Property Variation of Magnetic Mesoporous Carbon Modified by

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The property variation of magnetic mesoporous carbon modified by aminated hollow magnetic nanospheres: Synthesis, characteristic and sorption Xin Li, Weicheng Cao, Yunguo Liu, Guangming Zeng, Zeng Wei, Lei Qin, and Ting-ting Li ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.6b01207 • Publication Date (Web): 01 Dec 2016 Downloaded from http://pubs.acs.org on December 3, 2016

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ACS Sustainable Chemistry & Engineering

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The property variation of magnetic mesoporous carbon

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modified by aminated hollow magnetic nanospheres:

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Synthesis, characteristic and sorption

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Xin Li a, b, Wei-cheng Cao a, b*, Yun-guo Liu a, b,*, Guang-ming Zeng a, b, Wei-Zeng a, b, Lei

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Qin a, b, Ting-ting Li a, b

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a

Juzizhou Street，Changsha 410082, P.R. China

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College of Environmental Science and Engineering, Hunan University, Lushan South Road,

b

Key Laboratory of Environmental Biology and Pollution Control (Hunan University), Mini

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stry of Education, Changsha 410082, P.R. China

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* Corresponding author:

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Wei-cheng Cao, E-mail address: [email protected];

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Yun-guo Liu, Tel.: + 86 731 88649208; Fax: + 86 731 88822829; E-mail address:

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[email protected]

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ACS Paragon Plus Environment


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ABSTRACT

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Magnetic mesoporous carbon with particular morphology was fabricated by

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immobilizing uniform aminated hollow magnetic nanospheres (AHMNs) into oxidized

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mesoporous carbon (OC) matrix with different mass ratio (AHMOC-Y, Y = 1:1, 2:1, 5:1).

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This study devoted to explore the effect on morphology，surface charge and the adsorption

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capacity when AHMNs were immobilized onto OC. The morphology and surface properties

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were studied using SEM, BET, XRD, FTIR, XPS and VSM. Batch experiments were carried

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out to study the sorption behavior of methylene blue (MB) by AHMOC-Ys, indicating that a

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good adsorption capacity of cation dye could be obtained at a mild condition (at pH = 8

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compared with pH = 11) which was sustained by point of zero charge (pHpzc). The

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characterizations of adsorption behavior revealed that isotherm and kinetics synthesis were

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well-fitted respectively by pseudo-second-order model and Freundlich isotherm model. The

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rate-limiting step was mainly involved film diffusion and intra-particle diffusion for the

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whole reaction. Thermodynamic analysis indicated that the adsorption reaction was an

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endothermic and spontaneous process. The conclusion revealed that AHMOC-2:1 had

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advantages in adsorption capacity and separation feasibility compared with OC,

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AHMOC-1:1 and AHMOC-5:1, which could be preferable in practical application of

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environment purification.

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KEY WORDS: Mesoporous carbon; Hollow nanospheres; Magnetic; Methylene blue;

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Adsorption mechanism

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INTRODUCTION

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Mesoporous materials especially ordered mesoporous carbon materials, had been 2


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aroused significant concerns since Ryong Ryoo et al. first prepared and denoted as CMK

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materials in 1999 due to their high specific pore volume, high specific surface area, uniform

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pore size distribution, stability and good electrical conductivity.1-4 The ordered mesoporous

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carbon presented the uniform mesopores, larger surface area and ordered structure, which

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might be resulted in more anchoring sites for a variety of applications such as catalysts

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supporter,5 super-capacitor,6 medicine release involving the practical application of large

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hydrophobic molecules (ibuprofen, vitamins, dyes).7-10 However, it still suffered from

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disadvantages relating with its inherent properties, including poor hydrophilicity and difficult

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reclamation, which might possibly cause secondary pollution and severely hinder its

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practical application.11

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Numerous methods and techniques had been applied to modify ordered mesoporous

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carbon to overcome its defect. Previous studies indicated that nitric acid oxidation can

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introduce

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hydrophobic/hydrophilic balance.8 Furthermore, the incorporation of metallic elements (Pd,

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Zn, Co, Fe, Zr etc.) into ordered mesoporous carbon during the preparation or via

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post-synthetic methods that had been led to high performance such as adsorption, catalyst

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and magnetism.12-14 Noteworthily, majority researchers were committed to combine both the

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well-ordering of mesoporous materials with the magnetic property of nanoparticles for

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adsorption of dyes.15-18 Up to know, the current research of magnetic ordered mesoporous

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carbon was commonly synthesized by adding extra iron precursor such as FeSO4，Fe2(SO4)3

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or FeCl3.19-22 For instance, Shanmuga Kittappa et al.15 synthesized Fe3O4 nanoparticles firstly,

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then the nano-magnetite encapsulated with silicon dioxide (SiO2) was synthesized by Stöber

carboxyl

groups

into

the

carbon

material

3


and

change

its

surface


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method23 to fabricate nanocomposite mesoporous materials for removal of methylene blue. It

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exhibited the higher and faster adsorption capacity for MB than other silicic mesoporous

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materials. Lin Tang et al.11 synthesized ordered mesoporous carbon composite which was

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functionalized with carboxylate groups and iron oxide nanoparticles to adsorb

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2,4-dichlorophenoxyacetic acid. The materials exhibited good magnetic properties and large

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adsorption capacity at low pH and temperature. However, these studies only focused on

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magnetizing the ordered materials by impregnating ferric chloride solution into carbon/silica

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source.18, 19, 24 According to the above literatures, these synthesis methods couldn’t get the

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uniform and controllable morphology of magnetic nanoparticles, meanwhile, it might lead to

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instability and aggregation, which would lower the particle reactivity and mobility. In

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addition, the decrease in surface area of ordered mesoporous carbon after incorporating

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magnetic nanoparticles should be concerned because it was significant for contaminant

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removal successfully.

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It was worth mentioning that, hierarchically structured magnetic iron oxide materials

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especially hollow iron nanospheres had been paid extensive attentions as potential materials

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for sequestering hazardous dyes,25 toxic heavy metal,26 drug delivery27, 28 and so on.25, 26,

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29-35

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non-toxic, hydrophilic, chemically stable, easily in synthesis and excellent recycling

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capability.36 Manickam Sasidharan and his groups synthesized hollow α-Fe2O3 and Fe3O4

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nanospheres using FeCl3.6H2O as iron precursor and micelles of poly (styrene-b–acrylic

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acid-b–ethylene oxide) (PS–PAA–PEO) as soft template to release ibuprofen sustainably.33

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Mahmood Iram et al.25 fabricated Fe3O4 hollow nanospheres via a simple one-pot

The functionalized hollow iron nanospheres possessed the properties such as magnetic,

4


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template-free hydrothermal method and investigated their application as an adsorbent for

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neutral red dye contaminants removal from water. These studies were depend on that hollow

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Fe3O4 nanospheres enclosed a vacant volume at their centers and presented shell-like ordered

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independent nanoparticles outside. The structure provided a high surface area, high polarity,

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large interior space, quantum sizes, low density, thermal and chemical stability and high

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permeability. As a consequence, these properties provided a higher removal capacity for

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heavy metal ions and dyes as well.26, 37

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The above studies emphasized that the functional groups existed on the surface and

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metallic nanoparticles encapsulated within the porous carbons played a key role in particle

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application.8, 38 It might be an alternative choice to combine ordered mesoporous carbon with

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hollow iron nanospheres to affect the surface charge each other and improve separation

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capacity. However, grafting hollow iron nanospheres directly to ordered mesoporous carbon

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was instability and might greatly reduce adsorption capacity due to the aggregation of

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magnetic iron oxide. Therefore not only ordered mesoporous carbon should be improved but

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the hollow iron nanospheres should be modified as well before assembled with each other. In

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this study, ordered mesoporous carbon was oxidized by nitric acid and hollow iron

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nanospheres were aminated by urea in one-step simple template-free hydrothermal method.

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Then the synthesized aminated hollow magnetic nanospheres (AHMNs) were immobilized

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into the oxidized ordered mesoporous carbon matrix (OC) by post-synthesis.39 The

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functionalized AHMNs and OC obtained functional groups (e.g. carboxyl, amino) which

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possibly resulted in that the AHMNs well dispersed and encapsulated in the matrix of OC at

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the presence of ultrasound when they were compounded covalently with each other. The final 5



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sample was noted as AHMOC and the charged surface of the composition could greatly

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enhance the dispersion and morphology of the particles. In addition, the acylamino generated

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by combination of carboxyl and amino groups was also negative charged and favorable for

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catching cation dye molecules. This research synthesized a novel functional uniform

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magnetic carbon framework which not only maintained the structural and geometrical

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features of hollow Fe3O4 nanospheres and ordered mesoporous carbon, but also protected the

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magnetic spheres to a certain extent to guarantee its adsorption capacity.

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The primary objective of this research was not only to study the formation mechanism

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of AHMOC, including its physical and chemical properties, but also to identify the optimum

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synthesis proportion of material for removal of cationic dye (methylene blue) from aqueous

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solution. The innovative adsorbent (AHMOC) was prepared successfully with different mass

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ratio (1:1, 2:1, 5:1), and a batch equilibrium technique was applied to investigate the effect of

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various experimental factors, such as pH, initial dye concentration, ionic strength, contact

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time and temperature. The adsorption isotherms, kinetics and thermodynamics were

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performed to analyze the adsorption behaviors. The results indicated that AHMOC-2:1 was

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promising for further application as an efficient low cost and recyclable adsorbent for dye

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removal.

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EXPERIMENT SECTION (MATERIALS AND METHODS)

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The samples were synthesized by combining oxidized ordered mesoporous carbon (OC)

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and aminated hollow magnetic nanospheres (AHMNs) in a mild condition. Experiment

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section (Materials, methods, Materials characterization and Sorption experiments) and part

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of the Sample characterization (SEM and BET) were presented in Supplementary 6


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information.

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RESULTS AND DISCUSSION

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Sample characterization

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Scanning electron microscope (SEM) images displayed the surface morphologies of

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CMK-3, OC and AHMOC-Y (Y = 1:1, 2:1, 5:1), which were set out in Fig. 1. It was clear to

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find that the pristine CMK（Fig. 1a） displayed a typical ordered mesoporous structure,

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which was consistent with the previous studies.2, 4 However, the oxidized mesoporous carbon

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(Fig. 1b) consisted of many rope-like domains which exhibited a similar structure with initial

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mesoporous carbon in average diameter of 350−450 nm.8 Fig. 1c and Fig. 1d presented the

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special sphere-rhabditiform macrostructures which was resulted from distribution of AHMNs

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on OC with different density，indicating that the AHMNs were successfully grafted onto

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mesoporous carbon structure and uniformly dispersed on the surface of oxidized carbon

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rods.39 Fig. 1c presented that the load of AHMNs onto OC surface had a little agglomeration,

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which slightly aggravated the order of mesoporous but without destroying the skeleton

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structure of OC.40 It was obviously to find in the patterns of AHMOC-Ys that the AHMNs

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were not only grafted on the surface of OC (Fig. 1c), but also encapsulated by several

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segments of OC fabricating cage-like domains (Fig. 1d). These were attributed to the

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covalent binding between amino groups and carboxy groups. However, the SEM images of

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AHMOC-Ys were considerably different from other magnetic carbon.22, 41

7



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Fig. 1. SEM images of CMK (a), OC (b), AHMOC-2:1 (c) and (d), AHMOC-1:1 (e),

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AHMOC-5:1 (f).

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Fig. 2a displayed the Wide-angle X-ray diffraction (XRD) patterns of as-prepared each

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sample. It was noted that the pristine OC exhibited two widen scattering peaks at around 24°

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(002) and 44°(100), which were consistent with graphitic lattice of oxidized mesoporous

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carbon and demonstrated a high degree of graphitization.42, 43 However, the XRD patterns of 8


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AHMOC-Ys presented similar characteristic peaks at 2θ = 30.0º, 35.5º, 37.1º, 43.2º, 53.6º,

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57.1º and 62.7º, which were assigned to the (112), (211), (202), (220), (422), (511) and (440)

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reflection respectively.26, 44 These results corresponded to the standard orthorhombic phase of

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Fe3O4 (JCPDS card No.65−3107) indexing to the good crystallinity of these nanospheres

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with a face-centered cubic structure.45 It was obviously that these three samples using

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different amount of AHMNs exhibited the similar diffraction patterns, and all of the

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diffraction peaks could be indexed to the special structures of AHMOC-Ys and there were no

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evidence of impurities could be observed in the XRD patterns.44 In addition, the

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characteristic peaks of OC (2θ = 23.4º) in AHMOC-Ys which converted from 23.4º to 21.5º

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were also observed, which could be attributed to the polymerization between amino-iron and

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carboxyl groups onto the oxidized mesoporous carbon with different amount.42, 43 These

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results demonstrated that AHMOC-Ys were prepared successfully.

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Fig. 2b revealed the FTIR spectra of OC, AHMOC-Ys and AHMOC-2:1-MB and

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confirmed the functional groups on the surface of materials. Several common characteristic

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peaks were clearly observed in all samples, such as 3430 and 1122 cm-1, which corresponded

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to the N−H/O−H and C−O stretching vibration, respectively.46, 47 The band at 1640 cm-1 was

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also presented in the OC spectra, which could be attributed to the C=O stretching vibration.

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After modification, the intensity of N−H/O−H presented a relatively weak value, which

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could be assigned to the formation of ester group (1725 cm-1).39 Meanwhile, the new

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emergency of C−N (1233 cm-1) and NH−C=O (1590 cm-1) could be found, demonstrating

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that −NH2 might be introduced into the compound successfully attributed to the introduction

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of AHMNs into the OC matrix.48 It was clearly to find a characteristic peak centered at 580 9



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cm-1, which corresponded to the Fe−O and Fe−N stretching vibration, demonstrating that

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magnetic hollow nanoparticles were successfully modified by urea. The above discussions

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clearly revealed that AHMOC-Ys composites consisted of AHMNs and OC, which further

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supported that AHMNs were grafted on or encapsulated by the OC matrix (Fig. 1d).

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Compared the FTIR spectrum before and after adsorption, the adsorption behavior was

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revealed distinctly. Fig. 2b (OC and 2:1-MB) indicated that the band at 1590 cm-1 moved to

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1620 cm-1. The emerging of peaks ranged from 1122 cm-1 to 1590 cm-1 can be assigned to

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C=N stretching of MB molecule, which indicated the composites could combine with MB

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molecules successfully.

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Fig. 2. XRD patterns (a) and FTIR spectrum of OC，1:1, 2:1, 5:1 and AHMOC-2:1-MB (b).

10


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Fig. 3. XPS full-scan of OC and 2:1 (a); C1s XPS spectrum of OC (b) and 2:1(c); N1s XPS

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spectrum of OC (d) and 2:1 (e); The XPS spectrum of Fe 2p from the fractured surface of the

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Fe3O4 standard sample 2:1 (f);

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As the magnetite (Fe3O4) and maghemite (γ-Fe2O3) had the similar XRD patterns and

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both exhibited magnetic behavior, X-ray photoelectron spectrospecy (XPS) was carried out 11



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to gain further information about the chemical composition of OC and AHMOC-2:1. The

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core-electron lines of ferrous ions and ferric ions could both be detected and distinguished in

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XPS. Therefore, it was used to examine shell structure of the synthesized product.

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The full scan of OC and AHMOC-2:1 were displayed in Fig. 3a and showed the C and

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O peaks at the binding energy of 285.08 and 532.08 eV, respectively. While there were two

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additional peaks centered at 399.08 and 710.08 eV, corresponding to N and Fe respectively,

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which might be assigned to the introduction of amino-hollow magnetic iron nanoparticles.

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The results were consistent with the FTIR analysis.

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Fig. 3b and Fig. 3c displayed the spectra of C1s region for pristine OC and

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AHMOC-2:1. The C1s spectra of OC could be deconvoluted into four primary peak

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components, centered at 284.06, 285.63, 288.08 and 290.08 eV, which were assigned to pure

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graphitic sites C−C, C−O, O−C=O and carbonates, respectively.49 However, there were only

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three fitted-curves observed in AHMOC-2:1 spectrum. The binding energy of 283.73 eV

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corresponded to pure graphitic sites (C−C). 285.38 eV corresponded to sp2 carbon atoms

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bonded to nitrogen in the amorphous CN matrix (C−N). And 287.68 eV corresponded to sp3

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carbon atoms bonded to nitrogen (N−C=O).49 The difference of the two materials might be

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attributed to the reaction between –NH2 and –COOH.

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Fig. 3d exhibited the N1s spectrum of OC, which could be curve-fitted into two primary

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peak components at 401.18 eV, corresponding to quaternary (N-Q) (12.42 %), and 405.38 eV,

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corresponding to oxidized like nitrogen species (N-X) (87.58 %), respectively.50 The N1s

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XPS spectrum of AHMOC-2:1 was also revealed in Fig. 3e. The peaks of N-Q and N-X had

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been slightly shifted to 399.18 eV and 404.98 eV, correspondingly the content turned into 12


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78.90 % and 11.46 %, respectively. In addition, two new peaks appeared at 401.28 eV and

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402.68 eV, corresponding to acylamino, and indicated that AHMNs were successfully

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combined into the composites.50

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To further confirm the ingredient of magnetic particles, XPS spectrum of Fe 2p from the

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fractured surface of the Fe3O4 standard sample 2:1 was exhibited in Fig. 3f. The Fe 2p

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spectra of AHMOC-2:1 was deconvoluted into two primary peaks at 709.78 eV and 724.93

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eV which corresponded to the peaks of Fe 2p3/2 and Fe 2p1/2 in Fe3O4 respectively, and there

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were no satellite peaks around.25, 51 Therefore, the spectra were typical for stoichiometric

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Fe3O4, and demonstrated that the magnetic ingredient was mainly Fe3O4. The peaks centered

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at 722.88 and 713.13 eV, which attributed to amination of magnetic hollow nanospheres

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(Fe−N). These values were close to the report for Fe3O4 previously that the peak positions of

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Fe 2p3/2 and Fe 2p1/2 were 710.6 (S.D. = 0.05) and 724.1 eV (S.D. = 0.07), respectively.

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These results confirmed the analysis of XRD patterns, and proved that AHMOC-Ys were

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prepared successfully.

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Adsorption analysis

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The adsorption behavior of MB in aqueous solutions was applied to evaluate the effect

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of AHMNs on surface charge of OC and the adsorption ability of cation molecules by the

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three different adsorbents (1:1, 2:1, and 5:1). The adsorption process was performed in 30

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mL MB solution (200 mg/L) with 0.01 g samples at 318 K. The detailed process and

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mechanism figure of adsorption (Fig. S8) were set out in Supplementary information.

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Effect of initial pH. The initial pH of the solution was one of the most critical

232

parameters involved in adsorption capacity and mechanism. The previous reports of pH 13



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revealed that it not only affect the degree of deprotonation and the speciation of surface

234

functional groups on absorbent but also affect the status of adsorbate. So it had a significant

235

effect on electrostatic charge that was assigned to ionized dye molecules between adsorbent

236

and adsorbate. The experiments were carried out by varying solution pH range from 2 to 12

237

with different adsorbents (OC and AHMOC-Ys), and the results were presented in Fig. 4a.

238

The zeta potential of the each mesoporous carbon composites at different pH was also

239

measured and revealed in Fig. 4b. As indicated in Fig. 4a, the removal capacity was

240

significantly increased when pH increased from 3.0 to 8.0. However, there was a decreased

241

trend with increasing pH value from 8 to 12 by AHMOC-Ys, but the adsorption capacity of

242

OC was increased constantly. These results could be explained on the basis of the point of

243

zero charge (pHpzc)26. As depicted in Fig. 4b, the pHpzc of OC was nearly 4.5, however the

244

pHpzc of AHMOC-Ys were about 1.5. When pH＜pHpzc, the adsorbents were positively

245

charged due to the protonation of the surface functional groups, which generated electrostatic

246

repulsion force restricting the adsorption between adsorbent and adsorbate. In addition, the

247

excess hydrogen ions, pore filling and π–π electron donor–acceptor interactions also had

248

great competitiveness with dye cations for the adsorption sites, further resulted in the lower

249

adsorption capacity.52,53 However, when pH＞pHpzc, the surface of absorbents was negatively

250

charged, which could be beneficial for MB adsorption due to electrostatic attraction. When

251

the solution pH at about 8, the deprotonation of amine groups (on the surface of AHMNs)

252

and acylamino groups (between OC and AHMNs) resulted in the formation of anion and led

253

to a lower pHpzc value and higher removal capacity of AHMOC-Ys. While the pHpzc value of

254

AHMOC- Ys obviously increased when pH ＞8 owing to high concentration of OH−. It was 14


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255

the obstacle for deprotonation velocity of functional groups on AHMOC-Ys. That was also a

256

reason to why the pHpzc value and adsorption capacity presented a fluctuation. Otherwise, the

257

amide bond was negatively charged functional group, and the formation of amide bond was

258

favorable to capture cation molecules. The adsorption capacity of each sample was listed in

259

Table 1.

260

Table 1. Comparison of the maximum MB adsorption capacity of various adsorbents. Adsorbents

Adsorption capacity (mg·g–1)

References

Fe-CMK-3 Oxidized CMK-3 AHMOC-1:1 AHMOC-2:1 AHMOC-5:1

316 475.2 457.5 522.4 488.2

7 This study This study This study This study

261

The mechanism for the removal of the cation contaminant by metal component was

262

proposed to involve surface complexation and ion exchange between the iron oxide surface

263

and the cations ions in MB aqueous solution.25 The amino-iron oxides located in water had

264

≡Fe－OH and ≡Fe－NH－NH2 surface sites. The chemical structure (hydroxyl, amidogen

265

coordination and adsorption sites) on the surface depended on the oxidized morphology and

266

crystal structure. The ≡Fe－O

267

for the adsorption of MB. With the changing of pH, the ≡Fe－OH and ≡Fe－NH－NH2

268

groups acquire positive or negative charge by protonation or deprotonation. The magnetite

269

surface chemistry could be summarized by the following formulas:26

－

and ≡Fe－NH－NH− groups were pH dependent active sites

270

≡Fe－OH + H+ → ≡Fe－OH2+

271

≡Fe－NH－NH2 + H+→ ≡Fe－NH－NH3+ ( at pHpHpzc, deprotonation)

－

－

15



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274

－ The ≡Fe－O and ≡Fe－NH－NH− site were Lewis base and could be coordinated

275

with the MB molecules inner or outer of the complexes.26 The above analyses might explain

276

the variation of pHpzc values and adsorption capacity of each sample.

277 278

Fig. 4. The effect of initial solution pH on MB adsorption (a) and Zeta-potential of

279

AHMOC-Ys at different pH (b).

280

Effect of initial concentration and isotherm analysis. The initial MB concentration

281

was also an important parameter to affect adsorption capacity and removal efficiency, which

282

provided necessary driving force to surmount the resistance between the aqueous and solid

283

phases. The experiments were carried out with different initial concentrations (100-1000

284

mg·L−1) at pH = 8, and the results were set out in Fig. S3. It was obviously to find that the

285

adsorption capacity was enhanced with increasing initial MB concentrations, which might be

286

owing to the heterogeneous adsorption sites.

287

From Fig. S3a, the AHMOC-2:1 had higher adsorption capacity than other absorbents.

288

It could be due to that the co-incorporation of AHMNs and OC in suitable ratio provided

289

more functional groups, which could offer additional sites, much higher negative charge and

290

small opening. With the increase of initial MB concentration from 200 to 1000 mg·L−1, the 16


Page 16 of 33

Page 17 of 33

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291

removal efficiency of MB at equilibrium by AHMOC-2:1 decreased from 88.5 % to 55 %

292

(Fig. S3b). The increased adsorption capacity and decreased adsorption efficiency verified

293

that the available surface binding sites were finite and fully used for a fixed adsorbent dosage

294

with the excessive initial MB concentration. The results also demonstrated that AHMOC-2:1

295

had advantages in adsorption capacity compared with other texting materials, and it could be

296

better in practical application.

297

In order to further study the adsorption behaviors between solid and liquid phase at

298

equilibrium, two isothem models (Langmuir and Freundlich isotherm model) were selected

299

to analyze the equilibrium data. The Langmuir isotherm model could be expressed as:11

300

=

301

=

302

where (mg·g−1) is the maximun monolayer adsorption capacity of MB; (L·mg−1)

303

is the Langmuir adsorption free energy constant; (mg·g−1) and (mg·L−1) are the initial

304

and the equilibrium concentration, respectively; is the equilimbrium parameter which

305

can determine the types of isotherms as follows: favorable (1> >0), liner ( = 1),

306

unfavorable ( >1).

307

The Freundlich isotherm model is given as:

(1)

(2)

308

=

309

where (mg·g−1) is adsorption capacity at equilibrium concentration; (mg ∙ g ) (L ∙

310

mg )

311

adsorption intensity, respectively; (mg·L−1) is the equilibrium solute concentration. The

312

magnitude of n indicates the favorability of the process, and values of 10 > n > 1

# "

(3)

and n represent Freundlich isotherm constants relating to adsorption capacity and

17



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313

Page 18 of 33

represent favorable adsorption.11

314

The adsorption parameters of the Langmuir isotherm and the Freundlich isotherm

315

model were analysed using nonlinear regression method, and the results were displayed in

316

Fig. S4 and Table 2. It was obviously that the correlation coefficient ( of Freundlich

317

isotherm model at temperature 328 K of AHMOC-Ys were 0.9334 (1:1), 0.9961 (2:1),

318

0.9893 (5:1) accordingly, which were preferable than ( values of Langmuir isotherm

319

model suggesting that the adsoprtion behavior was more suitable for Freundilich model. The

320

results suggested that the adsorption onto heterogeneous surfaces with uniform energy

321

distribution was reversible attributing to the active groups, which was consistent with the

322

previous researchers.54 This predicted that as long as the dye concentration increased in the

323

solution the dye concentrated onto the materials increased correspondingly.9, 25, 52 In addition,

324

the n value was calculated in the range from 2.19 to 2.51, indicating that the adsorption

325

between adsorbents and MB molucules was favorable (10 > n > 1).25

326

Table 2. Langmuir and Freundlich isotherm parameters for adsorption of MB onto

327

AHMOC-Ys. AHMOC-Y 1:1 2:1 5:1

Langmuir model −1

Freundlich model

−1

2

qm (mg·g )

KL (L·mg )

R

KF (L·mg−1)

n

R2

1755.8 1919.4 1741.0

0.0056 0.0102 0.0085

0.8167 0.9107 0.9033

77.56 146.3 116.7

2.197 2.512 2.445

0.933 0.996 0.989

328

Effect of ionic strength. Generally, the wastewater of factories commonly contains

329

dyes with high salt concentration. The salt concentration of the solution was one of the

330

factors that had influence on the hydrophobic and electrostatic interaction between surface

331

functional sites of adsorbent and dye molecules.9, 52 18


Page 19 of 33

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332

The effect of ionic strength on the MB adsorption by AHMOC-2:1 was carried out by

333

adding different NaCl concentration range from 0 to 1.0 mol·L−1. As seen from Fig. S5, the

334

sorption of MB on AHMOC-2:1 slightly improved with increasing NaCl concentration from

335

0.00 to 0.01 mol·L−1 and then decreased mildly as the NaCl concentration increased from

336

0.01 to 1 mol·L−1. The increasing phenomenon could be explained by the following reasons:

337

1) the MB molecules might aggregated due to the increased intermolecular forces with

338

increasing NaCl concentrations; 2) the increased ionic strength might screen the electrostatic

339

interaction, which was beneficial for MB removal because of the depression of electrostatic

340

repulsive.9 However, the decreasing phenomenon occurred in the salt concentration exceeded

341

0.1 mol·L−1 and might be due to the competition adsorption between Na+ and MB molecules.

342

The addition of abundant NaCl could also enhanced the intermolecular force and effected the

343

diffusion of MB molecules.38 These results were consistent with the pH analysis, and further

344

proved that electrostatic attraction could be involved in proposal adsorption mechanism.

345

Effect of contact time and kinetics model. In order to evaluate the adsorption kinetics

346

and rate-limiting step in the adsorption process, two adsorption kinetic models including

347

pseudo-first and pseudo-second order rate models were selected to estimate the adsorption

348

mechanism and quantify the extent of uptake in adsorption process. These two equations

349

were employed to model the sorption data over the entire time range which could be

350

generally expressed as following equations, respectively:52

351

Pseudo-first-order model

352

)*( − , ) = )* − (.// 0

353

Pseudo-second-order model

-

(4)

19



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

354

, 1

=

-2 2

−

Page 20 of 33

,

(5)

355

where and , (mg·g−1) are donated as the amounts of MB adsorbented at equilibrium ,

356

at time t respectively. 3 (min·L−1) and 3( (g·mg−1·min−1) are the rate constant of

357

pseudo-first-order and pseudo-second-order adsorption model respectively.

358

Table 3. Correlation coefficients and Kinetic parameters for adsorption of MB onto

359

AHMOC-Ys.

AHMOC-Y 1:1

Pseudo-first-order qe,1 k1 (min−1) −1 (mg·g ) 472.7 0.098

R2 0.983

Pseudo-second-order qe,2 k2×103 −1 (mg·g ) (g/mg·min−1) 487.2 0.356

R2 0.963

2:1

513.4

0.129

0.921

531.4

0.423

0.989

5:1

497.6

0.165

0.926

511.5

0.617

0.976

360

The kinetic studies for MB adsorption on the AHMOC-Ys were carried out by varying

361

contact time from 0 to 48 h, and the results were displayed in Fig. S6. It was clearly to find

362

in Fig. S6a that the adsorption capacity was markedly increased in the first 30 min, which

363

might be assigned to the existence of plentiful of adsorption sites on the adsorbent surface.

364

And then, the removal capacity was slowly increased until the adsorption equilibrium was

365

reached in about 2 h. The result might be owing to the insufficient active sites after initial

366

adsorption process.55 The correlation coefficients and kinetic parameters were listed in Table

367

3 after fitted by the kinetic models.25 The pesudo-second order of AHMOC-2:1 presented

368

highest correction coefficients (（0.989）than others and the calculated adsorption capacity

369

(522.4 mg·g–1) was more agreement with the experimental data (531.4 mg·g–1). It indicated

370

that the rate-limiting step of adsorption mechanism might be physisorption which involved

371

valence force duringsharing or exchanging electrons.52, 54

372

Intra-particle diffusion theory could be identified complementary for the diffusion 20


Page 21 of 33

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373

mechanism. Therefore, intra-particle diffusion model was necessary to simulate the process

374

of MB transportation from aqueous solution to the surface of adsorbents. It could be

375

described as follows.10, 52

376

The interparticle diffusion model:

377

, = 345 0

378

where 345 is the interparticle diffusion rate constant (mg·g−1·h1/2), and C (mg·g−1) is the

379

constant varied instantly with the boundary layer thickness.

380

# (

+

(6)

The plots of , versus 0

# (

were revealed in Fig. S6c. It could be observed that the

381

plots were multi-linear and obtained two regions, indicating different stages in adsorption

382

process. In the first stage (slope 345 ), a large slope was presented, which might be owing to

383

the mass transfer from boundary film to the external surface of AHMOC-Ys by film

384

diffusion. However, the second phase (slope 345( ) was a gradual process, which presented

385

that the adsorbate migrated to the adsorbent internal structure by intra-particle diffusion. The

386

above elaborations demonstrated that rate-limiting step was mainly involved film diffusion

387

and intra-particle diffusion for the whole reaction.

388

Thermodynamic studies. The thermodynamic parameters involved Gibbs free energy,

389

enthalpy and entropy that were employed to explore whether the adsorption process was

390

endothermic or not. The thermodynamic data were simulated by the following equations:10

391

∆8 = −9 )*

392

)* = − ;< +

393

=

∆:

∆= ;

(7)

(8)

(9)

394

where ∆8 is change of Gibbs free energy (kJ·mol−1). ∆> (J·K−1· mol−1) and ∆? (kJ·mol−1) 21



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Page 22 of 33

395

are entropy and enthalpy, respectively. (8.314 J·mol−1·K−1) is the universal gas constant,

396

9 (K) is the Kelvin absolute temperature, represents the Langmuir constant, is the

397

equilibrium concentration.

398

Table 4. Thermodynamic parameters for MB adsorption onto AHMOC-2:1 T (K)

ln kL

∆G (kJ·mol−1)

∆S (kJ·mol–1·K−1)

298 K 308 K 318 K

1.8698 1.9811 2.1653

-4.6328 -4.9450 -5.7248

54.372

∆H (kJ·mol−1) 11.603

399

The plot of )* versus 1/9 was indicated in Fig. S7. Thermodynamic analysis was

400

investigated at three different temperatures (298 K, 308 K, 318 K) and the results were listed

401

in Table 4. Generally, the change in free energy for chemisorption was in a range of −80 to

402

−400 kJ·mol−1 and for physisorption was between −20 and 0 kJ·mol−1.52 The negative values

403

for ∆8 increased with increasing temperature at the range of 298 K to 318 K (from −4.63 to

404

−5.72 kJ·mol−1), which were within the range of −20 to 0 kJ·mol−1, indicating the

405

predominant mechanism of the adsorption process was physisorption.52 The positive value

406

for ∆? (11.60 kJ·mol−1) indicated that the adsorption process was an endothermic process

407

in nature, which could be further supported by the study of temperature effect. The positive

408

value of ∆> (54.37 kJ·mol–1·K−1) represented the higher order of reaction during the

409

adsorption of MB dye onto AHMOC-2:1, indicating that randomness at the solid/solution

410

interface increased during the adsorption process, which reflected some structural changes of

411

MB molecules and AHMOC-2:1. Additionally, as the adsorption process was endothermic,

412

electrostatic repulsion existed between the adsorbent and adsorbate. So the higher

413

temperature provided extra energy for the process to overcome the repulsion force to propel

414

the ionic dyes closed onto the adsorbent resulting in higher adsorption capacity. 22


Page 23 of 33

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415

CONCLUSION

416

In the presented study, we composed the aminated hollow magnetic nanospheres with

417

oxidized mesoporous carbon in different mass ratio and the final materials was noted as

418

AHMOC-Y (Y=1:1, 2:1, 5:1). The primary purpose was to explore the characteristics of

419

these new hybrid materials and rationalized the optimum proportion of material to gain a

420

good adsorption capacity of MB at mild condition. The characterizations of SEM, XRD,

421

VSM, FTIR, XPS, and Zeta-potential confirmed these kinds of samples were synthesized

422

successfully. And the results revealed the special morphology compared with initial oxidized

423

mesoporous carbon. The adsorption characteristics were also affected by the initial

424

concentration, ionic strength and temperature. The Zeta-potential and adsorption behavior at

425

different pH demonstrated that a high adsorption capacity was achieved at low pH value (pH

426

= 8). The adsorption process was well fitted with Freundlich model and pseudo-second-order

427

model. The thermodynamic study confirmed the endothermic and spontaneous nature of MB

428

adsorption process. Conclusions obtained from this study revealed the special morphology of

429

AHMOC and its high adsorption capicity in mild condition made it able to widely applied in

430

other domains.

431

ASSOCIATED CONTENT

432

Supporting information

433

Experiment details: Materials, Synthesis methods, Materials characterization, Sorption

434

experiments; Sample characterization: BET and VSM; Figures: Figure S1-Figure S8.

435

AUTHOR INFORMATION

436

Corresponding author: 23



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437

*Wei-cheng Cao, E-mail address: [email protected];

438

*Yun-guo Liu, Tel.: + 86 731 88649208; Fax: + 86 731 88822829;

439

E-mail address: [email protected]

440

Notes

441

The authors declare no competing financial interest.

442

ACKNOWLEDGEMENTS

443

This work greatly acknowledged the financial support that came from the National

444

Science Foundation of China (51521006, 51579097), Lake contamination and wetland

445

remediation (51521006), and the principle of electrical chemical enhancement of

446

Thiobacillus ferrooxidans bacterial ex-situ bioremediation of heavy metal contaminated

447

sediment (51579097).

448

ABBREVIATIONS

449

OC, oxidized ordered mesoporous carbon

450

AHMNs, aminated hollow magnetic nanospheres

451

AHMOC, aminated hollow magnetic nanospheres-oxidized ordered mesoporous carbon

452

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453

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Ando,

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Micro-structural,


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620

For Table of Contents Use Only

621

Title

622

The property variation of magnetic mesoporous carbon modified by

623

aminated hollow magnetic nanospheres: Synthesis, characteristic and

624

sorption

625

Xin Li, Wei-cheng Cao*, Yun-guo Liu*, Guang-ming Zeng, Wei-Zeng, Lei Qin, Ting-ting

626

Li

627

TOC figure

628 629

Synopsis

630

A green modification strategy was proposed for sustainable production of novel

631

functional mesoporous carbons. The samples exhibited special morphology and high

632

adsorption capacity at mild condition.

33


Property Variation of Magnetic Mesoporous Carbon Modified by

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