Adsorption of 2,4-D on magnetic graphene and mechanism study

Colloids and Surfaces A: Physicochem. Eng. Aspects 509 (2016) 367–375

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Colloids and Surfaces A: Physicochemical and Engineering Aspects journal homepage: www.elsevier.com/locate/colsurfa

Adsorption of 2,4-D on magnetic graphene and mechanism study Wei Liu, Qi Yang ∗ , Zhilin Yang, Wenjing Wang School of Water Resource and Environment, China University of Geoscience (Beijing), Beijing 100083, China

h i g h l i g h t s

g r a p h i c a l

a b s t r a c t

• The pHpzc of FGN was determined to be 4.7.

• The adsorption behaviors of 2,4-D on FGN was studied in detail.

• Density functional theory indicated that ␲-␲ interaction was enhanced when 2,4-D molecule dissociated into 2,4-D anion, which was beneficial to the adsorption. • According to Langmuir isotherm model, the maximum adsorption capacity of 2,4-D on FGN was 32.31 mg/g at 303.15 K and pH 3.

a r t i c l e

i n f o

Article history: Received 12 July 2016 Received in revised form 3 September 2016 Accepted 11 September 2016 Available online 12 September 2016 Keywords: Adsorption 2,4-D Graphene Fe3 O4 Mechanism

a b s t r a c t Magnetic Fe3 O4 @graphene nanocomposite (FGN) was synthesized by co-precipitation method. The ability and mechanism of FGN to remove 2,4-dichlorophenoxyacetic acid (2,4-D) from aqueous solution were evaluated. Results showed that the adsorption of 2,4-D on FGN exhibited a strong pH dependence and optimal pH value was 3. Based on density functional theory, ␲-electron density was enhanced when 2,4-D molecule dissociated into 2,4-D anion, which was beneficial to ␲-␲ interaction. The decreased 2,4-D adsorption uptake with increasing pH was mainly caused by electrostatic repulsion. The presence of NaCl (1–20 mmol/L) exerted ignorable influence on 2,4-D removal. Adsorption kinetics and isotherms could be better represented by pseudo-second kinetic and Langmuir adsorption isotherm, respectively. The maximum adsorption capacity was calculated to be 32.31 mg/g at 303.15 K according to Langmuir model. Thermodynamic parameters revealed that the adsorption was an exothermic and spontaneous process. The adsorption of FGN for 2,4-D was mainly driven by ␲-␲ interaction between benzene ring of 2,4-D and graphene, which was demonstrated by FTIR spectrums. Fe3 O4 contributed to the removal of 2,4-D through electrostatic attraction at low pH. Moreover, FGN still remained certain adsorption capacity following four desorption/regeneration cycles. © 2016 Elsevier B.V. All rights reserved.

1. Introduction

∗ Corresponding author. E-mail address: [email protected] (Q. Yang). http://dx.doi.org/10.1016/j.colsurfa.2016.09.039 0927-7757/© 2016 Elsevier B.V. All rights reserved.

The widely use of agrochemicals has drawn people’s great attention in recent years. Among numerous herbicides, 2,4dichlorophenoxyacetic acid (2,4-D) was extensively used due to its low cost and good selectivity [1,2]. However, 2,4-D has been

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W. Liu et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 509 (2016) 367–375

detected in surface and ground water all over the world [3]. It can not only pose a threat to aquatic life and ecosystem [4], but also was proved to be toxic to humans and animals [5]. Accordingly, the removal of 2,4-D from environment became an urgent issue. In this context, several technologies such as biological degradation [6], advance oxidation [7] and adsorption [8,9] have been developed to treat the wastewater contaminated by 2,4-D. Among these methods, adsorption was frequently used because of its unique advantageous, such as low cost, simplicity in operation and fewer harmful secondary products [10]. Scientists have investigated the removal of 2,4-D by various adsorbents e.g., activated carbon [3], chitosan [4], resin [5] and bentonite [11]. However, activated carbon, the main conventional adsorbent used in adsorption process, had a high production and regeneration costs [12]. Other adsorbents suffered the problem of low adsorption capacity. Therefore, there was an increasing demand in developing efficient adsorbent in low cost. Graphene was a novel two-dimensional carbon nanomaterial with a honeycomb structure that was sp2 -hybridized with single carbon atom thickness [13]. It exhibited many unique properties such as high thermal conductivity, excellent mechanical strength but ultra-light weight and huge specific surface area with a theorized value of 2630 m2 /g [14]. Owing to that, graphene has been studied intensively in many fields, ranging from electronic devices, solar cells, sensors to hydrogen storage [15]. Recently, graphene and its derivatives were widely utilized as adsorbent in wastewater treatment. In previous studies, graphene/graphene based materials exhibited excellent adsorption performance for inorganic anion [16], harmful gas [17], heavy metal ions [18], radionuclides [19] and numerous organic pollutants [20–23]. For example, Moussavi et al. [12] found that the maximum capacity of double-oxidized graphene oxide for acetaminophen estimated by Langmuir model was 704 mg/g. Wang et al. [13] investigated the adsorption of naphthalene, phenanthrene and pyrene on graphene and adsorption capacity were 127.7, 136.4 and 170.2 mg/g, respectively. Nevertheless, the application of graphene/graphene based materials on a large scale was a huge challenge for it was difficult to reclaim suspended graphene from water [24,25]. Fe3 O4 nanoparticle has long been used as adsorbent in environmental pollution remediation [26–28]. The most striking advantage of it was that it could be separated easily from the reaction system with an external magnet. Therefore, the composite of graphene with Fe3 O4 may also possess excellent magnetic character, which was beneficial to the recycle of graphene. The preparation of graphene based magnetic composites and their potential application in removing pollutants from contaminated environment have been reported by lots of researchers [29–32]. To our knowledge, only a few works regarding the removal of 2,4-D by graphene based materials have been performed [33]. Furthermore, in previous literatures, most researchers emphasized the excellent adsorption capacity of graphene. The role of Fe3 O4 in organics removal has been ignored. In this work, magnetic Fe3 O4 @graphene nanocomposite (FGN) was synthesized through a simple method, and characterized by scanning electron microscope (SEM), X-ray diffraction (XRD) as well as Fourier transform infrared spectra (FTIR). The impact of several parameters such as initial pH, contact time and ionic strength were evaluated. Meanwhile, adsorption kinetics, isotherms, thermodynamic and reusability were tested. The adsorption mechanism was also tentatively proposed.

2. Materials and methods 2.1. Materials All chemicals used in current study were of analytical grade. 2,4-D (>98.8%) was obtained from Merck (Germany). Graphite flake were produced by Alfa Aesar (China). Hydrazine hydrate (>80%) was supplied by Tianjin Guangfu Fine Chemical Research Institute (Tianjin, China). FeSO4 ·7H2 O, Fe2 (SO4 )3 and other reagents were purchased from Sinopharm Chemical Reagent Co. Ltd (Shanghai, China). Distilled water was used through the experiment. 2.2. Preparation of adsorbent Graphite oxide was synthesized using modified hummers method [34]. FGN was prepared according to previous literature [35] with some modifications. 0.35 g graphite oxide was dispersed in 400 mL deionized water by sonication for an hour. A solution containing 4.00 g Fe2 (SO4 )3 and 2.78 g FeSO4 ·7H2 O was added to graphite oxide suspension with vigorous stirring under N2 condition. Subsequently, aqueous ammonia (about 28%) was added drop-wise until solution pH reached 10. The solution was heated to 80 ◦ C and 3.5 mL of hydrazine hydrate was injected. After being kept five hours at 80 ◦ C, the solution was cooled down at ambient environment and then washed several times until pH of effluent close to neutral. Finally, the precipitation was dried in a vacuum freezing dryer. Pure Fe3 O4 and graphene were prepared separately by replacing graphite oxide solution or iron salts with equal volume of water using the same procedure mentioned above. 2.3. Characterization The morphology of prepared FGN was characterized by SEM (Hitachi S-4800). FTIR spectra were obtained using a FTIR spectrometer (V70 Hyper1000) at room temperature. XRD patterns were measured on an XRD diffract meter (D8-Advance, Bruker) with Cu K␣ radiation (40 kV and 40 mA). The potentiometric acid-base titration [36] of adsorbent was performed using an automatic titrator (HI902C, Hanna Instruments). Briefly, the suspension of FGN with certain ionic strength was prepared and pH was adjusted to 3 by adding HClO4 . Then, NaOH (0.01 mol/L) was added by titrator until pH attained to 11. Meanwhile, the volume of NaOH consumed and corresponding pH of suspension were recorded. The values of Gran function (G) were calculated as follows. Acidic side : Ga = (V0 + Va + Vb ) × 10−pH × 100 Alkaline side : Gb = (V0 + Va + Vb ) × 10

−(13.8−pH)

(1) × 100

(2)

Where V0 was the initial solution volume (mL), Va and Vb were the volume of acid and NaOH added, respectively (mL). TOTH, the total concentration of surface reactive sites [36], was determined by Eq. (3). TOTH = −

(Vb − Veb1 ) × Cb V0 + Vb

(3)

Where Cb was the concentration of NaOH used in titration process. Veb1 can be obtained from the results of linear regression analysis of the Gran plots. 2.4. Batch adsorption experiments For all adsorption experiments, 0.10 g FGN was accurate weighted and subjected into a 250 mL stoppered flask containing 200 mL 2,4-D with known concentration. Solution pH was adjusted varying from 3 to 11 and tested by a pH meter (Orion star A211,

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(311)

369

Graphene Fe3O4 FGN

Indensity(a.u.)

(220)

10

20

30

(400)

(511) (440) (422)

40 50 2 Theta(degree)

60

70

80

Fig. 1. XRD pattern of graphene, Fe3 O4 and FGN.

Thermo). Flakes were shaken in a thermostat shaker with a shaking speed of 150 rpm. At desired time, samples were withdrawn from flasks and filtered by PTFE filter (0.22 ␮m). To determine the minimum time required for adsorption equilibrium, kinetics studies were carried out ranging from 0 to 12 h. The effect of ionic strength (1–20 mmol/L) on 2,4-D adsorption was studied by adding different amounts of NaCl to the solution. Adsorption thermodynamic was performed under different temperatures (303.15, 313.15 and 323.15 K). In reused experiments, FGN was collected by a magnet at equilibrium time. It was immersed in 20 mL 0.01 mol/L NaOH for 6 h and washed by water several times. Then, it was reused in the same condition of the first time. Mechanism study was conducted at pH 3 and the dosage of adsorbent was 0.10 g with 10 mg/L of 2,4-D. The concentration of 2,4-D was determined by high performance liquid chromatogram (Waters 1525) equipped with an UV detector (Waters 2487) using a C18 column (4.6 × 150 mm, 5 ␮m) as stationary phase [7]. To assess reproducibility, each experiment was tested in duplicate. The adsorption amount (qe , mg/g) and removal (%) of 2,4-D were defined as follows: qe =

(C0 − Ce ) ×V W

Removal (%) =

C0 − Ce × 100% C0

Fig. 2. SEM images of graphene (a) and FGN (b).

d c

3350

1561 1723

1075

b

3329

1556

1038

1554

1057

(4) a

3330

(5)

Where C0 and Ce (mg/L) were the initial and equilibrium concentration of 2,4-D, respectively. V (mL) was the volume of solution and W (g) was the mass of adsorbent.

1616 1721

3359

4000

3500

3000 2500 2000 Wavelength(cm-1)

1500

1222 1043

1000

500

3. Results and discussion 3.1. Characterization XRD was used to identify the crystalline structure of species. The patterns of graphene, Fe3 O4 and FGN were shown in Fig. 1. Typical peaks of FGN at 2␪ values (30.1◦ , 35.6◦ , 43.2◦ , 53.5◦ , 57.1◦ , 62.9◦ ) were consistent with that of standard XRD data (PDF#19-0629) of Fe3 O4, indicating Fe3 O4 nanoparticles were formed on surface of graphene. However, no obvious diffraction peaks belonging to graphene (2␪ = 23.3◦ ) could be found in FGN signals. Similar phenomenon has been reported [37]. The result may be derived from the following reasons: (1) the formation of Fe3 O4 particles reduced the aggregation of graphene sheets, resulting in more monolayer graphene. Therefore, weaker peaks from carbon were observed; (2) the strong signals of Fe3 O4 crystals would cover the weak carbon peaks [37]. Fig. 2 presented the typical morphology of graphene and FGN. As could be seen, graphene was made up of randomly aggregated,

Fig. 3. FTIR spectrum of graphite oxide (a), graphene (b), FGN before adsorption (c) and FGN after adsorption (d).

wrinkled and thin sheets [14]. Compared with Fig. 2a, spherical nanoscale particles with a high density were deposited on the surface of graphene (Fig. 2b), suggesting that Fe3 O4 was successfully coated. In addition, lots of agglomerated Fe3 O4 nanoparticles were observed, which was in accordance with earlier studies [35,38]. FTIR spectrums of graphite oxide, graphene as well as FGN (before and after adsorption) were revealed in Fig. 3. Typical peaks could be observed and were consisted with earlier literatures [11,34,35,39]. The bands of graphite oxide at 1043, 1222 and 1616 cm−1 were ascribed to C O, C OH and aromatic C C, respectively. The peak located in 1721 cm−1 was characteristic of C O in carbonyl and carboxyl moieties. The band at 3359 cm−1 was assigned to the O H stretching vibrations. In contrast, the spectrum of graphene displayed peaks around 1057, 1554 and 3330 cm−1 which corresponded to the C O, aromatic C C and −OH, respec-

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10 8

G

6 4 Veb1=5.36mL

2 0 0

3

6 VNaOH(mL)

9

12

Fig. 4. Gran plots of the FGN (I = 0.01 mol/L NaClO4 , FGN = 0.05 g, V = 50 mL).

80 0.0010

0.0005

0.0000

40

-0.0005

TOTH

Removal(%)

60

20 -0.0010

pHpzc=4.7

0

2

4

6

8

10

according to Eq. (3) and relationship between pH and Vb . Obviously, TOTH was zero when Vb equaled Veb1 .The pH corresponded to Veb1 was pHpzc (point of zero charge). As illustrated in Fig. 5, TOTH declined with increasing pH and pHpzc was determined to be 4.7. At pH below 4.7, FGN surface was positively charged. Otherwise it turned to negative. Thus, even though the fraction of negatively charged 2,4-D species increased at higher pH, electrostatic attraction may become weaker. Moreover, repulsion between negatively charged FGN and anionic 2,4-D would be enhanced when pH above 4.7, which ultimately leaded to the decreased removal efficiency with increasing pH. The ␲-␲ interaction between benzene ring of 2,4-D and graphene consisting in FGN was another important factor that resulted in 2,4-D adsorption. In fact, the electron distribution of 2,4D also changed when 2,4-D molecule dissociated into 2,4-D anion, which would further influence the interaction between 2,4-D and FGN. Herein, the natural population analysis (NPA) was carried out by using OCRA (Version 3.03) program [43] with density functional theory at the level of (B3LYP)/6–311 + G** [44]. As depicted in Fig. 6, digital labeled on each atom denoted the number of NPA electron of the corresponding atom calculated by OCRA program. The total NPA electron on benzene ring (␲-electron) of neural 2,4-D was −0.521e and that of anionic 2,4-D was −0.549e. The increased ␲-electron density strengthened the attraction between benzene ring of 2,4-D and graphene through ␲-␲ interaction [45]. Therefore the removal efficiency of 2,4-D would be enhanced, which was contrast to the results we got. It further demonstrated that the enhanced electrostatic repulsion was the main reason that resulted in low removal efficiency at high pH. According to the result, subsequent studies were performed at pH 3 unless noted specially.

-0.0015

12

pH Fig. 5. Plot of removal efficiency and TOTH versus solution pH (303.15 K, 2,4D = 10 mg/L, FGN = 0.5 g/L).

tively. The absence of C O and C OH suggested that graphite oxide was reduced partly. For pristine FGN, peaks at 1038, 1556 and 3329 cm−1 were attributed to C O, C C and OH, respectively. A new peak at 1723 cm−1 was observed after its contact with 2,4-D. The new peak could be attributed to COOH, indicating 2,4-D was adsorbed on FGN. Moreover, FGN (after adsorption) also displayed peaks of C O (1075 cm−1 ), C C (1561 cm−1 ) and OH (3350 cm−1 ). Fig. 4 showed Gran plots for the titration data of FGN. Veb1 could be obtained from linear regression analysis of the Gran plots at acid side. The intersection of fitting line and horizontal ordinate (volume of NaOH consumed) was Veb1 , which was determined to be 5.36 mL. 3.2. Effect of initial solution pH pH was a significant variable which must be considered in adsorption. To examine its effect on removal efficiency of FGN for 2,4-D, a series of experiments were conducted with an initial pH ranged of 3–11. Results were depicted in Fig. 5. It was clear that the removal of 2,4-D by FGN was highly pH sensitive. The best performance was observed at pH 3.0 and 68.46% of 2,4-D was removed. Afterwards, a sharp reduction of removal efficiency which decreased from 68.46% to 2.48% when pH increased from 3 to 11 was observed. Tang et al. [9] and Hameed et al. [40] conducted the adsorption of 2,4-D by activated carbon separately and similar phenomena were found. Generally, pH influenced organic uptake by its effect on the speciation distribution of adsorbate and surface charge of adsorbent [41]. The pKa of 2,4-D was 2.7 [42]. The degree of dissociation of 2,4-D aggravated when pH above 2.7 at which 2,4-D existed mostly in anionic form. TOTH of FGN at various pH values were calculated

3.3. Effect of contact time To determine the equilibrium time for maximum adsorption capacity, 2,4-D concentration reserved in solution was examined at different time interval. Fig. 7 presented the time profile of 2,4-D adsorption onto FGN with various 2,4-D initial concentrations. Clearly, a fast adsorption process was observed within the first 20 min and adsorption capacity attained around 80% of the saturated state, followed by slow adsorption kinetics. It could be attributed to that there were numerous blank active sites on adsorbent at early stage, and fewer sites were available as the reaction processed [46]. Moreover, repulsive force may occur between anions of 2,4-D adsorbed on adsorbent and the ones in solution. Hence, the adsorption rate reduced gradually. Fig. 7 also illustrated that equilibrium was achieved within 12 h. Afterward, the amount of 2,4-D desorbing from adsorbent was in a state of dynamic equilibrium with that of 2,4-D being adsorbed [47]. As a result, no noteworthy increase in adsorption capacity was observed. At the same time, adsorption uptake improved with increasing initial concentration, which could be owing to that there was more collision between 2.4-D and FGN at higher 2,4-D concentration [47,48]. The values of qe at the 12th hour for four initial concentrations were 8.33, 13.50, 17.44, and 19.95 mg/g, respectively. 3.4. Effect of ionic strength In present study, different amounts of NaCl were dissolved in the mixture of 2,4-D and FGN to examine the effect of ionic strength (0–20 mmol/L) on 2,4-D adsorption. As depicted in Fig. 8, the presence of NaCl had ignorable effect on 2,4-D uptake. While, Jin et al. [29] found that the adsorption capacity of 4-nnonylphenol (4-n-NP) and bisphenol-A (BPA) on magnetic reduced graphene oxides increased slightly with increasing NaCl concentration (0–100 mmol/L). They suggested that the enhancement of

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371

Fig. 6. NPA of neutral 2,4-D (a) and anion 2,4-D (b).

adsorption uptake at high ionic strength could be caused by salting outing effect which decreased the solubility of organic compound in solution. Compared with the solubility of 4-n-NP (5.40 mg/L) and BPA (0.12–0.30 mg/L), that of 2,4-D was much higher (900 mg/L) [3]. Additionally, the initial concentration of 2,4-D was 10 mg/L, which was far less than its solubility. Therefore, ionic strength had no significant effect on adsorption capacity of 2,4-D on FGN. 3.5. Adsorption kinetics To investigate the adsorption kinetics, pseudo-first and second order models were applied to fit the datum obtained in experiment. The pseudo-first order, pseudo-second order equation were expressed as follows [41]: ln (qe − qt ) = ln qe − k1 t

(6)

1 t t = + qt qe k2 q2e

(7)

Where qe and qt were adsorption uptake (mg/g) of 2,4-D at equilibrium and time t (h), respectively. K1 and k2 were rate constant of pseudo-first order (1/h) and pseudo-second order adsorption (g mg−1 h−1 ), respectively. Fig. 9 revealed the results of linear form of two kinetic models. It was evident that pseudo-second order kinetic model fitted better than pseudo-first one for almost all the spots were on the line. This phenomenon was further confirmed by correlation coefficient (R2 ). Relevant parameters were summarized in Table 1. From it, we can discover that values of R2 for pseudo-second order were all greater than 0.99, while that of pseudo-first-order were around 0.7. Moreover, values of qe of pseudo-second order model agreed very well with the experimental ones. A better fit to pseudo-second order model meant that the adsorption rate should be more relying on the availability of adsorption sites rather than the concentration of adsorbate in medium [47]. Similar results were reported by previous works in which the adsorption of methylene blue [48], aniline [49] and p-chloroaniline [49] on magnetic Fe3 O4 @graphene nanocomposite followed the pseudo-second order kinetic model.

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Table 1 Parameters of kinetics at different initial concentrations. Initial concentration (mg/L)

qe (experiment data, mg/g)

5 10 15 20

Pseudo-first-order kinetic model

8.33 13.50 17.44 19.95

qe,cal (mg/g)

R2

k2 (g/mg h)

qe,cal (mg/g)

R2

0.34 0.35 0.31 0.48

1.61 4.04 4.51 5.12

0.573 0.738 0.631 0.790

1.22 0.44 0.39 0.46

8.32 13.51 17.39 20.00

1.000 0.999 0.999 1.000

20

(a)

3

5mg/L 10mg/L 15mg/L 20mg/L

2

15

10 5mg/L 10mg/L 15mg/L 20mg/L

5

1

ln(qe-qt)

qe(mg/g)

Pseudo-second-order kinetic model

k1 (1/h)

0 -1 -2

0 0

2

4

6 T(h)

8

10

0

12

1

2

3 T(h)

4

5

6

Fig. 7. Adsorption of 2,4-D on FGN as a function of contact time (303.15 K, 2,4-D (5,10,15,20 mg/L), FGN = 0.5 g/L, pH = 3).

1.5

(b)

5 mg/L 10mg/L 15mg/L 20mg/L

10

1.0

t/qt

qe(mg/g)

8 6

0.5 4 2 0

0.0 0

1 5 10 NaCl concentration(mmol/L)

0

20

Fig. 8. Influence of ionic strength on 2,4-D adsorption capacity (303.15 K, 2,4D = 10 mg/L, FGN = 0.5 g/L, pH without adjust (4.5)).

3

6 T(h)

9

12

Fig. 9. Kinetics of 2,4-D adsorption (a) pseudo-first order model and (b) pseudosecond order model (303.15 K, 2,4-D (5,10,15,20 mg/L), FGN = 0.5 g/L, pH = 3).

3.6. Adsorption isotherms 35

In present work, two classic adsorption models named Langmuir and Freundlich model were used to describe adsorption equilibrium and mathematical representations of them were given below [50]. qm KL Ce 1 + KL Ce

qe = KF Ce 1/n

(8) (9)

Where Ce (mg/L) was the equilibrium concentration of 2,4-D in flake, qe (mg/g) was the amount of 2,4-D adsorbed at equilibrium time, qm (mg/g) was the maximum adsorption capacity per unit weight of adsorbent. KL (L/mg) and KF [mg/g (L/mg)1/n ] were equilibrium constants of Langmuir and Freundlich models, respectively. And 1/n was the heterogeneity factor. Fig. 10 exhibited adsorption equilibrium of 2,4-D on FGN at different temperatures. As could be seen, adsorption capacity of FGN for 2,4-D increased with increasing 2,4-D equilibrium con-

25 qe(mg/g)

qe =

323.15K 313.15K 303.15K

30

20 15 10 5 0

5

10

15

20 25 Ce (mg/L)

30

35

40

Fig. 10. Isotherm model of Langmuir (solid lines) and Freundlich (dotted lines) at 303.15, 313.15 and 323.15 K (2,4-D (5–50 mg/L), FGN = 0.5 g/L, pH = 3).

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373

Table 2 Isotherm parameters of 2,4-D adsorbed on FGN. Temperate (K)

Experiment data (mg/g)

303.15 313.15 323.15

31.64 31.29 28.96

Langmuir model

Freundlich model

qm (mg/g)

KL (L mg−1 )

R2

NSD (%)

KF

1/n

R2

NSD (%)

32.31 30.75 29.69

0.209 0.203 0.184

0.876 0.860 0.867

5.09 5.51 3.62

9.28 8.61 7.99

0.332 0.338 0.345

0.991 0.968 0.978

99.91 102.46 102.40

centration. Relevant parameters including the value of normalized standard deviation (NSD (%), Eq. (10)) [51] were listed in Table 2. Commonly, NSD was utilized to validate whether the isotherm was fitted to experimental data. Even though R2 values of Freundlich model were slightly higher than that of Langmuir, the NSD values of Langmuir model at different temperatures were smaller and experiment adsorption uptakes were more closed to model-calculated adsorption capacity (qm ), indicating Langmuir model could match well to experiment data [51]. The adsorption uptake determined by Langmuir model at 303.15 K, 313.15 K and 323.15 K were 32.3, 30.8 and 29.7 mg/g, respectively. The tiny difference between qe values at different temperature approved that temperature had a minor effect on 2,4-D adsorption. Additionally, values of 1/n of Freundlich model varied from 0.332 to 0.345, which were smaller than other adsorbents [3,52]. All of values were lower than 1, which was a good indicator that 2,4-D had high affinity with FGN [4].





NSD (%) = 100 ×

exp

[(qe

Fig. 11. Recycling of FGN in the adsorption of 2,4-D (303.15 K, 2,4-D = 10 mg/L, FGN = 0.5 g/L, pH = 3).

exp 2

− qcal e )/qe ]

N−1

(10)

1.0

Where N was the number of measurements.

0.8

Graphene Fe3O4

0.6

FGN

Thermodynamics analysis provided an insight into the inherent energy changes that were associated with adsorption. In this study, thermodynamic parameters including the changes of enthalpy (H◦ ), entropy (S◦ ) and Gibbs free energy (G◦ ) were calculated by the following equations [53]. Kd =

C0 − Ce V × Ce m

ln K • =

S • H • − R RT

G • = −RT ln K •

(11) (12) (13)

Where C0 (mg/L) and Ce (mg/L) were the initial and equilibrium concentration of 2,4-D, respectively. V (L) was the volume of suspension and m (g) was the mass of adsorbent. R (8.3145 J mol−1 K−1 ) was ideal gas constant. T (K) was absolute temperature. The adsorption equilibrium constant, lnK◦ , could be determined by plotting lnKd verse Ce and extrapolating Ce to zero, the intercept was the value of lnK◦ . S◦ and H◦ were obtained from Eq. (12) by plotting lnK◦ verse 1/T. Values of G◦ , H◦ and S◦ were summarized in Table 3. The negative value of G◦ and H◦ implied that 2,4-D adsorbed on FGN should be a spontaneous, exothermic process [54]. The values of G◦ became more negative with declined temperature suggesting that lower temperature was more favored to adsorption [31]. Moreover, the negative value of S◦ (−0.02 kJ/K mol) indicated the decrease of randomness at the solidsolution interface [54]. 3.8. Recycling of adsorbent In this work, four consecutive desorption/regeneration cycles were performed to evaluate the reusability of adsorbent. As presented in Fig. 11, 2,4-D adsorption capacity decreased from 13.50

C/C0

3.7. Thermodynamic of adsorption

0.4 0.2 0.0 0

2

4

6 Time (h)

8

10

12

Fig. 12. Adsorption of 2,4-D on graphene, Fe3 O4 and FGN (303.15 K, 2,4-D = 10 mg/L, adsorbent = 0.5 g/L, pH = 3).

to 6.90 mg/g when adsorbent was reused 4 times. Meanwhile, FGN still remained excellent magnetic sensitivity after four recycling and could be separated conveniently by a magnet. Considering the low toxicity of NaOH and easy operation, the as-prepared adsorbent was a promising adsorbent for removing 2,4-D from wastewater. 3.9. Mechanism analysis In order to further ascertain the mechanism of removal of 2,4-D by FGN, adsorption of 2,4-D on graphene, Fe3 O4 and FGN were also studied. As displayed in Fig. 12, graphene exhibited the highest removal efficiency among three adsorbents. In fact, about 99% of 2,4-D was removed by it within 5 min. The removal efficiency of FGN for 2,4-D was 68.46% at the 12th h. While, Fe3 O4 showed the lowest removal efficiency toward to 2,4-D and 13.51% of 2,4-D was removed at equilibrium time. Asuha and co-workers [55] suggested that the surface of Fe3 O4 would be protonated at low pH, resulting in the positively charged surface groups (–FeOH2 + , Eq. (14)) and negatively charged CrO4 2− will be adsorbed. Other literatures [56,57] also reported that Fe3 O4 with different structures showed excellent adsorption per-

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Table 3 Thermodynamic parameters of 2,4-D adsorbed on FGN. T(K)

G◦ (kJ/mol)

H◦ (kJ/mol)

S◦ (kJ/K mol)

303.15 313.15 323.15

−1.43 −1.11 −1.04

−7.29

−0.02

formance for organic pollutants through electrostatic attraction. In current work (pH = 3), therefore, the main reason that resulted in 2,4-D removal was graphene. Simultaneously, Fe3 O4 also played a significant role through electrostatic attraction. −FeOH + H+ → −FeOH2 +

(14)

Several interactions have been proposed to illustrate the attraction between organic matter and graphene-based materials including electrostatic interaction [20,41,58], hydrogen bond [45,53,58] and ␲-␲ force [13,21,53,58]. As shown in the FTIR spectrum (Fig. 3), the C C peak of FGN after 2,4-D adsorption shifted from 1556 to 1661 cm−1 , which was due to the ␲-␲ interaction between the benzene ring of 2,4-D and graphene [13,29]. The –OH peak shifted from 3330 to 3350 cm−1 suggested the formation of hydrogen bonding [10]. However, hydrogen bonding was probably not the main cause of 2,4-D adsorption. On the one hand, hydrogen bonding between functional groups (e.g. hydroxyl which was showed in FTIR spectrum) on FGN and aromatic compound was much less than that between functional groups and water molecules [53]. On the other hand, it would be weakened seriously when the fraction of neutral 2,4-D decreased with increasing pH. Moreover, electrostatic attraction diminished for TOTH on the surface of FGN decreased with increasing pH until it was replaced by electrostatic repulsion. In contrast, ␲-␲ force would be enhanced for the higher ␲-electron density on the benzene ring of 2,4-D when neutral 2,4-D dissociated into 2,4-D anion. Accordingly, the removal of 2,4-D by FGN was driven primarily by ␲-␲ interaction between 2,4-D and graphene. Fe3 O4 also played an important role at low pH. 4. Conclusion In summary, magnetic Fe3 O4 @graphene nanoparticle (FGN) was prepared and utilized as adsorbent for removing 2,4-D from solution. It showed the best performance at pH 3. The kinetics and isotherm data fitted well with pseudo-second kinetic and Langmuir isotherm model, respectively. The Langmuir model correlated to the experimental data showing an adsorption uptake of 32.31 mg/g for 2,4-D at 303.15 K. Thermodynamic study signified that the adsorption reaction was spontaneous and exothermic. The mechanism for the adsorption of 2,4-D on FGN was mainly out of ␲-␲ interaction between graphene and the benzene ring of 2,4-D. Additionally, FGN remained certain adsorption capacity after being reused four times and could be reclaimed by a magnet. Therefore, considering relatively high adsorption capacity, rapid adsorption rate and excellent magnetic properties of FGN, it could be used as a promising adsorbent for 2,4-D removal from water. Acknowledgments This study was granted by the National Natural Science Foundation of China (50578151), the National Science and Technology Major Project of China (2012ZX07201-005-06-01; 2015ZX07406005), and the Fundamental Research Funds for the Central Universities (2652013101, 2652013086, 2652013087). The authors want to thank Dr. Tian Lu (Beijing Kein Research Center for Natural Sciences, Beijing, China) for the computational support in NPA.

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