Degradation of Methylene Blue Using Hydrothermally Synthesized α -Manganese Oxide Nanostructures as a Heterogeneous Fenton Catalyst

Lately, the upsurge in the liberation of synthetic dyes into the environment, primarily by the textile industries, is a threat to the natural habitat and existing ecosystem. Various methods such as adsorption and degradation with nanoparticles are currently being used to degrade those hazardous materials, but still, the yearning for novel methods continues. In this study, hydrothermal reactions were performed at 160 ° C to synthesize manganese dioxide nanostructures (MnNSs) under di ﬀ erent incubation periods that facilitated the comparison of the size, morphology, and crystallinity of MnNSs. The study revealed the change in crystallinity over the incubation period; MnNSs prepared at 24 hrs were highly crystalline among others. Additionally, the size and morphology of MnNSs changed from the sea-urchin or ﬂ ower-like structure, predominantly sheet/ layer form, to nanorods as the reaction proceeded for 24hrs. Characterization of MnNSs was followed by heterogeneous Fenton ’ s reaction, using α -manganese dioxide nanostructures, for the degradation of methylene blue (MB). To further understand the catalytic activity of MnNSs, the synthesized nanostructures were subjected to degrade MB at varied time intervals, both with and without hydrogen peroxide (H 2 O 2 ). Catalytically, MnNSs evinced good potential for degrading MB dye in the presence of H 2 O 2 ; MnNSs prepared at 24 hrs degraded MB up to 73% within 110 minutes.


Introduction
Effluents from the textile industries essentially contain synthetic dyes that contaminate the abiotic as well as biotic components of an ecosystem and so forth causing major environmental problems [1]. About 700,000 metric tons of dyes (azo, disperse, fast color bases, ingrain, organic pigment colors, sulfur dyes, etc.) encompassing greater than 10,000 types are abundantly produced and employed for imparting color in the textile industry. They have raised ecological concerns on account of their persistence, harmfulness, and bioaccumulation in living organisms [2]. Such detrimental effects of dyes are not exclusive to humans and are also noxious to different plants and creatures in the biological system, so these dyes should be dealt with using proper removal techniques. MB, also called methylthioninium chloride, is the most widely used nonbiodegradable recalcitrant dye from the textile industries that demands advanced methods for its removal from the water sources. Among the various methodologies, the viable and technological strategies involve advanced oxidation processes (AOPs) utilizing peroxide, peroxymonosulfate, ozone, and other oxidizing agents for producing reactive species to disintegrate dyes [3].
Fenton reaction is the typical hydroxyl-based, costeffective advanced oxidation process whose application is, however, restricted due to the low pH range and sludge generation during the reaction [4]. Heterogeneous Fenton reactions are used to overcome the problems of a homogenous reaction, in which, iron-based materials such as Fe 3 O 4 , Fe 2 O 3 , and FeOOH have been employed to decompose H 2 O 2 into OH • radical [5]. Nevertheless, the Fe-based process is still inefficient at the higher pH which highlights the demand for effective heterogeneous catalysts. The metal oxide nanostructurebased heterogeneous Fenton-like catalytic process is getting attention due to its large pH range and reduced sludge generations [6]. Among the various metal oxide-based nanoparticles, MnO 2 is a potential alternative due to its benign nature, diverse oxidation states, optimal surface area, flexible redox potential, and availability in various phases [7].
Manganese dioxide exists in varied crystalline phases according to the arrangement of MnO 6 octahedra in them. Among them, alpha (α) phases having a double-chained nanotube structure have displayed promising catalytic activities toward oxygen reduction reactions (ORR) [8]. Peng et al. reported that nanowires (NWs) of α-MnO 2 possess a substantially larger surface with a reduced negative surface charge density than α-MnO 2 nanotubes, which is presumably the fundamental explanation for their higher catalytic activity [9]. Similarly, NWs of α-MnO 2 showed outstanding Fenton-like catalysis for decolorizing the RB5 dye with H 2 O 2 [10]. Cao [12]. Additionally, Le et al. prepared MnNSs on laterite and demonstrated its good adsorption and heterogeneous Fenton catalytic oxidation activity in the degradation of MB [13].
Among the various synthetic approaches applied for the fabrication of MnO 2 nanostructures, the hydrothermal process is environment-friendly, and the simplest method, the size, shape, and phases of the particles, can be tuned according to the reaction temperature, duration, and composition of the reacting mixture [14]. Xu et al. reported the formation of hollow sphere and urchin of α-MnO 2 by hydrothermal process and studied the effect of the reaction time on their capacitance property [15]. Similarly, Bai et al. fabricated δ-MnO 2 and ɑ-MnO 2 nanostructures under hydrothermal conditions and revealed the formation of flower-like δ-MnO 2 at lower temperature and needle-like α-MnO 2 at higher temperature [16]. Herein, the effect of the incubation period on the morphology, crystallinity, and catalysis of MnNSs synthesized by the hydrothermal method was studied; then the nanostructures were employed for degrading MB as a heterogeneous Fenton catalyst. To illustrate the heterogeneous catalytic activity of MnNSs in the degradation of the MB, the reaction was carried out both in the presence and absence of H 2 O 2 .

Chemical Reagents.
Analytical grades of all chemicals including manganous sulfate, potassium permanganate, methylene blue, and hydrogen peroxide were purchased from Thermo Fisher Scientific (USA). They were used without further purification.      Journal of Nanomaterials   Journal of Nanomaterials (H1), 6 hrs (H2), 12 hrs (H3), and 24 hrs (H4), and the prepared nanostructures were separated by centrifuging at 5000 rpm and washed with ethanol three times. Then, the nanostructures were dried in an oven at 80°C to remove moisture followed by calcination in the muffle furnace at 300°C for 3 hrs.  [11,17]. In brief, 25 mL of MB, 60 mL of distilled water, and 15 mL of 30% H 2 O 2 were mixed. Then, 25 mg of MnNSs was added to the mixture. After that, the mixture was allowed to react in a shaking incubator at 28°C at 200 rpm. Then, 4 mL of the reaction mixture was taken at a certain time interval and centrifuged to remove MnNSs. Further, the absorbance of the reaction mixture was taken and monitored at the wavelength range of 300-800 nm.

Characterization of
2.7. Degradation in the Absence of Hydrogen Peroxide. 25 mL of MB and 75 mL of water were mixed, and 25 mg of MnNSs was added to the mixture, which was allowed to react in a shaking incubator at 28°C at 200 rpm. Then, 4 mL of the reaction mixture was taken at a certain time interval and centrifuged to remove nanostructures. Finally, the absorbance of the reaction mixture was measured at a wavelength ranging from 300-800 nm.  Figure 1. The well-defined diffraction peaks of the samples observed in Figure 1 are attributed to facets of tetragonal α-MnO 2 (JCPDS, card no:044-0141) in the XRD pattern shown in Table 1 [18]. It was seen that at lower reaction time, the peaks were broad which indicated the presence of the noncrystalline nature of the nanostructures [3]. However, the peaks at 24 hours were sharp and intense which was indicative of the highly pure and crystalline nature of MnNSs [18]. The gradual increment in the crystallinity with the increase in the reaction time is obvious and expected, finally leading to the formation of highly crystalline MnNSs at 24 hours of hydrothermal reaction.

SEM Analysis.
The morphology of the prepared nanostructures was analyzed by SEM. Figure 2 depicts the SEM images of MnNSs prepared at different times. First, the urchin-like structure is formed (Figure 2(a)). As the reaction time increased, due to greater surface energies, the inner cavity was progressively created by a core evacuation process. With the increase in the reaction time, the urchin-like structure (~500 nm) changed to a nanorod, the size and aspect ratio of individual nanorods (Figures 2(b) and 2(c)), and finally, the rod-like structure is seen (Figure 2(d)). The diameter and length of the nanorods are around 50 nm and a few micrometers, respectively. The time-dependent Ostwald ripening phenomenon can justify the discrepancies in MnNSs, in which, reaction time appears to be significant for the creation of hollow structured α-MnO 2 to the development of nanorods. As per the Ostwald ripening process, large numbers of nuclei are generated within a short period after which the crystal gradually grows following the reaction time in which the aggregate expands in size and density until it forms a spherical shape with a solid center [19]. The stage can take several hours, after which a core evacuation process progressively forms the internal cavity as a result of higher surface energy; the increase in reaction time not only completely damages the urchin-like structure but also expands the dimensions of each nanorod [15].

TEM Analysis.
The TEM analysis shows that with the increase in the reaction time, the sheet-like structures turned into crystalline nanorods which were also corroborated by the XRD and SEM analysis. In the TEM images, it can be seen that during a shorter reaction time of 3 hrs (Figure 3), 7 Journal of Nanomaterials nanosheets are formed, but as reaction time increases, the nanosheets disappear, and well-crystallized nanorods are seen progressively from 3 to 24 hr (Figures 3(a)-3(d)) reaction time with an aspect ratio of~27 (6 hrs),~29 (12 hrs), and~11 (24 hrs) [15].

Catalytic Activity.
The catalytic activity of the synthesized four different samples (H1, H2, H3, and H4) was tested for the degradation of the MB dye at 28°C and neutral pH. The degradation of MB dye was monitored with the gradual decrease in the absorption peak at 664 nm in the UV-Vis absorption spectra with the increase in reaction time (Figures 4(a)-4(h)), which is the characteristic absorption peak of MB.
When MnNSs were used in the presence of H 2 O 2 , there was a considerable decrease in the absorption peaks of the MB. Figures 4(a), 4(c), 4(e), and 4(g) show the decrease in absorbance of MB with the increase in the time when H1, H2, H3, and H4 were used, respectively, in the presence of H 2 O 2 . Similarly, Figures 4(b), 4(d), 4(f), and 4(h) represent the respective activity of H1, H2, H3, and H4 on the MB in the absence of H 2 O 2 . Here also, the peaks broaden with the timing, but the broadening of peaks was less compared to the degradation in the presence of H 2 O 2 . As more reaction time passes, the fall of MB peaks continues but at a slower rate compared to the previous drop. In general, the efficiency of degradation of MB molecules (r) in this study was calculated as follows: where C 0 (ppm) is the initial concentration of MB solution and H 2 O 2 solution and C (ppm) is the concentration of a mixture of solutions at a different time interval. By utilizing Equation (1), the degradation percentage of MB blue using different MnNSs is given in Table 2.
The time profile of MB degradation accompanied by H 2 O 2 (black) and the absence of H 2 O 2 (red) at different time intervals is demonstrated in Figures 5(a)-5(d). The rate of (percentage of MB) degradation accompanying H 2 O 2 increases in the order of H1(61%) < H2(63%) < H3(63.1%) < H4(73%) in 110 minutes. This increment in degradation rate is due to the increment in the crystallinity and the surface area of the synthesized MnNSs. Similarly, the decrease in the intensity of peaks in absence of H 2 O 2 may be due to the adsorption of the MB on the surface of the MnNSs. 8

Journal of Nanomaterials
The rate of adsorption is in the order of H4(10.25%) < H1(18.06%) < H2(27.87%) < H3(34.23). This order can be partly justified by the specific surface area of the particles; adsorption increases with the surface area increment for the same type of MnNSs [20]. However, TEM images (Figures 3(b) and 3(c) revealed that there is a mixture of MnNSs in H2 and H4. So, a comprehensive study of morphology, crystal structures, and surface conditions is required for a complete explanation of the above order [21]. When MnO 2 and H 2 O 2 were used together, the efficiencies of dye degradation were in the order of H4 > H2 > H3 > H1. The study shows that the catalytic activity of the synthesized MnNSs depends not only on their size but also on their morphology. In a highly compact and organized structure of well-crystallized materials, electron transfer might be effort-less and swift, eventually leading to better catalytic activity in crystalline α-MnO2 (H4) nanorods than sheets and flowerlike nanostructures, even though the former had a smaller surface area than the latter [3,7]. Table 3 illustrates the comparison of several catalytic systems for the degradation of MB under varied reaction conditions. The result obtained in this study shows that the ability of MnNSs to degrade MB in presence of H 2 O 2 is comparable to and often even superior to that of other catalytic systems.

Probable Mechanism of Degradation of Methylene Blue
(MB). Based on our research and literature, an adsorptionoxidation-desorption mechanism may lead to the degradation of MB [23]. The MB molecules and H 2 O 2 are first adsorbed on the surface of the MnNSs. The adsorption of   Journal of Nanomaterials MB on MnNSs may occur through the covalent bond between electron-pair on the nitrogen atom of MB and the manganese atom of manganese dioxide or through ionic interaction between the electropositive sulfur atom of MB and the electronegative oxygen atom of manganese dioxide [34] as depicted in Figure 6.
Similarly, H 2 O 2 is likely to be adsorbed on the surface of MnNSs via hydrogen bonding followed by its decomposition into free radical species such as OH • , HOO • , and O 2 ( Figure 7). Here, degradation is an important reaction preceded by a heterogeneous Fenton-like reaction; the MnNSs show mixed-valence states as its framework contains Mn 3+ , Mn 4+ , and possibly Mn 2+ , due to which the decomposition of H 2 O 2 as shown in reactions (2), (3), (4), (5), and (6) is thermodynamically feasible [35,36]. Moreover, MnNSs catalytic activity is good at neutral pH [37].
Several species, mainly free radicals, such as OH • , HOO • , and O 2 are believed to be responsible for the decoloration of dye. The OH • radical has a high oxidative capacity which decomposes adsorbed molecules on the nanostructures to generate CO 2 , H 2 O, and other small chemical entities. These small molecules can quickly leave the surface of the MnNSs (desorption) leading to the recovery of the catalyst, and since the catalyst is insoluble in water, it can be easily separated from the reaction mixture by centrifugation and reused. Similarly, if free radicals generated from the decomposition of H 2 O 2 are more than the free radical employed, they will    10 Journal of Nanomaterials combine to form molecules; therefore, the decomposition of H 2 O 2 to produce OH • should match the dye degradation to ensure efficient degradation [38].

3.2.2.
Reusability of the Catalyst. The reusability of the catalyst was evaluated by reusing the recovered MnNSs-H4 catalyst system for two more successive MB degradation reactions ( Figure 8). The used MnNSs catalysts were separated by centrifugation, recovered by filtration and utilized for the next cycles of MB degradation. The reusability test findings revealed that after replicating the catalytic reaction three times, 70.31% of the MB could still be degraded, demonstrating high stability of the MnNSs catalyst in the MB degradation process. The slight decline in the degradation rate for subsequent recycle reactions is mainly attributed to the blockage of active sites of the catalyst due to increasing by-products and the mass loss during the recovery process (filtration and transfer) [32].

Conclusions
From the study, it is concluded that the morphologies of MnNSs can be effectively tuned with reaction time in the hydrothermal process, and with the rise in reaction time ( Scanning electron microscopy TEM: Transmission electron microscopy UV-Vis: Ultraviolet-visible XRD: X-ray diffraction

Data Availability
The data used to support the findings of this study are included in the article.