Synthesis of carboxyl-functionalized magnetic nanoparticle for the removal of methylene blue

Colloids and Surfaces A 572 (2019) 58–66

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Synthesis of carboxyl-functionalized magnetic nanoparticle for the removal of methylene blue

T

Zou Jiaqi, Dai Yimin , Liu Danyang, Wang Shengyun, Zhou Liling, Zhou Yi ⁎

School of Chemistry and Food Engineering, Hunan Provincial Key Laboratory of Materials Protection for Electric Power and Transportation, Changsha University of Science and Technology, Changsha, 410114, PR China

GRAPHICAL ABSTRACT

A novel core-shell structure maleic anhydride (MAH) functionalized composite nanoparticle (Fe3O4@SiO2-EDA-COOH) was synthesized for removal of methylene blue (MB) from aqueous solution. The chemical structure, morphology and magnetic characteristics of the magnetic nanoparticle were investigated. The methylene blue adsorption effect factors of initial concentration, contact time and solution pH were investigated in detail. The magnetic composite with dendritic structure can increase the specific surface area of magnetic nanoparticles. The magnetic composite showed higher adsorption capacity for MB than other reported adsorbents, and it can be quickly separated from aqueous solution by external magnet.

ARTICLE INFO

ABSTRACT

Keywords: Fe3O4@SiO2-EDA-COOH Methylene blue Composite magnetic nanoparticle Core-shell structure Adsorption

In this paper, carboxylated ethylenediamine functionalization Fe3O4@SiO2 nanoparticles (Fe3O4@SiO2-EDACOOH NPs) have been developed in order to remove methylene blue (MB) from aqueous solution. The chemical structure, morphology and magnetic characteristics of the magnetic composite nanoparticles were characterized by Fourier transformed infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), High resolution transmission electron microscopy (HR-TEM), Scanning electron microscopy (SEM) and vibrating sample magnetometer (VSM). To explore the truth, some methylene blue adsorption influenced factors of initial concentration, contact time and solution pH were at length investigated. The results showed that the absorption capacity depended strongly on pH, and the maximum adsorption value for MB (43.15 mg/g) was obtained at pH 10. In addition, the equilibrium was attained within 60 min, and the adsorption kinetics and adsorption isotherms data can be well accorded with pseudo-second-order kinetic model and Freundlich isotherm model, respectively. Most importantly, remarkable enhancement in adsorption capacity for MB is achieved than other reported adsorbents. It is worth noting that adsorbents can be quickly recovered from

⁎

Corresponding author. E-mail address: [email protected] (D. Yimin).

https://doi.org/10.1016/j.colsurfa.2019.03.095 Received 22 November 2018; Received in revised form 27 February 2019; Accepted 31 March 2019 Available online 01 April 2019 0927-7757/ © 2019 Published by Elsevier B.V.

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aqueous solution by external magnet. Finally, the plausible adsorption mechanism was briefly discussed. Our results suggest that novel nanoparticle Fe3O4@SiO2-EDA-COOH is a promising adsorbent in improving adsorption performance of MB from aqueous solution.

1. Introduction

SiO2 core-shell structure is introduced, because it not only prevents oxidation and aggregation of Fe3O4 in a harsh environment but also improves chemical stabilities of Fe3O4 [23]. Besides, the Fe3O4@SiO2 can be easily modified with silane coupling agents or organic compounds [24–26]. Huang et al. reported that Fe3O4@SiO2@HKUST-1/BiI can selectively remove Hg2+ (Qmax = 264.0 mg/g) from water [27]. Wang et al. reported high adsorption performance of Fe3O4@SiO2 for congo red (Qmax = 96.4 mg/g) from contaminated water [28]. Song et al. reported excellent performance of amine/ Fe3O4-resin for Methyl Orange (Qmax = 222.2 mg/g) removal from wastewater [29]. In this study, we explored a new core-shell structure carboxyl-functionalized Fe3O4@SiO2 nanoparticle. The main advantages of Fe3O4@ SiO2-EDA-COOH are as follows : (1) Fe3O4@SiO2 core-sell can facile separation and prevent aggregation and chemical decomposition in solution [30]; (2) Carboxylic-functionalized nanomaterials are acidic and can effectively adsorb basic dye MB; (3) Fe3O4@SiO2 nanomaterials with dendritic structure can enlarge the specific area of magnetic nanoparticles and improve their adsorption performance, and the adsorbents can be scalable and cost-effectively in industrial production [31,32]. The research shows that the Fe3O4@SiO2-EDA-COOH adsorbents have high adsorption capacity and fast magnetic separation. We not only investigated the effects of solution pH, MB concentration, and equilibration time on sorption performance, but also investigated the kinetics and isotherms of MB adsorption. Besides, the regeneration capacity and adsorption mechanism of the adsorbent were also investigated in detail.

With the acceleration of industrialization, water pollution has become an important global issue facing human beings in this century [1,2]. Specially, the pollution of organic dyes is one of the most serious water pollution problems nowadays [3,4]. As a synthetic dye, methylene blue (MB) has been widely used in cotton dyeing, pesticides, rubbers, cosmetics, pharmaceuticals as well as leather tanning industries [5–7]. In addition, MB has high solubility, stability and biotoxicity in water, so it has aroused much attentions of scientific community and the public because it is difficult to be removed from water and can threaten the ecological balance even the survival of human owing to their toxicity, carcinogenicity and bioaccumulation [8]. In addition, MB can cause eye irritation in human as well as animals, allergy and cancer in mankind due to its toxicity [9,10]. Thus, it is necessary to develop a new adsorbent to remove MB from the waste water efficiently with highly selectivity. In order to remove the pollution of MB, various technologies are developed and applied, such as chemical coagulation, electrolysis, ion exchange, photo-degradation and membrane separation [11–14]. Among these widely adopted approaches, adsorption is considered to be one of the most feasible methods owing to simple operation, high efficiency and low cost [15]. Recently, some sorbents such as clay minerals, metal oxides, polymer and activated carbon have been used to remove dyestuffs from waste water [16,17]. However, the reported adsorbents have a number of disadvantages such as high investment, operating costs, regeneration difficulty, and long time to reach equilibrium of adsorption, thus limiting its application in real wastewater treatment [18,19]. Therefore, it is urgent to study some new recoverable adsorbents. Recently, magnetic nanoparticles (MNPs), especially Fe3O4, as a new type of adsorbent, has attracted attention of many scientists in removal of dyes from waste water solution, because of the excellent performance like easy to separation, ease of surface modification, non-toxicity and easy to regeneration [20]. However, raw Fe3O4 particles are easily oxidized by air especially in acidic solutions and easily aggregated in aqueous solution, thus the adsorption capacity was reduced [21,22]. In order to eliminate these drawbacks, a Fe3O4@

2. Materials and methods 2.1. Materials Iron chloride hexahydrate (FeCl3·6H2O), iron chloride tetrahydrate (FeCl2·4H2O), sodium hydroxide (NaOH), ammonia (NH3·H2O) and tetraethyl orthosilicate (TEOS) were purchased from Xiyu Chemical Reagent Co., Ltd. Anhydrous ethanol, 3-chloropropyltriethoxysilane (CPTES), polyvinyl pyrrolidone (PVP), ethylenediamine (EDA), triethylamine (Et3N), N,N-dimethylformamide (DMF), potassium

Fig. 1. The preparation process of Fe3O4@SiO2-EDA-COOH. 59

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2.4. Batch sorption experiments

a

b T%

Batch sorption experiments were employed in 250 mL polypropylene bottles. The amount of MB solution and adsorbent were kept at 50 mL and 20 mg, respectively. The pH value of MB solution was adjusted by adding negligible volumes of sodium hydroxide and hydrochloric acid solution. In order to achieve sorption equilibrium, the suspensions were shaken at room temperature for 4 h. After adsorption, the sorbents were separated from the mixture with magnet. The influence of pH value on MB adsorption was investigated in the range of 3–10. The MB percent removal (η%) and MB adsorbed on the adsorbent (qe (mg/g)) were calculated as follows:

1089

c

1564 1454

d

2921

2856

1641

2971 3427

1164

1560 1640 1093

886 520

(%) =

1049

qe = (C0

(c 0

ce ) c0 Ce ) ×

× 100% V M

(1) (2)

Where C0 (mg/L) and Ce (mg/L) are the initial and the equilibrium concentrations, respectively. V (mL) is the amount (mL) of the mixture, M (g) is the amount of adsorbent.

Fig. 2. The FT-IR spectrums of (a) Fe3O4@SiO2, (b) Fe3O4@SiO2-Cl, (c) Fe3O4@ SiO2-EDA and (d) Fe3O4@SiO2-EDA-COOH.

carbonate (K2CO3), and maleic anhydride (MAH) were purchased from Sinopharm Chemical Reagents Corporation. All chemicals were analytical grade and not further purified.

3. Results and discussion 3.1. Characterization

2.2. Synthesis of Fe3O4@SiO2-EDA-COOH

The FT-IR spectrums of synthesized (a) Fe3O4@SiO2, (b) Fe3O4@ SiO2-Cl, (c) Fe3O4@SiO2-EDA and (d) Fe3O4@SiO2-EDA-COOH nanoparticles are shown in Fig. 2, respectively. The band at ˜520 cm−1 is attributed to the stretching vibration of Fe-O bond. As is shown in Fig. 2a, the obvious band at˜1089 cm−1 corresponds to SieOeSi stretching vibrations [35]. Fig. 2b showed several signals in the area of 2920-2850 cm-1 originate from chloropropyl groups [36]. Fig. 2c provide more intuitive FT-IR spectra of Fe3O4@SiO2-EDA, the obvious band at ˜2921 cm−1 and ˜2856 cm-1 can be speculated to the C–H stretching vibrations of methyl [37]. The peak at 1635 cm−1 and 3430 due to -NH2 bending vibration and stretching vibration, respectively [38,39]. In Fig. 2d, the peaks at 1093 cm−1 and 1560 cm−1 are associated with C–O and C]C stretching vibration, respectively [40,41]. In addition, the appearance of new band at ˜886 cm−1 is attributed to = C–H bending vibration. The band at ˜1640 cm−1 is attributed to C]O stretching of the secondary amide, indicating the presence of the acylamino (NHeCO) derived from the COOH [42]. Above all, all of these evidences verified the assumption that the composite nanoparticle Fe3O4@SiO2-EDA−COOH was successful synthesized. In order to acquire more accurate results, the crystalline structures of all samples were characterized by XRD in Fig. 3, respectively. The characteristic peaks at 2θ = 62.8°, 57.1°, 53.5°, 43.2°, 35.5° and 30.2° are ascribed to the diffraction of Fe3O4 crystal with (440), (511), (422), (400), (311) and (220) planes, respectively [43]. These characteristic crystallographic planes confirm that Fe3O4 crystal is face-centered cubic (fcc) structure. Besides, a wide band at 2θ values from 20° to 26° can be attributed to amorphous silica [44]. These results clearly indicated that the Fe3O4@SiO2 composite nanoparticles are successfully synthesized and well retained the Fe3O4 crystal structure in the process of coating silicon and surface modification. Simultaneously, the same absorption peaks could be also observed for other composite nanoparticles, indicating the high crystalline phase stability of Fe3O4@SiO2 nanoparticles in the surface modification. For the purpose of examining the size distribution and microstructure of the synthesized core-shell composite nanospheres particles, the SEM and HR-TEM images of Fe3O4@SiO2-EDA-COOH were compared and illustrated in Fig. 4a and b, respectively. Form the SEM image in Fig. 4a, we can clearly see that the structure of Fe3O4@SiO2-EDACOOH is spherical microparticle and the particle has an average diameter of about 50 nm. Most nanoparticles have a smooth and

In this study, the Fe3O4@SiO2-Cl microspheres were prepared by ours previous work [33]. Firstly, the magnetic Fe3O4 nanoparticle was prepared by coprecipitation method. Secondly, magnetic Fe3O4 nanoparticle was coated with TEOS via a modified Stöber method. Finally, the Fe3O4@SiO2 nanoparticle was further modified with CPTES to obtain Fe3O4@SiO2-Cl. The Fe3O4@SiO2-EDA microspheres were prepared via a substitution reaction [34]. Briefly, 0.5 g Fe3O4@SiO2-Cl microspheres were dispersed in 200 mL toluene. Then 0.8 g Potassium carbonate and 1.0 ml EDA were added into the mixture under continuous stirring, and the mixture was refluxed for 6 h at 60℃. Finally, the Fe3O4@SiO2-EDA microspheres were washed three times with deionized water and anhydrous ethanol, and separated under a permanent magnet, dried in vacuum at 50℃ for 12 h prior to further use. The Fe3O4@SiO2-EDA-COOH particles were prepared as follows: 0.5 g Fe3O4@SiO2-EDA microspheres were dispersed in 200 mL DMF. Then 1.0 g maleic anhydride and 1.0 ml triethylamine were added slowly into the mixture under vigorous stirring. After reflux for 8 h at 303 K, the product was separated by an external magnet. Finally, the Fe3O4@SiO2-EDA-COOH microspheres were washed three times with deionized water and anhydrous ethanol, respectively, and dried under vacuum at 50℃ for 12 h. The preparation process of Fe3O4@SiO2-EDACOOH was illustrated in Fig. 1. 2.3. Characterization The size distribution and microstructure of Fe3O4@SiO2-EDA-COOH were observed by HELIOS NanoLab 600i Scanning electron microscopy (SEM) and TecnaiG2 F20 High resolution transmission electron microscopy (HR-TEM). Fourier transformed infrared (FT-IR) spectrums of nanoparticles were studied from KBr pellets by using an AVATAR 360 spectrometer. X-ray diffraction (XRD) patterns of nanoparticles were carried out by SEMENS D5000 X-ray diffractometer at 298 K. Energy dispersive spectrometers were recorded by HELIOS NanoLab 600i. Magnetic property measurements were investigated by using Lakeshore 7307 vibrating sample magnetometer (VSM) at 298 K. Persee TU-1810 ultraviolet-visible spectrophotometer (664 nm) was used to analyze the concentration of methylene blue after adsorption. 60

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Fe3O4@SiO2-EDA-COOH nanoparticles at room temperature. As shown in Fig. 5, Fe3O4@SiO2-EDA-COOH nanoparticles have not hysteresis in the magnetization. Neither remanence nor coercivity was observed, indicating that magnetic nanoparticles exhibit typical superparamagnetic. The saturation magnetization of Fe3O4@SiO2-EDACOOH was 58.7 emu/g. In addition, the illustration in Fig. 5 shows that the Fe3O4@SiO2-EDA-COOH nanoparticles can be expertly separated from the mixture by using an external magnet within 3 min. These imply that the Fe3O4@SiO2-EDA-COOH magnetic nanoparticles can be dispersed into aqueous solutions and separated from the mixture conveniently. The thermal degradation behavior of the magnetic nanoparticles is illustrated through the TGA and DTG curves (Fig. S2 in Supporting information). The weight losses on the TG curve of Fe3O4@SiO2-EDACOOH under a nitrogen atmosphere is shown in Fig. S2 blue line. The microspheres exhibited a two stage of weight loss process. The first stage of weight loss at a temperature less than 150℃ can be attributed to the evaporation of the solvent molecules or physically adsorbed water, and the second stage of weight loss from 150 to 600 °C was due to the degradation of organic components grafted on the surface of the silicon shell. The differential scanning calorimeter (DSC) curve recorded during the endothermic and exothermic process of Fe3O4@SiO2EDA-COOH is shown in Fig. S2 red line. The first exothermic peak between 250 and 450℃, accompanied by a significant mass loss on the TG curve, these can be ascribed to the oxidative decomposition of carbon on the silicon surface. The second endothermic peak between 450 and 700℃ was greatly associated with the pyrolysis of organic molecules on the silicon shell surface.

(311)

e

(220)

(400)

(511) (422)

(440)

d c b a

Fig. 3. The XRD patterns of (a) Fe3O4, (b) Fe3O4@SiO2, (c) Fe3O4@SiO2-Cl, (d) Fe3O4@SiO2-EDA and (e) Fe3O4@SiO2-EDA-COOH.

homogeneous surface morphology. Besides, very few the magnetite particles tend to aggregate owing to the ultrasmall size and magnetic dipole. According to the HR-TEM image of Fe3O4@SiO2-EDA-COOH (Fig. 4b), it can be clearly observed that a thin uniform ash color silica shell is coated on Fe3O4 nanoparticle. It is worth noting that the size and morphology of nanoparticles did not change significantly during modification with MAH. This fact can be attributed to two main factors. One is the modified reaction occurrence only on the surface of particles and the other is the grafting molecules are too thin to be observed by HR-TEM. As shown in Fig. 4c, the Fe3O4@SiO2-EDA-COOH nanoparticles displayed typical single crystalline according to the lattice fringes continue with homogenous array. Furthermore, the selected area electron diffraction (SAED) patterns (Fig. 4d) of Fe3O4@SiO2-EDACOOH can confirm their crystal structure. The results are good agreement with XRD. The SAED pattern shows bright discrete diffraction spots and concentric circles, indicating high crystallinity. In Fig. 4e, the EDS spectrum of Fe3O4@SiO2-EDA-COOH appears the obvious peaks of Fe, O and Si, indicating that these elements are the main components in Fe3O4@SiO2-EDA-COOH. In addition, the week peak of carbon and potassium were observed in the EDS spectra, indicating that low-level C and K are constituent in Fe3O4@SiO2-EDA-COOK. These results further indicated that maleic acid has been grafted onto the surface of nanoparticles. The above results are agreement with Fourier infrared spectroscopy. In order to obtain an insight into chemical composition and electronic structure of Fe3O4@SiO2-EDA-COOH, X-ray photoelectron spectroscopy (XPS) experiments were performed. The wide-scan XPS spectrum (Fig. S1a in Supporting information) of Fe3O4@SiO2-EDA-COOH exhibits five peaks at 710.8 eV, 532.1 eV, 399.8 eV, 284.8 eV, and 103.1 eV, which were assigned to Fe 2p, O 1 s, N 1 s, C 1 s and Si 2p, respectively, indicate the existence of Fe, O, N, C and Si elements. As shown in Fig. S1b (Supporting information), three different visible peaks centered at 288.4 eV, 286.2 eV and 284.8 eV were observed in the C1 s deconvolution spectrum of Fe3O4@SiO2-EDA-COOH which attributed to their corresponding C]O, C–O, and CeC groups, respectively [45]. Similar visible peaks were detected in Fig. S1c (Supporting information), the peaks centered at 401.2 eV and 399.8 eV are related to C–N and NeH groups, respectively [46,47]. In the Fe 2p spectrum (Fig. S1d in Supporting information), two clearly peaks at 724.6 eV and 710.8 eV were observed which were attributed to Fe 2p1/2 and Fe 2p3/2, respectively. The results indicated the successful coupling between Fe3O4@SiO2-EDA nanoparticles and COOH via an amide bond. High saturation magnetization and superparamagnetism of the magnetic adsorbent play a decisive role in evaluating their regeneration and recyclability. Fig. 5 depicts the magnetic hysteresis curve of the

3.2. Effect of pH for MB adsorption The initial solution pH is an important factor on affecting MB adsorption. We investigated the influence of pH on MB adsorption by adsorbents in the pH range of 3-10. As is show in Fig. 6, the sorption capacity of MB abruptly increased with the increase of pH from 3 to 5, while slightly increase from 21.4 to 28.3 mg/g with pH increasing from 5 to 10. This tendency is mainly controlled by electrostatic interactions between dyes and adsorbent surface charges. MB is a kind of cationic dye (pKa = 5.85), when the pH of methylene blue solution is less than pKa, MB is protonated in the solution. The protonated MB has electrostatic repulsion on the carboxyl group of the adsorbent, which results in the decrease of the adsorptive capacity of the adsorbent. When the pH of MB solution is higher than pKa, MB is deprotected and existed in the form of positive charge, so the electrostatic attraction between carboxylate anion and MB cation on the adsorbent increases. In addition, the abrupt decrease in the adsorption capacity of the adsorbent in Fig. 6 is due to the pH of the solution being lower than the pKa. Therefore, the adsorption capacity of the adsorbent can be increased at a high pH value, and desorption and regeneration of the adsorbent are facilitated at a low pH. 3.3. Sorption isotherm The sorption isotherm of MB on sorbent was carried out at 298 K and pH 10. As shown in Fig. S3 (Supporting information), the adsorption capacity of Fe3O4@SiO2-EDA-COOH increases gradually with the initial concentration of MB increases, the sorption capacity decreases slightly after reaching a maximum value (43.15 mg/g). The high adsorption capacity is mainly due to electrostatic attraction between the carboxyl anion on the surface of the adsorbent and the quaternaries cation on the MB. Sorption isotherm can be used to evaluate the sorption capacity of the adsorbent, and can also be used to describe the interaction between the adsorbents, it is essential to optimize the use of the adsorbent. In order to study the sorption mechanism better, the sorption isotherms data was fitted by Langmuir isotherm model and Freundlich isotherm 61

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Fig. 4. (a) SEM images, (b) HR-TEM images, (c) single crystalline images, (d) SAED patterns, and (e) EDS spectra of Fe3O4@SiO2-EDA-COOH.

model, respectively. The Langmuir isotherm model assumes that sorption is monolayer and each molecule occupies only one adsorption site, and there is no interaction between the adsorbed molecules. The Freundlich isotherm model mainly used for multilayer sorption, and can also be used for physical adsorption and chemical sorption. The Langmuir isotherm equation can be described as [48]:

Ce C 1 = e + qe qmax KL qmax

(3)

The Freundlich isotherm equation can be described as [49]:

lgqe = lgKF +

1 lgCe n

(4)

Where qmax (mg/g) is the maximum sorption capacity of the adsorbent, qe (mg/g) is the equilibrium sorption capacity of the adsorbent, Ce (mg/ 62

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Table 1 Sorption isotherm parameters for MB adsorption onto adsorbent (pH = 10, 298 K). Langmuir

Freundlich

qmax (mg/g)

KL (L/mg)

R2

KF (mg1−1/n∙L1/n/g)

n

R2

12.05

0.37

0.604

4.78

1.21

0.967

Table 2 Comparison of MB maximum sorption capacity of Fe3O4@SiO2-EDA-COOH with other adsorbents.

Fig. 5. Magnetic hysteresis curves of Fe3O4@SiO2-EDA-COOH. (The illustration showing magnetic separation photos using permanent magnets.).

Adsorbents

qmax (mg/g)

Refs.

Fe3O4@SiO2-CR Graphene Oxide Ruthenium nanoparticle loaded activated carbon Magnetic γ-Fe2O3/SiO2 (M- γFS) nanocomposite chitosan/ Fe3O4/graphene oxide nanocomposite Yellow passion fruit peel Mesoporous Fe3O4@SiO2 Fe3O4@SiO2-EDA-COOH

31.44 19.39 41.60 26.62 30.10 6.8 33.12 43.15

[33] [51] [52] [53] [54] [55] [56] This work

data, indicating that the sorption process is a multilayer sorption, and the sorption capacity of adsorbent increased with the increase of MB concentration. The sorption capacity of the prepared Fe3O4@SiO2-EDA-COOH and various adsorbents reported in literatures are shown in Table 2 for comparison. Obviously, our Fe3O4@SiO2-EDA-COOH shows higher ability of MB sorption than other reported adsorbents, indicating that the as-prepared Fe3O4@SiO2-EDA-COOH nanoparticles are more attractive than other adsorbents for MB removal. 3.4. Sorption kinetics The sorption kinetics of MB on Fe3O4@SiO2-EDA-COOH were studied in 20 mg/L MB solution at room temperature and pH = 10. The effect of contact time on Fe3O4@SiO2-EDA-COOH adsorption of MB was shown in Fig. 8. As can be seen that MB adsorption increased rapidly within the first hour of contact time then proceeded at a slow speed and finally kept the level. Rapid adsorption kinetics showed that the adsorbent have good practical application potential and can adsorb MB from a large amount of waste water. The short equilibrium time was mainly due to electrostatic attraction between anions and cations. After a period of time, due to the electrostatic repulsion between the adsorbed molecules, it is difficult for the adsorbent to continue to adsorb, and then the adsorption equilibrium is reached [57]. According to the kinetic data, 2 h was selected to ensure enough time to reach equilibrium adsorption. The adsorption process of MB on Fe3O4@SiO2-EDA-

Fig. 6. Effect of initial solution pH for MB adsorption.

L) is the equilibrium concentration, KL (L/mg) is the Langmuir adsorption equilibrium constant, KF (mg1−1/n⚫L1/n/g) is the Freundlich adsorption constant, and n is a constant indicating the dependence of the adsorption concentration on the equilibrium concentration. Fig. 7 shows the sorption isotherms of MB on adsorbent. The correlation coefficient (R2) and related parameters calculated by Eqs. (3) and (4) are shown in Table 1. The n value between 1˜10 indicates that MB adsorbed onto adsorbents is advantageous [50]. The Freundlich model is better than the Langmuir model for fitting adsorption isotherm (a)

(b)

lgqe

1.5

1.0

0.5

Fig. 7. (a) Langmuir isotherm and (b) Freundlich isotherm for MB adsorption (T = 298 K, pH = 10). 63

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Table 3 Kinetic parameters of MB sorption on Fe3O4@SiO2-EDA-COOH composites at 298 K, and pH = 11. pseudo-first-order model K1 (1/min) qe (mg/g) R2

0.040 4.907 0.664

pseudo-second-order model K2 (g/(mg∙min)) qe (mg/g) R2

0.011 28.57 0.999

Fig. 8. Effect of contact time on Fe3O4@SiO2-EDA-COOH adsorption of MB (T = 298 K, pH = 10, MB initial = 20 mg/L).

COOH was described by fitting the experimental data with pseudo-firstorder and pseudo-second-order kinetics models. The pseudo-first-order kinetics equation can be written as [58]:

ln(qe

qt ) = ln qe

(5)

k1 t

The pseudo-second-order kinetics equation is expressed as [59]:

t 1 t = + qt qe k2 qe2

(6)

Fig. 10. Adsorption cycles performance of Fe3O4@SiO2-EDA-COOH for removal of MB.

Where qt is the sorption amounts of MB (mg/g) at any time t (min), qe is the sorption amounts of MB (mg/g) at equilibrium time (min), k1 (1/ min) represents the pseudo-first-order kinetic rate constants, k2 (g/(mg/ min)) represents the pseudo-second-order kinetic rate constants. Fig. 9(a, b) shows pseudo-first-order kinetics and pseudo-secondorder kinetics for MB adsorption, respectively, and Table 3 shows the kinetic parameters calculated from Eqs. (5) and (6). Obviously, the pseudo-first-order poorly fitted adsorption data with a low correlation coefficient (R2 = 0.664) and the calculated equilibrium sorption capacities (4.907 mg/g) lower than the experiment adsorption capacity (29.02 mg/g). Whereas, the pseudo-second-order kinetic model fits adsorption data well with a high correlation coefficient (R2 = 0.999) and the calculated equilibrium adsorption capacities (28.57 mg/g) significantly approach to the experiment adsorption capacity (29.02 mg/g). All experimental results show that the pseudo-secondorder model has advantages in simulating the adsorption process of MB on Fe3O4@SiO2-EDA-COOH.

3.5. Regeneration study From an economics point of view, the regeneration and recyclability of the adsorbents is a key factor to evaluate the performance of adsorption materials. In order to study the regeneration and recyclability of Fe3O4@SiO2-EDA-COOH, five adsorption-desorption cycles were performed, and the results are shown in Fig. 10. The used Fe3O4@SiO2EDA-COOH nanoparticles were placed in 1 mol/L HCl solution for 30 min under ultrasonication, and then washed several times with deionized water to be reused in the next cycle. As is shown in Fig. 10, although the MB removal rate may decrease slightly during each cycle, it still possesses high adsorption capacity, indicating that the Fe3O4@ SiO2-EDA-COOH nanoparticles have excellent regeneration and reusability. In addition, it has the function of magnetic recovery. These results indicate that Fe3O4@SiO2-EDA-COOH can be effectively regenerated by 1 mol/L HCl, and MB adsorption capacity is slightly

(b)

(a)

t/qt(min g mg-1)

8 6 4 2 0

Fig. 9. (a) pseudo-first-order kinetics and (b) Pseudo-second-order kinetics for MB adsorption (T = 298 K, pH = 10, MB initial = 20 mg/L). 64

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Fig. 11. The plausible mechanism for the adsorption of MB from aqueous solution.

decreased. Therefore, Fe3O4@SiO2-EDA-COOH can be used for efficient and stable MB adsorbent with low replacement costs.

for MB is achieved than other reported adsorbents. The higher adsorption capacity can be attributed to the electrostatic interactions, hydrogen bonding and chemical sorption between MB and Fe3O4@ SiO2-EDA-COOH. The adsorption kinetic data and the absorption isotherm data were accorded well with pseudo-second-order kinetic model and Freundlich model, respectively. The adsorbed dyes could not only be desorbed with 1 mol/L hydrochloric acid solution but also the adsorbent can be quickly and easily separated from the MB solution by using an external magnetic field. Thus, the prepared Fe3O4@SiO2-EDACOOH adsorbent can be quickly and effectively applied to the removal of MB from wastewater.

3.6. Adsorption mechanism Three main mechanisms of electrostatic attraction, hydrogen bonding interaction and chemical sorption were proposed to explain the adsorption behaviors of MB on Fe3O4@SiO2-EDA-COOH. Electrostatic attraction is considered to be the main way of adsorption process, which depends primarily on the pH of zero point charge (pHzpc) and solution pH of the absorbent. When the pH value of pretreated solution is higher than that of pHzpc, the adsorption capacity of the adsorbent will increase. Whereas the pH value of pretreated solution is lower than that of pHzpc, the adsorption capacity of the adsorbent will decrease. Moreover, MB is a cationic dye with a positive charge, and the surface of the adsorbent has a large amount of negative charged carboxylate anion in the solution. The fast adsorption is mainly caused by strong electrostatic attraction between MB surface cations and adsorbent surface anions [60]. Therefore, the adsorption capacity is greatly affected by the pH of the solution and will be enhanced under alkaline conditions. The hydrogen bonding is another important factor affecting adsorption. The formation of hydrogen bonds between the adsorbent and the adsorbed molecules can enhance the sorption capacity of the adsorbent. And the hydrogen bond only occurs between two atoms with relatively large electronegativity. Nitrogen atoms in MB have large electronegativity and can form hydrogen bonds with hydrogen atoms on hydroxyl groups in the adsorbent, the capacity of absorption of MB is enhanced. Besides, chemical sorption plays an important role in the adsorption process. Chemical sorption mainly involves the electron exchange, sharing, transfer and form chemical bond in the adsorption process. In the process of adsorption, the carboxyl anion in the adsorbent and the nitrogen atom in MB undergo electron transfer, forming an adsorption chemical bond. Therefore, the adsorption capacity of MB has been enhanced. Therefore, the capacity of absorption of MB is further enhanced. The possible adsorption mechanism of MB on the Fe3O4@SiO2EDA-COOH was schematically illustrated in Fig. 11.

Acknowledgments The authors greatly acknowledge the financial support provided by the National Natural Science Foundation of China (21671026), the Natural Science Foundation of Hunan Province of China (2019JJ40310), Scientific Research Key Fund of Hunan Provincial Education Department (15A001). Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.colsurfa.2019.03.095. References [1] M. Hua, S. Zhang, B. Pan, W. Zhang, L. Lv, Q. Zhang, Heavy metal removal from water/wastewater by nanosized metal oxides: a review, J. Hazard. Mater. 211-212 (2012) 317–331. [2] P. Xu, G. Zeng, D. Huang, C. Feng, S. Hu, M. Zhao, C. Lai, Z. Wei, C. Huang, G. Xie, Z. Liu, Use of iron oxide nanomaterials in wastewater treatment: a review, Sci. Total Environ. 424 (2012) 1–10. [3] J. Li, Z. Shao, C. Chen, X. Wang, Hierarchical GOs/Fe3O4/PANI magnetic composites as adsorbent for ionic dye pollution treatment, RSC Adv. 4 (2014) 38192–38198. [4] B. Royer, N.F. Cardoso, E.C. Lima, V.S. Ruiz, T.R. Macedo, C. Airoldi, Organofunctionalized kenyaite for dye removal from aqueous solution, J. Colloid Interf. Sci. 336 (2009) 398–405. [5] J. Liu, G. Liu, W. Liu, Preparation of water-soluble β-cyclodextrin/poly(acrylic acid)/graphene oxide nanocomposites as new adsorbents to remove cationic dyes from aqueous solutions, Chem. Eng. J. 257 (2014) 299–308. [6] X. Wu, Y. Shi, S. Zhong, H. Lin, J. Chen, Facile synthesis of Fe3O4-graphene@mesoporous SiO2 nanocomposites for efficient removal of methylene blue, Appl. Surf. Sci. 378 (2016) 80–86. [7] L. Hu, Z. Yang, L. Cui, Y. Li, H.H. Ngo, Y. Wang, Q. Wei, H. Ma, L. Yan, B. Du, Fabrication of hyperbranched polyamine functionalized graphene for high-efficiency removal of Pb(II) and methylene blue, Chem. Eng. J. 287 (2016) 545–556. [8] S. Xu, J. Wang, R. Wu, J. Wang, H. Li, Adsorption behaviors of acid and basic dyes

4. Conclusions In this study, a new magnetic adsorbent Fe3O4@SiO2-EDA-COOH was successfully synthesized and its possibility to remove MB from aqueous solution. The remarkable enhancement in adsorption capacity 65

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