Fabrication and application of hierarchical porous carbon for the adsorption of bulky dyes

Microporous and Mesoporous Materials 290 (2019) 109651

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Fabrication and application of hierarchical porous carbon for the adsorption of bulky dyes

T

Changli Qi, Liheng Xu∗, Mengxue Zhang, Ming Zhang Department of Environmental Engineering, China Jiliang University, Hangzhou, 310018, China

A R T I C LE I N FO

A B S T R A C T

Keywords: Adsorption Water treatment Hierarchical porous carbon (HPC) Dye Intraparticle diffusion

To enhance the removal efficiency of bulky dyes from water and wastewater, a hierarchical porous carbon (HPC) is developed and evaluated as a superior adsorbent in this study. Characterization of the prepared HPC demonstrates that it adopts an interconnected hierarchical porous structure. Macropores with 800–1000 nm diameters interconnect each other via inner windows, and mesopores with an average diameter of 8.1 nm locate in the interwalls of the macropores. The Brunauer-Emmett-Teller (BET) surface area and the pore volume of the HPC reach 1058 m2/g and 2.16 cm3/g, respectively. The adsorption kinetics and isotherms of HPC towards selected dyes are investigated and analyzed by theoretical models. Compared to the commercial activated carbon (AC) and mesoporous carbon (MC), HPC exhibits superior adsorption rate and capacity towards bulky dyes. For the adsorption of all the selected dyes in this study, the intraparticle diffusion rates of HPC keep 2–4 times as high as those of AC and 1.5–2 times as high as those of MC. The interconnected hierarchical porous framework of HPC strongly improves the diffusion rate of guest molecules through the porous network, and consequently accelerates the adsorption rate. A maximum adsorbed amount of 584.32 mg/g of brilliant yellow is achieved by HPC, which is nearly 6 times the capacity of AC. The opened macro/meso porous structure of HPC is assumed to enhance the accessibility of the inner surface to bulky molecules.

1. Introduction Currently, water pollution has become one of the most concerned environmental issues worldwide [1]. As a family of common contaminants, dyes are usually discharged with the effluents from textile, cosmetic, leather, printing and other industries. Furthermore, dyes pose a serious threat to water quality and human health due to their deep color, high toxicity, potential carcinogenicity and low biodegradability [2–4]. Various treatment technologies have been employed to eliminate dyes from water and wastewater, including adsorption, membrane separation, chemical coagulation, and catalytic degradation [5–7]. Of these technologies, the adsorption method has attracted widespread attention for its high efficiency, simple operation and low cost, and a great variety of adsorbents have been developed for the removal of dyes [8–13]. Porous carbons, including activated carbon (AC), mesoporous carbon (MC), and nanostructured carbons, are considered as highly efficient adsorbents due to their large specific surface areas, adjustable pore size, and excellent chemical stability. They have been extensively explored in the removal of organic pollutants from water [14]. The geometrical structure of the pores, which includes such factors as

∗

diameter, length, and tortuosity, is a critical factor for the adsorption characteristics of carbonaceous porous materials. According to the international union of pure and applied chemistry (IUPAC), pores can be divided into three categories depending on their diameters: micropores (< 2 nm), mesopores (2 nm–50 nm), and macropores (> 50 nm) [15]. Studies have shown that micropores and small mesopores tend to provide high specific surface areas, while large mesopores and macropores benefit to the diffusion performance of guest molecules [16–18]. Therefore, it should not be surprising that porous carbon with unimodal pores inevitably faces some obstacles in practical application. For example, microporous AC has a relative large diffusion distance of up to 5 μm [17]. This will seriously decline the adsorption rate and subsequently reduce the adsorption efficiency especially in the industrial application with limited adsorption time. Walker et al. studied the adsorption of dyes on AC and found that only 14% of the total specific surface area is available for dye molecules [19]. The lower accessibility of the microporous surface will undoubtedly cut down the adsorption capacity. It is necessary to utilize the pore structures of carbonaceous adsorbents to overcome the limitations of low adsorption capacity and slow adsorption rate. Hierarchical porous carbons (HPCs), which possess pores of

Corresponding author. E-mail address: [email protected] (L. Xu).

https://doi.org/10.1016/j.micromeso.2019.109651 Received 14 December 2018; Received in revised form 8 August 2019; Accepted 13 August 2019 Available online 16 August 2019 1387-1811/ © 2019 Elsevier Inc. All rights reserved.

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Table 1 Structural characteristics of the selected dyes. Dye

Molecular structure

Molecular size (nm)

Molecular weight (Da)

Methylene blue (MB)

1.26 × 0.77 × 0.65

320

Rodamine B (RB)

1.59 × 1.18 × 0.56

478

Brilliant yellow (BY)

2.45 × 1.09 × 0.36

625

Fig. 1. Illustration of the fabrication process of HPC.

micropores contribute maximized space sites, mesopores provide fast mass transport pathways, and macropores can act as reservoirs to minimize the diffusion distances [22–24]. So far, studies of the application of HPCs have focused on catalyst supports [24,25], super capacitors and battery materials [22,23]. Despite their potential as highly efficient adsorbents, there are few studies on the application of HPCs for the adsorption of water contaminants [26,27]. The main goal of this study is to explore the performance of HPCs as adsorbents and to reveal the influence of the pore geometrical structure on the adsorption efficiency. To obtain HPCs, various fabrication strategies have been reported including templating, templateing/activation combination, and template-free methods [22]. Of the various routines, templating method is an effective and commonly used technique, including hard templating [28,29], soft templating [30,31] and hard/soft templating methods [24,32]. The pore networks and geometrical parameters of the prepared HPCs can be easily tailored by controlling the structure and integration of the templates. In this study, a hard/soft templating method is employed to produce HPC material, with poly (methyl methacrylate) (PMMA) particles act as both carbon source and template for macropores and silica nanospheres act as template for mesopores. Aiming to provide highly efficient adsorbents for bulky dyes in water, an HPC containing interconnected macro and mesopores is constructed using a dual-template method in this study. The adsorption behaviors of the HPC towards selected dyes are explored and analyzed

Fig. 2. FTIR spectra of the carbons. (a) PMMA-SiO2 template; (b) C–SiO2; (c) HPC; (d) MC; (e) AC.

multiple size levels, have emerged as promising candidate materials due to the combination of advantages of different sizes of pores in a synergistic way [20,21]. The design and application of HPCs have attracted increasing attention over the last decade. It is assumed that 2

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Fig. 3. Microscopic images of the carbons. (a, b) SEM images of HPC; Insert is the SEM image of PMMA template particles; (c, d) TEM images of HPC; (e) TEM images of MC; (f) SEM image of AC.

Fig. 4. N2 adsorption-desorption isotherms (a) and the pore size distributions (b) of HPC, MC and AC.

3

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Table 2 BET surface areas and pore distributions of the adsorbent samples. Sample

BET surface area (m2/ g)

Micropore area (m2/g)

External surface area (m2/g)

BJH pore volume (cm3/g)

BJH average pore diameter (nm)

HPC MC AC HPC-BY MC-BY AC-BY

1058 916 1022 468 498 732

120 171 471 37 47 280

938 745 551 430 451 452

2.16 1.88 0.44 0.94 0.95 0.36

8.1 8.5 2.9 7.1 7.1 2.9

Fig. 5. XRD patterns of prepared HPC, MC and AC.

with the Langmuir, Freundlich, pseudo second order and intraparticle diffusion models. It is anticipated that outstanding adsorption rate and capacity for bulky dyes would be achieved by the prepared HPC due to the specific hierarchical porous structure. 2. Materials and methods 2.1. Materials Poly (methyl methacrylate) (PMMA) with a molecular mass of 20000–23000 Da was obtained from Suzhou Ankesi Advanced Material Co. (China). The PMMA particles had diameters of approximately 1 μm. Silica nanospheres with an average diameter of 20 nm were obtained from Shenzhen Jingcai Chemical Co. (China) and were used as received. Commercial activated carbon (AC) powder was purchased from Sinopharm Chemical Reagent Co. (China). Methylene blue (MB), rhodamine B (RB) and brilliant yellow (BR) were analytical chemicals, and their properties are listed in Table 1.

Fig. 6. Adsorption kinetics curves of HPC, MC, and AC towards MB (a), RB (b), and BY (c) in water. Solid lines are the fitted results of the pseudo second order model.

2.2. Preparation of HPC

24 h to remove the nanosilica. The obtained HPC was filtered and washed with ultrapure water and then dried at 80 °C for 10 h. The preparation of mesoporous carbon (MC) followed the same procedure as above, with the exception of the carbonization procedure. The PMMA-SiO2 template was heated at 270 °C for 3 h to allow the PMMA particles to melt and become uniform throughout the template. Then, the temperature was further increased to 600 °C and maintained for 2 h for carbonization.

The hierarchical porous carbon (HPC) was prepared via a dualtemplate approach. First, 0.5 g of PMMA particles and 1 g of SiO2 nanospheres were mixed ultrasonically in 100 mL of ultrapure water for 4 h to form a uniform dispersion. The resultant dispersion was then poured into a petri dish and allowed to stand in a thermostatic drying oven at 50 °C for 8 h. The PMMA and SiO2 particles assembled to form a colloidal crystal template during this process, and the product of this step is noted as PMMA-SiO2 template. The PMMA-SiO2 template was carbonized at 600 °C for 2 h with a heating ramp rate of 1 °C/min under the protection of an N2 atmosphere in a muffled furnace. After carbonization, the sample was soaked in 5% hydrofluoric acid and shaken for 4

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Table 3 The fitting parameters of the adsorption kinetic curves of dyes on the adsorbents by the pseudo second order model. dye

adsorbent

k2x103 (g/mg min)

qe (mg/g)

R2

χ2

MB

HPC MC AC

1.86 1.94 16.98

143.07 152.75 186.01

0.9821 0.9045 0.9964

33.01 219.1 11.43

RB

HPC MC AC

1.31 0.89 1.40

281.97 218.92 178.86

0.9378 0.9108 0.9076

455.9 399.8 276.9

BY

HPC MC AC

1.21 0.60 0.66

221.20 188.55 133.01

0.9792 0.8406 0.9807

96.01 614.5 33.55

2.3. Characterization The prepared HPC and MC samples were investigated by scanning electron microscopy (SEM) using an SU 8010 (Hitachi, Japan) and the images were recorded at 10.0 kV. Transmission electron microscopy (TEM) images were obtained using a Tecnai G2 F30 S-Twin transmission electron microscope (Netherlands) at an accelerating voltage of 300 kV. Samples for TEM examination were prepared by dispersing the carbons on a carbon-coated copper grid. The surface area and pore size distribution of the carbons were characterized by Brunauer-EmmettTeller (BET) N2 adsorption-desorption experiments using a Gemini Ⅶ instrument (Micromeritics, US). The micropore surface and external surface were calculated by t-plot method depending on the N2 adsorption-desorption isotherm. X-ray diffraction (XRD) analysis was performed with an X-ray diffractometer (D2 PHASER, Germany). The FTIR spectra of the samples were recorded with a Nicolet 5700 spectrometer (Thermo, US) at a resolution of 2 cm−1. The zeta potentials of the carbons were examined using a Zetasizer Nano ZS instrument (Malvern, England).

2.4. Adsorption experiment MB, RB and BR were selected as typical dyes with which to examine the adsorption properties of the carbonaceous adsorbents in this study. Adsorption experiments were carried out using the batch equilibration technique. First, 0.01 g of HPC, MC or AC adsorbent was added to 20 mL of the dye solution with an initial concentration of 200 mg/L (100 mg/L for MB) in tubes with Teflon caps. Next, the tubes were shaken at 25 ± 0.5 °C on an orbital shaker at 150 rpm to allow the adsorption proceeding with the period from 3 min to 24 h. The pH values of all of the solution were ~7, and no notable variation was observed after the adsorption process. The solution and the solid phase were then immediately separated by centrifugation at 3500 rpm for 5 min, after which the supernatant was analyzed using a Shimadazu UV-1800 spectrophotometer to determine the residual concentration of dyes. As for the adsorption isotherm, the 10–400 mg/L initial concentrations of dyes were adopted, and the equilibrium time was set as 6 h according to result of the kinetics study. For the reversibility and regeneration study, the adsorbents after the adsorption and separation were added into 20 mL of ethanol. The suspensions were shaken for 2 h at 25 °C and 150 rpm followed by filtration. The adsorbents were then washed with Milli-Q water, collected and dried at 40 °C, and then used for another adsorption cycle. The wavenumbers for detection of MB, RB and BR were 665 nm, 554 nm and 400 nm, respectively. The adsorbed amounts of dye were calculated from the differences between initial and final solute concentrations. Control experiments demonstrated that the loss of dye in the adsorption process was negligible. All of the adsorption experiments were duplicated.

Fig. 7. Fitted plots of the adsorption of HPC, MC, and AC towards MB (a), RB (b), and BY (c) of the intraparticle diffusion model.

3. Results and discussion 3.1. Construction of HPC The fabrication procedure of HPC in this study is illustrated in Fig. 1. PMMA microballs act as both the carbon source and the macropore templates in this method, and the SiO2 nanoparticles act as mesopore templates. During the assembly process, dispersed PMMA microballs and SiO2 nanoparticles gradually settle at the bottom of the container forming a complex colloidal crystal template. After the carbonization and etching processes, it is clearly found from the EDS images in Fig. 1 that the O and Si components of the PMMA-SiO2 template are removed and a HPC material is successfully achieved. 5

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Table 4 The fitting parameters of the adsorption kinetic curves of dyes on the adsorbents by the intraparticle diffusion model. dye

adsorbent

ki1 (mg/(g min1/2))

C1 (mg/g)

R2

ki2 (mg/(g min1/2))

C2 (mg/g)

R2

MB

HPC MC AC

11.75 6.79 2.81

58.06 83.27 167.01

0.9498 0.9139 0.9420

2.56 1.86 0.37

115.78 132.73 186.04

0.7469 0.9118 0.7271

RB

HPC MC AC

22.88 14.75 11.44

58.07 86.42 80.65

0.8920 0.9621 0.9470

9.73 2.87 4.85

148.18 189.11 129.64

0.8279 0.8421 0.9137

BY

HPC MC AC

20.88 10.56 12.27

82.50 75.23 20.52

0.9939 0.9901 0.9819

3.43 3.62 2.34

179.19 151.32 97.32

0.9716 0.8184 0.9997

10% of the BET surface of HPC versus nearly half in the case of AC. Mesopores and macropores are assumed to be the major contributors to the surface area of HPC. The BET surface area of the prepared MC is slightly less than that of the HPC and also is dominated by mesopores as shown. Fig. 4(b) shows detailed pore size distributions of the studied carbonaceous materials, as calculated from the desorption branches of the N2 adsorption-desorption isotherms by the BJH method. A centralized distribution of pore sizes for the HPC in the range of 5–15 nm is investigated, with an average pore diameter of 8.1 nm. A similar mesopore-dominant structure is adopted by the prepared MC, which has an average pore diameter of 8.5 nm. On the other hand, the average pore diameter of AC is 2.9 nm, and nearly all pores are smaller than 4 nm, suggesting a typically microporous structure. The remarkable difference in pore size between the various carbons results in obvious differences in the pore volume. As listed in Table 2, the BJH pore volume of HPC reaches 2.16 cm3/g, nearly 5 times that of AC. Compared to HPC, the lower pore volume of MC of 1.88 cm3/g could be due to the absence of macropores in the porous network. The XRD patterns of the prepared HPC are recorded to characterize its structure. In Fig. 5, two broad diffraction peaks can be observed at approximately 23° and 44° (2θ) in the XRD pattern of HPC, which are attributed to typical reflections from the (002) and (100) planes of graphite, respectively [27,37]. The results show that the prepared HPC possesses a large amount of amorphous carbon structure, which are beneficial to serve as adsorption sites in the application process as adsorbents. Similar diffraction peaks are also observed in the XRD patterns of MC and AC, implying similar amorphous carbon structures. The zeta potential analysis results show that HPC, MC, and AC exhibit similar zeta potential distribution centralized around −15mV. Combining the similar FTIR spectra of the carbons, it is conducted that the carbons possess similar chemical composition and properties.

The FTIR spectra of the PMMA-SiO2 template, carbonized template (noted as C–SiO2), and the prepared HPC are shown in Fig. 2. For PMMA-SiO2 template, typical adsorption peaks can clearly be observed in the FTIR spectrum. The sharp peaks at 1112 cm−1 and 800 cm−1 are attributed to the Si–O stretching vibration of SiO2 [33], the characteristic bands at 2996 cm−1 and 2952 cm−1 can be attributed to the –CH3/-CH2 stretching vibration [34] of PMMA, and the characteristic bands at 1732 cm−1 correspond to the C]O stretching vibration [35] of PMMA. It is clearly shown that after carbonization, the adsorption bands belonging to PMMA molecules nearly disappear from the FTIR spectrum of C–SiO2. The bands belonging to SiO2 vanish from the FTIR spectrum of HPC due to the successful etching process. As shown, no obvious absorption peak is present in the spectrum of the prepared HPC, indicating that there are few functional groups in the product. The weak band at 1612 cm−1 may be attributed to the C]C vibration [13] of the obtained carbon. Similar FTIR spectra of MC and AC are obtained, meaning similar chemical composition of the carbons. 3.2. Structural properties of HPC Fig. 3(a) shows the SEM image of the morphology of the prepared HPC. A macro/meso bimodal porous structure of HPC can be clearly observed. Macropores with diameters of 800–1000 nm are found in dense arrays throughout the whole HPC particle. Windows with sizes of 50–100 nm are found locating on the bottoms and sidewalls of the macropores, which connect the macropores each other to form an interconnected porous network. The amplified SEM image (Fig. 3(b)) gives detailed structural information of HPC. It is noticed that the inner walls between the macropores are composed of abundant mesopores with diameters of approximately 10 nm. The total thicknesses of the inner walls are around 100 nm. The TEM images of the HPC (Fig. 3(c) and (d)) also clearly demonstrate the network of layered macropores and the abundant mesopores. For comparison, the TEM image of the prepared MC is shown in Fig. 3(e), and a unimodal mesoporous structure can be observed. The N2 adsorption-desorption isotherms of the studied carbonaceous adsorbents are shown in Fig. 4(a). According to the IUPAC classification, the isotherms of HPC and MC can be classified as type II with type H2 hysteresis loops, meaning macro or meso porous structure with cylindrical pores [36]. The N2 isotherm of AC can be classified as type I with type H3 hysteresis loop, indicating a microporous structure with slit shaped pores [36]. At low relative pressures (p/p0 < 0.3), monolayers of N2 molecules form in the pores of the carbons. At high relative pressures (p/p0 > 0.3), higher amounts of N2 are adsorbed by HPC and MC than by AC. Multilayers of N2 molecules and condensations may form in the mesopores and macropores at high pressure, thereby causing a sharp increase in the amount of adsorbed N2. The specific surface areas and pore distributions calculated from the adsorptiondesorption isotherms are summarized in Table 2. Although the prepared HPC and the selected commercial AC possess similar total surface areas (1058 m2/g and 1022 m2/g, respectively), micropores contribute only

3.3. Adsorption kinetics Adsorption kinetics, which demonstrate the dependence of the adsorption amount on the contact time, constitute an important factor by which to evaluate the adsorption performance. Data of the adsorption kinetics of HPC, MC, and AC towards the selected dyes are shown in Fig. 6. All of the kinetics curves demonstrate a sharp increase followed by a gradual increase until the equilibrium stage is reached. It is clear that for the adsorption of MB, the kinetics curve of AC gives a sharper increase in the initial period and attains a higher equilibrium value than those of HPC and MC. With regard to the adsorption of BY, on the other hand, the kinetics curve of HPC exhibits the steepest increase and the highest equilibrium adsorbed amount amongst the three adsorbents. A pseudo second order model is employed to analyze the kinetic data in this study. Its mathematical representation is as follows [38]:

qt = 6

k2 q e2t 1 + k2 qe t

(1)

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Compared to the molecular size of MB (1.26 nm × 0.77 nm × 0.65 nm), the pore size of AC (2.9 nm) is large enough for the MB molecules to easily enter the pores, and the micropores further facilitate the adsorption of these molecules on the effective sites. Thus, a fast overall adsorption rate is attained by AC. While, as for the bulky dye such as BY, the large molecular dimensions (2.45 nm × 1.09 nm × 0.36 nm) make the diffusion into micropores difficult due to the spatial and massrelated hindrances. The macro/meso porous frame of HPC with the average pore diameter of 8.1 nm could provide enough space for BY molecules to enter freely and subsequently result in a fast adsorption rate. Therefore, the k2 value of HPC towards BY is nearly twice that of AC. The kinetics performance of HPC is better than that of MC when adsorbing RB and BY, as shown in Table 3. The existence of macropores in the porous structure of the adsorbent enhances the diffusion of dye molecules to the inner surface. According to the previous studies, the dye adsorption process on porous solid can be divided into four steps [39,40]: diffusion of dye molecules through solution to the outer surface of adsorbents; adsorption on the external surface; intraparticle diffusion of dye molecules to the inner surface of the adsorbents and intraparticle adsorption. The first bulky diffusion step is affected by agitation of the solution. The outer adsorption and intraparticle adsorption steps are dependent mainly upon the molecular interaction between dyes and adsorbents. Therefore, the intraparticle diffusion step is usually considered the ratelimiting step during the dye adsorption process. An intraparticle diffusion model was proposed and adopted to analyze the adsorption of organic contaminants onto carbonaceous adsorbents [41,42]. The model can be expressed as:

qt = ki t 0.5 + C

(2)

where qt is the adsorbed amount (mg/g) of adsorbate on the adsorbent at any time t (min), ki (mg/(g min1/2)) is the diffusion rate constant and C (mg/g) is a constant related to the boundary layer thickness. According to this model, if the fitted plot of qt versus t0.5 is a straight line through the origin (C = 0), the intraparticle diffusion is the rate-limiting step in the overall adsorption process. As shown in Fig. 7, all of the simulated plots of qt versus t0.5 for the adsorption in this study exhibit two-stage lines with good linearity. This result indicates that the intrapartilce diffusion is an important step in the adsorption, while it is not the sole rate determining step because none of the fitting plots pass through the origin [26,43]. It is likely that surface adsorption and pore adsorption occur simultaneously. The large C1 value at the initial stages means the fast boundary layer diffusion rate and large instantaneous initial adsorption on the external surface. By estimating from the C1 values (in Table 4) and the equilibrium adsorption amount, all of the portions of initial adsorption in this study belong to the intermediately initial adsorption [42]. The occurrence of two linear sections in each plot indicates a stepwise transport through different sized pores in the adsorbents [41]. From the fitting parameters summarized in Table 4, it is obvious that the intrapartilce diffusion rate constants (ki1 and ki2 corresponding to the first and second linear section, respectively) of HPC are remarkably higher than those of MC or AC for all the studied dyes. The ki1 values of HPC maintain 2-4 times as high as that of AC, and 1.5–2 times as high as those of MC. It is easily revealed that the intraparticle diffusion of dye molecules in HPC is faster than that in MC or AC. Taking the large pore size and the connectivity of pores in HPC into account, it is acceptable that the guest molecules diffuse more freely in the porous network than in that of AC. Similar pore diameters and pore volumes are found in both MC and HPC. The hierarchical porous structure of HPC is found to benefit the diffusion performance. The interconnected macropores may cut down the transport distance of guest molecules, and consequently speed up the adsorption process.

Fig. 8. The adsorption isotherms of HPC, MC and AC towards dyes MB (a), RB (b) and YB (c). The solid lines are the fitted results of the Freundlich model, and the dotted lines are the fitted results of the Langmuir model.

where qe and qt are the adsorbed amounts (mg/g) of the adsorbate on the adsorbent at equilibrium and at any time t (min), respectively, and k2 (g/mg min) is the related adsorption rateconstant. The fitted curves are shown in Fig. 6 and the parameters are summarized in Table 3. The adsorption rate constant (k2) is found to depend strongly on both the type of adsorbent and the molecular structure of the dye. Given the similar chemical composition and properties of the three carbons, the pore structure of the adsorbent is assumed to be a key factor influencing the adsorption behavior. For the adsorption of MB, the k2 value of AC is 16.98 g/mg min, nearly 9 times as high as those of HPC and MC. 7

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Table 5 The fitting parameters of the adsorption isotherms of dyes on the adsorbents by the Langmuir model and the Freundlich model. dye

adsorbent

Langmuir model

Freundlich model

kL (L/mg)

qm (mg/g)

R

χ

2

2

kF (mg/g)/(mg/L)1/n

1/n

R2

χ2

MB

HPC MC AC

0.480 0.833 1.248

205.99 213.80 281.80

0.9513 0.9650 0.9595

205.6 163.9 512.9

98.3 109.7 175.4

0.1793 0.1711 0.1271

0.9529 0.9752 0.9861

199.2 118.8 134.9

RB

HPC MC AC

1.614 0.855 11.90

314.40 223.59 160.24

0.9482 0.8842 0.9559

796.8 711.2 154.4

163.7 115.9 114.5

0.1883 0.1895 0.1127

0.9751 0.9894 0.9712

383.1 65.14 100.9

BY

HPC MC AC

0.008 0.015 0.174

584.32 430.29 101.91

0.9909 0.9909 0.9764

91.40 81.53 21.46

13.9 23.2 44.3

0.6340 0.5175 0.1714

0.9875 0.9751 0.9664

125.5 222.6 30.57

The fitting results of all the adsorption isotherms are shown in Fig. 8 and the fitting parameters are summarized in Table 5. It should be noted that both the Langmuir model and the Freundlich model can well describe the adsorption isotherms of these dyes on the adsorbents in this study. The maximum adsorbed amount (qm) calculated from the Langmuir model of MB on HPC is slightly lower than that of the commercial AC. In contrast, the calculated qm of BY on HPC reaches a value of 584.32 mg/g, which is nearly 6 times as high as the qm on AC. It can obviously be concluded that the prepared HPC exhibits a prevailing capacity for adsorption towards relatively bulky dye molecules. The adsorption affinity constants (kL from Langmuir model and kF from Freundlich model) show that the adsorption affinity of each dye on AC is higher than that on HPC. Taking the spatial limitation into account, it is acceptable that the inner surfaces of small pores in AC tend to exhibit stronger interaction with dye molecules inside the pores, especially at low concentrations. The values of Freundlich exponential index (1/n) for HPC towards dyes are noticed to be higher than those of AC. Therefore, the adsorption ability of HPC in the high concentration region gradually becomes higher than AC. To obtain more detailed information on the adsorption behavior of bulky dyes on carbon adsorbents, the N2 adsorption-desorption isotherms of adsorbents loaded with BY molecules were collected, and the results are summarized in Table 2. After adsorption of BY, the average pore sizes of HPC and MC clearly decreased from 8.1 nm to 8.5 nm, respectively, to approximately 7.1 nm. The adsorbed BY molecules occupy the inner surfaces of the mesopores and then shrink the pore diameter. The sharp decrease in total pore volumes of HPC after adsorption from 2.16 cm3/g to 0.94 cm3/g also supports this interpretation. The pore occupation by the adsorbed BY molecules at the equilibrium adsorption are 56%, 50% and 18% for HPC, MC and AC, respectively. The average pore size of AC remains approximately the same after the adsorption of BY, and a micropore area of 280 m2/g is maintained after the adsorption. It is reasonable to assume that a considerable portion of the micropore surface of AC does not act as effective adsorption sites during the adsorption of BY. The narrow size of the micropores hinders the adsorption of bulky dyes.

Fig. 9. Recycling adsorption of HPC towards BY.

3.4. Adsorption isotherms The adsorption isotherms of dyes on HPC, MC, and AC are shown in Fig. 8. As the equilibrium concentration increases, the adsorbed amount of each dye on the adsorbents initially increases sharply, then shows a gradual increasing. As noticed, the adsorption of BY on the carbon adsorbents is lower than those of MB and RB, especially in low concentration region. The different charge density of dye molecules may account for this. Because of the negative zeta potential of carbon adsorbents, positive charged MB and RB molecules are benefit to get stronger affinity than negative charged BY molecules in the same circumstance. With regard to the adsorption of MB, the smallest dye molecule in this study, the adsorption ability of AC is obviously higher than those of HPC and MC. For the adsorption of the relatively larger dyes, RB and BY, the adsorption capacities of the carbon adsorbents demonstrate an order of HPC > MC > AC. The advantages of macro/ meso porous HPC adsorbent are fully exhibited in the adsorption of bulky molecules. The Langmuir model and Freundlich model are frequently employed to evaluate adsorption isotherms using the following equations [44,45]:

qe =

qm kL ce 1 + kL ce

qe = kF ce1/ n

3.5. Reuse of HPC The reusability of adsorbents is important to the practical application in water treatment. To evaluate the repeated availability of the prepared HPC, the adsorption of BY was conducted for many adsorption/regeneration cycles. It is obviously seen from Fig. 9, above half of the adsorption capacity of HPC is remained after 5 adsorption/regeneration cycles. This result indicates that HPC could be repeatedly used in the adsorption removal of bulky dye from water.

(3) (4)

where qm (mg/g) is the maximum adsorbed amount of adsorbate on the adsorbent, qe (mg/g) is the equilibrium adsorbed amount, ce (mg/L) is the equilibrium concentration of adsorbate in the aqueous, kL (L/mg) is the Langmuir adsorption constant, kF is the Freundlich affinity coefficient ((mg/g)/(mg/L)1/n) and 1/n is the Freundlich exponential index. 8

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4. Conclusion [19]

An HPC adsorbent with an interconnected macro and meso porous structure was successfully fabricated in this study, and the BET surface of the prepared HPC attained a value of 1058 m2/g. The prepared HPC exhibited superior intraparticle diffusion rate and adsorption capacity when adsorbing bulky dyes from a water solution. For the adsorption of the MB, RB, and BY dyes, the intraparticle diffusion rates of guest molecules in the porous network of HPC remained 2–4 times as high as those of AC and 1.5–2 times as high as those of MC. The maximum adsorbed amount of BY on the prepared HPC was nearly 6 times that on AC. It can be concluded that HPC adsorbents are promising candidates for the efficient removal of bulky dyes.

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Acknowledgements

[25]

The authors gratefully acknowledge the financial support from the National Natural Science Foundation of China (21677137).

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