Impact of adding vanadium pentoxide to Mn-doped magnetite for technological uses

Nanocomposites containing different contributions of vanadium oxide (V2O5) and magnetite (Fe3O4) modified with manganese (Mn) ions were fabricated upon the formula of xMn0.4Fe2.6O4/(1-x)V2O5, whereas x = 0.0, 0.2, 0.4, 0.5, 0.6, 0.8, and 1.0. The obtained compositions were characterized upon their structure, morphology, besides their magnetic susceptibility. The TEM micrographs depicted that V2O5 was formed as rod shapes with diameters of 20–30 nm and length of 450 nm, while Mn–magnetite was configured in ellipsoidal shapes with dimensions 25–40 nm. Moreover, the Maximum height of the roughness(Rt) changed from 507.0 to 220.6 nm, while the skewness (Rsk) increased from 0.104 to 0.122 for (x = 0.2, and x = 1.0), respectively. Furthermore, Curie Weiss constant (θ) increased from 830 to 890 K and then decreased to 520 K, for x = 0.2, 0.5, and x = 1.0, respectively. The photo-activated antibacterial activity was examined and showed that the inhibition zone increased significantly by increasing the V2O5 content as well as it increased also from dark to light conditions. It was changed from 17.4 ± 1.1 and 16.9 ± 1.3 mm to 19.2 ± 1.4 and 18.9 ± 1.5 mm for pure V2O5 (x=0.0) against E. coli and S. aureus, respectively. The degradation of methylene blue (MB) was tested, and the efficiency of removal reached around 97.1% for the highest contribution of V2O5 after 35 min of visible light exposure. Hence, the fabricated nanocomposites can be suggested for a deep investigation to be highlighted for disinfection and water treatment applications.


Introduction
In the last decades, the rapid development of printing and dyeing has caused a tremendous increase in environmental pollution [1,2]. The dyes, organic molecules, and the pharmaceutical reagents that reach the rivers and seas lead to significant consequences on the drinking water [3,4]. Furthermore, organic dyes such as methylene blue, congo-red, and malachite green are toxic compounds and can cause health problems against the kidney and the liver and even lead to cancer cell formation [5,6]. Therefore, an economical treatment methodology is necessary to keep the pollution heath level [7].
Numerous techniques have been developed to degrade dyes from aqueous solutions, such as absorption, coagulation, and photocatalysis. Among those, the photocatalysis approach can be suggested as promising for water treatment and dye degradation owing to its feasibility and low cost [8,9]. However, photocatalysts that can work only under ultraviolet (UV) consume intensive energy and attain low effectiveness. Thus, large bandgap and fast recombination of the formed charge carriers are considered the main drawbacks of the designed photocatalysts materials [10].
Therefore, composite materials could be suggested to gain high potency of dye degradation. Substances with low bandgaps can offer a high intensity of pumped electron-hole pairs [11]. However, a large number of electrons does not guarantee a high decolorization efficiency, because charge recombination might deteriorate the flow of electron current. Furthermore, the inability to collect the adsorbent agents after dye removal can cause secondary pollution. Therefore, the design of recoverable substances is crucial to deal with wastes. In addition, using an adsorbent material for one time could increase the cleaning costs [12,13]. Hence, versatile requirements are vital for an appropriate approach to manage wastes through the water.
On the other hand, keeping a high level of care system requires remaining surfaces clear from bacterial organisms. Photocatalytic materials can generate reactive oxygen species (ROS) with a high affinity to interact with cellular walls. Thus, photoactivated antibacterial agents can play a crucial role in the disinfection of surfaces [14,15].
In this regard, the development of nanomagnetic materials such as magnetite (Fe 3 O 4 ) due to its high magnetic properties, surface area, recoverability, and reusability has attracted intensive interest. Many attempts have been made to improve the properties of modified magnetite [16,17]. In general, the replacement of diamagnetic of paramagnetic ions into the magnetic structure induces lattice distortion, and thus, magnetic, electrical, and optical behaviors of the host composition can be changed significantly. Ionic substitution of magnetite doping can introduce a simple way to promote physical-chemical properties of ferrites. Among diverse metallic ions, Mn 2+ is considered one of the best candidates for magnetite ionic replacement. Due to its magnetic moment value (5μB), which is larger than that of iron ions (4μB), Mn-doped magnetite has a cubic spinel structure, which is stable chemically and has a wide range of technological applications [18][19][20]. Many attempts have been done for improving the properties of doped magnetite. In general, adding diamagnetic or paramagnetic materials to magnetic sample (Mn doped magnetite) causes modification in structure, magnetic, electrical behavior of the host sample. As, the grain size formation depends on impurities concentration [21,22]. Furthermore, V. A. R. Villegas et al. investigated the photocatalytic activity of nitrobenzene (NB) via magnetic nanoparticles under UV irradiation. The results depicted that the photocatalytic degradation reached around 73% after 2 h of irradiation at pH 2 [23]. Moreover, S. M. Sajadi et al. fabricated CuO@magnetite@hen bone using for the reduction of polycyclic aromatic hydrocarbons. The results illustrated that 7 mg of the nanocomposites were able to degrade methylene blue after 20 min of exposure [24]. On the other hand, M. Moztahida et al. used magnetite loaded into reduced graphene oxide for photocatalytic degradation of 2-methylisoborneol (MIB). The results showed that pure magnetite degraded only 22.5% of MIB, while the nanocomposite was able to remove around 99% after 180 min of continuous irradiation [25].
Vanadium pentaoxide (V 2 O 5 ) is one of the unique transition metal oxides because of its physical and chemical properties, which enable it to be used widely in various applications as gas sensors and photo-catalysis. Thus, nanocomposites based on vanadium oxide (V 2 O 5 ) have been investigated for dye removal for decades. For instance, H. EL-Sheshtawy et al. examined the removal of MB using composites of g-C3N4/V 2 O 5 modified with Ag nanoparticles. The results indicated that the composition was able to degenerate MB completely after 60 min of irradiation [26]. The bandgap of V 2 O 5 is around 2.6 eV, while its calculated conduction and valence edges are around 0.3 eV and 2.9 eV [27]. Concerning their biological utilization, W. Ma et al. used vanadium oxide nanorods for antibacterial applications. The results showed that the released ROS were able to degenerate E. coli and S. aureus bacterial cells and thus accelerated wound healing [28].
In this regard, this work aims to investigate the effect of compositional variation of Mn-magnetite/V 2 O 5 on their ability to degrade dyes, besides their antibacterial activity based on their magnetic.

Materials
Analytical grade of oxalic acid dihydrate, ammonium metavanadate, ferrous chloride tetrahydrate (FeCl 2 ·4H 2 O), nitric acid, ferric chloride hexahydrate (FeCl 3 ·6H 2 O), manganese chloride tetrahydrate (MnCl 2 ·4H 2 O), sodium metavanadate and ammonium chloride were used without further purification and were obtained from Sigma-Aldrich Company. Figure 1A reveals formation of Mn-doped magnetite by co-precipitation method. First, suitable amounts of (FeCl 2 ·4H 2 O), (MnCl 2 ·4H 2 O), and (MnCl 2 ·4H 2 O) have been dissolved in deionized water at room temperature. Then, the pH value was adjusted to be 11 by adding droplets of ammonia solution under vigorous stirring. Finally, the resultant precipitate was washed by ethanol and distilled water several times and then dried at 90 °C On the other hand, Fig. 1b shows that V 2 O 5 was synthesized upon the dissolving of sodium metavanadate in distilled water. Then, the ammonium chloride (1.2 g) was droped into the solution, followed by an increase of the temperature to 80 oC. Then, pH value was increased to 8. The obtained produced was then annealed at 600 oC for 4 h.

Synthesis of Mn 0.4 Fe 2.6 O 4 /V 2 O 5 nanocomposites
Different weights of both V 2 O 5 and Mn 0.4 Fe 2.6 O 4 were added into a Falcone tube containing 50 ml of distilled water. The solution was then sonicated under a high-frequency probe sonicator for 30 min. The obtained gel was then precipitated and dried at 50-70 °C.
The optical properties, absorbance, and reflection spectra were acquired via the UV-Vis spectrophotometer (Bio Aquarius CE 7250, UK) and information about the bandgaps.
The degradation of methylene blue (MB) was done under a visible light irradiation source of (500 W). First, 0.1 mg of each composite was added into a 10 mL solution of the MB that contains ~ 10 ppm (20 ml). The total time of exposure is 35 min; every 5 min, around 3 mL aliquot of the dye was taken for testing using the double beam spectrophotometer (Bio Aquarius CE 7250, UK). The degradation effectiveness (DR) was calculated [29,30]: where C o is the initial at t = 0.0 min; C t is the absorbance at any time of the interval.
The antibacterial effectiveness was investigated against Escherichia coli (E. coli) and Staphylococcus aureus (S. aureus). Standard agar plate assays were used, whereas the initial concentration of V 2 O 5 /Mn-magnetite powders was 0.5 mg/mL. The test was repeated in the dark and in the light conditions under visible light at 37 °C [60]. Each test was repeated three times to calculate the standard deviation. The results were compared with the standard sample, which was selected as amoxicillin with a concentration of 30 µg/mL. The control sample was considered as a bacterial disk without additional nanocomposites.

Crystal structure study
The crystal configuration of the fabricated nanocomposites was investigated by XRD, as obvious in Fig where D is the crystallite size, 0.9 represented the shape factor, λ is the wavelength of X-ray, β donates the full width at half maximum, and θ reveals to the Bragg angle.   Table 1. The data in Table 1 show that by increasing V 2 O 5 content, the crystallite size decreased as the lattice distortion increased. The calculated crystallite size depicts a decreasing behavior upon the increasing of V 2 O 5 contribution starting from 79 to 50 nm. Thus, the crystallinity of V 2 O 5 seems to be lower than that of the Mn-magnetite nanoparticles. This behavior is assigned to the difference in preparation methods.

Microstructural features
In Fig. 3a, b, TEM micrographs exhibit the formation of dual phases of Mn-magnetite and V 2 O 5 . It could be distinguished that V 2 O 5 was formed as rod shapes with diameters in the range of 20-30 nm and length exceed 450 nm. On the other hand, Mn doped magnetite seems to be configured with ellipsoidal shapes with dimensions around 25-40 nm. Good distribution of the particles can be detected, while the smaller size of V 2 O 5 than magnetite matches well with the calculated crystallite size from XRD results.   Table 2 reports the roughness parameters. It can be noticed that the roughness average (Ra) decreases from 40.3 significantly to 22.8, while the root mean square roughness (Rq) decreases from 57.8 to 29.2 nm from the lowest additional Mn-magnetite to the highest one. Furthermore, the maximum height of the roughness (Rt) starts from 507.0 and plunges to 220.6 nm, while the maximum roughness valley depth (Rv) begins with 281.3-117.6 nm. The high divergence between values of Rt and Rv indicates the inhomogeneity of roughness components. In other words, the surface topography can denote two components, heights, and notches. Herein, the expression of skewness might help   crystalline V 2 O 5 through the grains Mn-magnetite causes increasing of notches. Moreover, the presence of crystallographic defects on the grain surface acts as ionic traps for surrounding ions [32,33]. This ability to establish chemical bonds with the ambient environment is vital for degrading dyes or interact with cellular walls. Furthermore, the physical adhesion is promoted as a result of the heights, and it can be crucial to initiate the chemical one [34]. Controlling the compositional contribution is a good tool to manipulate the surface topography and thus promoting the dye removal effectiveness.

Magnetic study
Relation between molar magnetic susceptibility as a function of temperature of xMn 0.4 Fe 2.6 O 4 /(1-x)V 2 O 5 ; x = 0.0, 0.2, 0.4, 0.5, 0.6, 0.8, 1.0 is shown in Fig. 6. The magnetic susceptibility was measured upon the variation of temperature under different applied fields. It is obvious that curves start from different points on the axis of magnetic susceptibility. These values are inversely proportional to the applied field. The behavior of magnetic susceptibility seems to gradually deteriorate upon the increase of temperature reaching the zero value (Curie temperature). The raising of thermal energy due to the increase of temperature encourages thermal agitation and induces entropy through the system, which enlarges the dipoles disordered, and thus, susceptibility deteriorates. Furthermore, the inversely proportional between the magnetic susceptibility and magnetic field intensity is assigned to the disability of dipoles to be aligned with the field direction. The temperature point in which the dipoles cannot follow the direction of the applied field. By increasing V 2 O 5 content from 1 up to x = 0.6, the curie temperature decreased due to diamagnetic nature of V 2 O 5 . From x = 0.5 to x = 0.2, the curie temperature has higher value due to enhancement of grain growth which is attributed to existence of large number of cation vacancies presented by presence of V. +5 From Fig. 7, the straight part of all prepared samples fitted with Curie-Weiss law, from the paramagnetic region, some magnetic parameters as Curie constant, Curie Weiss constant and effective magnetic moment can be calculated, Fig. 7 Varying of reciprocal molar magnetic susceptibility on the temperature at different applied magnetic fields for nanocomposites of Mnmagnetite/V 2 O 5 using Faradays method from the linear fitting of the obtained curves, the paramagnetic region obey to Curie-Weiss law, and thus, Curie constant (C), Curie Weiss constant (θ), and effective magnetic moment (μ eff ) can be calculated by the following equations: The obtained magnetic constants were scheduled in Table 3. It could be noticed that the critical concentration of V 2 O 5 is x = 0.5, at this ratio, there is a fetal change in magnetic parameters, which is suitable for various applications.
Finally, the magnetic behavior of the nanocomposite with varied x values can be explained by the presence of (3) Curie const(C) = 1 slope diamagnetic behavior related to V 2 O 5 at all the indicated temperatures [10] but in another hand we found that V 2 O 5 has small ferromagnetic component due to the presence of small amount of V +4 in the sample in addition to enhancement of grain growth which is attributed to existence of large number of cation vacancies presented by presence of V +5 . Consequently, the curie temperature, Curie constant, curie Weiss constant and effective magnetic constant have their lowest values for x = 0.6 and 0.8.

Antibacterial activity
The improvement of the ability of materials to interact with the bacterial cell under light irradiation denotes its photoactivated antibacterial effectiveness. As obvious from Fig. 8, the antibacterial offense has been investigated in dark and light conditions. The inhibition zone increased upon the increasing of V 2 O 5 content through the nanocomposite starting from 17.4 ± 1.1 and 16.9 ± 1.3 mm to 10.6 ± 0.8 and 11.3 ± 1.4 mm in the dark, while it plunged from 19.2 ± 1.4 and 18.9 ± 1.5 mm to 10.5 ± 0.7 and 11.9 ± 0.9 mm under visible light, against E. coli and S. aureus, respectively. It can be noticed that there is a significant change in inhibition zones from dark to light conditions, whereas the inhibition zone increased from 17.4 ± 1.1 to be 19.2 ± 1.4 mm against E. coli from dark to light conditions. The release of ionic species [5,11,16,28,35] from the nanocomposite, including V 5+ , Mn 2+ , Fe 2+ , besides the oxyanions of OH − , O 2 − possess high reactivity against the cellular tissue of bacteria. It can be noticed that the incidents of photons due to Table 3 Magnetic constants of Curie temperature (T c ), Curie Weiss constant (θ), curie constant (c), and effective magnetic moment (μ eff )  light irradiation can generate charge carriers to be pumped through the conduction band (C.B.). These charges represent an additional source of ionic species which are able to degenerate bacterial cells.

Dye degradation
The degradation of methylene blue (MB) under visible light irradiation was examined using adsorbents of nanocomposites containing different contents of V 2 O 5 and Mn-magnetite. As shown in Fig. 9a The photocatalytic degradation is based on the ability of an adsorbent substance to pump electrons due to the bombarding of photons with moderate energy [35]. The generated photoelectrons jump to the C.B. to participate in the current. The increasing of V 2 O 5 with its low bandgap (2.28 eV) can facilitate the motivation of electrons. Furthermore, the high surface area of the fabricated compositions might decrease the charge collisions, and thus, the recombination ratio. The produced oxyanions, including OH − , O 2 − , and other cations, are responsible for the high degradability of the MB. The degradation behavior was studied using the pseudo-first and pseudo-second-order kinetics as follows [36]: Pseudo first-order model: Pseudo-second-order model: where k 1 and k 2 are the pseudo first-, second-order and inter-particle diffusion rate constants in (min − 1 ) and (g mg − 1 min − 1 ), respectively.
The kinetics constant (k) was calculated from the fitting of the equation and summarized in Table 4. The correlation coefficients related to the pseudo-second-order model (0.94-0.99) were higher than those for the pseudo-first-order model (0.74-0.94), which recommends that the adsorption process follows a pseudo-second-order model, and it was noticed that values of the kinetics constant correlated with the contribution of V 2 O 5 . The highest value was 8.3 × 10 -2 min − 1 and 0.191 g/ mg.min. − 1 , while its lowest value was 2.8 × 10 -2 min − 1 and 0.137 g/mg.min − 1 for the pseudo-first-and pseudo-secondorder constant respectively, which accompanied the highest and the lowest additional V 2 O 5 . Table 5

Conclusion
Nanocomposites based on different contents of V 2 O 5 / Mn-magnetite were fabricated. The TEM graphs exhibited that the V 2 O 5 were formed in a rod shape with diameters of Data availability Data sharing not applicable to this article as no datasets were generated or analyzed during the current study.

Conflict of interest
The authors declare that they have no conflict of interests.
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