A novel-green adsorbent based on betaine-modified magnetic nanoparticles for removal of methyl blue

Accepted Manuscript Article A novel-green adsorbent based on betaine-modified magnetic nanoparticles for removal of methyl blue Haowei Wang, Shangning Jia, Haojiang Wang, Bo Li, Wen Liu, Ningbo Li, Jie Qiao, Chen-Zhong Li PII: DOI: Reference:

S2095-9273(17)30056-7 http://dx.doi.org/10.1016/j.scib.2017.01.038 SCIB 63

To appear in:

Science Bulletin

Received Date: Revised Date: Accepted Date:

21 December 2016 21 January 2017 23 January 2017

Please cite this article as: H. Wang, S. Jia, H. Wang, B. Li, W. Liu, N. Li, J. Qiao, C-Z. Li, A novel-green adsorbent based on betaine-modified magnetic nanoparticles for removal of methyl blue, Science Bulletin (2017), doi: http:// dx.doi.org/10.1016/j.scib.2017.01.038

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Article Received 21 December, 2016; Revised 21 January; 2017Accepted 23 January, 2017

A novel-green adsorbent based on betaine-modified magnetic nanoparticles for removal of methyl blue Haowei Wanga, Shangning Jiaa, Haojiang Wanga, Bo Lia, Wen Liua, Ningbo Lia,*, Jie Qiaoa,*, Chen-Zhong Lib,* a

College of Basic Medicine, Shanxi Medical University, Taiyuan 030001, China, E-mail:

[email protected]; [email protected]; b

Department of Biomedical Engineering, Florida International University, Miami, FL 33174,

USA. E-mail: [email protected];

Abstract: A potential adsorbent based on betaine-modified magnetic iron oxide nanoparticles (BMNPs) was successfully synthesized by facile method, characterized and applied for methyl blue (MB) removal from aqueous solution. The characterization results of FTIR, transmission electron microscopy (TEM), X-ray diffraction (XRD) and vibrating sample magnetometer (VSM) showed that the prepared nanoparticles could be well dispersed in water and exhibited excellent superparamagnetism. These properties imply the potential to recycle BMNPs from wastewater through magnetic field. In the adsorption process, the effects of main experimental parameters such as pH of MB solution, initial concentration of MB, contact time, and adsorption capacity for MB were studied and optimized. These results demonstrated that large amounts of quaternary ammonium groups existing on the surface of BMNPs could promote absorption of MB via electrostatic forces. Additionally, the adsorption kinetics of MB was found to follow a pseudo-second-order kinetic model and the adsorption equilibrium data fitted very closely to the Langmuir adsorption isotherm model. The maximum adsorption capacity for MB was calculated to be 136 mg g−1 at room temperature. Moreover, the BMNPs showed good reusability with 73.33% MB adsorption in the 5th cycle.

Keyword: Iron oxide nanoparticles; Betaine; Adsorption; Methyl blue

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1. Introduction Synthetic dyes are extensively used in many industrial fields, such as textile, pharmaceuticals, leather, paper, printing, food, and cosmetics [1]. At the same time, dyestuff is one of the most troublesome contaminants among the various pollutants of effluents from industries because of their toxic and non-biodegradable chemical structure, including aromatic amine (Ar-NH2), naphthyl (C10H7CH2), phenyl (C6H5CH2), and azo (-N=N-) groups, which are all carcinogenic and mutagenic for the aquatic flora and fauna [2-7]. Therefore, treatment of the dyes containing wastewater before surface discharge is highly desirable from the perspective of environmental protection. In recent years, a number of methods such as photocatalytic degradation, advanced oxidation processes, adsorption, reactive extraction technique, catalytic ozonation, membrane filtration, ion exchange, etc., have been reported for removal of dyes from wastewater system [8-14]. Among these, adsorption is considered to be the most suitable and favorable method due to its low cost, simple operation, and good reusability [15]. A wide range of materials, such as tungsten oxide nanosheet [16], nano-porous Bi2WO6 hierarchical microcrystal [17], cellulose nanofibers reinforced sodium alginate-polyvinyl alcohol hydrogels [18], Ni functional MCM-41 [19], graphene [20], magnetic iron oxide nanospheres [21], etc., have been widely used as adsorbents for dyes removal from aqueous solution. In this regard, much attention has recently been focused on the use of nanosized materials due to their high surface area. Particularly, magnetic Fe3O4 nanoparticles as adsorbents have gained increasing attention during the past decade. The surface of iron oxide material is easy to be modified and exhibits super paramagnetic behavior, which makes them very versatile in the fields of MRI contrast agents [22], drug delivery [23], magnetic hyperthermia therapy [24], cell sorting [25], gene delivery [26], removal of hazardous substances [27], etc. Moreover, iron oxides possess good adsorption capacity and can be rapidly recovered from the medium in external magnetic field. Such excellent features enable the material to adsorb dyes from aqueous solution. For example, Rajabi et al. prepared Fe3O4 functionalized by aminopropyltriethoxysilane for dealing with sunset yellow [28]. Zargar et al. [29] have reported that the iron oxide nanoparticles modified by cetyltrimethylammonium bromide could quickly remove and

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recover amaranth. Parham et al. [30] used iron oxide nanoparticles modified by cetyltrimethylammonium bromide as an efficient adsorbent for removal of picric acid in water samples. Compared with the cetyltrimethylammonium bromide, betaine (trimethylglycine) has analogous functional groups (quaternary ammonium) and additional active carboxyl, which is prone to interact with hydroxyl group from pure iron oxide under certain conditions [31]. On the other hand, betaine is an important nutrient to humans widely distributed in nearly all living organisms, antibacterial, regulated without influencing on cellular functions, highly soluble and easily obtained [32-34]. Due to its cationic quaternary ammonium salt and carboxyl, it is capable of treating wastewater via adsorption technology. Very recently, betaine-modified cationic cellulose was reported for treatment of reactive dye wastewater by Ma et al. [35]. However, the use of betaine-modified iron oxide as an adsorbent for dyes removal has not been reported in the area. In this work, betaine-modified magnetic iron oxide nanoparticles (BMNPs) were successfully synthesized by sating a little too much coprecipitation method under aerobic conditions. The nanoparticles were characterized by Fourier transform nfrared (FT-IR), X-ray diffraction (XRD), vibrating sample magnetometer (VSM), and transmission electron microscopy (TEM). In the process, the effects of main experimental parameters such as pH of dye solution, initial concentration of dye, contact time, and amount of BMNPs on the removal of MB were studied and optimized. Moreover, the kinetics and isotherms of MB adsorption into iron oxide and reusability were as well as studied to demonstrate its adsorption ability and recycling potential. 2. Materials & methods 2.1. Material FeCl3·6H2O, FeCl2 ·4H2O, anhydrous trimethylglycine and NH3 ·H2O (25%; v/v) were used for the preparation and surface modification of iron oxide. For the adsorption experiments, methyl blue (MB) (cas: 28983-56-4; Mr = 799.80 g mol−1), hydrochloric acid (HCl) and sodium hydroxide (NaOH) were used. All reagents were of analytical grade and purchased from Aladdin Chemical Agent Company (China). Distilled water was used to prepare all aqueous solutions. 2.2. Synthesis of betaine-type iron oxide

3

At first, 200 mL distilled water was added to a three-neck rounded bottom flask and heated to approximately 90

using an electric jacket. FeCl3·6H2O (4.0086 g), FeCl2·4H2O (2.0014 g)

and anhydrous trimethylglycine (2.0033 g) in mass ratio of 2:1:1 were appended to above distilled water under aerobic condition during heating process. Subsequently, the ammonium hydroxide solution (8−9 mL, 25%; v/v) was added dropwise to the reaction mixture with vigorous mechanical stirring at about 90

, which aimed for adjusting pH to maintain a level

of 9−10. Meanwhile, there appeared a number of air bubbles in the mixture solution. It should be noted that the temperature must be decreased to 80

and after heated for forty minutes, all

bubbles converged on the center of the reaction solution without stirring, indicating that betaine-type magnetic nanoparticles (BMNPs) had been successfully synthesized. Then, the BMNPs were isolated from the reaction mixture in a magnetic field, and washed with distilled water about 3−5 times while also removing the impurities and unreacted materials. At last, the collected BMNPs were transferred to a beaker for freeze-drying. It was worth noting that the BMNPs were highly soluble due to their plenty of quaternary ammonium salt on surface. 2.3. Characterization methods The phase analysis of samples was performed by D/max-2500 Diffractometer (RIGAKU, Japan). FT-IR spectra was recorded with Fourier Transform Infrared Spectroscopy (FTIR, 8400s, Shimadzu, Japan) using a KBr wafer with the wave number ranging in 500−4,000 cm−1. Morphology and structure were investigated using JEM-1011 TEM (JEOL, Japan). VSM analysis was carried out by a BKT-4500Z, VSM (Lakeshore Company, USA) operated at room temperature to investigate the magnetic properties of samples. 2.4. Adsorption experiment All batch experiments were carried out in a beaker containing 50 mL dye solution, which was prepared by dissolving an appropriate amount of MB in distilled water. The beaker was then placed in a shaker (HY-2A, Youlian Instrument Research Institute, China) with the shaking speed of 300 r min−1 at room temperature. In order to investigate effect of the pH on adsorption process, the experiments were performed using the following procedure: (1) 300 mL distilled water was evenly poured into six identical beakers. (2) The pH values of the solution were adjusted in the range of 1.0−6.0 with dilute HCl and NaOH using a pH meter (PHS-25, Shengke instrument company, China). (3) The required amount of MB dye was

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dissolved in water solution so that the density of dye solution was 22 mg L−1. (4) 10 mg the damped BMNPs powders after ultrasonic treatment were added to the dye solutions. (5) After dye adsorption, BMNPs were quickly separated from the sample solutions using a magnet and (6) The residual dye concentrations in the supernatant clear solutions were determined by a UV-vis spectrophotometer (UV2550, Shimadzu company, Japan) at λmax= 600 nm. Decolorization efficiency (DE%) is calculated according to the following Eq. (1) [ 36].  C 0 − Ct   A0 − At  DE% =   × 100 =   × 100,  C0   A0 

(1)

where C0 and Ct are the concentration of MB at time 0 and t; A0 and At are the initial absorbance and the absorbance of the dye sample at time t, respectively. According to above steps, 1 L dye solution (20 mg L −1) was prepared at the optimal pH value and 5, 6, 7, 8, 9, 10, 11, and 12 mg BMNPs were employed for adsorption process so as to attain optimum quantity of adsorbents. Six different concentrations of MB (14.51, 22.02, 24.11, 27.19, 30.60, 31.33 mg L−1) with pH 1 were used to conduct the equilibrium experiments and kinetic adsorption studies. A 10 mg BMNPs were separated by a strong permanent magnet at twelve time point (1, 3, 5, 10, 15, 20, 30, 40, 50, 60, 70, 80 min). The removal percentage of dye was calculated using Eq. (1). The residual MB concentration was analyzed using a UV-vis spectrophotometer calibrated with the standard curve of MB. The amount of pollutant adsorbed at each time point per unit mass of the adsorbent qt (mg g−1) was calculated by the following Eq. (2):

qt =

C 0 − Ct ×V , W

(2)

where V (L) is the volume of the solution and W (g) is the mass of the adsorbent. Adsorption isotherm studies were executed with six different concentrations of MB that was the same as above solutions. The amount of adsorbents were kept constant (10 mg) and equilibrium time was 20 min. The following Eq. (3) was employed to quantify the amount of adsorbed MB per unit mass of adsorbent at equilibrium qe (mg g−1):

qe =

C 0 − Ce ×V , W

(3)

where, Ce (mg L−1) refers to the equilibrium concentration of MB in solution. The BMNPs containing MB were shaken with NaOH (0.1 mol L−1) and washed with distilled water several times until the water appeared colourless. Subsequently, the adsorbents

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were used again for removal of MB. This process was repeated for each recycle. 3. Results and discussion 3.1 Characterization of BMNPs According to the IR spectrum of pure Fe3O4 described in Fig. 1, a typical characterization of adsorption bands corresponding to Fe+2-O−2 streching vibration appears at 597 cm−1 [37], which belongs to intrinsic vibration mode of magnetite inverse spinel structure. Similarly, there appears a peak near 597 cm−1 in the spectrum of BMNPs in Fig. 1 indicating that resultants contain Fe3O4. Two peaks located in 2,854 and 2,929 cm−1 are associated with symmetric and antisymmetric streching vibration of betaine alkyl chains (CH) [38], respectively. C=O streching vibration is confirmed by characteristic bands between 1,543 and 1,649 cm−1. Additionally, a low-frequency peak at 1,467 cm−1 related to C-N streching vibration suggests that anhydrous trimethylglycine is successfully introduced to the surface of Fe3O4 [39]. It should be noted that adsorptions at 2,367 and 3,431 cm−1 are not representative of the chemical structure, which originate from background interference, i.e., surface adsorption of water molecules and carbon dioxide.

Fig. 1 (Color online) FTIR spectra of Fe3O4 and BMNPs

The X-ray diffraction pattern of BMNPs is displayed (Fig. S1 online). There are six characteristic peaks at 2θ = 30.19°, 35.78°, 43.25°, 53.74°, 57.30°, 62.90° referring to the lattice parameters of (220), (311), (400), (422), (511), (440), which are assgined to inverse spinel structure with regular interplanar spacing of Fe3O4 [40]. Moreover, the most intense peak is applied to estimate particle size on average by Debye-Scherrer formula (D=0.9λ/βcosθ), where D is the average crystalline size, λ is ascribed to the wavelength of the incident X-ray, β corresponds to the full width at half of the maximum height from (311) peak, and θ is related to Bragg’s diffraction angle. The calculated average particle size was 3.38 nm,

6

which was further analyzed by TEM. The range around 10 nm in size presented in Fig. 2 shows that nanoclusters may be composed of small nanocrystals and have relatively good dispersity. Slight condensation may be associated with concentration of BMNPs [41].

Fig. 2 TEM images of BMNPs

The magnetic properties of the BMNPs were analyzed at room temperature VSM with an applied field of −20.0≤H≤20.0 kOe (Fig. S2 online). The results showed magnetic behavior for BMNPs did not have the hysteresis behavior, and the saturation magnetization value (Ms) is about 48.9 emu g−1. It could also be seen from Fig. S2 (online) that BMNPs could be separated from solution under an external magnetic field. 3.2 Effect of pH Generally, pH as a variable plays a vital role in the adsorption process of various dyes [42, 43]. Thus, in order to understand the influence of pH on the removal percentage of MB, the experiments were conducted at initial concentration of 22.02 mg L−1 in the range of pH 0.96−5.91 with 10 mg BMNPs. This is because the MB solution will gradually fade until it becomes colourless when the pH value is greater than 6.0. As shown from Fig. 3, removal efficiency (93.05%, 90.49%) at pH 0.96, 1.95 were significantly higher than 79.40%, 62.11%, 60.66%, 19.91% at pH 2.97, 4.01, 5.07, 5.91, respectively. The result showed an obvious trend following the variation of pH. It is evident to know that the protonated quaternary ammonium may be beneficial for the interaction between positively charged surface and negatively charged sulfonate groups of MB via electrostatic attraction at lower pH. The reason for this may be that lower pH makes active surface sites on BMNPs increase, while competing hydroxyl ions capture partial quaternary ammonium groups leading to a decrease in surface charge density at higher pH. It is consistent with a recent report by Rajabi group

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about the adsorption of sunset yellow using aminopropyltriethoxysilane functionalized iron oxides [28]. Therefore, the above adsorption mechanism made us conclude that pH 1.0 was the optimal pH value for all of the following experiments.

Fig. 3 The influence of pH on removal of MB at the presence of BMNPs

3.3 Effect of sorbent concentration In order to obtain the optimal sorbent dosage, various sorbent dosages were used to conduct a quantitative analysis at the concentration of 20 mg L−1 MB, pH 1.0 and allowing reaction time of 60 min. The interrelation between adsorption capacity and sorbent dosage was shown in Fig. 4. Increases is parabolic until it reaches 10 mg, followed by a slight decrease at 12 mg. the reason may be that the amount of MB adsorbed per unit weight BMNPs can decrease with increasing adsorbents [44]. When the quantity of BMNPs increases to a certain value, removal efficiency doesn’t depend on adsorbent dosage anymore. The reason for this may be that the MB molecule will be unable to occupy all the sites as the dosage increases leading to more active sites on the surface of BMNPs being exposed to each other, which promotes agglomeration and produces a large number of inactive sites [45]. Therefore, 10 mg dosage was chosen as the optimum amount of BMNPs for the following studies

Fig. 4 Removal effects of MB solution by BMNPs at different dosages

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3.4 Adsorption equilibrium To explore the time dependency of the adsorption process, MB solutions were prepared at six different initial concentrations and pH 1 was used to interact with BMNPs (10 mg) after twelve time points. The function of the relationship between contact time and removal percentage was presented in Fig. 5. DE% increases quite rapidly within the first 5 min. Afterwards it increases slowly and remains stable at 20 min, which exhibits the exceptional and rapid adsorption capacity of BMNPs. This phenomenon is due to that the prepared nanosize particles have adequate specific surface area for absorption to occur [28]. In addition, it can be seen that the rate of MB uptake at low initial concentration is quicker than at high initial concentration, which indicates that the more MB molecules existing in the solution, more adsorbents are needed. In other words, a number of BMNPs can provide adequate active sites, which are bound with large amount of MB molecules.

Fig. 5 (Color online) Effect of initial MB concentration on the removal of MB

3.5 Adsorption isotherm In this section, pH 1.0, room temperature and a reaction of 20 min were selected as the reaction conditions for batch experiments. A series of data were obtained and employed for studying equilibrium adsorption isotherms. In order to analyze adsorption properties and loading capacity, all experimental data were fitted using three common models: Langmuir (Eq. (4)), Freundlich (Eq. (5)), and Temkin (Eq. (6)). These three models are important for establishing on optimal sorption scheme and described in order as follows:

9

Ce 1 Ce = + , qe K L qm qm

(4)

1 ln qe = ln KF + ln Ce, n

qe = A + B ln Ce,

(5) (6)

where q m (mg g−1), the maximum sorption quantity, decides removal efficiency; KL (L mg−1), the Langmuir adsorption equilibrium constant, is dependent on the energy of adsorption; KF (L mg−1), the Freundlich constant, stands for the adsorption capacity; 1/n, the empirical constant, is on behalf of the sorption intensity; B, the Temkin canstant, denotes the heat of sorption. According to these three equations, Ce/qe, lnqe, qe were taken as vertical coordinates, while Ce, lnCe, lnCe were selected as abscissa, respectively. All experimental data is displayed in Fig. 6. The plot of Ce/qe vs. Ce in Fig. 6a is matched more clearly with real data than two other plots and also has a higher correlation coefficient (R2 = 0.9982) than Freundlich models (0.9274) in Fig. 6b and Temkin models (0.9634) in Fig. 6c, which indicates that the adsorption behaviour is best fitted with Langmuir models. Therefore, in sorption process, single MB molecule only combines with one specific surface site on homogeneous surface of BMNPs in a certain direction and there is no interparticle reaction among BMNPs in the nearby active site. In addition, the sorption energy does not change with MB coverage area on BMNPs. 1/n was calculated and applied for deciding what category the sorption reaction belongs to. The value of 1/n is 4.2248 larger than 1, manifesting that ambient conditions are beneficial for conducting experiments and adsorption is ascribed to physical and chemical reactions [46]. In order to optimize effective use of BMNPs, it is necessary to determine the maximum sorption quantity with the help of all the important parameters from three models. The significant parameters are listed in Table 1. In comparison with previous adsorbents presented in Table 2, the BMNPs surpass other adsorbents in the field of effortless separation, high adsorption capacities and producing little environmental pollution. Thus, the BMNPs have an adequate performance and are suitable to become a candidate for MB removal.

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Fig. 6 Isotherm model plots for adsorption of methyl blue onto BMNPs. (a) Langmuir, (b) Freundlich, (c) Temkin

Table 1 Isotherm model parameters for the adsorption of methyl blue onto BMNPs Models

Parameters

BMNPs

Langmuir

qm

135.69 mg g−1

KL

1.83 L mg−1

R2

0.9982

n

4.22

KF

84.48 L mg−1

R2

0.9274

R2

0.9634

Freundlich

Temkin

Table 2 Maximum adsorption efficiency for methyl blue by various adsorbents Types of adsorbent

qe (mg g−1)

Ref.

Bi2WO6 microcrystal

60.08

[17]

Mn/MCM-41

45.38

[36]

Ni0.5Zn0.5Fe2O4 nanoparticles

54.7

[47]

CuO-Al2O3 nanomaterials

97.04

[48]

Betain-type iron oxides

135.69

This work

3.6 Adsorption kinetics To further evaluate sorption capacity of BMNPs, it is important to understand reaction mechanisms and know the sorption diffusion form. Pseudo-first-order (Eq. (7)),

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pseudo-second-order (Eq. (8)), and intraparticle diffusion (Eq. (9)) models were used to analyze the rate-control process. Three kinetic models are simplified as below:

ln ( qe − qt ) = ln qe − K 1t ,

(7)

t t = 1 2+ , K * q qt qe 2 e

qt = K 3it

1/ 2

(8)

+ Ii,

(9)

where qt (mg g−1) is defined as the adsorbed quantity of MB to BMNPs at time t (min); k1, k2, k3i , the rate constant of three kinetics models; Ii, the intercept of intraparticle diffusion models in stage i. The plots of t/qt vs. t in Fig. 7b yield a set of straight lines in agreement with experimental data by linear regression, while other plots of lnqe vs. t in Fig. 7a and qt vs. t1/2 in Fig. 7c have not been observed. Some parameters were calculated with the slope and intercept and presented in Table 3. As observed, pseudo-second-order best describes adsorption kinetics with higher correlation coefficients versus the others two models, which means that chemical bonding stems from electron transfer between dye molecules and active sites of nanoadsorbents plays a major role in driving the sorption reaction [38]. In addition, it can be seen in Fig. 7a that intermolecular competing sorption at lower concentrations is stronger than that at higher concentrations [49]. It is necessary to note that intraparticle diffusion model has an important effect on predicting rate-limiting process [50]. The plots of qt vs. t1/2 in Fig. 7c are consist of two linear portions using piecewise linear regression. The first portion suggests that dye molecules are distributed on external interface layer between solid phase and liquid phase by external mass transfer pertaining to chemisorptions. The second portion has reached equilibrium state where adsorption quantity is equal to desorption quantity. Therefore, there is no twisty micro tunnel existing in adsorbents. Furthermore, the intercepts are much larger than 1, which proves that the thickness of the boundary layer is great enough to have a remarkable impact on sorption.There is no straight line passing through its origin, revealing that external mass transfer controls the rate-limiting process. Consequently, adsorption is governed by electrostatic attraction conforming to Section 3.2 and has a dominant role in the rate-control process.

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Fig. 7 (Color online) Kinetics model plots for adsorption of methyl blue onto BMNPs (a) pseudo-first-order (b) pseudo-second-order (c) intraparticle diffusion

Table 3 Kinetic parameters for the removal MB by BMNPs MB

Pseudo-first-order

Pseudo-second-order

Intraparticle diffusion

(mg g−1) K1

qe (mg g−1)

R 12

K2

qe (mg g−1)

R 22

Stage 1

Stage 2

(Rd12 )

(Rd2 2)

14.51

0.001

67.45

0.3700

0.078

72.52

0.9999

0.8371

0.0445

22.02

0.002

36.11

0.4570

0.044

105.2

0.9999

0.9637

−0.025

24.11

0.004

28.58

0.3587

0.051

113.1

0.9999

0.9382

−0.162

27.19

0.005

22.00

0.4083

0.048

119.8

0.9998

0.8243

−0.131

30.60

0.015

14.60

0.6268

0.022

130.7

0.9999

0.9834

0.3611

31.33

0.012

14.97

0.5997

0.022

138.1

0.9999

0.9527

0.2333

3.7 Reusability of BMNPs study In order to design a good protocol for dye treatment and grasp reuse properties, regeneration experiments were carried out under ambient conditions. Since prepared magnetic nanoparticles have good water-solubility, water was selected as a washing solvent for dye desorption. Moreover, dye sorption is mainly governed by electrostatic attraction between positively charged quaternary ammonium salts and negatively charged sulfonate groups. In basic solution, the quaternary ammonium is deprotonated, leading to break the chemical bond originated from electrostatic attraction. The desorption step was conducted with 0.1 mol L−1 NaOH solution and repeated for five recycles. As shown in Fig. 8, the first removal

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percentage of MB can reach up to 92.40%, and the fifth removal percentage can be still remained at 73.33% for BMNPs. The slight decrease of the removal percentage may be due to the coverage of the adsorbed MB on the adsorbent surface. To sum up, the good reusability is vital to the practical applications in MB removal because the overall cost will be reduced.

Fig. 8 Reusability study for adsorption of methyl blue onto BMNPs

3.8 The adsorption mechanism of BMNPs for MB removal The adsorption process of BMNPs and adsorption mechanism were shown in Fig. S3 (online), it can be seen the operation process for MB removal is simple and BMNPs are easy to be separated. The mechanism can be explained by the protonation of amino groups [38]. As seen from IR spectra, there exist a certain amount of quaternary ammonium groups on the surface of BMNPs. In acid condition, the amino groups of BMNPs were protonated to possess positive charges. The amino cations of modified BMNPs could interact with sulfonate anions of MB by electrostatic adsorption. Thus quaternary ammonium groups are considered as vital active sites during adsorption process. 4. Conclusion In summary, the novel magnetic iron oxide nanoparticles BMNPs were prepared and characterized, in order to evaluate their use as a magnetic nanoadsorbent. BMNPs have high adsorption capacity for MB due to the introduction of anhydrous trimethylglycine. The adsorption of MB onto BMNPs followed Langmuir isotherm model or distribution and the maximum adsorption capacity for MB was calculated to be 136 mg g−1 at room temperature. Batch

adsorption

experiments

showed

that

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the

adsorption

process

followed

pseudo-second-order kinetic model. The BMNPs, which possess efficient and fast adsorption capabilities, as well as the simple and convenient magnetic separation, can be utilized as an environmentally friendly adsorbent for the removal of dyes from aqueous solution and are suitable for industrial applications as cost effective alternative to other dye removal products. Acknowledgments This work was supported by the Natural Science Foundation of Shanxi Province (2013011012-5), the 331 Early Career Researcher Grant of Shanxi Medical University (201421), Shanxi Province Hundred Talent Project of China and Startup funds of Shanxi Medical University (03201501).

Conflict of interest The authors declare that they have no conflict of interest.

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In this report, a novel adsorbent based on betaine-modified magnetic iron oxide nanoparticles (BMNPs) was successfully synthesized by facile method and characterized, and applied for methyl blue (MB) removal from aqueous solution. The results showed BMNPs had strong adsorption capacity and good reusability.

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