Fenton-like reaction-induced degradation of Methylene Blue by using supermacroporous ferrimagnetic nanorings

The removal and/or degradation of methylene blue (MB) from dyestuff wastewater has attracted widespread attention. Utilization of environmental purification nanomaterials is an effective means in the field of environmental remediation, and degradation efficiency under different circumstances is always a high priority for the nanoagents. In this study, uniform supermacroporous ferrimagnetic Fe3O4 nanorings (sFe3O4-NRs) were fabricated for high-efficiency MB degradation. Typically, the sFe3O4-NRs were prepared by a convenient hydrothermal method. Subsequently, the morphology structure of the sFe3O4-NRs was characterized by scanning electron microscope (SEM), transmission electron microscope (TEM) and highresolution transmission electron microscopy (HRTEM), respectively. Next, a Fenton-like reaction-induced MB degradation was performed in the present of sFe3O4-NR nanocatalysator and hydrogen peroxide (H2O2). Meanwhile, the sFe3O4-NRs showed excellent Fenton-catalytic activity for degradation of MB in a wide range of pH (3-11). Moreover, because of the magnetic property of the sFe3O4-NRs (saturation magnetization of 34.26 emu/g), the used sFe3O4-NR could be rapidly separated from the reaction medium by using a magnet, the sFe3O4-NR resented relative high catalytic activity even after 10 times reuse. The main conclusion from this work was that the as-synthesized sFe3O4-NRs nanoagent was a type of desirable Fenton catalyst to degenerate MB from wastewater.


Introduction
During the past several decades, with the fast developing on chemical industry, organic dyes have been extensively used in the industrial production of textiles, paper, plastics, leather, food, printing, cosmetics, pharmaceuticals, and other industries [1,2]. Most of organic dyes are harmful, toxic, carcinogenic, or even fatal [3,4]. However, the organic dyes in general are exceptionally stable and resistant to biodegradation and photolysis [5]. What's more, as hypertoxic and carcinogenic pollutants, most of dyes pose a grave threat to water ecological security and human health, some water-soluble dyes are harmful under even minute quantities (e.g., less than 1 ppm) [6]. Wherein, Methylene blue (MB), is one of the most often utilized basic dyes in industry which could cause serious public health issues, including vomiting, tissue death, limb paralysis, and even death when released into the drinking  [7]. Furthermore, because of the stable and complex aromatic molecular structure, MB is hard to be removal or degraded under natural conditions. Therefore, there is an urgent need for efficient treatment of the MB effluent.
According to previous reports, multifarious physical and/or chemical techniques have been exploited for the treatment of dyestuff wastewater containing MB, such as membrane separation, physical adsorption, biological degradation, chemical oxidation, electrocatalytic/photocatalytic degradation, and Fenton-like catalytic degradation [8], and so on. Among these purification techniques, the Fenton-like catalytic degradation is demonstrated to be a preferred method for degradation of dyes to harmless products by using hydrogen peroxide (H 2 O 2 ) as a pro-oxidant. Typically, by using Fentonlike agents, such as iron oxides nanomaterials, the Fenton-like catalytic degradation reactions have been revealed to produce free radicals (•OH) via catalyzing H 2 O 2 with ferrous ions (Fe 2+ /Fe 3+ ) [9]. In previous studies, the accepted mechanism of •OH generated in Fenton-like reaction includes a series of cyclic reactions, which can be described by the classical Haber-Weiss cycle (Eqs. (1) -(7)) [10].
Herein, we presented a feasible one-step hydrothermal method to fabricate supermacroporous ferrimagnetic Fe 3 O 4 nanorings (sFe 3 O 4 -NRs). With the assistance of H 2 O 2 , Fe 2+ /Fe 3+ released from the sFe 3 O 4 -NRs were used as a Fenton-like agents for MB degradation by a Fenton-like catalytic reaction. Moreover, the sFe 3 O 4 -NRs could be easily separated from the processed medium with an external magnet for reuse. All characterizations indicate that the sFe 3 O 4 -NRs were uniform, well dispersed, supramaximal porous and favourable magnetizable, which make them ideal for use as Fenton-like catalytic degradation. Scheme 1 shows the Fenton-like degradation mechanism of sFe 3 O 4 -NRs. Scheme 1. Overview of MB degradation mechanism of sFe 3 O 4 -NRs.

Synthesis of sFe 3 O 4 -NRs
Firstly, supermacroporous α-Fe 2 O 3 nanorings were synthesized via a simple hydrothermal method and then used as a precursor to fabricate sFe 3 O 4 -NRs [11]. In this experimental reaction procedure, FeCl 3 (0.03 M), NH 3 H 2 PO4 (0.25 mM) and Na 2 SO 4 (0.64 mM) were dissolved into 75 mL distilled water. The maple solution was stirred for 30 min under room temperature with N 2 protection. Until the ferric salt is dissolved completely, the mixture solution was then transferred into a 100 mL Teflon-lined stainless-steel autoclave. The hydrothermal reaction was carried at 230 °C for 24 h. Next, the autoclave was cooled down to room temperature, the yellow precipitates were separated and collected by centrifugation at 7500 rpm. The final products (α-Fe 2 O 3 -NRs) were washed with deionized water and ethanol for eight times and then dried at 60 °C for 10 hours.
To prepare sFe 3 O 4 -NRs, a reduction process was taken by using α-Fe 2 O 3 -NRs as the precursor. In brief, 100 mg α-Fe 2 O 3 -NRs were annealed in a tube furnace at 400 °C under a continuous reducing gas flow (v/v: H 2 /Ar = 5%) for 10h. The resulting sFe 3 O 4 -NRs was finally collected with an external magnet and washed with distilled water and ethanol each for eight times, respectively.

Catalytic activity measurements
Next, the synthesized sFe 3 O 4 -NRs were evaluated through the oxydic degradation of MB in the presence of H 2 O 2 . Typically, effects of pH (3.0-11.0) were measured during the degradation process. Meanwhile, initial concentration of MB was investigated for dyes degradation. Typically, sFe 3 O 4 -NRs (5.0 mg) was firstly dispersed into MB solution that containing 1.0 ml H 2 O 2 at room temperature. Then, 1 µL sample was measured in time course by monitoring the absorbance at 665 nm with a UVvis spectrophotometer.

Statistical analysis
Significance test was taken by using an analysis of variance and Tukey's test (OriginPro, version 9.0, OriginLab Corporation, America). Statistical significance was established at p < 0.05.

Morphology of the sFe 3 O 4 -NRs
The morphology of as-synthesized sFe 3 O 4 -NRs was observed by SEM ( Figure.

Magnetization
To measure the magnetic properties of sFe 3 O 4 -NRs, hysteresis loops text was taken under room temperature. As shown in Figure 5, saturation magnetization of the obtained sFe 3 O 4 -NRs was about 58.6 emu/g, which revealed that the sFe 3 O 4 -NRs have excellent Magnetic strength and can be easily separated by an external magnet.

X-ray diffraction pattern
Then composition of the magnetic nanorings were further detected by X-ray diffraction (XRD) . As shown in Figure 6, the results indicated that all characteristic peaks (30.12 °, 35.51 °, 43.12 °, 53.41 °, and 57.04 °) matched well with pure spinel

MB Degradation
The quantitative MB degradation efficiency of the sFe 3 O 4 -NRs was determined by UV-vis. The decoloration optical image was shown in Figure 7, briefly, after 6 hours Fenton-like catalysis, with the assistance of H 2 O 2 , sFe 3 O 4 -NRs exhibited excellent performance in MB degradation under different pH ( Figure 8). Finally, the MB molecule was degraded to various inorganic ions ( Figure 9) such as SO 4 2− , NH 4 + , and NO 3 − , respectively. Accordingly, with the assistance of H 2 O 2 , the sFe 3 O 4 -NRs exhibited excellent Fenton catalytic efficiency for dyes decoloration though a typical Fenton-like reaction. Moreover, the reuse ability is very important for nanomaterials, Figure 10 shows the reuse performance of sFe 3 O 4 -NRs in MB degradation, the magnetic nanorings can retain above 85% of its original activity after being used for 10 cycles.

Conclution
In summary, sFe 3 O 4 -NRs were successfully prepared and used as Fenton-like nanoagents for MB degradation. The sFe 3 O 4 -NRs exhibited excellent Fenton-like catalytic activity under a wide range of pH, and therefore could overcome the shortcoming of traditional Fenton reaction in narrow pH ranges. Moreover, the sFe 3 O 4 -NRs could be separated from solution for reuse by an external magnetic field.