Facile green synthesis of L-methionine capped

Journal of Molecular Liquids 224 (2016) 713–720

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Facile green synthesis of L-methionine capped magnetite nanoparticles for adsorption of pollutant Rhodamine B Neway Belachew a, D. Rama Devi b, K. Basavaiah a,⁎ a b

Department of Inorganic & Analytical Chemistry, Andhra University, Vishakapatnam 530003, India A.U. College of Pharmaceutical Sciences, Andhra University, Visakhapatnam -530003, India

a r t i c l e

i n f o

Article history: Received 28 June 2016 Received in revised form 29 September 2016 Accepted 17 October 2016 Available online 19 October 2016 Keywords: Green synthesis Magnetite nanoparticles Amino acid Capping agents Co-precipitation Dye adsorption

a b s t r a c t Surface modified magnetite nanoparticles (Fe3O4 NPs) were synthesized by the one pot co-precipitation method using L-methionine (L-Met) as a capping agent. As synthesized Fe3O4 NPs have been characterized by using UV– Visible spectroscopy, FTIR spectroscopy, powder XRD, SEM-EDX and TEM. Magnetic property of L-Met capped Fe3O4 NPs was investigated by vibrating sample magnetometer (VSM). The XRD results indicated that formation of pure phase Fe3O4 NPs. The spectroscopic (FTIR and UV- Vis spectroscopy) results further confirm the effective capping of Fe3O4 NPs using L-Met. The VSM curve clearly depicts the superparamagnetic behavior of L-Met capped Fe3O4 NPs with saturation magnetization value of 65 emu/g. The kinetic studies for Rhodamine B (RhB) adsorption showed rapid adsorption within the first 40 min and sorption process best linear fitted to second-order kinetic model, suggesting chemisorption mechanism. Dye adsorption equilibrium data were fitted well to the Langmuir isotherm. © 2016 Published by Elsevier B.V.

1. Introduction During the past decade, magnetic nanoparticles have attracted much attention beyond to basic scientific interest due to many technological applications such as magnetic fluids, data storage, catalysis, waste water treatment and bio-medical applications [1–6]. More recently, there have been an increased investigations based on iron oxide nanoparticles in particularly magnetite nanoparticles (Fe3O4 NPs), maghemite (γ-Fe2O3 NPs), etc. Among all iron oxide nanoparticles, Fe3O4 NPs emerged as a potential candidate for advanced technological applications due to their biocompatibility, low toxicity and high magnetic saturations as well as potential applications [7–9]. All these applications requires Fe3O4 NPs to be superparamagnetism with particle size less than the critical particle size (20 nm), well dispersed in aqueous solution without any aggregation [10]. However, Fe3O4 NPs can be easily aggregated because of their large specific surface area, strong magnetic dipole–dipole interaction and Van der Waals forces between the nanoparticles and hence change in magnetic properties and low dispersibility in aqueous solution, which restricts the application of Fe3O4 NPs [11]. In order to overcome these limitations, Fe3O4 NPs should be capped with capping materials such as polymers [12], surfactants [13], inorganic metal or metal oxides [14], silica [15], polyaniline [16], dodecyl amine [17], etc. The protective capping materials not ⁎ Corresponding author. E-mail address: [email protected] (K. Basavaiah).

http://dx.doi.org/10.1016/j.molliq.2016.10.089 0167-7322/© 2016 Published by Elsevier B.V.

only arrest aggregation of Fe3O4 NPs but also act as a platform for functionalization to enhance their surface properties for technological applications. However, to exploit the potential application of Fe 3 O4 NPs especially in biomedical application, it is essential for synthesis of a Fe3 O 4 NPs through facile and green synthesis approaches. It has been reported that several papers on the synthesis of capped Fe3O4 NPs using biocompatible capping agents, such as dextran [18, 19], polyvinyl alcohol [20,21], poly (ethylene-glycol) [22], pullulan [11], dimercaptosuccinic acid [23], ascorbic acid [24] and proteins, like albumin [25] and transferrin [26]. Recently, among all biocompatible capping agents for Fe3O4 NPs, amino acids besides to biocompatibility and physiological importance, in the synthesis point of view have greater potential as a capping of Fe 3 O4 NPs. The amino (\\NH2) and carboxyl (\\COOH) groups are the most common surface capping agents frequently used for capping of Fe3O4 NPs [24, 27]. However, synthesis of Fe3O4 NPs by using common capping materials such as polymers [12] and silica [15] showed significantly increased the particle size, no uniform particle size distribution and reduce the magnetic saturation value. In this study, we report a simple and environmentally friendly method for the preparation of L-Met capped Fe3O4 NPs through a one pot synthesis approach using L-Met as a capping agent. L-Met capped Fe3O4 NPs was synthesized in the presence of different molar concentration of L-Met. Finally, the optimum adsorption conditions of L-Met capped Fe3O4 NPs for removal of RhB dye, adsorption isotherms and kinetic studies were extensively studied.

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2. Experimental

2.4. Batch mode adsorption studies

2.1. Materials

Batch adsorption efficiency of L-Met capped Fe3O4 NPs experiments were carried out at room temperature (300 K) using RhB as a model dye. Typically, 50 mg of L-Met capped Fe3O4 NPs was mixed with 50 mL of known concentration of dye solution. The solution pH was adjusted by NaOH (0.1 M) or HCl (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. The L-Met capped Fe3O4 NPs were collected by an external magnet. The residual dye concentration was determined by UV − Visible spectrophotometer by measuring the absorbance at a wavelength of maximum (λmax = 554 nm) absorbance of RhB. The amount of adsorbed RhB (q) was expressed in mg of dye per gram of

Iron (III) chloride hexahydrate (FeCl3·6H2O) and Iron (II) sulfate heptahydrate (FeSO4 ·7H2O) used for synthesis of Fe3 O4 NPs were purchased from Merck used as without further purification. L-Met was purchased from HiMedia, India. NH3 (25%) and other reagents were analytical grade and used without further purification. Milli-Q water was used throughout the whole synthesis process.

2.2. Synthesis of L-methionine capped Fe3O4 nanoparticles

L-Met

The synthesis scheme of Fe3O4 NPs was followed single pot coprecipitation reaction under the presence of L -Met as a capping agent. Typically, 2:1 M ratio of metal salts Fe+ 3 (0.54 g FeCl3·6H2O) and Fe+ 2 (0.278 g FeSO4·7H2O) was added in a 250 mL round bottomed flask respectively. Allow the reaction under nitrogen atmosphere at 80 °C for 1 h at a continuous constant string. Then 5 mL of 2 mol L − 1 L -Met was added to the reaction mixture and after 30 min 5 mL ammonia solution (25%) was rapidly injected into a reaction mixture. The reaction was preceded for 2 h at 80 °C under constant string. Then finally the black precipitate was obtained and magnetically separated, washed several times with Milli-Q water, and then dried vacuum at room temperature. In the same analogy uncapped Fe3O4 NPs was synthesized in the absence of L -Met. The same procedure was adopted for preparation of L -Met capped Fe3O4 NPs by varying the concentration of L-Met while all other reaction parameters were kept constant and their detailed preparation conditions were shown in Table 1.

2.3. Characterization The UV–Visible absorption spectra were recorded using a Shimadzu 2450 – SHIMADZU spectrometer. Fourier transform-infrared (FTIR) spectra were recorded over the range of 400–4000 cm− 1 using a SHIMADZU-IR PRESTIGE-2 Spectrometer. Powder samples were mixed thoroughly with KBr and pressed into thin pellets. X-ray diffraction (XRD) patterns were recorded by PANalytical X'pert pro diffractometer at 0.02 °/s scan rate using Cu-kα1 radiation (1.5406 Å, 45 kV, 40 mA). Transmission electron microscopy images were obtained (TEM model FEI TECNAI G2 S-Twin) at an accelerating voltages of 120 and 200 kV. Surface morphology was examined by scanning electron microscopy (SEM) using JEOL-JSM6610 LV equipped with an electron probe-micro analyzer. Thermogravimetric analysis (TGA) curves were obtained using a Shimadzu thermogravimetric analyzer (DTG-60H) with the temperature ranging from 25 to 800 °C under a stable N2 flow (50 mL/min) and with a heating rate set at 10 °C min−1. Room temperature magnetization measurements versus applied magnetic field were carried out using vibrating sample magnetometer (VSM), Lakeshore 665, USA.

capped Fe3O4NPs, as shown by the Eq. (1):

q ¼ ðC0 −CÞV=m

ð1Þ

where C0 (mg/L) represents the initial RhB concentration, C (mg/L) is the RhB concentration in solution after adsorption, V (L) is the volume of the aqueous solution and m (g) is the mass of L-Met capped Fe3O4 NPs. The mechanism of adsorption and kinetics were correlated by common adsorption isotherms and kinetic models respectively. 2.5. Recyclability experiment To test the recyclability or reusability of the L-Met capped Fe3O4 NPs for removal of RhB from water, 50 mg of the L-Met capped Fe3O4 NPs was added to 50 mL of RhB dye solution (5 ppm) and the mixture was stirred for 180 min at room temperature. After the separation of the L-Met capped Fe3O4 NPs via an external magnet, the supernatant dye solution was measured by UV–Visible spectrophotometer. Then the RhB adsorbed on L-Met capped Fe3O4 NPs was washed with 25 mL of ethanol several times at room temperature. The L-Met capped Fe3O4 NPs was collected by an external magnet and reused for the second RhB adsorption experiment. The reusability experiments were performed 5 times. 3. Result and discussion Fig. 1 shows UV–Visible spectra of uncapped Fe3O4 NPs and L-Met capped Fe3O4 NPs. Fig. 1(a) shows two absorption bands at 251 and 351 nm for uncapped Fe3O4 NPs. The band at 250 nm is attributed to the absorption and scattering of light by magnetic NPs while the peak at 351 nm is due to the aggregation of fine Fe3O4 NPs [28]. The absorption spectrum of L-Met capped Fe3O4 NPs [Fig. 1(b)] reveals the strong absorption band at 200 nm is ascribed to the charge transfer between ligand (L-Met) to the unsaturated Fe atom in Fe3O4 NPs. The absence of the band at 351 nm in L-Met capped Fe3O4 NPs confirms the successful synthesis of aggregate free Fe3O4 NPs [29]. The UV–Vis spectra of capped Fe3O4 NPs with different concentration of L-Met were shown in †ESI Fig. S1. The blue shift of Fe3O4 NPs peak with increasing molar ratio of L-Met probably attributed to the decreasing size of the Fe3O4 NPs. The crystalline structures of L-Met capped Fe3O4 NPs were characterized by XRD diffraction.

Table 1 Synthesis conditions of L-Met capped Fe3O4 NPs. Sample

Precursor solution

Temperature (°C)

Total reaction time

2:1:2 M ratio M-1 2:1:4 M ratio M-2 2:1:8 M ratio M-3 Uncapped Fe3O4

Fe3+:Fe2+:L-Met molar ratio 2:1:2

80

3h

Fe3+:Fe2+:L-Met molar ratio 2:1:4

80

3h

80

3h

80

3h

3+

Fe

:Fe

2+

:L-Met molar ratio 2:1:8

Fe3+:Fe2+ molar ratio 2:1


Fig. 1. The UV–Visible spectra of (a) uncapped Fe3O4 NPs and (b) L-Met capped Fe3O4 NPs (sample M-3).

Fig. 2 shows the XRD patterns of uncapped Fe3O4 NPs and L-Met capped Fe3O4 NPs. The diffraction patterns were centered at 2θ = 30.23°, 35.77°, 43.3°, 57.13°, and 62.83°, could be indexed as (220), (311), (400), (511), and (440) characteristics planes of magnetite (JCPD file No. 19-0629). The average crystalline size of L-Met capped Fe3O4 NPs estimated based on Scherrer formula [30] is ~ 10.5 nm which is smaller than uncapped Fe3O4 NPs (32.6 nm). This attributed to the L-Met act as a protective material for the growing of Fe3O4 NPs during the synthesis process and which is in accordance to the reported literatures [31]. However, the estimated crystalite size have a notable difference compared to average particles sized calculated by TEM (4.59 nm). This is probably because of the assumptions made by Scherrer equation as well as L-Met capped Fe3O4 NPs was contained many sub-grains which are not detected accurately by XRD, causes for the difference size. Fig. 3 depicts the FTIR spectra of uncapped Fe3O4 NPs and L-Met capped Fe3O4 NPs. The FTIR spectrum of uncapped Fe3O4 NPs has to two prominent peaks at 1641 cm−1 and 3429 cm−1 due to the O\\H deforming and stretching vibration respectively. The O\\H group of Fe3O4 NPs comes from the water molecule, when it form coordination bond with unsaturated Fe atoms in Fe3O4 NPs [31]. The FTIR spectrum of pure L-Met (ESI†: Fig. 1) show an intense broad band ~ 3014 cm−1 which is due to O\\H stretch of carboxylic group. The C\\H and N\\H stretch bands overlap with O\\H group. In addition L-Met shows that

Fig. 2. X-ray power diffraction patterns of (a) L-Met capped Fe3O4 (sample M-3) and (b) uncapped Fe3O4 NPs.

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Fig. 3. FTIR spectra of (a) uncapped Fe3O4 NPs, and (b) L-Met capped Fe3O4 NPs (sample M-3).

the band around 1600 cm− 1, 1510 cm−1, and 1315 cm−1 which are due to C_O stretch, O\\H bend, C\\O stretch vibrations respectively [32]. L-Met capped Fe3O4 NPs (Fig. 3b) show the characteristic peaks at ~ 1574 cm−1 and ~ 1402 cm−1 which are ascribed to the C_O and C\\O stretching vibrations of carboxyl of L-Met respectively [33]. Because of the reaction carried out under alkaline media (above pI of the carboxylic acid fully deprotonated (COO−). On deprotonation, vibration of C_O shifts to lower energy (1600 cm− 1 to 1574 cm−1) as its vibrational mode becomes coupled to that of the other oxygen, giving rise to an asymmetric feature. Similarly, the C\\OH band of carboxyl shift to higher energy (1315 cm−1 to 1402 cm−1) on deprotonation, yielding a symmetric COO− mode vibration at 1402 cm− 1. The differences between the asymmetric and symmetric stretching of carboxyl groups in L-Met capped Fe3O4 give hints about the nature of interaction of carboxyl to the metal surface. Hence, the differences between the asymmetric and symmetric stretching frequencies (Δνa − s = 1574–1402 cm− 1 = 172 cm− 1) were observed less than 200 cm− 1, corresponding to the bi-dentate complexing of carboxyl group with unsaturated Fe cation [34]. The strong bands at 580 and 401 cm − 1 were attributed to the characteristic of the intrinsic Fe\\O vibrations of L-Met capped Fe3O4 NPs [16,29]. These results confirm that the L-Met molecules are adsorbed on the surface of Fe3O4 NPs. The capping of L-Met onto the surface of Fe3O4 NPs was further confirmed by TGA as shown in Fig. 4. L-Met capped Fe3O4 NPs (M2 and M3), TGA thermograms show a continuous weight loss in the temperature L-Met)

Fig. 4. TGA thermograms of (a) M-2 and (b) M-3 L-Met capped Fe3O4 NPs samples.

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Fig. 5. (a, b and c) Representative SEM images and (d) Energy dispersive X-ray spectroscopy (EDS) of L-Met capped Fe3O4 (sample M-3).

range of 250–450 °C, which is the range of decomposition temperature of pure L-Met (ESI, †Fig. S3). The weight losses of samples M2 and M3 are 19.42 and 30.82%, respectively. The percentage of weight loss

of L-Met capped Fe3O4 NPs prepared at high L-Met concentration is more than that formed at low concentration, and this is in accordance with literature results [33].

Fig. 6. (a and b) Representative TEM images and (c) Selected Area Electron Diffraction (SAED) and size distribution histogram of L-Met capped Fe3O4 NPs (sample M-3).


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opposite end at high pH (≫ pI) de-protonation of functional groups takes place (− NH2 and –COO−). It indicates that at high pH values, L-Met

Fig. 7. Vibrating sample magnetometer (VSM) curve of L-Met capped Fe3O4 NPs (M-3) at room temperature.

Morphology of L-Met capped Fe3O4 NPs was investigated using SEM. Fig. 5 shows the L-Met capped Fe3O4 NPs have nearly spherical shape. EDX spectra (Fig. 5(d)) reveals that the presence of Fe and O atoms with 75% and 15% weight value respectively, which is near to the theoretical percent weight value of pure magnetite. EDX spectra also show that the presence of C (5.21%), N (3.8%) and S (~1%) which are sourced from L-Met, confirms that L-Met is effectively capped on the surface of Fe3O4 NPs. TEM micrographs of as prepared L-Met capped Fe3O4 NPs dried powder dispersed in dimethyl sulfoxide (DMSO), are presented in Fig. 6. TEM micrographs depicted in Fig. 6(a and b) clearly show the spherical like morphology of L-Met capped Fe3O4 NPs. Selected area electron diffraction (SAED) pattern obtained from the particles given in Fig. 6(c) is in good agreement with the characteristic electron diffraction pattern of Fe3O4 NPs of spinel structure. Fig. 6(d) shows the particle size distribution of L-Met capped Fe3O4 NPs with average particle size 4.59 nm and 2.55 nm standard deviation. Magnetic property of L-Met capped Fe3O4 NPs was measured by a vibrating sample magnetometer (VSM) at room temperature. The room temperature magnetization curve of the L-Met capped Fe3O4 NPs is presented in Fig. 7. Neither coercivity nor hysteresis loops with remanence was observed, which affirms the superparamagnetic nature of L-Met capped Fe3O4 NPs. The saturation magnetization (Ms) values found to be 65 emu/g at room temperature, which is lower than bulk Fe3O4 (92 emu/g) [29]. The lower Ms value of L-Met capped Fe3O4 NPs ascribed to size, surface affect and capping of non-magnetic L-Met on the surface of Fe3O4 NPs.

3.1. Plausible mechanism of interaction of L-Met on Fe3O4 surface In general, charge of amino acid is highly depended on pH of solution. However, very small variation in pH could cause the deviation of charge of amino acid. L -Met at a pH = pI (5.74) found dominantly in its Zwitterion form. Scheme 1 clearly shows that the pH below pI L -Met is protonated (− NH + 3 , –COOH) and the

could form a bond with an electron deficient species. Nanoparticles in general due to high surface to volume ratio they have fully of unsaturated surface atoms and undergo different types of interaction to gain their stability. Fe3O4 NPs is an inverse spinel structure, where Fe3 + occupied both Td and half of Oh sites and Fe2+ half of Oh of the unit cell. Unsaturated Fe3 + surface atoms form a bond with nucleophilic ligands to attain its stability. In this regards L-Met in alkali media readily to make bond with Fe3 + surface through COO– and NH2. Hence, in this study synthesis of Fe3O4 NPs was carried on under alkaline media (pH = 11), and the possible interaction of L-Met functional groups supposed through N and O donor atoms as shown Fig. 8. Carboxylate ion can interact to Fe3 + by either bidentate or unidentate modalities (Fig.8(I) and (II) respectively, however FTIR analysis through bidentate more likely than unidentate [33]. Amine group (Fig.8(III) through N atom is also a potent donor group to make a coordinate covalent bond with Fe3+. Sulphur atom is also possibly a potent donor atom due to its lone pair electrons. 3.2. Adsorption studies Fig. 9(a) shows UV–Visible absorption spectra of RhB adsorbed by L-Met

capped Fe3O4 NPs as a function of contact time, which clearly demonstrate that L-Met capped Fe3O4 NPs have a potential for adsorption RhB. At the adsorption equilibrium (180 min), adsorption of L-Met capped Fe3O4 NPs (M3) for RhB [at 300 K, pH = 7.4], were found to be 86.06%, 65.5%, 46.4%, 37.58% and 26.91% for concentration of 5 ppm, 10 ppm, 15 ppm, 20 ppm and 30 ppm RhB respectively. The high adsorption efficiency by the L-Met capped Fe3O4 NPs is ascribed to the high affinity of the positively charged surface to the negatively charged dye species, as well as the contribution of physical adsorption [35]. 3.3. Adsorption kinetics Fig. 9(b) shows the adsorption rate of L-Met capped Fe3O4 NPs at different initial concentration of RhB, pH at 7.4 and 300 K as a function of contacting time of adsorption. The adsorption process is rapid in the first 40 min and gradually reached to equilibrium at the end of 180 min. The kinetics of adsorption of RhB on L-Met capped Fe3O4 NPs was validated by pseudo-first-order Eq. (2) [36] and the pseudo-second-order Eq. (3) [37] kinetic models: logðqe −qt Þ ¼ logqe −ðK1 =2:303Þ t

ð2Þ

t=qt ¼ 1=K2 qe 2 þ ð1=qe Þ t

ð3Þ

where k1 and k2 are the rate constants of the pseudo-first-order and pseudo-second-order kinetic models, respectively, and qe denotes the amount of dye adsorption at equilibrium, which can be obtained independently from equilibrium experiments. The validity of the two kinetic models were evaluated with the results as shown in Fig. 10(a) and (b), by linearizing Eqs. (2) and (3), respectively. A correlation coefficient value (R2) as shown in Table S1 ESI† obtained from linear fitting curve

Scheme 1. Charge of amino acid as a function of pH.

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Fig. 8. Plausible Mechanism of interaction of L-Met on Fe3O4 surface through carboxylate bidentate and S-atom (I), carboxylate Unidentate and S-atom (II), and carboxylate Unidentate, and S-atom and amine group (III).

by applying the pseudo-second-order kinetic model (0.98 and above) is much higher than that by using pseudo-first-order model (0.92–0.95), indicating that the kinetic behavior of the RhB adsorption process onto L-Met capped Fe3O4 NPs (M3) is better approximated to pseudosecond-order kinetics, whose rate-determining step predominantly chemisorptions [38].

3.4. Adsorption isotherm To evaluate the maximum adsorption capacity of L-Met capped Fe3O4 NPs (M3), the equilibrium adsorption of RhB further investigations about the adsorption isotherm were analyzed by three widely used isotherm models: Langmuir [39], Freundlich [40] and Tempkin [41] isotherm models. The Langmuir equation can be expressed in the linearized form (Eq. (4)): Ce =qe ¼ 1=Q o bL þ Ce =Q o

ð4Þ

where q e (mg/g) is the equilibrium amount of dye adsorption, Qo (mg/g) refers to the maximum adsorption capacity of L-Met capped Fe3O4 NPs, and bL (L/mg) is the Langmuir equilibrium constant related to the enthalpy of the process. The Freundlich isotherm can be applicable for modeling the adsorption of dye on heterogeneous surfaces and the linearized form of

isotherm is expressed as (Eq. (5)): logqe ¼ logK f þ ð1=nÞ logCe

ð5Þ

KF (mg/g) is Freundlich constant representing the adsorption capacity at unit equilibrium concentration that is defined as the adsorption or distribution coefficient, and 1/n is related to the adsorption intensity. The value of Freundlich constant ‘n’ is greater than 1.0 indicating that RhB favorably adsorbed on L-Met capped Fe3O4 NPs. Tempkin isotherm is based on the assumption that the heat of adsorption would decrease linearly with the increase of coverage of adsorbent is expressed as (Eq. (6)). qe ¼ ðRT=bT Þ lnaT þ ðRT=bT Þ lnCe

ð6Þ

where aT and bT Tempkin constants. The adsorption isotherms described by these three models for RhB adsorption on the L -Met capped Fe 3 O 4 NPs are shown in Fig.11(b, c and d), and the parameters and R 2 values calculated from the three models are shown in Table S2 ESI†. From these results, the regression coefficient values of Langmuir model (R2 = 0.996), Freundlich model (R2 = 0.939) and Tempkin model (R 2 = 0.970) were obtained for the L-Met capped Fe3O4 NPs, indicating that the data best fit the Langmuir isotherm, that is, the RhB adsorption on L-Met capped Fe3O4 NPs is monolayer adsorption. The monolayer adsorption capacity was theoretically calculated at 8.33 mg/g by

Fig. 9. (a) UV–Vis absorption spectrum of RhB (10 mg·L−1) adsorbed on L-Met coated Fe3O4 nanoparticles (sample M-3) at different time and (b) the percent (%) color removal rate. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)


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Fig. 10. (a) Pseudo-first-order and (b) Pseudo-first-order plots for adsorption of RhB on L-Met capped Fe3O4 (sample M-3) at varying initial concentration of RhB. (Error bar represents 5% error).

Langmuir model fitting. The saturated adsorption efficiency of L-Met capped Fe3O4 NPs and different adsorbents towards RhB were compared (as shown ESI Table S3). The naturally occurring Kaolinite and Fe3 O4 /RGO nanocomposite have good adsorption efficiency 37.26 and 30 mg/g towards RhB respectively. Some natural adsorbents, such as raw orange peel and mango leaf powder are not inexpensive, but their adsorption capacities for Rhodamine molecules are low. Furthermore, the L-Met capped Fe3O4 NPs is magnetically recoverable and recycle for RhB removal from aqueous solution. The recyclability of the L-Met capped Fe3O4 NPs in RhB removal was tested for 5 times and the results are depicted in Fig. 12. Although the adsorption capacity

of the L-Met capped Fe3O4 NPs for RhB removal decreased slightly for each successive run, it was obtained more than 78% removal efficiency after 5 successive cycles. 4. Conclusion L-Met capped Fe3O4 NPs were effectively synthesized based co-precipitation in alkaline media. It was synthesized pure phase magnetite nanoparticles. VSM characterization reveals superparamagnetic nature of L-Met capped Fe3O4 NPs. It has 65 emu/g magnetization values. The adsorption behavior of L-Met capped Fe3O4 NPs towards RhB was followed a pseudo-second order kinetic model, suggesting

Fig. 11. (a) The amount of dye adsorbed at various concentration of RhB; (b), (c) and (d) the Langmuir, Freundlich and Tempkin adsorption isotherm plots of RhB respectively. (Error bar represents 5% error).

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Fig. 12. The recyclability of the L-Met capped Fe3O4 NPs (sample M-3) in the RhB removal from aqueous solution for 5 successive runs. (L-Met Capped Fe3O4 NPs = 50 mg and RhB = 5 ppm).

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