J Mater Sci (2016) 51:7016–7028
Synthesis of mesoporous carbon nanospheres for highly efficient adsorption of bulky dye molecules Aibing Chen1,*, Yuetong Li1, Yifeng Yu1, Yunqian Li1, Kechan Xia1, Yuying Wang1, and Shuhui Li1 1
College of Chemical and Pharmaceutical Engineering, Hebei University of Science and Technology, Shijiazhuang 050018, China
Received: 5 January 2016
ABSTRACT
Accepted: 15 April 2016
A facile synthesis route to the mesoporous carbon nanospheres (MCNs) with tunable size has been prepared via silica cooperative self-assembly using resorcinol as a polymer precursor, tetraethyl orthosilicate as an inorganic precursor, and hexadecyl trimethyl ammonium bromide and triblock copolymer Pluronic F127 as a co-template. The sizes of carbon nanospheres are uniform and easily controlled in the range of 80–170 nm by varying the content of F127. The MCNs exhibit promising properties for adsorption of bulky dye molecules due to their high specific surface area (1481 m2 g-1), large pore volume (2.55 cm3 g-1), and dual mesoporous texture (4.1 and 24.1 nm). Kinetic and isothermal analysis demonstrates the strong interactions between dye molecules and the MCNs. Furthermore, the regenerated MCNs show quite stable adsorption performance, which can be reused for dye removal. The results demonstrated in this work should give a useful enlightenment for the design of adsorbent to remove organic pollutant in wastewater.
Published online: 25 April 2016
Ó
Springer Science+Business
Media New York 2016
Introduction Dyes are considered as one of the most important group of water contaminants and have caused increasing environmental problems [1]. Various treatments for dye removal including adsorption, oxidation processes, nanofiltration, ozonation, and coagulation have been reported [2–6]. Because of the low cost and flexibility in design and operation, the adsorption process is recognized as the effective and versatile technique that has been successfully employed for dye removal from wastewater [7]. The adsorption is a surface phenomenon in which the
adsorbates are attracted to the surface of solid adsorbent and form attachments via physical or chemical bonds without any external force [8, 9]. Numerous materials have been used for absorbing dyes, which either do not have sufficient adsorption capacity or need relatively long adsorption contact time [10–12]. With the tunable and appropriate pore size, mesoporous carbon materials are desirable for the adsorption of bulky molecules [13]. Recently, mesoporous carbon nanospheres (MCNs) have attracted great attention for its high surface areas, open pore structure, large pore size, and excellent adsorption capacity [14]. The considerable adsorption capacity
Address correspondence to E-mail:
[email protected] DOI 10.1007/s10853-016-9991-7
7017
J Mater Sci (2016) 51:7016–7028
and short contact time are superior to the previous materials and other morphological mesoporous carbons [15, 16]. Therefore, many efforts have been made to fabricate MCNs using different methods, including Sto¨ber-based method, hard templating, and organic–organic self-assembly strategy [17–19]. Zhao and coworkers synthesized MCNs with adjustable diameters from co-assembly of resol and F127 by a low-concentration hydrothermal route, which had a low yield and difficult procedures for isolating the product [20]. Liu et al. [21] reported a simple method to prepare MCNs using hexamine instead of formaldehyde during the synthesis, whereas the uneven particle sizes ranged from hundreds of nanometers to a few micrometers. Soon afterwards, Dai et al. developed a general, effective approach for high-yield MCNs and controlled particle size with a wide range of 180–850 nm via ‘‘silicaassisted’’ strategy, while exhibiting relatively low pore volumes (\1 cm3 g-1) that devalued their practical application [22]. Moreover, MCNs attract sustained research interests especially when the particle size is smaller than 200 nm which can provide short pathways for mass transport and minimize the viscous effects [23]. Thus, it is significant to develop an effective and simple route to synthesize mesoporous carbon nanospheres with a small size, high specific surface area, and larger pore volume. In the present work, we report a facile synthesis route to MCNs with regular spherical morphology and uniform tunable size, especially with macropore on the surface. The process uses surfactant of hexadecyl trimethyl ammonium bromide (CTAB) and triblock copolymer Pluronic F127 as a co-template to control the mesoporous structure, ammonia as a catalyst to catalyze the polymerization of resorcinol (R) and formaldehyde (HCHO) that comes from the hydrolysis of hexamethylenetetramine (HMT), and tetraethyl orthosilicate (TEOS) as an agent to increase specific surface area and macropore. Etching of the silica is followed by carbonization in the carbon/silica nanocomposite, resulting in the formation of MCNs. The mesostructure of MCNs is retained, while the spherical sizes are tuned from 80 to 170 nm by varying the content of F127. Additional macropores are produced by removing the silica aggregates. Three dyes, methyl orange (MO), fuchsin basic (FB), and methylthionine chloride (MC), are employed as model dye molecules. The MCNs with high specific surface area and larger pore volume exhibit
properties that are promising for use in dye adsorption. Furthermore, the adsorption capacity and behavior are discussed in terms of surface area, pore volume, and structural matching between adsorbent and adsorbate in detail. In addition, isotherm equilibrium model and adsorption kinetics model are also used to determine the adsorption process.
Experimental section Reagents Poly(propylene oxide)-block-poly(ethylene oxide)block-poly(propylene oxide) triblock copolymer Pluronic F127 (Mw = 12600, PEO106PPO70PEO106) and hexadecyl trimethyl ammonium bromide were purchased from Sigma-Aldrich Corp. Resorcinol, hexamethylenetetramine, ammonium hydroxide solution (28 wt%), tetraethyl orthosilicate, ethanol, methyl orange, fuchsin basic, and methylthionine chloride were purchased from Tianjin Yongda Chemical Corp. All chemicals were used as received without any further purification. Deionized water was used in all experiments.
Synthesis of mesoporous carbon nanospheres (MCNs) In a typical synthesis, CTAB (0.5 g) and F127 (2.05 g) were dissolved in ammonia aqueous solution (NH4OH, 11 mL) mixed with ethanol (EtOH, 43 mL) and deionized water (H2O, 96 mL), then stirred at room temperature. After complete dissolution, R (0.25 g) and HMT (0.159 g) were added and continually stirred for 30 min. Subsequently, TEOS (1.93 mL) was added into the reaction solution at once. After 10 min of mechanical stirring at 1000 rpm, the mixture was kept at static condition for 24 h at room temperature for primary resorcinol polymerization and further silica condensation. The resultant homogenous solution was poured into a Teflon-lined autoclave and then transferred into an oven at 100 °C for 24 h. The solid product was obtained by ultrahigh-speed centrifuge, washed with water, and air-dried at 80 °C overnight. Carbonization was carried out in a tubular furnace at 600 °C for 3 h under the condition of N2 flow with the temperature ramp of 1 °C min-1. The mesoporous carbon nanospheres were recovered after dissolution of the silica component in silica–
7018 carbon composite by 10 wt% HF solution at room temperature, filtration, washing with water, and drying at 100 °C. In the synthesis of other samples, different dosages of resorcinol or F127 were used. The obtained carbon materials were designated as MCNs-x–y, where x represents the mass of F127 (i.e., 0, 2.05, 3.37, or 4.75) and y represented the mass of resorcinol (i.e., 0.15, 0.25, 0.325, or 0.4).
Characterization Scanning electron microscopy (SEM) was performed on a HITACHI S-4800-I scanning electron microscope. High-resolution transmission electron micrographs (HR-TEM) were obtained on a JEOL JEM-2010 electron microscope. Samples for TEM studies were prepared by placing a drop of the suspension of sample in ethanol onto a carbon-coated copper grid, followed by evaporating the solvent. N2-sorption experiments were performed on a Micromeritics TriStar 3020 volumetric adsorption analyzer at -196 °C. The samples were degassed at 120 °C overnight before the measurement. The Brunauer– Emmett–Teller (BET) method was utilized to calculate the specific surface area of each sample and the average pore size distribution was derived from the adsorption branch of the corresponding isotherm through the Barrett–Joyner–Halenda (BJH) method. The total pore volume was estimated from the N2 amount adsorbed at a relative pressure of P/ P0 = 0.99. The concentrations of dyes were measured by Shimadzu Ultraviolet–Visible spectrophotometer (UV-1700). Fourier transform infrared (FTIR) spectra were collected on a Perkin Elmer Spectrum GX I using KBr pellets. Zeta potential was measured at 25 °C using a Zeta Meter (Zetasizer Nano Z) equipped with a microprocessor unit.
J Mater Sci (2016) 51:7016–7028
continuously shaken in a shaking bath with a speed of 120 rpm at 30 °C until the equilibrium was reached (typically 24 h). After adsorption, the suspension was separated using a 0.45-lm Uniflo filter and the filtrate was analyzed with a UV–Vis spectrophotometer. The wavenumber for MO, FB, and MC is 465, 543, and 665 nm, respectively. The molecular structure and characteristic properties of the three dyes are shown in Fig. S1. The equilibrium adsorption capacity Qe (mg g-1) was calculated according to the following formula: Qe ¼
ðC0 Ce ÞV ; m
ð1Þ
where C0 (mg L-1) is the initial concentration of dye solution, Ce (mg L-1) is the equilibrium concentration of dye, V (L) is the volume of the solution, and m (g) is the weight of the adsorbent. To study the effect of contact time on adsorption, we continuously oscillated the mixture containing 0.02 g of MCNs in 50 ml of 400 mg L-1 dye solution for different times and analyzed it as Qt ¼
ðC0 Ct ÞV ; m
ð2Þ
where Qt (mg g-1) is the adsorption amount at time t (min), C0 (mg L-1) is the initial concentration of dye solution, Ct (mg L-1) is the concentration of dye solution at time t, V (L) is the volume of the solution, and m (g) is the mass of the adsorbent.
Recycling test for dye adsorption The dye-loaded MCNs were dispersed in ethanol for 12 h and then washed with ethanol and distilled water for several times. After that, the materials were dried in a vacuum oven at 100 °C.
Batch mode adsorption and analysis
Results and discussion
The batch mode adsorption studies for three dyes, including methyl orange (MO), fuchsin basic (FB), and methylthionine chloride (MC), were carried out with 0.02 g of MCNs in 50 ml of dye solution. All dyes were dissolved in distilled water with no pH adjustment of dye solutions treated by adsorption onto MCNs in this study. The pH of MO, FB, and MC in this experimental condition is 6.1, 5.9, and 5.8, respectively. The concentration of various dyes ranged from 50 to 600 mg L-1. The mixture was
The possible formation of the MCNs is proposed as shown in Fig. 1. Firstly, F127 and CTAB were added into the mixture solution of H2O, NH4OH, and EtOH. The F127 covered the surface of CTAB micelles to suppress the micelle growth [24–26]. Subsequently, the resorcinol coupled with the CTAB micelles via electrostatic interactions to form spherical composite micelles when resorcinol and HMT were added [25]. After the addition of TEOS, the silicate oligomers and the resorcinol interacted with CTAB through
J Mater Sci (2016) 51:7016–7028
7019
Figure 1 Schematic illustration of the formation of MCNs.
electrostatic interactions to co-assemble the mesostructures that bounded to hydrophilic PEO segments of F127 [27]. During subsequent hydrothermal treatment, plenty of HCHO released via the hydrolysis of HMT. The formaldehyde molecules could condense with the resorcinol molecules to form resorcinol–formaldehyde (RF) resin which accelerated to rearrange and crosslink into interpenetrating 3D rigid frameworks [28, 29]. After carbonization and removal of silica, the MCNs with dual mesoporous texture could be achieved. The resorcinol amount is a significant parameter in the formation of MCNs. However, there are no obvious differences in the size of these MCNs (Fig. 2). When the mass of resorcinol is changed to 0.15, 0.25, 0.325, and 0.4 g, the average particle sizes are 160, 175, 180, and 170 nm for MCNs-2.05-0.15, MCNs2.05-0.25, MCNs-2.05-0.325, and MCNs-2.05-0.4, respectively. It is found that all MCN samples with macropores are monodisperse and uniform. The irregular macropore appears on the surface, which may result from the formation of small silica aggregates during the hydrolyzation process of TEOS. The morphology of the final product of MCNs can be tuned by adding F127 in our case. As is shown in Fig. 3a, in the absence of F127, MCNs-0-0.25 has irregular spherical morphology and some blocks could be found. Increasing the initial F127 mass to 2.05, 3.37, and 4.75 g can systematically decrease the
particle size to 175, 125, and 85 nm, respectively (Fig. 3b–d). Uniform spheres are achieved with continuing to raise the mass of F127, suggesting that F127 has a positive effect on the formation of spherical nanoparticles and affects the nucleation and growth between silicate oligomers and RF resin cluster [30, 31]. As shown above, it is believed that F127 plays a very significant role in forming uniform spheres and controlling the size. Further structural characterization of MCNs with different mass of F127 was performed by TEM. Figure 4a shows uniform isolated spheres with diameters of approximately 170 nm which is close to the SEM measured size of MCNs-2.05-0.25. Meanwhile, sporadic macropore can be found in Fig. 4b. With the increase of F127, corresponding particle sizes decrease from 120 to 80 nm as shown in Fig. 4c, d. The carbon nanospheres tend to congregate together when the particle size decreases to 80 nm which is in agreement with SEM (Fig. 3d). Besides, the sample with no CTAB is also prepared. Unfortunately, there is no product formed after hydrothermal process, indicating that CTAB is necessary in this system. Because both RF resin and silicate oligomer negatively charged in the presence of a base catalyst can interact with the positively charged CTAB templates by electrostatic interactions to form RF polymer, silica interpenetrated polymer/silica/surfactant composites [28].
7020
J Mater Sci (2016) 51:7016–7028
Figure 2 SEM images of a MCNs-2.05-0.15, b MCNs2.05-0.25, c MCNs-2.050.325, and d MCNs-2.05-0.4.
Figure 3 SEM images of a MCNs-0-0.25, b MCNs2.05-0.25, c MCNs-3.37-0.25, and d MCNs-4.75-0.25.
The specific surface area and pore volume are the main influencing factors of evaluation criterion on adsorption capacity. Thus, the N2 adsorption–desorption is used to measure the porosity of the obtained samples. As shown in Fig. 5a, b, N2 adsorption–desorption isotherms of MCNs-2.05-0.25 exhibit representative type IV curves with obvious capillary condensation steps at P/P0 [ 0.4, suggesting a uniform mesopore [32]. Furthermore, at a
higher relative pressure (P/P0 [ 0.9) a hysteresis loop appears, which may reflect the interparticle texture between the carbon nanospheres [20]. Similar to this sample, the N2 adsorption–desorption isotherms of other as-synthesized MCNs with different mass of resorcinol follow a similar behavior. Among all these samples, MCNs-2.05-0.25 possesses the highest specific surface area of 1481 m2 g-1 as well as the largest total pore volume of 2.55 cm3 g-1. The
J Mater Sci (2016) 51:7016–7028
7021
Figure 4 TEM images of a and b MCNs-2.05-0.25 at different magnifications, c MCNs-3.37-0.25, and d MCNs-4.75-0.25.
Figure 5 N2 adsorption– desorption isotherms (A) and pore size distribution curves (B) of a MCNs-2.05-0.15, b MCNs-2.05-0.25, c MCNs2.05-0.325, and d MCNs-2.050.4.
specific surface areas of other carbon spheres are in the range of 1216–1049 m2 g-1 as the resorcinol mass increased (Table 1), which are higher than those of the mesoporous carbon spheres reported in literature (427 m2 g-1) [33]. The pore size of these samples is about 4.1–4.3 nm which is calculated based on the BJH model, and the pore volumes are calculated to be as large as 0.75–2.55 cm3 g-1, which are also much
larger than those of mesoporous carbon nanospheres as has been reported in literature (0.19–0.57 cm3 g-1) [31]. These samples with different mass of F127 are also tested. The corresponding N2 adsorption–desorption isotherms and pore size distributions are shown in Fig. 6. The clear hysteresis loop in P/P0 [ 0.4 is also observed (Fig. 6a), suggesting that the samples with
7022 Table 1 Structural and textual properties of MCNs with different resorcinol mass
J Mater Sci (2016) 51:7016–7028
Sample
BET surface area (m2 g-1)
Pore volume (cm3 g-1)
Pore size (nm)
MCNs-2.05-0.15 MCNs-2.05-0.25 MCNs-2.05-0.325 MCNs-2.05-0.4
1070 1481 1216 1049
1.95 2.55 2.23 1.89
4.1 4.1/24.1 4.1 4.3
Figure 6 N2 adsorption– desorption isotherms (A) and pore size distribution curves (B) of a MCNs-0-0.25, b MCNs-2.05-0.25, c MCNs3.37-0.25, and d MCNs-4.750.25.
different mass of F127 have the similar mesoporous structure as above. With increasing the amount of F127, the specific surface area of the as-synthesized samples with the values of 818 m2 g-1 (MCNs-00.25), 1481 m2 g-1 (MCNs-2.05-0.25), 1433 m2 g-1 (MCNs-3.37-0.25), and 1212 m2 g-1 (MCNs-4.750.25). In addition, pore volume is also changed to 0.79, 2.55, 1.50, and 1.03 cm3 g-1. Except for MCNs-00.25, all samples show dual mesoporous structure with identical small mesopores with 4.1 nm size (Fig. 6b). The size of large mesopores diminishes from 24.1 to 9.5 nm with the amount of F127 increasing from 0 to 0.4 g. These samples with high surface area and pore volume are listed in Table 2. The FTIR spectrum of MCNs-2.05-0.25 which has the maximum specific surface area and pore volume is shown in Fig. S2. The broad band at 3420 cm-1 is observed and it is attributed to the O–H stretching vibration of the adsorbed water molecules. The absorbance band at around 1560 cm-1 corresponds to the C–H stretching vibrations and the stretching skeletal vibrations of benzene rings [13]. The bands at 1130 and 1080 cm-1 are assigned to the phenolic –OH groups. The zeta potential of MCNs-2.05-0.25 measured in a wide range of pH is shown in Fig. S3. The MCNs-2.05-0.25 exhibits the point of zero charge at a pH of about 4. To investigate the adsorption capacity of dyes, the MO, FB, and MC are chosen as model pollutants for
the adsorption experiment. Figure 7a, b depicts the adsorption capacity versus the adsorption time (concentration: 400 mg L-1) and various dye concentrations on these dyes over the representative sample MCNs-2.05-0.25. A predominant increase in the initial stage is found, which demonstrates the fast adsorption rate (Fig. 7a). During the first 5 min, the removal efficiency of MO, FB, and MC increased to 90, 91, and 88 % of the total amount, respectively. Saturated adsorption is achieved after approximately 30 min. With a long contact time (24 h), the adsorption amount is on the same order as above for the MCNs-2.05-0.25. This phenomenon gives further evidence on the high affinity between the adsorbate and the adsorbent. Figure 7b shows the equilibrium adsorption curves of MCNs-2.05-0.25 for dyes at 30 °C. The amount of adsorption dramatically increases at a lower final solution concentration, suggesting a high affinity between the dye molecule and the carbonaceous adsorbent surface, and then they diffuse into the porous structure of the adsorbent [34, 35]. It also can be seen that MCNs-2.05-0.25 exhibits very high adsorption capacities for these dyes. For all dyes, the adsorption amounts are higher than 99.9 % when their concentration is 50 mg L-1, suggesting a complete removal (Fig. 7 optical photos). Even if the initial concentration is 100 mg L-1 of MC, MCNs-2.050.25 could also reveal highly efficient adsorption
7023
J Mater Sci (2016) 51:7016–7028
Table 2 Structural and textual properties of MCNs with different F127 mass
Table 3 Comparison of adsorption capacities for different dyes on carbon materials from the literature
Sample
BET surface area (m2 g-1)
Pore volume (cm3 g-1)
Pore size (nm)
MCNs-0-0.25 MCNs-2.05-0.25 MCNs-3.37-0.25 MCNs-4.75-0.25
818 1481 1433 1212
0.79 2.55 1.50 1.03
4.1 4.1/24.1 4.1/16.8 4.1/9.5
Material
Mesoporous carbon nanofibers FDU-15 Activated carbon CMK-3 MCNs-2.05-0.25
capacity which may pave the way for the practical use. The maximum adsorption capacity of MO, FB, and MC is 329, 489, and 791 mg g-1, respectively. For each individual target dye, the value of MCNs-2.050.25 is higher than or at least comparable with those of the reported porous carbon (Table 3). In addition, MCNs-2.05-0.25 has better adsorption on basic dyes (MC and FB) than acidic/azo dyes (MO) which is consistent with the result that active carbon has poor adsorption onto acidic/azo dyes [36, 40]. Meanwhile, the best adsorption on MC illustrates that the diameter of dye molecule (seen in Fig. S1) also plays an important role in adsorption. It has been reported that dyes with small diameter (such as MC) can easily enter into the mesoporous carbon nanofibers [13], which is similar to MCNs. On the other hand, the pore blockage may occur due to the aggregation of large molecules in the orifice of small mesopore. However, the larger mesopore may weaken the negative effects. The equilibrium adsorption under identical adsorption condition was also tested for other as-synthesized samples and the corresponding adsorption amounts are listed in Table 4. We find that the amounts of the adsorbed dyes positively correlate with specific surface area and pore volume (Fig. S4). These results demonstrate that the adsorption properties for dyes are sensitively dependent not only on specific surface area, pore volume, and mesopore texture, but also on dye nature and molecular size. To better describe the mechanism of dye adsorption onto MCNs, the equilibrium experimental
Adsorption capacities (mg g-1)
References
MO
FB
MC
– – 243 294 329
– 207 362 / 489
567 540 396 420 791
[13] [36] [36, 37] [38, 39] This work
adsorption data of MCNs-2.05-0.25 are fitted by Langmuir and Freundlich isotherm models. Langmuir model assumes the monolayer coverage of adsorbate over a homogenous adsorbent surface. The adsorption isotherm is based on the assumption that there are fixed numbers of active sites on the surface of the adsorbent. Each site is only available for one adsorbate molecule to be occupied. Once an adsorbate molecule occupies a site, no further adsorption can take place at the same site, with no interaction between adsorbed species [41], which can be expressed as Ce Ce 1 ¼ þ ; qe qm KL qm
ð3Þ
where Ce is the equilibrium concentration of dye (mg L-1), qe is the amount of adsorbate adsorbed per unit mass of adsorbent (mg g-1), qm is the theoretical maximum adsorption capacity (mg g-1), and KL is the Langmuir constant (L mg-1). The applicability of the isotherm equation to describe the adsorption process is contingent on the correlation coefficients, R2 values. The linear Langmuir isotherms of Ce/qe versus Ce are shown in Fig. 8a and the parameters of Langmuir KL and qm are given in Table 5. All correlation coefficient R2 values are above 0.99, indicating that the adsorption behavior shows well linear Langmuir isotherm. The essential characteristics of the Langmuir isotherm can be also expressed in terms of the separation factor (RL), a dimensionless factor, which is defined by the following equation:
7024
RL ¼
J Mater Sci (2016) 51:7016–7028
1 ; 1 þ KL C0
ð4Þ
where KL is the Langmuir constant (L mg-1) and C0 is the initial dye concentration (mg L-1). The value of Table 4 Adsorption capacities of MCNs for dye molecules Adsorption capacities (mg g-1)
Sample
MCNs-2.05-0.15 MCNs-2.05-0.25 MCNs-2.05-0.325 MCNs-2.05-0.4 MCNs-0-0.25 MCNs-3.37-0.25 MCNs-4.75-0.25
MO
FB
MC
277 329 299 272 261 308 280
432 489 454 425 406 469 450
722 791 753 716 696 758 740
RL indicates the type of the isotherm to be either unfavorable (RL [ 1), linear (RL = 1), favorable (0 \ RL \ 1), or irreversible (RL = 0). The value of RL is calculated as in Table 4 for different dyes. RL \ 1 further indicates that Langmuir isotherm is favorable for adsorption of all dyes under the conditions used in this study. Freundlich model has been used to characterize adsorption on the heterogeneous surface through a multilayer adsorption mechanism [42], which is shown as 1 ln qe ¼ ln KF þ ln Ce ; n
ð5Þ
where KF and n are Freundlich constants related to the adsorption amount (mg/g) and intensity, respectively. The slope of 1/n ranging between 0 and
Table 5 Langmuir and Freundlich constants for the adsorption of MO, FB, and MC on MCNs-2.05-0.25 Dye
MO FB MC
Langmuir
Freundlich
qm (mg g-1) g-1)
KL (L mg-1)
RL
R2
KF [(mg g-1) (L mg-1)1/n]
1/n
R2
327.9 497.5 793.7
0.1582 0.1107 0.4013
0.0125–0.1122 0.0177–0.1530 0.0041–0.0475
0.997 0.995 0.995
168.1 167.0 422.7
0.1155 0.1958 0.1189
0.943 0.965 0.948
Figure 7 Effect of contact time (a) and various initial concentrations (b) on the adsorption of three dyes over MCNs-2.05-0.25, and the corresponding optical photos of the concentration (mg L-1)dependent dye after adsorption for 24 h (right).
7025
J Mater Sci (2016) 51:7016–7028
Figure 8 Langmuir (a) and Freundlich (b) adsorption isotherms of MCNs-2.05-0.25 for MO, FB, and MC at 30 °C.
Figure 9 Pseudo-first-order (a) and pseudo-second-order (b) kinetic models of MCNs2.05-0.25 for MO, FB, and MC.
1 is a measure of adsorption intensity or surface heterogeneity, becoming more heterogeneous as its value gets closer to zero. A value for 1/n below one indicates a normal Langmuir isotherm, while 1/ n value above one is indicative of cooperative adsorption. For the Freundlich isotherm (Fig. 8b), the plots of log qe against log Ce exhibit linearity with slope of 1/n with a value of 0.1155, 0.1958, and 0.1189, as seen in Table 4, which shows that adsorption of the three dyes is favorable. Besides, Freundlich constants KF and n are listed in Table 5. Compared to Freundlich isotherm, the Langmuir isotherm has higher correlation coefficient. That is to say, in this paper, the adsorption behavior belongs to the monolayer adsorption. Comparing between experimental maximum adsorption quantity and experimental value, we find that the data are more in line with the Langmuir adsorption. That further testifies the Langmuir adsorption of these dyes onto these porous carbon materials. Furthermore, to better understand the relationship between adsorption rate and equilibrium time in the adsorption process, the pseudo-first-order and
pseudo-second-order kinetic models are used to test the experimental data [43]. The pseudo-first-order model can be expressed as lnðqe qt Þ ¼ ln qe K1 t;
ð6Þ
where qe and qt are the adsorption capacities at equilibrium and at time t and k1 is the first-order rate constant. The values of adsorption capacity, rate constant, and correlation coefficient (R2) are calculated from the plot of ln (qe - qt) versus t. The pseudo-second-order model is given as t 1 t ¼ þ ; qt k2 q2e qe
ð7Þ
where k2 is the second-order rate constant. The values of adsorption capacity and rate constant are derived using the intercept and slope of the plot of t/qt versus t. The adsorption kinetic plots are displayed in Fig. 9 and all the kinetic parameters obtained are listed in Table 6. It can be seen that the regression coefficients are close to unity for pseudo-second-order kinetic model, suggesting a chemisorption process [44]. For
7026
J Mater Sci (2016) 51:7016–7028
Table 6 Pseudo-first-order and pseudo-second-order kinetic model constants for the adsorption of MO, FB, and MC onto MCNs-2.050.25 Dye
MO FB MC
C0 (mg L-1)
400 400 400
qe,exp (mg g-1)
310.7 476 768.5
Pseudo-first-order
Pseudo-second-order
qe1,cal (mg g-1)
K1 (g mg-1 min-1)
R2
qe2,cal (mg g-1)
K2 (g mg-1 min-1)
R2
15.0 34.5 15.2
4.5 9 10-3 6.5 9 10-3 3.2 9 10-3
0.261 0.521 -0.005
310.6 476.2 769.2
3.2 9 10-3 1.6 9 10-3 2.2 9 10-3
0.999 0.999 0.999
Figure 10 Recycling of MCNs-2.05-0.25 for adsorption of MC.
the theoretical uptakes from kinetic models, the values of qe2,cal are much higher than those of qe1,cal for all dyes. The qe2,cal also fits the experimental data well, which indicates that the adsorption process onto these dyes can be well described by pseudosecond-order kinetic model. Practically, the MCNs-2.05-0.25 and the MC molecule can be easily and simply regenerated and recovered by washing with ethanol due to the high solubility of the MC molecules in ethanol. After five adsorption and regeneration cycles (Fig. 10), the adsorption capacity could still reach 664 mg g-1 and only about 16 % deterioration with quite stable performance. These demonstrate that the MCNs may become a promising adsorbent in environmental application. Based on the above discussion, we proposed a possible mechanism for the adsorption of dyes onto MCNs. The adsorption of dye molecules from aqueous solution depends not only on the nature of the dye and the surface characteristics of the MCNs, but it is also greatly affected by the mesoporous structure
of MCNs. The dye molecules would cover the surface of MCNs by capillary force. The MCNs that have high specific surface area and pore volume would adsorb more dye molecules. The electrostatic attraction between the surface of carbon material and dye molecule would also influence the adsorption capacity [13]. The surface of MCNs in this experimental condition is negatively charged (zeta potential in Fig. S3) which is beneficial to adsorb cationic dye, such as FB and MC. Subsequently, the dye adsorption would occur in the mesopores of MCNs. The capillary force inside the mesopore facilitates the mass transfer of dyes which enhances the total adsorption capacity. Thus, the dye adsorptions onto MCNs involve interplay between capillary force and electrostatic interaction.
Conclusions In summary, highly efficient adsorption of bulky dye molecules onto MCNs prepared from a facile synthesis route has been demonstrated. The synthesis can be carried out using resorcinol as a polymer precursor, tetraethyl orthosilicate as an inorganic precursor, and hexadecyl trimethyl ammonium bromide and triblock copolymer Pluronic F127 as a cotemplate. The sizes of MCNs are uniform and easily controlled in the range of 80–170 nm by varying the content of F127. The MCNs has dual mesoporous texture (4.1 and 24.1 nm), high specific surface area (1481 m2 g-1), and large pore volume (2.55 cm3 g-1). Highly efficient adsorption of MO, FB, and MC from aqueous solution has been demonstrated on MCNs, which reaches adsorption equilibrium in less than 30 min. A good performance in fading has been shown regardless of the dye nature, including basic, acidic, or azo dyes. The maximum adsorption capacity can reach 329, 489, and 791 mg g-1, respectively. The adsorption data are described well by
7027
J Mater Sci (2016) 51:7016–7028
Langmuir adsorption model and pseudo-secondorder kinetic model for dyes. Moreover, the MCNs can be easily regenerated with stable cyclic performance. Hence, these behaviors may offer good opportunities for MCNs as an adsorbent to remove organic pollutant in wastewater.
[8]
[9]
Acknowledgements This work was financially supported by the Natural Science Foundation of China (20906019), the Science and Technology Research Projects in Hebei Universities (QN20131069, QN2014142, ZD20131032, and Z2013001), and Five Platform Open Fund Projects of Hebei University of Science and Technology (2014PT86).
[10]
[11]
[12]
Electronic supplementary material: The online version of this article (doi:10.1007/s10853-016-99917) contains supplementary material, which is available to authorized users.
[13]
References [1]
[2]
[3]
[4]
[5]
[6]
[7]
Purkait MK, DasGupta S, De S (2005) Adsorption of eosin dye on activated carbon and its surfactant based desorption. J Environ Manag 76:135–142 Zhou L, Gao C, Xu WJ (2010) Magnetic dendritic materials for highly efficient adsorption of dyes and drugs. ACS Appl Mater Interfaces 2:1483–1491 Kusvuran E, Gulnaz O, Irmak S et al (2004) Comparison of several advanced oxidation processes for the decolorization of reactive red 120 azo dye in aqueous solution. J Hazard Mater B 109:85–93 Chakraborty S, Purkait MK, DasGupta S et al (2003) Nanofiltration of textile plant effluent for color removal and reduction in COD. Sep Purif Technol 31:141–151 Punzi M, Nilsson F, Anbalagan A et al (2015) Combined anaerobic-ozonation process for treatment of textile wastewater: removal of acute toxicity and mutagenicity. J Hazard Mater 292:52–60 Liang CZ, Sun SP, Li FY et al (2014) Treatment of highly concentrated wastewater containing multiple synthetic dyes by a combined process of coagulation/flocculation and nanofiltration. J Membr Sci 469:305–315 Wang H, Yuan XZ, Zeng GM et al (2014) Removal of malachite green dye from wastewater by different organic acid-modified natural adsorbent: kinetics, equilibriums,
[14]
[15]
[16]
[17]
[18]
[19]
[20]
mechanisms, practical application, and disposal of dye-loaded adsorbent. Environ Sci Pollut Res 21:11552–11564 Wang H, Yuan XZ, Zeng GM et al (2015) Three dimensional graphene based materials: Synthesis and applications from energy storage and conversion to electrochemical sensor and environmental remediation. Adv Colloid Interface Sci 221:41–59 Wang H, Yuan XZ, Wu Y et al (2013) Graphene-based materials: fabrication, characterization and application for the decontamination of wastewater and wastegas and hydrogen storage/generation. Adv Colloid Interface Sci 195–196:19–40 ¨ (2012) Anionic dye removal from Alver E, Metin AU aqueous solutions using modified zeolite: adsorption kinetics and isotherm studies. Chem Eng J 200–202:59–67 Rehman MSU, Kim I, Han JI (2012) Adsorption of methylene blue dye from aqueous solution by sugar extracted spent rice biomass. Carbohyd Polym 90:1314–1322 Roy A, Chakraborty S, Kundu SP et al (2012) Adsorption of anionic-azo dye from aqueous solution by lignocellulosebiomass jute fiber: equilibrium, kinetics, and thermodynamics study. Ind Eng Chem Res 51:12095–12106 Teng MM, Qiao JL, Li FT et al (2012) Electrospun mesoporous carbon nanofibers produced from phenolic resin and their use in the adsorption of large dye molecules. Carbon 50:2877–2886 Chang BB, Guan DX, Tian YL et al (2013) Convenient synthesis of porous carbon nanospheres with tunable pore structure and excellent adsorption capacity. J Hazard Mater 262:256–264 Gong JM, Liu T, Wang XQ et al (2011) Efficient removal of heavy metal ions from aqueous systems with the assembly of anisotropic layered double hydroxide nanocrystals @ carbon nanosphere. Environ Sci Technol 45:6181–6187 Ma J, Yu F, Zhou L et al (2012) Enhanced adsorptive removal of methyl orange and methylene blue from aqueous solution by alkali-activated multiwalled carbon nanotubes. ACS Appl Mater Interfaces 4:5749–5760 Choma J, Jamiola D, Augustynek K et al (2012) New opportunities in Sto¨ber synthesis: preparation of microporous and mesoporous carbon spheres. J Mater Chem 22:12636–12642 Wang S, Li WC, Hao GP et al (2011) Temperature-programmed precise control over the sizes of carbon nanospheres based on benzoxazine chemistry. J Am Chem Soc 133:15304–15307 Liu J, Yang TY, Wang DW et al (2013) A facile soft-template synthesis of mesoporous polymeric and carbonaceous nanospheres. Nat Commun 4:2798–2804 Fang Y, Gu D, Zou Y et al (2010) A low-concentration hydrothermal synthesis of biocompatible ordered
7028
[21]
[22]
[23]
[24]
[25]
[26]
[27]
[28]
[29]
[30]
[31]
[32]
mesoporous carbon nanospheres with tunable and uniform size. Angew Chem Int Ed 49:7987–7991 Liu D, Lei JH, Guo LP et al (2011) Simple hydrothermal synthesis of ordered mesoporous carbons from resorcinol and hexamine. Carbon 49:2113–2119 Qiao ZA, Guo BK, Binder AJ et al (2013) Controlled synthesis of mesoporous carbon nanostructures via a ‘‘silicaassisted’’ strategy. Nano Lett 13:207–212 Tang J, Liu J, Li CL et al (2015) Synthesis of nitrogen-doped mesoporous carbon spheres with extra-large pores through assembly of diblock copolymer micelles. Angew Chem Int Ed 54:588–593 Suzuki K, Ikari K, Imai H (2004) Synthesis of silica nanoparticles having a well-ordered mesostructure using a double surfactant system. J Am Chem Soc 126:462–463 Zhang XH, Lia YN, Cao CB (2012) Facile one-pot synthesis of mesoporous hierarchically structured silica/carbon nanomaterials. J Mater Chem 22:13918–13921 Chen JC, Xue ZT, Feng SS et al (2014) Synthesis of mesoporous silica hollow nanospheres with multiple gold cores and catalytic activity. J Colloid Interface Sci 429:62–67 Liu D, Lei JH, Guo LP et al (2014) One-pot aqueous route to synthesize highly ordered cubic and hexagonal mesoporous carbons from resorcinol and hexamine. Carbon 50:476–487 Liu J, Qiao SZ, Liu H et al (2011) Extension of the Sto¨ber method to the preparation of monodisperse resorcinolformaldehyde resin polymer and carbon spheres. Angew Chem Int Ed 50:5947–5951 Wang J, Liu HY, Gu XM et al (2014) Synthesis of nitrogencontaining ordered mesoporous carbon as a metal-free catalyst for selective oxidation of ethylbenzene. Chem Commun 50:9182–9184 Sun XM, Li YD (2004) Colloidal carbon spheres and their core/shell structures with noble-metal nanoparticles. Angew Chem Int Ed 43:597–601 Wang J, Liu HY, Diao JY et al (2015) Size-controlled nitrogen-containing mesoporous carbon nanospheres by onestep aqueous self-assembly strategy. J Mater Chem A 3:2305–2313 Xue CF, Tu B, Zhao DY (2009) Facile fabrication of hierarchically porous carbonaceous monoliths with ordered
J Mater Sci (2016) 51:7016–7028
[33]
[34]
[35]
[36]
[37]
[38]
[39]
[40]
[41]
[42]
[43] [44]
mesostructure via an organic–organic self-assembly. Nano Res 2:242–253 Yan Y, Zhang FQ, Meng Y et al (2007) One-step synthesis of ordered mesoporous carbonaceous spheres by an aerosolassisted self-assembly. Chem Commun 27:2867–2869 Vinu A, Hossain KZ, Kumar GS et al (2006) Adsorption of L-histidine over mesoporous carbon molecular sieves. Carbon 44:530–536 Tan IAW, Ahmad AL, Hameed BH (2008) Adsorption of basic dye on high-surface-area activated carbon prepared from coconut husk: equilibrium, kinetic and thermodynamic studies. J Hazard Mater 154:337–346 Zhuang X, Wan Y, Feng CM et al (2009) Highly efficient adsorption of bulky dye molecules in wastewater on ordered mesoporous carbons. Chem Mater 21:706–716 Do MH, Phan NH, Nguyen TD et al (2011) Activated carbon/Fe3O4 nanoparticle composite: fabrication, methyl orange removal and regeneration by hydrogen peroxide. Chemosphere 85:1269–1276 Mohammadi N, Khani H, Gupta VK et al (2011) Adsorption process of methyl orange dye onto mesoporous carbon material-kinetic and thermodynamic studies. J Colloid Interface Sci 362:457–462 Dong Y, Lin HM, Jin QM et al (2013) Synthesis of mesoporous carbon fibers with a high adsorption capacity for bulky dye molecules. J Mater Chem A 1:7391–7398 Asouhidou DD, Triantafyllidis KS, Lazaridis NK et al (2009) Sorption of reactive dyes from aqueous solutions by ordered hexagonal and disordered mesoporous carbons. Micro Meso Mater 117:257–267 Jia YF, Thomas KM (2000) Adsorption of cadmium ions on oxygen surface sites in activated carbon. Langmuir 16:1114–1122 Barka N, Assabbane A, Nounah A et al (2009) Removal of textile dyes from aqueous solutions by natural phosphate as a new adsorbent. Desalination 235:264–275 Ho YS, Ng JCY, McKay G (2000) Kinetics of pollutant sorption by biosorbents: review. Sep Purif Rev 29:189–232 Shi S, Fan YW, Huang YM (2013) Facile low temperature hydrothermal synthesis of magnetic mesoporous carbon nanocomposite for adsorption removal of ciprofloxacin antibiotics. Ind Eng Chem Res 52:2604–2612
