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Magnetic Activated Carbon Derived from Biomass Waste by Concurrent Synthesis: Efficient Adsorbent for Toxic Dyes André L. Cazetta, Osvaldo Pezoti, Karen C. Bedin, Taís L. Silva, Andrea Paesano Junior, Tewodros Asefa, and Vitor C. Almeida ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.5b01141 • Publication Date (Web): 13 Jan 2016 Downloaded from http://pubs.acs.org on January 20, 2016

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ACS Sustainable Chemistry & Engineering

Magnetic Activated Carbon Derived from Biomass Waste by Concurrent Synthesis: Efficient Adsorbent for Toxic Dyes

André L. Cazetta,† Osvaldo Pezoti,† Karen C. Bedin,† Taís L. Silva,† Andrea Paesano Junior,§ Tewodros Asefa,‡,#,* and Vitor C. Almeida†,*

†

Laboratory of Environmental and Agrochemistry, Department of Chemistry, Universidade

Estadual de Maringá, Av. Colombo 5790, Maringá, Paraná 87020-900, Brazil.

§

Department of Physics, Universidade Estadual de Maringá, Av. Colombo, 5790 Maringá,

Paraná 87020-900, Brazil.

‡

Department of Chemistry and Chemical Biology, Rutgers, The State University of New Jersey,

610 Taylor Road, Piscataway, New Jersey 08854, USA.

#

Department of Chemical and Biochemical Engineering, Rutgers, The State University of New

Jersey, 98 Brett Road, Piscataway, New Jersey 08854, USA.

* Corresponding Authors: (T.A.) [email protected]; (V.C.A.) [email protected]

ACS Paragon Plus Environment

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ABSTRACT: The development of advanced carbon nanomaterials that can efficiently extract pollutants from solutions is of great interest for environmental remediation and human safety. Herein we report the synthesis of magnetic activated carbons via a simultaneous activation and magnetization processes using carbonized biomass waste from coconut shells (Cb’s) and FeCl3.6H2O as precursor. We also show the ability of the materials to efficiently extract toxic organic dyes from solutions and their ease of separation and recovery from the solutions using a simple bar magnet. Textural characterization shows that the materials are microporous. Further analyses of the deconvoluted XPS spectra and X-ray diffraction patterns reveal that the materials possess magnetite, maghemite and hematite. SEM and TEM images show that an increase in the ratio of FeCl3.6H2O:Cb leads to an increase in the material’s magnetic properties. The point of zero charge (pHpzc) indicates that the materials have acidic characteristics. Adsorption kinetic studies carried out onto MAC1 indicates that the Elovich model can satisfactorily describe the experimental data at low initial concentrations and the pseudo-second order model can best fit the data at higher initial concentrations. Moreover, adsorption equilibrium studies reveal that the Langmuir model adequately allows the determination of the materials’ adsorption capacity. Our adsorption and equilibrium fit of the data include non-linear more models, and are thus more informative compared with recent related works, in which only linear fits have been presented. Extensive mechanistic studies for the adsorption processes are also included in the work.

Keywords: Coconut shell, Magnetic activated carbon, Adsorption, Sunset Yellow, Food dye.


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INTRODUCTION Magnetic activated carbons (MACs) are a class of carbon materials that possess high surface areas and magnetic properties. Compared with conventional carbons, MACs are generally more suitable for adsorption applications due to their magnetic property, which allows them to be easily removed from solutions along with the pollutants they adsorb, simply by applying a bar magnet around them. This process, in turn, makes lengthy filtration and/or centrifugation step(s) unnecessary when the MACs are applied for adsorption of environmental pollutants.1,2 MACs with such a property have been shown to remove various common environmental pollutants from solutions, including tetracycline,3 methylene blue,1,4-5 arsenic,2 hexavalent chromium,6-8 zinc,9 anionic dyes,10 p-nitrophenol,11-12 Acid Yellow 17 dye,13 and 2,4,6-trinitrophenol.14 MACs have been commonly synthesized from various precursors that possess high contents of carbon, such as graphene oxide,15-16 carbon nanotubes,6,17-18 glucose19 and commercially available cellulose;20 however, the synthesis of MACs from such non-renewable precursors is relatively costly and unfeasible for possible utilization of the materials in large-scale applications.14,21 Thus, research is being conducted worldwide to use alternative, abundant, renewable precursors for MACs.2-3,9 To this end, coconut shells or their lignocellulose, which are waste plant byproducts that constitute a large fraction (or approximately 35%) of the total weight of coconut fruits, can be good candidate precursors for MACs. While coconut shells have been shown to serve as precursor for making activated carbons (ACs),22-24 to the best our knowledge, there has not been any report on the application of coconut shells for making MACs. Besides the type of precursor, the synthetic method itself is another important issue that needs to be considered when making MACs. Generally, the synthesis of MACs is achieved by


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using either an iron co-precipitation7,14 or a hydrothermal treatment method.3,19,25-26 In both cases, the syntheses involve the steps of making AC, followed by iron-impregnation into the AC and pyrolysis of iron-impregnated iron/carbon composite material. An alternative to this synthetic method involves the preparation of the AC and iron-impregnated magnetic materials in a single step simultaneously, wherein the iron serves both as an activating agent for the AC and as a precursor for magnetically active species on the material’s surfaces. With such concurrent synthetic route, the synthetic steps can be reduced and the synthesis can become more facile.2,27-28 Herein we report the application of concurrent activation and magnetization processes on the biochars of coconut shells—an agricultural waste that is widely available at many places around—for making MAC materials that are proven to serve as efficient adsorbents for toxic dyes. Moreover, by using different relative ratios of coconut shells and ferric chloride (FeCl3), MACs with different structural features as well as optimal adsorption properties were obtained. The MACs were characterized by different analytical techniques, and their adsorption properties for the Sunset Yellow (SY) food dye were evaluated. More importantly, the materials were found to have high adsorption capacity for SY. This is important given the fact that SY is a dye that is widely used by the food and pharmaceuticals industries to provide color and good appearance to various food products, but it is also documented to cause allergic reaction to organisms including humans and some potential health effects at higher doses mainly due to its azo (N=N) group.29-30 For this very reason, SY has in fact been completely banned from use as food additive in some countries.29 Additionally, a dye such as SY can have severe environmental impacts by the virtue for its severe inherent resistance to undergo biodegradation, and thereby difficultly to remediate if it makes it into the environment.31 Hence, effectively removing SY from different food and drug industrial effluents using the types of materials we have developed here is the most feasible


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way to protect the environmental, human beings and various organisms from the dye’s potentially adverse effects. Moreover, we chose SY in our studies, as it can serve as a good model dye for elucidation of the adsorption behaviors of various materials toward various organic pollutants.

EXPERIMENTAL SECTION Synthesis of MACs. The proximate analysis of coconut shells and the procedure for obtaining the carbonized coconut shells (Cb’s) (i.e., the parent material that leads to the coconut shell-derived MACs) have been described in our previous report.22 We prepared the MAC materials possessing different structure and composition by impregnating different relative amounts of ferric chloride (FeCl3.6H2O) into Cb and then carbonizing the resulting composite materials. Specifically, three different ratios of FeCl3.6H2O:Cb (wt.:wt.) were employed, giving three different MAC materials, namely 1:1 (MAC-1), 2:1 (MAC-2), and 3:1 (MAC-3). The mixtures of Cb and FeCl3.6H2O in 10 mL of distilled water were placed in a stainless steel reactor and subjected the mixtures to magnetic stirring for 2 h. The mixtures were then placed in oven at 120 °C until the water was completely evaporated from them. The resulting solid materials were placed in a muffle furnace and heated from room temperature to 700 °C at a rate of 20 °C min-1 and kept at 700 °C for 1.5 h under a flow of N2 at a rate of 100 cm3 min-1. After letting the furnace to cool to room temperature, the carbonized materials were washed with aqueous HCl solution (1.0 mol L-1) (prepared in distilled water) under reflux, until the pH of the solution reached a value of ~6.5-7.0. The solid materials were recovered by filtration, washed with water and dried in oven at 100 °C for 24 h, yielding the desired MAC materials.


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Characterizations. Textural properties of all the MAC materials synthesized above and the Cb were investigated with N2 adsorption-desorption measurements at 77 K using a Quantachrome Nova 1200e surface area analyzer. The BET surface area (SBET) was calculated by applying the Brunauer-Emmett-Teller (BET) equation on the data collected in the range of relative pressure of 0.05 to 0.20. The total pore volume (Vt) (which is defined as the maximum volume of N2 adsorbed at relative pressure p/po = 0.99) and the micropore volume (Vµ) of the materials were determined using the deBoer method.32 The mesopore volumes of the materials were calculated from the difference between the total pore volume and the micropore volume (Vt - Vµ). The average pore sizes of the materials (Aps) were determined with the equation Aps = 4Vt/SBET, and the pore distributions of the materials were analyzed using density functional theory (DFT).33 The surface functional groups in the MAC materials were determined using Fourier transform infrared spectroscopy (FT-IR) of samples prepared as pellets with KBr. The spectra were obtained with a resolution of 4 cm-1 and an acquisition rate of 20 scans min-1 in a range of 4000 to 400 cm-1 using a Bomem MB-100 spectrometer. X-ray photoelectron spectroscopy (XPS) spectra were obtained using a Kratos Axis Ultra Spectrometer with monochromatic X-ray source of Al Kα operating at 1486.7 eV with emission current of 10 mA. The XPS peaks were deconvoluted as necessary, and then analyzed. The crystallinity of the MACs was investigated with X-ray diffraction (XRD) using a Shimadzu XRD-7000 diffractometer operating with Cu Kα (λ = 1.540598 Å) as the radiation source. The diffractograms were obtained in the 2θ range of 5 to 80° with a step size of 0.02°. Magnetic measurements of the samples were performed with a vibrating sample magnetometer (VSM) at room temperature. The point of zero charge (pHpzc) of the MACs was determined by employing the methodology described in Prahas et al.34 The morphological characteristics of the materials were elucidated from the images obtained by scanning electron microscopy (SEM) using a Zeiss Sigma SEM microscope. Transmission


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electron microscope (TEM) images were acquired for the materials with a FEI Tecnai T-12 TEM instrument. Batch adsorption studies. The adsorption properties of the MAC materials for removal of dye molecules from aqueous solutions were then evaluated. This was conducted by mixing the MAC material that possessed the highest BET surface area with aqueous solutions of the Sunset Yellow food dye (SY; Acid Yellow 6; C16H10N2O7S2Na2; MW is 452.36 g mol-1; and whose chemical structure is shown in Figure S1) in different concentrations, and then measuring/analyzing the amount of SY adsorbed on the MAC material in different intervals of time. For adsorption studies, a stock solution of SY with a concentration of 1.0 g L-1 was prepared by dissolving an appropriate mass of the dye in distilled water. Working solutions were then prepared by diluting the stock solution with distilled water to desired concentrations. The effect of pH on the adsorption properties of MAC toward SY was studied using a dye solution with an initial concentration of 80 mg L-1, and then adjusting the pH of the solution in the range of 3.0 to 10.0 using NaOH or HCl solution (0.1 mol L-1). In a typical study, 20.0 mg of MAC and 20 mL of SY were mixed in 50 mL flask, and the mixture was shaken constantly (at 150 rpm) for 240 min. After centrifugation of the mixtures, the concentrations of the SY remaining in the solutions were determined by UV-Vis spectroscopy (using PerkinElmer, Lambda 25). Specifically, the intensity of the absorption maximum (λmax) at 483 nm corresponding to SY was measured, and the values were compared with a pre-determined calibration curve. The maximum amount of SY adsorbed on MAC (qe) was then calculated using Eq. (1).

q =

(1)


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where Co and Ce (mg L-1) are the initial and equilibrium liquid-phase concentrations of SY, respectively, V (L) is the volume of the solution, and m (g) is the mass of dry adsorbent used for the adsorption study. For adsorption kinetic studies, the volume of SY solution and the mass of MAC were both set the same as those in the experiments described above. Moreover, the initial concentrations of SY were set as 10, 50 and 80 mg L-1, and the contact times were varied between 2.5 and 240 min. The amount of SY adsorbed on the MAC at t time was then calculated using Eq. (2):

q =

(2)

where, Co and Ct (mg L-1) are the initial concentration of SY and the liquid-phase concentrations of SY at time t, respectively, V (L) is the volume of the solution, and m (g) is the mass of dry adsorbent used. Adsorption equilibrium studies were carried out by mixing 20 mL of SY solutions with different concentrations (5-100 mg L-1) of MAC (20.0 mg) in 50 mL flasks. The mixtures were shaken constantly at 150 rpm for 240 min, and the amount of SY adsorbed on the MAC (in mg g1

) was then calculated using Eq. 1 above. Additionally, the non-linear kinetic models of pseudo-

first order, pseudo-second order and Elovich (Table S1), and the non-linear models of Langmuir, Freundlich and Dubinin-Radushkevich (Table S2) were fitted to experimental data using Origin 6.1® software. Thermodynamic studies were carried out at temperatures of 308, 318 and 328 K with SY dye at initial concentration of 100 mg L-1 in a 20 mL solution containing 20.0 mg of MAC that was shaken constantly at 150 rpm for 240 min. The amount of SY adsorbed on the MAC (in mg g-1) at each temperature was then calculated by using Eq. 1 above and by employing the thermodynamic parameters calculated from the equations listed in Table S2.


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RESULTS AND DISCUSSION Textural properties. Figure 1a shows the N2 adsorption-desorption isotherms of the MAC materials. According to the results, the isotherms of MAC2 and MAC3 could be classified as Type I, or also known as Langmuir type, which is characteristic of microporous materials.35 The isotherm for MAC1, on the other hand, could be classified as Type IV as it showed a capillary condensation step at higher relative pressure, which is typically observed in materials possessing mesoporous structures. Moreover, there was significant volume of N2 adsorbed by the MAC1 material at higher relative pressure, which is due to the presence of a substantial amount of mesopores in this particular material.12,35 140 130

0.030 0.025

120 110

(b)

(a)

MAC1

DVp (cm3 Å-1 g-1)

3

-1

Adsorbed volume (cm g )

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


100 MAC2

90 80

0.020

MAC1 MAC2 MAC3

0.015 0.010 0.005

70 MAC3

60 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 0

Relative pressure (p/p )

0.000 15 20 25 30 35 40 45 50 55 60 65 70 Pore size (Å)

Figure 1. N2 adsorption (closed symbols) and desorption (opened symbols) isotherms (a), and pore size distribution obtained by the DFT-based method (b) for the different MAC materials we have synthesized (MAC1, MAC2 and MAC3). In Table 1 the textural properties of the Cb and MAC materials have been compiled. The SBET values of Cb, MAC1, MAC2 and MAC3 were found to be 6.22, 372, 337 and 238 m2 g-1, respectively. Thus, the concurrent activation and magnetization synthetic processes can be said to


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be effective in producing highly porous MAC materials from charred biomass waste (e.g., MAC1 has 60 times higher surface area (SBET) than the parent material Cb). Furthermore, it was observed that the increase in the ratio of FeCl3.6H2O:Cb could lead to a decrease in the values of surface area and total pore volume of the MAC materials. Additionally, as can be seen in Table 1, the increase in the FeCl3.6H2O:Cb caused a decrease in the average pore size of MACs from 2.26 to 1.98 nm. The reason for the materials to show a decrease in surface area as the amount of iron is increased most likely because the iron ends up as iron oxide-based particles occupying some of the void spaces in the materials (vide infra). Table 1. Textural properties of Cb and MACs synthesized in different impregnations ratio 1:1 (MAC1); 2:1 (MAC2) and 3:1 (MAC3). Textural properties

Cb

MAC1

MAC2

MAC3

(SBET) – Surface area (m2 g-1)

6.22

372

337

238

(VT) – Total pore volume (cm3 g-1)

0.099

0.210

0.172

0.118

(Vµ) – Micropore volume (cm3 g-1)

0.000

0.156

0.141

0.101

(Vm) – Mesopore volume (cm3 g-1)

0.099

0.054

0.031

0.017

Percentage of micropore (%)

0.00

74.29

81.98

85.59

Percentage of mesopore (%)

100

25.71

18.02

14.41

(Aps) – Average pore size (nm)

6.34

2.26

2.04

1.98


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It is worth noting here that the MAC materials we have synthesized show slightly higher/comparable surface areas than/as some related materials reported by others. For instance, the magnetic carbon materials synthesized by Bastami and Entezari11 using carrot dross as precursor via a chemical co-precipitation technique showed SBET values between 340-435 m2 g-1 and an average pore size of ~2.20 nm. In another case, using a chemical co-precipitation method Han et al.12 obtained MACs modified with iron that gave SBET values between 278-192 m2 g-1. It is also worth adding that the surface areas of the materials we have synthesized are much higher than other carbon materials prepared by other related methods. For example, the surface area of copper oxide-loaded activated carbon prepared in one-step by Ghaedi et al.36 is only ~83 m2 g-1. Based on pore size distribution analyses (Figure 1b), the pores in the MAC3 were found to be in the range of 17 to 19 Å, while those in MAC2 were found to be predominantly ca. 17 Å, with some more in the range of 25 to 40 Å. On the other hand, based on the pore size distribution analysis of MAC1, a significant portion of the pores in this particular material (25.71% of them) was found to be in the range of 23 to 43 Å (or pores that are considered as mesopores) and another large fraction was found to be distributed around 18 Å (Figure 1b). Chemical, Morphological and Magnetic Characterization. Figure 2 shows the FT-IR spectra of MAC1, MAC2 and MAC3. The spectra for the three materials appear similar, revealing their similar compositions. It is worth noting that the characteristic O-H stretching vibration peak at ca. 3500 cm-1 is absent in the spectra, which is indicative of the absence of “free” hydroxyl groups on the materials. The result is also indirectly indicative of the conversion of the oxygen-containing moieties in the organic backbone of the parent materials to inorganic oxides (magnetite, maghemite and/or hematite, vide infra). The peaks at ca. 1700, 1580 and 1220 cm-1 are attributable to the stretching vibration of -C=O (carbonyl groups), C=C (aromatic rings),


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and C-O (ether moieties), respectively.15,21,37-38 The peak at ca. 600 cm-1 can be attributed to the Fe-O bond stretching vibration; this peak is actually common in iron containing oxides, such as maghemite (γ-Fe2O3).8,12,14

MAC1

Transmittance (a.u.)

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

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MAC2

MAC3

4000

3500

3000

2500

2000

1500

1000

500

-1

Wavenumber (cm )

Figure 2. FT-IR spectra for the MACs obtained using different ratios of FeCl3:Cb. The XRD patterns of the MAC materials we have synthesized are presented in Figure 3ac. Based on the results, it is possible to discern the presence of two distinct crystalline phases in the materials: one characteristic of hematite and the other characteristic of magnetite. The hematite is a crystalline solid with hexagonal compact structure whereas the magnetite is a crystalline oxide with cubic structure of inverse spinel type possessing two sites that are nonequivalent in symmetry, i.e., tetrahedral (A) and octahedral (B). The peaks observed at 2θ of 24.3, 33.3, 41.0, 49.6 and 57.5o correspond to the 012, 104, 113, 024 and 122 basal planes of the hematite portion of the materials.1,39 The peaks at 2θ of 30.4, 35.8, 43.5, 57.6 and 63.1o, on the other hand, are characteristics of the 220, 311, 400, 511 and 440 basal planes of the crystalline magnetite portion of the materials.1,5,10 Furthermore, the magnetite portion of the materials is observed to have higher degree of crystallinity. It is worth adding here that there is another


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member of the family of iron oxides called maghemite, which too has cubic structure of inverse spinel type and a similar structure as that of magnetite, except it exclusively contains Fe(III) oxide. Due to the structural similarity between magnetite and maghemite phases, discerning the magnetite phase from the maghemite phase unambiguously with XRD alone is, therefore, inherently difficult. Hence, XPS spectroscopy was employed to distinguish between these two components, as described below and in Supporting Information section.

(a)

(b)

Hematite Magnetite

10

20

30

40 50 o 2θ ( )

Hematite Magnetite

60

Relative intensity (a.u.)


70

10

80

(c)

20

30

40 50 2θ (°)

60

70

80


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


Hematite Magnetite

10

20

30

40

50 2θ (°)

60

70

80

Figure 3. XRD patterns for MAC1 (a), MAC2 (b) and MAC3 (c).


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Figure S2a-c displays the XPS survey spectra of the MACs we have synthesized. As can be seen from the spectra, the MAC materials contain carbon, nitrogen, oxygen, and iron. Specifically, peaks with binding energy values of ca. 285.08, 401.08 and 531.08 eV, which correspond to C1s, N1s and O1s, respectively,4,40-41 are observed. The double peaks in the range between ca. 712.08 to ca. 724.08 eV are characteristics of Fe2p3/2 and Fe2p1/2, respectively.1-3,40 The elemental compositions of the materials obtained from the XPS spectra of MACs are compiled in Table S3. The results show that MAC3 has the highest amount of Fe (4.93%) and O (29.2%), due most likely to the higher ratio of FeCl3.6H2O:Cb used for making it. Dastgheib et al.28 also reported that for a given pyrolysis temperature (600 °C) and N2 flow (500 cm3 min-1), increasing the amount of FeCl3 yields materials with lower carbon and higher oxygen content. Figure S3a-c shows the deconvoluted XPS spectra for different Fe species present on the surface of MACs. From the results, a peak corresponding to Fe0, which is typically observed at a binding energy of ca. 708 eV,40 is not seen; this result is indicative of the absence of Fe0 in the MAC materials. On the other hand, peaks at 710.9 and 713.1 eV (Fe3/2), which are characteristics of Fe3+ and Fe2+, are observed, confirming the presence of magnetite (Fe3O4) in the materials. Additionally, two other peaks at 724.1 and 725.9 eV, corresponding to Fe1/2 or the hematite (αFe2O3) and maghemite (γ-Fe2O3) portions, respectively, were seen on the XPS spectra of the MAC materials.40,42-43 The magnetic properties of the MAC materials were evaluated by obtaining magnetic hysteresis curves for them, and the results are displayed in Figure 4. Specific pertinent data corresponding their magnetic properties are also compiled in Table 2. As can be seen from the results, all the MAC materials showed ferromagnetic properties with saturation magnetization (Ms) values of 17.06, 16.56 and 28.69 emu g-1 for MAC1, MAC2 and MAC3, respectively. It is


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worth noting here that these values are comparable or higher than those of a few other MAC materials recently reported by other groups.3,7,14,26 For example, MAC materials obtained from almond shells and reported by Mohan et al.14 showed a saturated magnetization of only 4.47 emu g-1 at 300 K. A magnetic porous carbon material reported by Zhu et al.,3 which was obtained via simultaneous activation and magnetization, showed even a lower saturation magnetization values of only 0.76 emu g-1 at 300 K.3 On the other hand, Cui et al. reported γ-Fe2O3/carbon hollow spheres possessing a saturation magnetization of 34.7 emu g-1 at room temperature.26 Nethaji et al. reported in Bioresource Technology 134 (2013) 94 that a corn cob-activated carbon coated with nano-sized magnetite particles have a surface area of 143 m2 g-1 and a saturation magnetization value of 48.43 emu g-1. It is worth adding here that based on the values of coercivity (Hc), remanence (Mr), and the ratio between remanence and saturation magnetization compiled in Table 2, it is possible to conclude that the MAC materials we have synthesized all possess superparamagnetic properties at room temperature, as their ratios of Mr/Ms were found to be less than 25%.1,13,44

30 20 -1

MS (emu g )

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


MAC1 MAC2 MAC3

10 0 -10 -20 -30 -15000 -10000

-5000

0

5000

10000

15000

HC (Oe)

Figure 4. Magnetic hysteresis loops of MAC1, MAC2 and MAC3.


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The morphological features of the MAC materials were investigated by obtaining SEM images for them, which are displayed in Figure 5a-c. From the images in Figure 5a-b, it can be seen that MAC1 and MAC2 possess some cracks and porosity in their structure. The SEM image for MAC3 (Figure 5c), on the other hand, shows surfaces with higher degree of homogeneity and no apparent cracks (which is perhaps the reason for its lower surface area, Table 1); this may be due to the relatively higher amount of FeCl3 used for making this particular material, or the relatively higher amount of iron oxides it contains (Table S3). Additionally, some particles on the surface of the materials, which are presumably iron oxide nanoparticles, are also observed in the SEM images of all the samples. Table 2. Magnetic properties of MAC1, MAC2 and MAC3. MAC materials

Ms (emu g-1)

Mr (emu g-1)

Hc (Oe)

Mr / Ms

MAC1

17.01

3.61

136.58

0.21

MAC2

16.55

1.79

66.99

0.11

MAC3

28.74

3.04

75.27

0.11

The same types of particles are also observed on the TEM images of MACs (Figure 6a-c). The result is similar to the aggregated iron oxide particles observed on the surfaces of an iron oxide-containing MAC material derived from industrial activated carbon and iron(II) gluconate that was reported by Ranjithkumar et al.13


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Figure 5. SEM images for MAC1 (a), MAC2 (b) and MAC3 (c).


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Figure 6. TEM images for MAC1 (a), MAC2 (b) and MAC3 (c). Such types of iron oxides (hematite, magnetite and maghemite) are expected to form from the hydrolysis of FeCl3 in aqueous solution, in a process involving the formation of mono-, di-, and poly-nuclear iron hydroxide species via different elementary reaction steps, including the ones described in Scheme 1 (Eq. 3a-3g below).


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Scheme 1. Mechanism of iron oxides formation and porosity development on MAC materials.

In the course of these reactions that lead to iron oxides, the pH of the solution dictates what the dominant Fe3+ species in the aqueous media would be.45 In the synthetic procedure reported herein, the initial steps (Eq. 3a-3b) occur in the early stages upon stirring the mixture of FeCl3 and Cb. These reaction steps promote the hydrolysis of FeCl3, giving rise to different ferric hydroxide ionic species and colloidal particles. The reactions shown in Eq. 3b, 3c and 3d, in particular, could be extremely fast and produce various species that coexist in equilibrium.45 When the resulting solution is placed in oven at 110 °C, evaporation of water and transformation of the soluble hydroxide species to goethite occur (Eq. 3e). Upon further heating of the solid mixture (goethite and Cb) in an inert atmosphere to 200 °C, further dehydration and the conversion of goethite to maghemite, as described by Eq. 3f, take place. Heating the material further at higher temperatures would make the maghemite to undergo some structural rearrangement and convert to hematite.46 Heating the material at temperatures at >567 °C can make the hematite also to undergo some structural change and become part of the iron


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oxide/carbon nanocomposite materials.47-48 This process can also directly help the hematite to transform to magnetite, especially when CO is removed from the reaction as fast as it is formed, which can be facilitated by the constant flow of N2.49 Additionally, the formation of porosity within our MAC materials can be explained based on Eq. 3g, in which the conversion of C(s) to CO(g) occurs and the release of carbon as CO leaves behind some void spaces within the structure of the material. This also accounts for MACs’ higher surface area compared with Cb’s (Table 1). The acidity/basicity of the MAC materials was elucidated by probing their point of zero charge (pHpzc). pHpzc corresponds to the maximum pH value at which the difference between pHinitial and pHfinal for a given material is zero, i.e., pHinitial – pHfinal = 0.34 Figure S4a-c shows the graphs and the values of pHpcz for MAC1 (a), MAC2 (b) and MAC3 (c). Accordingly, the pHpzc values for the MAC1, MAC2 and MAC3 were obtained to be 4.51, 4.12 and 4.10, respectively. This means, as the impregnation ratio of FeCl3:Cb decreases the acidity of MAC materials increases. Moreover, the values of pHpzc are close to those reported for other related materials in the literature.8,50 We also believe that the magnetite portion of our MAC materials is responsible for their acidic properties because the presence of magnetite on carbon lattice has been reported to provide acidity to the composite material.51 Effect of Initial pH Value on MACs’ Adsorption Properties toward SY Dye. The effect of pH on the ability of the MAC materials to remove SY dye from aqueous solutions was then investigated, and the results are presented in Figure S5. For detailed studies, MAC1 was selected due to its higher surface area as well as higher amount of iron and magnetic properties among the three materials we synthesized. As SY is an acid dye, it can be expected to form negatively charged species over a wide pH range in aqueous solutions.30 Considering the value of pHpzc for the MAC1 of 4.51, MAC1 (the adsorbent) acquires negative charge in the range of


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pHsolution > pHpzc, or over a large pH range including pH 7.00. Conversely, MAC1 possesses positively charged surfaces if pHsolution < 4.51. So, as can be seen in Figure S5, unsurprisingly the removal of SY by MAC1 was attained well in values of pH < pHpzc of MAC. The decrease in the adsorption capacity of MAC1 toward SY upon an increase in pH of the solution is associated with the unfavorable electrostatic interaction between the negatively charged surfaces of MAC1 at higher pH and the negatively charged SY molecules. Although electrostatic interaction is important, other mechanism such as hydrogen bonding and electron dispersion at π-conjugated systems (see below for further explanations) may also exert some influence in the adsorption processes between MAC1 and SY molecules.30 Adsorption Kinetics. Adsorption of adsorbates in porous solid materials often present complex kinetics, in which the adsorption rate can be directly influenced by parameters such as the reactivity of the adsorbent, the homogeneity/heterogeneity of the adsorbent’s surface, the pH and temperature of the solutions, the surface and compositional features of the adsorbate molecules, and many other factors. Elucidation of these factors by investigating the experimental data using different kinetic models may allow one to obtain information about the possible mechanism of adsorption processes as well as the different transition state leading to the formation of complex adsorbent-adsorbate moieties.52-53 Hence, to determine further the materials’ adsorption properties toward SY molecules under non-acidic or natural conditions, equilibrium and kinetic of adsorption studies were carried out at the natural pH of common aqueous solutions (~5.88). In Figure 7a-c the kinetic curves of adsorption of SY on MAC1 are presented. As can be seen, the adsorption process reaches equilibrium in approximately 75 min for the three different initial concentrations that we investigated. Analysis of kinetics parameters, presented in Table 3, shows that the pseudo-first


21


order model does not describe well the experimental data as its corresponding R2 value is relatively low (between 0.9153-0.9471) and also because the theoretical adsorption capacity (qe) expected by the model is significantly different from the one obtained experimentally. 10

4.0 (a)

9

3.5

7

2.5 Experimental data Pseudo-first order Pseudo-second order Elovich

2.0 1.5

6

-1

qt (mg g )

-1

(b)

8

3.0 qt (mg g )

1.0

Experimental data Pseudo-first order Pseudo-second order Elovich

5 4 3 2

0.5

1

0.0

0 0

25

50

75 100 125 150 175 200 225 250 Time (min)

0

25

50

75 100 125 150 175 200 225 250 Time (min)

16 (c)

14 12 10

-1

qt (mg g )

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

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Experimental data Pseudo-first order Pseudo-second order Elovich

8 6 4 2 0 0

25

50

75 100 125 150 175 200 225 250 Time (min)

Figure 7. Adsorption kinetic for SY on MAC1 at different initial concentrations: 10 mg L-1 (a), 50 mg L-1 and (b) and 80 mg L-1 (c).


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Table 3. Kinetics parameters of models pseudo-first order, pseudo-second order and Elovich calculated from non-linear fits. Co (mg L-1)

qe,exp (mg g-1)

Pseudo-first order

Pseudo-second order

Elovich

10

3.32

qe = 3.12

qe = 3.38

α = 2.58

k1 = 0.12

k2 = 0.049

β = 1.97

ho = 0.37

ho = 0.56

R2 = 0.9153

R2 = 0.9695

R2 = 0.9718

qe = 8.55

qe = 9.14

α = 20.28

k1 = 0.19

k2 = 0.028

β = 0.85

ho = 1.62

ho = 2.34

R2 = 0.9336

R2 = 0.9359

R2 = 0.8813

qe = 12.59

qe = 13.54

α = 18.40

k1 = 0.17

k2 = 0.017

β = 0.53

ho = 2.09

ho = 3.02

R2 = 0.9471

R2 = 0.9541

50

80

8.93

13.12

R2 = 0.9093

qe = mg g-1, k1 = min-1, k2 = g mg-1 min-1, ho = mg g-1 min-1, α = g mg-1 min-2 and β = mg g-1min-1.

As can be seen in Table 3, a pseudo-second order model gave the best fit to the experimental data, with R2 values of 0.9359 and 0.9541 for initial concentrations of 50 and 80 mg L-1, respectively. Additionally, the values of qe were found to be close to those determined experimentally. So, the results of adsorption processes taking place on our MAC materials for these two concentrations can be interpreted more reasonably on the basis of the pseudo-second order. The pseudo-second order model can be interpreted as Langmuir adsorption kinetics as it takes into consideration the same assumptions as those employed in Langmuir isotherm model.54


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Besides this though, the pseudo-second order model considers the adsorption mechanism to occur via chemisorption.53-54 On the other hand, for initial concentration of 10 mg L-1 the Elovich model presented the best fit, with R2 of the 0.9718, as can be seen in Table 3. This model also considers that the mechanism of adsorption occurs via chemisorption, but on non-homogeneous surfaces.53 The parameters α and β for Elovich model are related to initial adsorption rate and rate of desorption, respectively. As can be seen in Table 3, the adsorption kinetics of SY on MAC1 occurred at higher initial rate, as verified by the lower values of α and ho, and the short time the process took to reach of equilibrium (~75 min). Additionally, the values of β were obtained to be between 1.97 and 0.53, and they decreased as the initial concentrations increased; this fact, proves the presence of effective interaction between MAC1 and SY. Adsorption Isotherm. Besides allowing the determination of the adsorption capacity of a given material, analysis of adsorption isotherms can provide insights into how the adsorbate molecules are distributed at the solid/liquid interfaces, when equilibrium is established.55 The adsorption isotherm obtained for SY over MAC1 and the non-linear fit of the theoretical models of Langmuir, Freundlich and Dubinin-Radushkevich are shown in Figure 8a. The Langmuir model considers that the adsorption of adsorbate molecules occurs in monolayers on homogeneous surfaces that have energetically equivalent sites available for interaction with the adsorbate, where the trans-migration of the adsorbate molecules on the adsorbent does not take place after monolayer formation.5,12 As shown in Table 4, the Langmuir model gives a better fit to experimental data (with R2 value of 0.9824) than either the Freundlich model or the Dubinin-Radushkevich. Additionally, the dependence of separation factor (RL) with


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the range of concentration investigated (5-100 mg L-1) is presented in Figure 8b. This nondimensional parameter provides important information related to the nature of adsorption. Accordingly, the values of RL ranging between 0.93-0.41 obtained (Figure 8b) indicate that the adsorption of SY on MAC1 is favorable. 14

1.0 (a)

(b)

12

0.9

10

0.8 8

RL

-1

qe (mg g )

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


6 Experimental data Freundlich Langmuir Dubinin-Radushkevich

4 2

0.7 0.6 0.5

0

0.4 0

10

20

30

40 50 60 -1 Ce (mg L )

70

80

90

0

10

20

30

40 50 60 -1 Co (mg L )

70

80

90 100

Figure 8. Adsorption models and adsorption isotherms of SY onto MAC1 (a) and separation factor (RL) obtained from Langmuir model (b). The empirical theory of Freundlich describes satisfactorily the adsorption processes that occur as multilayers on heterogeneous surfaces.2,8 Figure 8a shows the non-linear fit of Freundlich model to experimental data, and Table 3 contains the parameters obtained from the non-linear fit. The Freundlich model gave a good fit, with R2 of 0.9807, despite it is lower than that the one obtained with the Langmuir model. By analyzing nF and the ratio 1/nF, information about the adsorption intensity and heterogeneity of surface could also be obtained.56 The values of nF and 1/nF we obtained for the MAC materials we have synthesized were 1.606 and 0.023, respectively. The values, in turn, suggest that the adsorption processes occurring on the MACs


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can be influenced by physisorption and the MAC’s surfaces present a low degree of heterogeneity, respectively. Table 4. Isothermal parameters of the Langmuir, Freundlich and DubininRadushkevich calculated from the non-linear fits. Langmuir

Freundlich

Dubinin-Radushkevich

Qm = 22.31 mg g-1

kF = 0.7835 mg g-1

Qm = 15.87 mg g-1

Ka = 0.0142 L mg-1

nF = 1.606

KDR = 0.00514 E = 9.86 kJ

R2 = 0.9824

R2 = 0.9807

R2 = 0.9363

The Dubinin-Radushkevich model allows estimation of free energy of adsorption on energetically non-uniform surfaces. By using the value of free energy, whether the adsorption process occurs via chemisorption or physisorption could be determined.57-58 According the Figure 9a and Table 4, the Dubinin-Radushkevich model gave R2 values less than those observed based on the Langmuir and Freundlich models. Additionally, the value of free energy (E) shows that the adsorption process occurs with a significant contribution from chemisorption. Moreover, the value indicates that MAC1 possesses surfaces with low degree of heterogeneity, in accordance with the value of 1/nF calculated from the Freundlich model. Our adsorption equilibrium studies indicated that the Langmuir model is more adequate to describe the process, presenting a maximum adsorption capacity of 22.30 mg g-1. Despite different adsorbates have been employed in many different studies and simple comparison may thus not be precise, we can still say that the adsorption capacity of our MAC1 are either comparable or higher than many other related materials. For example, Zhang et al.2 reported that the MAC material synthesized from biochar with a one-step synthetic method gave an adsorption


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capacity of ca. 3,000 mg kg-1 for arsenic. In another work, Zhu et al.3 showed that the adsorption of tetracycline onto magnetic porous magnetic carbon synthesized by simultaneous activation and magnetization had adsorption capacity of 25.44 mg g-1. In a third example, the maximum adsorption of methylene blue onto magnetic graphene-carbon nanotube composite materials synthesized by Wang et al. was 40 mg g-1.5 In the work of Ghaedi et al.59 copper oxide/activated carbon composite gave an adsorption capacity of 10.55 mg g-1 for methylene blue dye. In the work of Bonetto et al.60 for magnetic composite materials, an adsorption capacity of 20.04 mg g-1 for methyl violet 2B dye was reported. Finally, in the work of Ghaedi et al.61 the adsorption capacity of activated carbon prepared from wood of orange tree for SY was reported to be only 2.27 mg g-1. It is also worth adding that the MAC material we have synthesized could easily be removed from solutions, along with the SY that material it adsorbed, by applying a simple bar magnet around the solution, as shown in Figure 9.

(A)

(B)

Figure 9. Digital images of solutions containing MAC1 and SY before (A) and after (B) a bar magnet is placed around the solutions. In Figure S6 and Table S4, the thermodynamics parameters for adsorption of SY onto MAC material are compiled. The negative values of the change in Gibbs free energy (∆Go) confirm the feasibility and spontaneous nature of the adsorption process. The positive value of


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enthalpy change (∆Ho) suggests the endothermic nature of adsorption process, which favors the adsorption of SY at higher temperature. Additionally, the positive value of entropy change (∆So), which favors the stability of the adsorption, suggests the higher randomness produced in the system as a result of the adsorption process. Adsorption Mechanism. Determining the adsorption mechanisms of adsorbates on adsorbents can be a challenging task because various factors often play important roles simultaneously toward the adsorbent-adsorbate interactions. The interactions and how quickly they reach equilibrium could also depend on the textural properties, surface chemistry, functional groups on the adsorbent, molecular structure of adsorbates, etc. Specifically, the interactions between a carbonaceous material and adsorbate molecules can occur through π-π stacking, hydrogen bond interaction, formation of complex, and acid-base and hydrophobic interactions.6265

Based on the molecular structure of SY, shown in Figure S1, and the MACs’ functional groups

we determined by FT-IR spectra, we proposed a mechanism for the adsorption of SY on our MAC materials and the interactions involved using the illustration depicted in Scheme 2.


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Scheme 2. Proposed mechanism for SY adsorption onto MAC1.

Different kinetic and equilibrium models have been fitted to the experimental data, and the models have been supported by theory to explain the observed results. The fact that the pseudo-first order kinetic model is the best fit, along with the free energy values obtained from the Dubinin-Radushkevich model, suggests that the chemisorption plays significant role on the adsorption of SY on the MACs;66-67 however, it is not the only model that can explain the outcomes. As described above, the pH of the solution in which the adsorption was carried out was 5.88, where the MAC’s surface is negatively charged. Thus, the main mechanism of interaction between the material and the SY molecules (which are also negatively charged under this condition) could be mainly through π-π stacking and H-bonding interactions, where the principal interaction would be between π-electron system on MAC surface and the aromatic ring of the SY structure. This conclusion is made on the basis of reported by other research groups before.51,62 Additionally, the MAC’s micro- and mesopores would be expected to help the material with its adsorption properties, where these small pores provide higher capillary effect


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and promote better migration of the adsorbate molecule throughout the MAC’s porous structure.64

CONCLUSIONS Magnetic activated carbons (MACs) were synthesized in a simultaneous process using carbonized coconut shell as precursor in the presence of different amounts of FeCl3. The resulting materials were characterized by various analytical techniques. The synthesis involved FeCl3 impregnation and activation simultaneously and led to high surface area carbons with the magnetically active iron oxide species (magnetite, maghemite and superparamagnetic hematite nanoparticles). Due to their high surface area and magnetic properties, the MAC materials exhibited good batch adsorption capacity for Sunset Yellow (SY) food dye and enabled the removal of the organic dye from solutions by applying a bar magnet. At low concentration of SY the Elovich model gave the best fit to experimental data of the adsorption studies, whereas at higher initial concentration of SY the pseudo-second order model showed the best fit to the experimental data. The equilibrium study was fitted best with the Langmuir isotherm model, and based on the results it was possible to conclude that the MACs possess surfaces with high degree of homogeneity and surfaces to significantly impart adsorption and chemisorption processes. We thus believe that the MAC we have synthesized can serve as excellent, easily recoverable adsorbents for removal of SY as well as many other types of organic pollutants and dyes from different solutions.


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ASSOCIATED CONTENT Supporting Information Information as mentioned in text. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Authors *(T.A.) Tel: +1-848-445-2970; Fax: +1-732-445-5312; e-mail: [email protected] *(V.C.A) Tel: +55 44 3011-4500; Fax: +55 44 3011-4449; e-mail address: [email protected]

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS The authors acknowledge the financial support provided by CAPES, CNPq, Fundação Araucária in Brazil, Center for Global Advancement and International Affairs (GAIA) program at Rutgers University, and NSF (NSF NanoEHS-1134289) in USA.


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Table of Contents (ToC) Graphic and Synopsis Magnetic Activated Carbon Derived from Biomass Waste by Concurrent Synthesis: Efficient Adsorbent for Toxic Dyes André L. Cazetta, Osvaldo Pezoti, Karen C. Bedin, Taís L. Silva, Andrea Paesano Junior, Tewodros Asefa, and Vitor C. Almeida

Synopsis: Easily recoverable and recyclable magnetic activated carbons (MACs) are successfully synthesized from the shells (waste byproducts) of coconut (Cocos nucifera).

Fe(III)

700 °C, N2

Carbonized Precursor

Fe(III) Impregnated

MACs (MAC1, MAC2, MAC3)


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Magnetic Activated Carbon Derived from Biomass Waste by

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