Preparation of Efficient Carbon-Based Adsorption Material Using

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Preparation of efficient carbon-based adsorption material using asphaltenes from asphalt rocks Zhenwei Han, Shunli Kong, Jing Cheng, Hong Sui, Xingang Li, Zisheng Zhang, and Lin He Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.9b02143 • Publication Date (Web): 18 Jul 2019 Downloaded from pubs.acs.org on July 18, 2019

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Industrial & Engineering Chemistry Research

1

Preparation of efficient carbon-based adsorption

2

material using asphaltenes from asphalt rocks

3 4

Zhenwei Han a,b,cζ, Shunli Kong a,bζ, Jing Chengd, Hong Sui a,b,c, Xingang Li a,b,c, Zisheng Zhang

5

a,b,e

6

a

7

China

8

b

National Engineering Research Centre of Distillation Technology, Tianjin 300072, China

9

c

Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), 300072,

, Lin He a,b *

School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072,

10

China

11

d

China Petroleum Pipeline Engineering Co. Ltd. Tianjin Branch, Tianjin 300000, China;

12

e

Department of Chemical and Biological Engineering, University of Ottawa, Ottawa K1N

13

6N5, Canada

14

*

15

E-mail address: [email protected]

16

ζ

Corresponding author at: (Lin He).

These two authors contribute equally to this work.

1

ACS Paragon Plus Environment

Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

17

ABSTRACT

18

In this work, the asphaltenes from natural Indonesia asphalt rocks were taken as raw

19

materials for the preparation of micro-mesoporous enriched carbon material through

20

pyrolysis (< 500 ℃) and KOH activation (< 900 ℃) processes. It is found that, during the

21

pyrolysis process, the asphaltenes could be converted to non-condensable gas (36.02%),

22

pyrolytic tar (26.57%) and residual char (37.44%). When the char was mixed together with

23

KOH for heating, more carbons would be released due to the activation reaction, forming

24

a carbon network. The optimal activation conditions were obtained at KOH/char ratio of

25

3:1 and 800℃ for 30min. Results also show that almost all of the nitrogen atoms stay in

26

the carbon solid during heating without releasing to the gas or liquid products. The final

27

obtained porous carbon materials were determined to possess a specific surface area of

28

1735 m2/g with rich micropores (~2.0 nm). Instrumental characterizations showed that

29

there are abundant heteroatomic groups, including S=O, —OH, —N=, on the activated

30

carbon surface. Further tests by adsorption indicated that the adsorption of methylene blue

31

on the porous carbon material is monolayer adsorption. The maximal adsorption capacity

32

was determined to be at 556.00 mg/g, much higher than that of some commercial activated

33

carbons. It is also indicated that the adsorption kinetics follows the pseudo-second-order

34

kinetic model. These findings suggest that the asphaltene derived carbon material would

35

be promising efficient adsorbents. It also sheds lights on the resourcilization of asphaltenes.

36

Keywords: Asphaltenes; Pyrolysis; Activated carbons; Adsorption;

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GRAPHIC ABSTRACT

39 40

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41

1. INTRODUCTION

42

It is reported that the unconventional oil ores account for about 70% of the total

43

petroleum reserves in the word 1, 2. These unconventional oils are considered as promising

44

alternative fuels or chemical raw materials. However, during the exploitation of

45

unconventional oil resources, the high content (even up to 80% of the heavy oils, such as

46

Iran asphalt rocks) of asphaltenes (the heaviest fraction in heavy oil) often leads to the high

47

difficulty in separation or upgrading 3-5. From the aspects of environmental protection and

48

resources utilization, it is necessary and promising to convert these natural asphaltenes to

49

other high value products or chemicals.

50

Actually, asphaltenes are the most complex fractions of bitumen or petroleum, which

51

would lead to some problems during oil production, processing and transportation6. During

52

the past decades, great efforts have been made on revealing the molecular structure of the

53

asphaltenes7-10. Previous studies suggested that the light oil fractions and resins would

54

interact with asphaltenes, allowing the asphaltenes to be self-aggregated or dispersed11-13.

55

The dominant molecular architectures in asphaltenes, very complex mixtures of polycyclic

56

aromatic hydrocarbons, are often described by “archipelago” model and “island” (Yen-

57

Mullins) model. Generally, the asphaltenes consist of approximately 40% to 45% aromatic

58

carbon with alkane branched chains possessing an average of four to five carbons long

59

chain14. The polycyclic aromatic hydrocarbons were variably substituted with heteroatoms

60

mainly in the form of hydroxyl, thiofuran and pyridine structure 15, 16.

4


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61

Because of the high aromaticity, the asphaltenes are in highly stable, which are

62

difficult to be degraded or converted under normal conditions. Up to now, the thermal

63

conversions, such as pyrolysis, combustion, catalytic decomposition, etc., are considered

64

as the potential methods to convert the asphaltenes. For example, Janna et al. used the

65

pyrolytic treatment to decompose the Maya asphaltene17. They separated pyrolysis

66

products into saturated, aromatics and polar compounds. Similarly, some of previous works

67

are also focused on maximizing the production of escaped pyrolytic products, such as gas

68

products

69

inspired from the previous work, an idea comes to us on utilizing the residual solids by

70

converting them to high value carbon-based materials 20, such as adsorbents, graphene, etc.

71

In this work, our primary attempts are to prepare high efficient adsorbent using the

72

asphaltenes from the natural unconventional oil ores.

18, 19

. However, little attention has been paid on the pyrolytic solids. Herein,

73

In practice, many different raw materials could be used to prepare the activated

74

carbons through thermal treatment (e.g., pyrolysis, activation by KOH), such as biomass,

75

coal tar pitch, petroleum coke, etc. shown in Table 1. The activated carbons obtained from

76

different sources possess some differences in their physichemical properties, such as

77

specific area, surface chemistry, microporosity, etc21-25. As stated above, the asphaltenes

78

from petroleum or unconventional oils show big difference in elemental composition,

79

functional groups, etc. However, little published information have touched the asphaltene-

80

based adsorption materials, especially made from the natural unconventional oil ores.

5



81

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Table 1 Studies of activated carbons from different raw materials Raw materials

SBET (m2/g)

Vtol (cm3/g)

Vmic (cm3/g)

Microporosity (%)

Cassava peel21

1567

1.18

0.25

21.2

Vetiver roots22

1272

1.19

0.39

32.8

Wood23

1039

0.56

0.34

60.8

Petroleum coke24

1129

0.47

0.46

99.1

Anthracite coal25

838

0.37

0.35

93.0

82 83

Accordingly, in this work, the asphatlenes from natural unconventional oil ores

84

(Indonesian asphalt rocks, with asphaltene content up to 30%) are selected as raw materials

85

for the preparation of high-valued products. Specific purposes are to (i) test the feasibility

86

of converting asphaltenes to high value pyrolysis products and carbon-based materials, (ii)

87

optimize the operational conditions in preparing carbon-based adsorption materials, (iii)

88

understand the pyrolysis and activation mechanism of carbon-based adsorption materials,

89

(iv) apply the activated carbon material for the adsorption of dyes from water and reveal

90

the adsorption mechanism of tested materials on the carbon-based adsorption materials.

91

2. MATERIALS AND METHODS

92

2.1 Materials

93

The asphaltene was extracted from Indonesia asphalt rocks. N-hexane and toluene

94

(analytical grade) were supplied by Tianjin Yuanli technology Co. Ltd., China. Potassium 6


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95

hydroxide and methylthionine chloride were purchased at their analytical grade from

96

Tianjin Rianlon Bohua pharmaceutical and chemical Co. Ltd., China and Tianjin Kermel

97

chemical reagent Co. Ltd., China respectively.

98

The bitumen was extracted from unconventional oil ores by organic solvents (toluene

99

or carbon tetrachloride). The asphaltenes were precipitated using the following procedures:

100

40ml n-heptane was mixed with 1g bitumen26, followed by 24 hours settling and

101

subsequent centrifugation at 8000 rpm for 15 min. The solid materials precipitated at the

102

bottom of the centrifuge tube were taken out and washed by n-heptane until the supernatant

103

was colorless27. The precipitated asphaltenes were dried in vacuum oven until the weight

104

is constant.

105

2.2 Pyrolysis of asphaltenes

106

The pyrolysis of asphaltenes was conducted in a tube furnace (OTF-1200X, KJ GROUP,

107

China). The pyrolysis device is consisted of gas supply system, heat system, gas collecting

108

system, liquid oil collecting system (Figure S1, shown in Supporting Information). The

109

gases, such N2, CO2, O2, etc. are supplied by gas cylinders. The heating system contains

110

sample tube, electric resistance heating wire, temperature control system and other

111

supporting facilities. The asphaltene sample was put in the tube being heated at constant

112

heating rate (5 ℃/min) until the temperature reached up to 500 ℃ for 60 min in an inert

113

atmosphere (N2). The asphaltenes will be cracked and converted into char, liquid oil and

114

gases. The oil was cooled in the heat exchanger by water and collected as liquid product.

7



115

The gas products were collected using a gas sample bag. The collected gas and liquid

116

products were analyzed by GC-MS (Thermos ISQ LT; SGE Pyroject-Ⅱ, Thermos Fisher

117

Scientific, USA). The solid residue in the tube was collected to prepare the carbon material.

118

2.3 Preparation of Porous Carbon Material

119

Preparation method. Figure 1 presented the process of transforming asphaltene-based

120

char to porous carbon material by chemical activation method. The potassium hydroxide

121

was used to react with the powder char at high temperature, corroding the char to form

122

porous material 28. To mold the product, a small amount of binder (pyrolysis tar) has been

123

added into the powder char together with the alkali. The above activation process was

124

carried out at a heating rate of 5 ℃/min up to 800 ℃. The sample was kept at 800 ℃ for

125

30 min. The final carbon material was washed by hydrochloric acid to remove the residual

126

potassium compound. The washed carbon materials were dried and collected as product.

127 128 129

Figure 1 The chemical activation process for preparing activated carbon Optimization of operational conditions. The activation experiments were conducted in

8


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130

batch mode to test the influence of the quantity of alkali and activation temperature on the

131

adsorptive property of product. The mixing ratios (KOH: char) selected in this study were

132

1:1, 2:1, 3:1, and 4:1 at specific temperatures (i.e., 700, 800, 900 ℃). To be simplified, the

133

newly prepared carbon samples are named as AC-Tx-y, where x is activation temperature

134

and y is mixing ratio (KOH: char). For example, AC-T800-1 stands for the carbon material

135

was obtained at 800 ℃ with the KOH: char ratio of 1:1.

136

Properties Characterization of the carbon material. The morphologies of As-M, Ch-

137

T500, and AC-T800-3 were observed using S-4800 scanning electron microscopy (SEM,

138

Hitachi S-4800, Japan). The elemental composition (C, H, O, S, N) measurements were

139

performed on a Vario EL cube (Elementar Analysensysteme, Germany). The standard

140

deviation was controlled less than 0.1% by weighing using analytical balance.

141

The adsorption properties of the carbon materials were accordingly measured. After a

142

treating processes of vacuum degassing for 6 hours at 150 ℃, the standard adsorption-

143

desorption isotherm was measured at 77 Fahrenheit (-196.15 ℃) using an automatic gas-

144

sorption analyzer (BELSORP-max, MicrotracBEL, Japan). The specific surface area and

145

pore diameter distribution were calculated by the Brunauer-Emmett-Teller (BET) model

146

and the Nonlocal Density Functional Theory（NLDFT）equilibrium model, respectively.

147

The total volume (Vm) of pores was gained through nitrogen-adsorption at relative pressure

148

(p/p0=0.994).

149

The chemical characteristics of the carbon materials were determined by infrared

9



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150

spectroscopy using Nicolet 6700 (IR, Thermo Nicolet Corporation, USA) which was

151

equipped with attenuated total reflection and diffuse reflection receiver. Before the

152

characterization analysis, the sample (1mg) was mixed with 100mg KBr. The infrared

153

spectroscopy was recorded from 4000 cm-1 to 400 cm-1 and the spectral resolution was

154

superior to 0.1cm-1.

155

2.4 Adsorption Tests

156

Adsorption of dyes. The adsorption capacity of the obtained carbon materials was

157

determined by the decrement of methylene blue (MB) in solution which was acquired by

158

concentration calculation. The concentration of MB solutions was calculated according to

159

absorbance measured by a UV spectrometer at 665 nm. The standard curve was obtained

160

through measuring sample concentration (1.0, 2.0, 3.0, 4.0, 5.0 mg/L) and the

161

corresponding absorbance using UV spectrometer (TU-1810, Pgeneral, China). The batch

162

adsorption experiments were conducted by mixing 0.1 g porous carbon material and MB

163

solutions. During magnetic stirring process, we suck out samples at different time and

164

tested the absorbance after diluting them by 100 times. The uptake of dyes by the carbon

165

materials at time t, qt (mg/g), was determined as: qt =

C0 − Ct V w

(1)

166

where C0 and Ct represent methylene blue concentrations (mg/L) at the initial time and

167

the time t, respectively; V is the volume of solution (ml) and W denotes the weight of

168

adsorbent (g). The equilibrium absorption capacity, q e (mg/g), was calculated by

10


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169


analogue formula: qe =

C0 − Ce V w

(2)

170

where Ce is methylene blue equilibrium concentration concentrations (mg/L). The

171

adsorption process was conducted with an equilibrium time of 6h. The solutions with

172

different initial dye concentrations (100, 200, 300, 400, 500 mg/L) were prepared by

173

dissolving methylene blue in deionized water. The batch kinetic tests were undertaken

174

under the same conditions.

175

Adsorption isotherm. The adsorption isothermal curves are important data for the

176

understanding of the adsorption equilibrium and adsorption mechanism. There are different

177

theoretical models for describing the adsorption of adsorbate on the adsorbent, such as the

178

Langmuir and Freundlich models, etc.

179 180

The Langmuir adsorption model is mostly used for describing the typical monolayer adsorption, as expressed in Eqs (3) qe =

q m bCe 1 + bC𝑒

(3)

181

Where qe represents the equilibrium absorption capacity (mg/g), qm is the maximum

182

absorption capacity (mg/g), b denotes the Langmuir constant (L/mg) being relevant to

183

adsorption free energy, Ce is equilibrium concentration of MB (mg/L). The equation could

184

also be linearized: 1 1 1 = + 𝑞𝑒 𝑞m 𝑞m 𝑏𝐶e

185

(4)

Differing from the Langmuir model, the Freundlich adsorption model is used to

11



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186

describe the non-ideal and reversible adsorption process (e.g., multilayer heterogeneous

187

adsorption), shown as follows: 1⁄ 𝑛

𝑞𝑒 = 𝐾𝑓 𝐶𝑒

(5)

188

where Kf represents the coefficient ((mg/g)•(L/g)) being related to adsorption capacity, n

189

is the Freundlich index (non-dimensional).

190

Adsorption kinetics. The adsorption kinetics were predicted in the process of removing

191

methylene blue from solution by sample AC-T800-3. Two of the well-recognized models,

192

pseudo-first-order kinetics equation and pseudo-second-order kinetics equation, were used

193

to analyze the experimental data.

194

The pseudo-first-order adsorption kinetics equation is given as: ⅆq t = k1 (q e − q t ) ⅆt

195

(6)

According to the initial condition, the above equation could be integrated as: ln (

𝑞𝑒 𝑘1 )= 𝑡 𝑞𝑒 − 𝑞𝑡 2.303

(7)

196

Where k1 (min-1) is the constant of this model and can be acquired by calculating the slope

197

of the plotting ln (𝑞𝑒 − 𝑞𝑡 ) to t.

198

Another widely used model is the pseudo-second-order kinetics equation: ⅆq t = k 2 (q e − q t )2 ⅆt

199

With the initial condition, it could be converted to: 𝑡 𝑡 1 = + 𝑞𝑡 𝑞𝑒 𝐾2 𝑞𝑒2

200

(8)

(9)

Where k2 (g·mg-1·min-1) is the constant of pseudo-second-order kinetics equation and can

12


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𝑡

201

be acquired by calculating the intercept of

202

3. RESULT AND DISCUSSION

203

3.1 Pyrolysis of asphaltenes

204

Figure 2 presented the results of pyrolysis of asphaltenes. It was found that over 36.02%

205

of the asphaltenes had been converted to gases, including hydrogen (34%), methane (57%),

206

ethane (6%), ethylene (2%) and propylene (1%) (Figure S2 in Supporting Information).

𝑞𝑡

to t plotting.

207 208

Figure 2 The content of non-condensable gas, pyrolytic tar and pyrolysis residue obtained

209

from pyrolysis of asphaltenes

210

The solid product was char, accounting for about 37.44% of the original asphaltenes.

211

Another pyrolysis product was the liquid oil. The liquid oil products were detected to

212

contain different components, including benzene series, sulfur organic compounds,

213

oxygenated compounds, naphthenic hydrocarbons, alkanes and alkenes, shown in Figure

214

3. The alkenes are the dominant products in pyrolytic oil, account for 38% of the total oil.

215

The second major products in oil are the oxygenated chemicals (about 19%). The

216

proportions of other components obtained through pyrolysis process were 12% or less.

13



217

Furthermore, the pyrolytic volatile matter contained a large proportion of alkene and some

218

sulfur organic compounds, as well as a small amount of nitrogen-containing compounds.

219

The major proportion of pyrolytic tar were 2-Butene, 1,1-dimethyl-cyclopropane, 3-

220

methyl-hexane, 3,4-dimethyl-1-hexene and 1,4-hexadiene (Table S1 in Supporting

221

Information).

222 223

Figure 3 The content of different varieties of organic compounds from pyrolytic tar

224

3.2 Porous Carbon Material

225

Optimization of operational activation conditions. It was observed that KOH acts as a

226

good activating agent because it could produce more micropores, allowing the micropore

227

volume and surface area to be higher. The porous characteristics of carbon materials

228

obtained from asphaltenes under different conditions were summarized in Figure 4. The

229

activated carbon samples were detected with a BET surface area ranging from 521 to 1735

230

m2/g, a total pore volume ranging from 0.30 to 0.92 cm3/g and an average pore size ranging 14


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231

from 2.1 to 2.9 nm. It was obvious that the activated carbons were mainly characterized as

232

micro-mesopores, where the micropore volume was dominant.

233

The characteristics of carbon samples were found to be highly dependent on the

234

preparation conditions. The two major factors for preparation of the porous carbon

235

materials were the addition of activating agent and activation temperature. When the KOH:

236

As-M ratio increased from 1:1 to 3:1 (by weight), the pore volume increased by over 2

237

times (0.7 cc/g). The surface area of the porous samples was also observed to be improved

238

significantly (>1700 m2/g). However, continuing increasing the KOH: As-M ratio up to 4:1

239

was found to deteriorate the carbon materials, leading to the reduction of pore volume and

240

surface area. This is because more carbons of the char have been reacted with the KOH,

241

allowing higher loss of carbon after the activation. Therefore, in the following tests, this

242

optimal KOH to carbon ratio of 3:1 was used.

243 244

Figure 4 The BET surface area, micropore area and micropore volume as a function of (a)

245

the addition of activating agent, and (b) activation temperature

15



246

In addition to the activation agent, the activation temperature is also found to exert

247

significant influence on changing the property of the porous carbon materials. Figure 4b

248

shows that the specific surface area and micropore volume are sensitive to the activation

249

temperature. These porous carbons were prepared at the activation temperature from 700℃

250

to 900℃ at KOH/char radio of 3:1 and activation time of 30 min. Results show that the

251

largest specific surface area and biggest micropore volume were obtained at 800℃.

252

Actually, the activation energy increased with the increase of temperature, allowing more

253

carbon atoms to react with KOH. However, when the activation temperature is too high,

254

the CO2 will be generated by K2CO3 pyrogenic decomposition. At the same time, the

255

potassium compounds were reduced to potassium in zero valence state. The potassium was

256

gasified at high temperature and accelerated the reaction with carbon atoms. These

257

processes were favorable for enlarging specific surface area, as shown in Figure 4b. When

258

the temperature was higher than 800℃, the sharp reaction occurs and the pores were

259

destroyed because of the erosion of pore wall. Subsequently, the specific surface area and

260

adsorption capacity gradually declined with the rise of temperature.

261

Properties of porous carbon materials. Figure 5 shows the surface morphology of

262

various carbon materials obtained at different stages. The surface of asphaltenes (As-M,

263

Figure 5a) was smooth. The pyrolytic char (Ch-T500, Figure 5b) presented irregular and

264

porous structure with 2-10um pore diameter because of high-temperature pyrolysis

265

shrinking. The surface of activated carbon (AC-T800-3, Figure 5c) was rough due to the

16


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266


sharp reaction of carbon and KOH under high temperature.

267 268

Figure 5 SEM images of (a) As-M (b) Ch-T500 (c) AC-T800-3

269

The nitrogen adsorption-desorption isotherms for the sample AC-T800-3 prepared

270

under optimum conditions was shown in Figure 6a. The specific surface area and pore

271

volume of this material were calculated. For this sample, a rapid increased adsorption at

272

low relative pressures was followed by near constant adsorption value at higher relative

273

pressures (Type I isotherm). It indicated that the adsorbent possesses rich micropores. A

274

discernible hysteresis loop existing in adsorption-desorption isotherm was associated with

275

wedged mesoporosity. The pore diameter distribution acquired by NLDFT method was

276

shown in Figure 6b. Obviously, most of the pores in the carbon materials were located in

277

the range from 1 to 5 nm. The average peak appeared near 2.0 nm, suggesting that the pores

278

are mainly micro pores.

279

Table 2 Physio-chemical characteristics of asphaltene-based and commercial activated

280

carbon. Carbon

BET surface area

Total pore volume

Micropore volume

Microporosity

sample

(m2/g)

(cm3/g)

(cm3/g)

(%)

17



Page 18 of 34

Ch-T500

11

0.01

-

-

AC-T800-3

1735

0.92

0.69

74.6

WS-480a

1231

0.66

0.41

62.0

Coke-1073-2b

990

0.60

0.55

91.7

281

a

WS48029 was a commercial activated carbon prepared by Calgon Carbon company

282

(USA)

283

b

284

temperature was 1073 K (800℃).

Coke-1073-230 was activated by KOH at double amount addition and activation

285

The other properties, such as specific surface area, the total pore volume, micropore

286

volume and microporosity, were shown in Table 2. The product in this work was compared

287

with reported activated carbon: Coke-1073-2, which took petroleum coke as raw materials.

288

Compared with the commercial activated carbons, AC-T800-3 possessed larger specific

289

surface area and total pore volume. In addition, the microporosity determined for this

290

porous carbon material was 74.6% which is higher than that of WS480.

291

18


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292

Figure 6 The BET characterization of AC-T800-3 (a) The nitrogen adsorption-desorption

293

isotherms (b) The pore diameter distribution

294

Table 3 presents the elemental analysis of raw material (As-M) and processed samples

295

(Ch-500, AC-T800-3). During the carbonization, with the process of pyrogenation, the

296

relative content of hydrogen was observed to be gradually decreased. This is because the

297

pyrogenation process was the main reaction when the temperature was lower than 500 ℃.

298

At this stage, most of branches in asphaltene molecules were converted to light

299

hydrocarbons, resulting in the loss of the most saturated carbons and hydrogens. Further

300

increase of temperature to 800 ℃, the dehydrogenation process became the main reaction

301

in the system. During this stage, the hydrogens in the aromatic rings were further broken

302

and released with or without the carbons. Some of the aromatic carbons were poly-

303

condensed to form carbon network. This is why the relative contents of hydrogen and

304

carbon reduce with the process of pyrogenation. However, different things happen to the

305

heteroatoms (i.e., N, S, O) in the carbon materials during the pyrolysis. As mentioned above,

306

there are some oxygenated organics or sulfurous organics in the gas products and liquid

307

products from the pyrolysis of Asphaltenes. Consequently, the oxygen and sulfur contents

308

in the residual carbon materials changed after pyrolysis. However, the nitrogen content in

309

the residual carbon solids was found to be increased during the pyrolysis. It is evidenced

310

that almost all of nitrogen-atoms stay in the solid carbon materials even this asphaltene

311

carbon material were heated at high temperature. This is also confirmed by the liquid and

19



312 313

Page 20 of 34

gas products analysis (without nitrogenous organics or inorganics). Table 3 Elemental analysis of As-M, Ch-T500 and activated carbon AC-T800-3 Elements ( % ) Samples C

H

O

S

N

As-M

73.71

9.27

4.92

9.64

2.45

Ch-T500

80.23

3.01

3.28

9.39

4.11

AC-T800-3

76.24

1.14

5.16

9.75

7.70

314 315

The infrared spectroscopy of asphaltene material (As-M), pyrolytic char (Ch-T500)

316

and chemically activated porous carbon (AC-T800-3) were given in Figure 7. As-M

317

possessed a broad band peak at 3424 cm-1 which is attributed to the stretching vibration of

318

the hydroxyl groups. The bands at 2925 cm-1 and 2851 cm-1 were ascribed to the stretching

319

vibration of methylene group, while the bending vibrations of -CH2- were presented in

320

Figure 7 at 1456 cm-1 and 1376 cm-1. The peaks at 1602 cm-1 and 1098 cm-1 were

321

corresponded to pyridine clusters and S=O functional group vibration, respectively. The

322

bands located around 1600-1400 cm-1 indicated the presence of the aromatic skeletal

323

structure.

324

The structural and chemical changes from raw materials to char and activated carbon

325

are shown in Figure 7b (Ch-T500) and Figure 7c (AC-T800-3). For Ch-T500 and AC-

326

T800-3, the stretching vibration absorption intensity of -OH at about 3428 cm-1 changed 20


Page 21 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


327

slightly, which was ascribed to the no destruction of the hydroxyl groups. While, the series

328

of bands intensity attributed to methylene group stretching vibration and bending vibrations

329

bands were much weaker than those of asphaltenes. Furthermore, no vibration bands

330

assigned to -CH2- were observed in AC-T800-3. This phenomenon occurred because of the

331

dehydrogenation of char at high temperature. The high temperature treatment also removed

332

hydrogenous organics and translated secondary carbon into graphitized carbon. There was

333

no obvious band intensity change for all spectrum of different samples at 1602 cm-1 and

334

1098 cm-1. It proves the presence of similar pyridine structure or S=O functional group in

335

samples after pyrolysis and carbonization activation. The band during 1600-1400 cm-1 had

336

changed into relatively flat band on Ch-T500 and disappeared on AC-T800-3, which

337

indicated the aromatic skeletal had crossed bonding and translated into graphite carbon

338

structure gradually.

21



339 340

Figure 7 The infrared spectroscopy of As-M, Ch-T500 and AC-T800-3

341

3.3 Adsorption tests

342

Adsorption of MB on activated carbons

343

The newly prepared activated carbons have been used for the adsorption of MB from water.

344

As shown in Figure 8, increasing the KOH/char ratio from 1:1 to 3:1 is found to

345

significantly improve the adsorption capacity of the carbon materials to MB. While, once

346

the KOH/char ratio increased up to 4:1, the adsorption capacity of carbon material is

347

evidenced to be sharply decreased. This is mainly attributed to the decreased specific

348

surface area and pore volume, shown in Figure 4. At the optimal KOH addition, the pores

349

were fully developed and distributed. These pores provide adsorption sites for the

350

adsorption of dyes.

22


Page 22 of 34

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351 352

Figure 8 Adsorption of MB on activated carbons generated at different conditions: (a)

353

different addition of activating agent, (b) different activation temperature

354 355

Adsorption isotherm. Figure 9a depicted the nonlinear fitting isotherm results of dyes

356

adsorption on the prepared carbon materials (AC-T800-3) using Langmuir and Freundlich

357

models. All of the fitting parameters were listed in Table 5. The good fitting results by the

358

Langmuir isotherm equation suggested that the adsorption process of methylene blue on

359

AC-T800-3 was homogeneous. The Langmuir isotherm expression proved that there was

360

equably adsorption potential on the surface of AC-T800-3 and confirmed the monolayer

361

coverage of methylene blue onto it. It is also observed that the maximal adsorption capacity

362

was determined to be 556.00 mg/g. This result also shows that the AC-T800-3 perform

363

better in adsorption than some reported or commercial carbon products, which were shown

364

in Table 4. The Langmuir coefficient b related to the adsorption affinity is 0.5062, which

365

is larger than those in literature 31. This result suggests the affinity of MB on AC-T800-3

366

was stronger than those of normal activated carbons. 23



367

Page 24 of 34

Table 4 MB adsorption capacities for reported activated carbons and commercial products. Raw material

Preparation condition

Adsorption capacity (mg/g)

Straw32

Chemical-physical activation

472

Durian shell33

Chemical activation

289

Waste tire34

Physical activation

227

Coconut husk32


435

Bamboo35


454

Furfuryl alcohol36

vapor-deposition polymerization

380

Filtrasorb 40037

commercial activated carbons

299

368 369

A dimensionless constant (the separated factor RL, also known as equilibrium

370

parameter) could be used to express the essential feature and feasibility of the Langmuir

371

isotherm. The parameter RL can be used to predict if an adsorption system is “favorable”

372

or “unfavorable”. RL =

1 1 + bC0

(11)

373

Where RL represents the separation factor, C0 is initial concentration of MB (mg/L) and b

374

refers to the Langmuir constant (dm3/mg). The shape of isotherm is indicated by the values

375

of RL to be unfavorable (RL > 1), linear (RL = 1), favorable (0 < RL < 1) or irreversible (RL

376

= 0).

24


Page 25 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


377 378

Figure 9 Analysis of adsorption data (a) Fitting isotherms of Langmuir and Freundlich. (b)

379

Separation factor for methylene blue onto AC-T800-3. The adsorption experiments were

380

conducted at particular temperatures of 25℃ and constant stirring speed.

381

According to Figure 9b, the values of RL were in range from 0 to 1. In addition, it

382

also showed that lower initial dye concentrations corresponds to higher RL values. It is

383

obvious that theAC-T800-3 is favorable for adsorption of methylene blue under the

384

conditions studied especially at lower initial concentrations.

385

Table 5 Isotherms constants for MB adsorption on AC-T800-3 Langmuir isotherm parameters

Freundlich isotherm parameters

qm (mg/g)

b

R2

Kf (mg/g)(L/g)

n

R2

556.00

0.5062

0.9866

284.52

7.5616

0.8883

386

Adsorption kinetics. The amount of methylene blue adsorbed on the AC-T800-3 at

387

specific contact time was shown in Figure 10. It is observed that the adsorption rate of

388

methylene blue on AC-T800-3 is found to be highly dependent on the initial concentration

389

of methylene blue. At low initial concentration, the adsorption process could reach the 25



390

equilibrium state quickly (

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