Adsorption Mechanism and Electrochemical Characteristic of Methyl Blue onto Calcium Ferrite Nanosheets

A rapid combustion process was applied to prepare CaFe2O4 nanomaterials using CaBr2·xH2O and Fe(NO3)3·9H2O as raw materials and CaFe2O4 nanomaterials were characterized by SEM, TEM, VSM, XRD, and FTIR techniques. The results showed that the prepared nanomaterials had a sheet-like structure, and for larger adsorption capacity of dyes, CaFe2O4 nanosheets prepared at 700°C for 2 h with average grain size was 93.3 nm, a thickness of 8.4 nm, and the saturation magnetization of 8.15 emu/g were employed as adsorbate for the removal of methyl blue (MB). The adsorption performance of MB onto CaFe2O4 nanosheets was investigated; CaFe2O4 nanosheets displayed favorable adsorption capacity, and the adsorption conformed to the pseudo-second-order model and the Freundlich model, which demonstrated that the adsorption process of MB on CaFe2O4 nanosheets belonged to multilayer chemisorption process. When the pH value reached 3, the adsorption capacity of MB by CaFe2O4 nanosheets kept maximum value of 478.07 mg/g; and after 5 regenerations, the removal efficiency of MB was reduced to 59.06% of the first time. The electrochemical behavior of MB onto the nanosheets was evaluated through CV in conjunction with EIS. The CaFe2O4 nanosheets revealed a promising prospect for the adsorption of dyes.


Introduction
Magnetic materials have widespread applications, such as electronics, automotive, environmental protection, and new energy sources [1][2][3][4][5][6][7][8]. Magnetic materials are divided into categories by nature: metallic and nonmetallic, the former mainly having metallic magnets such as Nd 2 Fe 14 B and nickel-based alloys [9][10][11][12], the latter mainly being ferrite nanomaterials [13,14]. Compared to metallic materials, ferrite, as a nonmetallic material, is cheap, technical maturity and stable performance, and is, therefore, used in a significant amount. In the realm of magnetic nanomaterials, ferrite nanomaterials are composite oxide consisting of iron in combination with one or more other suitable metallic elements. The most applied ferrite is the spinel type with the chemical formula MFe 2 O 4 or MO·Fe 2 O 3 (M = Mg, Mn, Co, Zn, Cu, Ca, etc.). M is composed of a divalent mental ion or a plurality of mental ions whose average valence is divalent [15,16]. Among them, CaFe 2 O 4 have received extensive attention because Ca 2+ is not a heavy metal [17].
CaFe 2 O 4 is p-type semiconductor material, which has the advantages of nontoxicity, environmental friendliness, and good biocompatibility [18][19][20]. At present, there are many methods for preparing CaFe 2 O 4 nanomaterials, such as solid-phase reaction [21], coprecipitation [22], one-step hydrothermal [23], sol-gel [24], and electrospun [25]. The above methods, however, prepare ferrite products with poor homogeneity, high costs, and complex reactions. Rapid combustion process is a convenient and effective way to prepare polymetallic and monometallic oxides [26]. Compared with the other methods, it has the advantages of cheaper cost, simple manipulation, and controllable particle size.
Today, there is an increasing demand for dyes and the discharge of dye wastewater is gaining widespread attention. Dye wastewater contains harmful chemicals and is highly toxic, posing a serious risk to human health and safety, [27][28][29] such as methyl blue (MB) can cause skin irritation, profuse sweating, and cancer, [30]. The removal of dyes from wastewater is therefore urgent. The current methods of treating dyes are adsorption [31], photodegradation [32], and advanced oxidation [33]. Of these, adsorption method is the simplest and most effective technique due to its simplicity of operation, the variety of adsorbents, and environmental friendliness [34][35][36]. Ferrite nanomaterials have received a lot of attention from researchers as emerging adsorption materials. With a large specific surface area and many active sites, ferrite can quickly achieve solid-liquid separation under the action of an applied magnetic field, effectively avoiding secondary contamination [37].
In this work, an environmentally friendly adsorbent was obtained experimentally with the concept of regeneration and environmentally friendly utilization. CaFe 2 O 4 nanomaterials were prepared by a rapid combustion approach and characterized, and the adsorption mechanism of MB removal from simulated dye wastewater onto CaFe 2 O 4 nanosheets was investigated. The prepared CaFe 2 O 4 nanomaterials could efficiently remove MB from the solutions, which provided a reference for the application of ferrite nanomaterials in the treatment of dye wastewater, and had certain research significance. Then the mixed solution was burned and put into the programmed temperature-control furnace after the fire was extinguished. The calcination was carried out at 400-800°C, respectively. Finally, the nanomaterials after calcination were ground to get the powders.

Preparation and
Analyzing the characteristic of nanomaterial morphology by scanning and transmission electron microscopy (SEM, JSM-7001F, JEOL Ltd., Japan; TEM, JEM-2100, JEOL Ltd., Japan), X-ray diffraction (XRD, D8 ADVANCE, Germany) was applied to research the physical phase analysis and crystallinity with Cu-Kα and the diffraction angles range from 20-80°. The saturation magnetization was measured by the vibrating sample magnetometer (VSM, HH-15, Physcience Optoelectronics Co., Ltd., Beijing, China). Analysis of the functional groups and chemical bonds of the nanomaterials before and after adsorption by Fourier transform infrared spectra (FTIR, FTIR-370, Nicolet Avatar, America).    (1)(2)(3)(4)(5)(6)(7)(8)(9), different initial mass concentrations (800-3000 mg/L), and regeneration times on adsorbent adsorption performance. After the adsorption was completed by centrifugation at 10000 rpm, the supernatant was taken to measure the absorbance using ultraviolet spectrophotometer (UV), and then the remaining concentration was calculated from the MB standard curve, and the corresponding capacity of adsorption was calculated by

Adsorption
where C 0 and C (mg/L) were the solution concentrations before and after the adsorption of MB, respectively; V (mL) was the MB solution volume; m (g) was the mass of the adsorbent.

Electrochemical
Experiments. An electrochemical workstation was employed to research the electrochemical behavior of CaFe 2 O 4 nanosheets. Glassy carbon electrode, Ag/AgCl electrode, and platinum wire electrode with good electrical conductivity and chemical stability were employed as experimental electrodes, which play the roles of working, reference, and auxiliary. Cyclic voltammetry (CV) was observed at voltages of −0.2-0.7 V and 100 mV/s scan rate. Electrochemical impedance spectroscopy (EIS) was obtained for signal amplitudes for 5 mV and frequencies in the range 0.1-10000 Hz.

Characterizations of Magnetic CaFe 2 O 4 Nanomaterials.
The characteristics of as-prepared CaFe 2 O 4 nanomaterials were shown in Figure 1. SEM morphology (Figure 1(a)) showed that CaFe 2 O 4 nanomaterials were irregularly flaky textures. The average grain size of 40.2 nm for CaFe 2 O 4 nanosheets, but the actual average grain size was 93.3 nm, which indicated that superposition occurred during the preparation process. The average grain size was calculated by the Scherrer formula [13]: Where D was the average size of CaFe 2 O 4 nanosheets (nm); λ was the wavelength of the nanosheets for XRD measurements; β was the half-height width of the diffraction peak of the nanosheets; θ was the Bragg diffraction angle. The  (Figure 1(d)) was 8.15 emu/g, which was beneficial to the subsequent solidliquid separation.
As could be seen from Figure 2(a), the CaFe 2 O 4 was not formed at 400-500°C. When the calcination temperature reached 600°C, the CaFe 2 O 4 begin to form gradually, but the crystallinity was very poor. With the calcination temperature was further risen to 700-800°C, the intensity of diffraction peaks was enhanced and the crystallinity was fine. Figure 2(b) showed the relationship between different calcination temperatures and saturation magnetizations. The saturation magnetization decreased with the calcination temperature increasing from 400 to 500°C. When the calcination temperature was between 600 and 700°C, CaFe 2 O 4 started to form gradually and therefore the saturation magnetization   Figure 3 showed adsorption kinetics curves of CaFe 2 O 4 nanosheets with different MB concentrations. It could be seen that with the beginning of adsorption, the adsorption capacity increased rapidly, and after a certain time, the adsorption capacity gradually slowed down until the adsorption achieved equilibrium. The reason was that at the beginning of the adsorption process, the material surface had many vacant adsorption and binding sites, and the greater the concentration of dye was, the stronger the mass transfer drive, resulted in a greater chance of contact between the MB molecules and the material. As the concentration of dye decreased, the driving force decreased and the adsorption sites were largely occupied, and the adsorption slowly reached equilibrium. The largest adsorption amount was 436.01 mg/g with the initial concentration of 1100 mg/ L. The adsorption time was 60 min to reach adsorption equilibrium.

Adsorption Kinetics.
To further investigate the effect of adsorption time on adsorption capacity, the kinetics models were adopted to the data for fitting. The fitted curves for different kinetics models were shown in Figure 4. The correlation coefficients    (R 2 ) of the pseudo-second-order kinetics model were the largest compared with the pseudo-first-order kinetics and intraparticle diffusion models, which indicated that the pseudo-second-order kinetics model could better describe the adsorption of on MB. According to the mechanism of the pseudo-second-order kinetics model, the adsorption of CaFe 2 O 4 nanosheets on MB was chemisorption with electron gain and loss. The arguments fitted to three kinetics models were summarized as Table 1. 3.3. Adsorption Isotherms. The adsorption behaviors of the adsorbent for the dye can be evaluated with the adsorption isotherm model ( Figure 5). The fitted parameters for the three models were displayed from Table 2. A comparison of the correlation coefficients (R 2 ) showed that the Freundlich model had the largest correlation coefficient of 0.9907. This suggested that the Freundlich model was consistent with isothermal adsorption processes. According to Freundlich theory, the adsorption between the dye molecules and the surface of the CaFe 2 O 4 nanosheets was nonuniform. 1/n was in the range of 0.1-0.5, which indicated that the adsorption performance of CaFe 2 O 4 nanosheets was excellent.
The maximum capacity was investigated by comparing the MB adsorption with other articles, it was observed that the CaFe 2 O 4 nanosheets adsorption capacity of MB was much larger than those of the other adsorbents, which reflected that the superior adsorption performance of CaFe 2 O 4 nanosheets, and the relevant data were listed in Table 3.

Effect of pH and Thermal Regeneration Performance.
The experiment kept the constants of CaFe 2 O 4 nanosheets mass and MB concentration, and then the pH effect on the CaFe 2 O 4 nanosheets adsorption of MB was explored, the results were shown in Figure 6(a). When the pH of the solution was below 3.0, the CaFe 2 O 4 nanosheets exhibited an enhanced adsorption of MB as the pH value enhanced. The excellent capacity was observed at pH 3.0 with the adsorption capacity of CaFe 2 O 4 nanosheets was 478.1 mg/g. This might be due to the fact that the CaFe 2 O 4 nanosheets and dye molecules had positively charged at the same time when the pH was less than 3.0, which led to the strong electrostatic repulsion between CaFe 2 O 4 nanosheets and dye molecules, thus the adsorption capacity of the CaFe 2 O 4 nanosheets was decreased. With the increase of pH value, the positive charge held by the dye molecules progressively dropped, and the electrostatic repulsion also dropped, resulted in a gradual improvement in the capacity of adsorption of CaFe 2 O 4 nanosheets. The adsorption of MB capacity by the adsorbent remained largely unchanged when pH was greater than 3, mainly attributable to the electrostatic attraction of the negatively charged dye and the positively charged CaFe 2 O 4 and the saturation adsorption capacity of the CaFe 2 O 4 nanosheets [36,42].   (Figure 7(a)). Figure 7(b) showed the FTIR spectrum of MB, the spectrum described some characteristic peaks at 1172 cm −1 , 1126 cm −1 , 1033 cm −1 , and 1002 cm −1 . The characteristic peaks of MB appeared on the FTIR spectrum of the adsorbed CaFe 2 O 4 nanosheets indicated that MB was successfully adsorbed onto the CaFe 2 O 4 nanosheets (Figure 7(c)). The regeneration results of the adsorbed CaFe 2 O 4 nanosheets were presented in Figure 7(d). After calcination, the characteristic peak of MB disappeared completely, which indicated that the nanosheets could be regenerated by the mean of calcination.  Figure 8(a), where the bare MGCE had the highest peak current. The peak currents decreased sequentially with material and material modifications on the glassy carbon electrode. Ferrite nanomaterials were semiconductor materials with a high resistivity. When the materials were modified on the electrode surface, a certain spatial potential resistance was created, making it difficult for [Fe(CN) 6 ] 3-/4to approach the electrode surface, resulted in a decrease in peak current. After adsorption of MB, the nanomaterial was modified on the electrode surface and the negative charge carried by the MB and the [Fe(CN) 6 ] 3-/4were mutually repelled in the electrolyte, prevented it from approaching the surface of the electrode for electron switching and led to a peak current that was further reduced.
The electrochemical impedance spectrum consists of two parts, the semicircular part and the linear part. The electron transfer resistance can be obtained from the half-circle at high frequencies, and the linear at low frequencies corresponds to the diffusion process. As can be seen in Figure 8(b), the impedance of the bare electrode was lower compared to the impedance of the material and the material after adsorption of MB, and the impedance of the material after adsorption of MB was the highest. The results obtained were consistent with the CV plots.  average grain size of 93.3 nm, a thickness of 8.4 nm, and a saturation magnetization of 8.15 emu/g. The adsorption of MB by CaFe 2 O 4 nanosheets was eligible for the pseudo-second-order model and Freundlich model, which suggested that the adsorption process was multimolecular layer chemisorption and easy to adsorb. When the pH value was 3.0, the maximum adsorption capacity of CaFe 2 O 4 nanosheets for MB was 478.07 mg/g. After five regenerations, the CaFe 2 O 4 nanosheets still had a relative removal rate of 59.06%, which indicated excellent recyclability. This demonstrates that the prepared CaFe 2 O 4 nanosheets were an adsorbent for dye wastewater with practical applications.

Data Availability
The research data used to support the finding of this study are included within the article.