graphene oxide nanocomposites for removal of nickel ions, methylene blue from water

graphene oxide nanocomposites for removal of nickel ions, methylene blue from water

Journal Pre-proofs Fabrication of manganese ferrite/graphene oxide nanocomposites for removal of nickel ions, methylene blue from water Lu Thi Mong Th...

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Journal Pre-proofs Fabrication of manganese ferrite/graphene oxide nanocomposites for removal of nickel ions, methylene blue from water Lu Thi Mong Thy, Nguyen Hoan Kiem, Tran Hoang Tu, Lu Minh Phu, Doan Thi Yen Oanh, Hoang Minh Nam, Mai Thanh Phong, Nguyen Huu Hieu PII: DOI: Reference:

S0301-0104(19)31221-2 https://doi.org/10.1016/j.chemphys.2020.110700 CHEMPH 110700

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Chemical Physics

Received Date: Revised Date: Accepted Date:

14 October 2019 22 January 2020 28 January 2020

Please cite this article as: L. Thi Mong Thy, N. Hoan Kiem, T. Hoang Tu, L. Minh Phu, D. Thi Yen Oanh, H. Minh Nam, M. Thanh Phong, N. Huu Hieu, Fabrication of manganese ferrite/graphene oxide nanocomposites for removal of nickel ions, methylene blue from water, Chemical Physics (2020), doi: https://doi.org/10.1016/j.chemphys. 2020.110700

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FABRICATION OF MANGANESE FERRITE/GRAPHENE OXIDE NANOCOMPOSITES FOR REMOVAL OF NICKEL IONS, METHYLENE BLUE FROM WATER Lu Thi Mong Thy1,3, Nguyen Hoan Kiem2, Tran Hoang Tu1, Lu Minh Phu1, Doan Thi Yen Oanh4, Hoang Minh Nam1,2, Mai Thanh Phong2, Nguyen Huu Hieu1,2* 1VNU-HCM

Key Laboratory of Chemical Engineering and Petroleum Processing (CEPP Lab) Ho Chi Minh City University of Technology (HCMUT), VNU-HCM 2Faculty of Chemical Engineering, HCMUT, VNU-HCM 3Faculty of Chemical Engineering, Ho Chi Minh City University of Food Industry 4Publishing House for Science Technology, Vietnam Academy of Science and Technology *Corresponding author: Email: [email protected]

Abstract Nowadays, environmental pollution has increasingly serious effects to human health, especially water pollution. Heavy metals and organic dyes are major environmental pollutants. In this study, manganese ferrite/graphene oxide (MnFe2O4/GO) nanocomposites were fabricated by in situ method and used as an adsorbent for the removal of nickel ions (Ni2+), methylene blue (MB) from water. The MnFe2O4/GO nanocomposite with a mass ratio of MnFe2O4 to GO of 1:1 (FGO2) had high adsorption capacity and was easily separated by external magnetic field. The adsorption data of FGO2 fitted well with a pseudo-secondorder kinetic and Langmuir isotherm models. The maximum adsorption capacities of FGO2 for Ni2+ and MB were determined to be 152.67 and 89.29 mg/g, respectively. MnFe2O4/GO nanocomposite was characterized by Fourier transform infrared spectroscopy, X-ray diffraction, transmission electron microscopy, Brunauer-Emmett-Teller (BET) specific surface area, and vibrating sample magnetometer (VSM). FGO2 had the BET specific surface area of 78.46 m2/g and VSM of 28.74 emu/g. Based on these results, the FGO2 could be considered as a highly efficient adsorbent for removing Ni2+, MB from water. KEYWORDS: manganese ferrite; graphene oxide; nanocomposite; adsorption; nickel, methylene blue.

1. Introduction The developments of industrialization, civilization, and rise in population have led to a decline in the quality of water resources. Water pollution is a global problem and poses challenges in many developing countries owing to the shortage of suitable water treatment technologies. Concentrations of various pollutants such as heavy metals, organic dyes, and oils in water have rapidly increased beyond the acceptable limits [1]. Nickel ions (Ni2+) have been identified as heavy metal ions and widely used in metallurgical, metal plating, battery, and alloy industries [2]. Ni2+ ions can damage the lungs and kidneys and cause gastrointestinal distress, pulmonary fibrosis, skin dermatitis, and cancer when penetrating the human body [2]. Additionally, methylene blue (MB) is an organic dye used in many industries such as plastics, paper, textile, foodstuffs, and cosmetics. However, MB is associated with serious health problems including skin lesions, digestive diseases, cyanosis, tissue necrosis, and intestinal cancer [3]. Therefore, the removal of Ni2+ and MB from water is necessary. Many technologies including physical, chemical, and even biological approaches have been developed for heavy metal ion, dye removal from aquatic media [4]. Among these approaches, adsorption turns out to be one of the most effective solution due to its simple treatment process, low cost, and capability to adsorb a broad range of removal efficiently [5]. The adsorbents such as activated carbon, zeolite, metal oxides, and biomaterials are difficult to separate and do not have high adsorption capacities [6,7]. The study of adsorbent materials, especially magnetic materials (Fe3O4, NiFe2O4, MnFe2O4, etc.), has been enhanced. Manganese ferrite oxide (MnFe2O4) nanoparticles with good magnetism and functional surface are widely used as adsorbent for removing heavy metals in aqueous solution. Nevertheless, the materials still remain some defects, which include instability and agglomeration [8,9]. Specifically, MnFe2O4 nanoparticles have some drawbacks including as instability, agglomeration, low mechanical strength and oxidative capability in acidic environment, which requires it to be attached on substrate surface. Graphene

oxide (GO) contains a large number of oxygen-containing functional groups render GO strongly hydrophilic which has good dispersibility in water [10]. Besides, due to its large specific surface area, GO has high negative charge density on the surface to link of pollutants. These unique properties of GO enables them as ideal substrates to anchor MnFe2O4 nanoparticles for adsorption applications. The combination of MnFe2O4 NPs and GO to form nanocomposite has advantages of reusability, magnetic separation and high removal efficiency, which become promising adsorbents for removal of Ni2+, MB from water. In this study, manganese ferrite/graphene oxide (MnFe2O4/GO) nanocomposites were prepared by in-situ method and applied in removing Ni2+, MB from water. The suitable MnFe2O4:GO mass ratios in the nanocomposites were determined through the adsorption capacity and separation ability. The formation mechanism of MnFe2O4 particles on GO surface was discussed. The selected nanocomposite was used to investigate the effects of single factors including pH, contact time, and initial concentration on the adsorption capacity. The adsorption process was studied using pseudo-first and pseudo-second-order kinetic; Langmuir, Freundlich, and Temkin isotherm models. 2. Materials and methods 2.1. Materials Graphite powder with CAS number of 7782-42-5 was purchased from Sigma Aldrich, Germany. Analytical grade chemicals including FeCl3.6H2O, MnCl2.4H2O, NaOH, HCl, Ni(NO3)2.6H2O, and C2H5OH were purchased from VinaChemsol, Vietnam. KMnO4, H2SO4, MB (C16H18ClN3S), and H3PO4 were purchased from Xilong Chemical, China. Double-distilled water was used in all experiments. 2.2. Synthesis of MnFe2O4/GO nanocomposites MnFe2O4 nanoparticles (MnFe2O4 NPs) were synthesized in situ method [11]. GO was synthesized by Tour method [12]. MnFe2O4/GO nanocomposites were synthesized by in situ method with different mass ratios of MnFe2O4 to GO including 1:0.5 (FGO1), 1:1 (FGO2), 1.5:1 (FGO3), 2:1 (FGO4), and 2.5:1 (FGO5) [13]. Briefly, FeCl3.6H2O (2.52 g) and MnCl2.4H2O (0.93 g) were added into 200 mL of GO suspension. The mixture was sonicated for 30 min. Next, pH of the mixture was adjusted to 10. The mixture was stirred and heated at 80 °C for 60 minutes. The precipitation was washed and dried to obtain the product. 2.3. Determine of appropriate MnFe2O4 to GO ratio The nanocomposites were carried out to determine adsorption capacity and separation ability, thereby comparing the obtained results to find the appropriate weight percentage. Adsorption experiments were performed at room temperature to investigate the effect of MnFe2O4 to GO ratios on the adsorption capacities of MnFe2O4/GO nanocomposites to remove Ni2+, MB. Then, 0.02 g adsorbent was added to the solution of 20 mL Ni2+, MB with the concentration of 200 mg/L at pH 6, and shacked on the shaker (100 rpm) for about 12 h. After the adsorption, the Ni2+, MB concentrations in the solution were determined by Inductively coupled plasma mass spectrometer (ICP-MS Model 7500, Agilent, USA) and UV-VIS spectrophotometer (Horiba Dual FL UV-VIS, Japan), respectively. The adsorption capacity (q) (mg/g) was calculated by the following equation (1): (Co ― Ce)V (1) q= m where Co and Ce (mg/L) are the initial and equilibrium concentrations of pollutants, respectively; V (mL) is the volume of the solution; and m (mg) is the mass of the adsorbent. Moreover, the morphologies of materials were analyzed by TEM images using JEM-1400 microscope (JEOL) with a magnification of 1,500,000x and a resolution of 20-200 nm. The saturation magnetization of suitable nanocomposite was determined by using vibrating sample magnetometer (MicroSense Easy VSM v. 9.13 L) with magnetic range of 10-100 emu and magnetic field strength of 0-32 Oe. 2.4. Characterization The functional groups on the surface of materials were studied by FTIR spectra (FTIR Alpha–E, Bruker) with a resolution of 0.2 cm-1, wavenumber region of 400-4000 cm-1, and temperature of 30 oC. The sample was dried and pelleted with KBr powder. The crystalline phase of materials was studied using XRD pattern (Advanced X8, Bruker) with operation conditions including wavenumber length of 1.5406 nm, scan angle of 5-80 o, humidity of 70 %, temperate of 30 oC. The structural defects of materials were studied by Raman spectra (LabRAM HR Evolution, Horiba, Japan). The elemental composition of materials was investigated

by using scanning electron microscope coupled with energy-disperse X-ray analyzer (SEM/EDX) (Jeol JMS 6490, JEOL - Japan). The BET specific surface area of the suitable nanocomposite was measured through N2 adsorption isotherm curve at 77.3 K and po = 765 mmHg (Nova 3200e, Quantachrome). The sample is degassed at 300 °C for 4 h prior to the adsorption process. 2.5. Investigate the effects of factors on the adsorption ability of nanocomposite Factors including contact time, pH, and initial concentration were selected to investigate the effects on the Ni2+, MB adsorption capacities of suitable nanocomposite. The Ni2+ adsorption experiments were performed with a time interval of 10-480 minutes, pH 3-8 and initial concentration of 50-400 mg/L. For MB, the intervals of factors were changed as the time to 30-480 minutes, pH of 4-9, and initial concentration of 50-450 mg/L, respectively. The pseudo-first-order and pseudo-second-order models were used to investigate the kinetic properties of Ni2+, MB adsorption as shown in equation (2) and (3), respectively. ln (qe ― qt) = ln qe ― k1t (2) t 1 t = + qt k2q2e qe

(3)

where qe and qt are the adsorption capacities at equilibrium and at time t (mg/g), respectively; k1 (min-1) and k2 (g/mg.min) are the rate constants [14,15]. The zeta potential (pHpzc) values of MnFe2O4/GO was determined by pH drift method [16]. Besides, Langmuir, Freundlich, and Temkin isotherm models were applied for the analysis of the adsorption type and the maximum adsorption capacity. The linear equations for models are expressed in equation (4), (5), and, (6), respectively: Ce Ce 1 (4) = + qe qm qmkL 1 ln qe = ln kF + ln Ce n

(5)

qe = Bln kT + Bln Ce

(6)

where qm (mg/g) is the maximum adsorption capacity; kL (L/mg) is the Langmuir constant; n and kF (L/mg) are the Freundlich constants; and B and kT are the Temkin constants [15]. 3. Results and discussion 3.1. Appropriate MnFe2O4 to GO ratio The effect of MnFe2O4 to GO ratios on the Ni2+, MB adsorption capacities of MnFe2O4/GO nanocomposites were shown in Figure 1. The Ni2+, MB adsorption capacities decreased when the ratios of MnFe2O4 to GO were enhanced. The key reason is the adsorption mechanism due to oxygen-containing groups on GO’s surface. Moreover, the increase in ratio of MnFe2O4 to GO led to the enhancement in aggregation of MnFe2O4, resulting in the reduction of surface area and adsorption capacity [10]. Thus, the FGO1 and FGO2 were considered for Ni2+, MB adsorption.

(a) (b) Figure 1. Effect of MnFe2O4 to GO ratios on the adsorption capacities of MnFe2O4/GO for (a) Ni2+, (b) MB The morphologies of GO, MnFe2O4, FGO1, and FGO2 are shown in Figure 2. GO sheets were flat, which showed the strong oxidation and exfoliation processes of GO. However, the wrinkles and folding of GO appeared, which showed the stacking of sheets. In Figure 2 (b), the agglomeration process of MnFe2O4 nanoparticles was observed. For FGO1 and FGO2, the MnFe2O4 nanoparticles were distributed evenly on the GO surface with the average size of 10-25 nm [17]. The combination of MnFe2O4 with GO was based on the electrostatic interactions, which helped to prevent the agglomeration and stabilization of nanoparticles. In addition, TEM results show that when adding MnFe2O4 into GO structure, the distance between GO layers increases, which prevents the adhesion of GO sheets. This increases the active sites, which shows that the nanocomposites have higher adsorption ability than precursors. Thus, FGO1 and FGO2 were chosen for investigating magnetic properties through VSM.

(a)

(b)

(d) (e) Figure 2. TEM images of (a) GO, (b) MnFe2O4 NPs, (c) FGO1, and (e) FGO2 Figure 3 shows that FGO2 samples are more easily and completely recovered by an external magnetic field than FGO1 after the adsorption process That FGO2 is better than FGO1 because of its higher oxide ratio, which could help the nanocomposite recover after using. Also, the magnetic curves of nanocomposites have S-like shape and the saturation magnetism (Ms) of FGO1 and FGO2 were determined to be 18.30 and 28.74 emu/g from VSM data. This result explains that the distribution of MnFe2O4 on GO surface was reduced in particles size, so the Ms value of nanocomposites were smaller than that of MnFe2O4 (66.70 emu/g) [14]. Additionally, the small coercivity value (Hc = 0.36 Oe and Hc = 1.35 Oe) proves that FGO2 is easily magnetized.

FGO1

FGO2

Figure 3. The magnetic properties of FGO1 and FGO2

Figure 4. The VSM data of FGO1 and FGO2

The size of nanoparticles and Ms values of FGO2 in this study are shown in Table 1. The FGO2 had smaller size nanoparticles and recovered easily. Table 1. The size of nanoparticles and Ms values of FGO2 and other nanocomposites Materials

Method

FGO2

in situ Co-precipitation Co-precipitation Solvothermal Co-precipitation

MnFe2O4/GO MnFe2O4/Activated carbon MnFe2O4/Reduced GO MnFe2O4/Cellulose aerogel

Size of

nanoparticles (nm) 10-15 10-15 40-70 150 30-50

Ms (emu/g)

References

28.74

In this study

27.10

[13]

14.76

[18] [19] [20]

40.50 37.8

The size of FGO2 nanoparticle is smaller than that of the others, which reduces its magnetic properties. The size of nanoparticle is affected by the nucleation and growth stages. In aqueous solution, Fe3+ and Mn2+ are hydrolysed and produce hexa-aq ions. These ions were linked with the negative oxygen-containing groups on GO surface (-OH and -COOH) based on the electrostatic interactions, which made it form more nuclei during the nucleation period, following equation (7) - (10): Fe3+ + 6H2O ⇌ Fe(H2O)63+ (7) 2+ 3+ Mn + 6H2O ⇌ Mn(H2O)6 (8) M(H2O)63+ + GO-OH ⇌ M(H2O)63+ (O-GO)3 (9) M(H2O)63+ + GO-COOH ⇌ M(H2O)63+ (OOC-GO)3

(10)

Then, the hexa-aq ions are hydrolyzed and formed nuclei sites at pH 10-11 and 80-90oC. These hydroxides simultaneously precipitated to form MnFe2O4 crystalline nanoparticles on GO surface to create nanocomposite in the following equation (11) – (13): Fe(H2O)63+ + OH- ⇌ Fe(OH)3 Mn(H2O)63+ + OH- ⇌ Mn(OH)2 Fe(OH)3 + Mn(OH)2 ⇌ MnFe2O4

(11) (12) (13)

With the increasing number of nuclei sites, the amount of Mn2+ and Fe3+ ions in solution for growth period decreases, resulting in the formation of smaller nanoparticles (equation (14)) [21]. Moreover, the use of ultrasound stage in the formation process of nanocomposite is to disperse nanoparticles on GO surface and reduce particle size. GO + Mn2+ + 2Fe3+ + 8OH- ⇌ MnFe2O4/GO

(14)

The Ms value of FGO2 is lower than other nanocomposites in other studies. This result is explained that the size of particles effects on the magnetic properties of nanocomposite as shown in the following equation (15): β MS = Ms (bulk)[1 ― ] D

(15)

where MS is the saturation magnetization of a sample, Ms (bulk) is the bulk saturation magnetization value, β is the constant, and D is the average size. Based on these results, FGO2, which was synthesized by in situ method, was chosen for investigating factors affecting adsorption ability and characteristic analysis. 3.2. Characterization of FGO2 3.2.1. XRD patterns In Figure 5 (a), GO had a sharp peak at 2 = 11.3o (d= 0.80 nm), which shows that the oxygen-containing groups were linked on GO surface, and the interlayer distance between GO sheets was increased [22]. FGO2 exhibited diffraction peaks at 2 = 27.07o, 32.83o, 45.38o, 56.47o, and 66.56o that were assigned to the (220), (311), (222), (400), (511), and (440) planes, respectively. These peaks were in agreement with the standard data of MnFe2O4 (JCPDS File Card No. 10-0319) [23]. These results showed that MnFe2O4 nanoparticles were successfully decorated on GO surface.

(b) (a) Figure 5. XRD patterns of (a) GO, (b) MnFe2O4 and FGO2 3.2.2. FTIR spectra The functional groups of GO and FGO2 were studied by FTIR spectra as shown in Figure 6. For FGO2, several peaks at 3449, 1722.41, 1628.32, 1383.46, and 750.19 cm-1 corresponded to the vibrations of -O-H, -C=O, -C=C, and -C-O- groups of GO, respectively [24]. Besides, two peaks at 663.27 and 481.23 cm-1 were ascribed to the Fe-O- or Mn-O- bonds of MnFe2O4 structure. These results indicated that the MnFe2O4 nanoparticles were successfully linked to the surface of GO sheets [25].

Figure 6. FTIR spectra of GO and FGO2 3.2.3. Raman spectra

Figure 7. Raman spectra of GO and FGO2

Figure 7 shows the Raman spectra of the GO and FGO2. The G peak is a result of in-plane vibrations of sp2 bonded carbon atoms whereas the D peak is due to out of plane vibrations attributed to the presence of structural defects [26]. The ratio of intensity of D/G peaks (ID/IG) presents the defect of materials. The ID/IG values of GO and FGO2 were calculated to be 0.93 and 1.12, respectively. The ID/IG of FGO2 is higher than of GO shows that the defect degree in materials increased, showing MnFe2O4 partilces were linked with oxygen groups of GO [27]. 3.2.4. EDX results The elemental analysis of GO and FGO2 were confirmed by EDX analysis as shown in Table 2. For FGO2, the appearance of Mn and Fe elements were recorded and the C/O ratio was determined to be 0.89. The results show the formation of MnFe2O4 linked to the functional groups on GO surface [23,25]. Besides, the weight percentage of MnFe2O4 in FGO2 was calculated to be 48.61 %, is slightly closed to the mass ratio of MnFe2O4 to GO of 1:1. Table 2. The elemental compositions in GO and FGO2 GO FGO2 Element C 50.09 30.95 O 49.91 34.58 Mn 12.15 Fe 23.31 3.2.5. BET specific surface area The BET specific surface area of FGO2 was determined to be 78.46 m2/g, which was higher than that of MnFe2O4. The MnFe2O4 nanoparticles were linked with oxygen-containing groups on GO surface, reducing the aggregation of MnFe2O4, increasing the surface area of FGO2 [17,23]. However, the distribution of MnFe2O4 nanoparticles on GO surface reduced the number of N2 adsorption sites; thus, the surface area of FGO2 was lower than that of GO. The specific surface area of FGO2 was in medium range in comparison with other graphene-based nanocomposites as shown in Table 3. Table 3. The BET specific surface areas of FGO2 and other materials Materials

BET specific surface area (m2/g)

References

FGO2 GO MnFe2O4 MnO2/Fe3O4/GO

78.46 84.68 42.41 60.10

In this study In this study In this study [28]

qt (mg/g)

3.2. Effects of factors on the Ni2+, MB adsorption capacities of FGO2 3.2.1. Contact time As shown in Figure 8, the Ni2+, MB adsorption capacities increased with adsorption time. The Ni2+ uptake quickly increased for the first 50 minutes and reached the equilibrium after 270 minutes. For MB, the adsorption process also took place rapidly for the first 100 minutes, then increased slowly and reached equilibrium after 270 minutes. The active sites in the FGO2 structure were gradually filled with Ni2+ or MB, which leaded to a decrease in the adsorption capacity when increasing the time. Thus, the effective contact time of FGO2 for Ni2+, MB were both determined to be 270 minutes.

(a) (b) Figure 8. Effect of contact time on the adsorption capacities of FGO2 for (a) Ni2+, (b) MB adsorption Kinetic parameters were obtained from various models for Ni2+, MB adsorption of FGO2 as shown in Table 4. The kinetic data fitted well with the pseudo-second-order model with the correlation coefficient (R2) higher than 0.950 as shown in Figure 9. The Ni2+, MB adsorption was mainly dominated by the chemical adsorption mechanism. So in addition to the diffusion period, the process rate also depends on the interaction between the active sites on adsorbents surface and Ni2+, MB. Table 4. The parameters of kinetic models of FGO2 for Ni2+, MB adsorption Pseudo-first-order Pseudo-second-order -1 2 𝐤𝟏(min ) k2 (g/mg.min) R22 R1 Ni2+ 0.0156 0.982 0.00012 0.9985 MB 0.0136 0.968 0.00990 0.9786

Y = 0.0099x + 0.6066 R2 =0.9786

(a)

(b) Figure 9. The pseudo-second-order linear plot of FGO2 for (a) Ni2+, (b) MB adsorption 3.2.2. pH Figure 10 shows that the Ni2+, MB adsorption capacities of FGO2 enhanced with the increase of pH values, which can be explained as following pHpzc. The pHpzc of FGO2 was determined to be 5.5 as shown in Figure 11. Hence, under the pH of 5.5, the surface of FGO2 is positive while nickel ions is formed as Ni2+,

which means it is difficult for the nanocomposite material to link with these ions. Further, at low pH condition, the number of H+ ions in the solution increases and −OH groups become positively charged to form −OH2+, which decreases the adsorption capacity of Ni2+ ions and MB on the surface of the adsorbent [8]. As the pH increases above the pHpzc, the FGO2 surface was highly negatively charged, which ascribed to the deprotonation of oxygen-containing groups on FGO2 as shown in equation (16)-(18). GO–OH + H2O → GO-O− + H3O+ GO–COOH + H2O → GO-COO− + H3O+ MnFe2O4–OH + H2O → MnFe2O4-O− + H3O+

(16) (17) (18)

The FGO2 surface carries negative charged groups to provide active sites on FGO2 surface. The justification is also in accordance with surface complex and electrostatic interactions formation theory. This theory states that an increase in the pH leads to decrease the competition between metal ions and protons favoring the metal ion adsorption [29]. Besides, more H+ ions were released and neutralized in basic solution after the equilibrium adsorption, improving the removal efficiency of FGO2 [30]. Therefore, the suitable pH for Ni2+, MB removal of FGO2 was 8.

(b) (a) Figure 10. Effect of pH on the adsorption capacities of FGO2 Figure 11. Zeta potential of GO, for (a) Ni2+, (b) MB adsorption MnFe2O4, and FGO2 3.2.3. Initial concentration Figure 12 shows that the uptake correlated with the initial concentration of Ni2+ and MB. The curve went up then leveled off horizontally, indicating that the adsorption capacity of FGO2 increased as the concentration of Ni2+, MB increased and saturated at 350 mg/L. The reason was that the material had a constant amount of active sites while the amount of adsorbed substance increased and then occupied sites.

(b) (a) Figure 12. Effect of initial concentration on the adsorption capacities of FGO2 for (a) Ni2+, (b) MB adsorption The adsorption data of Langmuir, Freundlich, and Temkin isotherm models are summarized in Table 5.

The determination coefficients (R2) values of Langmuir model for Ni2+ and MB were 0.9964 and 0.9976, respectively, which are higher than those of other models. Thus, the adsorption process of FGO2 for Ni2+, MB was a homogeneous monolayer adsorption.

Ni2+ MB

Table 5. The parameters of isotherm models of FGO2 for Ni2+, MB adsorption Langmuir Freundlich Temkin kL qm RL2 n kF RF2 kT B 0.0589 152.67 0.9964 5.24 51.27 0.9356 6.27.10-10 0.0443 0.3578 89.29 0.9976 5.54 40.21 0.8807 1.2446 11.076

RT2 0.9482 0.9536

The maximum adsorption capacities from Langmuir model of FGO2 for Ni2+ and MB were determined to be 152.67 and 89.29 mg/g, respectively. Table 6 shows the adsorption capacities of FGO2 were higher than those of other precursors. This result could be explained as the following reason: the attachment of MnFe2O4 nanoparticles on GO surface increased the number of adsorption sites to link with Ni2+ and MB [17,31]. Table 6. Maximum adsorption capacities of FGO2 and other materials for Ni2+, MB adsorption Materials qm (mg/g) References Ni2+ FGO2 152.67 In this study GO 101.72 In this study MnFe2O4 50.05 In this study Activated carbon 62.50 [32] CoFe2O4/Graphene 105.20 [22] NiFe2O4/Graphene 74.62 [22] MB FGO2 89.29 In this study GO 253.13 In this study MnFe2O4 32.67 In this study Fe3O4/Graphene 95.31 [33] Fe3O4/GO 64.23 [33] Fe3O4/SiO2/GO 111.10 [33] TiO2/GO 83.26 [33] 3.4. Adsorption mechanism The mechanism of Ni, MB adsorption was studied by using SEM-EDX analysis and FTIR spectra of adsorbent after adsorption. The elemental compositions of FGO2 before and after adsorption are shown in Table 7 and Figure 13. The presence of Ni2+ ions on the FGO2 after Ni2+ adsorption was recorded with the Ni content of 13.48 wt.%. For MB adsorption, the increase of C and O contents in nanocomposite was compared than FGO2. These results indicate that the Ni2+, MB are successfully linked to FGO2 surface, and the adsorption is a series of interaction produced by functional groups of FGO2 and Ni2+, MB. Table 7. The elemental compositions in FGO2 before and after Ni2+, MB adsorption FGO2_MB FGO2 FGO2_Ni Element C 30.95 16.04 35.95 O 34.58 32.69 34.58 Mn 12.15 10.10 10.15 Fe 23.31 27.69 19.32 Ni 13.48 The SEM-mapping images indicated that the pollutants changed the morphology of adsorbent, the pores were filled and the edges became fuzzy after adsorption. However, the active sites were not evenly distributed, indicating that the Ni2+ ions are interacted with only selected functional groups on nanocomposite surface. After MB adsorption, the surface of adsorbent has smooth, thus the adsorption is a series of interaction created by FGO2 groups and MB [28,34].

(a) (b) Figure 13. Elemental mapping of FGO2 after (a) Ni2+ and (b) MB adsorption FTIR spectra of FGO2 before and after adsorption Ni2+, MB were used to study adsorption mechanism as shown in Figure 14.

Figure 14. FTIR spectra of FGO2 (a) before and after (b) Ni2+, (c) MB adsorption The results show the significant change of FTIR spectra of FGO2 after Ni2+, MB adsorption. The decrease in the intensity of –OH, –C=O, and –C-O peaks was reported, thus the acidic protons of oxygen-containing groups existed on FGO2 surface were interacted with Ni2+, MB. The electrostatic attraction between the positively charged Ni2+, MB-N+ and negatively charged FGO2 provides a driving force for the adsorption, equation (19) – (20).

FGO2-COO- + M+ → FGO2-COO-M+

(19)

(20) FGO2-O- + M+ → FGO2-O-M+ Besides, the ions chelated with –COOH and –OH groups on FGO2 surface as shown in follow reactions (21) – (24) [26]: (21) FGO2-COOH + Ni2+ → (FGO2-COO)2-Ni2+ + H+ Ni2+

FGO2-COOH + Ni2+ + H2O → FGO2-COO-NiOH + + 2H+

(22)

FGO2-OH + Ni2+ → (FGO2-O)2-Ni2+ + H+

(23)

(24) FGO2-OH + Ni2+ + H2O → FGO2-O-NiOH + + 2H+ Additonal, the intensity of aromatic C=C peak was weakened than that of FGO2 after adsorption, thus the cation- π and π-π interactions between hexagonal lattice of GO and Ni2+, MB were affected to the adsorption capacity of FGO2 [26]. Based the characterization results and adsorption data, the adsorption mechanism of FGO2 for Ni2+, MB could be explained by as follows: (1) the electrostatic interactions between the negative charges of FGO2 surface and positive Ni2+, MB; (2) the interactions between hexagonal lattice of GO and Ni2+, MB [17,31]. 4. Conclusions MnFe2O4/GO nanocomposite was successfully fabricated by in-situ method with the suitable MnFe2O4:GO mass ratio of 1:1 (FGO2). The MnFe2O4 nanoparticles were linked to the surface of GO with the average size of 10-15 nm. The saturation magnetization value (Ms) was found to be 28.74 emu/g, which proves that it can be easily separated by external magnetic field. The adsorption process was well-fitted to pseudo-second-order kinetic and Langmuir isotherm models. The maximum adsorption capacities of FGO2 for Ni2+ and MB calculated from Langmuir model were 152.67 and 89.29 mg/g, respectively. Therefore, FGO2 nanocomposite could be considered as a potential adsorbent for Ni2+, MB from water. Acknowledgments This research is funded by Vietnam National University Ho Chi Minh City (VNU-HCM) under grant number C2019-20-31. References

AUTHOR STATEMENT Manuscript

title:

FABRICATION

OF

MANGANESE

FERRITE/GRAPHENE

OXIDE

NANOCOMPOSITES FOR REMOVAL OF NICKEL IONS, METHYLENE BLUE FROM WATER Author names: Lu Thi Mong Thy, Nguyen Hoan Kiem, Tran Hoang Tu, Lu Minh Phu, Doan Thi Yen Oanh, Hoang Minh Nam, Mai Thanh Phong, Nguyen Huu Hieu Authors’ individual contributions: Conceptualization; Lu Thi Mong Thy, Nguyen Hoan Kiem, and Doan Thi Yen Oanh Data curation; Lu Thi Mong Thy, Tran Hoang Tu, and Nguyen Huu Hieu Formal analysis; Nguyen Hoan Kiem and Lu Thi Mong Thy Funding acquisition; Mai Thanh Phong and Nguyen Huu Hieu Investigation; Nguyen Hoan Kiem and Lu Minh Phu Methodology; Lu Thi Mong Thy and Tran Hoang Tu Project administration; Hoang Minh Nam, and Nguyen Huu Hieu Resources; Hoang Minh Nam , Mai Thanh Phong, and Nguyen Huu Hieu Software; Lu Minh Phu and Doan Thi Yen Oanh Supervision; Nguyen Huu Hieu and Mai Thanh Phong Validation; Lu Minh Phu and Doan Thi Yen Oanh Visualization; Mai Thanh Phong and Hoang Minh Nam Roles/Writing – original draft; Lu Thi Mong Thy, Tran Hoang Tu, Lu Minh Phu, and Doan Thi Yen Oanh Writing – review & editing: Lu Thi Mong Thy, Tran Hoang Tu, and Nguyen Huu Hieu Please address all correspondence concerning this manuscript to corresponding author at [email protected] Thank you for your consideration of this manuscript. Sincerely, Nguyen Huu Hieu

FABRICATION OF MANGANESE FERRITE/GRAPHENE OXIDE NANOCOMPOSITES FOR REMOVAL OF POLLUTANTS FROM WATER