A facile strategy for preparation of Fe3O4 magnetic nanoparticles using Cordia myxa leaf extract and investigating its adsorption activity in dye removal

This study demonstrates the successful, facile, and cost-effective preparation of magnetic Fe3O4 nanoparticles (MNPs) via green procedure using Cordia myxa leaf extracts for efficient adsorption of methylene blue (MB) as a model of organic pollutant. The formation of Fe3O4 NPs was confirmed by a range of spectroscopy and microscopy techniques including FT-IR, XRD, FE-SEM, TEM, EDS, VSM, TGA, and BET-BJH. The synthesized spherical nanoparticles had a high specific surface area of 115.07 m2/g with a mesoporous structure. The formed Fe3O4 MNPs exhibited superparamagnetic behavior with saturation magnetization of 49.48 emu/g. After characterization, the adsorptive performance of the synthesized MNPs toward MB was evaluated. To achieve the maximum removal efficiency, the effect of key parameters such as adsorbent dosage (MNPs), initial adsorbate concentration, pH, and contact time on the adsorption process was evaluated. A maximum adsorption capacity of 17.79 mg/g was obtained, after one-hour incubation at pH 7.5. From the pHPZC of 7.1 of the synthesized adsorbent, the electrostatic attraction between MB and Fe3O4 NPs plays an important role in the adsorption process. The adsorption experimental data showed the closest match with the pseudo-second-order kinetic and Langmuir isotherm. The prepared Fe3O4 NPs were easily recovered by an external magnet and could be reused several times. Therefore, the synthesized MNPs seem to be excellent adsorbents for the removal of MB from aqueous solution.


Chemicals and materials
Ferric chloride hexahydrate (FeCl 3 .6H 2 O) and ferrous sulfate heptahydrate (FeSO 4 .7H 2 O), methylene blue (MB), all with analytical grades, and sodium hydroxide (NaOH, > 97.0%) were purchased from Merck chemical company (Germany).All chemicals used in synthesis or application steps were of analytical reagent grade and utilized as received.In the synthesis procedures, the distilled was of deionized grade.All glassware was thoroughly cleaned with aqua regia and rinsed with deionized water.

Collection of the plant and preparation of the extract
This study complies with relevant institutional, national, and international guidelines and legislation.Fresh and healthy Cordia myxa L. leaves were collected from Bandar Abbas, Hormozgan province, Iran (57° 33′ E 227° 30′ N, 1050 m).The guidelines for collecting plants botany 440/540 (available at https:// herba rium.eku.edu/) were used for plant collection.Dr. Mansoore Shamili (Horticulture Department, University of Hormozan, email: shamili@ut.ac.ir) identified the plant species.Accession number 386 was assigned to the plant sample in the university herbarium.Fresh Cordia myxa L. leaf extract was prepared via the green procedure which means not using any toxic or dangerous chemical additives.No cultivation of the plant was done, and there was no genetically modified organism (GMO) procedure.The collected leaves were exhaustively washed with tap water and deionized water to remove surface pollutants and dust particles.Drying the leaves was done in the shade at room temperature for 10 days.A powder was prepared from the dried leaves via an electric blender.10.0 g of the powdered leaf was added to a 250 ml round-bottom flask containing 100 ml deionized water and refluxing was done for 60 min at 80 °C.After cooling, the mixture was filtered using Whatman No. 1 filter paper, and the final extract was kept in a refrigerator for the next characterization or studies.

Synthesis of Fe 3 O 4 MNPs
The preparation of Fe 3 O 4 MNPs was conducted via the following easy and environmentally-friendly procedure.In a typical reaction, 1.11 g of FeCl 3 .6H 2 O (0.004 mol) and 0.53 g of FeSO 4 .7H 2 O (0.002 mol) (amounting to a Fe 3+ :

Instrumentation and characterization
The obtained Fe 3 O 4 MNPs were analyzed by X-ray powder diffraction (Bruker D8 Advance powder diffractometer) with Ni-filtered Cu-K α radiation (λ = 1.5406Å) at a setting of 40 kV/30mA with a scan rate of 0.02° per minute in the angular range (2θ) of 20 to 70°.FTIR spectra of C. myxa leaves extract and synthesized Fe 3 O 4 MNPs were obtained in the range of 4000-550 cm −1 using a Bruker alpha FT-IR spectrometer (Germany) equipped with a Diamond attenuated total reflection (ATR) accessory at room temperature.The morphologies of the samples were observed using field emission scanning electron microscope images (FE-SEM) on a TE-SCAN MIRA3 SEM with primary electron energy of 15 kV.The chemical composition of the obtained Fe 3 O 4 MNPs was investigated by Energy Dispersive X-ray Spectroscopy (EDS) performed in SEM.TEM studies were carried out on a Zeiss-EM10C instrument with an accelerating voltage of 100 kV.The magnetic properties of the synthesized Fe 3 O 4 MNPs were identified at room temperature using a vibrating sample magnetometer (VSM; Meghnatis Daghigh Kavir Company LBKFB).The pore diameter and specific surface area were measured using a Brunauer-Emmett-Teller surface area analyzer (Microtracbel Corp BELSORP Mini).Nitrogen adsorption measurements were done on samples that were priory degassed at 150°C.Thermogravimetric analysis (TGA) was done with a heating program from room temperature up to 700 ºC with an increasing slope equal to 10 °C min −1 under a nitrogen atmosphere using a TGA-7 analyzer (Perkin-Elmer, USA).UV-Vis spectra were obtained at room temperature with a SCINCO-S-3100 spectrophotometer (Scinco Co., Korea) equipped with a 1.0 cm quartz cell.

Point of zero charge (PZC)
Determination of the point of zero charge (pH PZC ) for the synthesized Fe 3 O 4 MNPs was done using the pH drift method 48 .It is well-known that the pH PZC is the pH at which the surface charge of the synthesized Fe 3 O 4 MNPs is equal to zero.To a series of 100 ml falcon tubes, 50.0 ml of 0.01 M NaCl (as an inert electrolyte) was added, for adjusting the ionic strength throughout the experiments.Next, the pH values of the solutions (pH initial ) were brought to a pH value in the range between 2.0 and 10.0 with intervals of one by adding either HCl or NaOH.0.010 g of the synthesized Fe 3 O 4 MNPs was then added to each falcon tube and they were closed.The resulting mixtures were allowed to equilibrate for 12 h in a shaker kept at room temperature.After 12 h, the synthesized Fe 3 O 4 MNPs were magnetically separated from the solution and the pHs of the remaining solutions were measured (pH final ).The plot of ∆pH (pH final −pH initial ) versus pH initial was drawn and the point of pH final −pH initial = 0 was considered as the pH PZC value 49 .

Batch experiments
To evaluate the adsorption of methylene blue by the synthesized Fe 3 O 4 MNPs, 15 mg of Fe 3 O 4 MNPs were stirred for 2 h in 30.0 ml of MB solution (10 mg L −1 in distilled water) in a shaker at room temperature.The reaction was monitored by UV-Vis spectrophotometer at time intervals of 10 min by taking 3.0 ml of the mixture and separating the Fe 3 O 4 MNPs from the reaction solution through an external magnet.The MB concentration remaining in the reaction mixture was determined by UV-Vis spectrophotometry at a wavelength of 664 nm.The precision of UV-Vis measurements before and during work was checked with a spectrophotometer, and the RSD of absorbance was not higher than 3%.The efficiency of the Fe 3 O 4 MNPs in MB removal was obtained using Eq.(1) 50 : where A 0 is the initial absorbance of MB in the solution and A t is its absorbance at time t.
Effects of several experimental factors on the adsorption efficiency of MB by Fe 3 O 4 MNPs including pH (3.0, 5.0, 6.5, 7.5, 9.0, and 11.0), amount of Fe 3 O 4 MNPs (0.17, 0.25, 0.33, 0.50, 0.67, and 0.83 mg ml −1 ), and initial MB concentration (5.0, 10.0, 12.0, and 15.0 mg L −1 ) were investigated.Each experiment was performed three times and the mean ± SD was reported.The adjustment of the pH was performed using HCl or NaOH solutions as required.
The capacity of the synthesized Fe 3 O 4 MNPs to adsorb MB was calculated using Eq.(2) 41,51 : where q t is the adsorption capacity per gram of magnetic adsorbent (synthesized Fe 3 O 4 MNPs) at desired time t.C 0 and C t denote the dye concentrations (mg L −1 ) in the aqueous phase at the start of the experiment and at the desired time (t), respectively.The volume of the dye solution (L) is also shown by V in Eq. ( 2) and m denotes the mass (g) of the magnetic adsorbent.It is worth mentioning that at t in equilibrium contact time, C t is equal to C e and q t is equal to q e respectively. (

Powder XRD analysis
The crystallinity of the synthesized Fe 3 O 4 MNPs was characterized by X-ray powder diffraction (XRD) (Fig. 1a).XRD analysis showed six major Bragg diffraction peaks at 30.24°, 35.64°, 43.42°, 53.52°, 57.26° and 63.68° (2θ), which correspond to the (220), (311), (400), (422), ( 511) and (440) crystal indices (corresponds to the standard XRD pattern of Fe 3 O 4 from JCPDS 75-0033) 52 .These diffraction peaks are very similar to the peaks of the cubic spinel structure of Fe 3 O 4 crystals 52,53 .No other distinct peaks of metal hydroxides or α-Fe 2 O 3 (hematite) were observed, indicating the pure crystalline phase of the synthesized Fe 3 O 4 MNPs and confirming the complete formation of Fe 3 O 4 MNPs.The average crystallite size of the as-prepared Fe 3 O 4 MNPs was estimated from the full width at half-maximum (FWHM) of the (311) reflection peak using Debye-Scherrer's equation (Eq. 3) 35 : In Debye-Scherrer's equation, 0.89 is the shape factor, D denotes the average particle size and λ is the wavelength of the Cu-Kα irradiation.β shows the full width at half maximum intensity of the obtained diffraction peak and θ is the diffraction angle for the (311) peak of the Fe

FE-SEM, EDS, and TEM of Fe 3 O 4 MNPs
The size and shape of synthesized Fe 3 O 4 MNPs were established by FE-SEM.As can be seen in Fig. 2a, the synthesized nanoparticles were spherical in shape and uniform in size with a size range of 21-32 nm, which was similar to the XRD result.The composition of the Fe 3 O 4 MNPs was established by EDS in SEM, showed that iron, oxygen, carbon, and nitrogen are the four main elements with weight percentages of 58.22, 31.86,7.58 and 2.34%, respectively (Fig. 2b).The elements of carbon and nitrogen were derived from the phytochemicals found in C. myxa leaf extract.The presence of these elements proves that the prepared nanoparticles are coated with phytochemicals 54 .Also, the source of excess oxygen can be from flavonoids and phenolics of C. myxa leaf extract or the physical adsorption of oxygen from air on the surface of synthesized Fe 3 O 4 MNPs.Furthermore, TEM images of the prepared nanoparticles confirmed the formation of spherical particles (Fig. 2c).

FTIR analysis
FT-IR analysis was performed to identify possible functional groups of the C. myxa leaf extract on the surface of prepared magnetic nanoparticles.The FT-IR spectrum of C. myxa leaf extract (Fig. 3a) displays peaks at 3500 to 3000 (centered at 3250), 2919, 1723, 1583, 1385, 1260, and 1062 cm −1 , corresponding to free OHs and OH group forming hydrogen bonds, aliphatic C-H stretching vibrations, aromatic ring C=C stretching vibrations, amide C=O stretching vibrations, nitrogen N-O bending vibrations, C-OH stretching vibrations, and C-N stretching vibrations of amine groups, respectively 3,4,55,56 .These peaks indicate the presence of flavonoids and phenolics in C. myxa leaf extract, which could reduce metal ions to metal nanoparticles and stabilize the formed nanoparticles 57 .In Fig. 3b, the peak at 586 cm −1 is the characteristic Fe-O peak, confirming the successful formation of Fe 3 O 4 MNPs 3,4,10,56 .Furthermore, the differences between the FT-IR spectrum of the C. myxa leaves extract and the synthesized Fe 3 O 4 MNPs indicate that the iron cations interact with the phytochemicals (Fig. 3b).The shifted peaks at about 3372, 2926, 1587, 1356, 1130, and 923 cm −1 correspond to the O-H functional groups, C-H stretching, C=O stretching, nitrogen N-O bending, and C-N stretching vibrations, respectively 50 .FT-IR results indicate that the flavonoids and phenolics in C. myxa leaf extract act as capping agents for the formed Fe 3 O 4 MNPs and prevent their aggregation through surface adsorption via π-electron interaction in the absence of other strong capping agents.A possible mechanism for the formation of Fe 3 O 4 MNPs can be proposed as follows 58 and is also illustrated in Fig. 4 to highlight the role of functional groups that modify the surface of MNPs: The FT-IR spectrum of the modified Fe 3 O 4 MNPs after adsorption of MB is also shown in Fig. 3c.By comparing this spectrum with the spectrum of modified Fe 3 O 4 MNPs (Fig. 3b), it can be observed that the C=C bond (located around 1600 cm −1 ) in Fig. 3c is slightly reduced compared to Fig. 3b, but this change was not significant after MB adsorption.Due to the repetition of this fact in different runs, it can be suggested that the contribution of π-π interaction is not significant in this interaction 59 .On the other hand, the reduction of C-O-H bond at about 1070 cm −1 has a significant loss in intensity after MB adsorption which indicates the possibility of electrostatic interactions and H-bonding during this phenomenon.

Magnetic measurements
The shape, size, and morphology of nanomaterials, which are strongly dependent on the applied synthetic method, could affect the magnetic behavior of the nanomaterials 55 .Therefore, the magnetic properties of the prepared Fe 3 O 4 MNPs were investigated at room temperature via a vibrating sample magnetometer (VSM), with a field sweeping from − 15,000 to + 15,000 Oe. Figure 5 shows the superparamagnetic behavior of the synthesized Fe 3 O 4 as the magnetic hysteresis loop shows an S-like curve 60 .The observed saturation magnetization (Ms) was ~ 49.48 emu/g.The remnant magnetization (Mr) and coercivity (Hc) of the synthesized Fe 3 O 4 MNPs were 2.25 emu/g and ~ 30 Oe, respectively (upper left inset of Fig. 5).These low values of Mr and Hc indicate the superparamagnetic behavior of the synthesized MNPs 53 .Moreover, the sufficient saturation magnetization of the synthesized Fe 3 O 4 MNPs allowed easy and rapid separation (within seconds) of these MNPs from the mixture by an externally applied magnet, with the solution becoming clear (bottom right inset of Fig. 5).This property is very important in the reusability of the sorbent.

Thermogravimetric analysis
To investigate the thermal stability of the prepared Fe 3 O 4 MNPs, a thermogravimetric analysis (TGA) of Fe 3 O 4 MNPs with and without leaf extract was done (Fig. S1a,b), supporting information).The synthesized Fe 3 O 4  www.nature.com/scientificreports/MNPs in the presence of leaf extract show three weight loss steps (Fig. S1a).In the first step (below 100 °C), the weight loss is due to the dehydration of the sample, the remaining weight of which is about 96%.The weight loss steps at 230 °C and at 420 °C can be attributed to the decomposition of adsorbed phyto-compounds of C. myxa leaf extract that act as capping agents 55 .At temperatures higher than 520 ºC, the phytochemicals of C. myxa leaf extract were completely degraded, and the MNPs did not show further weight loss up to 700 ºC.For the MNPs caped with C. myxa leaf extract, the residual weight is 86% after 520 ºC (Fig. S1a).In contrast, the residual weight of the Fe 3 O 4 MNPs synthesized without plant extract is about 96% at 520 ºC to 700 ºC (Fig. S1b

Surface area and pore distribution
The surface area and porous nature of the synthesized MNPs were investigated by determining the adsorption-desorption isotherm at 77 K using liquid N 2 as adsorbent, as shown in Fig. S2 (Supporting information).The synthesized Fe 3 O 4 MNPs exhibited hysteresis loops with intensities associated with capillary condensation at relatively high pressures, which are characteristic of type IV isotherms with H3 type hysteresis loops, according to the IUPAC classification 10,61 .The calculated Brunauer-Emmett-Teller (BET) surface area of the prepared green-coated MNPs was about ~ 115.07 m 2 /g, which is clearly higher than that of many other Fe 3 O 4 MNPs 10 .
The single-point adsorption total volume at P/P 0 = 0.990 was 0.3357 cm 3 g −1 .The values of surface area and pore volume of the synthesized Fe 3 O 4 MNPs indicate the potential of the proposed method in preparing the Fe 3 O 4 MNPs with superior catalytic or adsorption activity.Moreover, the pore size distribution from the Barrett-Joyner-Halenda (BJH) analysis (inset of Fig. S2), indicates the mesoporous nature of the synthesized Fe 3 O 4 MNPs, with a wide pore size distribution.Overall, the high BET-specific surface area and the BJH pore-size distribution analysis confirmed that these one-pot synthesized mesoporous Fe 3 O 4 MNPs have the potential to be used for the adsorption of pollutants such as dyes and toxic metals from wastewater.

Adsorption of methylene blue
Next, the synthesized Fe 3 O 4 MNPs were used to remove MB, as a model of an organic dye pollutant, from aqueous solution.The effects of various parameters on the adsorption capacity of the Fe 3 O 4 MNPs were followed: including the pH of the solution, the amount of sorbent, the dye concentration, and the adsorption time.

pH dependence studies
The pH of the solution is a key factor in the adsorption of dye from water because pH affects the surface charge of the adsorbent as well as the structure and ionization value of the dye molecules 62 .Using an initial dye concentration of 10 mg L −1 and 10 mg MNPs in 30.0 ml dye solution (0.33 mg ml −1 ) the removal efficiency of MB by Fe 3 O 4 MNPs was studied at pHs from 3.0 to 11.0.According to Fig. 6a, the removal efficiency increases with the increase of initial pH solution (from pH 3.0 to 7.5) and remains almost constant at higher pHs.Similar remarks have been observed in the adsorption of methylene blue by other adsorbents [63][64][65] .This result can be explained by the pH PZC value of the adsorbent.In pH PZC , the electric charge density on the sorbent surface immersed in the electrolyte solution is zero.At pH < pH PZC , the net surface charge of adsorbent is positive and the adsorption of anions dominates, while, at pH > pH PZC , the net surface charge is negative and thus allows the trapping of cations 66 .The pH PZC of the synthesized Fe 3 O 4 MNPs in the presence of C. myxa leaf extract was found to be 7.1 (see Fig. S3, Supporting information).MB is a cationic dye with pK a = 3.8 and has a permanent positive charge in the studied pH range.Hence, at pH lower than pH PZC , the adsorption of MB onto Fe 3 O 4 MNPs decreases due to the positive charge of adsorbent and electrostatic repulsion.In addition, the H + concentration, which is high at lower pH, competes with the positively charged MB for vacant adsorption sites.Thus, at low pH, the adsorption is very low.However, at pH higher than pH PZC , the surface charge of MNPs is negative, due to the deprotonation of carboxyl groups and adsorption of OH − on the surface of the adsorbent, and the adsorption of MB increases due to electrostatic attraction between adsorbent and MB.These results confirm that electrostatic attraction plays a key role in the adsorption process.The maximum adsorption is achieved at a pH close to the pH PZC of the adsorbent 63,67 , which explains the optimum pH (= 7.5).

Effect of the amount of Fe 3 O 4 MNPs
Since the adsorption of MB takes place at the Fe 3 O 4 -H 2 O interface, the amount of adsorbent has a great influence on the adsorption capacity.According to Fig. 6b, increasing the amount of adsorbent from 0.17 to 0.50 mg ml −1 increases the removal of MB (10 mg L −1 ) from 51 to 88%.This is related to the increased surface area of the adsorbent and access to a large number of adsorption sites for MB 68 .The removal efficiency remains constant at higher amounts of adsorbent (0.67 and 0.83 mg ml −1 ).

Effect of the initial concentration of MB
Figure 6c shows the influence of the initial dye concentration on the removal efficiency of MB at a fixed dosage of Fe 3 O 4 MNPs (0.50 mg ml −1 ) and at pH 7.5.Enhancing the concentration of MB from 5.0 to 12.0 mg L −1 did not affect the removal efficiency.However, a further increase of the MB concentration to 15.0 mg L −1 clearly reduced the removal efficiency.This can be justified by the saturation of the MB binding sites: at a given dosage of MNPs, the number of MB binding sites is constant.Hence, with the increase in MB concentration, the adsorption of MB molecules becomes a competitive process, which leads to a decrease in removal percentage.For any adsorbent, it is desirable that its adsorption capacity remains more or less constant during regenerated and reuse.This property makes the use of the adsorbent economically sustainable, which is especially important for commercial and industrial applications.Therefore, in each cycle, after the adsorption process, the utilized Fe 3 O 4 MNPs are magnetically separated from the solution, washed with ethanol to remove the adsorbed MB, dried, and reused in the next cycle.From Fig. 7, it can be concluded that the removal efficiency remained more or less constant after three successive runs.After the 4 th cycle, only about 11% of its removal efficiency was lost, indicating the stability and reusability of the Fe 3 O 4 MNPs.

Kinetics of dye adsorption
Most adsorption processes are time-dependent.In this study, equilibrium was reached after about 60 min, after which the adsorption efficiency remained almost constant (Fig. 6).Kinetic models are used to investigate different mechanisms controlling the adsorption of the dye molecules from the aqueous solutions; that is, these models help to describe the adsorption rate of solutes from the solute-solution interface.In fact, adsorption kinetics provide valuable information for the design of the adsorption process for practical applications.To investigate the kinetic parameters of the adsorption process in more detail, three kinetic models were considered: pseudofirst-order or Lagergren model (shown in Eq. 4) 69,70 , pseudo-second-order (shown in Eq. 5) 71 , and intraparticle diffusion (shown in Eq. 6) 72,73 .
The parameter q e (mg/g) is the adsorption capacity in the equilibrium and q t is the adsorption capacity at time t.q e and q t show the amount of MB adsorbed on the magnetic adsorbent.The k 1 (min -1 ), k 2 (mg/g min), and k id (mg/g min 0.5 ) are rate constants for pseudo-first-order kinetics, pseudo-second-order kinetics, and intraparticle diffusion, respectively 69,71,72 .The C (mg/g) in Eq. ( 6) is a constant value related to the thickness of the boundary layer.Kinetic studies of the adsorption process were performed with an initial concentration of MB‫‬ of 12 mg L -1 , Fe 3 O 4 MNPs dosage of 0.50 mg ml -1 , and pH 7.5.All the kinetic parameters were calculated by fitting the experimental data to different kinetic models and are presented in Table 1 and Fig. 8.The highest R 2 value (above 0.99) from fitting the experimental data to the pseudo-second-order kinetic shows that this model describes the process in the best way compared to the others.Moreover, the q e value calculated from the second-order model is close to the experimental value and demonstrates a smaller deviation compared to the first-order model, which further confirming that the adsorption mechanism is second order.The verification of this model suggests that both adsorbent and adsorbate concentrations are associated with the rate-determining step of the adsorption process along with chemisorption, via valence forces through the exchange or sharing of electrons between the dye and nanoparticles, chelation, coordination and/or complexation 74,75 .

Adsorption isotherm
The analysis of the adsorption equilibrium models can provide useful information about the adsorption mechanism, surface properties, and affinity of the applied adsorbent 76 .In order to understand the interaction behavior between adsorbate (MB dye) and adsorbent (green synthesized Fe 3 O 4 MNPs), isotherm experiments were conducted at room temperature and the obtained results were studied with Langmuir and Freundlich models as two common.The Langmuir isotherm is suitable for monolayer adsorption on a surface containing a finite number of identical sites.This model assumes uniform adsorption energies on the surface and there is no transmigration of the adsorbate in the plane of the surface.During the adsorption process, an active site adsorbs a dye molecule and then does not allow any additional adsorption on the occupied active site [76][77][78] .The linearized Langmuir isotherm is expressed as 74 : where q e is the equilibrium adsorption capacity as denoted previously, C e (mg L −1 ) is the equilibrium concentration of MB in solution, q max (mg g −1 ) is the maximum adsorption capacity, and b (L mg −1 ) is the Langmuir constant.
(  The Freundlich model assumes that with an increase in the concentration of the adsorbate, the adsorbate concentration on the adsorbent surface also increases and, correspondingly, the sorption energy decreases exponentially with the completion of adsorption sites of adsorbent.On the other hand, this isotherm was used to describe the adsorption characteristics of multilayer and heterogeneous surfaces with unequally available adsorption sites that have different adsorption energies 76,77 .The Freundlich adsorption isotherm is given as 74 : where n and k f are the Freundlich adsorption isotherm constants related to the adsorption intensity and the adsorption capacity, respectively. The obtained Freundlich and Langmuir constants from regression analysis are represented in Table 2. Based on the R 2 values, the Langmuir equation describes the adsorption of MB onto Fe 3 O 4 MNPs better than the Freundlich equation.Herein, the q max of the MNPs was found to be 17.79 (± 0.06) mg g −1 in three repeated sets of experiences.Based on the obtained results, it is clear that the modification of MNPs with the phytochemical compounds enhanced the adsorption capability of adsorbent.To show the importance of modifying the surface of MNPs with the C. myxa leaf extract in the adsorption capacity, the q max of bare MNPs was calculated in similar conditions and it was found that in spite of weak correlation Langmuir model, the q max was significantly lower than q max of modified MNPs (< 65%).The important role of functional groups obtained from the extract is schematically shown in Fig. 9.

Comparison with other adsorption studies using Fe 3 O 4 MNPs
The commercial applicability of an adsorbent depends on its adsorption capacity, specific surface area, availability, and compatibility with the environment and the user.Table 3 shows a comparison of the adsorption capacity of various green synthesized Fe 3 O 4 MNPs reported in the literature [79][80][81][82] for the removal of MB from aqueous solutions.Based on Table 3, Fe 3 O 4 MNPs prepared with C. myxa leaf extracts have better or comparable adsorption capacity compared with other MNPs, with the exception of MNPs prepared with Cress seed mucilage.Therefore, the synthesized Fe 3 O 4 MNPs produced with C. myxa leaf extract appear to be a sustainable adsorbent for the removal of MB from aqueous solutions.It is clear that q max is one of the most important criteria for the application of nanosorbents in real applications.On the other hand, by comparing the required pH of our suggested MNP with previous green synthesized ones, it can be observed from Table 3 that the pH of the current work is not too alkaline or acidic which makes it a good applied sorbent for application without serious need of pH adjustment.The highlight point of the current work was the using C. myxa leaf to enhance the surface properties of Fe 3 O 4 MNPs which changes its adsorption ability in comparison with the previous similar reports.MNPs were easily recovered from the solution by an external magnet and could be successfully be reused several times and no significant decrease in removal performance was observed.The nontoxic and magnetically separable green synthesized Fe3O4 MNPs could be applied as a cost-effective adsorbent with possible wide application in wastewater treatment technologies and removal of organic water pollutants.However, the study of the thermodynamics of the adsorption process was not within the scope of this work but its investigation in future studies can enhance our knowledge about the adsorption property of the proposed nanosorbent.The presented work was not the best in comparison with the previous studies but can obtain an alternative way to decrease the dye pollution in aqueous media using stable and green-stabilized MNPs which are simply and easily prepared.

3 O 4
MNPs.The calculated crystallite size of the Fe 3 O 4 MNPs was ~ 25.3 nm.To ensure the ability of the chemical method used to prepare MNPs without using the plant extract, the XRD of Fe 3 O 4 MNPs prepared in the absence of C. myxa leaf extract is also represented in Fig. 1b.According to the results, the indicative peaks of MNPs in the XRD pattern show the construction of desired particles.

Figure 1 .
Figure 1.X-ray powder diffraction pattern of (a) Fe 3 O 4 MNPs synthesized in the presence of C. myxa extracts and (b) without using the C. myxa extracts.

Figure 2 .
Figure 2. (A) Field emission scanning electron microscope image, (B) Energy Dispersive X-ray Spectroscopy spectrum, and (C) Transmission Electron Microscopy image of the synthesized Fe 3 O 4 MNPs.

Figure 4 .
Figure 4. Proposed mechanism for synthesis of Fe 3 O 4 MNPs in the presence of C. myxa extract.
), which is close to the residual weight of the Fe 3 O 4 MNPs synthesized with leaf extract after the first step of weight loss.These results indicate that the Fe 3 O 4 MNPs synthesized without extract contain only adsorbed water without any capping agent.

Figure 5 .
Figure 5. Room temperature magnetization-hysteresis (M-H) loops of synthesized Fe 3 O 4 MNPs.The inset (upper-left) is an enlarged hysteresis loop; the lower-right inset shows M-H loops of the solution before and after separation by an external magnet.

Figure 6 .
Figure 6.Adsorption efficiency of the synthesized Fe 3 O 4 MNPs versus time as a function of (a) solution pH (other experimental conditions: MNPs dosage = 0.33 mg ml −1 and MB conc.= 10.0 mg L −1 ), (b) concentration of synthesized Fe 3 O 4 MNPs (at pH = 7.5 and MB conc.= 10.0 mg L −1 ), and (c) initial MB concentration (at pH = 7.5and MNPs dosage = 0.50 mg ml −1 ).Inset: The color change of MB solution with the concentration of 12.0 mg L −1 before and after the adsorption process (at the optimum pH of 7.5 and adsorbent dosage of 0.50 mg ml −1 ).

Figure 9 .
Figure 9. Illustration of the proposed mechanism for adsorption of MB by synthesized Fe 3 O 4 MNPs.

Table 1 .
Kinetics constants for pseudo-first, pseudo-second order and intraparticle diffusion models.

Table 2 .
Adsorption isotherm constants for binding of MB to the synthesized Fe 3 O 4 MNPs.

Table 3 .
Comparison of various plant synthesized Fe 3 O 4 MNPs as adsorbent for the removal of MB from aqueous solution with proposed Fe 3 O 4 MNPs adsorbent.