Carbonaceous nanomaterials as effective and efficient platforms for removal of dyes from aqueous systems

Journal Pre-proof Carbonaceous nanomaterials as effective and efficient platforms for removal of dyes from aqueous systems Wandit Ahlawat, Navish Kataraia, Neeraj Dilbaghi, Ashrafs Aly Hassan, Sandeep Kumar, Ki-Hyun Kim PII:

S0013-9351(19)30701-7

DOI:

https://doi.org/10.1016/j.envres.2019.108904

Reference:

YENRS 108904

To appear in:

Environmental Research

Received Date: 13 February 2019 Revised Date:

22 October 2019

Accepted Date: 7 November 2019

Please cite this article as: Ahlawat, W., Kataraia, N., Dilbaghi, N., Hassan, A.A., Kumar, S., Kim, K.H., Carbonaceous nanomaterials as effective and efficient platforms for removal of dyes from aqueous systems, Environmental Research (2019), doi: https://doi.org/10.1016/j.envres.2019.108904. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Inc.

1

Carbonaceous nanomaterials as effective and efficient platforms for removal of dyes from

2

aqueous systems

3

Wandit Ahlawat1, Navish Kataraia2, Neeraj Dilbaghi1, Ashrafs Aly Hassan3,4, Sandeep

4

Kumar1,3* and Ki-Hyun Kim5*

5

1

6

Hisar-Haryana, 125001, India;

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Jambheshwar University of Science and Technology, Hisar- Haryana, 125001, India; 3Department of

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Civil Engineering, University of Nebraska Lincoln, P.O. Box 886105, Lincoln, NE 68588-6105, United

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States; 4Department of Civil & Environmental Engineering, United Arab Emirates University, P.O. Box

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15551, Al Ain, UAE. 5Department of Civil & Environmental Engineering, Hanyang University, 222

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Wangsimni-Ro, Seoul 04763, Republic of Korea

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*Corresponding authors:

13

[email protected], Phone: 911662-263378, Fax: 911662-276240

14

[email protected], Phone: 82-2-2220-2325, Fax: 82-2-2220-1945

Department of Bio and Nano Technology, Guru Jambheshwar University of Science and Technology, 2

Department of Environmental Science and Engineering, Guru

15 16 17

Abstract

18

In this study, the feasibility of using carbonaceous nanomaterials was explored for adsorptive

19

removal of methylene blue (MB) and methyl orange (MO) dyes from contaminated water under

20

dark conditions. The morphology and crystalline nature of synthesized carbonaceous

21

nanomaterials (e.g., multi-walled carbon nanotubes (MWCNTs), activated carbon (AC), and

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their nanocomposite) were characterized by different microscopic and spectroscopic techniques.

23

Furthermore, adsorption experiments were carried out by controlling several key parameters

24

including solution pH, adsorbent dosage, dye concentration, contact time, and temperature. First,

25

the adsorptive behavior of MWCNTs was explained with the aid of adsorption isotherms and

26

kinetics. Thereafter, the adsorptive performance of MWCNTs was compared with those of AC

27

and MWCNTs/AC, and the maximum adsorption capacity (mg/g) of MB/MO was in the order of

28

MWCNTs/AC nanocomposite (232.5/196.1) > MWCNTs (185.1/106.3) > AC (161.3/78.7). The 1

29

improved adsorption performance (e.g., in terms of adsorption capacity and partition coefficient)

30

of the MWCNTs/AC nanocomposite could be attributed to the presence of more active sites on

31

its surface. Furthermore, their reusable efficiency was in the order of MWCNTs/AC

32

nanocomposite (90.2%), MWCNTs (81%), and AC (67%) after the first step of recovery. The

33

performance of these adsorbents was also evaluated for real field samples. In comparison to

34

MWCNTs and AC, the MWCNTs/AC sorbents offered excellent performance in both single and

35

binary systems, i.e., ~99.8% and 98.7% average removal of MB and MO, respectively.

36 37

Keywords: Multi-walled Carbon Nanotubes, Methylene Blue, Methyl Orange, Adsorption,

38

Kinetics.

39 40 41

1. Introduction

42

Potable water for living organisms is a serious issue worldwide as numerous anthropogenic

43

activities have resulted in contamination of water resources. Among different pollutants, the

44

evacuation of dyes into the environment is a matter of concern from toxicological and esthetical

45

viewpoints. Dyes contribute significantly to water pollution as colored effluents are released

46

from associated industries, e.g., the pulp, textile, tanning, and pharmaceutical industries, printing

47

factories, etc. Dye-contaminated water can intrude into the food cycle of living organisms to

48

cause phenotypic and genotypic disorders in humans, plants, and animals (Kumar et al., 2014a;

49

Kumar et al., 2014b; Kataria et al., 2016). Globally, more than 105 types of dyes are available

50

commercially, with an annual production of 7×105 tons (Hareesh et al., 2012).

51

Among the different dyes, methylene blue (MB) is commonly used in dyeing silk, cotton, and

52

wood. The consumption of MB-contaminated water was reported to cause severe health hazards

53

such as diarrhea, jaundice, restlessness, increased heart rate, vomiting, shock, quadriplegia, tissue

54

necrosis, and cyanosis (Saini et al., 2018). In addition, methyl orange (MO) is widely used as a

2

55

pH indicator due to its water solubility. It may directly enter the human body by ingestion,

56

wherein intestinal microorganisms metabolize the azo dye into aromatic amines, leading to fatal

57

mutagenesis (Ghaedi et al., 2015).

58

The effluent containing dyes needs to be treated efficiently to overcome the detrimental

59

consequences in the environment and living beings. The degradation of dyes by natural

60

substances is not easy due to their high physicochemical, optical, and thermal properties (Yang et

61

al., 2018). A number of techniques (such as flocculation, coagulation, membrane separation,

62

adsorption, ozone treatment, and photo-catalysis) have been proposed for removal of dye

63

substances (Wang et al., 2018; Li et al., 2018). Among these, adsorption remains a convincing

64

technique for eradication of dyes due to its eco-friendly nature, ease of operation, and low cost

65

(Saini et al., 2017).

66

Numerous forms of adsorbents have been investigated extensively, such as activated carbon

67

(AC) (Silva et al., 2018), metal oxides (Chen et al., 2016), polymers (Blanco et al., 2017), and

68

zeolites (Habiba et al., 2018). The extraordinary physicochemical properties and porous structure

69

of carbonaceous nanomaterials make them more attractive as adsorbents. Among these materials,

70

activated carbon (AC) has been used most abundantly as an adsorbent. However, the low

71

regeneration efficiency of AC puts limitations on its usage as an adsorbent. To solve this

72

limitation, enormous research efforts have been conducted to develop alternative adsorbents with

73

high reusability at low cost.

74

As an alternative, carbon nanotubes (CNTs) are rolled up graphitic sheets that exhibit several

75

interesting properties in terms of hydrophobic nature, large specific surface area, hollow tube

76

structure, layer-by-layer arrangement, high porosity, and π-conjugative structures (Kumar et al.,

77

2015). All these features make them an excellent adsorbent for various dyes, heavy metals, and

78

other organic contaminants (Kumar et al., 2014c; Sarkar et al., 2018). MWCNTs have already 3

79

been reported for adsorption of endocrine disruptors (i.e., 4-Tert-octylphenol), offering an

80

adsorption capacity of 270.27 µg/g at 35 °C (ALOthman et al., 2019). Moreover, the potential of

81

CNTs has been explored for the adsorption of heavy metal ions to confirm the enhanced

82

capabilities of CNTs under normal water situations (Ali, 2018). However, there are several

83

parameters that can affect the adsorption process and regeneration efficiency of CNTs, such as

84

morphology, active sites, and presence of defects. Therefore, research efforts have been directed

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toward the optimal application of CNTs as an effective adsorbent either by diverse

86

functionalization (Keller et al., 2018) or through the formation of nanocomposites (Goscianska

87

and Ciesielczyk, 2019).

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In this study, MB and MO were chosen as the target adsorbates as representative cationic

89

(basic) and anionic (acidic) dyes over carbonaceous materials, respectively. Three different

90

forms of carbon nanomaterials were used in this study including AC, multi-walled carbon

91

nanotubes (MWCNTs), and MWCNTs/AC nanocomposite. MWCNTs were synthesized by the

92

chemical vapor deposition (CVD) technique. The adsorptive removal of both MB and MO by

93

MWCNTs is interesting due to their porous nature. The performance of the adsorption process

94

by MWCNTs was evaluated in batch mode experiments with due consideration of different

95

variables, such as pH, adsorbent dose, adsorbate concentration, contact time, and temperature.

96

Different models of isotherms and kinetics were used to evaluate the adsorption capacity and the

97

associated mechanism. The comparative adsorption performance of the carbonaceous

98

nanomaterials was also examined in terms of their reusability and adsorptive capabilities in real

99

water samples. In this study, we made a step toward fully exploring the potential of carbon

100

nanomaterials for quantitative adsorption of both cationic and anionic dyes.

101 102

2. Materials and methods 4

103

2.1. Synthesis and characterization of CNTs

104

Dyes (both MB and MO) and ferrocene were procured from Himedia Laboratory Pvt. Ltd.,

105

India. AC and toluene were purchased from S. D. Fine Chemicals Ltd. and Sisco Research

106

Laboratories Pvt. Ltd., India, respectively. All of the procured chemicals were of analytical

107

grade. Deionized water (DW) was prepared in the laboratory for use in experimental and stock

108

solutions. The real field samples (e.g., groundwater (GW) and tap water (TW)) were collected

109

from Hisar, Haryana, India. Table 1 shows the physicochemical properties of the MB and MO

110

dyes.

111

MWCNTs were synthesized via the chemical vapor deposition (CVD) technique (Kumar et al.

112

2013) with two heating zones using a mixture of ferrocene and toluene (1:3) through quartz tube

113

with fixed temperatures of 350 °C (first zone) and 750 °C (second zone). The obtained

114

MWCNTs were dried and then washed with H2SO4 as well as DW to remove debris from the

115

pores of the nanotubes. This step made the nanotubes completely hollow, ultimately contributing

116

to formation of a large surface area. The nanocomposite of MWCNT and AC was further

117

prepared by overnight stirring of the mixture of MWCNT and AC (1:1) in DW. Then, the

118

collected sample was dried and stored at room temperature for further characterization. The

119

schematic for the synthesis procedures of carbonaceous nanomaterials (i.e., MWCNTs and AC-

120

MWCNTs) and their potential use for dye removal is depicted in Fig. 1.

121 122

Table 1

123

Properties and structure of Methylene Blue (MB) and Methyl Orange (MO) dyes Order

Dye

1

Molecular

Methylene Blue formula

and C16H18CIN3S

5

Methyl Orange C14H14N3NaO3S

structure

2

Molecular weight (g/mol)

319.85

327.33

3

Classification

Basic dye

Azo dye

4

C.I. no

52015

13025

5

C.I. name

Basic blue 9

Gold orange

6

Melting point (˚C)

190

300

7

Dye content (%)

95%

95%

8

λ max (nm)

664

463

9

Physical state

Dark blue to brown powder

Red to orange powder

124 125

Scanning electron microscope (SEM) and energy dispersive X-ray (EDX) (Carl Zeiss, EVO 18

126

SEM, Germany) were used to determine the surface topography and elemental composition of

127

the MWCNTs and MWCNTs/AC nanocomposite, respectively. The morphology of synthesized

128

MWCNTs was determined by transmission electron microscopy (TEM) (TECNAI 200 Kv TEM,

129

FEI Electron Optics, USA). The Brunauer-Emmett-Teller (BET) (Quantachrome Nova, 2000e,

130

USA) technique was used to determine the surface area and pore size distribution of the samples.

131

Fourier transform infrared (FTIR) spectroscopy (Shimadzu IR AFFINITY-I, Japan) was

132

employed to record the presence of functional groups on the surface of sorbent samples. The

133

structure of the samples was examined with the help of X-Ray diffraction (XRD) (PANalytical

134

X'Pert Pro Multipurpose Diffractometer powered by a Philips PW3040/60 X-ray generator,

135

Netherlands) with Cu Kα X-ray radiation scanned from 10˚ to 80˚. The salt addition method was

136

used to calculate the point of zero charge (pHpzc) of MWCNTs (Kataria and Garg, 2017). The

137

concentration of dye solution was examined using an UV-VIS spectrophotometer (UV-VIS 3000 6

138

LABINDIA, India) capable of UV-vis scanning from 190 to 1,100 nm. The dye concentrations

139

were measured at particular wavelengths, i.e., 664 nm for MB and 463 nm for MO.

140 141

Fig. 1. Schematic of the synthesis procedures of MWCNTs and their composite with AC and

142

their applications for MB and MO dye removal.

143 144

2.2 Adsorption studies

145

All of the adsorption experiments were performed in batch mode. Adsorption experiments were

146

performed in conical flasks under absolute dark conditions in an orbital shaker with a speed of

147

180 rpm at 27 ˚C. Adjustments to the pH were made using 0.1 M HCl and NaOH solutions. The

148

effects of various parameters (e.g., pH, MWCNTs dose, time, dye concentration, and

149

temperature) on adsorption performance of the carbon-based nanomaterials were examined. The

7

150

percentage of dye removal and the adsorption capacity were calculated using the following

151

equations:
  % =

152

153

   

× 100 1

and      =



!

−  # 2 

154

where Co and Ce are the initial and final dye concentration (mg/L). The amount of dye adsorbed

155

on the surface of MWCNTs with mass (g) at equilibrium in a given volume (L) of dye was

156

defined as qe (mg/g). Furthermore, the statistical validation of adsorption performance of all of

157

the adsorbents was confirmed using partition coefficient (PC) calculations. PC is the ratio of

158

adsorbate concentration onto an adsorbent solid surface to the adsorbate concentration in the

159

liquid phase in a solid/liquid interface (Nehra et al., 2019). It is a more objective performance

160

metric (e.g., relative to adsorption capacity) in that PC denotes the strength of solid

161

adsorbent/liquid adsorbate interactions by normalizing the effects of varying adsorbate inputs

162

between different studies.

163 164

3. Results and discussion

165

3.1. Characterization of MWCNTs

166

The XRD patterns of MWCNTs, AC, and MWCNTs/AC are given in Fig. 2a. In the case of

167

MWCNTs, the main diffraction peaks corresponding to 2θ = 26.2°, 43°, and 44.6° resemble the

168

diffraction planes (002), (100), and (101), respectively, which confirm the graphite structure of

169

the MWCNTs. The obtained pattern matched the standard diffraction data for MWCNTs (Cao et

170

al., 2001). For AC, the main XRD diffraction peaks were observed at 2θ = 23° and 43° for the 8

171

diffraction planes of (002) and (101), respectively. These XRD peaks confirmed the graphitic

172

crystallite phase of carbon. In the case of the MWCNTs/AC nanocomposite, the same XRD

173

peaks were recorded as those of MWCNTs, but at a lower intensity due to the presence of

174

amorphous AC (Wang et al., 2015). FTIR spectroscopy was an effective tool to examine the

175

functional groups present on the hexagonal structure of MWCNTs (refer to Fig. 2b). The wide

176

band at 3,427 cm-1 denoted the O-H stretching vibrations of the carboxylic acid group (Nguyen

177

et al., 2015). The peaks at 2,925 and 2,857 cm-1 could be assigned to the symmetric and

178

asymmetric stretching vibrations of C-H, respectively (Sayyah et al., 2015). The peaks at 1737,

179

1637, and 1544 cm-1 denoted C=O stretching (Atieh et al., 2010), and the peak at 676 cm-1 was

180

attributed to C-H stretching (Gurses et al., 2014). The nitrogen adsorption/desorption isotherms

181

for MWCNTs and the MWCNTs/AC nanocomposite are plotted in Fig. 2c. The details of BET

182

surface area, pore size, and volumes of the synthesized MWCNTs and MWCNTs/AC are listed

183

in Table 2. The BET surface area and pore size distribution details of AC are available elsewhere

184

(Suresh et al., 2012). The addition of AC into MWCNTs increased the surface area through

185

addition of extra active sites (Birch et al., 2013; Mojoudi et al., 2019; Lu et al., 2014).

186

9

187

10

188

Fig. 2. Characterization details of the synthesized sorbents: (a) XRD pattern (MWCNTs, AC,

189

and MWCNTs/AC nanocomposite), (b) FTIR spectrum (MWCNTs), and (c) nitrogen

190

adsorption/desorption isotherm at 77 K (MWCNTs and MWCNTs/AC).

191 192 193

Table 2

194

Surface properties of synthesized MWCNTs and MWCNTs/AC Order Parameters

Values for MWCNTs

Values for MWCNTs/AC

1

BET Surface Area

638.135 m2/g

1,021.154 m2/g

2

Total Pore Volume Summary Pore volume

5.982 cm3/g

6.587 cm3/g

BJH Desorption Summary Surface area Pore volume Pore diameters

89.338 m2/g 0.318 cm3/g 3.8 nm

100.698 m2/g 0.197 cm3/g 3.8 nm

Average Pore Size Summary Average pore diameter

3.7 nm

2.5 nm

3

4

195 196 197

The SEM technique was utilized to examine surface morphology of synthesized MWCNTs at a

198

scale of 20 µm and 200 nm (Fig. 3a & b). The surface view of MWCNTs clearly indicated their

199

formation in the form of nanotubes piled over one another. Figures 3c & d show the SEM

200

imaging of composite MWCNTs/AC at a scale of 100 nm and 200 nm. The elemental

201

composition of MWCNTs and MWCNTs/AC was confirmed by EDX spectrum (refer to Fig. 3e

202

& f). The clear and differentiating formation of MWCNTs was confirmed by TEM (Fig. 4a & b).

11

203

The magnified images of MWCNTs at 38000X indicate that the synthesized MWCNTs have a

204

width of ~5-20 nm and a length of several micrometers.

205 206

207 208

Fig. 3. SEM/EDX results of synthesized sorbents: (a, b) SEM image of MWCNTs, (c, d) SEM

209

image of MWCNTs/AC, and (e, f) EDX of synthesized MWCNTs and MWCNTs/AC.

210 211 212 213

12

214 215

Fig. 4. TEM image of synthesized MWCNTs.

216 217

3.2 Adsorption experiment

218

The point of zero charge (PZC) calculations are critical to understand the behavior of

219

adsorbents in terms of anion or cation exchangers. The pHpzc value of synthesized MWCNTs was

220

6, confirming that the positive charge-acquired surface of MWCNTs was at a pH < pHpzc, while

221

a negatively charged surface was at a pH > pHpzc. The pHpzc value of synthesized MWCNTs,

13

222

AC and MWCNTs/AC were found to be 6, 5.25 and 5 (refer to Fig. 5). Therefore, the surface of

223

MWCNTs, AC and MWCNTs/AC acquired positive charge at pH < pHpzc and negative charge

224

at pH > pHpzc.

1.2

MWCNT AC MWCNT/AC

1.0 0.8

Delta pH

0.6 0.4 0.2

6

5 5.25 0.0 -0.2 2

3

4

5

6

7

8

9

10

pH 225 226

Fig. 5. pH at a point of zero charge plot between ∆pH and pHi.

227 228

3.2.1. Effects of pH

229

Adsorption of dye depends significantly upon the pH of the dye solution. A change in the pH

230

affects (a) the degree of ionization of the solution, (b) the charge on tfhe hexagonal structure of

231

MWCNTs, and (c) the specifications of the adsorbates. The pH of the solution was controlled in

232

a range from 2-10 when the experiments were conducted with the following conditions: 50 mL

233

dye solution (10 mg/L), 0.01 g adsorbent dose, 10 mg/L adsorbate concentration, 27 oC 14

234

temperature, and 120 min contact time. The percentage of dye removal increased from 31% to

235

83% for MB with change in pH from 2 to 10 (Fig. 6a). On the other hand, the percentage

236

removal of MO decreased from 67.4% to 13% with pH change from 4.5 to 10, as can be seen in

237

Fig. 6a. The adsorption of MO dye also decreased to 32% in strong acidic conditions (pH 2).

238

This was due to the contrasting degree of ionization, charge on the adsorbent, and the nature of

239

the adsorbate under different pH conditions. At pH > pHpzc, the surface of MWCNTs contained a

240

negative charge. The presence of COOH¯ and OH¯ groups supported the favorable adsorption of

241

cationic dyes over anionic dyes. On the other hand, the surface of MWCNTs had a positive

242

charge (H+ and H3O+) at pH < pHpzc to facilitate the adsorption of anionic dyes. Therefore, the

243

maximum removal of MB dye was achieved at a higher pH due to enhanced affinity of the

244

negatively charged surface of the adsorbent for cationic dyes through favorable electrostatic

245

interactions (Gurses et al., 2014). The acidic pH of the solution hindered the adsorption of MB

246

mainly due to the excess H+ ions. These H+ ions competed with MB cations for the adsorption

247

sites that were available over the adsorbent surface. In the context of MO dye, a maximum of

248

67.4% removal was achieved at pH 4.5. At higher pH levels, the presence of OH⁻ ions in the

249

solution as well as on the surface of MWCNTs competed with the MO molecules (Mahmoodian

250

et al., 2014). In strong acidic conditions, the adsorption of MO dye decreased, confirmed by the

251

color change in dye solution from orange to red.

252

3.2.2. Effects of MWCNTs dose

253

From an economical point of view, the effective dose of MWCNTs should be low enough to

254

remove adsorbate from the aqueous solution. Figure 6b shows the effects of MWCNTs dose on

255

removal of MB and MO at 27 oC, 50 mL dye solution (10 mg/L), stirring speed of 180 rpm, time

256

of 120 min, and pH 8 (for MB) or pH 4.5 (for MO). These pH values correspond to the 15

257

maximum adsorption capacity of MWCNTs, as seen in Fig. 6a. An increase in percentage of dye

258

removal was observed with an increase in the amount of MWCNTs in the dosage range of 5 to

259

25 mg. The large amount of MWCNTs confirmed the presence of more active sites on the

260

hexagonal structure of MWCNTs for adsorption. The increment in adsorbent dosage beyond a

261

certain limit could decrease the adsorption capacity by blockage or overlap of active sites due to

262

agglomeration of MWCNTs (Liu et al., 2012; Saleh and Gupta et al., 2012). In the batch

263

experiments, dye removal was found to be almost constant after a 10 mg dosage of MWCNTs.

264

This dosage of 10 mg was thus selected for further batch experiments. a)

b) 100 Dye removal (%)

Dye removal (%)

80

60 MB MO

40

20

60 40

MB MO

20 0

0 2

4

6

8

10

0.000

0.005

pH

265

0.015

0.020

0.025

d) 100 Dye removal (%)

MB MO

80

Dye removal (%)

0.010

Adsorbent dose (g/50mL)

c) 100

60 40 20

80 60 MB MO

40 20 0

0 0

266

80

10

20

30

40

0

50

20

40

60

80

100 120 140 160 180

Time (min)

Dye Conc. (mg/L)

16

Dye removal (%)

e) 100 90 80 70

MB MO

60 50 40 30 10

20

30

40

50

60

Temperature (°C)

267 268

Fig. 6. Factors affecting the removal of MB and MO using MWCNTs: a) pH, b) adsorbent

269

dosage, c) dye concentration, d) contact time, and e) temperature.

270

271

3.2.3. Effects of adsorbate concentration

272

The effects of adsorbate concentration for both MB and MO were examined from 5 to 50 mg/L,

273

while keeping the other experimental parameters constant (i.e., MWCNTs dose of 0.01 g,

274

temperature of 27 o C, contact time of 120 min, stirring speed of 180 rpm, pH of 8 for MB, and

275

pH of 4.5 for MO). There was a significant decrease in percentage removal of MB (from 84 to

276

57%) and MO (from 78 to 38%) with increase in dye concentration from 5 to 50 mg/L (refer to

277

Fig. 6c). For a lower adsorbate concentration (5 mg/L), the fixed amount of adsorbent (0.01 g)

278

offered more binding affinity toward adsorbate, thereby resulting in higher percentage removal

279

of dyes, i.e., 84% for MB and 78% for MO. On the other hand, at a higher adsorbate

280

concentration (50 mg/L), adsorbate molecules saturated the binding sites on the adsorbent (of the

281

fixed amount of 0.01 g) surface; thereby resulting in a decreased percentage removal of dyes,

282

i.e., 57% for MB and 38% for MO.

17

283

284

3.2.4. Effects of contact time

285

Adsorption experiments were also performed by changing the contact time from 15 to 180 min

286

with the other parameters remaining constant. Both dyes showed a gradual increase in percentage

287

removal with an increase in contact time from 15 to 180 min. The percentage of dye removal for

288

MB and MO increased from 51 to 83% and 36 to 70%, respectively (refer to Fig. 6d). A fast

289

increase in dye adsorption was observed in the beginning of the experiment due to the presence

290

of numerous free active sites on the surface of the MWCNTs. With the progression of time,

291

adsorption slowed with saturation of active sites to attain near-equilibrium conditions

292

(Mahmoodian et al., 2014; Duman et al., 2016).

293

294

3.2.5. Effects of temperature

295

The dye adsorption performance was also evaluated as a function of temperature in the range of

296

17 to 57 oC. MWCNTs showed an increase in percentage removal with an elevation in

297

temperature (Fig. 6e). It was evident that the process of adsorption for both dyes was

298

endothermic in nature. With a rise in temperature, the mobility and diffusion of the dye

299

molecules in solution increased as the interaction of adsorbate molecules with the active sites of

300

the adsorbent increased simultaneously (Ma et al., 2014). Therefore, maximum adsorption (i.e.,

301

~86% for MO and ~97% for MB) was observed at a higher temperature (57 oC). All experiments

302

were performed in closed vessels.

303

18

304

3.3 Adsorption isotherms

305

An adsorption isotherm explains the interaction between adsorbate and adsorbent. With due

306

consideration of MWCNTs as adsorbents, Langmuir, Freundlich, and Temkin adsorption models

307

were employed to explain the adsorption behavior of MWCNTs for both MB and MO dyes.

308

Later, the adsorption behavior of MWCNTs was also compared with those of AC and the

309

composite, as detailed in section 3.7. The Langmuir adsorption model explained monolayer

310

adsorption of the dye molecules on a homogeneous surface of MWCNTs on active sites (Kataria

311

et al., 2016). A linear form of the Langmuir isotherm is given as follows: 



=

1

%&' (

+



%&'

3

312

where ( is the Langmuir constant to explain the affinity of binding sites (L/mg), and qmax is the

313

maximum adsorption capacity for monolayer adsorption (mg/g). The linear plot between

314

 ⁄ vs.



helped in calculation of the adsorption model parameters for both the MB and MO

315

dyes (Fig. 7a). The values of qmax, b, the correlation coefficient (R2), and the separation factor

316

(RL) for MB and MO are given in Table 3.

317 318

RL can be calculated as follows:

,- =

1 1+(

.

4

319

where ( denotes the Langmuir constant, and the Langmuir adsorption model is favorable for 0

320


321

adsorbate molecules onto the MWCNTs active surface (Fig. 7b). Multiple layer adsorption

322

occurred due to the heterogeneous nature of active sites available on the surface of MWCNTs

323

(Wang et al., 2018). A linear form of the Freundlich isotherm can be expressed as follows: 1

0 = 012 + 0  5 

19

324

where 12 is the Freundlich constant related to the adsorption capacity [mg/g. (L/mg)1/n], and  is

325

the heterogeneity factor related to the adsorption intensity. The Freundlich adsorption model was

326

favorable when 1< n <10 (Table 3).

20

a)

0.35

MB MO

C e /qe

0.30 0.25 0.20 0.15 0.10 0.05 0

5

10

15

20

25

30

Ce

327

b) 2.2

MB MO

log qe

2.0 1.8 1.6 1.4 1.2 0.0

0.5

328

c) 160

1.0

1.5

MB MO

120

qe (mg/g)

log Ce

80

40

0 0

329

1

2

ln Ce

3

4

330

Fig. 7. a) Langmuir adsorption isotherm, b) Freundlich adsorption isotherm, and c) Temkin

331

adsorption isotherm for MB and MO dyes onto MWCNT. 21

332 333 334 335

Table 3 Different adsorption isotherm models for MB and MO dyes calculated from linear equations Order Isotherm model 1 Langmuir

Linear equation  / = 1/%&' ( +

2

Freundlich

log = 012 + 1⁄ 0

3

Temkin

 = 4 15 + 4 





 /%&'



Parameters qmax (mg/g) b (L/mg) R2 Co (mg/L) RL n Kf (mg/g) R2 B KT (L/g) R2

MB 185.1 0.144 0.995 50 0.12 1.72 26.28 0.986 37.26 1.78 0.981

MO 106.3 0.148 0.974 50 0.11 2.19 19.58 0.987 21.29 1.87 0.968

336 337 338

According to the Temkin and Pyzhev isotherm models, adsorption on the adsorbent with a

339

hexagonal surface and heterogeneous active sites occurred due to formation of a single layer of

340

adsorbate (Karim et al., 2014). The uniform distribution of binding energy was also confirmed.

341

The heat of adsorption decreased linearly due to the interaction between adsorbate and

342

MWCNTs. A linear form of the Temkin isotherm can be expressed as follows:  = 4 15 + 4   6

343

where B = RT/b, R is the universal gas constant (i.e., 8.314), T is the temperature, b is the

344

Temkin constant expressing the heat of adsorption (J/mol), and 15 is the equilibrium binding

345

constant (L/g). The adsorption parameters were evaluated from the linear plot of  vs. 

346

(refer to Fig. 7c).



347

Upon comparing the adsorption isotherms, the Langmuir model was best fitted for MB dye

348

adsorption, with a correlation coefficient (R2) of 0.995. The maximum adsorption capacity of 22

349

MB was 185.1 mg/g through monolayer adsorption of basic dye molecules on the homogeneous

350

active sites of MWCNTs (Pathania et al., 2017). On the other hand, the Freundlich model was

351

the best fit for MO dye adsorption, with a correlation coefficient (R2) of 0.987. This result

352

suggests multilayer adsorption of MO on heterogeneous active sites of MWCNTs (Yan et al.,

353

2016).

354

355

3.4 Adsorption kinetics

356

To determine the rate of adsorption onto MWCNTs, Lagergren first order equation, pseudo-

357

second order equation, and intra-particle diffusion equation were applied to the obtained

358

practical data. The linear form of the Lagergren model with the first order and pseudo-second

359

order can be expressed as follows (Yagub et al., 2014; Ho and McKay, 1999): => log − ; = 0  − < @  7 2.303  1  = + 8 B ; =B  

360

where ; denotes the adsorption capacity at time t (mg/g), => denotes the pseudo-first order

361

kinetic rate constant (min-1), and =B denotes the pseudo-second order kinetic rate constant (g mg-1

362

min-1). In the pseudo-first order model, the values of => and  of both MB and MO dyes were

363

determined by plotting the graph between log − ; versus  (refer to Fig. 8a). Similarly, in

364

the pseudo-second order model, the values of =B and  for the dyes (MB and MO) were

365

calculated via the linear plot between ⁄; versus  (Fig. 8b and Table 4).

23

a) 1.2 MB MO

log (qe- qt )

0.8

0.4

0.0

-0.4 0

366

30

60

90

b) 6

150

180

MB MO

5 4

t/qt

120

t (min.)

3 2 1 0 0

50

100

150

200

t (min.)

367

c) 45 40

qt (mg/g)

K-II

MB MO K-I

35

K-III K-II

30 K-I

25 20 3

368

6

1/2

9

12

15

t (min.)

369

Fig. 8. a) Pseudo-first order model, b) Pseudo-second order model, and c) Intra-particle diffusion

370

models for MB and MO removal by MWCNTs. 24

371

The values obtained from different kinetic models confirmed that the   values were best

372

fit with the  exp values obtained from the pseudo-second order kinetic model (refer to Table

373

4). The pseudo-second order model significantly favored adsorption of both MB and MO dyes,

374

with higher correlation coefficients of R2 ≥ 0.9984 and 0.9988, respectively (Yagub et al., 2014).

375

Table 4

376

Kinetic model parameters and values for MB and MO dyes Order

Kinetic models

Parameters

MB

MO

1

Pseudo-first order

k1 ( min-1) qe (cal) R2

0.0019 17.542 0.9007

0.0020 14.89 0.8271

2

Pseudo-second order

k2 (g/mg min) qe (cal) R2

0.00183 44.444 0.9984

0.00214 36.9 0.9988

3

Intraparticle diffusion

kid (mg g-1min-1/2) C R2

1.4044 24.149 0.9718

1.2704 18.977 0.9262

4

Experimental data

qe (exp)

41.7

34.59

377 378

The intra-particle diffusion model could be used to explain the kinetics of intra-particle diffusion

379

as follows (Yagub et al., 2014):

380

; = =JK  >⁄B + 9 ,

381

where ; denotes the equilibrium dye uptake at time t (mg/g), =JK denotes the intra-particle

382

diffusion rate constant (mg g-1min-1/2), and

383

boundary layer effect (mg/g). The values of =JK and

384

calculated by plotting a graph between ; and =JK (refer to Fig. 8c). Intra-particle diffusion was

385

only a rate controlling step where the plot did not pass through the origin ( ≠0) (Kataria and

denotes the intercept showing the thickness of the

25

for both dyes (MB and MO) were

386

Garg, 2018a,b). The correlation coefficient (R2) values of both MB and MO dyes for the intra-

387

particle diffusion model were slightly lower than those of the pseudo-second order model (<

388

0.9718 and 0.9262, respectively) (refer to Table 4). Based on comparison of the correlation

389

coefficient values of all of the models, it was clear that the pseudo-second order model was

390

better suited for adsorption of MB and MO dyes onto MWCNTs than the pseudo-first order

391

kinetic model and intra-particle diffusion model.

392

3.5. Thermodynamic study

393

Batch experiments were performed at varying temperatures (17-57 ˚C) with other parameters

394

kept constant to elaborate the effects of temperature on adsorption of both dyes. The Van't Hoff

395

equation was used to evaluate entropy (MN O ), enthalpy (MP O ), and change in Gibb’s free

396

energyMQ O as follows: MQ O = −,R 1K 10 1K =

& 

11

MQ O = MP O – RMN O 12

1K =

MP O MN O – 13 ,R ,

397

where MQ O is the change in Gibb's free energy (kJ mol-1), , is the gas constant (8.314 J mol-1 K-

398

1

399

concentration on the adsorbent (mg/L).

), R is the temperature (K), 1K is the equilibrium constant, and

&

is the amount of dye

400

The plot of 1K versus 1⁄R was helpful to evaluate the values of MP O and MN O as shown in

401

Table 5. The negative value of MQ O for both dyes showed that the adsorption process was 26

402

spontaneous; for example, adsorption increased with a rise in temperature. The positive value of

403

MP O confirmed the endothermic nature of the adsorption process, while the positive value of

404

MN O indicated the affinity of MWCNTs toward both MB and MO dyes (Pathania et al., 2017).

405 406 407

Table 5 Thermodynamic parameter details for MB and MO dye removal by MWCNTs Order

Temp.

Methylene Blue (MB)

(K)

Methyl Orange (MO)

1

290

∆S⁰⁰ ∆H⁰⁰ ∆H⁰⁰ ∆G⁰⁰ ∆G⁰⁰ ∆S⁰⁰ -1 -1 -1 -1 -1 -1 -1 (k J mol ) (J mol K ) (k J mol ) (k J mol ) (J mol K ) (k Jmol-1) -1.178 210.4 59.9 0.172 141.1 40.7

2

300

-3.695

-1.988

3

310

-4.870

-3.408

4

320

-6.747

-4.458

5

330

-10.255

-5.564

408 409

410

3.6. Mechanism of adsorption onto MWCNTs

411

In general, adsorption depended on the pH of the solution and the surface properties of

412

MWCNTs. The mechanism of dye adsorption involved interaction between the adsorbate and

413

adsorbent in the solution. This interaction may have occurred in the following ways: electrostatic

414

bonding, ᴨ-ᴨ bonding, and hydrogen bonding. Carbon nanotubes are hollow tubes with

415

hexagonally arranged carbon atoms with sp2 hybridization, offering ᴨ-ᴨ bonding between the

416

side wall of MWCNTs and C=C bond of dye molecules. Hydrogen bonding mainly occurred

417

between the dyes and MWCNTs due to the presence of hydroxyl, carboxyl, and amide groups on 27

418

MWCNTs. Electrostatic bonding occurred between the negatively charged surface of MWCNTs

419

and MB dye molecules at higher pH (Gupta et al., 2013). The anionic groups of MO dye

420

molecules were electrostatically attracted to the positive hydronium group on the surface of the

421

MWCNTs. The interactions between both of the dyes and the MWCNTs are schematically

422

drawn in Fig. 9.

423 424

Fig. 9. Schematic for the mechanisms of adsorption of both MB and MO dyes onto the

425

MWCNTs.

426

427

3.7. Comparative adsorption study with other adsorbents

28

428

For both dyes (MB and MO), activated carbon (AC) and MWCNT/AC were also used to

429

describe their adsorptive behavior across varying concentrations of dyes (e.g., 5 to 50 ppm).

430

Adsorption of MB over AC was found to be equivalent to that of MWCNTs, whereas adsorption

431

of MO over AC was not. Therefore, it was concluded that MWCNTs offered higher adsorption

432

for both of the dyes than AC (i.e., 161.3 mg/g for MB and 78.7 mg/g for MO) due to their

433

structure being favorable for inducing a stronger interaction with adsorbate molecules. With the

434

use of MWCNT/AC, much higher levels of adsorption were achieved for MB and MO, 232.5

435

mg/g and 196.1 mg/g, respectively. At fixed concentrations of MWCNTs and nanocomposites

436

(0.01 g) for 10 ppm dye solutions, the MWCNTs/AC nanocomposite offered 100% removal of

437

MB dye in comparison to 81% removal using MWCNTs. Similarly, the percentage removal of

438

MO (10 ppm) increased from 67 to 100% with the use of MWCNT/AC nanocomposite over

439

MWCNTs. The FTIR spectra for different adsorbents (i.e. AC, MWCNT, MWCNT/AC) were

440

also recorded after MB and MO adsorption and are shown in Figure 1S (Supplementary

441

Information). Figure 10 depicts the comparative adsorption capacity of MB and MO dyes with

442

MWCNT, AC, and MWCNT/AC.

29

443 444 445

Fig. 10. Adsorption capacities of MWCNT, AC, and MWCNTs/AC for a) MB dye and b) MO dye.

446

The comparative effect of pH and adsorbent dose on the removal efficiency of MB and MO

447

using different carbonaceous adsorbents has been presented in Figure 2S (Supplementary

448

Information). The comparative performance of different adsorbents towards dye concentration,

30

449

contact time, and temperature has been elucidated in Figure 3S (Supplementary Information).

450

The different adsorption isotherms as applicable to these experimental observations have been

451

depicted in Figure 4S (Supplementary Information). The values obtained from different kinetics

452

model for making comparative analysis of carbonaceous materials are shown in Figure 5S

453

(Supplementary Information) and the related data are presented in Table 1S and 2S

454

(Supplementary Information).

455

3.8. Reusability

456

The reusability of adsorbents (i.e., MWCNTs, AC, and MWCNTs/AC) for MB and MO dyes

457

over successive cycles was investigated. First, MWCNTs, AC, and MWCNTs/AC were collected

458

from aqueous solution left over after adsorption experiments. They were then subjected to

459

repetitive washing with ethanol and double distilled water and dried in an oven at 100 ˚C. The

460

dried adsorbents were used again for batch mode experiments of MB and MO dye removal at

461

optimized conditions, such as dye concentration 10 mg/L, dose 0.01 g, time 120 min,

462

temperature 27 ˚C, 180 rpm, and optimum pH 4.5 (MO) and 8 (MB). Such tests were conducted

463

for a total of five cycles against both dyes. Remarkable results were observed for both of the

464

dyes in comparison to the data reported in the existing literature (Gupta et al., 2013). Reusability

465

of MWCNTs/AC and MWCNTs makes them better candidates for dye removal compared to AC

466

with low reusability (Fig. 11).

31

467

468

32

469 470

Fig. 11. Reusability of MWCNTs, AC, and MWCNTs/AC for MB and MO removal.

471

3.9. Adsorption efficiency in real water systems

472

The standard dye solutions were prepared in single and binary systems using GW, TW, and

473

DW. MWCNT and AC were equally efficient for MB removal in all samples, while MWCNT

474

was more efficient for MO removal. In comparison to CNT and AC, the MWCNT/AC composite

475

offered enhanced removal efficiency for both dyes, whether it was a single or binary system

476

(refer to Fig. 12).

33

477 478

Fig. 12. Dye adsorption efficiency of carbonaceous nanomaterials in real water samples: (a) and

479

(c) for MB dye for GW, TW, and DW systems spiked with single and binary components; (b)

480

and (d) for the MO dye counterparts of (a) and (c), respectively.

481

4. Role of partition coefficient in the present work

482

The adsorption performance of the carbonaceous nanomaterials was compared to that in

483

existing literature in terms of adsorption capacity (refer to Table 6). However, the adsorption

484

capacity of adsorbents varied with a change in optimum operating conditions. For instance, the

485

adsorption capacity increased with an increase in the initial concentration of dyes. Therefore,

486

there was a need for true practical performance metrics to evaluate the effectiveness of

487

adsorbents for removal of MB and MO dyes. The partition coefficient can offer significant 34

488

practical basis to explore the actual performance of the adsorbents regardless of alteration in

489

optimum conditions. The partition coefficients in the existing literature and present work are

490

compared in Table 6.

491

Table 6

492 493

Partition coefficient calculations of previously reported papers and the present work of different dyes using MWCNTs Order

Adsorbent

Adsorbate

Adsorption

Final analyte

Optimum

Partition

capacity

concentration

adsorption

coefficient

(mg/g)

(mg/L)

conditions

(mg.g-1.µM-1)

Reference

(Temp. (°C), pH) 1

MWCNTs +

Methylene

42.3

35

25, 6

0.39

Fe2O3 2

MWCNTs

(Qu et al. 2008)

Methylene blue

132.6

4.36

37, -

9.72

(Shahryari et al., 2010)

3

G-CNTs

Methylene blue

81.7

0.3

--

86.91

(Li and Jiang, 2012)

4

Magnetic

Methylene blue

48

46.13

25, -

0.33

MWCNTs 5

MWCNTs +

(Shirmardi et al, 2012)

Methyl orange

66.9

--

--

--

Fe2O3+

(Ai et al., 2011)

Chitosan 6

Magnetic

Methylene blue

48.1

1

25, 7

15.36

modified

(Zhu et al., 2010)

MWCNTs 7

CNTs

Methylene blue

188.6

17.3

25, 9

3.49

8

MWCNTs +

Methyl orange

12.5

--

--

--

Calcium

Methylene blue

606.1

Methyl orange

544.99

(Elsagh et al., 2017) (Sui et al., 2012)

alginate 9

MWCNTs +

62

25, 4.5

2.36

Fe2O3 +

(Zhao et al., 2015)

polyaniline 10

Functionalized

Methyl orange

42.85

0.3

CNTs loaded

25, 6.5

46.52

(Ahmad et al., 2017)

TiO2

35

11

12

13

MWCNTs

AC

MWCNTs/AC

Methylene blue

185.1

21.65

27, 8

2.09

Present

Methyl orange

106.3

31.19

27, 4.5

0.99

work

Methylene blue

161.3

21.01

27, 8

2.21

Present

Methyl orange

78.7

38.01

27, 4.5

0.48

work

Methylene blue

232.5

3.8

27, 8

19.58

Present

Methyl orange

196.1

11.78

27, 4.5

5.31

work

494

495

4.1 Conclusion

496

In summary, the performance of carbonaceous materials were evaluated for removal of MB and

497

MO dyes using batch mode experiments. Langmuir isotherm and Freundlich isotherm models

498

were best fit for MWCNTs to MB and MO, respectively. These isotherms confirmed the

499

monolayer adsorption of MB dye onto the homogenous active sites of MWCNTs. On the other

500

hand, multilayer adsorption of MO dye occurred on the heterogeneous surface of MWCNTs. The

501

adsorption process was spontaneous and endothermic in nature, as confirmed by thermodynamic

502

study. The rate of adsorption for both MB and MO dyes was in good agreement with the pseudo-

503

second order model. The carbonaceous nanomaterials offered the maximum adsorption capacity

504

in the order of MWCNTs/AC ˃MWCNTs ˃AC for MB and MO, respectively. The adsorption

505

capacity values (mg/g) of MB by MWCNTs (185.1), AC (161.3), and MWCNTs/AC (232.5)

506

were larger than those of MO (106.3, 78.7, and 196.1 mg/g, respectively). These observations

507

could be attributed to the cationic exchange capacity of the adsorbents. In addition, MWCNT/AC

508

recorded the best performance for MB and MO dye adsorption in terms of partition coefficients

509

of 19.58 and 5.31 mg.g-1.µM-1, respectively. Reusability experiments illustrated that MWCNTs

510

and their nanocomposites could resolve the poor reusability of AC. Therefore, the studied

511

MWCNTs and their nanocomposites could be highly economical and efficient for removal of

512

both cationic and anionic dyes from aqueous solutions. 36

513

514

Acknowledgements

515

Sandeep Kumar would like to thank DST, Govt. of India, University of Nebraska Lincoln

516

(UNL), the Daugherty Water for Food Institute (DWFI), and Indo-US Science and Technology

517

Forum (IUSSTF) for financial support through the Water Advanced Research and Innovation

518

(WARI) (research grant vide letter No. IUSSTF/WARI/2018/F-029-2018 dated 03-01-2018)

519

along

520

HSCST/R&D/2018/2103 dated 01-08-2018), and the DST-PURSE sanctioned to GJUS&T,

521

Hisar under PURSE program No. SR/PURSE Phase 2/40(G). Wandit Ahlawat would like to

522

acknowledge DST INSPIRE, New Delhi, India, for providing financial assistance through the

523

Senior Research Fellowship. KHK would like to acknowledge support made in part by a grant

524

from the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT,

525

& Future Planning (No. 2016R1E1A1A01940995). This study was also supported by the

526

"Cooperative Research Program for Agriculture Science & Technology Development (Project

527

No. PJ014297)," Rural Development Administration, Republic of Korea.

with

HSCST,

Govt.

of

Haryana,

India

(research

grant

vide

letter

No.

528 529

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44

Highlights •

Multi-walled carbon nanotubes (MWCNTs) synthesized by chemical vapour deposition.

•

Batch mode experiments to study removal of methylene blue and methyl orange dyes.

•

Comparative studies of MWCNTs with AC and their composites for real water samples.

•

The maximum adsorption capacity of MWCNTs, AC, MWCNTs/AC was different for dyes.

•

Reusability of MWCNTs/AC for both the dyes was excellent.

Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

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