Liquid adsorption of basic dye using silica aerogels with different textural properties

Journal of Non-Crystalline Solids 356 (2010) 250–257

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Liquid adsorption of basic dye using silica aerogels with different textural properties Guoqiang Liu, Ru Yang *, Min Li State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing 100029, China

a r t i c l e

i n f o

Article history: Received 23 May 2008 Received in revised form 27 September 2009 Available online 23 December 2009 Keywords: Porosity Colloids Silica Aerogels Adsorption

a b s t r a c t Monolithic silica aerogels with different pore structure were prepared by sol–gel process and CO2 supercritical drying by changing the cycle pressure of liquid CO2 which was used to replace ethanol in the autoclave before supercritical state of CO2 was obtained. N2 adsorption at 196 °C was used to characterize the texture properties of the silica aerogels as prepared. Equilibrium adsorption of methylene blue (MB) onto silica aerogels was investigated. It was found that silica aerogel prepared at 4.5 MPa possesses mainly macroporous structure, while samples prepared at 6.5 and 8.5 MPa are typical mesoporous material. These materials exhibit significant difference in dye adsorption behavior depending on the different porous structure. The MB adsorption capacity and adsorption rate for sample prepared at 4.5 MPa is higher than samples prepared at 6.5 and 8.5 MPa, and the silica aerogel prepared at 8.5 MPa possesses the lowest adsorption capacity and adsorption rate. All samples are best fitted by Langmuir equation, but the fitting of Freundlich equation shows that the adsorption is also favorable. The adsorption on macroporous silica aerogel prepared at 4.5 MPa is best described by the pseudo-first-order model, while mesoporous silica aerogels prepared at 6.5 and 8.5 MPa follow the pseudo-second-order kinetics model. Surface energy distribution obtained by using DFT method indicated that the surface energy distributions of all samples are very wide, and the surface heterogeneity not only affects the dye adsorption capacity but also the adsorption rate, i.e., both the adsorption capacity and adsorption rate are higher for the sample with more heterogeneous surface. Ó 2009 Elsevier B.V. All rights reserved.

1. Introduction It is a common approach to remove organic solvent pollutants from water by filtration using porous materials [1]. The unique combination of high porosity and small pore size of silica aerogel leads it to be a novel adsorbent. Basso et al. [2] verified the applicability of aerogel for the adsorption and entrapment of hydrolases to be used in organic media at controlled water activity and assayed the applicability of aerogel as a solid support for immobilization of the enzymes PGA, thermolysin and a-chymotrypsin. The application of lipase encapsulated in silica aerogels to transesterification reaction was studied by Rassy et al. [3], and the effect of aerogels on the kinetic mechanism was deeply discussed. Silica aerogels were also used as oral drug delivery systems and loaded with several drugs. It was found that the drugs adsorbed on hydrophilic silica aerogels dissolve faster than the corresponding crystalline drugs [4]. Woignier et al. investigated the effect of the porous network features of silica aerogel on the ability of the material to soak up long life nuclear wastes. Neodymium and cerium oxides were used to simulate the actinide oxides in the nuclear wastes * Corrsponding author. Tel./fax: +86 10 64436736. E-mail address: [email protected] (R. Yang). 0022-3093/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2009.11.019

[5,6]. Wang et al. [7] investigated the dynamic adsorption of methylene blue (MB) on mesoporous silica with different structures. They found that dye adsorption presents reversible or irreversible behavior in dye solution depending on the structure of mesoporous materials. The research showed that the ordered mesoporous silica adsorbents prepared by grafting amino- and carboxylic-containing functional groups onto MCM-41 maintained selectivity of dyes. The amino-containing OMS-NH2 adsorbent has a large adsorption capacity and a strong affinity for the Acid blue 25. It can selectively remove Acid blue 25 from a mixture of dyes (i.e., Acid blue 25 and MB), while the OMS-COOH is a good adsorbent for MB displaying excellent adsorption capacity and selectivity for the dye [8]. Although a lot of work has been done using mesoporous materials for gas adsorption, however, few investigations have been reported on employing mesoporous materials for organic adsorption in aqueous solution [7]. Moreover, the relationship between porous structure of silica aerogels and liquid adsorption behavior were not elucidated in depth. Therefore, the investigation of organics adsorption on silica aerogels with different porous structures in aqueous system is very important to their application in practical areas. In the present paper, crack-free monolithic silica aerogels were prepared. N2 adsorption isotherm was used to

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characterize the textural properties. We report the results for kinetic and thermodynamic equilibrium adsorption studies of MB onto silica aerogels as prepared. Kinetic and equilibrium isotherm models were used to establish the rate of adsorption, adsorption capacity, and the mechanism for MB adsorption. 2. Experimental procedure 2.1. Preparation of silica aerogels The silica alcogel utilized throughout this study was prepared by the ‘two-step process’ developed by Brinker and co-workers [9,10], and all the chemicals used in this study were analytical reagent. The first step, hydrochloric acid was used as an acid catalyst of the hydrolysis of tetraethylorthosilicate (TEOS). TEOS, anhydrous ethanol (EtOH), distilled water and hydrochloric acid were mixed in a 250 mL beaker at a molar ratio of 1:6:4:103 and by stirring for 30 min, and a hydrolysis solution was obtained. The second step, ammonia, as a base catalyst, was slowly dropped into the solution resulting from the first step, at the molar ratio of TEOS: NH4OH equals to 1:102. The mixture was further stirring for 2 min, and then kept for gelation. As the prepared wet gels have the structure filled with water and alcohol, and water is poorly soluble in CO2, the alcogels were immersed in excess of anhydrous ethanol at room temperature and maintained for 1 day in order to remove the water in the structure. This process was repeated several times in order to replace the solution contained in the alcogels completely. The silica alcogels were dried by CO2 (99.99% purity) supercritical drying (CO2-SCD) based on Ref. [11] in two steps. First, the alcogels were placed in a 500 mL autoclave which had been filled with ethanol in order to minimize the evaporation of the solvent from the alcogel and to avoid the cracks during pressure build up. Subsequently, the autoclave was airproofed and then liquid CO2 was pumped into the autoclave and continuously flowed through the vessel at a fixed pressure, which was defined as ‘cycle pressure’ in this work. When the liquid CO2 was pumped through the autoclave and mixed with ethanol, the mixture was separated by a separator. The gaseous CO2 is liquefied and recycled to the extractor, while the ethanol was drown off from the separator and was measured to make sure that it has been replaced completely. The corresponding temperature was fixed at room temperature with very small undulation until all ethanol had been removed from the vessel. Second, when the autoclave was ethanolfree, more liquid CO2 was pumped into the vessel to reach a pressure to 10 MPa which is above the critical pressure of CO2 (6.9 MPa), and then heated to 42 °C, which is above the critical temperature of CO2. During heating, the pressure of autoclave was maintained using a back-pressure regulator. Once 42 °C was reached, the system was held at this supercritical condition (10 MPa, 42 °C) for 3.5 h to extract the ethanol inside the gel. After the supercritical extraction, the autoclave was depressurized by the slow, controlled release of CO2 to ambient pressure and aerogels were obtained. Three simples were prepared by operating the cycle pressure of liquid CO2 at 4.5 MPa, 6.5 MPa and 8.5 MPa. The as prepared silica aerogels were identified as ‘SA’ followed by the cycle pressure (in 105 Pa), i.e. SA45, SA65 and SA85. All samples were thermal treated at 350 °C in a muffle furnace for 16 h and then were kept in a vacuum container before dye adsorption tests. 2.2. Characterization The textural properties of the samples were studied by adsorption of nitrogen at 196 °C with automatic instrument (ASAP2020, Micromeritics) over a wide relative pressure range from about 106

to 0.995. The density employed for adsorption at 196 °C was 0.808 g/cm3. Prior to the measurements, all samples were degassed at 350 °C for 16 h under vacuum to ensure that the silica aerogels samples were clean and free of moisture. When the degassing was completed, the sample was carefully moved from the degas port to the analysis port for gas adsorption measurement. The equilibrium time between each nitrogen gas dosing during the measurement was set at 30 s to make sure a good gas–liquid equilibrium between the two consecutive doses. The specific surface area of prepared silica aerogels were calculated by the Brunauer–Emmett– Teller (BET) (software available in the ASAP2020) method, using N2 adsorption data in the relative pressure ranging from 0.05 to 0.35. The total pore volume (Vt) was calculated by converting the amount of nitrogen adsorbed at a relative pressure of 0.99 to the volume of liquid adsorbate. Pore size distributions (PSDs) and various pore volumes including micropore volume (Vmi), mesopore volume (Vme) and macropore volume (Vma) are calculated by using the density functional theory (DFT) Plus Software provided by Micromeritics Instrument Corporation, applying a technique which is based on calculated adsorption isotherms for pores of different sizes. This program performs an inversion of the integral equation for the overall adsorption isotherm with respect to the PSD. 2.3. Dye adsorption test A typical basic dye MB was used. The molecular structure of the MB is shown in Fig. 1. The molecular size of MB is 1.5 nm, and the characteristic wavelength is 665 nm [7,8]. Stock solution was prepared by dissolving accurately 0.5000 g of MB (measured by a sophisticated electronic balance) in 1 L of distilled water in a 1 L volumetric flask, and then the bottle was closed tightly to avoid any evaporation of liquid. To prevent decolorization by direct sunlight, the stock solutions were stored in dark bottle and kept in dark place before being used. All the solutions for adsorption tests were prepared from the stock solution to the desired concentrations (40– 350 mg/L) by using volumetric flasks of suitable volume according to the experiment demand. Adsorption experiments for all samples were undertaken in a batch equilibrium technique in a thermostatic shaker bath, and each experiment was repeated four times to make it accurate. The determination of dye concentrations for all dye adsorption experiments were done on a spectrophotometer by measuring absorbance at maximal characteristic wavelength 665 nm. The influence of contact time on the efficiency of MB adsorption has been studied by placing 0.02 g silica aerogels into 20 mL MB solution (40 mg/L) in 50 mL flasks, and adsorption was conducted at 25 °C and 65 °C by immersing the flaks in the thermostatic shaker bath until equilibrium. The efficiency of adsorption (g) is defined as:



g ð%Þ ¼ 1 

 Ct  100: C0

ð1Þ

As Ct is the MB solution concentration at time t, C0 is the initial concentration of the MB solution. The effect of temperature on the amount adsorbed was investigated under isothermal conditions in the temperature range of 25– 95 °C in the thermostatic shaker bath. Silica aerogels (0.01 g) were

H3C

CH3 N

ClS+

CH3 N CH3

N Fig. 1. Molecular formula of methylene blue.

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placed in 10 mL MB (40 mg/L) aqueous solution for 16 h then the adsorption amounts were calculated by the determination of concentrations using spectrophotometer. The adsorption isotherm study of MB was performed by mixing 0.02 g silica aerogels and 20 mL MB aqueous solution with different initial concentrations (40–350 mg/L) in a set of 50 mL Erlenmeyer flasks. All the flasks were kept in a thermostatic shaker bath (25 °C) and shaken for enough time to reach equilibrium. The data obtained from the adsorption tests were then used to calculate the adsorption capacity, qe (mg/g), of the adsorbent by a mass–balance relationship, which represents the amount of adsorbed dye per the amount of silica aerogels. The kinetic study of MB adsorption was performed by mixing 0.02 g silica aerogels and 20 mL (40 mg/L) MB aqueous solution in 50 mL flasks immersed in the thermostatic shaker bath (25 °C). At various time intervals, the concentration of solution was calculated by measuring absorbance at maximal characteristic wavelength 665 nm. 2.3.1. Thermodynamic isotherm model In general, the adsorption isotherm describes how adsorbates interact with adsorbents and the correlation of equilibrium data by either a theoretical or an empirical equation is essential to the practical design and operation of an adsorption system [12,13]. Two isotherm equations are used in this work to analyze the adsorption data. One is the Langmuir isotherm equation, which has been widely applied to describe experimental adsorption data based on the assumption that adsorption on a homogeneous surface. The maximum adsorption corresponds to a saturated monolayer of adsorbate molecules on the adsorbent surface with constant energy and no transmigration of adsorbate in the plane of adsorbent surface [13]. That is to say, the surface consists of identical sites, equally available for adsorption and with equal energies of adsorption, and that the adsorbent is saturated after one layer of adsorbate molecules forms on its surface [14]. The Langmuir isotherm equation can be represented as [15]:

Ce 1 1 ¼ Ce; þ qe Q m b Q m

1 log C e ; n

k1 t: 2:303

ð4Þ

A pseudo-second-order [19] model is also based on the sorption capacity of the solid phase and also can be used to describe the kinetics of adsorption. Contrary to the pseudo-first-order model, it predicts the behavior over the whole time of adsorption and is in agreement with an adsorption mechanism being the rate controlling step, which may involve valence forces through sharing or exchange of electrons between dye anions and adsorbent. This model can be expressed as

qt ¼

k2 q2e t : 1 þ k2 qe t

ð5Þ

The intraparticle diffusion model [20] assumes that the film diffusion is negligible and intraparticle diffusion is the only rate controlling step, which is usually true for well-mixed solutions. It is described as

qt ¼ K p t 0:5 :

ð6Þ

In order to check the applicability of the above models in describing the kinetic data, the least-squares correlation coefficient (R2) and the sum of error squares (SSE, %) were calculated for each experiment, the SSE is given as

sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi P ðqe;exp  qe;cal Þ2 SSE ¼ : N

ð7Þ

In Eqs. (3)–(6), qe and qt (mg/g) are amount of adsorbed MB onto silica aerogels at equilibrium and at time t; k1 the pseudo-first-order rate constant (h1); k2 is the pseudo-second-order rate constant (g/mg h); kp (mg/gmin0.5) is defined as the intraparticle diffusion rate constant and is related to the intraparticle diffusivity; qe,exp and qe,cal are the measured and calculated MB adsorbed equilibrium, respectively, and N is the number of data points. 3. Results

ð2Þ 3.1. Texture properties

where qe is the equilibrium MB concentration on the adsorbent (mg/g); Ce, the equilibrium MB concentration in solution (mg/L); Qm, the monolayer capacity of the adsorbent (mg/g); b, the Langmuir constant (L/g) and related to the free energy of adsorption. The other isotherm equation is Freundlich isotherm equation [16]. This equation is the earliest known relationship describing the sorption equation and can be used for nonideal sorption that involves heterogeneous surface energy systems [14]:

log qe ¼ log K F þ

logðqe  qt Þ ¼ log qe 

ð3Þ

where KF is the Freundlich constant (mg/g) indicative of the relative adsorption capacity of adsorbent; 1/n (dimensionless) is the heterogeneity factor which has a lower value for more heterogeneous surfaces, giving an indication of the favorability of adsorption. 2.3.2. Kinetic isotherm models In order to investigate the controlling mechanisms of adsorption process such as chemical reaction, diffusion control and mass transfer [17], three empirical kinetic models, namely, pseudo-firstorder, pseudo-second-order and intraparticle diffusion model were tested to fit the adsorption kinetics data. The pseudo-first-order equation [18] does not fit well for the whole range of contact time and is generally applicable only over the initial stage of adsorption processes. It is based on the adsorption capacity which has the form:

Fig. 2 shows nitrogen adsorption–desorption isotherms measured on as prepared silica aerogels. As can be seen from Fig. 2(a), the isotherm of aerogels prepared at 4.5 MPa belongs to a mixed type in the IUPAC classification [21]. The part of isotherm at high relative pressure P/P0 > 0.8 corresponds to type II, typical of non-porous or macroporous materials, and represents a process of monolayer–multilayer adsorption. A hysteresis loop can be seen in the multilayer range P/P0 > 0.6, associated with capillary condensation in mesopores, which is characteristic of type IV isotherms. Further increasing the pressure to 6.5 and 8.5 MPa, the isotherms of these aerogels are of type IV with hysteresis loops at P/P0 > 0.6, typical characteristic of mesoporous materials. The N2 uptakes of the initial part for all the isotherms are very low, indicating that the micropores may be ignored in these samples. The highest adsorption amount at P/P0 close to unity was observed for silica aerogel prepared at 4.5 MPa, and the isotherms do not reach a clear plateau as the pressure is increased up to 5.5 MPa. Increasing the pressure to 6.5 MPa provokes a strong decrease in N2 uptake, and the isotherm attains a plateau at P/P0 > 0.9. A further rise in the pressure to 8.5 MPa results in a minimum in the amount adsorbed. Fig. 2 also shows that increasing the pressure results in significant changes in the type of adsorption hysteresis found. Samples prepared at 4.5 MPa has a high-pressure hysteresis loop of type H3 [22] without an adsorption limit at P/P0 close to 1. This type hysteresis loop is typical of aggregated particles that form plates and give rise to formation of such rifts or wedges. The closure of the loop is

253

1600

0.4

0.6

0.8

1400

1200

(b)

Quantity Adsorbed(cm 3/g)

3600 3200 (a) 2800 2400 2000 1600 1200 800 400 0 0.0 0.2

Quantity Adsorbed(cm 3/g)

Quantity Adsorbed(cm 3/g)

G. Liu et al. / Journal of Non-Crystalline Solids 356 (2010) 250–257

1200 1000 800 600 400 200 0

1.0

1000 800 600 400 200 0

0.0

0.2

Relative Pressure (P/P0 )

0.4

0.6

0.8

(c)

1.0

0.0

0.2

0.4

0.6

0.8

1.0

Relative Pressure (P/P 0 )

Relative Pressure (P/P 0 )

Fig. 2. N2 adsorption–desorption isotherms of silica aerogels prepared by CO2-SCD at different cycle pressure of liquid CO2 (full symbols, desorption; open symbols, desorption). (a) SA45; (b) SA65 and (c) SA85.

6

3.2. Liquid-phase adsorption study 3.2.1. Effect of contact time and temperature Fig. 5 displays the relationship between the contact time and adsorption efficiency at 25 °C and 65 °C. It can be seen that the

(b)

Differential Pore Volume(cm 3/g)

(a)

tary pores (mainly micropores). Although the intensity of peak at 36 nm becomes stronger when increase pressure to 8.5 MPa, the intensity of entire PSD is much weaker than that of other samples. Consequently, the increasing of liquid CO2 cycle pressure can cause narrowing of PSD from macropore region to mesopore region. Furthermore, textural parameters listed in Table 1 shows that the surface area (SBET), average pore size (D) and total pore volume (Vt) have the same change trend with each other: they all decrease with the increase of cycle pressure. Both of the micropore volume (Vmi) and mesopore volume (Vme) increase with cycle pressure reaching up to 6.5 MPa, and then decrease by further pressing to 8.5 MPa. The macropore volume (Vma) of SA45 is much lager than other samples, and the macropore volumes of samples prepared at 6.5 and 8.5 MPa are more comparable. In addition to providing a measure of the specific extent of surface, the adsorption isotherm conveys a great deal of information about the energetic heterogeneity and geometric topology or porosity of a solid substrate [25]. The surface energy distribution silica aerogels prepared by different drying process are presented in Fig. 4, in which e is the Lennard–Jones interaction potential of the adsorptive with the surface and k is Boltzmann’s constant; e/ k has unit of degree Kelvin and is related to the isosteric heat of adsorption [26]. As shown in Fig. 4, the surface energy distributions of all samples are very wide, display a multimodal distribution with several incontinuous peaks, suggesting that there are several adsorption sites with different surface energy, and the surfaces are very heterogeneous. The dramatic change of textural parameters predicts that difference exists in porous structure of silica aerogels as prepared, and it may leads to different performance in liquid adsorption of organic dye such as MB.

3

10

Differential Pore Volume(cm /g)

Differential Pore Volume(cm 3/g)

gradual, and this confirms the existence of mesopores formed by parallel plates or wedge-shaped sites where desorption occurs due to capillary evaporation [23]. Further increasing of pressure up to 6.5 MPa, a H2 type hysteresis loop was observed, which is usually attributed to a difference in mechanism between condensation and evaporation processes occurring in pores with narrow necks and wide bodies, often referred to as ink-bottle pores [24]. According to the definition of pore size by the IUPAC, pores within porous materials are classified as macropores (>50 nm), mesopores (2–50 nm) and micropores (<2 nm), and micropores can be divided into ultramicropores (<0.7 nm) and supermicropores (0.7–2.0 nm) [22]. The micropore volume (Vmi), mesopore volume (Vme) and macropore volume (Vma) are calculated by DFT method to N2 adsorption data according to the pore size classification of IUPAC mentioned above. Fig. 3 displays the PSDs obtained by applying the DFT method to N2 adsorption data of prepared silica aerogels. As shown in Fig. 3, all samples yielded roughly multimodal PSDs with distinct maxima in the micropore and/or meso-/ macropore regions. The PSD was shifted to smaller pore diameters by increasing the pressure of liquid CO2 from 4.5 MPa to 8.5 MPa, and the intensity of peaks decrease dramatically. The PSDs of samples dried at 4.5 MPa possesses complex PSDs over the entire micro-meso-macro range, with few micropores, large fraction of mesopores and macropores distributing in the region of 1–2 nm, 6–50 nm, and larger than 50 nm, respectively. It is worthy to note that there is no intersection with abscissa for the curve of aerogel prepared at 4.5 MPa, suggesting that there is macropores larger than 100 nm in this sample, corresponding to the tail formation in isotherm shown in Fig. 2. Increasing the pressure up to 6.5 MPa, two distinct maxima of the PSD curve are observed. The one is located in the pore width range below 2 nm, the other is located at about 24 nm, and no peaks are observed in the macropore region. The PSDs centered at about 24 nm in mesopore region represents the ordered mesopores, and the intensity of peaks in the range between 1 and 2 nm is stronger than the samples prepared below 6.5 MPa, which indicates the presence of small complemen-

8 6 4 2 0 0.1

1

10 Pore width(nm)

100

5 4 3 2 1 0 0.1

1

10 Pore width(nm)

100

2.4

(c)

2.0 1.6 1.2 0.8 0.4 0.0 0.1

1

10 Pore width(nm)

100

Fig. 3. DFT pore size distribution of silica aerogels prepared by CO2-SCD at different cycle pressure of liquid CO2. (a) SA45; (b) SA65 and (c) SA85.

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G. Liu et al. / Journal of Non-Crystalline Solids 356 (2010) 250–257

Table 1 Textural parameters deduced from N2 adsorption at 196 °C of silica aerogels prepared at different cycle pressure of liquid CO2. Sample code

SBET (m2/g)

D (nm)

Vt (cm3/g)

Vmi (cm3/g)

Vme (cm3/g)

Vma (cm3/g)

SA45 SA65 SA85

948.54 829.96 677.96

21.76 10.63 9.61

5.16 2.20 1.63

0.0196 0.0955 0.0786

1.45 1.96 1.39

3.69 0.14 0.17

80

120 100 80 60 40 20 0

0

20

40 60 80 Energy(ε/k)

10 0

120

(b)

70

Incremental Surface Area (m 2/g)

(a)

140

Incremental Surface Area (m 2/g)

Incremental Surface Area (m 3/g)

Note: SBET: BET surface area; D: average pore width; Vt: total pore volume; Vmi: micropore volume; Vme: mesopore volume; Vma: macropore volume.

60 50 40 30 20 10 0

0

20

40 60 80 Energy(ε/k)

(c)

100 80 60 40 20

100

0

0

20

40

60

80

100

Energy(ε/k)

Fig. 4. Surface energy distribution deduced from the nitrogen adsorption isotherm at 196 °C on the silica aerogels prepared at different cycle pressure of liquid CO2 (a) SA45, (b) SA65 and (c) SA85.

100

100

(a)

(b)

80

60 40 SA45

25 oC

20

SA65

Efficiency (%)

Efficiency (%)

80 60 40

SA45

65 oC

20

SA65 SA85

SA85

0

0 0

10

20

30 40 50 Time (h)

60

70

0

10

20

30 40 50 Time (h)

60

70

80

Fig. 5. Effects of contact time on the adsorption of MB onto silica aerogels prepared at different cycle pressure of liquid CO2 (a) 25 °C and (b) 65 °C. Experimental condition: MB 20 mL (40 mg/L); SA 0.02 g.

temperature up to 75 °C and then decrease with further heating to 95 °C. 80 70 60

SA45 SA65 SA85

50 q (mg/g)

adsorption efficiency increases with increasing contact time at both of the test temperature. The adsorption curve of silica aerogel drying at 4.5 MPa is higher than that of drying at 6.5 MPa, and the one dried at 8.5 MPa is the lowest, which is independent of temperature, indicates that the adsorption capacity of sample prepared at 4.5 MPa is higher than the other two samples. There is another useful information we can get from Fig. 4. It takes about 9 h for sample prepared at 4.5 MPa to get the adsorption efficiency about 60% at 25 °C, while only 4.5 h was taken at 65 °C. Similar trend can be observed for samples prepared at 6.5 and 8.5 MPa, suggests that the adsorption rate is faster at high temperature. In addition, for both of the adsorption temperature, the adsorption efficiency at the same contact time follows the similar order as was observed for adsorption capacity, i.e. S45 > SA65 > SA85, reveals that the adsorption of MB is faster for samples with large pores. Although the adsorption capacity is influenced by temperature, it does not change the relative adsorption capacity of these materials. Sample prepared at 4.5 MPa possesses the highest adsorption capacity while the one prepared at 8.5 MPa has the lowest, being consistent with the order of textural parameters such as SBET and Vt. Fig. 6 shows the temperature dependence of the adsorption amount (q). It can be observed that the magnitude of adsorption is obviously influenced by the temperature. For all the samples, the q values of the MB dye increase continuously with the increasing of

40 30 20 10 0 20

30

40

50 60 70 o Temperature ( C)

80

90

100

Fig. 6. Effect of temperature on the adsorption of MB onto silica aerogels prepared at different cycle pressure of liquid CO2. Experimental condition: 25–95 °C; MB 10 mL (40 mg/L); SA 0.01 g; time 16 h.

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G. Liu et al. / Journal of Non-Crystalline Solids 356 (2010) 250–257

(a)

200

(b)

120

(c)

160 140

100

120 80

Exp.data Langmuir equation Freundlich equation

40 0 0

5

10

15

20

25

120

80

100

qe(mg/g)

qe(mg/g)

160

qe(mg/g)

180

140

240

60

Exp.data Langmuir equation Freundlich equation

40 20 0

30

0

10

20

Ce(mg/L)

30

80 60

Exp.data Langmuir equation Freundlich equation

40 20 0

40

0

10

20 30 Ce(mg/L)

Ce(mg/L)

40

50

Fig. 7. Equilibrium adsorption isotherms of silica aerogels prepared at different cycle pressure of liquid CO2 (a) SA45, (b) SA65 and (c) SA85.

Table 2 Isotherm parameters and sum of error squares (SSE, %) in Langmuir and Freundlich equations for adsorption of MB at 25 °C on silica aerogels prepared at different cycle pressure of liquid CO2. Adsorbents

Langmuir isotherm

Freundlich isotherm

SA45

Qm (mg/g) b (L/mg) R2 SSE

218.82 0.1348 0.9782 8.61

1/n KF ((mg/g)(mg/L3)n) R2 SSE

0.3982 30.06 0.9656 3.14

SA65

Qm (mg/g) b (L/mg) R2 SSE

133.69 0.1608 0.9857 5.57

1/n KF ((mg/g)(mg/L3)n) R2 SSE

0.4336 26.74 0.9703 1.88

Qm (mg/g) b (L/mg) R2 SSE

111.86 0.1893 0.9724 4.39

1/n KF ((mg/g)(mg/L3)n) R2 SSE

0.6889 11.25 0.9641 1.76

SA85

Table 3 Kinetic parameters and sum of error squares (SSE, %) in kinetic models for adsorption of MB at 25 °C on silica aerogels prepared at different cycle pressure of liquid CO2. Kinetic models and parameters

3.2.2. Adsorption isotherms The adsorption capacity variation vs. concentration for the different adsorbents is shown in Fig. 7, the parameters and correlation coefficients calculated for the models are given in Table 2. It can be seen that the least-squares correlation coefficient (R2) values of Langmuir model are higher than those of Freundlich model for all samples, indicates that the isotherms of these samples are best fitted by Langmuir equation. From another point of view, the Freundlich isotherm parameters 1/n shown in Table 2 indicates that SA45 (1/n = 0.3982; R2 = 0.9656), SA65 (1/n = 0.4336; R2 = 0.9703) and SA85 (1/n = 0.6889; R2 = 0.9641) are also favorable adsorption, for which 0 < 1/n < 1.

50

35

(a)

30

40

SA45

SA65

SA85

Pseudo-first-order

qe,exp (mg/g) qe,cal (mg/g) k1 (1/h) R2 SSE

39.76 39.00 0.0719 0.9547 0.38

27.73 25.72 0.0438 0.9571 1.01

24.62 20.54 0.0690 0.9498 2.04

Pseudo-second-order

qe,exp (mg/g) qe,cal (mg/g) K2 (g/(mg h)) R2 SSE

39.76 38.65 0.017 0.8322 0.56

27.73 26.02 0.0091 0.9609 0.85

24.62 21.44 0.0105 0.9832 1.59

Intraparticle diffusion

qe,exp (mg/g) qe,cal (mg/g) Kp (mg h0.5/g) R2 SSE

39.76 37.76 3.7852 0.8494 1.0

27.73 25.07 2.7065 0.9361 1.33

24.62 20.13 2.4854 0.9200 2.24

3.2.3. Kinetic study of adsorption The kinetic adsorption data are analyzed by pseudo-first-order, pseudo-second-order and intraparticle diffusion models. The experimental data and best fits of these models are shown in Fig. 8, and the parameters for the model fitting results are summarized in Table 3. In all adsorbent cases, three phases are observed: (a) the first phase, where a rapid adsorption appeared within 10 h contact time; (b) the second phase, where a progressive adsorption occurred thereafter; (c) the adsorbed amount remained constant

30

(b)

Exp.data pseudo 1st order pseudo 2nd order interparticle diffusion model

10 0 0

20

40

60

Time (h)

80

100

120

20

q (mg/g)

q (mg/g)

q (mg/g)

20

(c)

25

25 30

Adsorbents

20 15 Exp.data pseudo 1st order pseudo 2nd order interparticle diffusion model

10 5 0 0

20

40

60

Time (h)

80

100

120

15 10

Exp.data pseudo 1st order pseudo 2nd order interparticle diffusion model

5 0 0

20

40

60

80

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Time (h)

Fig. 8. The adsorption kinetics of silica aerogels prepared at different cycle pressure of liquid CO2 (a) SA45, (b) SA65 and (c) SA85.

120

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after about 30 h of contact time, implying that equilibrium is reached. It is found that the adsorption kinetics of SA45 is best described by the pseudo-first-order model with relatively low SSE (0.38) and high correlation coefficient R2 (0.9547) values, while the adsorption of MB on SA65 and SA85 follows the pseudo-second-order kinetics not the pseudo-first-order and intraparticle diffusion model based on the parameter R2 and SSE, agreeing with the solid curve in Fig. 8.

4. Discussion It is known that dyes adsorption mechanisms depend on the adsorbent structure, dye’s molecular structure, and temperature, especially if there are significant differences in their specific surface areas and porosity. There are three consecutive mass transport steps associated with the adsorption of solute from solution by porous adsorbent, i.e., the adsorbate migrated through the solution to the external surface of the adsorbent particles by molecular diffusion, solute movement from the particles surface into internal sites by pore diffusion and the adsorbate is adsorbed onto active sites at the interior of the adsorbent particles [16]. Therefore, as the relationship between contact time and adsorption amount at different temperature shown in Fig. 5 reveals, the whole adsorption process for all silica aerogels takes relative long time to reach equilibrium at both adsorption temperatures. When the adsorption process reaches equilibrium at 25 °C, the highest adsorption capacity, about 97%, was observed for SA45, and about 64% and 60% for SA65 and SA85, respectively. When the adsorption equilibrium was performed at 65 °C, the adsorption capacity for all simples is close to each other, about 80%. The difference in MB adsorption can be attributed to the different pore structures of the samples and adsorption temperature. It is easy to observe from the N2 adsorption isotherms and PSDs shown in Figs. 2 and 3 that the porous structures of sample prepared at 6.5 and 8.5 MPa are similar with each other, but significant difference can be found for the sample prepared at 4.5 MPa. When the dye adsorption is performed at 25 °C, the MB molecule motion is relatively slower than that at 65 °C, and there is enough time for the MB molecules adsorbed onto the surface and into the pores of silica aerogels. As a result, the adsorption efficiency at equilibrium for SA65 and SA85 is close to each other but much lower than SA45, and the equilibrium adsorption amount may be dominated by the textural properties such as surface area and pore volume at low temperature. While the dye adsorption is performed at 65 °C, dye molecule motion is much faster, both the adsorption and desorption process are easy to take place on macroporous and mesoporous silica aerogels at high temperature [27], and the equilibrium can be achieved in a short time. Moreover, such high temperature may easily cause partial desorption of dyes from SA45 which possesses much larger pore size than other samples. On contrary, the release rate could be restricted by the small pore size and the narrow necks of pores with ink-bottle shape for samples prepared at 6.5 and 8.5 MPa. In conclusion, the adsorption temperature may play the major role in the case of high adsorption temperature, and the adsorption efficiency is close to each other for all samples. Parameters in Table 2 show that no matter Langmuir or Freundlich model is used, the adsorption capacity of as prepared materials all follow the same order SA45 > SA65 > SA85 by using the two models, agreeing with the change trend of SBET and Vt shown in Table 1, reveals that the porous characteristics play an important role in the adsorption process. The Freundlich isotherm describes adsorption where the adsorbate has a heterogeneous surface with adsorption sites that have different energies of adsorption, and the parameter 1/n is the heterogeneity factor which has a lower value for more heterogeneous surfaces [14]. The Freundlich parameters

1/n in Table 2 show that they follow an order that 1/n (SA45) < 1/n (SA65) < 1/n (SA85), suggests that the sample SA45 has a more heterogeneous structure than SA65 and SA85, while SA85 is the most homogeneous one in all samples as the parameter 1/n has a lower value for more heterogeneous surfaces. Energetic heterogeneity of solid surfaces has been incorporated into the density functional theory to improve the fitting of adsorption isotherms and induce information on the surface structure, and the degree of heterogeneity also can be described by different surface energy distributions calculated by the DFT method [28]. The surface energy distributions in Fig. 4 show that the amount of energy peaks is 24, 19 and 17 for the three samples SA45, SA65 and SA85, respectively. The amounts of energy peaks represent the amounts of different kinds of adsorption sites with different adsorption potential, and the more amount of adsorption sites the surface possesses, the more heterogeneous the surface is [29]. Therefore, the degree of heterogeneity for the silica aerogels follows the order SA45 > SA65 > SA85, which is consistent with the conclusion obtained by applying Freundlich equation. It is worthy to note that the adsorption capacity also follows the same order observed for surface heterogeneity, indicates that the heterogeneous surface with more adsorption sites would result in higher adsorption amount, which is also observed for surface area and pore volume. The kinetic data obtained by fitting all kinetic models shown in Table 3 reveals that the adsorption rate follows the order that SA45 > SA65 > SA85, and the adsorption kinetics of SA45 was best described by the pseudo-first-order model, while the adsorption of MB on SA65 and SA85 follows the pseudo-second-order kinetics not the pseudo-first-order and intraparticle diffusion model. As the pseudo-first-order equation does not fit well for the whole range of contact time and is generally applicable only over the initial stage of adsorption processes [18], it is not difficult to understand that the macroporous structure of SA45 leads to a relatively faster adsorption process rate than other two samples, and the adsorption would be finished at the initial stage of adsorption, being fitted by pseudo-first-order equation. Comparing with SA45, sample SA65 and SA85 possesses relative regular porous structure with PSDs mainly distributing in the mesoporous region, the adsorption rate is much slower. As a result, the adsorption will take longer time to reach equilibrium and the adsorption rate may be a dominant factor, being consistent with the adsorption mechanism of the pseudo-second-order model, which predicts the behavior over the whole time of adsorption and is in agreement with an adsorption mechanism being the rate controlling step. In general, the fast adsorption could be attributed to a surface reaction process, whereas the progressive decrease of adsorption sites results in a slower adsorption reaction [30], further proving the above analysis of surface heterogeneity by applying DFT to N2 adsorption. In conclusion, the degree of surface heterogeneity also affects the adsorption rate of dye adsorption.

5. Conclusions Monolithic silica aerogels with different pore structure were prepared by sol–gel process and CO2 supercritical drying. The analysis of N2 adsorption isotherms shows that the pore structure of silica aerogels depend on the cyclic pressure of liquid CO2 which was used to replace ethanol in the autoclave before supercritical state of CO2 was obtained. N2 adsorption at 196 °C shows that silica aerogel prepared at 4.5 MPa possesses mainly macroporous structure, while samples prepared at 6.5 and 8.5 MPa are typical mesoporous material. MB adsorption tests in aqueous solution show that these materials exhibit significant difference in adsorption behavior depending on the structure. The MB adsorption capacity and adsorption rate for sample prepared at 4.5 MPa is

G. Liu et al. / Journal of Non-Crystalline Solids 356 (2010) 250–257

higher than samples prepared at 6.5 and 8.5 MPa, and the silica aerogel prepared at 8.5 MPa possesses the lowest adsorption capacity and adsorption rate. All silica aerogels are best fitted by Langmuir equation, but the fitting of Freundlich equation shows that they are also favorable adsorption. The adsorption on macroporous silica aerogel prepared at 4.5 MPa was best described by the pseudo-first-order model, while mesoporous silica aerogels prepared at 6.5 and 8.5 MPa follow the pseudo-second-order kinetics model. The surface energy distributions of all samples are very wide, both the adsorption capacity and adsorption rate in MB adsorption are higher for the sample with more heterogeneous surface. Acknowledgment The authors gratefully acknowledge the support of Beijing Natural Science Foundation Program (2052017). References [1] L.W. Hrubesh, P.R. Coronado, J.H. Satcher Jr., J. Non-Cryst. Solids 285 (2001) 328. [2] A. Basso, L.D. Martin, C. Ebert, L. Gardossi, A. Tomat, M. Casarci, O.L. Rosi, Tetrahedron Lett. 41 (2000) 8627. [3] H.E. Rassy, A. Perrard, A.C. Pierre, J. Mol. Catal. B: Enzym. 30 (2004) 137. [4] I. Smirnova, S. Suttiruengwong, W. Arlt, J. Non-Cryst. Solids 350 (2004) 54. [5] T. Woignier, J. Reynes, J. Phalippou, J.L. Dussossoy, N.J. Francillon, J. Non-Cryst. Solids 225 (1998) 353. [6] J. Reynes, T. Woignier, J. Phalippou, J. Non-Cryst. Solids 285 (2001) 323. [7] S.B. Wang, H.T. Li, Micropor. Mesopor. Mater. 97 (2006) 21.

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