Facile synthesis of FeFe2O4 magnetic nanomaterial for removing methylene blue from aqueous solution

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Facile synthesis of FeFe2O4 magnetic nanomaterial for removing methylene blue from aqueous solution Van-Phuc Dinha,∗, Ngoc Quyen Tranb,c, Nguyen-Que-Tran Led, Quang-Huy Trand, Trinh Duy Nguyene, Van Tan Lef a Future Materials and Devices Laboratory, Institute of Fundamental and Applied Sciences, Duy Tan University, 10C, Tran Nhat Duat, District 1, Ho Chi Minh City, 700000, Vietnam b Institute of Applied Materials Science, VAST, TL29, ThanhLoc Ward, Dist. 12, Ho Chi Minh City 700000, Vietnam c Graduate University of Science and Technology, VAST, TL29, Thanh Loc Ward, Dist. 12, Ho Chi Minh City 700000, Vietnam d Le Quy Don–LBT Highschool, Bien Hoa City, Dong Nai province, 76000, Vietnam e Center of Excellence for Green Energy and Environmental Nanomaterials (CE@GrEEN), Nguyen Tat Thanh University, 300A Nguyen Tat Thanh, District 4, Ho Chi Minh City 755414, Vietnam f Industrial University of Ho Chi Minh City, 12 Nguyen Van Bao, Go Vap Disc., HCM City, 700000, Vietnam

A R T I C LE I N FO

A B S T R A C T

Keywords: FeFe2O4 magnetic nanomaterial Adsorption Methylene blue Adsorption mechanism

A facile and simple chemical precipitation method was utilized to synthesize FeFe2O4 magnetic nanomaterial for removing methylene blue (MB) from aqueous solution. Characterizations of this material were examined by using XRD, SEM, FT-IR, TGA-DTG and BET methods. Factors affecting the uptake of methylene blue, including pH, adsorption time, and MB initial concentration were also studied. The measured data were fitted by using four isotherm models including Langmuir, Freundlich, Sips and Temkin. Results showed that the Sips model offers the best fit to the data. In addition, the monolayer MB adsorption capacity obtained from the Langmuir model was 42.35 mg g−1 under the optimal conditions of pH = 10 and adsorption time = 80 min. In particular, the uptake of MB onto FeFe2O4 magnetic nanomaterial obtained from the kinetic and mechanism studies of the adsorption was in good agreement with prediction of the pseudo-first-order model, whereas the electrostatic interaction was found to play a primary mechanism.

1. Introduction Industrialization leads to the release of toxic chemicals into the environment, especially heavy metals and organic substances. Among them, dyes are most common chemicals found in wastewater from leather, textile, paper, cosmetics, dyeing and pharmaceutical manufactories. Dyes can cause serious diseases of eye, skin, respiratory and cancer [1,2]. The high concentration of methylene blue (MB) can cause nausea, vomiting, profuse sweating, anemia, hypertension mental confusion and methemoglobinemia [3,4]. Hence, removing these toxic chemicals has become environmentally important. Among physico-chemical methods, which have been applied to remove dyes from aqueous solution such as membranes, coagulation flocculation, adsorption and ultrasonic, adsorption is a promising method due to its advantages over other processes, e.g., simple operation, cheap and having a large number of adsorbents [5]. Oxide nanomaterials used as adsorbents have been attracted much attention

∗

from researchers owing to their high adsorption capacity [6–9]. Recently, magnetic iron oxide nanomaterial (FeFe2O4) has been considered as one of the potential materials to remove dyes from aqueous solution, owing to its magnetic properties, high surface area, chemical stability, easy synthesis and low toxicity [10]. Moreover, this material can be synthesized via many different methods, such as sol-gel method [11], precipitate and coprecipitate [12–14], ultrasonic [15], hydrothermal [16] …. Among these methods, chemical precipitate is a common method since it is simple, fast and convenient. In this report, magnetic iron oxide nanomaterial (FeFe2O4) synthesized via a fast and simple chemical method is considered to remove MB from aqueous solution. In addition, factors affecting the uptake of MB such as pH, adsorption time, ion strength and initial concentration have also been investigated. Furthermore, the isotherm and kinetic models including Langmuir, Freundlich, Sips, Temkin, pseudo-first-order, pseudo-second-order and Intra-diffusion models have been applied to calculate the maximum adsorption capacity and to estimate the

Corresponding author. E-mail address: [email protected] (V.-P. Dinh).

https://doi.org/10.1016/j.pnsc.2019.11.009 Received 5 November 2019; Accepted 26 November 2019 1002-0071/ © 2020 Chinese Materials Research Society. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/BY-NC-ND/4.0/).

Please cite this article as: Van-Phuc Dinh, et al., Progress in Natural Science: Materials International, https://doi.org/10.1016/j.pnsc.2019.11.009

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Fig. 1. Diagram of the experimental procedure.

2.4. Data analysis

mechanism of the adsorption.

Non-linear method has been used to determine parameters from isotherm and kinetic equations. To solve these non-linear equations, the Solver Add-in program of Microsoft Excel software is utilized. The chisquare (χ2), the root mean square error (RMSE) and the coefficient of determination (R2), which are the main error functions of the nonlinear methods, are determined as follows [17,18]:

2. Materials and methods 2.1. Chemicals All chemicals used without any further purification were of analytical grade. We have used Nitric acid (HNO3) of 63% and sodium hydroxide (NaOH) of 98% purchased from Merck MB (C16H18ClN3S) purchased from Sigma-Aldrich.

n

R2 = 1 − n

χ2 =

2.2. Synthesis of FeFe2O4 nanomaterial

∑ n=1

Fig. 1 shows the diagram of a synthesis of FeFe2O4 nanomaterial. In the first experimental stage, the FeFe2O4 magnetic nanomaterial has been synthesized via a fast and simple chemical method at room temperature. Here, the solution of 2.0 g sodium hydroxide was gradually added to the solution of 6.975 g FeSO4·7H2O. The obtained mixture was shaken in 5 h at 1000 revolutions per minute (rpm). The reaction solution was changed from light blue to brown-black. Then, the brownblack mixture was dried at 100 °C in 8 h to obtain black FeFe2O4 magnetic nanomaterial, prior to be characterized by XRD, SEM, FT-IR, TGA and BET methods.

RMSE =

(qe, meas − qe, calc ) 2 qe, calc 1 n−1

2

, (03)

, (04)

n

∑ (qe,meas − qe,calc )2

.

j=1

Fig. 2 shows the X-ray diffraction pattern of the FeFe2O4 with some characteristic peaks at 2θ degree = 18.31°, 30.19°, 35.53°, 43.18°, 53.59°, 57.13° and 62.68° (JPDS–19–0629), indicating that FeFe2O4 magnetic nanomaterials have successfully been synthesized at room temperature via a facile and simple chemical method. The sample's morphology was investigated by using the Scanning Electron Microscope (SEM) and shown in Fig. 3. It is easily to see in Fig. 3 that the material has many nanospheres with 60–80 nm in size

(01)

whereas the percentage of the uptake is given as:

% Removal =

(Co − Ce) .100% , Co

(05)

3.1. Characteristic properties of FeFe2O4 magnetic nanomaterial

To investigate the MB adsorption onto FeFe2O4 magnetic nanomaterial, the batch adsorption experiment is shown in Fig. 1. Here, 0.1 g of this material is placed into a flask of 100 mL including 50 mL of the MB solution. Then, the mixture is shaken with a speed of 150 rpm. Factors, which affect the uptake of MB onto this material, such as pH (2–11), adsorption time (5–240 min), ion strength (0–0.5 M of KCl) and MB initial concentration, are investigated. The MB concentration in the solution before and after the adsorption is determined by UV–Vis method at 664 nm. The adsorption capacity (Qe) (mg/g) at equilibrium is calculated by using the following equation:

(Co − Ce ). V , m

n

∑n = 1 (qe, meas − qe, calc )

3. Results and discussion

2.3. Batch adsorption experiment

Qe =

∑n = 1 (qe, meas − qe, calc )2

(02)

where C0 is the initial concentration (mg/L), Ce is the equilibrium concentration of MB (mg/L), V is the volume (L) of the solution, and m is the mass of FeFe2O4 magnetic nanomaterial (g).

Fig. 2. XRD of FeFe2O4 magnetic nanomaterial. 2

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Fig. 3. SEM image of FeFe2O4 magnetic nanomaterial.

Fig. 5. Plot of Point of zero charge of FeFe2O4 magnetic nanomaterial.

of anion. By contrast, the surface of material will be negatively charged when the solution has pH to be higher than the pHPZC value. As a result, the adsorption of cations will be prioritized [22–25]. 3.2. Effects affecting the adsorption of MB 3.2.1. The UV–Vis spectra of MB The MB, which was used as a cationic dye model, forms a solution at room temperature with a maximum absorbed wavelength at 664 nm. The UV–Vis spectra shown in Fig. 6 presents fours specific peaks of MB at the wavelengths of 246, 292, 613 and 664 nm. 3.2.2. Effect of pH It has been well-known that pH has important influence on the removal of MB from aqueous solution. According to the diagram in Fig. 7a, the absorbance decreases sharply with increasing pH from pH = 2 to pH = 10, before climbing gradually back up, indicating that the adsorption of MB on FeFe2O4 has increased with increasing pH and obtained its maximum value at pH = 10 (Fig. 7b). These results correspond to the changes in the charge of this FeFe2O4 material from the positive to the negative values when pH values vary from 2 to 11.

Fig. 4. The FT-IR spectra of FeFe2O4 magnetic nanomaterial before (a) and after (b) the adsorption of MB.

and nanorods with 200–300 nm in length and 20–30 nm in wide. The surface of material is heterogeneous satisfying with the adsorption. Fig. 4a [black line] presents the FT-IR spectra of FeFe2O4 nanomaterial before the adsorption of MB. As can be seen from this figure, the peaks at 3138 cm−1 and 1637 cm−1 are related to the –O–H groups of physico–chemical water molecules, whereas those at 1116 cm−1, 893 cm−1, 796 cm−1 and 618 cm−1 are associated with Fe–O bonds. Surface area of FeFe2O4 magnetic nanomaterial examined by BET analysis is shown in Table 1. As a result, this material's surface area is 57.19 m2/g which is higher than some others synthesized by different precursors and methods. Fig. 5 plots the point of zero charge (pHPZC), which is defined as the value of pH where the charge of material's surface is zero. The value of pHPZC obtained from Fig. 5 is 7.2. This value means that when pH of solution is smaller than 7.2, adsorption sites will be protonated, which results in the positively charged surface satisfying with the adsorption

3.2.3. Effect of adsorption time Fig. 8a shows effects of adsorption time on the removal of MB from aqueous solution by FeFe2O4 magnetic nanomaterial. As can be seen, the uptake started climbing steadily, peaking at 80 min, and being saturated at a level of approximately 35% of the removal with the high MB initial concentration of 100 mg/L. This result can be explained by

Table 1 BET analysis of FeFe2O4 nanomaterials synthesized from different methods and precursors. Synthesized methods

Co-precipitation Electrodeposition and reduction methods Solvothermal synthesis Precipitation Precipitation

Precursors

Surface area (m2/g)

References

Fe3O4 commercial FeCl2·4H2O and FeCl3·6H2O Fe2O3

6.8 12.7

[12] [10]

39–45

[19]

FeCl3 FeSO4·7H2O FeSO4·7H2O

6.4 47.07 57.19

[20] [21] This study

Fig. 6. The UV–Vis spectra of methylene blue (MB) at room temperature. 3

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Fig. 7. Effect of pH on the adsorption of MB on FeFe2O4 nanomaterial: UV–Vis spectra (a) and plot of Qe (mg/g) versus pH of solution (b). (MB initial concentration = 100 mg/L, adsorption time = 120 min, adsorbent dosage = 0.1 g, error bar = SD, n = 3).

the fact that at the early stage, only a little mount of MB molecular was absorbed on its surface, resulting in a large value of equilibrium concentration Ce and small value of adsorption capacity (Qe). The uptake will then reach a saturation after 80 min when the adsorption sites are fully occupied [26]. 3.2.4. Effect of initial concentration The effects of MB initial concentration on the adsorption capacity of FeFe2O4 magnetic nanomaterial have been also investigated and the results are shown in Fig. 8b. This figure indicates that there is an increase in the adsorption capacity when the MB initial concentration increases from 15 mg/L to 320 mg/L. The reason is that if the MB concentration is low, the amount of MB being adsorbed is small, resulting in the small adsorption capacity, as explained above. In contrast, the percentage of the removal increases with increasing the MB concentration due to the increase of the adsorption sites. Notwithstanding, the uptake will be flattened if the adsorption sites obtain the saturation.

Fig. 9. Effect of ion strength on the adsorption of MB onto FeFe2O4 magnetic nanomaterial. (MB initial concentration = 100 mg/L, adsorptiontime = 120 min,pH = 10, adsorbent dosage = 0.1 g).

3.2.5. Influence of ion strength The effect of ion strength on the removal of MB by FeFe2O4 magnetic nanomaterial is shown in Fig. 9. The diagram depicts that there is a significant decrease in the percentage of the removal of MB from approximately 64% to about 5% with increasing the ion strength expressed by the change in the concentration of KCl from 0 to 0.5 mol/L. Clearly, the presence of electrolyte reduces the adsorption of MB onto FeFe2O4 magnetic nanomaterial, owing to the decrease in the screening

effect (known as the electrostatic screening) between the negatively charged material surface and MB molecules. This finding is consistent with some other reports [1,27]. Therefore, it can be concluded that the adsorption has been controlled by the electrostatic force.

Fig. 8. Adsorption capacity Qe of MB on FeFe2O4 nanomaterial versus the adsorption time (a) and initial concentration (b). (MB initial concentration = 100 mg/L, pH = 10, adsorbent dosage = 0.1 g, error bar = SD, n = 3). 4

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3.3. Adsorption isotherm

Table 2 Parameters of the non-linear isotherm models.

The Langmuir isotherm model, has been employed in order to describe the homogeneous adsorption, in which each MB molecule possesses one adsorption site, without having interacted with the adsorbed molecules [18,28]. The non-linear equation of this model is given as:

qe =

Q m .KL .Ce , 1+KL .Ce

where qe (mg.g ) denotes the amount of adsorbate in the adsorbent at the equilibrium; Qm (mg.g−1) characterizes the maximum adsorption capacity of the adsorbent; and KL (L.mg −1) stands for the Langmuir adsorption constant. In addition, the Freundlich isotherm model, an empirical model, which describes the heterogeneous adsorption systems, in which MB molecules interact with each other to form the multi-layers [18,28], has been also used. The non-linear equation of this model reads:

q e = KF .C1/n e ,

Langmuir

KL qm(mg.g−1) RMSE R2 χ2

0.03314 42.35 1.823 0.9725 1.325

Freundlich

n KF RMSE R2 χ2

3.02 6.58 3.443 0.9018 5.53

Sips

Qs αs βs RMSE R2 χ2

1.264 0.0302 1.035 1.819 0.9726 1.236

Temkin

KT (L.mg−1) bT(kJ.mol−1) RMSE R2 χ2

0.4543 0.30 2.4290 0.9511 2.0370

(6)

with KF being the Freundlich isotherm constant (mg/g) associated with the adsorption capacity and n being the adsorption intensity. Since the Langmuir and Freundlich models are limited by the concentration of adsorbed molecules, the Sips model with three parameters has been developed by combining both models [18] and its non-linear equation is expressed as:

QS .CβeS 1+

Non-linear isotherm parameters

(5)

−1

qe =

Non-linear isotherm models

α s.CβeS

,

−1

MB onto FeFe2O4 magnetic nanomaterial is favorable [29,30]. Moreover, the maximum MB monolayer adsorption capacity of this material obtained from the Langmuir model is 42.35 mg g−1. Therefore, it is obvious that the FeFe2O4 magnetic nanomaterial can be used as a novel adsorbent to remove the MB from aqueous solution.

(7) −1

where QS (L.g ) is the Sips constant; αS (L.mg ) is the Sips isotherm model constant and βS is the Sips isotherm model exponent. Fig. 10 presents the plots of these non-linear isotherm models, whose non-linear isotherm parameters are given in Table 2. According to the method of least squares, if the smallest values of χ2 and RMSE and highest value of R2 are obtained from the model calculations, one can conclude that the models have achieved the best fit to the experimental data. Our model analyses given in Fig. 10 and Table 2 indicate that the Sips model (χ2 = 1.236, RMSE = 1.819 and R2 = 0.9726) offers the best fit to the experimental adsorption isotherm data than the Langmuir (χ2 = 1.325, RMSE = 1.823 and R2 = 0.9725) and Freundlich (χ2 = 5.53, RMSE = 3.443 and R2 = 0.9018) models. This agreement of the Sips model with the experimental data suggests that the surface of adsorbent is a heterogeneous system [18], which is in accordance with the observation of its morphology in Fig. 2. The value of n calculated from the Freundlich model is about 3.02. This value of n, which is within the interval of [1,10], illustrates that the adsorption of

3.4. Kinetic models To identify the information on the nature of the adsorption of MB on FeFe2O4 magnetic nanomaterial, three kinetic adsorption models have been used within the present work, namely the pseudo-first-order, pseudo-second-order, and intra-diffusion kinetic models. Equations and kinetic parameters of these models are shown in Table 3, whereas their plots are presented in Fig. 11. Here, one can easily see that the pseudosecond-order kinetic model gives the best fit to the experimental data because of its highest R2 and smallest RMSE and χ2 values. Moreover, while the pseudo-first-order and pseudo-second-order models cannot Table 3 Kinetic parameters. Kinetic models

Non-linear parameters Co (mg/L) qe

Fig. 10. Plots of non-linear isotherm models. (Adsorbent dosage = 0.1 g, adsorption time = 120 min, MB initial concentration from 15 to 320 mg/L, temperature = 303 K). 5

(exp)

(mg.g−1)

100 18.30

Pseudo-first-order model

qe (cal) (mg.g−1) k1(min−1) RMSE R2 χ2

18.88 0.0224 1.8313 0.9043 5.080

Pseudo-second-order model

qe (cal) (mg.g−1) k2 (g.mg−1.min1) RMSE R2 χ2

22.70 0.0011 1.895 0.8976 4.561

Intra-diffusion model

KP C RMSE R2 χ2

1.213 2.593 2.334 0.8445 4.326

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Fig. 13. TGA-DTG of the FeFe2O4 magnetic nanomaterial before and after the adsorption.

Fig. 11. Plots of non-linear kinetic models. (Adsorbent dosage = 0.1 g, adsorption time from 5 min to 240 min, MB initial concentration = 100 mg/L, temperature = 303 K).

isotherm constant. Using equation (8), we have obtained a KT value of 0.3 kJ mol−1. This obtained value of KT clearly indicates that the uptake of MB on FeFe2O4 magnetic nanomaterial follows a physical process. These results can be also confirmed by using the FT-IR spectrum and TGA-DTG measurement before and after the adsorption. The FT-IR spectra in Fig. 5 show that there is a slight change in the intensities of the specific peaks of FeFe2O4 nanomaterial with small shifts from 1637, 1116 and 618 cm−1 to 1634, 1123 and 627 cm−1, respectively. These shifts demonstrate that the primary mechanism of the adsorption of MB onto this material is electrostatic attraction corresponding to the effects of pH and ion strength on the uptake [27]. Additionally, a comparison between the TGA-DTG curves before and after the adsorption is depicted inFig. 13. Obviously, before the adsorption, there are two endothermic peaks related to the weight losing process of absorbed water with about 4% of weight loss at 243.4 °C and the decomposition of Fe3O4 into FeO with approximately 3% of weight loss at 265.5 °C. However, after the uptake, these temperatures change to 254.1 °C and 334.4 °C with roughly 9% and 3% of weight loss, respectively. It might be concluded here that the MB molecules have been successfully adsorbed onto the FeFe2O surface, similar to the results reported in Refs. [31,32]. All the above results allow us to conclude that the removal of MB from aqueous solution by FeFe2O4 magnetic nanomaterial has certainly followed the physical process, in which the electrostatic attraction has played as a primary mechanism.

Fig. 12. Plot of the intra-diffusion kinetic model. (Adsorbent dosage = 0.1 g, adsorption time from 5 min to 240 min, MB initial concentration = 100 mg/L, temperature = 303 K).

describe exactly the mass transfer of the uptake of MB onto FeFe2O4 magnetic nanomaterial, the intra-diffusion model can be utilized to identify mechanism of the diffusion in the adsorption process. Fig. 12 shows that there are three stages in the uptake of MB on FeFe2O4 magnetic nanomaterial. In the first stage, MB molecules are transferred rapidly to the adsorbent surface, whereas the intra-diffusion process occurs in the second stage. In the last stage, the adsorption obtains a saturation. The intra-diffusion parameters are listed in Table 3. From this table, it is apparent that the value of C calculated from this model is not zero, indicating that the adsorption should follow not only the intraparticle diffusion but also two or more extra diffusion mechanisms [28].

4. Conclusion The FeFe2O4 magnetic nanomaterial has been successfully synthesized via the facile and simple chemical precipitation methods at room temperature. The synthesized material has rough surface with two kinds of shape including nanospheres and nanorods, which are satisfied with the adsorption. The isotherm studies for the synthesized material have shown that the Sips model offers the best fit to the experimental data, confirming the heterogeneous characteristics of the adsorption systems. At the same time, the kinetic and mechanism studies have indicated that the electrostatic attraction plays as a primary mechanism for the removal of MB from aqueous solution by FeFe2O4 magnetic nanomaterial.

3.5. Mechanism of the adsorption To study the mechanism of the adsorption of MB on FeFe2O4 magnetic nanomaterial, theoretical models are combined with empirical spectroscopes. Among them, Temkin isotherm model has been used in order to determine the adsorption heat. This model is presented as following equation:

qe =

RT ln(KT Ce ), bT

Acknowledgment This research is funded by Vietnam National Foundation for Science and Technology Development (NAFOSTED) under grant number 103.02-2018.368.

(8)

where bT (kJ mol−1) is the heat of the adsorption and KT is the Temkin 6

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[18] [19]

[20]

[21]

[22]

[23]

[24] [25]

[26] [27]

[28]

[29]

[30]

[31]

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