fulltext3Performance of Poly(Styrene–Divinylbenzene) Magnetic Porous Microspheres Prepared by...

8/13/2019 fulltext3Performance of Poly(StyreneDivinylbenzene) Magnetic Porous Microspheres Prepared by Suspension Poly

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641

Performance of Poly(StyreneDivinylbenzene) Magnetic Porous Microspheres

Prepared by Suspension Polymerization for the Adsorption of

2, 4-Dichlorophenol and 2, 6-Dichlorophenol from Aqueous Solutions

Ping Yu1,2, Qilong Sun3,*, Jianming Pan2, Zhenjiang Tan1,*, Jiangdong Dai4, YongshengYan2 and Feng Cheng1 (1) School of Computer Science, Jilin Normal University, 1301 Haifeng Street, Siping136000, China. (2) School of Chemistry and Chemical Engineering, Jiangsu University, Zhenjiang 212013, China. (3)

School of Management, Jilin Normal University, 1301 Haifeng Street, Siping 136000, China. (4) School of Materials,

Jiangsu University, Zhenjiang 212013, China.

(Received 14 May 2013; revised form accepted 2 July 2013)

ABSTRACT: Poly(styrenedivinylbenzene)/Fe3O4 (Fe3O4@StDVB) magneticporous polymer microspheres based on suspension polymerization were

prepared for adsorption of 2, 4-dichlorophenol (2, 4-DCP) and 2,

6-dichlorophenol (2, 6-DCP) from aqueous solution. The as-prepared product

was characterized by Fourier transform infrared spectroscopy, Raman, X-ray

diffraction, scanning electron microscopy, thermogravimetric analysis,

differential thermal analysis and elemental analysis. The results showed that the

composite material has both porous and magnetic characteristics, which aid in

faster separation and increase in adsorption capacity. The adsorption

performance of the Fe3O4@(StDVB) magnetic porous microspheres was

manifested by batch mode adsorption experiments with respect to pH, initial

concentration, contact time and temperature. The adsorbent was found to be

sensitive to changes in pH, and the adsorption capacity of 2, 4-DCP is higher

than that of 2, 6-DCP. The kinetics experimental data fitted well with the

pseudo-second-order model, as three steps belonged to the pseudo-second-

order adsorption process. The adsorption isotherms were also described by the

Langmuir and Freundlich isotherm models, respectively. It was found that the

Langmuir isotherm model fitted the equilibrium data superior to the Freundlich

model. Thermodynamic parameters were calculated by the Gibbs free energy

function, confirming that adsorption process was spontaneous and

endothermic. In addition, the reusability performance of the Fe3O4@(StDVB)

magnetic porous microspheres was demonstrated by four repeated cycles.

1. INTRODUCTION

Chlorophenols, such as 2, 4-dichlorophenol (2, 4-DCP) and 2, 6-dichlorophenol (2, 6-DCP) are

multi-duty raw materials in chemical industries and are widely used in many areas such as

petroleum, chemical, petrochemical, coal gasification and carbonization, pharmaceutical, wood,

plastic, paper, textile, rubber, photographic, dye, disinfectant and pesticide industries (An et al.

2012). However, the Environmental Protection Agency has listed chlorophenols and its

derivatives as toxic pollutants because they are highly toxic and harmful to different biologicalprocesses. Chlorophenols give an unpleasant taste and odour to drinking water, and even small

amounts of the chemical in water will greatly harm aquatic life and peoples health. Chlorophenols

*Author to whom all correspondence should be addressed. E-mail: [email protected] (Q. Sun).


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can encroach the nerve centre, causing different degree of headache, vomiting and other adverse

symptoms. Most of these compounds are known or suspected to be human carcinogens (Kieth and

Telliard 1979). Therefore, removal of chlorophenols from aqueous solution is a crucial theme in

the field of environmental protection.

Various techniques and methods can be used to remove chlorophenols from aqueous solution,such as biodegradation (Stehlickova et al. 2009), photocatalytic degradation (Deshpande and

Madras 2010), chemical oxidation (Zazo et al. 2009), membrane separation (Ng et al. 2010) and

adsorption (Matynia et al. 2003). Among these methods, adsorption is an economic and effective

method. Recently, a variety of adsorbents have received increased attention for their application

in the removal of chlorophenols. For example, functional chitosan (Kumar et al. 2010) and fly ash

(Andini et al. 2008) have been used as sorbents for the adsorption of chlorophenols from aqueous

solutions. However, common adsorbents have to face the problems of low adsorption capacity and

it is difficult to reuse them. Therefore, searching for reproducible adsorbents that have comparable

capacity and simple separation process is highly desired (Matynia et al. 2003).

Recently, porous polymer adsorbents were synthesized and widely applied to removechlorophenols from aqueous solution (Zeng et al. 2010), due to their characteristic properties such

as low density, high porosity and easy functionalization. The methods of synthesis of porous

polymer microspheres commonly included suspension polymerization, emulsion polymerization,

dispersion polymerization, etc. Of all these methods, suspension polymerization has been proved

to be the most simple yet powerful method. Suspension polymerization not only has the advantage

of simple operation, but also the purity of the product is high. In addition, the porous polymer

adsorbents prepared by suspension polymerization, as a kind of special surface morphology

modification method, have large specific surface area and increased adsorption ability. To the best

of our knowledge, there were no studies about the adsorptive removal of chlorophenols by porous

polymer adsorbents prepared by suspension polymerization.In order to solve the difficulty in collecting adsorbents from their dispersing media, special

attention has been directed to the combination of iron oxide magnetic nanoparticles and

conventional adsorbents (Pan et al. 2011). A series of absorbents composed of magnetic

particles and inorganic matters has been prepared. For example, -Fe2O3 and cerium atomiccomposite magnetic nanoparticles (Haviv et al. 2010) and the Fe2O3 nanoparticles have been

introduced into the multi-walled carbon nanotubes to synthesize magnetic adsorption material

(Qu et al. 2008). The magnetic particles showed significant advantages in the separation

process, easy to separate and no longer required additional centrifugal separators or filters.

However, the aforementioned magnetic particles are very limited due to the lack of high

magnetic leakage and difference in acid resistance. Use of functional polymer-coated magnetic

particle preparation of a magnetic adsorbent is a useful method to improve this limitation, by

which we can use the polymer-coated layer to bind pollutants and using polymer layer internal

magnetic particles to realize the response of the magnetic field. The preparation of magnetic

porous polymer microspheres by suspension polymerization not only has high specific surface

and excellent adsorption performance, but also can be realized by simple separation in

additional magnetic field.

Inspired by these works, we herein report the synthesis of magnetic porous polymer

microspheres based on suspension polymerization of styrene (St) and divinylbenzene (DVB) with

Fe3O4 coated by oleic acid. The Fe3O4@(StDVB) magnetic porous polymer microspheres werethen used for the absorption of 2, 4-DCP and 2, 6-DCP from aqueous solution. The adsorption

properties such as adsorption equilibrium, kinetics, thermodynamics and regeneration

performance were manifested by batch mode adsorption experiments.

642 Ping Yu et al./Adsorption Science & Technology Vol. 31 No. 7 2013


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2. EXPERIMENTAL SECTION

2.1. Materials

All solvents used in this work were of analytical or high-performance liquid chromatographygrade. St, toluene, sodium chloride, sodium acetate, ethanol, acetone, methylene dichloride and

methyl alcohol were supplied by Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). The

2,4-DCP and 2,6-DCP were purchased from Tianda Chemical Reagent Factory (Tianjin, China).

Azodiisobutyronitrile (AIBN), DVB, cyclohexanol, iron(III) chloride and ethylene glycol were

obtained from Aladdin Reagent Co., Ltd. (Shanghai, China). Hydroxyethyl cellulose (HEC) was

supplied by Shanghai Hualing Resin Co., Ltd (China Shanghai).

2.2. Equipment

Fourier transmission infrared spectra (FTIR) of the Fe3O4@(StDVB) magnetic porous polymermicrospheres were recorded on a Nicolet NEXUS-470 FTIR apparatus. The shape and the surface

morphology of the magnetic porous polymer microspheres were obtained using a scanning

electron microscope (XL-30ESEM, Philips). An elemental analyzer (Elementary, Hanau,

Germany) was used to investigate the surface elemental composition of the Fe3O4@(StDVB)

magnetic porous microspheres. Magnetic measurements were carried out using a vibrating sample

magnetometer (VSM) (7300, Lakeshore) under a magnetic field up to 10 kOe. The

thermogravimetric analysis (TGA) and DSC of the samples (powdered form, weighing about 10 mg)

were performed using a Diamond TG/DTA instrument (PerkinElmer) under a nitrogen atmosphere

up to 800 C with a heating rate of 5 l minute1. The photographs of the magnetic separation were

obtained using Canon IXUS125 HS. The identification of crystalline phase was performed usinga Rigaku D/max-B X-ray diffractometer with monochromatized Cu-K radiation over a 2 rangeof 2070 at a scanning rate of 7 minute1, Raman spectroscopy was used to analyze the chemical

composition and crystallographic structure of the nanoparticles.

2.3. Preparation of Oleic AcidModified Fe3O4 Magnetic Fluid

The oleic acidmodified Fe3O4 nanoparticles were prepared according to our previous work

(Laurenti et al. 2011). In brief, 1.35 g of FeCl36H2O and 7.2 g sodium acetate were dissolvedin 40 ml of ethylene glycol under nitrogen gas protection with vigorous stirring at 160 C for

1 hour. Thereafter the mixture was transferred into a 50-ml reaction flask and heated to 200 C

for 10 hours. After cooling the mixture to room temperature, the suspension was washed three

times with ethyl alcohol. Subsequently, the products obtained were dried under vacuum at 60 C

and finely ground in a mortar. Finally, 4 ml oleic acid was added to 0.1 g of Fe3O4 particles and

the mixture was continuously stirred for 1 hour to obtain the magnetic fluid.

2.4. Preparation of Fe3O4@(StDVB) Magnetic Porous Microspheres

The preparation of Fe3O4@(StDVB) magnetic porous microsphere was followed according to a

previously described procedure (Li et al. 2006) with a few modifications. The microspheres weremainly used to adsorb phenol from aqueous solution. In order to achieve the optimal morphology

and performance of the microspheres, the dosage of the porogen and surfactant was varied. Table 1

shows the characterization data of Fe3O4@StDVB microspheres. In this work, Sample 3 was

Magnetic Porous Microspheres for DCP Adsorption 643


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used. St, DVB, AIBN, toluene and cyclohexanol were mixed with oleic acidmodified Fe3O4magnetic fluid to form the organic phase. Pre-specified concentrations of sodium chloride and

HEC were dissolved in 80 ml of deionized water to form the water phase. After complete stirring,

the two phases were mixed together by ultrasonic treatment (t = 1530 minutes) in order to forman emulsion. This emulsion was then transferred into a 250-ml three-necked flask equipped with

a condenser, a N2 inlet and stirred for 1 hour under nitrogen gas protection. Thereafter, the

temperature was increased to 70 C and the reaction was carried out with a stirring rate of 800 rpm

for 24 hours. Finally, the brown magnetic emulsion was obtained and the magnetic porous

microspheres were separated by an additional magnetic field.

The obtained polymer microspheres were filtrated and washed with deionized water, ethanol

and acetone to remove water-soluble impurities, followed by extraction with methylene dichloride

in a Soxhlet extractor to remove the linear polystyrene porogen and organic impurities. The

purified beads were washed three times with deionized water and methanol and dried under

vacuum for 24 hours (Li et al. 2006; Tai et al. 2011).

2.5. Effect of Solution pH on 2,4-DCP and 2,6-DCP Adsorption

Fe3O4@(StDVB) microspheres (10 mg) were added to eight centrifuge tubes containing 10 ml of

100 mg l1 2, 4-DCP or 2, 6-DCP solution. The solution pH was adjusted to 2, 3, 4, 5, 6, 7, 8, 9 by

adding 1 mol l1 HCl or NH3H2O solutions. After stewing in a thermostatic water bath for 12 hours

at 298 K, the supernatant fluid was obtained by centrifuging the solution at high speeds. Ultraviolet

spectroscopy was used to analyze the remnant concentrations of 2, 4-DCP and 2, 6-DCP.

2.6. Equilibrium Studies

Equilibrium experiments were carried out by treating 5 mg Fe3O4@(StDVB) microspheres with

10 ml of 2, 4-DCP (pH = 2) or 2, 6-DCP (pH = 2) solution of different initial concentrations

(10, 30, 50, 80, 100, 150, 200, 250, 300 and 400 mg l1). Several samples of the mixture were

added to 10-ml centrifuge tubes and stewed in a thermostatic water bath for 12 hours at 298 K.

The equilibrium adsorption capacity of (Qe, mg g_1) was calculated according to equation (1)

(1)

where C0 (mg l1) is the initial concentration and Ce (mg l

1) is the equilibrium concentration of

the adsorbate (2,4-DCP or 2,6-DCP); V (l) is the solution volume and W (g) is the weight of the

adsorbent.

Q(c c )V

W

e

0 e=


TABLE 1. Characterization Data of Poly(StyreneDivinylbenzene)@Fe3O4 Microspheres

Organic phase Water phase

NaCl Deionized AIBN Cyclohexanol

(m mol) HEC (g) water (ml) St (ml) DVB (ml) (m mol) Toluene (ml) (ml)

Sample 1 25.67 0.9 80 5 7.5 1.52 4.165 8.33

Sample 2 17.11 0.6 80 5 7.5 1.52 4.165 8.33

Sample 3 25.67 0.9 80 5 7.5 1.52 4.165 6.25

Sample 4 17.11 0.6 80 5 7.5 1.52 4.165 6.25


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2.7. Adsorption Kinetics

Adsorption kinetics experiment was studied over a reaction time of 10720 minutes. Approximately

5 mg of Fe3O4@(StDVB) microspheres were added into various centrifuge tubes containing 10 ml

of 2, 4-DCP or 2, 6-DCP solution (solution pH = 2 for both 2, 4-DCP and 2, 6-DCP; concentration= 100 mg l1). The solutions were stewed in a thermostatic water bath and the reaction time was

varied from 10 to 720 minutes. The mixtures were centrifuged and using ultraviolet spectroscopy

the residual concentrations of 2, 4-DCP and 2, 6-DCP were analyzed.

2.8. Desorption and Regeneration

Desorption and regeneration experiments of the adsorbent were carried out to verify the

regeneration capability of the magnetic porous microsphere. Desorption of the adsorbed 2, 4-DCP

and 2, 6-DCP from Fe3O4@(StDVB) was also studied by batch experiment, using acetic acid and

methyl alcohol solution as eluent. The Fe3O4@(StDVB)-adsorbed 2, 4-DCP and 2, 6-DCP wereplaced in the eluent and the remnant concentrations of 2, 4-DCP and 2, 6-DCP were studied using

ultraviolet spectroscopy, after drying under vacuum for 12 hours. The Fe3O4@(StDVB)

microspheres[CE1] were once again added to 100 mg l1 2, 4-DCP (pH = 2) and 2, 6-DCP (pH = 2)

solutions. The mixture was allowed to react for 12 hours. This adsorptiondesorption cycle was

repeated four times in order to evaluate the regeneration performance of the microspheres.

3. RESULTS AND DISCUSSION

3.1. Characterization of Fe3O4@(StDVB)

FTIR spectra of the porous polymer microsphere StDVB and magnetic porous polymer

microsphere Fe3O4@(StDVB) were measured and are shown in Figures 1(a and b), respectively.


100

80

60

40 3029

1610

1489

1453

755599

694

3000 2500 2000 1500 1000 500

3065

3090

Wave number (cm1)

Transmittance(%)

(a)

(b)

Figure 1. FTIR spectra of the (a) porous microsphere St-DVB and (b) magnetic porous microsphere Fe3O4@(St-DVB).


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The main functional groups of the predicted structure can be observed with the corresponding

infrared absorption peaks (Pan et al. 2011). In Figure 1(b), compared with StDVB particles,

Fe3O4@(StDVB) microspheres showed new absorption bonds at 599 cm1 corresponding to the

FeO groups. The absorption peaks at 3090, 3065 and 3029 cm1 were attributed to the stretching

vibration of the CH bond of the benzene ring in polystyrene (Li et al. 2006), whereasthe absorption peaks at 1453 and 1489 cm1 were attributed to the skeletal vibration of the benzene

ring. Asharp peak was observed at 694 and 755 cm1, which indicates the presence of polystyrene

in the polymer (Torres et al. 2012). In addition, the strong peaks at 1610 cm1 were assigned to

the C = C groups of DVB monomer (Tai et al. 2011).

Raman spectrum of the Fe3O4@(StDVB) composites was shown in Figure 2. There were two

peaks at 621 and 642 cm1 in the wavelength range of 200800 cm1, and these peaks could prove

the existence of Fe3O4.


200

0

10

20

30

40

300 400 500 600 700 800

Wave number/cm1

Intersity

621

642

Figure 2. Raman spectrum of Fe3O4@(StDVB) magnetic porous polymer microspheres.

Figure 3 shows the X-ray diffraction (XRD) patterns of Fe3O4 and Fe3O4@(StDVB)

microspheres. In the 2 range of 2565, six characteristic peaks corresponding to maghemite(2 = 30.00, 35.45, 42.98, 53.41, 57.03 and 62.55) were observed in the Fe3O4 XRDpattern, and the peak positions could be indexed to (2, 2, 0), (3, 1, 1), (4, 0, 0), (4, 2, 2), (5, 1,

1) and (4, 4, 0) (Zhu et al. 2010). Moreover, the corresponding peaks also appeared in the

pattern suggestive of Fe3O4@(StDVB), but presented in slightly lower wave numbers than

those of Fe3O4. The results showed that Fe3O4 was introduced into StDVB without changing

its crystal structure.

The detailed morphologies of Fe3O4@(StDVB) microspheres and regenerated microspheres

were obtained using scanning electron microscope and the images are shown in Figure 4. As can

be seen from Figure 4(a), the microspheres were spherical with a rough surface, with particle-size

distribution value of approximately 20 m. Figure 4(b) shows that there are many pores on thesurface of the magnetic composite microspheres. Furthermore, the spectral image in Figure 4(c)

further confirms the presence of magnetic materials in the microspheres. Figure 4(d) shows the

morphologies of the microspheres after four repeated uses. The surface of the microspheres


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appeared peeled owing to the effect of eluent acidity, but as can be seen from Figure 4(e), their

surface still has many cavities, ensuring that the microspheres still have good adsorption capacity

even after recycling several times.

The TGA and differential thermal analysis (DTA) curves were obtained to determine the

thermal stability and the magnetic content of the Fe3O4@(StDVB) polymer microspheres. As

can be seen from Figure 5(a), in the temperature range of approximately 350460 C, the TGA

curve exhibited one weight-loss step, which was accompanied with an endothermic peak at

approximately 430 C in the DTA curve due to the decomposition of organic polymer. Moreover,when the temperature reached 480 C, there was an exothermic peak in the DTA curve, which

can be ascribed to the rapid decomposition of the polymer. At temperatures above 550 C, no

weight loss was noted, and therefore, it was very easy to determine the content of magnetic


6555453525

2(degree)

Intensity

(220)

30.00

(311)

35.45

(400)

42.98

(422)

53.41

(511)

57.03

(440)

62.55

(b)

(a)

Figure 3. XRD patterns of (a) Fe3O4 and (b) Fe3O4@(StDVB) magnetic porous polymer microspheres.

100 nm

ba

c

FeFe

Fe

e

d

1 m1 m

1 m0

020406080

100120

1 2 3 4 5 6 7 8

Figure 4. Scanning electron microscopic images of (a) Fe3O4@(StDVB) microspheres, (b) surface detail, (c) energy

spectrum diagram, (d) after adsorption of microspheres and (e) surface detail.


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materials in the polymer. From Figure 5(a), it can be seen that the magnetic content incorporated

in magnetic polymer composites was 5.37%. Figure 5(b) shows the percentage of polymer

composition elements, with carbon and hydrogen in the polymer being 86.66% and 7.94%,

respectively.

The magnetic hysteresis loop of Fe3O4@(StDVB) polymer microspheres was recorded by VSMat room temperature and is shown in Figure 6(a). From the curve, it can be observed that the


Temperature (C)

TG

(%)

0

0

10

20

30

40

50

60

70

80

90

100

110

100 200 300 400 500 600 700 800 900

DTA(mW/mg)

10

8

6

4

2

5.402835

7.941285

86.65588

Carbon Hydrogen Magnetic material

0

(a)

(b)

Figure 5. The TG/DTA curve of (a) Fe3O4@(StDVB) microspheres and (b) the microspheres element analysis.

Magnetic field (kOe)

100002000

1500

1000

500

0

500

1000

1500

5000 0 5000

2 3 4 5 6 7

0.004

0.002

0.000

0.002

0.004

10000

Magnetization(e

mu/g)

Initial pH

Magneticconcentration(mg/l)

(a)(b)

(c)

Figure 6. (a) Magnetization curve of Fe3O4@(StDVB) microspheres, (b) photographs of microspheres suspended in the

presence of an externally placed magnet and (c) curve of magnetic leakage.


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3.3. Adsorption Isotherm

Adsorption isotherm can well describe the adsorption process and adsorption capacity of the

adsorbent at a constant temperature. Several equations have been adopted to describe the

experimental data of adsorption isotherms, which indicate when the adsorption process reaches anequilibrium state (Li et al. 2010). The equilibrium data of adsorption of 2, 4-DCP and 2, 6-DCP

were fitted to the Langmuir and Freundlich isotherm equation. The applicability of the isotherm

models to the adsorption behaviours was studied by analyzing the correlation coefficient (R2). The

Langmuir equation can be expressed as linear and non-linear forms by equations (2) and (3),

respectively.

(2)

(3)

where Qm (mg g1) is the maximum adsorption capacity; Ce is the equilibrium concentration of

adsorbate (mg l1); Qe is the equilibrium adsorption capacity (mg g1) and KL represents the

affinity constant.

For predicting the favourability of an adsorption system, the Langmuir equation can also be

expressed in terms of a dimensionless separation factor RL [equation (4)], which is defined as

follows (Zhang et al. 2009):

(4)

where Cm is the maximal initial concentration of the adsorption solution. The RL indicates the

favourability and the capacity of adsorption system. When 0 < RL < 1, a good adsorption

performance is indicated.

The Freundlich isotherm equation can be expressed as linear and non-linear forms by equations

(5) and (6), respectively.

(5)

(6)

where KF (mg g1) and n are the adsorption constants that indicate the adsorption capacity. A value

of 1/n ranging from 0.1 to 1 represents a favourable adsorption condition.

Table 2 lists the adsorption isotherm constants at room temperatures. Furthermore, a

comparison of the Langmuir and Freundlich isotherm models for 2, 4-DCP and 2, 6-DCP

adsorption onto Fe3O4@(StDVB) microspheres was illustrated in Figure 8. From Table.2, it isobvious that the correlation coefficient that fitted to the Langmuir isotherm is higher than that of

the Freundlich isotherm model. From Figure 8 it can be seen that with the increase in the solution

concentration, the equilibrium adsorption capacity (Qe) for 2, 4-DCP and 2, 6-DCP increased

Q K Cee F

1/n=

lnQ lnK1

nlnC

e F e= +

RL=+

1

1 C Km L

QK Q C

1 K Ce

L m e

L e

=+

C

Q

1

Q K

C

Q

e

e m L

e

m

= +



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sharply at first, and then increased slightly, and finally reached to maximum point. The

experimental data were fitted to the Langmuir and Freundlich isotherm equations. Figure 8 shows

that the Langmuir isotherm model fitted the equilibrium data much better than the Freundlich

model. In addition, Figure 8 also revealed that equilibrium concentration of 2, 4-DCP is a littlehigher than that of 2, 6-DCP.

3.4. Adsorption Kinetics

The kinetic behaviour of the adsorbents was examined by performing a series of experiments with

different contact time; the adsorption kinetic data were analyzed by pseudo-first-order and

pseudo-second-order equations (Anirudhan and Suchithra 2010). The pseudo-first-order equation

can be expressed as linear and non-linear forms by equations (7) and (8), respectively.

(7)

(8)Q Q Q et e ek t1=

ln(Q Q ) lnQ k te t e 1 =


TABLE 2. Adsorption Isotherm Constants for 2, 4-DCP and 2, 6-DCP onto Microspheres at 298 K

Adsorption isotherm models Contents 2, 4-DCP 2, 6-DCP

Langmuir R2 0.997 0.996

equation Qm,c (mg g1) 132.834 128.519

KL (l mg1) 0.153 0.14

RL 0.017 0.017

Freundlich R2 0.702 0.626

equation KF (mg g1) 25.244 27.219

N 2.732 3.025

Note: Sorbent dose = 0.01 g; solution volume = 10.0 ml; agitation speed = 600 rpm; solution pH = 2 for 2, 4-DCP and 2

for 2, 6-DCP and contact time = 12 hours.

180

(a)

160

140

120

100

80

60

40

20

00 50 100

Ce(mg g1)

Qe

(mgg

1)

150 200

Experimental data at 298 K

Langmuir fit

Freundlich fit

180

(b)

160

140

120

100

80

60

40

20

00 50 100

Ce(mg g1)

Qe

(mgg

1)

150 200


Langmuir fit

Freundlich fit

Figure 8. Comparison of Langmuir and Freundlich isotherm models for (a) 2,4-DCP adsorption onto microspheres and

(b) 2,6-DCP adsorption onto microspheres.


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The pseudo-second-order equation can be expressed as linear and non-linear forms by

equations (9) and (10), respectively

(9)

(10)

where Qe and Qt (mg g1) are the amounts of 2, 4-DCP and 2, 6-DCP adsorbed onto

Fe3O4@(StDVB) polymer microspheres at the equilibrium and at time t (minutes), respectively.

K1 (minutes1) is the equilibrium rate constant, the parameter k1 can be obtained by plotting ln

(Qe Qt) versus t. K2 (g mg1 minute1) is the second-order-model rate constant and can be

calculated from the plot of t/Qt versus t (Li et al. 2010).The experiment data were calculated using pseudo-first-order and pseudo-second-order

equations and the results are shown in Figure 9. Compared with pseudo-first-order, pseudo-

second-order equation showed better results for experimental kinetic data with higher values of

Qk Q t

1 k Q tt

2 e

2

2 e

=+

t

Q

1

K Q

t

Qt 2 e2

e

= +


140(a)

120

100

80

60

40

0 100 200 300 400 500 600 700

t (min)

Qt(mgg

1)


Pseudo-first-order rate model

Pseudo-second-order rate model


Pseudo-first-order rate model

Pseudo-second-order rate model

140(b)

120

100

80

60

40

0 100 200 300 400 500 600 700

t (min)

Qt(mgg

1)

Figure 9. Comparison of pseudo-first-order and pseudo-second-order kinetic models for (a) 2,4-DCP adsorption onto

microspheres and (b) 2,6-DCP adsorption onto microspheres.

correlation coefficient (R2 > 0.99), which indicated that the adsorption of 2, 4-DCP and 2, 6-DCP

onto Fe3O4@(StDVB) polymer microspheres followed second-order kinetics. Figure 9 also

shows that the time required to achieve adsorption equilibrium was approximately 100 and 70

minutes for 2, 4-DCP and 2, 6-DCP, respectively, which concludes that 2, 6-DCP could reach the

balance status faster than 2, 4-DCP.

Table 3 summarizes the comparison of the adsorption of 2,4-DCP onto different adsorbents

reported in the literature (Li et al. 2009a; Sathishkumar et al. 2009; Pan et al. 2011). Obviously,

although the equilibrium time was slightly longer than that of the other adsorbents, the adsorption

capacity of Fe3O4@(StDVB) microspheres was prominent. The magnetic porous polymermicrospheres not only had excellent adsorption performance, but can also be separated by

efficiently applying an external magnetic field. The microspheres have broad application in

wastewater treatment.


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3.5. Adsorption Thermodynamics

Thermodynamic parameters such as change in Gibbs free energy (G), enthalpy (H) andentropy (S) were calculated using equations (11) and (12) (Li et al. 2009b)

(11)

(12)

where R is the gas constant (8.314J mol1 K1) and T is the absolute temperature (K). G is Gibbsfree energy, S and H represent entropy and enthalpy, respectively. The S and H valuesare obtained from the slope and intercept of the line plotted by ln(Qe/Ce) versus 1/t, respectively.

A G value < 0 represents that the adsorption process is spontaneous, whereas H > 0represents that the process is endothermic.

Table 4 lists the obtained thermodynamic parameters. A negative G shows that themicrosphere adsorption of 2, 4-DCP and 2, 6-DCP was spontaneous. As 2, 4-DCP and 2, 6-DCP

were adsorbed onto the surface of microspheres, the number of water molecules in the solution

decreased, but the degrees of freedom of the water molecule increased (Mellah et al. 2006). The

positive values of entropy change (S) suggested increased randomness at the solidsolutioninterface during the adsorption 2, 4-DCP and 2, 6-DCP onto the surface of the microspheres (Pan

et al. 2010). The positive value of enthalpy change (H) showed that the adsorption process wasendothermic. As the temperature was increased, the adsorption of 2, 4-DCP and 2, 6-DCP onto

Fe3O4@(StDVB) microspheres gradually enhanced.

G H T= S

ln QC

SR

HRT

e

e

=


TABLE 3. Comparison of the Adsorption Properties for 2, 4-DCP on Different Adsorbents

Adsorbent Qmax(mg g1) T (minutes) Reference

-Cyclodextrin/attapulgite composites 23 400 [26]Maize cob carbon composites 7 40 [31]

Non-covalent imprinted microspheres 80 50 [32]

Fe3O4@(StDVB) magnetic porous microspheres 125 150 This work

TABLE 4. Thermodynamic Parameters for the Adsorption of 2, 4-DCP and 2, 6-DCP onto Microspheres

Thermodynamic parameters

phenol Temperature(K) G (kJ mol1) H (kJ mol1) S (J mol1 k1)

2, 4-DCP 298 19.55 23.28 65.68308 20.21318 20.93

2, 6-DCP 298 18.066 19.95 60.69308 18.673318 19.28

Note: Sorbent dose = 0.01 g; solution volume = 10.0 ml; initial concentration = 50 mg/l; solution pH = 2 for 2, 4-DCP

and 2 for 2, 6-DCP; contact time = 12 hours; temperature = 298 K, 308 K, 318 K.


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3.6. Regenerability

Regenerability is a key factor for an effective absorbent. The adsorption and desorption

experiments were repeated four times (sequentially) in order to verify the regenerability

performance of the magnetic porous microspheres. Both acetic acid and sodium hydroxide wereused as desorption agents, but acetic acid showed better desorption effect than sodium

hydroxide. The adsorption and desorption capacities of Fe3O4@(StDVB) polymer

microspheres are shown in Figure 10. As can been seen from the figure, the adsorption capacity

of the microsphere decreased after four cycles. In the previous two cycles, the adsorption of

phenol could almost be completely cleared. The rate of adsorption loss was calculated using

equation (13) as follows:


50

(a)

(b)

40

30

20

10

0

0 1 2

Recycle numbers

Qe

(mgg

1)

3 4

50

40

30

20

10

0

0 1 2

Recycle numbers

Qe

(mgg

1)

3 4

Adsorption

Desorption

Adsorption

Desorption

Figure 10. Adsorption and desorption experiments at 298 K to evaluate the regeneration capacity of the adsorbent. Acetic

acid: methanol (vol: vol) = 1: 49 as the eluent; sorbent dose = 0.01 g; solution volume = 5 ml and contact time = 12 hours.


15/16

(13)

where CF is the adsorption capacity of the first time, and CL is the adsorption capacity of the last

time. The adsorption capacity of microspheres for 2, 4-DCP was about 15.6% loss and 20.06%loss for 2, 6-DCP, which confirm that the microspheres had favourable adsorption capacity even

using them repeatedly.

4. CONCLUSIONS

In this study, Fe3O4@StDVB magnetic porous microspheres were successfully synthesized based

on suspension polymerization, and then the novel adsorbent Fe3O4@(StDVB) efficiently

adsorbed 2, 4-DCP and 2, 6-DCP from solution systems. The resulting Fe3

O4

@(StDVB)

microspheres had a spherical structure, porous and saturation magnetization, and all these

attributes are suggestive of improved adsorption performance. A series of adsorption experiments

showed that the adsorption capacity of the Fe3O4@(StDVB) microspheres for phenols was

largely influenced by the pH value, the initial concentration of the solution and temperature.

Between pH 2 and 9, the adsorption capacity of 2, 4-DCP is higher than that of 2, 6-DCP. The

kinetic data fitted well with pseudo-second-order kinetic model than pseudo-first-order kinetic

model, and 2, 6-DCP achieved balance status faster than 2, 4-DCP. The adsorption isotherms were

also described by Langmuir and Freundlich isotherm models, respectively. It was found that the

Langmuir isotherm model fitted the equilibrium data much better than the Freundlich model, and

the equilibrium adsorption capacity of 2, 4-DCP is higher than 2, 6-DCP. The Gibbs free energyfunction showed that the adsorption process was spontaneous and endothermic. Furthermore,

desorption experiment indicated that Fe3O4@(StDVB) microspheres were excellent reusable

adsorbents. Therefore, there is a broad application prospect for these microspheres in the phonetic

wastewater treatment.

ACKNOWLEDGEMENTS

This work was financially supported by grants from the National Natural Science Foundation of

China (Grant Nos. 21077046, 21107037, 21176107, 21174057 and 21004031) and NaturalScience Foundation of Jiangsu Province (Grant Nos. BK2011461, SBK2011459 and

BK2011514).

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