1 Fe 5 @SiO 2 is 5.68, and the pH of the RhB solution is 6.96; therefore, the surface charge of Mn 1 Fe 5 @SiO 2 is negative in pH higher than pHzpc. Thus, there will be an attractive force between positive charge RhB and negative charge nanocomposite, which enhances the removal of RhB due to simultaneous adsorption /degradation at neutral pH. Complete removal of RhB was achieved under the heterogeneous photo-Fenton process. The results of this study have been compared with similar studies in Table 2.

Conclusions
In this study, a series of novel metal oxide @silica nanocomposites (MnxFey@SiO2) were synthesized for the first time via the sol-gel/combustion technique with different Mn and Fe contents (x,y = 1-20%). The results showed that the nanocomposite with the highest metal content (Mn20Fe20@SiO2) had better morphology (uniformed semi-spherical shape). Interestingly, BET results revealed a different behavior compared to the literature, and the lowest and highest surface area was expected to be M20Fe20@SiO2 and M1Fe1@SiO2,, respectively. However, in this study, among all the nanocomposites, the lowest surface area (SBET = 320.74m 2 /g) belongs to M5Fe1@SiO2, which has a high Mn and low Fe content, and the highest specific surface area of 654.95 m 2 /g belongs to the sample with more Fe and less Mn content (M1Fe5@SiO2). From these results, it could be concluded that increasing Mn has a more substantial effect compared to Fe in decreasing the surface area. Moreover, the

Conclusions
In this study, a series of novel metal oxide @silica nanocomposites (Mn x Fe y @SiO 2 ) were synthesized for the first time via the sol-gel/combustion technique with different Mn and Fe contents (x,y = 1-20%). The results showed that the nanocomposite with the highest metal content (Mn 20 Fe 20 @SiO 2 ) had better morphology (uniformed semi-spherical shape). Interestingly, BET results revealed a different behavior compared to the literature, and the lowest and highest surface area was expected to be M 20 Fe 20 @SiO 2 and M 1 Fe 1 @SiO 2, , respectively. However, in this study, among all the nanocomposites, the lowest surface area (S BET = 320.74 m 2 /g) belongs to M 5 Fe 1 @SiO 2, which has a high Mn and low Fe content, and the highest specific surface area of 654.95 m 2 /g belongs to the sample with more Fe and less Mn content (M 1 Fe 5 @SiO 2 ). From these results, it could be concluded that increasing Mn has a more substantial effect compared to Fe in decreasing the surface area. Moreover, the ratio of Mn and Fe appears to have a more tangible effect on surface characteristics compared to the effectiveness of Fe and Mn content. UV-Visible DRS data showed that all the nanocomposites have strong absorbance in the entire UV-Visible range with bandgap energy of 1.44-1.78 eV. XRD and ESR results indicated that Mn existed in a mixture valence of Mn(III) (Mn 2 O 3 ) and Mn(II) (Mn 3 O 4 ) in all the nanocomposites.
The application of these nanocompo"Ite' was demonstrated by the decolorization of Rhodamine B (RhB) under a photocatalytic Fenton-like reaction. In the optimized experimental conditions, the Mn 1 Fe 5 @SiO 2 showed the highest dye removal due to a higher number of active sites, which could increase dye degradation. Additionally, electrostatic attraction between the surface of the nanocomposite with a negative charge and positive charge RhB made the nanocomposite dual functional material where the adsorption/degradation could occur, simultaneously enhancing the removal performance. Thus, this study recommends a promising method for fabricating an efficient nanomaterial with potential applications to remove complex organic pollutants at neutral pH.