Preparation of Efficient Carbon-Based Adsorption Material Using

Jul 18, 2019 - ... account for about 70% of the total petroleum reserves in the word. ... feasibility of converting asphaltenes to high value pyrolysi...
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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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Preparation of efficient carbon-based adsorption

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material using asphaltenes from asphalt rocks

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

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a,b,e

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a

7

China

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b

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

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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,

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China

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d

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

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e

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

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6N5, Canada

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*

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E-mail address: [email protected]

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ζ

Corresponding author at: (Lin He).

These two authors contribute equally to this work.

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ABSTRACT

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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Keywords: Asphaltenes; Pyrolysis; Activated carbons; Adsorption;

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GRAPHIC ABSTRACT

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1. INTRODUCTION

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

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

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

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

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

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

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

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other high value products or chemicals.

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

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

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

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

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

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

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

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

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

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chain14. The polycyclic aromatic hydrocarbons were variably substituted with heteroatoms

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

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

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

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

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

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pyrolytic treatment to decompose the Maya asphaltene17. They separated pyrolysis

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

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

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products

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

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

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

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asphaltenes from the natural unconventional oil ores.

18, 19

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

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

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

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

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

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

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

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

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

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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

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

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

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

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

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

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

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

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

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2. MATERIALS AND METHODS

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2.1 Materials

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

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

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hydroxide and methylthionine chloride were purchased at their analytical grade from

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

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chemical reagent Co. Ltd., China respectively.

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

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

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

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

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

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

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is constant.

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2.2 Pyrolysis of asphaltenes

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

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

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

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

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

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

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

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

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

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

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

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

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2.3 Preparation of Porous Carbon Material

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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(p/p0=0.994).

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The chemical characteristics of the carbon materials were determined by infrared

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

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

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

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

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superior to 0.1cm-1.

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2.4 Adsorption Tests

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

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

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

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

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

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

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

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

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

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

C0 − Ct V w

(1)

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

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

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

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analogue formula: qe =

C0 − Ce V w

(2)

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

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

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

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

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under the same conditions.

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

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

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

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Langmuir and Freundlich models, etc.

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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)

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

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

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

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

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(4)

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

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

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adsorption), shown as follows: 1⁄ 𝑛

𝑞𝑒 = 𝐾𝑓 𝐶𝑒

(5)

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

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is the Freundlich index (non-dimensional).

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

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

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

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to analyze the experimental data.

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

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(6)

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

𝑞𝑒 𝑘1 )= 𝑡 𝑞𝑒 − 𝑞𝑡 2.303

(7)

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

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of the plotting ln (𝑞𝑒 − 𝑞𝑡 ) to t.

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

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

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(8)

(9)

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

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𝑡

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be acquired by calculating the intercept of

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3. RESULT AND DISCUSSION

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3.1 Pyrolysis of asphaltenes

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

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

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

𝑞𝑡

to t plotting.

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

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from pyrolysis of asphaltenes

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

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

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

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

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

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

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

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

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

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

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

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Information).

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

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3.2 Porous Carbon Material

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

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

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

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

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

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

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micro-mesopores, where the micropore volume was dominant.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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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℃.

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

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

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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

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adsorption capacity gradually declined with the rise of temperature.

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

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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

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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

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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

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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)

(%)

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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

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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

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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

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Industrial & Engineering Chemistry Research

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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.

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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.

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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

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was stronger than those of normal activated carbons. 23

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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

Chemical-physical activation

435

Bamboo35

Chemical-physical activation

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).

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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

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390

equilibrium state quickly (