Property Variation of Magnetic Mesoporous ... - ACS Publications

Research Article pubs.acs.org/journal/ascecg

Property Variation of Magnetic Mesoporous Carbon Modified by Aminated Hollow Magnetic Nanospheres: Synthesis, Characterization, and Sorption Xin Li,†,‡ Wei-cheng Cao,*,†,‡ Yun-guo Liu,*,†,‡ Guang-ming Zeng,†,‡ Wei Zeng,†,‡ Lei Qin,†,‡ and Ting-ting Li†,‡ †

College of Environmental Science and Engineering, Hunan University, Lushan South Road, Juzizhou Street, Changsha 410082, P.R. China ‡ Key Laboratory of Environmental Biology and Pollution Control, Ministry of Education, Hunan University, Changsha 410082, P.R. China S Supporting Information *

ABSTRACT: Magnetic mesoporous carbon with particular morphologies was fabricated by immobilizing uniform aminated hollow magnetic nanospheres (AHMNs) in an oxidized mesoporous carbon (OC) matrix with different mass ratios (AHMOC-Y, Y = 1:1, 2:1, 5:1). This study was devoted to exploring the effects on morphology, surface charge, and adsorption capacity when AHMNs were immobilized onto OC. The morphology and surface properties were studied using SEM, BET, XRD, FTIR, XPS, and VSM. Batch experiments were carried out to study the sorption behavior of methylene blue by AHMOC-Ys, indicating that a good adsorption capacity of cation dye could be obtained at mild conditions (at pH = 8 compared with pH = 11), which was consistent to the point of zero charge (pHpzc). Characterization of the adsorption behavior revealed that kinetics and isotherm synthesis were well-fitted respectively by the pseudo-second-order model and the Freundlich isotherm model. The rate-limiting step mainly involved film diffusion and intraparticle diffusion for the whole reaction. Thermodynamic analysis indicated that the adsorption reaction was an endothermic and spontaneous process. The conclusion reveals that AHMOC-2:1 has advantages in terms of adsorption capacity and separation feasibility compared with OC, AHMOC-1:1, and AHMOC-5:1, which could make it preferable in practical applications for environmental purification. KEYWORDS: Mesoporous carbon, Hollow nanospheres, Magnetic, Methylene blue, Adsorption mechanism

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INTRODUCTION Mesoporous materials, especially ordered mesoporous carbon materials, have aroused significant interest since Ryoo et al. first prepared them and denoted them as CMK materials in 1999, due to their high specific pore volume, high specific surface area, uniform pore size distribution, stability, and good electrical conductivity.1−4 The ordered mesoporous carbon presented uniform mesopores, large surface area, and ordered structure, which offer more anchoring sites for a variety of applications such as catalyst supporter,5 supercapacitor,6 and medicine release, involving the practical application of large hydrophobic molecules (e.g., ibuprofen, vitamins, dyes).7−10 However, the ordered mesoporous carbon still suffered from disadvantages related to its inherent properties, including poor hydrophilicity and difficult reclamation, which might possibly cause secondary pollution and severely hinder its practical application.11 Numerous methods and techniques have been applied to modify ordered mesoporous carbon to overcome its defects. © 2016 American Chemical Society

Previous studies indicated that nitric acid oxidation can introduce carboxyl groups into the carbon material and change its surface hydrophobic/hydrophilic balance.8 Furthermore, the incorporation of metallic elements (Pd, Zn, Co, Fe, Zr, etc.) into ordered mesoporous carbon during the preparation or via postsynthetic methods led to high performance, for example, in adsorption, catalysis, and magnetism.12−14 Notably, the majority of researchers were committed to combining the good ordering of mesoporous materials with the magnetic properties of nanoparticles for adsorption of dyes.15−18 Up to now, research on magnetic ordered mesoporous carbon commonly involved adding extra iron precursors, such as FeSO4, Fe2(SO4)3, or FeCl3.19−22 For instance, Kittappa et al.15 synthesized Fe3O4 nanoparticles first, and then the nanomagnetite encapsulated with silicon dioxide (SiO2) was Received: May 31, 2016 Revised: November 13, 2016 Published: December 1, 2016 179

DOI: 10.1021/acssuschemeng.6b01207 ACS Sustainable Chem. Eng. 2017, 5, 179−188

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ACS Sustainable Chemistry & Engineering

Figure 1. SEM images of (a) CMK, (b) OC, (c,d) AHMOC-2:1, (e) AHMOC-1:1, and (f) AHMOC-5:1.

synthesized by the Stöber method23 to fabricate a nanocomposite mesoporous material for removal of methylene blue (MB). It exhibited higher and faster adsorption capacity for MB than other silicic mesoporous materials. Tang et al. 11 synthesized an ordered mesoporous carbon composite which was functionalized with carboxylate groups and iron oxide nanoparticles to adsorb 2,4-dichlorophenoxyacetic acid. The material exhibited good magnetic properties and large adsorption capacity at low pH and temperature. However, these studies only focused on magnetizing the ordered materials by impregnating ferric chloride solution into a carbon/silica source.18,19,24 According to the literature, these synthesis methods could not achieve uniform and controllable morphology of magnetic nanoparticles; meanwhile, they might lead to instability and aggregation, which would lower the particles’ reactivity and mobility. In addition, the decrease in surface area of ordered mesoporous carbon after incorporating magnetic nanoparticles should be a concern, because it was significant for successful contaminant removal. It is worth mentioning that hierarchically structured magnetic iron oxide materials, especially hollow iron nanospheres, have received extensive attention as potential materials for

sequestering hazardous dyes25 and toxic heavy metals,26 drug delivery,27,28 and so on.25,26,29−35 The functionalized hollow iron nanospheres possess desirable properties: they are magnetic, nontoxic, hydrophilic, chemically stable, and easily synthesized, and they offer excellent recycling capability.36 Sasidharan’s group synthesized hollow α-Fe2O3 and Fe3O4 nanospheres using FeCl3·6H2O as iron precursor and micelles of poly(styrene-block-acrylic acid-block-ethylene oxide) (PSPAA-PEO) as soft template to release ibuprofen sustainably.33 Iram et al.25 fabricated Fe3O4 hollow nanospheres via a simple one-pot template-free hydrothermal method and investigated their application as an adsorbent for removal of neutral red dye contaminants from water. These studies were dependent on hollow Fe3O4 nanospheres enclosing a vacant volume at their centers and presenting shell-like ordered independent nanoparticles outside. The structure provided a high surface area, high polarity, large interior space, quantum sizes, low density, thermal and chemical stability, and high permeability. As a consequence, these properties offered a higher capacity for removal of heavy metal ions as well as dyes.26,37 The above studies emphasized that the functional groups existed on the surface, and metallic nanoparticles encapsulated 180


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

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within the porous carbons played a key role in particle application.8,38 It might be an alternative choice to combine ordered mesoporous carbon with hollow iron nanospheres to affect the surface charge of each other and improve separation capacity. However, grafting hollow iron nanospheres directly to ordered mesoporous carbon leads to instability and might greatly reduce their adsorption capacity due to the aggregation of magnetic iron oxide. Therefore, not only should ordered mesoporous carbon be improved, but the hollow iron nanospheres should be modified as well before they are assembled together. In this study, ordered mesoporous carbon was oxidized by nitric acid, and hollow iron nanospheres were aminated by urea, in a simple one-step template-free hydrothermal method. The synthesized aminated hollow magnetic nanospheres (AHMNs) were then immobilized into the oxidized ordered mesoporous carbon matrix (OC) by postsynthesis.39 The functionalized AHMNs and OC had functional groups (e.g., carboxyl, amino) which may have resulted in the AHMNs being well dispersed and encapsulated in the OC matrix in the presence of ultrasound when they were compounded covalently with each other. The final sample was denoted as AHMOC, and the charged surface of the composite could greatly enhance the dispersion and morphology of the particles. In addition, the acylamino generated by combination of carboxyl and amino groups was negatively charged and favorable for catching cationic dye molecules. This research led to a novel functional uniform magnetic carbon framework which not only maintains the structural and geometrical features of hollow Fe3O4 nanospheres and ordered mesoporous carbon but also protects the magnetic spheres to a certain extent, to guarantee the adsorption capacity. The primary objective of this research was not only to study the formation mechanism of AHMOC, including its physical and chemical properties, but also to identify the optimum proportions for the synthesis of a material for removal of cationic dye (MB) from aqueous solution. The innovative adsorbent (AHMOC) was prepared successfully with different mass ratios (1:1, 2:1, 5:1), and a batch equilibrium technique was applied to investigate the effects of various experimental factors, such as pH, initial dye concentration, ionic strength, contact time, and temperature. The adsorption isotherms, kinetics, and thermodynamics were studied to analyze the adsorption behaviors. The results indicate that AHMOC-2:1 is promising for further application as an efficient low-cost and recyclable adsorbent for dye removal.

EXPERIMENTAL SECTION

The samples were synthesized by combining OC and AHMNs in mild conditions. Experimental details and some of the sample characterization (SEM and BET) are presented in the Supporting Information.

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RESULTS AND DISCUSSION Sample Characterization. Scanning electron microscopy (SEM) images displayed the surface morphologies of CMK-3, OC, and AHMOC-Y (Y = 1:1, 2:1, 5:1), which are set out in Figure 1. It is clear that the pristine CMK (Figure 1a) displayed a typical ordered mesoporous structure, which was consistent with the previous studies.2,4 However, the oxidized mesoporous carbon (Figure 1b) consisted of many rope-like domains which exhibited a similar structure, with the initial mesoporous carbon having an average diameter of 350−450 nm.8 Figure 1c,d presents the special sphere−rhabditiform macrostructures which resulted from distribution of AHMNs on OC with different densities, indicating that the AHMNs were successfully grafted onto the mesoporous carbon structure and uniformly dispersed on the surface of oxidized carbon rods.39 Figure 1c shows that the loading of AHMNs onto the OC surface caused a little agglomeration, which slightly aggravated the order of the mesoporous structure but without destroying the skeleton of OC.40 It is obvious in the patterns of AHMOCYs that the AHMNs were not only grafted on the surface of OC (Figure 1c) but also encapsulated by several segments of OC, fabricating cage-like domains (Figure 1d). These were attributed to the covalent binding between amino groups and carboxy groups. However, the SEM images of AHMOC-Ys were considerably different from thosse of other magnetic carbon.22,41 Figure 2a displays the wide-angle X-ray diffraction (XRD) patterns of each sample, as-prepared. It is noted that the pristine OC exhibited two wide scattering peaks at around 24° (002) and 44° (100), which were consistent with a graphitic lattice of oxidized mesoporous carbon and demonstrated a high degree of graphitization.42,43 However, the XRD patterns of AHMOC-Ys presented similar characteristic peaks at 2θ = 30.0°, 35.5°, 37.1°, 43.2°, 53.6°, 57.1°, and 62.7°, which were assigned to the (112), (211), (202), (220), (422), (511), and (440) reflections, respectively.26,44 These results corresponded to the standard orthorhombic phase of Fe3O4 (JCPDS card no. 65-3107), the indexing indicative of the good crystallinity of these nanospheres with a face-centered cubic structure.45 It was obvious that these three samples, prepared using different 181


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Figure 3. (a) XPS full scans of OC and AHMOC-2:1. (b,c) C1s XPS spectra of OC (b) and AHMOC-2:1 (c). (d,e) N1s XPS spectra of OC (d) and AHMOC-2:1 (e). (f) Fe2p XPS spectrum of the fractured surface of the Fe3O4 standard sample AHMOC-2:1.

were clearly observed in all samples, such as 3430 and 1122 cm−1, which correspond to the N−H/O−H and C−O stretching vibrations, respectively.46,47 The band at 1640 cm−1 in the OC spectra could be attributed to the CO stretching vibration. After modification, the intensity of the N−H/O−H of peaks became relatively weak, which could be assigned to the formation of ester groups (1725 cm−1).39 Meanwhile, the emergence of new C−N (1233 cm−1) and NH−CO (1590 cm−1) vibrations was seen, demonstrating that −NH2 was successfully introduced into the compound, which could be attributed to the introduction of AHMNs into the OC matrix.48 A characteristic peak was clearly found centered at 580 cm−1, which corresponded to the Fe−O and Fe−N stretching

amounts of AHMNs, exhibited similar diffraction patterns, and all of the diffraction peaks could be indexed to the special structures of AHMOC-Ys, with no evidence of impurities observed in the XRD patterns.44 In addition, the characteristic peak of OC (2θ = 23.4°) in AHMOC-Ys which converted from 23.4° to 21.5° was also observed, which could be attributed to the polymerization between amino-iron and carboxyl groups onto the oxidized mesoporous carbon in different amounts.42,43 These results demonstrate that AHMOC-Ys were prepared successfully. Figure 2b reveals the FTIR spectra of OC, AHMOC-Ys, and AHMOC-2:1-MB and confirms the functional groups on the surface of the materials. Several common characteristic peaks 182


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Figure 4. Effects of initial solution pH on (a) MB adsorption and (b) zeta potential of AHMOC-Ys.

corresponding to oxidized-like nitrogen species (N−X, 87.58%).50 The N1s XPS spectrum of AHMOC-2:1 is shown in Figure 3e. The peaks of N−Q and N−X were slightly shifted to 399.18 and 404.98 eV, and correspondingly the content turned to 78.90% and 11.46%, respectively. In addition, two new peaks appeared at 401.28 and 402.68 eV, corresponding to acylamino, and indicated that AHMNs were successfully incorporated into the composites.50 To further confirm the ingredients of the magnetic particles, the Fe2p XPS spectrum from the fractured surface of the Fe3O4 standard sample 2:1 is exhibited in Figure 3f. The Fe2p spectrum of AHMOC-2:1 was deconvoluted into two primary peaks at 709.78 and 724.93 eV, which corresponded to the peaks of Fe2p3/2 and Fe2p1/2 in Fe3O4, respectively, and there were no satellite peaks around.25,51 Therefore, the spectra were typical for stoichiometric Fe3O4 and demonstrated that the magnetic ingredient was mainly Fe3O4. The peaks centered at 722.88 and 713.13 eV were attributed to amination of magnetic hollow nanospheres (Fe−N). These values were close to those reported for Fe3O4 previously, where the peak positions of Fe2p3/2 and Fe2p1/2 were 710.6 (SD = 0.05) and 724.1 eV (SD = 0.07), respectively. These results confirmed the analysis of XRD patterns and proved that AHMOC-Ys were prepared successfully. Adsorption Analysis. The adsorption behavior of MB in aqueous solutions was studied to evaluate the effect of AHMNs on surface charge of OC and the adsorption ability of the three different adsorbents (1:1, 2:1, and 5:1) toward cation molecules. The adsorption process was performed in 30 mL MB solution (200 mg/L) with 0.01 g samples at 318 K. The detailed procedure and a figure showing the adsorption mechanism (Figure S8) are provided in the Supporting Information. Effect of Initial pH. The initial pH of the solution is one of the most critical parameters determining the adsorption capacity and mechanism. Previous reports revealed that pH not only affects the degree of deprotonation and the speciation of surface functional groups on the absorbent but also affects the status of the adsorbate. Thus, it had a significant effect on the electrostatic charge that was assigned to ionized dye molecules between adsorbent and adsorbate. The experiments were carried out by varying the solution pH range from 2 to 12 with different adsorbents (OC and AHMOC-Ys), and the

vibration, demonstrating that magnetic hollow nanoparticles were successfully modified by urea. The above discussion clearly reveals that the AHMOC-Y composites consisted of AHMNs and OC, which further supported that AHMNs were grafted onto or encapsulated by the OC matrix (Figure 1d). Comparing the FTIR spectra before and after adsorption, the adsorption behavior is revealed distinctly. Figure 2b (OC and 2:1-MB) shows that the band at 1590 cm−1 moved to 1620 cm−1. The emerging peaks ranging from 1122 to 1590 cm−1 can be assigned to CN stretching of the MB molecule, which indicated that the composites could combine with MB molecules successfully. As the magnetite (Fe3O4) and maghemite (γ-Fe2O3) have similar XRD patterns and both exhibited magnetic behavior, Xray photoelectron spectroscopy (XPS) was carried out to gain further information about the chemical composition of OC and AHMOC-2:1. The core−electron lines of ferrous ions and ferric ions could both be detected and distinguished in XPS. Therefore, it was used to examine the shell structure of the synthesized product. The full scans of OC and AHMOC-2:1 are displayed in Figure 3a and show the C and O peaks at binding energies of 285.08 and 532.08 eV, respectively. Two additional peaks centered at 399.08 and 710.08 eV, corresponding to N and Fe, respectively, might be assigned to the introduction of aminohollow magnetic iron nanoparticles. The results are consistent with the FTIR analysis. Figure 3b,c displays the spectra at the C1s region for pristine OC and AHMOC-2:1. The C1s spectrum of OC could be deconvoluted into four primary peak components, centered at 284.06, 285.63, 288.08, and 290.08 eV, which were assigned to pure graphitic sites CC, CO, OCO, and carbonates, respectively.49 However, only three fitted curves were observed in the AHMOC-2:1 spectrum: the binding energy of 283.73 eV corresponded to pure graphitic sites (CC), 285.38 eV corresponded to sp2 carbon atoms bonded to nitrogen in the amorphous CN matrix (CN), and 287.68 eV corresponded to sp3 carbon atoms bonded to nitrogen (NCO).49 The difference between the spectra for the two materials might be attributed to the reaction between −NH2 and −COOH. Figure 3d exhibits the N1s spectrum of OC, which could be curve-fitted into two primary peak components at 401.18 eV, corresponding to quaternary (N−Q, 12.42%), and 405.38 eV, 183


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ACS Sustainable Chemistry & Engineering results are presented in Figure 4a. The zeta potentials of each mesoporous carbon composite at different pH values were also measured and are given in Figure 4b. As indicated in Figure 4a, the removal capacity was significantly increased when the pH increased from 3.0 to 8.0. However, there was a decreasing trend with increasing pH value from pH 8 to 12 for AHMOCYs, but the adsorption capacity of OC showed a constant increase. These results could be explained on the basis of the point of zero charge (pHpzc).26 As depicted in Figure 4b, the pHpzc of OC was nearly 4.5; however, the pHpzc values of the AHMOC-Ys were about 1.5. When pH < pHpzc, the adsorbents were positively charged due to the protonation of the surface functional groups, which generated electrostatic repulsion force, restricting the adsorption between adsorbent and adsorbate. In addition, the excess hydrogen ions, pore-filling, and π−π electron donor−acceptor interactions were highly competitive with the dye cations for the adsorption sites, further resulting in a lower adsorption capacity.52,53 However, when pH > pHpzc, the surface of the absorbents was negatively charged, which could be beneficial for MB adsorption due to electrostatic attraction. When the solution pH was about 8, the deprotonation of amine groups (on the surface of AHMNs) and acylamino groups (between OC and AHMNs) resulted in the formation of anions and led to a lower pHpzc value and higher removal capacity of AHMOC-Ys. The pHpzc value of AHMOC-Ys obviously increased when pH > 8 owing to high the concentration of OH−, which was an obstacle for the deprotonation velocity of functional groups on AHMOC-Ys. That was also a reason for the fluctuations in the pHpzc value and adsorption capacity. Otherwise, the amide bond was a negatively charged functional group, and the formation of an amide bond was favorable for capturing cation molecules. The adsorption capacity of each sample is listed in Table 1.

FeNHNH 2 + H+ → FeNHNH3+ (at pH < pH pzc , protonation) FeOH + OH− → FeO−

FeNHNH 2 → FeNHNH− (at pH > pH pzc , deprotonation)

The FeO− and FeNHNH− sites are Lewis bases and could coordinate with the MB molecules inside or outside of the complexes.26 The above analyses might explain the variation of pHpzc values and adsorption capacities of the different samples. Effects of Initial Concentration and Isotherm Analysis. The initial MB concentration was also an important parameter affecting adsorption capacity and removal efficiency, which provided the necessary driving force to surmount the resistance between the aqueous and solid phases. The experiments were carried out with different initial concentrations (100−1000 mg· L−1) at pH = 8, and the results are set out in Figure S3. It was obvious that the adsorption capacity was enhanced with increasing initial MB concentrations, which might be due to the heterogeneous adsorption sites. From Figure S3a, it can be seen that the AHMOC-2:1 had a higher adsorption capacity than the other absorbents. That could be due to the fact that the co-incorporation of AHMNs and OC in a suitable ratio provided more functional groups, which could offer additional sites, much higher negative charge, and small opening. With the increase in the initial MB concentration from 200 to 1000 mg·L−1, the removal efficiency of MB at equilibrium by AHMOC-2:1 decreased from 88.5% to 55% (Figure S3b). The increased adsorption capacity and decreased adsorption efficiency verified that the available surface binding sites were finite and fully used for a fixed adsorbent dosage with the excessive initial MB concentration. The results also demonstrated that AHMOC-2:1 had advantages in adsorption capacity compared with other tested materials, and it could be better in practical applications. In order to further study the adsorption behaviors between solid and liquid phase at equilibrium, two isotherm models (Langmuir and Freundlich) were selected to analyze the equilibrium data. The Langmuir isotherm model can be expressed as11

Table 1. Comparison of the Maximum MB Adsorption Capacity of Various Adsorbents adsorbents

adsorption capacity (mg·g−1)

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

316 475.2 457.5 522.4 488.2

ref 7 this this this this

study study study study

qe =

The mechanism for the removal of the cation contaminant by the metal component was proposed to involve surface complexation and ion exchange between the iron oxide surface and the cations in the MB aqueous solution.25 The amino-iron oxides located in water have FeOH and FeNH NH2 surface sites. The chemical structure (hydroxyl, amidogen coordination, and adsorption sites) on the surface depends on the oxidized morphology and crystal structure. The FeO− and FeNHNH− groups are pH-dependent active sites for the adsorption of MB. With the changing of pH, the  FeOH and FeNHNH2 groups acquire positive or negative charge by protonation or deprotonation. The magnetite surface chemistry can be summarized by the following formulas:26

RL =

qmax KLCe 1 + KLCe 1 1 + KLC0

(1)

(2)

−1

where qmax (mg·g ) is the maximun monolayer adsorption capacity of MB, KL (L·mg−1) is the Langmuir adsorption free energy constant, C0 (mg·g−1) and Ce (mg·L−1) are the initial and the equilibrium concentrations, respectively, and RL is the equilibrium parameter, from which the isotherms can be described as favorable (1 > RL > 0), linear (RL = 1), or unfavorable (RL > 1). The Freundlich isotherm model is given as qe = KFCe1/ n

(3) −1

where qe (mg·g ) is the adsorption capacity at equilibrium concentration, KF (mg g−1) (L mg−1)1/n and n are Freundlich

FeOH + H+ → FeOH 2+ 184


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ACS Sustainable Chemistry & Engineering Table 2. Langmuir and Freundlich Isotherm Parameters for Adsorption of MB onto AHMOC-Ys Langmuir model

Freundlich model

AHMOC-Y

qm (mg·g−1)

KL (L·mg−1)

R2

KF (L·mg−1)

n

R2

1:1 2:1 5:1

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

Table 3. Correlation Coefficients and Kinetic Parameters for Adsorption of MB onto AHMOC-Ys pseudo-first-order

pseudo-second-order

AHMOC-Y

qe,1 (mg·g−1)

k1 (min−1)

R2

qe,2 (mg·g−1)

k2 × 103 (g/mg·min−1)

R2

1:1 2:1 5:1

472.7 513.4 497.6

0.098 0.129 0.165

0.983 0.921 0.926

487.2 531.4 511.5

0.356 0.423 0.617

0.963 0.989 0.976

and further proved that electrostatic attraction could be involved in the proposed adsorption mechanism. Effect of Contact Time and Kinetics Model. In order to evaluate the adsorption kinetics and rate-limiting step in the adsorption process, two adsorption kinetic models, i.e., pseudofirst-order and pseudo-second-order rate models, were selected to estimate the adsorption mechanism and quantify the extent of uptake in the adsorption process. These two equations were employed to model the sorption data over the entire time range, which could be generally expressed as follows:52 pseudo-first-order model

isotherm constants relating to adsorption capacity and adsorption intensity, respectively, and Ce (mg·L−1) is the equilibrium solute concentration. The magnitude of n indicates the favorability of the process, and values of 10 > n > 1 represent favorable adsorption.11 The adsorption parameters of the Langmuir isotherm model and the Freundlich isotherm model were analyzed using nonlinear regression, and the results are displayed in Figure S4 and Table 2. It can be seen that the correlation coefficients R2 of the Freundlich isotherm model at a temperature of 328 K for AHMOC-Ys are 0.9334 (1:1), 0.9961 (2:1), and 0.9893 (5:1), which were better than the corresponding R2 values from the Langmuir isotherm model, suggesting that the adsoprtion behavior was more suitable for the Freundlich model. These results suggest that the adsorption onto heterogeneous surfaces with uniform energy distribution is reversible, attributable to the active groups, which was consistent with previous work by other researchers.54 This predicts that, as long as the dye concentration increases in the solution, the dye concentrated onto the materials will increase correspondingly.9,25,52 In addition, the n value was calculated to be in the range from 2.19 to 2.51, indicating that the adsorption between adsorbents and MB molucules was favorable (10 > n > 1).25 Effect of Ionic Strength. Generally, the wastewater from factories commonly contains dyes with high salt concentration. The salt concentration of the solution was one of the factors that influenced the hydrophobic and electrostatic interactions between surface functional sites of adsorbent and dye molecules.9,52 The effect of ionic strength on the MB adsorption by AHMOC-2:1 was analyzed by adding different NaCl concentrations, ranging from 0 to 1.0 mol·L−1. As can be seen from Figure S5, the sorption of MB on AHMOC-2:1 slightly improved with increasing NaCl concentration from 0.00 to 0.01 mol·L−1 and then decreased mildly as the NaCl concentration increased from 0.01 to 1 mol·L−1. The increasing phenomenon could be explained by the following reasons: (1) the MB molecules might be aggregated due to the increased intermolecular forces with increasing NaCl concentrations, and (2) the increased ionic strength might screen the electrostatic interaction, which was beneficial for MB removal because of the depression of electrostatic repulsion.9 However, the decrease in the salt concentration exceeded 0.1 mol·L−1 and might be due to a competition for adsorption between Na+ and MB molecules. The addition of abundant NaCl could also enhance the intermolecular forces and affect the diffusion of MB molecules.38 These results were consistent with the pH analysis

ln(qe − qt ) = ln qe −

k1 t 2.303

(4)

pseudo-second-order model t 1 t = − 2 qt q k 2qe e

(5)

where qe and qt (mg·g−1) are the amounts of MB adsorbed at equilibrium and at time t, respectively, and k1 (min·L−1) and k2 (g·mg−1·min−1) are the rate constants of the pseudo-first-order and pseudo-second-order adsorption models, respectively. The kinetic studies for MB adsorption on the AHMOC-Ys were carried out by varying the contact time from 0 to 48 h, and the results are displayed in Figure S6. It is clear in Figure S6a that the adsorption capacity increased markedly in the first 30 min, which might be assigned to the existence of plentiful of adsorption sites on the adsorbent surface. The removal capacity then slowly increased until the adsorption equilibrium was reached after about 2 h. This result might be due to the insufficient amount of active sites after the initial adsorption process.55 The correlation coefficients and kinetic parameters listed in Table 3 are those fitted by the kinetic models.25 The pesudo-second-order model of AHMOC-2:1 presented the highest correction coefficients R2 (0.989), and its calculated adsorption capacity (522.4 mg·g−1) was more in agreement with the experimental data (531.4 mg·g−1). This indicates that the rate-limiting step of the adsorption mechanism might be physisorption, which involves valence forces during sharing or exchanging of electrons.52,54 Intraparticle diffusion theory could identify complementary information for the diffusion mechanism. Therefore, an intraparticle diffusion model was used to simulate the process of MB transportation from aqueous solution to the surface of adsorbents and can be described as follows:10,52 185


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ACS Sustainable Chemistry & Engineering qt = k idt 1/2 + C

electrostatic repulsion existed between the adsorbent and adsorbate. So the higher temperature provided extra energy for the process to overcome the repulsion force to propel the ionic dyes closed onto the adsorbent, resulting in higher adsorption capacity.

(6)

where kid is the interparticle diffusion rate constant (mg·g−1· h1/2) and C (mg·g−1) is the that constant varies with the boundary layer thickness. Plots of qt versus t1/2 are presented in Figure S6c. It can be observed that the plots are multi-linear and have two regions, indicating different stages in the adsorption process. The first stage (slope kid1) presented a large slope, which might be due to the mass transfer from the boundary film to the external surface of AHMOC-Ys by film diffusion. However, the second phase (slope kid2) was a gradual process, which shows that the adsorbate migrated to the adsorbent internal structure by intraparticle diffusion. The above elaborations demonstrate that the rate-limiting step mainly involves film diffusion and intraparticle diffusion for the whole reaction. Thermodynamic Studies. The thermodynamic parameters of Gibbs free energy, enthalpy, and entropy were employed to explore whether the adsorption process was endothermic. The thermodynamic data were simulated by the following equations:10 ΔG = −RT ln KL ln KL = −

KL =

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CONCLUSION In the presented study, we composed the aminated hollow magnetic nanospheres with oxidized mesoporous carbon in different mass ratios, and the final materials were noted as AHMOC-Y (Y = 1:1, 2:1, 5:1). The primary purpose was to explore the characteristics of these new hybrid materials and rationalize the optimum proportion of materials to gain a good adsorption capacity for MB at mild conditions. Characterizations based on SEM, XRD, VSM, FTIR, XPS, and zeta potential confirmed that these kinds of samples were synthesized successfully. The results revealed a special morphology compared with that of the initial oxidized mesoporous carbon. The adsorption characteristics were also affected by the initial concentration, ionic strength, and temperature. The zeta potential and adsorption behavior at different pH values demonstrated that a high adsorption capacity was achieved at low pH value (pH = 8). The adsorption process was well fitted with the Freundlich model and a pseudo-second-order model. The thermodynamic study confirmed the endothermic and spontaneous nature of the MB adsorption process. Conclusions obtained from this study revealed the special morphology of AHMOC, and its high adsorption capacity in mild conditions make it able to be widely applied in other domains.

(7)

ΔH ΔS + RT R

(8)

qe Ce

(9) −1

where ΔG is change of Gibbs free energy (kJ·mol ), ΔS (J· K−1· mol−1) and ΔH (kJ·mol−1) are entropy and enthalpy, respectively, R (8.314 J·mol−1·K−1) is the universal gas constant, T (K) is the Kelvin absolute temperature, KL represents the Langmuir constant, and qe is the equilibrium concentration. The plot of ln KL versus 1/T is shown in Figure S7. The thermodynamics were investigated at three different temperatures (298, 308, and 318 K), and the results are listed in Table 4. Generally, the change in free energy for chemisorption was in

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* Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.6b01207. Experimental details of materials, synthesis methods, sorption experiments, and sample characterization by BET and VSM, including Scheme S1, Table S1, and Figures S1−S8 (PDF)

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

ln kL

ΔG (kJ·mol−1)

ΔS (kJ·mol−1·K−1)

ΔH (kJ·mol−1)

298 308 318

1.8698 1.9811 2.1653

−4.6328 −4.9450 −5.7248

54.372

11.603

ASSOCIATED CONTENT

S

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AUTHOR INFORMATION

Corresponding Authors

*W.C.: E-mail [email protected]. *Y.L.: Tel. + 86 731 88649208; fax + 86 731 88822829; E-mail [email protected].

a range of −80 to −400 kJ·mol−1, and that for physisorption was between −20 and 0 kJ·mol−1.52 The negative values for ΔG increased with increasing temperature in the range of 298−318 K (from −4.63 to −5.72 kJ·mol−1) and were within the range of −20 to 0 kJ·mol−1, indicating that the predominant mechanism of the adsorption process was physisorption.52 The positive value for ΔH (11.60 kJ·mol−1) indicated that the adsorption process was endothermic in nature, which could be further supported by studying the temperature effect. The positive value of ΔS (54.37 kJ·mol−1·K−1) represented a higher order of reaction during the adsorption of MB dye onto AHMOC-2:1, indicating that randomness at the solid/solution interface increased during the adsorption process, which reflected some structural changes of the MB molecules and AHMOC-2:1. Additionally, as the adsorption process was endothermic,

ORCID

Wei-cheng Cao: 0000-0002-1496-0817 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors greatly acknowledge the financial support of this work from the National Science Foundation of China (51521006, 51579097), “Lake contamination and wetland remediation” (51521006), and “The principle of electrical chemical enhancement of Thiobacillus ferrooxidans bacterial exsitu bioremediation of heavy metal contaminated sediment” (51579097). 186


Research Article


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ABBREVIATIONS OC, oxidized ordered mesoporous carbon; AHMNs, aminated hollow magnetic nanospheres; AHMOC, aminated hollow magnetic nanospheres-oxidized ordered mesoporous carbon

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