Facile green synthesis of Fe3O4 nanoparticles using aqueous leaf extract of Zanthoxylum armatum DC. for efficient adsorption of methylene blue

ABSTRACT We report a facile and green route for the synthesis of superparamagnetic magnetite nanoparticles (Fe3O4 NPs) using aqueous leaf extract of Zanthoxylum armatum DC. for efficient adsorption of organic pollutant, methylene blue (MB). The phytochemicals present in the aqueous leaf extract Zanthoxylum armatum DC. are not only responsible for the formation of Fe3O4 NPs, but also act as capping agent for Fe3O4 NPs. The formation of Fe3O4 NPs was confirmed by a range of spectroscopy and microscopy techniques such as UV–visible spectroscopy, Fourier transform infrared spectroscopy, powder X-ray diffraction (XRD), field emission-scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM) and vibrating sample magnetometer. The spectroscopic results indicated the successful formation of Fe3O4 NPs. FE-SEM and TEM images revealed the spherical shape with particle size of 17 nm. Fe3O4 NPs showed superparamagnetism. Green synthesized Fe3O4 NPs shows enhanced adsorption efficiency toward MB from contaminated water. The kinetic data of adsorption fitted very well with the pseudo-second order. The adsorption isotherm data fitted well to Langmuir isotherm and it was found to be 10.4712 mg/g adsorption capacity and it implies that the chemisorptions of MB on to Fe3O4 NPs.


Introduction
During the past decade, there has been increasing concern about the organic pollutants, heavy metals and pathogens in an aquatic environment due to their wide distribution and potential adverse health effects [1]. Especially, the presence of toxic organic dyes in water effluent, even at parts per billion (ppb) levels, is extremely harmful and undesirable. Various treatment approaches for removing organic dyes from contaminated water are available, such as adsorption, bioremediation, precipitation, membrane filtration, reverse osmosis, and photocatalysis [2]. However, these methods have proved to be ineffective for removal of dyes from water effluents due to high chemical stability of dyes, high cost and low efficiency of these processes. Among these chemical and physical methods, the adsorption method is more versatile and efficient, and has been successfully used for removal of dyes from contaminated water due to its simplicity of design, wide adaptability, convenience and ease of operation. Especially, nano-adsorbents offer the possibility of efficient removal of organic dyes owing to smaller size and higher adsorptive surface area. From the past few years, there have been an increased investigations based on iron oxide nanoparticles such as magnetite nanoparticles (Fe 3 O 4 NPs), maghemite (γ-Fe 2 O 3 NPs), etc., for the removal of organic and inorganic contaminants. Among all magnetic nanoparticles, Fe 3 O 4 NPs have emerged as potential candidate for technological applications such as magnetic fluids, high-density magnetic storage, environmental remediation, cancer therapy, MRI contrast enhancement, drug delivery, tissue repair engineering, antibacterial, catalysis, lithium-ion battery due to their small size, biocompatibility, low toxicity and high saturation magnetism [1][2][3][4][5][6][7].
Numerous methods have been reported for synthesis of Fe 3 O 4 NPs such as co-precipitation, thermal decomposition of organic iron precursor, sol-gel method, Polyol method, surfactant-or polymer-assisted precipitation including reverse micelle, co-polymer templatesassisted synthesis, solvothermal synthesis and hydrothermal synthesis [8][9][10][11][12][13][14][15][16][17][18]. All these synthetic methods have employed the chemicals such as sodium borohydride, dimethyl formamide, hydrazine and carbon monoxide as reducing agents and also they are highly reactive and have both biological and environmental negative effects. Therefore, it is highly desirable to prepare Fe 3 O 4 NPs via green synthesis approach using non-toxic, environmentally benign materials, which offers numerous benefits of eco-friendliness and compatibility for pharmaceutical, biotechnological and biological applications [19]. More recently, plant-mediated green synthesis of nanoparticles has attracted much attention due to cost effective, environmentally friendly alternative to chemical and physical methods. There are successful reports for plant-mediated synthesis of Fe 3 O 4 NPs using various plant extracts such as seed extract of Grape proanthocyanidin [20], leaf extract of Tridaxprocumbens [21], seaweed of Kappaphycusalvarezii [22], Caricaya papaya [23], peel extract of plantain [24], and seaweed of Sargassummuticum [25]. However, no literature reports are available for the synthesis of Fe 3 O 4 NPs using aqueous leaf extract of Zanthoxylum armatum DC. The plant Zanthoxylum armatum DC. belongs to rutaceae family and is extensively used in traditional medicinal practices of the Khasi tribe in North-Eastern India and in neighboring regions including South-East Asia. The Zanthoxylum armatum DC. is an important medicinal plant used for anti-helminthic, controlling hyperglycaemia and also used for treatment of cholera, tonic for fever, remedy for skin diseases, diseases of the mouth and teeth; and possesses antioxidant, antidiabetic, antifungal and antimicrobial activities [26,27].
In this paper, for the first time, we report a simple and environmentally friendly method for the preparation of Fe 3 O 4 NPs using the aqueous leaf extract of Zanthoxylum armatum DC. for the adsorption of organic dye, methylene blue (MB). The synthesized Fe 3 O 4 NPs have been characterized by using UV-visible (UV-vis) spectroscopy, X-ray diffraction (XRD), Fourier transforms infrared (FTIR) spectroscopy, field emission-scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM) and vibrating sample magnetometer (VSM). Furthermore, as synthesized Fe 3 O 4 NPs were investigated for removal of organic dye, MB, from contaminated water. Finally, to find the optimum adsorption conditions of Fe 3 O 4 NPs for removal of MB dye, adsorption isotherms and kinetic studies were extensively investigated. Milli-Q water (Visakhapatnam, India) was used throughout the synthesis process.

Preparation of plant extract
The fresh leaves of Zanthoxylum armatum DC. were washed thoroughly with water and followed by Milli-Q water to remove the dust particles and then shade dried for 15 days to remove the moisture. Then the dried leaves were cut into small pieces and powdered using blender. 5 g of powder was transferred to an Erlenmeyer flask containing 100 ml of Milli-Q water, and boiled for 20 min at 60°C. Then, the resultant extract was filtered by Whatman No. 1 filter paper and the filtrate was stored at 4°C for further use.

Synthesis of Fe 3 O 4 NPs
Fe 3 O 4 NPs were synthesized based on co-precipitation method in the presence of aqueous leaf extract of Zanthoxylum armatum DC. as a reducing and capping agent. In a typical procedure, 2:1 molar ratio of FeCl 3 ·6H 2 O (0.11 g) and FeSO 4 ·7H 2 O (0.556 g), respectively, were mixed with 85 mL Milli-Q water with the assistance of ultrasonication in 250 mL Erlenmeyer flask. Then, the reaction was allowed for 60 min at 80°C with constant stirring under nitrogen atmosphere. After that, 10 mL of 5% aqueous extract of Zanthoxylum armatum DC. leaf was quickly added into the reaction mixture. After 30 min of refluxing, 5 mL of 25% NH 3 was added drop wise to the reaction mixture to adjust the pH to 10. Then black precipitate, Fe 3 O 4 NPs, was obtained and filtered by using bar magnet. The product was washed with Milli-Q water for three times and once with ethanol and dried in the vacuum at room temperature for further use.

Characterization
UV-vis spectra of synthesized Fe 3 O 4 NPs were recorded by Shimadzu (2450 -SHIMADZUCorporation, Tokyo, Japan) spectrometer. By using SHIMADZU-IR PRESTIGE-2 Spectrometer, FTIR spectra were recorded in the range of 400-4000 cm −1 . The powder XRD patterns were recorded by PANalyticalX'pert pro diffractometer at 0.02 degree/s scan rate with Cu-kα 1 radiation (1.5406 Å, 45 kV, 40 mA). The morphologies of the samples were characterized using FE-SEM (Zeiss Ultra-60, Carl Zeiss Promenade, Jena, Germany) equipped with X-ray energy dispersive spectroscopy (EDS). TEM images were obtained (TEM model FEI Technai 20 U Twin, ThermoFischer Scientific, Hillsboro, OR, USA) at an accelerating voltages of 120 and 200 kV. In order to study the magnetic behavior of Fe 3 O 4 NPs at room temperature, magnetization measurements were recorded using vibrating sample magnetometer (VSM Lakeshore 665, LakeShore CRYOTRONICS, East Lakeshore Drive, USA).

Batch mode adsorption studies
The absorption behavior of MB at various pHs was studied by measuring the absorbance of MB solutions at 664 nm in primary experiments. To attain maximum adsorption efficiency, various parameters affecting the MB dye removal were studied and optimized. The adsorption efficiency of synthesized Fe 3 O 4 NPs were investigated at room temperature (300 K) using MB as a model water pollutant. Typically, 50 mg of Fe 3 O 4 NPs was mixed with 50 mL of known concentration of dye solution. The solution pH was adjusted by HCl (0.1 M) or NaOH (0.1 M). The flasks were stirred for the specified time period and sample from each flask were withdrawn at the desired time of reaction. After the adsorption experiment, Fe 3 O 4 NPs were recovered by an external magnet. The residual dye concentration was determined by UV-vis spectrophotometer by measuring the absorbance at a wavelength of maximum (λ max = 664 nm) absorbance of MB. The following equation was applied to calculate the dye removal efficiency: where C 0 and C r are the initial and residual concentrations of the dye in the solution (mg L −1 ), respectively The amount of adsorbed MB (q) was expressed in mg of dye per gram of synthesized Fe 3 O 4 NPs, as shown by Eq. (2): where q is adsorption capacity of MB in (mg/g) at time t, C 0 represents the initial MB concentration in mg/L, C is the concentration of MB in solution after adsorption in mg/L, V is the volume of the aqueous solution in L, and m is the mass of synthesized Fe 3 O 4 NPs in grams. The mechanism of adsorption and kinetics were correlated by adsorption isotherms and kinetic models, respectively.

Characterization
Phytochemical qualitative analysis of aqueous leaf extract of Zanthoxylum aramatum DC. was listed in  The intense reflection at (311) plane in contrast to the other planes may specify the growth direction of the Fe 3 O 4 NPs. The average crystallite sizes of the Fe 3 O 4 NPs was calculated from the full width at halfmaximum (FWHM = 0.466) of the (311) reflection by the Debyee-Scherrer's equation [37].
where D is particle diameter size, k is a constant equals 1, λ is wavelength of X-ray source (0.1541 nm), β is the FWHM and θ is the diffraction angle corresponding to the lattice plane. The average crystallite size of Fe 3 O 4 NPs according to Scherrer equation with the width of (311) plane is found to be 20.    Figure 6. The magnetization curve reveals that the sigmoidal shape without hysteresis loop demonstration of superparamagnetic behavior with the saturation magnetization (Ms) value 128 emu/g at room temperature [36]. The higher saturation magnetization is correlated with the previous literature [38]. The magnetization of synthesized Fe 3 O 4 NPs is strong enough that they can be easily concentrated and separated by applying a small magnet from the solution.

Plausible mechanism for synthesis of Fe 3 O 4 NPs using aqueous leaf extract of Zanthoxylum armatum DC
A possible formation mechanism of Fe 3 O 4 NPs by using aqueous leaf extract of Zanthoxylum armatum DC. is presented in Figure 7. The reaction mixture consists of aqueous leaf extract of Zanthoxylum armatum DC., ferric chloride (FeCl 3 ·6H 2 O) and ferrous chloride (FeCl 2 ·4H 2 O) in one aqueous phase. The aqueous leaf extract of Zanthoxylum armatum DC. contains alkaloids, phenolic acid, flavonoid, tannins, terpenoids and carbohydrates. It is reported that the gallic acid is a major phytochemical present in aqueous leaf extract of Zanthoxylum armatum DC. [28]. Moreover, the FTIR results also (Figure 2) supported the presence of carboxylic group in aqueous leaf extract and C=O of carboxylic group (gallic acid) in leaf extract form complex with Fe 3+ and Fe 2+ . As the reaction was carried out at 80°C , OH − of NH 4 OH would be involved in the reaction and thus, a competition between of COO − …Fe 3+ and COO − …Fe 2+ bonds and the formation of HO − . . .

Adsorption studies
The UV-vis absorption spectra of unadsorbed MB in the solution at time "t" after the adsorption by Zanthoxylum armatum DC. synthesized Fe 3 O 4 NPs is shown in Figure 8. When the adsorption was progressed, 71% MB (5 mg L −1 ) dye removal efficiency was obtained at the end of 180 min. The adsorption after 180 min was not brought further adsorption of MB and which ascribed to adsorption equilibrium was attained at the end of 180 min. Figure 9(a,b) depicts that at the adsorption equilibrium (180 min), adsorption of Fe 3 O 4 NPs for MB [at 300 K, pH 11] was found to be 71%, 62.1%, and 48.4% for the concentration of 5 mg L −1 , 10 mg L −1 , and 15 mg L −1 MB, respectively.

Adsorption kinetics
Adsorption kinetics gives the valuable information for designing adsorption process for practical application. Hence, in this study, the adsorption kinetics of MB on synthesized Fe 3 O 4 NPs was conducted at different concentration of MB. Figure 9(a,b) shows that the adsorption rate of MB on to synthesized Fe 3 O 4 NPs at different initial concentrations of 5, 10, and 15 mgL −1 of MB (at pH 11 and 300 K). The dye removal efficiency is highly dependent on the initial dye concentration. The time variation plot   specifies that the removal of dye is rapid in preliminary stages but when it approaches equilibrium, it slows down gradually. This may be due to the availability of vacant surface sites during the initial stage of adsorption, and after a certain time period the vacant sites get occupied by dye molecules which create a repulsive force between the adsorbate (MB dye) on the adsorbent surface. When the initial dye concentration increased from 5 to 15 mg/L at 300 K and pH 11, the adsorption capacity increased from 3.5524 to 7.2587 mg/g while the removal percentage of dye decreased from 71% to 48%. At a low dye concentration there will be more unoccupied surface sites on the adsorbent, and when the initial dye concentration increases, there will be insufficient sites for the adsorption of dye molecules, thus decreasing the dye removal efficiency [39]. In contrast, the increase in initial dye concentration will cause an increase in the loading capacity of the adsorbent and this may be due to the high-driving force for mass transfer at a  high initial dye concentration [40]. As shown in Figure 9(b), the adsorption process is rapid in the first 30 min and gradually reached to equilibrium at the end of 180 min. The kinetics of adsorption of MB on synthesized Fe 3 O 4 NPs was investigated by pseudo-first-order [41] Eq. (4) and the pseudo-second-order Eq. (5) kinetic models. The pseudo-firstorder kinetic model is described by the equation: where k 1 (min −1 ) is the pseudo-first-order rate constant of adsorption, and q t (mg/g) and q e (mg/g) are the amount of the MB adsorbed at time t and at equilibrium, respectively. k 1 (min −1 ) was determined from the slope of the plot of log(q e − q t ) vs t (Figure 10(a)) and it was obtained that the correlation coefficient value (R 2 ) is 0.9660. Moreover, to validate the kinetic of adsorption of MB pseudo-second-order kinetic model was also investigated.
where k 2 (g/mg min −1 ) is the pseudo-second-order rate constant, and q t (mg/g) and q e (mg/g) are the amount of the MB adsorbed at time t and at equilibrium, respectively. The values of q e and k 2 can be calculated from the slope and intercept of a plot of t/ q t vs t (Figure 10(b)). For this plot, correlation coefficient value (R 2 ) was found to be 0.9849. It is concluded that the adsorption of MB on synthesized Fe 3 O 4 NPs was found to be pseudo-second-order kinetic model; this implies that the adsorption process will more likely proceed through electrostatic interaction of MB and surface of synthesized Fe 3 O 4 NPs, and this process is called as chemisorption process [42].

Adsorption isotherm
Adsorption isotherm analyses are used to investigate the plausible adsorption mechanism and determine the maximum adsorption capacity of synthesized Fe 3 O 4 NPs. The equilibrium adsorption isotherm parameters of MB on synthesized Fe 3 O 4 NPs were analyzed using Langmuir and Freundlich isotherm models [43,44]. The Langmuir equation can be expressed as (Eq. 6): where q e (mg/g) is the equilibrium adsorption capacity of MB on the adsorbent, C e (mg/L) is the equilibrium concentration of MB in the solution, Q 0 (mg/ g) is the maximum capacity of adsorbent, and b (L/ mg) is the equilibrium constant relating to the enthalpy of process. Figure 10(c) shows that the experimental data well-fitted to the Langmuir adsorption isotherm and it was found to be 10.4712 mg/g maximum adsorption capacity. As shown in ESI Table 7, the correlation coefficient (R 2 ) and equilibrium constant (b L ) values were found to be 0.9932 and 0.3416, respectively. Besides, dimension-less factor (R L ) of the Langmuir adsorption isotherm parameter was calculated using Eq. (7). In this study, the R L value obtained was 0.3693, which is in between 0 and 1. This indicates that the adsorption of MB on Fe 3 O 4 NPs is thermodynamically favorable.
where C 0 (mg/g) is initial MB concentration and b L (L/mg) is the Langmuir constant. For unfavorable sorption, R L > 1; for favorable sorption, 0 < R L < 1; for irreversible sorption R L < 0; and for linear sorption, R L < 1 The Freundlich isotherm (Eq. 7) can be expressed as: log q e ¼ log k f þ1=n log C e (8) where q e (mg/g) is the equilibrium adsorption capacity of MB on the synthesized Fe 3 O 4 NPs; k f (mg/g) and 1/n are Freundlich constants representing the adsorption capacity (adsorption coefficient) at unit equilibrium concentration and adsorption intensity, respectively. The value of Freundlich constant "n" is 2.6781. For this adsorption, n > 1 indicating that MB favorably adsorbed on synthesized Fe 3 O 4 NPs. However, as shown in ESI Table 7, the correlation coefficient values of Langmuir model (R 2 = 0.9932) were much closer to unity than Freundlich model (R 2 = 0.9492), which indicates that the experimental data best fit with the Langmuir isotherm. The monolayer adsorption maximum adsorption capacity (Q 0 ) of synthesized Fe 3 O 4 NPs was found to be 10.4712 mg/g. This figure indicates that Zanthoxylum armatum DC. extract-assisted synthesized Fe 3 O 4 NP has comparative adsorption efficiency toward organic dyes (ESI , Tables 4 and 8).

Conclusion
In