Preparation of magnetic hierarchical porous carbon spheres with graphitic features for high methyl orange adsorption capacity

Accepted Manuscript Preparation of magnetic hierarchical porous carbon spheres with graphitic features for high methyl orange adsorption capacity Adisak Siyasukh, Yothin Chimupala, Nattaporn Tonanon PII:

S0008-6223(18)30344-0

DOI:

10.1016/j.carbon.2018.03.093

Reference:

CARBON 13036

To appear in:

Carbon

Received Date: 22 January 2018 Revised Date:

30 March 2018

Accepted Date: 31 March 2018

Please cite this article as: A. Siyasukh, Y. Chimupala, N. Tonanon, Preparation of magnetic hierarchical porous carbon spheres with graphitic features for high methyl orange adsorption capacity, Carbon (2018), doi: 10.1016/j.carbon.2018.03.093. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. 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.

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Preparation of magnetic hierarchical porous carbon spheres with graphitic features for high methyl orange adsorption capacity

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Adisak Siyasukh*,1, Yothin Chimupala1, Nattaporn Tonanon2

Department of Industrial Chemistry, Faculty of Science, Chiang Mai University, Chiang Mai

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50200, Thailand

Department of Chemical Engineering, Faculty of Engineering, Chulalongkorn University,

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Bangkok 10330, Thailand

Corresponding author. Tel: 66-53-943405. E-mail: [email protected] (Adisak Siyasukh)

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Graphical abstract

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Abstract

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This report describes the successful use of a template-free method in a microemulsion system

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for the preparation of magnetic hierarchical porous carbon (MHPC) spheres, which possess

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high adsorption capability for methyl orange (MO) in aqueous solution, easy separability

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from water by magnetic force after the MO removal, and reusability that allowed MO to be

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adsorbed several times. MHPC spheres are produced by water-in-oil emulsification coupled

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with sol-gel polymerization of resorcinol containing Fe(NO3)3 and formaldehyde to make

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spherical carbon precursors with a macroporous framework inside, followed by carbonization

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with N2 or CO2 activation to transform them into MHPC spheres. Carbonization at

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temperatures higher than 800 °C resulted in not only graphitic feature formation, but also

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both micro- and mesopores in the macroporous framework. Moreover, CO2 activation at 900

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°C both remarkably enhanced the graphitic features and increased the mesopore volume to

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2.16 cm3/g. The magnetic property could be developed by both carbonization and CO2

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activation at 900 °C. MHPC spheres obtained from CO2 activation developed tremendous

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adsorption capacities, as high as 1522.6 mg/g under neutral conditions, which were suitable

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to be reused for MO removal for at least four consecutive cycles without efficiency loss.

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Keywords: Hierarchical porous carbon; graphitic carbon; magnetic porous carbon; methyl

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orange removal

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1. Introduction

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Water pollution caused by expansion of urbanized areas, industrial sectors, and agricultural

3

activities is a concerning environmental problem, which currently threatens the quality of

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human life. Dyes employed in various industries contribute to the threats posed to the

5

environment and human health by industrial waste water, especially the azo dyes, which are

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toxic, mutagenic, and even carcinogenic owing to the presence of amines [1-3]. Methyl

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orange (MO), which is widely used in the textiles, printing, the food and pharmaceutical

8

industries, and research laboratories, is among the water-soluble azo dyes [1,4]. Adsorption,

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an effective technology for dye removal, is most often used because of its simple design, low

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cost, ease of operation, and reusable features [5,6]. Hence, much research has focused on

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creating new approaches for preparing adsorbents with improved properties to enhance

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adsorption efficiency. Activated carbons are popularly used as an adsorbent for MO removal

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from wastewater [7,8]. Nonetheless, there are some disadvantages to using activated carbons.

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For instance, the majority of pore sizes are in the range of micropores; these possess large

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specific surface areas but provide low accessibility for the adsorption of large dye molecules,

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e.g. MO, resulting in low adsorption performance [9-12]. Also, as activated carbons are

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usually used in the form of powders, they are difficult to separate from wastewater for reuse.

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To overcome such problems, hierarchical porous carbon-based adsorbents responsive to

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magnetic force are promising materials. Their distinctive magnetic properties provide an easy

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method for removing and recycling the adsorbents using an external magnet [13-15]. The

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hierarchical porous structure is composed of interconnected macroporous frameworks (pore

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size > 50 nm), whose walls contain micropores (pore size < 2 nm) and/or mesopores (2 ≤

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pore size ≤ 50 nm). Mass transport can be enhanced via the macroporous frameworks, and

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large specific surface areas are obtained from the existing micro- and mesopores [16-23].

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This work aims to introduce a convenient method for the preparation of magnetic

4

hierarchical porous carbon (MHPC) spheres, which possess high capability for MO

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adsorption from aqueous solution, easy separability from water by magnetic force after the

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MO removal, and reusability to adsorb MO several times.

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In the past few years, many research groups have created various macroscopic shapes of

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MHPCs, for instance, preparing MHPC monoliths derived from synthesized polymer rods by

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carbonization [24,25] or hydrothermal treatment [26], fabricating MHPCs with irregular

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structures by thermal treatment of commercial activated carbons impregnated with iron

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compounds [27,28], producing MHPC spheres (or beads) by carbonization of industrial

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resins containing iron sources [29,30] or by solvothermal methods [31]. Spheres or beads

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have advantages compared with other shapes, especially for use in adsorption columns,

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because they are easy to handle on a large scale and have high resistance to attrition [32, 33].

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Although some of the referenced articles describe interesting methods for making MHPC

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spheres from commercial macroporous resins [29,30], a disadvantage of these methods is the

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inability to tailor the macroporous morphology inside the spheres because of the inherent

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structure of the precursors. To the best of our knowledge, the fabrication of MHPC spheres

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without using any template materials or without resorting to prefabricated spherical

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precursors has never been reported [33-38].

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The main aim of this work is to introduce a convenient template-free method for the

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preparation of MHPC spheres. The spheres were fabricated by water-in-oil emulsification

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coupled with sol-gel polymerization of resorcinol containing ferric (III) nitrate (Fe(NO3)3)

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and formaldehyde to make spherical carbon precursors with a macroporous framework

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inside. Whereas the emulsification system provides the spherical shape of the precursors,

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Fe(NO3)3 generates interconnected macroporous frameworks within the spheres. The

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prepared precursors were further converted to carbon spheres by carbonization with nitrogen

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or activation with carbon dioxide (CO2 activation), by which either micro- or mesopores

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could be generated within the macroporous frameworks. The distinctive properties of the

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prepared MHPC spheres – very high mesopore volumes (2.16 cm3/g) on the macroporous

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framework, the graphitic features, and magnetic responsivity – were obtained by CO2

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activation at 900 °C.

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In addition, to demonstrate that the prepared MHPC spheres can be utilized as an effective

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adsorbent for MO removal, we also investigated their adsorption performance. The

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experiments were conducted at neutral pH only, to avoid any requirement to readjust the pH

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to neutral after MO removal. The prepared MHPC spheres obtained by CO2 activation had

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tremendous adsorption capacity (1522.6 mg/g), higher than that of many carbon-based

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adsorbents recently reported elsewhere [39–44]. The results also highlighted the role of

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mesopores within hierarchical porous structures in enhancing the effective performance of

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MO removal. Furthermore, the prepared MHPC spheres were reusable several times. This

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reusability is a significant feature for practical applications.

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2. Experimental

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Research-grade resorcinol (C6H6O2, Sigma-Aldrich, ≥ 99%), formaldehyde (CH2O, Loba

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Chemie, 37%), ferric (III) nitrate nonahydrate (Fe(NO3)3.9H2O, AnalaR BDH, ≥ 99%),

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cyclohexane (C6H12, RCI Labscan, 99.5%), SPAN80 (nonionic surfactant) (C24H44O6,

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Sigma-Aldrich), acetone (C3H6O, RCI Labscan, 99.5%), MO (C14H14N3NaO3S, Sigma-

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Aldrich, dye content 85%), and ethanol (RCI Labscan, 96%) were used.

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2.1 Preparation of hierarchical porous carbon monoliths and carbon spheres

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First, resorcinol was dissolved in deionized water with a constant mass ratio of 0.41. When

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dissolution was complete, 0.1 M of Fe(NO3)3 solution was added into the resorcinol solution

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with various mass ratios of Fe(NO3)3 to obtain the total masses of resorcinol and the

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formaldehyde (Fe/RF-ratio) shown in Table 1. The mixture was continuously stirred at

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ambient temperature until its color turned from light yellow to purple. Next, formaldehyde

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was added to the mixture with a constant mole ratio of formaldehyde to resorcinol of 2 to

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initiate sol-gel polymerization, by which the mixture was transformed to resorcinol-

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formaldehyde (RF) gels. Carbon precursors were prepared for both monolithic and spherical

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shapes to produce the hierarchical porous carbon monoliths and carbon spheres, respectively.

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To prepare the carbon monoliths, the mixture was poured into a cylindrical glass mold until

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the sol-gel reaction was completed, followed by removal from the mold and further drying in

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a hot-air oven at 75 °C until a constant weight was reached. All monolithic gels were

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carbonized at 900 °C for 2 h with a heating rate of 10 °C/min under ambient nitrogen

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conditions. These monolithic carbons were denoted as CF0-900, CF1-900, CF2-900, and

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CF3-900, as listed in Table 1. Note that these monolithic carbons were synthesized only to

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investigate how Fe(NO3)3 influenced the macroporous morphology and size (see discussion

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in Section 3.1).

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To prepare the carbon spheres (Fig. 1), the mixture (water phase) was poured into

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cyclohexane (oil phase) containing SPAN80 (a surfactant) before losing its flow property,

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resulting in a water-in-oil emulsion system. The volume ratio of RF solution and cyclohexane

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was fixed at 0.1. The mixture was continuously agitated at 300 rpm until it became a reddish-

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brown suspension, which was dispersed throughout the cyclohexane solution. Then, it was

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filtered out and immersed in acetone three times, for 6 h each time. Finally, the carbon

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precursors with spherical shape were obtained by drying in a hot-air oven at 75 °C until the

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weight was constant. The precursors were converted to carbon spheres by carbonization

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under ambient nitrogen conditions. The carbonization was conducted in a tube furnace under

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nitrogen flow at 50 cm3/min at a heating rate of 10 °C/min until the desired temperature was

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reached; this temperature was then maintained for 2 h. The precursors without acetone

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treatment were also carbonized at 900 °C for comparison. CO2 activation was carried out in a

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similar manner to the carbonization procedure, except for changing the ambient gas from

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nitrogen to carbon dioxide. The carbon sphere samples were denoted as CF3-500, CF3-600,

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CF3-700, CF3-800, CF3-850, CF3-900, and ACF3-900, as listed in Table 1.

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2.2 Methyl orange adsorption experiment

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CF3-700, CF3-900, and ACF3-900 were selected for the MO adsorption experiments. Batch

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adsorption was conducted by adding spherical carbons into the MO solution with carbon

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dosages and initial concentrations varying from 0.05 to 0.4 g/L and 1 to 600 mg/L,

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respectively. As this work mainly focused on using the prepared MHPC spheres for

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removing contaminated MO from aqueous solution under neutral pH only, examination of

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the effect of pH on adsorption performance was beyond the scope of this work. It should be

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noted, however, that under the range of initial concentrations of MO used in the experiment

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(1 to 600 mg/L), the pH of the MO solution remained constant at 7.1 ± 0.2.

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All samples were shaken at 200 rpm at 30 °C for 48 h. MO solution was then withdrawn to

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analyze the equilibrium concentration using an ultraviolet-visible light (UV-vis)

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spectrophotometer at a wavelength of 463 nm. The amount of adsorbate at equilibrium ( qe )

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was determined by Eq. 1:

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qe =

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A removal efficiency or % removal of MO was calculated using Eq. 2:

% r emoval =

( C0 − Ce ) ×100 C0

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( C 0 − C e )V

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where C0 and Ce are the initial and equilibrium concentrations of MO (mg/L), and V and W

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are the volume of the MO solution (L) and the weight of spherical carbons (mg), respectively.

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The kinetics study was performed by shaking the MO solution containing the carbon spheres

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with a carbon dosage of 0.4 g/L. The initial concentration of MO, shaking speed, and

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temperature were 20 mg/L, 200 rpm, and 30 °C, respectively, for all experiments. MO

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solution samples were taken at pre-set time intervals and their concentrations were

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determined. The uptake of MO at time qt (mg/g) was calculated by Eq.3:

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qt =

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where Ct is the concentration of MO at any time.

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In the thermodynamics study, the influence of different temperatures (30, 40, 60, and 80 °C)

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was investigated. The experiment was conducted by adding spherical carbons into MO

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( C 0 − C t )V W

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solution with carbon dosage and initial concentration of 0.4 g/L and 20 mg/L, respectively.

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All samples were shaken at 200 rpm at the desired temperature for 48 h. Then MO solution

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was withdrawn to analyze the equilibrium concentration.

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2.2.1 Equilibrium isotherm models

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Three equilibrium adsorption isotherm models, Langmuir, Freundlich, and Dubinin-

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Radushkevich (D-R), were compared to elucidate the adsorption mechanism underlying the

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experimental equilibrium adsorption data. The Langmuir model [45] is given in Eq. 4:

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qe =

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where Q0 and b are the monolayer adsorption capacity (mg/g) and the Langmuir constant

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(L/mg), respectively. The Freundlich model [46] is represented in Eq. 5:

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qe = K f Ce1 n

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where Kf (mg/g)(l/mg)1/n and n are Freundlich adsorption constants related to the adsorption

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capacity and the intensity of adsorption, respectively. The D-R isotherm model [47] is

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expressed mathematically by Eqs. 6 and 7:

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q e = q DR exp ( − βε 2 )

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ε = RT ln 1 +

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where qDR, β, ε, R, and T are the maximum adsorption capacity, a constant related to the

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adsorption energy, the Polanyi potential, the gas constant (J/mol K), and temperature (K),

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 

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respectively. β is useful for obtaining the mean adsorption energy E (kJ/mol), which can be

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expressed by Eq. 8:

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E=

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The parameters of the three above-mentioned isotherms were determined by nonlinear

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regression analysis, using MS Excel [48]. The correlation coefficient (R2) was calculated as a

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measure of the compatibility of the experimental data and the adsorption isotherm models,

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using Eq. 9:

R

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∑(q = 1− ∑(q

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where qe, qe,calc, and qe,avg are the experimental data, the calculated, and the average values of the amount of MO adsorbed at equilibrium.

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2.2.2 Kinetics adsorption models

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Pseudo-first-order (PFO) and pseudo-second-order (PSO) kinetic models were employed to

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test the consistency of the kinetic data. R2 was calculated using Eq. (9) to determine the

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compatibility of the experimental data and the adsorption isotherm models.

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The PFO model [49,50] is generally expressed as Eq. 10:

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qt = qe1 (1 − exp ( −k1t ) )

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The PSO introduced by Ho and Mckay [51] can be expressed as Eq. 11:

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qt =

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k 2 qe22t (1 + k2 qe 2t )

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(11)

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where k2 is the PSO rate constant and qe2 is the equilibrium adsorption amount determined

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from the PSO model. At the initial condition, the initial rate of adsorption of the PSO can be

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expressed by Eq. 12:

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h2 = k2qe22

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where h2 is the initial rate constant (mg/g·min).

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2.2.3 Thermodynamic adsorption model

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To evaluate the thermodynamic feasibility of the adsorption process, three basic

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thermodynamic parameters, standard enthalpy (∆H0), standard entropy (∆S0), and standard

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free energy (∆G0), were calculated using Eqs. 13 and 14 [52]:

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∆G 0 = ∆H 0 − T ∆S 0

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where qe/Ce is the adsorption affinity.

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2.3 Characterizations

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The micro- and nanostructures were imaged using a field emission scanning electron

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microscope (FE-SEM, JSM-6335F, JEOL) equipped with an energy-dispersive X-ray

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spectroscopy (EDS) detector (Inca, Oxford), operated at 200 kV. The interconnected

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macroporous size distribution and volume were measured by mercury intrusion (Pore-Sizer-

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9320, Micromeritics). Nitrogen adsorption-desorption isotherms at -196 °C were carried out

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by nitrogen adsorption apparatus (Autosorb-1-MP, QuantaChrome). The specific surface area

(13)

(14)

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q   ∆H 0  1 ∆S 0 log  e  = −   Ce  2.303R  2.303R  T

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(SBET) was determined by the Brunauer-Emmett-Teller (BET) method. The mesopore width

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distribution and mesopore volume (Vmeso) were calculated from the desorption branch of the

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corresponding isotherm using the Barrett-Joyner-Halenda (BJH) method. The Dubinin-

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Radushkevich (D-R) model was applied to calculate the micropore volume (Vmic). The

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particle size and distribution were measured by laser particle size analyzer (Mastersizer S,

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Malvern). Powder X-ray diffraction (PXRD) was performed using an X-ray diffractometer

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(D8 Advance, Bruker). The diffractometer employs a Cu Kα tube with λ = 1.545 Å as the X-

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ray source. Raman results were collected at 25 °C and conducted on a Raman spectrometer

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(Jobin-Yvon T64000, HORIBA) with an excitation wavelength of 532 nm. Images were

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captured by a transmission electron microscope (TEM, JEM-2010, JOEL). The lattice

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spacing of the graphitic samples was measured using the ImageJ Software. The magnetic

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property was tested using a vibrating sample magnetometer (VSM) (Model-73098, Lake

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Shore). The concentrations of MO were measured with a UV-vis spectrophotometer

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(SPECORD® 50 PLUS, Analytik Jena).

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3. Results and discussion

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3.1 Effect of ferric (III) nitrate on the macroporous morphology

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The gelation step in the preparation process of CF0-900 (without Fe(NO3)3) took almost 2

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days at room temperature, whereas the precursors of CF1-900, CF2-900, and CF3-900 (with

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Fe/RF-ratios of 0.003, 0.010, and 0.023, respectively), require only 45, 25, and 5 min,

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respectively. This suggests that Fe(NO3)3 in resorcinol solution at the first period can

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strongly catalyze sol-gel polymerization. However, the Fe/RF ratio should not be greater than

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0.03, because the sol-gel reaction would be very severe, resulting in sudden complete gel

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formation that would cause practical difficulties in the preparation process.

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Prior to the preparation of hierarchical porous carbon spheres, in order to clearly understand

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the effects of Fe(NO3)3 on the microporous morphology, the samples (CF0-900, CF1-900,

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CF2-900, and CF3-900) were formed as monoliths instead of spherical powders to

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investigate how Fe(NO3)3 influences the macroporous morphology and size by the mercury

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intrusion method. This was necessary as the technique has a limitation in that fine particles

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cannot be measured, because pressure exerts the mercury to flow through spaces between

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compacted particles before penetration into the macropores [37].

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The macroporous textures of CF0-900, CF1-900, CF2-900, and CF3-900 are shown in Fig. 2.

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(a). It can be seen that CF1-900, CF2-900, and CF3-900 possess macroporous frameworks,

19

which are interconnected voids of dumbbell-like microparticle networks, while the

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macroporous morphology of CF0-900 (without the presence of Fe(NO3)3) exhibits

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interconnected voids of spherical-like microparticles. As shown in Fig. 2. (b), CF1-900, CF2-

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900, and CF3-900 had narrow macropore size distributions in the ranges of 9.5–32.1, 4.1–

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15.8, and 4.5–10.9 µm with median pore sizes of 10.9, 8.6, and 6.6 µm, respectively.

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However, CF0-900 had a wider distribution in a range of 1.5–38 µm with a median pore size

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of 14.5 µm. It is worth noting that the existence of Fe(NO3)3 plays an important part as a

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macropore size controller, and thus, increasing Fe(NO3)3 can decrease the macropore size.

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3.2 Preparation of carbon spheres containing hierarchical porous structure

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3.2.1 Template-free method for preparation of RF spherical gels and carbon spheres

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As described in section 3.1, carbon monoliths with hierarchical porous structures can be

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prepared by sol-gel polymerization of RF solution containing Fe(NO3)3, followed by

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carbonization. This synthesis approach can be adapted to fabricate carbon spheres containing

4

hierarchical porous structures, if the sol-gel-polymerized area is confined within droplets

5

(water phase) made from a water-in-oil emulsion system until the completion of the reaction.

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Fig. 3 (a) shows the microsphere formation of RF gel droplets (water phase), which are

7

enveloped by emulsifier (SPAN80) to minimize interfacial tension at the boundary of the two

8

immiscible phases. SPAN80 forms microscopic droplets dispersed in cyclohexane (oil phase)

9

and prevents coalescence of the droplets by generating a repulsive force between them. These

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droplets are thus stably dispersed throughout the oil phase by agitation. As the reaction

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proceeds, the droplets transform into gel spheres, and the RF spherical gels can be obtained

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as shown in Fig. 3 (b). After carbonization to convert the gels into carbon spheres, both the

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spherical morphology and 3D interconnected macroporous structure remained unchanged, as

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shown in Fig. 3 (c). This confirms that the microemulsion is a key important step to prepare

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carbon spheres with hierarchical porous structures. To tailor the spherical size, dosages of

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SPAN80 in cyclohexane were adjusted in the range of 0.2 to 0.4 %Vol. Fig. 3 (d) shows the

17

particle size distributions of RF spherical gels and carbon spheres prepared from different

18

%Vol of SPAN80. The mean diameter of the spherical RF gels became lesser when the %Vol

19

was higher. The mean sizes were 302, 222, and 141 µm for %Vol of 0.2, 0.3, and 0.4,

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respectively. A possible interpretation of this result is that an increase of SPAN80 reduces

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the interfacial tension between the two phases, allowing the droplets to adjust their surface

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area to be as small as possible, and the resulting small surface area causes a decrease in the

23

size of the spherical drops [53]. All the obtained spherical carbons shrunk after carbonization

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compared with their initial sizes. The mean sizes of the carbon spheres are 259, 141, and 103

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µm for %Vol of 0.2, 0.3, and 0.4, respectively.

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3.2.2 Effect of acetone treatment

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To investigate the effects of acetone treatment, the RF spherical gels prepared from sol-gel

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emulsification with 0.4 %Vol of SPAN80 were further suddenly immersed in pure acetone

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before carbonization at 900 °C (CF3-900). For comparison, the RF spherical gels without

4

immersion in acetone were carbonized under the same conditions. It was observed that the

5

CF3-900 without acetone treatment had a thin-film layer encapsulating its exterior, as shown

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in Fig. 4 (a), but this disappeared with acetone treatment, as shown in Fig. 4 (b). The

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formation of the thin film covering is similar to that reported in Yamamoto's work [54], in

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which mesoporous carbon spheres were synthesized by an emulsification method using

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SPAN80 as the emulsifier. It may be presumed that copious SPAN80 gathers at the oil/water

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interface, such that the micelles can construct the thin layer covering the exterior of the

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droplets. As the SPAN80 was dissolved in acetone immediately after completion of the sol-

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gel polymerization within the droplets, leaching with acetone could easily break the micelle

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thin film. This result confirms that acetone treatment is an important step to reveal the true

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structure of hierarchical porous carbon spheres. Fig. 4 (c) shows a comparison of average

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diameters and particle size distributions of CF3-900 with and without acetone treatment.

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These results indicate that acetone treatment does not affect the particle size distribution. The

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mean diameters were 103 and 102 µm for the samples with and without the acetone

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treatment, respectively.

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3.2.3 Effect of carbonization and CO2 activation on the porous properties of spherical

2

carbons

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Pore development of the micro/mesoporous structures in the macroporous walls was

2

determined using the N2 adsorption-desorption isotherms, as shown in Fig. 5 (a) and (b). At a

3

temperature for carbonization of 500 °C (CF3-500), the isotherm started to uptake a small

4

amount of nitrogen, indicating that pore development had begun to occur on the wall of the

5

macropores. When the temperature further rose to 600 and 700 °C (CF3-600 and CF3-700,

6

respectively), the isotherms showed a higher uptake of nitrogen adsorption and exhibited the

7

characteristic Type-I shape according to IUPAC classification. This implies that a

8

microporous structure will only form in the macroporous wall if the carbonization

9

temperature is higher than 500 °C. The Vmic values of CF3-600 and CF3-700 were 0.23 and

10

0.24 cm3/g, respectively, as presented in Table 2. This development of micropores results in

11

an increase in SBET from 569 to 584 m2/g for CF3-600 and CF3-700, respectively. Focusing

12

on the isotherms of CF3-800, CF3-850, and CF3-900 in Fig. 5 (b), they can be seen to

13

display a combination of Type-I and Type-IV shapes, including the H3-hysteresis loop that

14

indicates the existence of both micropores and slit-shaped mesopores within the walls of the

15

macropores. This result implies that the mesopores can be generated at carbonization

16

temperatures higher than 800 °C. However, without the presence of Fe(NO3)3, the N2

17

isotherm of CF0-900, as shown in Fig. 5 (b), provides a Type-I shape even if the

18

carbonization temperature is as high as 900 °C. This is evidence of the crucial role of

19

Fe(NO3)3 in generating the mesopores.

20

As shown in Table 2, the Vmeso increased as the temperature of carbonization increased,

21

reaching values of 0.19, 0.21, and 0.21 cm3/g for CF3-800, CF3-850, and CF3-900,

22

respectively. On the other hand, the Vmic decreased when the temperature of carbonization

23

rose, to 0.22, 0.19, and 0.16 cm3/g for CF3-800, CF3-850, and CF3-900, respectively. This

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indicates that the presence of Fe(NO3)3 could destroy the micropore structure, constructing

2

the expanded pore size in a range of mesopores. Likewise, SBET could be reduced by

3

increasing the carbonization temperature, to 553, 467, and 400 m2/g for CF3-800, CF3-850,

4

and CF3-900, respectively, because of micropore decrease.

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The SEM images, as illustrated in Fig. 6 (a) and (b), reveal the spherical morphology of CF3-

18

900 and CF3-700. These results confirm the existence of mesoporosity within the

19

macroporous framework of CF3-900, and the lack of such mesoporosity in CF3-700. This

20

evidence is consistent with the N2 isotherm result as discussed above.

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ACF3-900 was prepared similarly to CF3-900 except for the use of CO2 activation for the

8

former instead of carbonization. The particle size distribution of ACF3-900, as shown in Fig.

9

7 (a), was roughly between 7 and 120 µm, with a median size of 35 µm. One can see that the

10

size distribution and the median size of ACF3-900 were reduced compared with those of

11

CF3-900. The evidence of the mesoporosity improvement by CO2 activation can be seen in

12

the N2 adsorption-desorption isotherm shown in Fig. 7 (b). This isotherm shows the high-

13

volume uptake of liquid nitrogen and exhibits hybridization of type-I and type-IV with a H3-

14

hysteresis loop, which indicates the presence of both micropores and a large amount of slit-

15

shaped mesopores. The mesopore width distribution of the ACF3-900, as shown in Fig. 7

16

(b)-inset, suggests that the mesopore widths were between 2.00 to 4.87 nm, with a peak at

17

2.54 nm. The porosities, as listed in Table 2, indicate that ACF3-900 possessed a very high

18

mesopore volume (Vmeso = 2.16 cm3/g) compared with CF3-900 (Vmeso = 0.21 cm3/g).

19

Meanwhile, ACF3-900 had Vmic and SBET of 0.2411 cm3/g and 603 m2/g, respectively. Fig. 7

20

(c) presents the SEM images of ACF3-900, which depict the spherical morphology (upper

21

image) and high mesoporosity located in the macroporous framework (middle and lower

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images). It is clear that the mesopores were significantly enhanced by CO2 activation, which

2

is consistent with the results of the N2 adsorption-desorption isotherm.

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3.3 Graphitic features and magnetic properties of the prepared carbon spheres

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The carbon spheres were characterized using Raman spectroscopy, owing to the high

19

sensitivity of this technique for the characterization of graphitic materials. The results are

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displayed in Fig. 8 (a). A D-band at ∼1360 cm-1 and G-band at ∼1580 cm-1 of the Raman

2

spectra were observed for all samples. The D-band could be attributed to the local defects

3

and disorders at the edges of the graphitic materials [55]. The peak intensities at the D-band

4

were stronger when the samples were treated at higher temperatures, from 500 to 900 °C,

5

indicating phase transformation from amorphous carbon to a graphitic structure. The high

6

intensities of the D-band in CF3-900 and ACF3-900 indicate that a high number of π bonds

7

(sp2 carbon atom hybridization) were replaced with C-O and/or C-C σ bonds (sp3), resulting

8

in structural defects from idiomatically perfect graphite/graphene [55,56]. The higher values

9

of the ID/IG ratios in CF3-900 and ACF3-900 compared with those of other samples indicate

10

the higher degree of defects in the graphitic structure [55]. The Raman-active mode located

11

at ∼2700 cm-1 (2D-band), which was observed only in CF3-900 and ACF3-900, supports the

12

presence of a graphitic phase that was indicated by the XRD results. The two additional

13

active modes of the graphitic structure at approximately 2400 cm-1 of the G* band and 3000

14

cm-1 of the D+D* band appeared clearly only for ACF3-900 [40]. This confirms that the CO2

15

activation process encourages the phase formation of graphitic structure in ACF3-900 more

16

than in the other samples, which were synthesized via the carbonization process. To further

17

confirm the influence of Fe(NO3)3 on structural formation in CF3-900, the Raman spectra of

18

CF3-900 and CF0-900 (controlled conditions of carbonization at 900 °C in the absence of

19

Fe(NO3)3) were also analyzed, showing that the condition without Fe(NO3)3 does not give the

20

high-intensity peak at the D-band or 2D-band. This can be attributed to the fact that Fe(NO3)3

21

promotes the replacement of π bonds (sp2) with σ bonds (sp3) in the graphitic structure.

22

The XRD patterns of the samples are shown in Fig. 8 (b). The XRD spectra of CF3-800,

23

CF3-900, and ACF3-900 showed a sharp peak at 26.57°, which could be assigned to the

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graphite phase (JCPDS 01-089-8487), together with a broad peak at approximately 44°. The

2

calculated crystallite (grain) sizes of the graphite phases derived using XRD line broadening

3

at 26.57° together with Scherrer’s equation were 3.71 and 3.42 nm for CF3-800 and CF3-

4

900, respectively; the crystallite size of ACF3-900 could not be evaluated owing to the

5

overlap between the peak of graphite and the broad pattern of the amorphous carbon.

6

However, CF3-500, CF3-700, and CF0-900 all showed only the broad pattern of amorphous

7

carbon, with a weak diffraction peak of graphite at 26.41° for CF3-700 and CF0-900. This

8

indicates that the graphitic phase formation is supported by the presence of Fe(NO3)3 co-

9

operating with reaction temperatures higher than 800 °C in both CO2 activation and

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carbonization.

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Fig. 8 (c) and (d) shows bright-field TEM images for CF3-900 and ACF3-900. The graphitic

12

samples were tangled with each other by bending and folding of multilayer graphitic sheets,

13

which provided spaces between the folded graphitic sheets to form the mesoporous

14

structures. This confirms the existence of mesopore volumes within both CF3-900 and

15

ACF3-900, as discussed in section 3.2.3. Also, good distribution of multilayer sheets led to

16

the increase of mesoporosities. The enlarged square regions given as insets in the Figure

17

show the multi-layer graphitic sheets with a lattice fringe spacing of 4.21 ± 0.05 Å and 4.33 ±

18

0.05 Å, respectively, consistent with the (001) spacing of expanded graphite structures [58].

19

The lattice spacings of both samples were greater than the general spacing (3.34 Å) of

20

multilayer graphene structures, owing to the structural expansion resulting from the

21

replacement of sp2 carbon atoms with sp3 carbon, as indicated by the Raman spectra.

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To confirm the Fe3O4 phase formation, therefore, pure Fe(NO3)3 powders were subjected to

23

the same synthesis conditions as CF3-900 and ACF3-900, resulting in products that were

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labeled CFe-900 and AFe-900, respectively. Fig. 9 (a) and (b) show comparisons of the XRD

2

patterns of CF3-900 vs. CFe-900 and ACF3-900 vs. AFe-900, respectively, to elucidate the

3

structural change in Fe(NO3)3. The XRD patterns of both CFe-900 and AFe-900 showed the

4

peak pattern of Fe3O4, indicating that Fe(NO3)3 was easily transformed to the Fe3O4 structure

5

after both carbonization and CO2 activation at 900 °C. Moreover, the influence of CO2

6

seemed to promote the phase formation of Fe3O4 with higher crystallinity compared with N2,

7

owing to the higher oxidation firing of CO2. The characteristic peaks of Fe3O4 (JCPDS 00-

8

019-0629) also appeared, as an intense peak at 35.42° with five other weaker peaks at 30.09,

9

43.05, 53.40, 56.94, and 62.57° on ACF3-900, which confirms that ACF3-900 consists of a

10

mixed composition of graphite and Fe3O4. However, the Fe3O4 phase with very low

11

%loading of Fe (III), less than 0.6% weight in the starting mixture (Fe/RF-ratio of 0.023 g/g

12

as listed in Table 1), would normally be obstructed by the domain phase (graphite) and

13

background in the XRD pattern, resulting in the lower relative intensity of Fe3O4 in both

14

CF3-900 and ACF3-900 in the XRD patterns.

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The SEM-EDS results, as listed in Table 3, revealed the presence of C, O, and Fe with

23

atomic% of 96.53, 3.12, and 0.35, respectively, on CF3-900; and 95.06, 4.47, and 0.47,

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respectively, on ACF3-900. Furthermore, the well distributions of Fe3O4 particles in CF3-900

2

and ACF3-900 were measured using SEM-EDS mapping as shown in Fig. 9 (c) and (d),

3

respectively. The results illustrated that both samples contained a large number of carbon

4

atoms with equally good distributions of both iron and oxygen atoms. There were particles of

5

Fe3O4 dispersed randomly in the matrix of the carbon structure of both samples,

6

corresponding to the phase formation of Fe3O4 indicated by the XRD results.

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Fig. 10 (a) shows the magnetization curves, in which the magnetization (emu/g) and

2

magnetic field (Oe) are plotted. The observed hysteresis loop corresponds to the

3

ferromagnetism property, which indicates the presence of Fe3O4 in both samples. The

4

saturation magnetization (Ms), coercive force (Hc), and chemical compositions as

5

characterized by EDS are presented in Table 3. The Ms of ACF3-900 was 5.8 emu/g with Fe

6

content of 2.10 (weight%), compared with 0.4 emu/g and 1.57 weight% of Fe for CF3-900.

7

This result suggests substantial improvement in the magnetic property of ACF3-900, in

8

agreement with the phase formation of Fe3O4 having higher crystallinity than that of CF3-

9

900, as previously discussed. It also confirms that our method was successful for fabricating

10

the MHPC spheres, whose inner structures contained the interconnected macroporous

11

framework and whose surface possessed both micro- and mesoporosities, especially in the

12

case of CF3-900 and ACF3-900. Moreover, graphitic features could be found in both

13

samples. Fig. 10 (b) and VDO 1 (video clip) illustrate the performance of ACF3-900 for MO

14

removal from water at an initial concentration of 20 mg/L; the suspended solution rapidly

15

turned transparent, showing an easy separation that confirms the good magnetic response of

16

the MHPC spheres to an outer magnet. This also reveals that the prepared MHPC spheres are

17

a promising adsorbent for MO removal from aqueous solution.

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3.4 Application of the prepared carbon spheres for methyl orange adsorption

2

MHPC spheres with different porous characteristics were chosen to adsorb MO at neutral pH.

3

ACF3-900 and CF3-900, which possess Vmeso values of 2.16 and 0.21 cm3/g, respectively,

4

were used as representative MHPC spheres. For comparison, hierarchical porous carbon

5

spheres without mesopores (CF3-700) were also studied.

6

3.4.1 Effects of carbon dosages and initial concentrations on MO removal efficiency

7

The effects of carbon dosage on the removal percentage of MO was studied by varying the

8

amount of carbon in the MO solution from 0.05 to 0.4 g/L, with initial concentration,

9

temperature, stirring rate, and contact time kept constant at 20 mg/L, 30 °C, 200 rpm, and 48

10

h, respectively. The results, presented in Fig. 11 (a), show that CF3-700 (with an absence of

11

mesopores within its structure) had a poor removal efficiency of MO adsorption even when

12

the carbon dosage was as high as 0.4 g/L (2.51% removal), whereas the MHPC spheres

13

containing mesoporous structures (CF3-900 and ACF3-900) provided markedly better MO

14

removal performance. The results also show that the removal percentage of MO was

15

improved by increasing the carbon dosage (from 97.29 to 99.88% and 36.84 to 95.95% for

16

ACF3-900 and CF3-900, respectively), because the available surface area was enhanced,

17

consequently increasing the adsorption. MO adsorption reached its highest removal

18

percentage at a carbon dosage of 0.4 g/L for all MHPCs. Therefore, the remaining

19

experiments were carried out with 0.4 g/L carbon dosage.

20

The effects of initial concentration (varied from 10 to 300 mg/L) on the removal percentage

21

of MO are shown in Fig. 11 (b). Notably, the removal percentage decreased when initial

22

concentrations of MO were increased, from 99.91 to 77.46%, 97.22 to 12.50%, and 3.27 to

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1.46% for ACF3-900, CF3-900, and CF3-700, respectively. The decrease in removal

2

percentage was attributed to there being insufficient adsorption sites on the adsorption

3

surface at the higher initial concentrations [59]. The slight decrease in removal efficiency for

4

ACF3-900 compared with the others, even at higher initial concentrations, was due to its

5

abundant surface adsorption sites, resulting from the tremendous mesoporosities within its

6

hierarchical porous structure.

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3.4.2 Equilibrium isotherms, kinetics, and thermodynamics of MO adsorption

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Adsorption equilibrium isotherms, kinetics, and thermodynamics were adopted in this study

2

to help understand the mechanism of MO adsorption onto MHPC spheres. Fig. 12 (a) – (c)

3

shows the equilibrium adsorption data for MO adsorption, fitted to the Langmuir, Freundlich,

4

and D-R isotherms by a nonlinear regression method at 30 °C and carbon dosage of 0.4 g/L.

5

The R2 values, as well as the related parameters of Langmuir (Q0 and b), Freundlich (Kf and

6

n), and D-R (qDR and E), are summarized in Table 4.

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To describe the adsorbate-adsorbent interaction mechanism, i.e., mono- or multilayer

13

adsorption, Langmuir and Freundlich isotherms were analyzed by fitting with the

14

experimental data. The equilibrium data were found to be in accordance with both isotherms,

15

as evidenced by the values of R2 being close to unity for all carbon spheres, with notably

16

slightly higher values for Freundlich. Thus, it is possible that adsorption onto heterogeneous

17

sites and multilayer mechanism are the main processes involved, instead of monolayer

18

adsorption onto homogeneous sites [60].

19

To consider the potential for adsorption on carbon surfaces, the monolayer adsorption

20

capacity (Q0) of the Langmuir isotherm should also be discussed. As shown in Table 4, the

21

values of Q0 were 1522.6, 77.1, and 16.2 mg/g for ACF3-900, CF3-900, and CF3-700,

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respectively. Consequently, ACF3-900 appears to have tremendous capacity for MO

2

adsorption, higher than that of many carbon-based adsorbents recently reported by others, as

3

listed in Table 7. However, while monolayer adsorption is assumed for Langmuir, it is also

4

reasonable to explain the adsorption capacity of MO in terms of the ratio Q0/SBET. As shown

5

in Table 4, the values of this ratio were 2.5 and 0.2 mg/m2 for ACF3-900 and CF3-900,

6

respectively, whereas CF3-700 had a smaller value of 0.3 x 10-1 mg/m2. Clearly, the

7

existence of mesoporosities within hierarchical porous structures can significantly enhance

8

the Q0/SBET of MO adsorption, especially for MO adsorption on ACF3-900. The excellent

9

adsorption capacity of ACF3-900 could possibly be explained by the presence of more

10

available adsorptive sites resulting from the very high mesopore volumes (Vmeso = 2.16

11

cm3/g), whose pore widths are between 2.00 to 4.87 nm, suitable for the molecular size of the

12

adsorbate. Previous reports have given the molecular size of MO as approximately 1.2 nm

13

[61, 62], and it may be 2.6 nm owing to ionic micelles forming in aqueous solution [63].

14

The D-R isotherm can give insight into the adsorption energy, E, which provides information

15

about the adsorption process, i.e., whether it involves physisorption, ion-exchange, or

16

chemisorption. Values of E lower than 8 kJ/mol correspond to physical adsorption, while

17

values between 8 and 16 kJ/mol suggest ion-exchange adsorption, and those higher than 16

18

kJ/mol indicate a chemisorption mechanism [64]. As shown in Table 4, the E values were 4.1

19

and 3.5 kJ/mol for ACF3-900 and CF3-700, respectively, indicating that the adsorption of

20

MO was physisorption, whereas the E value of 12.0 kJ/mol for CF3-900 indicated an ion-

21

exchange adsorption process.

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To further explore the thermodynamic properties, three basic properties, ∆H0, ∆S0, and ∆G0,

6

were determined and are presented in Table 5. The negative values of ∆H° obtained for all

7

samples show that the adsorption process was exothermic in nature and favorable at low

8

temperatures [65]. In addition, the value of ∆H° can give an indication of the type of

9

adsorption, i.e., physical or chemical. Values of ∆H° larger than -84 kJ/mol indicate physical

10

adsorption, while those between -80 and -420 kJ/mol are associated with chemisorption [65-

11

67]. The obtained ∆H0 values were -42.4, -72.9, and -27.8 kJ/mol for ACF3-900, CF3-900,

12

and CF3-700, respectively; these results indicate that the MO adsorption onto all carbon

13

spheres was physisorption. However, MO adsorption onto CF3-900 tended to be a relatively

14

strong adsorbate-adsorbent interaction, which is in agreement with the ion-exchange

15

mechanism previously determined from the D-R isotherm. Detailed studies of the role of

16

surface chemistry to understand the in-depth adsorption mechanism of the prepared carbon

17

spheres would be worth considering in further research; however, these are beyond the scope

18

of this work.

19

Furthermore, the negative values of ∆S0 for all carbon spheres indicate a decrease in degrees

20

of freedom of MO after adsorption, resulting from the restriction of molecular movement on

21

the surface of the adsorbent [68]. Finally, the negative values of ∆G0 indicate the spontaneity

22

and feasibility of adsorption, while a decreased value of ∆G0 illustrates more favorable

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adsorption [69,70]. These results confirm that MO adsorption onto ACF3-900 and CF3-900

2

is spontaneous at all temperatures, and is less favorable at high temperatures. On the

3

contrary, ∆G0 of MO adsorption onto CF3-700 had a positive value at all temperatures,

4

indicating unfavorable adsorption of MO. This explains why the MO removal efficiency by

5

CF3-700 was very low.

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Fig. 12 (d) – (f) shows the kinetic adsorption data for MO onto ACF3-900, CF3-900, and

12

CF3-700 at a temperature of 30 °C, carbon dosage of 0.4 g/L, and initial concentration of 20

13

mg/L. MO adsorption onto both ACF3-900 and CF3-900 reached near-equilibrium within

14

approximately 20 min, implying an excellent kinetic property for MO adsorption, whereas

15

CF3-700 required at least 200 min to reach equilibrium. It is clear that the presence of

16

mesoporosities within hierarchical porous structures can enhance the rate of MO adsorption.

17

PFO and PSO kinetic models were tested to understand the dynamic mechanism of the MO

18

adsorption rate. The experimental data were fitted to PFO and PSO kinetic models by a

19

nonlinear regression method. The R2 values and the related parameters of PFO (k1 and qe1)

20

and PSO (k2, qe2, and h) are summarized in Table 6. PSO fit better with the experimental data

21

compared with PFO for all samples, indicating that MO adsorption onto all carbon spheres

22

followed the pseudo-second-order kinetic mechanism [39]. The initial rate constants (h2), as

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listed in Table 6, were 36.8, 10.6, and 0.3 x 10-1 mg/g·min for ACF3-900, CF3-900, and

2

CF3-700, respectively. These results reveal that the existence of mesoporosities within

3

hierarchical porous structures has a crucial role in the enhancement of the adsorption kinetic

4

rate.

5

3.4.3 Reusability of MHPC spheres

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Reusability is an important performance index for assessing the practical applicability of

16

MHPC spheres. In order to study the reusability of the adsorbents, recycling experiments

17

were conducted with each cycle. MHPC spheres were separated by a magnet after adsorption

18

and regenerated by desorption in pure ethanol at 30 °C with shaking at 200 rpm several times

19

until the solution became colorless. Then, the treated carbon spheres were used to re-adsorb

20

MO at the initial concentration and temperature of 20 mg/L and 30 °C, respectively, for each

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cycle. As shown in Fig. 13, after four consecutive cycles, the MO removal efficiency of

2

ACF3-900 still remained over 90%. On the contrary, the removal percentage of CF3-900

3

drastically decreased after the first run compared with ACF3-900; this was probably owing to

4

more MO residuals covering the surface after regeneration, resulting from a strong adsorbate-

5

adsorbent interaction. Moreover, it is probable that the immense mesoporosities of ACF3-

6

900 could provide abundant adsorption sites for MO removal, thus having more-than-

7

adequate available adsorption sites after regeneration for several cycles, despite the

8

remaining MO residuals. These results suggest that ACF3-900 could be regarded as a high-

9

performance and reusable adsorbent for MO removal from aqueous solution. Moreover, both

10

ACF3-900 and CF3-900 can remain responsive to an external magnet despite being used for

11

four consecutive regeneration cycles.

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Table 7 gives a comparison of the maximum monolayer adsorption capacities (Q0) of MO for

21

various carbon-based adsorbents, together with their magnetism and reusability. Clearly, the

ACCEPTED MANUSCRIPT -41-

MHPC spheres prepared in this work have tremendous adsorption capacity (1522.6 mg/g),

2

which is higher than that of many carbon-based adsorbents recently reported by others

3

[29,39-44]. This material also features good magnetism and reusability.

4

4. Conclusion

5

The successful preparation of MHPC spheres by a template-free method in a microemulsion

6

system is presented here. The prepared MHPC spheres possess high capability for adsorption

7

of MO from aqueous solution, easy separability from water by magnetic force after use, and

8

reusability to adsorb MO several times. The results can be summarized as follows.

9

(1) Water-in-oil emulsification coupled with sol-gel polymerization of resorcinol containing

10

Fe(NO3)3 and formaldehyde is a crucial factor to make spherical carbon precursors

11

containing a macroporous framework. The prepared precursors can be further converted to

12

MHPC spheres by carbonization or CO2 activation; in this way, either micro- or mesopores

13

can be generated within the macroporous frameworks.

14

(2) Another important step in the synthesis of MHPC spheres not covered by a thin film is

15

treatment with acetone immediately after completion of the sol-gel polymerization.

16

(3) Carbonization at temperatures above 800 °C can not only generate both micro- and

17

mesopores in the macroporous framework, but can also impart graphitic features, whereas

18

carbonization at lower temperatures can only generate micropores, not graphitic features.

19

(4) CO2 activation at 900 °C can result in both high mesopore volume (2.16 cm3/g) within the

20

macroporous framework and remarkable graphitic features. Good magnetic properties can

21

also be obtained by both carbonization and CO2 activation at 900 °C; however, CO2

22

activation can better enhance the magnetic responsivity.

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(5) The MHPC spheres obtained by CO2 activation possess tremendous adsorption capacities,

2

as high as 1522.6 mg/g, for MO removal, and are easily separated from the suspension by

3

magnetic force. Moreover, the spheres are a high-performance and reusable adsorbent that

4

can be used for at least four consecutive cycles of regenerations.

5

Acknowledgement

6

This work is fully supported by CMU Junior Research Fellowship Program.

7

References

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Table captions

2

Table 1. Synthesis conditions of the prepared carbon spheres and monoliths derived from RF

3

gels.

4

Table 2. Type of isotherm, specific surface area, micropore volume, and mesopore volume

5

of prepared carbons obtained from N2 adsorption-desorption isotherms at -196 °C.

6

Table 3. Chemical compositions and magnetic properties of CF3-900 and ACF3-900.

7

Table 4. Langmuir, Freundlich, and D-R isotherm parameters for MO adsorption.

8

Table 5. Thermodynamic properties of MO adsorption onto the prepared carbon spheres.

9

Table 6. Pseudo-second-order (PSO) and pseudo-first-order (PFO) kinetic parameters of MO

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adsorption onto the prepared carbon spheres.

11

Table 7. Comparison of maximum monolayer adsorption capacity (Q0) of MO, magnetism,

12

and reusability of various carbon-based adsorbents.

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Figure captions

2

Fig. 1. Schematic representation of carbon sphere fabrication.

3

Fig. 2. (a) SEM images showing the macroporous textures; (b) macropore size distributions

4

determined by the mercury intrusion method.

5

Fig. 3. (a) Schematic illustration of the synthetic strategy to fabricate spherical RF gels

6

(carbon precursors) via sol-gel polymerization in a water-in-oil emulsion system; (b) SEM

7

image of the obtained spherical RF gels; (c) SEM image of carbon spheres (CF3-900)

8

derived from carbonization at 900 °C; (d) particle size distributions of RF spherical gels (red)

9

and carbon spheres (black) with different % Vol. of SPAN80 at 0.2 % (rectangular), 0.3 %

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(triangle), and 0.4 % (circle).

11

Fig. 4. Demonstration of effect of acetone treatment before carbonization; (a) SEM image of

12

CF3-900 without acetone treatment; (b) SEM image of CF3-900 with acetone treatment; (c)

13

comparison of particle size distributions.

14

Fig. 5. (a), (b) Nitrogen adsorption-desorption isotherms of CF0-900, CF3-500, CF3-600,

15

CF3-700, CF3-800, CF3-850, and CF3-900; (c) mesopore width distributions of CF3-800,

16

CF3-850, and CF3-900.

17

Fig. 6. SEM images of (a) CF3-900 and (b) CF3-700, showing the surface morphology of

18

spherical carbons with and without mesoporous structure, respectively.

19

Fig. 7. Effect of CO2 activation on the physical morphology and porous properties of ACF3-

20

900; (a) particle size distribution; (b) N2 adsorption-desorption isotherm and mesopore width

21

distribution (inset-picture); (c) SEM image of ACF3-900 at different magnifications.

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Fig. 8. (a) Raman spectra showing the develop of the graphitic structure with respect to

2

reaction temperature and synthesis conditions, (b) XRD data showing the graphitic phase

3

formation of the samples, (c) and (d) TEM images with HRTEM inset of CF3-900 and

4

ACF3-900, respectively, showing the multilayer sheet of the samples.

5

Fig. 9. (a) and (b) Comparison of XRD patterns of CF3-900 vs CFe-900 and ACF3-900 vs

6

AFe-900 to study the Fe3O4 phase formation; (c) and (d) SEM-EDS mapping of CF3-900 and

7

ACF3-900, respectively.

8

Fig. 10. (a) VSM results showing magnetization curves of CF3-900 and ACF3-900; (b)

9

pictures for illustration of convenient separation of MHPC spheres (ACF3-900) by a magnet

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after MO removal.

11

Fig. 11. (a) Effects of carbon dosages and (b) initial concentrations on MO removal

12

efficiency of carbon spheres.

13

Fig. 12. (a)–(c) Isotherm fitting curves for MO adsorption onto carbon spheres; (d)-(f) kinetic

14

models fitting curves for MO adsorption onto carbon spheres.

15

Fig. 13. Reusability of ACF3-900 and CF3-900 at four cycles for MO removal (initial

16

concentration and carbon dosage of 20 mg/L and 0.4 g/L, respectively).

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Table 1.

Conditions Carbonized or activated temperature (°C) 900 900 900 900 850 800 700 600 500 900

EP AC C

Macroscopic shape

Monolith Monolith Monolith Monolith and sphere Sphere Sphere Sphere Sphere Sphere Sphere

SC

Carbonization or CO2-activation Carbonization with N2 Carbonization with N2 Carbonization with N2 Carbonization with N2 Carbonization with N2 Carbonization with N2 Carbonization with N2 Carbonization with N2 Carbonization with N2 Activation with CO2

TE D

CF0-900 CF1-900 CF2-900 CF3-900 CF3-850 CF3-800 CF3-700 CF3-600 CF3-500 ACF3-900

Fe/RFratios (g/g) 0.003 0.010 0.023 0.023 0.023 0.023 0.023 0.023 0.023

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Synthesis conditions of the prepared carbon spheres and monoliths derived from RF gels.

ACCEPTED MANUSCRIPT

Table 2. Type of isotherm, specific surface area, micropore volume, and mesopore volume of prepared carbons obtained from N2 adsorption-desorption isotherms at -196 °C. Type of isotherm

Vmic (cm3/g)

Vmeso (cm3/g)

685 400 467 553 584 569 12 603

0.27 0.16 0.19 0.22 0.24 0.23 0.4 x 10-2 0.24

N/D* 0.21 0.21 0.19 N/D* N/D* N/D* 2.16

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CF0-900 I CF3-900 I + IV CF3-850 I + IV CF3-800 I + IV CF3-700 I CF3-600 I CF3-500 I ACF3-900 I+IV Remark: *N/D = not determined

SBET (m2/g)

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Table 3. Chemical compositions and magnetic properties of CF3-900 and ACF3-900. Chemical composition* Atomic % Weight % C O Fe C O Fe 96.53 3.12 0.35 94.36 4.07 1.57 95.06 4.47 0.47 92.13 5.77 2.10

Magnetic properties**

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Hc (Oe) CF3-900 134.3 ACF3-900 234.0 Remarks: *The chemical compositions of carbon (C), oxygen (O), and Iron (Fe) are characterized by EDS. **The magnetic properties were tested by VSM.

Ms (emu/g) 0.4 5.8

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Table 4. Langmuir, Freundlich, and D-R isotherm parameters for MO adsorption.

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ACF3-900 CF3-900 CF3-700

Q0 (mg/g) 1522.6 77.1 16.2

D-R E (kJ/mol) 4.1 12.0 3.5

R2 (-) 0.9874 0.9905 0.9879

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Samples

Isotherm model constants and correlation coefficients Langmuir Freundlich b Q0/SBET R2 Kf n R2 qDR (L/mg) (mg/m2) (-) (mg/g)(L/mg)1/n (-) (-) (mg/g) 1.0 x 10-2 2.5 0.9941 63.0 1.9 0.9983 3.6 x 10-3 1.5 0.2 0.9940 39.2 6.3 0.9948 2.8 x 10-4 4.6 x 10-3 0.3 x 10-1 0.9977 0.2 1.4 0.9976 2.9 x 10-5

ACCEPTED MANUSCRIPT

Table 5.

Samples

∆S° (J/mol-K) -90.8 -197.2 -106.5

30 °C -14.9 -13.1 4.5

∆G° (kJ/mol) 40 °C 60 °C -14.0 -12.2 -11.2 -7.2 5.5 7.7

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ACF3-900 CF3-900 CF3-700

∆H° (kJ/mol) -42.4 -72.9 -27.8

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Thermodynamic properties of MO adsorption onto the prepared carbon spheres. 80 °C -10.3 -3.3 9.8

R2 (-) 0.9951 0.9655 0.9578

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Table 6. Pseudo-second-order (PSO) and pseudo-first-order (PFO) kinetic parameters of MO adsorption onto the prepared carbon spheres.

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ACF3-900 CF3-900 CF3-700

k2 (g/mg· min) 1.5 x 10-2 4.4 x 10-3 1.6 x 10-2

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Samples

Adsorption kinetics parameters and correlation coefficients Pseudo-second order (PSO) Pseudo-first order (PFO) qe2 h2 R2 k1 qe1 (mg/g) (mg/g· min) (-) (min-1) (mg/g) 49.7 36.8 0.9794 3.8 x 10-1 48.2 49.2 10.6 0.9833 1.2 x 10-1 46.4 1.3 0.3 x 10-1 0.9912 1.8 x 10-2 1.1

R2 (-) 0.9379 0.9718 0.9843

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Table 7.

Maximum adsorption capacity (mg/g)

References

yes

No report

152.0

[29]

yes no

yes No report

182.8 570.0

[39] [40]

yes

No report

no

yes

no yes yes

yes

EP AC C

SC

Reusability

181.2

[41]

545.5

[42]

680.2

[43]

yes

81.0

[44]

yes

1522.6

This work

TE D

Magnetic hierarchical porous carbon sphere Magnetic porous carbon Carbon nanosphere Carbon dots decorated magnetic nanoparticle Hematite nanoparticles decorating graphene nanosheet Porous carbon derived from pomelo peel MWCNTs Magnetic hierarchical porous carbon (MHPC) sphere

Magnetism

M AN U

Carbon-based adsorbents

RI PT

Comparison of maximum monolayer adsorption capacity (Q0) of MO, magnetism, and reusability of various carbon-based adsorbents.

AC C

EP

TE D

M AN U

SC

RI PT

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