Synthesis and characterization of mesoporous carbon nanofibers and its adsorption for dye in wastewater

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Contents lists available at ScienceDirect

Advanced Powder Technology journal homepage: www.elsevier.com/locate/apt

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Original Research Paper

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Synthesis and characterization of mesoporous carbon nanofibers and its adsorption for dye in wastewater

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Shengbiao Li 1, Zhigang Jia ⇑, Ziyu Li, Yanhua Li, Rongsun Zhu

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School of Chemistry and Chemical Engineering, Anhui University of Technology, No. 59 Hudong Road, Ma’anshan 243002, Anhui Province, PR China

11 10 12 1 2 4 6 15 16 17 18 19 20 21 22 23 24 25

a r t i c l e

i n f o

Article history: Received 7 May 2015 Received in revised form 2 January 2016 Accepted 25 January 2016 Available online xxxx Keywords: Mesoporous carbon nanofibers Calcination Hydrothermal method Adsorption

a b s t r a c t Mesoporous carbon nanofibers (MCNFs) were prepared using ferric-nitrilotriacetate (Fe-NTA) nanofibers as the precursors via annealing under enclosed condition at 500 °C. The precursors were synthesized by hydrothermal method with FeSO4
7H2O and nitrilotriacetic acid as the raw materials. The obtained mesoporous carbon nanofiber materials were characterized by means of X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), scanning electron microscopy (SEM), transmission electron microscope (TEM) and nitrogen adsorption–desorption measurement. Characterization results indicated that the mesoporous carbon nanofiber samples with long fibrous morphology, high surface area and mesoporous structure. Due to the porous structure, the obtained MCNFs exhibited excellent performance for the adsorption of methylene blue (MB) and methyl orange (MO) from aqueous solution. The adsorption kinetics and isotherms of the adsorbent were investigated in detail. The experimental equilibrium data of MB and MO were well fitted to the Langmuir isotherm model. The adsorption kinetic data of both dyes were found to follow the pseudo-second order kinetic. The calculated thermodynamic parameters namely Gibbs free energy (DGo), enthalpy (DHo) and entropy (DSo) showed that the adsorption process of MB and MO onto MCNFs was spontaneous and endothermic in nature. Ó 2016 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.

27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44

45 46

1. Introduction

47

Currently, with increasing population, industrialization level and agricultural activities, large amount of freshwater is consumed and generates substantial amount of waste water, and the waste water situation of world will become more severe in the future [1–3]. These wastewaters are highly colored and the main reason is related to various dyes [4–7]. It is well known that organic dyes are toxic and carcinogenic even at relatively low concentrations. So the polluted water is a serious threat to human beings and animals [8–10]. Hence, it is necessary to develop a kind of environmentfriendly technology to prevent further dyes contamination. Various techniques, such as membrane separation [11], extraction [12], adsorption [13], coagulation [14], photodecomposition [15] and electrochemical oxidation [16], have been used for the treatment of these dyes. Among these methods, adsorption is a promising technique for removing dyes from waste water due to its relatively simplicity, efficiency and environmental friendliness [17,18].

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⇑ Corresponding author. 1

A great number of mesoporous carbon nanofibers have attracted considerable attention, which can be widely used for treatment of waste water due to their relatively low-cost with respect to other adsorbents, wide availability and high performance in adsorption processes [19,20]. Mesoporous carbon nanofibers show excellent adsorption performances due to their high specific surface area, ordered pore structure and large pore volumes [21–23]. These mesoporous carbon nanofibers are mostly prepared by electrospinning technology [24,25]. However, experimental condition and special device limit its wide application. Moreover, mesoporous carbon nanofiber structure and adsorption properties are sensitive to their preparation conditions. Therefore, more endeavors need to be devoted to fabricate mesoporous carbon nanofiber. In this work, we synthesized MCNFs with large surface area and mesoporous structure by calcining ferric-nitrilotriacetate nanofiber precursor, which was prepared by solvothermal method. The adsorption properties in terms of adsorption capacity of MCNFs for MB and MO removal were studied. Adsorption kinetics, isotherms and thermodynamic studies for the adsorption of MB and MO were investigated in detail.

E-mail addresses: [email protected] (S. Li), [email protected] (Z. Jia). Tel.: +86 555 2311807; fax: +86 555 2311551.

http://dx.doi.org/10.1016/j.apt.2016.01.024 0921-8831/Ó 2016 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.

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2. Experimental

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2.1. Preparation of mesoporous carbon nanofibers

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All the chemicals used in this study were of analytical grade and used as received without further purification. Distilled water was prepared in the lab by a water treatment device. In a typical experimental procedure, 3 mmol of FeSO4
7H2O was dissolved in the mixed solvents which composed of 50 mL distilled water and 15 mL isopropyl alcohol with stirring at room temperature. After FeSO4
7H2O was dissolved fully, 9 mmol nitrilotriacetic acid was added to the above mixed solution. Then the above composition was transferred into an 80 mL-capacity teflon-lined stainlesssteel autoclave and hydrothermally treated at 180 °C for 24 h. After the reaction was completed, the autoclave was then allowed to cool naturally to room temperature. The resulting precipitates collected by suction filtration, washed with distilled water and absolute ethanol thoroughly and then vacuum-dried at 40 °C. In the next step, the white as-prepared precursors were annealed in enclosed condition at 500 °C for 2 h at the heating rate of 5 °C min1. The obtained carbon nanofibers (CNFs) were cooled in vacuum condition and soaked by hydrochloric acid. Finally, the obtained mesoporous carbon nanofiber samples were washed by distilled water several times and dried at 80 °C in air.

88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106

107

2.2. Characterization of sample

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117

X-ray diffraction (XRD) patterns of all samples were recorded on a Bruker D8 Focus diffractometer using Cu Ka radiation and operated at 40 kV and 40 mA. The Fourier transformed infrared (FT-IR) adsorption spectra were collected using a Thermo Nicolet 6700 in the range of 4000–280 cm1. Scanning electron microscope (SEM) images and transmission electron microscope (TEM) images were obtained on a JEOL-2010 and JSM-6360LV, respectively. The Brunauer–Emmett–Teller (BET) surface area and pore size distribution were measured with an accelerated surface area and porosimetry system (ASAP 2010).

118

2.3. Adsorption experiments

109 110 111 112 113 114 115 116

119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140

The adsorption of dyes from an aqueous solution was frequently used to test the adsorption capability of different adsorbents [26,27]. Methylene blue and methyl orange, have also been used as model pollutants to determine the adsorption capacity of the adsorbents [28–31]. Therefore, the aim of this study was the investigation of the adsorption capability of MCNFs with respect to MB and MO. In the isotherm experiments, 10 mg adsorbents and 20 mL of MB and MO solution of different initial concentrations were transferred in a series conical flasks of 250 mL, the dispersion were carried out at natural pH (ca. 7). The conical flasks were then sealed and placed in a Air Bath Thermostatic Rotary Shaker (HZQ-F, Harbin Donglian Electronic & Technology Development Co., Ltd.) at various temperatures (298 K, 308 K, and 318 K), shaking at 120 rpm (at natural pH). After adsorption was completed, the flasks were then removed from thermostatic bath and the final concentrations of MB and MO in the above solution were measured at maximum absorption wavelengths of MB (667 nm) and MO (462 nm) by 721 spectrophotometer (Shanghai Precision & Scientific Instrument Co., Ltd., China). The amount of MB and MO at adsorption equilibrium qe (mg g1) on the adsorbent samples were calculated using the following equation:

141 143

qe ¼

ðC o  C e ÞV W

ð1Þ

where Co and Ce are the initial and equilibrium concentrations (mg L1) of the dye, respectively. V is the volume of the solution (L) used for adsorption experiments and W is the mass of adsorbent used (g). The adsorption kinetic experiments were conducted via adding 10 mg adsorbent to MB solution (40 mg L1) and MO solution (40 mg L1), respectively, at room temperature. The above aqueous samples were taken to pre-set time intervals and the concentrations of MB and MO were similarly measured. The amount of adsorption at time t, qt (mg g1), were calculated by the following equation:

ðC o  C t ÞV qt ¼ W

144 145 146 147 148 149 150 151 152 153 154

155

ð2Þ

where Co and Ct represent the liquid phase concentrations (mg L1) of MB and MO at initial and anytime t, respectively, V and W represent the volume of the solution (L) and the mass of adsorbent used (g), respectively.

157 158 159 160 161

3. Results and discussion

162

3.1. Characterization of the MCNFs

163

The phase and composition of the as-obtained products were characterized by XRD. Fig. 1a represents a typical XRD pattern of the precursors calcined at 500 °C in enclosed condition for 2 h at the heating rate of 5 °C min1. The main peaks at 2h = 30°, 35.9°, 43.5°, 57.8° and 63.2° corresponding to the (2 0 6), (4 1 6), (1 6 0), (1 6 4) and (2 0 1), respectively, can be assigned to the spinel structure of Fe3O4. However, as shown in Fig. 1b, after soaked by hydrochloric acid solution, the typical diffraction peaks of the calcined samples were disappeared, which may be due to the Fe3O4 has been removed completely and the pure carbon materials can be obtained. The FT-IR spectra of Fe-NTA nanofiber precursors, CNFs and MCNFs are shown in Fig. 2. The FT-IR spectrum of Fe-NTA nanofiber precursor is show in Fig. 2a. The band at 3469 cm1 associated with the stretching vibration of hydrogen-bonded surface water molecules and hydroxyl groups. The bands in the range of 1729– 1643, 1580 and 1413 cm1, correspond to the presence of C@O stretching vibration, OAH in-plane and CAO stretching vibration of carboxyl group, respectively. The appearance of the peaks at 583 and 684 cm1 can be attributed to the FeAO stretching vibrations of Fe3O4 (Fig. 2b). However, as shown in Fig. 2c, the peaks at 583 and 684 cm1 disappear after being soaked by hydrochloric acid solution suggest the iron oxides are removed completely, and the result in correspondence with XRD. The micrographs of Fe-NTA nanofiber precursors, CNFs and MCNFs were characterized by scanning electron microscope (SEM) and the results were shown in Fig. 3. As shown in Fig. 3a, the Fe-NTA nanofiber precursors present uniform and long fibrous morphology with the diameter in the range of 150–200 nm. In Fig. 3b, the calcined nanofibers still can keep the same morphology with the Fe-NTA nanofiber precursors. In Comparison with the FeNTA nanofiber precursors (Fig. 3a), the MCNFs well maintain the nanofibrous morphology and no obvious breakage or scattered particles can be observed, as shown in Fig. 3c. That is to say, the architecture and morphology of the precursors could be wellpreserved in a series of chemical reactions. To get more information about the porous structures, the obtained MCNFs were further investigated by TEM. Fig. 3d reveals the surface of fibers is quite rough and some porous can be observed, due to the removing of iron oxide. The average size of the porous was about 4 nm and the diameter was about 200 nm in accordance with the SEM results.

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492

0.5

416 184

b 206

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160

201

a

Transmission

Intensity (a.u.)

339

a

1729

1.0

1413 1580

1.5

1385

3469

1643

b 3421

1620

634

2.0

c

583 2.5 3.0

1385 1434

3429

1630 1585

3.5 10

20

30

40

50

60

70

80

2 Theta (degree) Fig. 1. XRD patterns of the (a) CNFs and (b) MCNFs.

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The typical nitrogen adsorption–desorption isotherms and the pore size distributions of MCNFs are shown in Fig. 4. As shown in Fig. 4a, typical IV isotherms with obvious hysteresis loops at relative pressure (P/Po) between 0.4 and 1.0 were observed for MCNFs, indicating its mesoporous feature. The BET specific surface areas of the fibers were calculated to be 392.3 m2 g1. The pore size distribution (Fig. 4b) was obtained from the adsorption branch of the isotherms by the BJH method which further confirms a mesoporous structure (3–5 nm in size). The formation of the nanoporous structures are attributed to the removal of Fe3O4. The higher specific surface area and large pore volume is crucial for its high adsorption performance.

1000

2000

3000

4000

Wavenumber (cm-1) Fig. 2. FT-IR spectra for (a) Fe-NTA precursors, (b) CNFs and (c) MCNFs.

3.2. The adsorption comparison of CNFs and MCNFs for dyes

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The advantage of the higher specific surface area and large pore volume is clearly demonstrated by the adsorption capacity comparison of CNFs and MCNFs for MO and MB, respectively. As shown in Fig. 5, the adsorption capacity of CNFs and MCNFs for MO is 34.025 mg g1 and 50.0689 mg g1, respectively. The adsorption capacity of CNFs and MCNFs for MB is 35.578 mg g1 and 58.992 mg g1, respectively. Obviously, the adsorption capacity of MCNFs for MO and MB is higher than that of CNFs. We also notice that the adsorption capacity of MCNFs for MB is higher than that for MO. This difference can be ascribed to the electrostatic attraction between dye molecular and adsorbent. The carboxyl group on the surface of MCNFs negatively charged in solution and is more

219

Fig. 3. SEM images of (a) Fe-NTA precursors, (b) CNFs, (c) MCNFs and (d) TEM micrograph of MCNFs.

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600

3.5

(a)

(b)

3.0

Pore volume (dV/dD)

500

2

Volume Absorbed (cm /g STP)

4

400 300 200 100

2.5 2.0 1.5 1.0 0.5 0.0

0 0.0

0.2

0.4

0.6

0.8

1.0

0

10

20

30

40

50

60

70

80

Pore Diameter (nm)

p/p0

Fig. 4. N2 adsorption–desorption isotherms of (a) MCNFs, and the picture (b) is pore size distribution curve.

55 60

(a)

50

(b)

45

50

q t (mg g -1)

q t (mg g -1)

40 40 30

35 30 25 20

20 MCFs CFs

10 0

20

40

60

80

100

15

MCFs CFs

10 0

120

20

40

t (min)

60

80

100

120

t (min)

Fig. 5. Effect of contact time for the adsorption capacity of MO (a) and MB (b) onto CNFs and MCNFs at room temperature (25 °C) and pH value at 7.0.

232

attractive for the cation dye than anionic dye, which result in the higher adsorption capacity of MCNFs for MB than that for MO.

233

3.3. Adsorption kinetics

234

The pH of the aqueous solution is an important parameter in adsorption process, because it can affect the surface charge of the adsorbent and the functional groups of the adsorbate. The adsorption capability of the MB and MO dyes was studied in the range of pH values from 2.5 to 11.0. As shown in Fig. 6, the adsorption amount of MB onto MCNFs enhanced with the increasing of pH value, and there is on obvious enhancement when the pH value is higher than 7. For methyl orange, the adsorption capacity is higher in the pH range of 4–8 than other pH condition. In order to investigate the adsorption mechanism of MB and MO onto MCNFs, the adsorption process of the MB and MO onto MCNFs was studied as a function of contact time. As shown in Fig. 7a, the adsorption for two dyes is very fast during the first 20 min. However, with increasing adsorption time, the amount of MB and MO adsorbed gradually increases. Typically, the adsorption capacity for the two dyes reached equilibrium at 75–85 min and the color of MB (Fig. 7b) and MO (Fig. 7c) became colorless. The fast MB and MO uptake observed during the first 20 min can be attributed to the abundant availability of active porous on the surface of the MCNFs, and with the gradual occupancy of these porous the adsorption rate become low. The adsorption kinetics of the dyes were investigated using the pseudo-first order, pseudo-second order and intra-particle diffusion model [32,33]. These models can be expressed as:

231

55 54

236 237 238 239 240 241 242 243 244 245 246 247 248 249 250 251 252 253 254 255 256 257

52

qe ( mg g-1)

235

53 MB MO

51 50 49 48 47 46 45 2

4

6

8

10

12

pH Fig. 6. The influence of pH value for the adsorption capacity for MO and MB onto MCNFs at room temperature (25 °C) and pH value at 7.0.

Pseudo-first order equation:

lnðqe  qt Þ ¼ ln qe  k1 t

258

ð3Þ

Pseudo-second order equation:

t 1 t ¼ þ qt k2 q2e qe

259 261 262

263

ð4Þ

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Table 1 The pseudo-first order, pseudo-second order and intra-particle diffusion kinetic parameters for the adsorption of MB and MO at room temperature.

b

a

qt (mg g-1)

50 40

c 30 MB MO

20 10 0

20

40

60

80

100

120

t (min) Fig. 7. (a) Adsorption of MB and MO at different contact time in room temperature, (b and c) are the photographs of MB and MO solutions before and after the adsorption, respectively.

274

MO

40.00 59.14 41.54 0.04659 0.9923

40.00 50.99 30.02 0.04098 0.9488

Pseudo-second-order Co (mg L1) qe,exp (mg g1) qe,cal (mg g1) K2 (104 g mg1 min1) R2

40.00 59.14 64.93 16.024 0.9978

40.00 50.99 56.18 17.86 0.9967

Intra-particle diffusion Co (mg L1) qe,exp (mg g1) C (mg g1) Ki (mg g1 min1/2) R2

40.00 59.14 21.57 4.064 0.7575

40.00 50.99 17.69 3.603 0.7645

(mg g1) is the intercept of intra-particle diffusion model. The experimental kinetic data for MB and MO adsorption onto MCNFs, calculated from Eqs. (3)–(5) are summarized in Table 1. The corresponding results of the experimental data to the models are shown in Fig. 8a–c. As shown in Table 1, the values of R2 of the pseudo-second order for the two dyes adsorption are higher than

where qe and qt refer to the amounts of dye adsorbed at equilibrium (mg g1) and at time, t (min), k1 is the overall adsorption rate constant of pseudo-first order adsorption (min1), k2 is the rate constant of pseudo-second order adsorption (g mg1 min1), ki is the intra-particle diffusion rate constant (mg g1 min1/2) and C

2.6

(a)

4

(b)

2.4 2.2

3

2.0

2 1 0 MB MO

-1

1.8

t/q t (min g mg-1)

273

ð5Þ

MB

Pseudo-first-order Co (mg L1) qe,exp (mg g1) qe,cal (mg g1) K1 (min1) R2

1.6 1.4 1.2 1.0 0.8

MB MO

0.6 0.4

-2

0.2 0

20

40

60

80

100

120

0

20

40

t (min)

60

80

100

120

t (min)

60

(c)

50

40

t

272

qt ¼ ki t 1=2 þ C

q (mg g-1)

270 271

Intra-particle diffusion equation:

ln (q e -qt)

266

267 269

Adsorption kinetics

30 MB 20

MO

10 2

4

6

8 1/2

t (min

10

12

)

Fig. 8. (a) Pseudo-first order, (b) pseudo-second order and (c) intra-particle diffusion kinetic model plots for the adsorption of MB and MO onto MCNFs.

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pseudo-first and intra-particle diffusion model. It reveals that the pseudo-second order kinetic model better represented the adsorption kinetics and the calculated qe values agree well with the experimental values (Table 1). These results indicate that the adsorption of MB and MO onto MCNFs follows pseudo-second order model for all of the adsorption process.

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3.4. Adsorption isotherms

288

305

The adsorption isotherm studies for the two dyes were carried out at 298, 308 and 318 K, respectively. The qe (mg g1) and Ce (mg L1) denote the amounts of dyes adsorbed at equilibrium and dyes concentration, respectively. As shown in Fig. 9, higher temperature was favorable for the adsorption of MB and MO onto MCNFs, which showed an endothermic adsorption process. It was explained that as temperature increased, the surface activity and kinetic energy of MB and MO also increased which caused the interaction forces between the solute and adsorbent to become stronger than solute and solvent. Adsorption isotherm models are fundamental in revealing the mechanisms of adsorption. The equilibrium isotherm models of the Langmuir adsorption isotherm and the Freundlich isotherm are widely used to describe the interactions between the amount of the adsorbate and adsorbent [34,35]. The linear form of Langmuir and Freundlich isotherm equation can be expressed as follows: Langmuir isotherm:

308

Ce Ce 1 ¼ þ qe qm K L qm

290 291 292 293 294 295 296 297 298 299 300 301 302 303 304

306

ln qe ¼ ln K f þ ð1=nÞ ln C e

ð6Þ

140

ð7Þ

where Ce is the equilibrium concentration of adsorbate (mg L1) and qe is the equilibrium adsorption amount (mg g1), KL is the Langmuir constant (L mg1), qm is the theoretical maximum adsorption capacity (mg g1), Kf is the Freundlich constant (mg11/n L1/n g1) and n is the heterogeneity factor. According to Eq. (6), the linear form of the Langmuir adsorption for the two dyes can be obtained (Fig. 10). The linear form of the Freundlich expression for the dyes (Fig. 11) can be obtained by Eq. (7). The constants of the Freundlich and Langmuir isotherms obtained from the slope and intercept of the plots of each isotherm at different temperatures were summarized in Table 2. By comparing the correlation coefficients R2, it can be deduced that the experimental equilibrium adsorption data are well described by the Langmuir equation compared with Freundlich model, indicating the monolayer coverage of the pore surface of MCNFs by methylene blue and methyl orange molecules. The maximum adsorption capacities of MB are 119.21, 130.33, and 137.25 mg g1 at the adsorption temperature of 298, 308 and 318 K, respectively. For MO adsorption, the maximum adsorption capacities are 101.35, 106.40 and 110.99 mg g1 at the adsorption temperature of 298, 308 and 318 K, respectively. The adsorption capacity of the as-prepared MCNFs and other adsorbents for MB and MO adsorption are compared and listed in Table 3. A comparison with other reported adsorbents (Table 3) shows that the qm value for the MCFs is one of the highest, suggesting that the MCFs is a good candidate for dye removal from aqueous solutions. 120

(a)

(b)

110

120

100 90

100

qe (mg g-1)

289

309

80 60 40

0

20

40

60

80

100

120

140

70 60 50 40

298K 308K 318K

20

80

298K 308K 318K

30 20 160

0

20

40

60

80

100 120 140 160 -1

-1

Ce (mg L )

Ce (mg L )

Fig. 9. Equilibrium adsorption isotherms of (a) MB and (b) MO on the MCNFs at various temperature.

1.2

(a)

1.4

(b)

1.2

1.0 0.8

1.0

-1

285

Freundlich isotherm:

Ce /qe ( g L )

284

qe (mg g-1)

283

-1

282

Ce /qe (g L )

281

0.6 0.4 298K 308K 318K

0.2 0.0

0.8 0.6 0.4 298K 308K 318K

0.2 0.0

0

20

40

60

80

100 120 140 160 -1

Ce (mg L )

0

20

40

60

80

100

120

140

160

-1

Ce (mg L )

Fig. 10. Langmuir isotherm linear plots for the adsorption of (a) MB and (b) MO, respectively.

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Table 3 Comparison of maximum adsorption capacity of MB and MO onto various adsorbents.

(a)

4.8

Adsorbents

Dyes

qm (mg g1)

Reference

4.4

Polyaniline hydrogel

71.2

[37]

4.2

Pyrophyllite

Methylene blue Methylene blue Methylene blue Methylene blue Methyl orange Methyl orange Methyl orange Methyl orange

3.71

[38]

65.81

[39]

119.21 16.83

Present work [40]

35.4–64.7

[41]

87.03

[42]

101.35

Present work

lnqe

4.6

4.0

Acid-treated pyrolytic tire char (PTC) MCFs

3.8 3.6 298K

3.4

Graphene oxide

308K

3.2

Carbon nanotubes

318K

3.0 1.5

2.0

2.5

3.0

3.5

4.0

4.5

Organo-modified silkworm exuviae MCFs

5.0

lnc e

4.8

(b) Table 4 Thermodynamic parameters for the adsorption of MB and MO onto MCNFs.

4.6 4.4

Adsorbate

Temperature (K)

DG o (kJ mol1)

D Ho (kJ mol1)

DSo (kJ mol1 K1)

MB

298 308 318

3.577 4.155 4.724

13.53 – –

57.41 – –

MO

298 308 318

3.207 3.639 4.114

10.28 – –

45.25 – –

lnqe

4.2 4.0 3.8 298K

3.6

308K 3.4

318K

3.2

DGo ¼ RT ln K d 1.5

2.0

2.5

3.0

3.5

4.0

4.5

DGo DSo DHo ln K d ¼  ¼  RT R RT

Fig. 11. Freundlich isotherm linear plots for the adsorption of (a) MB and (b) MO, respectively.

338

3.5. Thermodynamic studies

339

347

The temperature influence is an important factor for the adsorption of MB and MO on MCNFs. In order to investigate whether the dye adsorption process will occur spontaneously, the thermodynamic parameters such as change in standard free energy (DGo), enthalpy (DHo) and entropy (DSo) of the adsorption of MB and MO onto MCNFs are obtained from experiments at three different temperatures from 298 K to 318 K and different initial concentrations of MB and MO solution using the following equations [36]:

350

q Kd ¼ e ce

341 342 343 344 345 346

348

ð8Þ

ð10Þ

Temp (K)

356

where Kd is the equilibrium constant, T is the adsorption temperature (K), R is the gas constant (8.314 J mol1 K1). The values of DHo and DSo were calculated from the slope and intercept, respectively. The calculated values of thermodynamic data of MB and MO adsorption onto MCNFs are given in Table 4. The negative DGo values for the two dyes at all the studied temperatures suggest that the adsorption of MB and MO onto MCNFs were feasible and thermodynamically spontaneous. In Table 4, the positive values of DHo for the two dyes confirm the endothermic nature of dye adsorption. The positive value of DSo reflect an increase in randomness at the solid/solution interface during the adsorption of MB and MO onto MCNFs.

357

4. Conclusion

369

In conclusion, Fe-NTA complex nonofibers were used as precursors for the production of MCNFs by the combination of hydrother-

370

Table 2 Values of Langmuir and Freundlich constants for adsorption of MB and MO onto MCNFs. Adsorbate

351 353 354

5.0

lnc e

340

ð9Þ

Langmuir constants

Freundlich constants

qm (mg g1)

KL (L mg1)

R2

Kf (mg11/n L1/n g1)

n

R2

MB

298 308 318

145.8 156.3 162.1

0.04542 0.05159 0.05966

0.9139 0.9323 0.9426

13.23 15.87 19.27

1.954 2.024 2.151

0.6519 0.6629 0.6531

MO

298 308 318

120.5 121.9 126.6

0.05125 0.06210 0.07258

0.9852 0.9920 0.9904

13.25 16.19 18.52

2.168 2.350 2.423

0.8397 0.8672 0.8762

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S. Li et al. / Advanced Powder Technology xxx (2016) xxx–xxx

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mal and calcination method. The characteristics of MCNFs as well as their adsorption capacities for MB and MO were investigated. The equilibrium, kinetics and thermodynamics of MB and MO adsorption were studied. The kinetic parameters indicate the adsorption process of MB and MO followed pseudo-second order kinetic model. The experimental equilibrium data for MB and MO on prepared MCNFs were both well fitted to the Langmuir isotherm model. Thermodynamics study further confirmed that adsorption process for both dyes were spontaneous and endothermic. The as-obtained MCNFs showed exceptional adsorption capability for the two dyes. It is indicated that the MCNFs obtained in this study has potential to be applied as an efficient absorbent for water purification.

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Acknowledgments

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This work is supported by National Natural Science Foundation of China (20907001), University Natural Science Research Project of Anhui Province (KJ2010A336), Student Research Training Program of AHUT (2013034Y) and Outstanding Innovation Team of Anhui University of Technology (TD201202).

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References

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