Adsorption behavior of methylene blue on halloysite nanotubes

Adsorption behavior of methylene blue on halloysite nanotubes

Available online at www.sciencedirect.com Microporous and Mesoporous Materials 112 (2008) 419–424 www.elsevier.com/locate/micromeso Adsorption behav...

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Available online at www.sciencedirect.com

Microporous and Mesoporous Materials 112 (2008) 419–424 www.elsevier.com/locate/micromeso

Adsorption behavior of methylene blue on halloysite nanotubes Mingfei Zhao, Peng Liu

*

State Key Laboratory of Applied Organic Chemistry and Institute of Polymer Science and Engineering, College of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou, Gansu 730000, China Received 22 August 2007; received in revised form 9 October 2007; accepted 11 October 2007 Available online 18 October 2007

Abstract The halloysite nanotubes (HNTs) were used as nano-adsorbents for the removal of the cationic dye, methylene blue (MB), from aqueous solutions. The dye adsorption experiments were carried out by using bath procedure. Experimental results have shown that the basic pH, increasing initial dye concentration and lower temperature favored the adsorption. The dye adsorption equilibrium was rapidly attained after 30 min of contact time. The factors controlling the adsorption process were also calculated and discussed. And a maximum adsorption capacity of 84.32 mg/g of methylene blue was achieved. It was noted that the dye adsorbed halloysite nanotubes had poor stability in aqueous suspension and deposited completely within 30 min while the original aqueous suspension of halloysite nanotubes remained stable for months. Ó 2007 Elsevier Inc. All rights reserved. Keywords: Adsorption; Removal; Methylene blue; Halloysite nanotubes; Nano-adsorbent

1. Introduction Cationic dyes are widely used in industries such as textiles, pulp mills, leather, dye synthesis, printing, food, and plastics, etc. Since many organic dyestuffs are harmful to human being and toxic to microorganisms, removal of dyestuffs from wastewater has received considerable attention over the past decades. The dyes include a broad spectrum of different chemical structures, primarily based on the substituted aromatic groups. Due to the complex chemical structure of these dyes, they are resistant to breakdown by chemical, physical and biological treatments. Furthermore, any degradation by physical, chemical or biological treatments may produce small amount of toxic and carcinogenic products. Adsorption is known to be a promising technique, which has great importance due to the ease of operation and comparable low cost of application in the decoloration process [1].

*

Corresponding author. Tel.: +86 931 8912516; fax: +86 931 8912582. E-mail address: [email protected] (P. Liu).

1387-1811/$ - see front matter Ó 2007 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2007.10.018

Many kinds of adsorbents such as activated carbon [2], silica [3], natural polymeric materials [4] and sewage sludge [5], etc have been developed for various applications. However, their operating costs are high. This has led to searches for unconventional adsorbents as alternative adsorbents. The adsorbents with high surface areas, such as mesoporous material [6–17], microporous materials [18,19], carbon nanotubes [20,21] and titania nanotubes [22], had been applied to decrease the dosage of adsorbent. Most recently, the clay minerals were reported to be unconventional adsorbents for the removal of dyes from aqueous solutions due to their cheap and abundant resources, higher surface areas [23]. Furthermore, the regeneration of these low-cost substitutes is not necessary whereas regeneration of activated carbon is essential because of the abundant resources. Clay materials with sheet-like structures [24–27] and needle-like structure [28–31] have been increasingly gaining attention because they are cheaper than activated carbons and their also provide highly specific surface area. Halloysite nanotubes (HNTs) is a kind of aluminosilicate clays with hollow nanotubular structure mined from

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natural deposits in countries such as China, America, Brazil, France and so on. It possesses a regular nanotubular morphology, bulk structure and rich mesopores and nanopores [32,33]. Recently, it was used as adsorbents [34] and nanotemplates or nanoscale reaction vessels instead of carbon nanotubes or boron nitride nanotubes [35–37]. In the present study, halloysite nanotubes (HNTs) were used as adsorbents for the removal of cationic dye from an aqueous solution. Methylene blue was selected as a model compound in order to evaluate the capacity of HNTs for the removal of dyes from aqueous solutions. The aim of this study was to investigate the adsorption of methylene blue onto HNTs, which is a low-cost adsorbent for the removal of dyes. 2. Experimental and methods 2.1. Raw materials Halloysite clay was obtained from Hebei Province, China. It was pretreated by the followed procedure: 250 g raw halloysite clay and 500 ml water were mixed and milled with SF400 type wit sand-grinding dispersing machine at 4000 rpm for 2 h. The halloysite nanotubes (HNTs) suspension was centrifugated (3000 rpm for 3 min) to throw away the deposit. The stable suspension was used for further experiments. Its TEM image was shown in Fig. 1 (The morphology of the HNTs was characterized with a JEM-1200 EX/S transmission electron microscope (TEM). The bare HNTs suspended colloid solution and was deposited on a copper grid covered with a perforated carbon film). Methylene blue (MB) (CI: 52015; chemical formula: C16H18ClN3S; molecular weight:

319.86; maximum wavelength: 662 nm) supplied by Merck, was not purified prior to use. 2.2. Methylene blue absorption All of the methylene blue solution was prepared with distilled water. The pH of the solution was adjusted with 0.1 N HCl or 0.1 N NaOH by using a Model 3C Digital pH-meter with a combined pH electrode. The pH-meter was standardized with NBS buffers before every measurement. The effect of contact time on the amount of dye adsorbed was investigated at 159.93 mg/l (0.50 m mol/l) initial concentration of dye and at different temperatures (20, 45 and 70 °C). Fifteen milliliter HNTs suspension (including 0.235 g HNTs) was mixed with 85 ml MB dye solution. For the other adsorption experiments, 15 ml HNTs suspension (including 0.235 g HNTs) was added into 85 ml of dye solution with known initial concentration at desired pH and temperature and stirred with a rate of 150 rpm for 180 min. A constant bath was used to keep the temperature constant. At the end of the adsorption period, the solution was centrifuged for 10 min at 15,000 rpm. After centrifugation, the dye concentration in the supernatant solution was analyzed using a UV spectrophotometer (Shimadzu UV-260) by monitoring the absorbance changes at a wavelength of maximum absorbance (662 nm). The samples were pipetted from the medium reaction by the aid of a very thin point micropipette, which prevented the transition to the solution of the HNTs samples. Preliminary experiments showed that the effect of the separation time on the amount of adsorbed dye was negligible. The amounts of dye adsorbed on HNTs at any time, t, were calculated from the concentrations in solutions before and after adsorption. At any times, the amount of MB adsorbed (mol/g) (qt), onto HNTs was calculated from the mass balance equation as follows: qt ¼ V ðC 0 –C e Þ=W

ð1Þ

where qt is the amount of adsorbed dye on HNTs at any time (m mol/g); C0 and Ce are the initial and equilibrium liquid-phase concentrations of MB (m mol/l), respectively; V is the volume of MB solution, and W is the mass of HNTs sample used (g) [29,38]. 2.3. Sedimentation time

Fig. 1. TEM image of halloysite nanotubes.

The sedimentation time of the methylene blue adsorbed halloysite nanotubes suspensions were investigated by the following method: 15 ml HNTs suspension sample (including 0.235 g HNTs) was mixed with 35 ml water or 35 ml 159.93 mg/l MB aqueous solution in a 50 ml measuring cylinder, respectively. Then the transparence of the suspensions was observed.

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3. Results and discussion

421

66.8 66.4

3.1. Adsorption rate

66.0

3.1.2. Effect of pH Effect of pH on the removal rate of MB by HNTs is shown in Fig. 3. As the pH increased, the removal rate increased. The pH value of the dye solution plays an important role in the whole adsorption process and particularly on the adsorption capacity. The zeta-potential behavior of halloysite nanotubes is mostly negative at pH 2–12 due to the surface potential of SiO2 with a small contribution from the positive Al2O3 inner surface [36,39,40] (the chemical properties on the halloysite nanotubes outermost surface are similar to the properties of SiO2 while the properties of the inner cylinder core could be associated with Al2O3). And it exhibits a strongly pH-dependent surface charge. With the increasing of pH, the surface of HNTs becomes more negatively charged, thereby increasing electrostatic attractions between positively charged dye anions and negatively charged adsorption sites and causing an increase in the dye adsorption. It was also found

pH=4 pH=7 pH=10

65.6

qt (mg/g)

3.1.1. Effect of contact time and initial dye concentration The effect of concentration on contact time was also investigated as a function of initial dye concentration. The effect of initial dye concentration and contact time on the removal rate of MB by HNTs is shown in Fig. 2. As shown, the adsorption increases with increasing initial dye concentration. The results show that dye uptake is rapid for the first 15 min and finally attains saturation within about 30 min. The equilibrium was attained at 30 min. The amount of MB adsorbed at equilibrium increases from 0.1276 to 0.2588 m mol/g (40.81–82.78 mg/g) by increasing the initial MB concentration from 0.30 to 0.70 m mol/l with the adsorption condition of initial pH 7 and 20 °C. It is near to the sepiolite and much higher than those other clay minerals [28].

65.2 64.8 64.4 64.0 63.6 63.2 0

30

60

90 120 t (min)

150

180

Fig. 3. Effect of contact time and initial pH on the removal rate of MB onto HNTs from aqueous solutions (20 °C, C0: 159.93 mg/l).

that the adsorption equilibrium was attained faster with the higher initial pH condition. 3.1.3. Effect of temperature Fig. 4 exhibits contact time versus adsorbed amount graph at different temperatures. The initial pH was selected as 7 in order to avoid the impossible breakage of HNTs at higher temperatures because SiO2 is the main component. The equilibrium adsorption capacity of MB onto HNTs was found to decrease with increasing temperature, decreasing from 0.2081 m mol/g at 20 °C to 0.2058 m mol/g at 70 °C indicating that the dye adsorption on the adsorbent was favored at lower temperatures. And the adsorption equilibrium was attained slowly with the higher temperature. 3.2. Adsorption kinetics 3.2.1. The pseudo-first-order kinetic model The pseudo-first-order kinetic model has been widely used to predict dye adsorption kinetics. A linear form of pseudo-first-order model was described by Lagergren [41]:

90

66.9

78

66.6

72

66.3

66 60

95.96mg/L 159.93mg/L 223.90mg/L

54 48 42

qt (mg/g)

qt (mg/g)

84

66.0 65.7 65.4

o

20 C o 45 C o 70 C

65.1

36

64.8

30 0

30

60

90 120 t (min )

150

180

Fig. 2. Effect of contact time and initial dye concentration on adsorption of MB onto HNTs from aqueous solutions (20 °C, pH 7).

0

30

60

90 120 t (min)

150

180

Fig. 4. Effect of contact time and temperature on the removal rate of MB onto HNTs from aqueous solutions (C0: 159.93 mg/l, pH 7).

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logðqeq  qt Þ ¼ log qeq  k pf t=2:303

ð2Þ

where qt is the amount adsorbed at time t (mg/g), and kpf is the equilibrium rate constant of pseudo-first-order adsorption (min1). The values of log(qeqqt) were calculated from the kinetic data (Fig. 4). The calculated qeq, kpf, and the corresponding linear regression correlation coefficient r21 values are shown in Table 1. It was observed that the rate constant kpf increased first with an increase in temperature and then decreased. It was also observed that correlation coefficients were lower for all temperatures. This shows no applicability of the pseudo-first-order model in predicting the kinetics of the methylene blue adsorption onto HNTs particles. 3.2.2. The pseudo-second-order kinetic model The kinetic data were further analyzed using Ho’s pseudo-second-order kinetics, represented by [42] t=qt ¼ 1=ðk ps q2eq Þ þ t=qeq

ð3Þ

where kps is the rate constant of second-order adsorption (g/mol min). The values were calculated from the kinetic data (Fig. 2). A plot between t/q versus t gives the value of the constants k2 (g/mg h) and also qeq (mg/g) can be calculated. If the second-order kinetics is applicable, then the plot of t/qt versus t should show a linear relationship. The curves of the plots of t/qt versus t were given in Fig. 5 and the calculated qeq, kps, and the corresponding linear regression correlation coefficient r22 values are summarized in Table 1. The linear plots of t/qt versus t show good agreement between experimental and calculated qeq values. The correlation coefficients for the second-order kinetics model (r22 ) are greater than 0.999, indicating the applicability of this kinetics equation and the second-order nature of the adsorption process of methylene blue onto HNTs.

5

0

0.011 66.55 0.7912

Pseudo-second-order qea, mg/g kps, g/mg min qeb, mg/g r22

C0 (mg/l) 95.96 40.82 3.97 40.82 0.9999

Intraparticle diffusion kid, mg/g min1/2 C r23

T (°C) 0.037 66.09 0.8424

a b

Calculated. Experimental.

30

60

90 120 t (min)

150

180

Fig. 5. Ho pseudo-second-order kinetics for MB onto HNTs (20 °C, pH 7).

3.2.3. Intraparticle diffusion The adsorbate species are most probably transported from the bulk of the solution into the solid-phase through an intraparticle diffusion process, which is often the ratelimiting step in many adsorption processes. The possiblity of intraparticle diffusion was explored by using the intraparticle diffusion model [43] qt ¼ k id t1=2 þ C

ð4Þ

where C is the intercept and kid is the intraparticle diffusion rate constant (mol/g min1/2). The values of qt were found to be linearly correlated with the values of t (Fig. 2). Plots between t/q versus t1/2 were given in Fig. 6. The values kid, C, and the corresponding linear regression correlation coefficient r23 values are given in Table 1. The intraparticle rate constants calculated from Fig. 6 are 0.037, 0.064, and 0.095 mg/g min1/2 at 20, 45, and 70 °C, respectively. From Table 1, it is observed that kid increased with increasing temperature. 66.6 66.3

70

0.023 66.46 0.8753

0.017 65.82 0.9976

159.93 66.53 0.195 66.55 0.9999

223.90 84.32 0.019 86.35 0.9999

0.063 65.77 0.8404

0.095 64.62 0.9914

66.0

q t (mg/g)

kpf, min1 qeb, mg/g r21

45

2

0

T (°C) 20

3

1

Table 1 Adsorption kinetic parameters of methylene blue onto HNTs Pseudo-first-order

95.96mg/L 159.93mg/L 223.90mg/L

4

t/qt (min g/mg)

422

65.7 65.4

o

20 C o 45 C o 70 C

65.1 64.8 0

3

6 1/2

t

9

12

15

1/2

(min )

Fig. 6. Intraparticle diffusion plots for MB onto HNTs (C0: 159.93 mg/l, pH 7).

M. Zhao, P. Liu / Microporous and Mesoporous Materials 112 (2008) 419–424

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and temperature of the solution. Adsorption kinetic follows pseudo-second-order kinetic model confirming the chemosorption of methylene blue (MB) on HNTs. The methylene blue adsorbed halloysite nanotubes aggregated and deposited completely within 30 min. It is expected that the halloysite nanotubes could be used as low-cost unconventional nano-adsorbent for the removal of dyes from aqueous solutions. References

Fig. 7. The photos of the HNTs suspensions after 30 min: (a) in water and (b) in methylene blue aqueous solution.

3.3. Sedimentation time The clays in nano-scale usually have excellent dispersibility in aqueous-phase and the suspensions had good stability because of the hydrophilic surface of the natural clay minerals. The clay nano-adsorbents were separated from the aqueous solutions via filtration or centrifugation processes after the adsorption of dye in those reported works. In this work, it was interested that the methylene blue adsorbed halloysite nanotubes aggregated together to mm scale particles and deposited completely within 30 min while the initial HNTs suspension with the same solid content remains stable in months (Fig. 7). It might be due to the hydrophobic surface of the methylene blue adsorbed halloysite nanotubes. It is expected that the filtration or centrifugation processes could be past over so the cost of the separation could be saved. It is also an advantage of the application of the natural halloysite nanotubes as low-cost unconventional nano-adsorbent. 4. Conclusion Hylloysite nanotubes (HNTs) have been proved to be an effective nano-adsorbent for the removal of cationic dye via adsorption from aqueous solution. The equilibrium adsorption was reached within 30 min. A maximum adsorption capacity of 84.32 mg/g of methylene blue was achieved. It is much higher than those other clay minerals. The adsorption is highly dependent on concentration, pH

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