Preparation, characterization, and adsorption evaluation of chitosan-functionalized mesoporous composites

Microporous and Mesoporous Materials 193 (2014) 15–26

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Microporous and Mesoporous Materials journal homepage: www.elsevier.com/locate/micromeso

Preparation, characterization, and adsorption evaluation of chitosan-functionalized mesoporous composites Qiang Gao a,b,⇑, Hao Zhu a, Wen-Jun Luo a, Shi Wang a, Cheng-Gang Zhou a a b

Department of Chemistry, Faculty of Material Science and Chemistry, China University of Geosciences, Wuhan 430074, PR China Engineering Research Center of Nano-Geo Materials of Ministry of Education, China University of Geosciences, Wuhan 430074, PR China

a r t i c l e

i n f o

Article history: Received 1 August 2013 Received in revised form 18 January 2014 Accepted 12 February 2014 Available online 22 February 2014 Keywords: Mesoporous SBA-15 Chitosan-functionalized Adsorption Acid red 18

a b s t r a c t Polymer-modified mesoporous silica materials are of practical interest due to their great potential for adsorption-related applications. In the present work, composites of natural polymer chitosan (CTS) and siliceous mesoporous SBA-15, i.e. SBA-15/CTS(5%), SBA-15/CTS(10%), and SBA-15/CTS(20%), were facilely prepared by prehydrolysis of tetraethyl orthosilicate in the presence of pore-directing agent and subsequent cocondensation with an appropriate amount of CTS-based organosilane. The texture and composition of pure SBA-15 and CTS-functionalized mesoporous products were characterized using various techniques such as TEM, XRD, N2 adsorption/desorption, 29Si MAS NMR, FT-IR, and TGA measurements. To disclose the adsorption properties of the composites, anionic compound acid red 18 (AR18) was selected as model adsorbate. The effects of pH, ionic strength, contact time, adsorption temperature, and initial concentration of AR18 on adsorption efficiency were investigated. It was found that pure SBA-15 had a negligible adsorption capacity while the CTS-functionalized composites showed large adsorption capacities (up to 232.6 mg g1) with rapid adsorption kinetics (less than 120 min). It was also observed that the adsorption capacity increased with increase in CTS content of the composite. Results of comparative analysis indicated that SBA-15/CTS(20%) had better adsorption capacity than most of common adsorbents. Experimental kinetic and isotherm data were analyzed by theoretical models including pseudo-first-order and pseudo-second-order kinetics, Weber–Morris diffusion, Freundlich and Langmuir models. Moreover, adsorption thermodynamics has also been evaluated. The study suggests that the CTSfunctionalized mesoporous composites are prospective adsorbents for adsorption of anionic compounds, and somewhat exemplifies their adsorbent function for adsorbing some other adsorbates. Ó 2014 Elsevier Inc. All rights reserved.

1. Introduction Adsorption is one of the most fundamental processes in a wide range of industrial physicochemical operations such as separation and heterogeneous catalysis [1]. For an adsorption system, adsorbent with high adsorption capacity and rapid adsorption kinetics is very critical. In the last fifteen years, many organic-modified mesoporous SBA-15 materials have been found to possess great potential as highly efficient adsorbents because of their large mesopores, high surface area, and functionalized pore wall [2]. The large pore diameter allows fast mass transfer during adsorption, and the high surface area with numerous surface active sites allows the binding of a large number of adsorbates [2,3].

⇑ Corresponding author at: Department of Chemistry, Faculty of Material Science and Chemistry, China University of Geosciences, Wuhan 430074, PR China. Tel.: +86 159 7221 6195; fax: +86 027 6788 3731. E-mail address: [email protected] (Q. Gao). http://dx.doi.org/10.1016/j.micromeso.2014.02.025 1387-1811/Ó 2014 Elsevier Inc. All rights reserved.

In general, two methods can be used for organic-modification of SBA-15, namely grafting organic groups onto the pore wall of mesoporous SBA-15 (called as ‘‘postsynthesis grafting’’ method) and direct incorporation of organic moieties into the silica framework (called as ‘‘cocondensation’’ method) [4]. The postsynthesis grafting can avoid damaging the silica frameworks, but it has two major shortcomings: (i) it is difficult to control the loading of the functional groups; (ii) the functional groups easily form local clusters and narrow the pore outlets [5]. The cocondensation is an alternate route for the preparation of such materials, in which the organic component can be incorporated into the silica framework by cocondensation of siloxane and organosilane precursors in the presence of pore-directing agent. Unlike the postsynthesis grafting, the cocondensation allows functionalized SBA-15 to have an uniform organic group distribution on the pore wall, and to avoid the necking of the pore channels [6]. However, the organosilane usually disrupts the cocondensation assembly system, which affects even destroys the ordered meso-structure of SBA-15 [7]. To

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overcome this shortcoming, an improvement over this cocondensation procedure has been recently proposed. That is, siloxane is prehydrolyzed and then cocondensated with organosilane [8]. Nevertheless, the incorporation amount of organic moieties is still limited [9]. Moreover, one organosilane incorporated generally yields only one adsorptive group [10]. Therefore, further investigations are still needed regarding the different possibilities of SBA-15 functionalization. Incorporating organic polymer within SBA-15 hosts is a good strategy to make organic-modified SBA-15 composites due to the vast functionalization possibilities of organic polymer chemistry. To date, many SBA-15/polymer composites have been prepared mainly by blending or surface polymerization methods [11]. In these systems, however, polymers tended to ultimately clog the mesopores of SBA-15, which greatly lowered their adsorption ability. Madhugiri et al., for the first time, reported that SBA-15/polymer could be prepared via an electrospinning process [12]. In this approach, the organic polymers were incorporated into the silica framework, which could result in unfilled mesopores. But the inorganic matrix and organic species were physically bonded, implying that the composite should be not mechanically strong. In spite of this, it would be interesting to move ahead with this concept by introducing chemical cross-linkages between the reactive groups of organic polymer and the siliceous skeleton. If so, a more effective bonding between the organic and inorganic phases will be achieved, improving the mechanical and chemical stabilities. Chitosan (CTS), typically obtained by deacetylation of chitin, is one of the most abundant natural polymers, being second only to cellulose on the earth [13]. Amine groups of CTS, especially, are attractive for several adsorption-related applications in base catalysis, CO2 capture, removal of dyes or heavy metal ions or as binding sites for bioactive molecules and controlled drug release [14]. Because of its solubility in acidic conditions and unsatisfied mechanical property, it is generally required to immobilize CTS onto solid particle surface or enable CTS to react with inorganic precursor to form hybrid composite [14,15]. Recently, Silva et al. demonstrated that the CTS could covalently bond with the organosilane 3-isocyanatopropyltriethoxysilane (ICPTES), and the resultant CTS–ICPTES could further form CTS–silica hybrids by hydrolysis and subsequent condensation processes [16]. Silva’s finding is of interest, and more importantly, it might provide an idea to fabricate a new SBA-15/polymer, i.e. CTS-modified SBA-15 (SBA-15/CTS), via

cocondensation route using siloxane and CTS–ICPTES as precursors. If this objective is achieved, the resultant SBA-15/CTS would be endowed with some attractive properties such as mechanical and chemical stabilities, unique surface chemistry, and unobstructed mesopore channels. Thus, the aim of this work is to fabricate the expected SBA-15/ CTS composites, and explain their adsorption behavior to see if desired adsorption properties such as high adsorption capacity and rapid adsorption kinetics, have been satisfied. As shown in Fig. 1, the SBA-15/CTS composites were prepared by prehydrolysis of tetraethyl orthosilicate (TEOS) with the addition of pore-directing agent (triblock copolymer P123) and subsequent cocondensation with an appropriate amount of CTS–ICPTES, where CTS–ICPTES was formed via the reaction between CTS and ICPTES according to the method of Silva. This is the first study, to our knowledge, to suggest it may be possible to prepare mesoporous silica/polymer composite by cocondensation method. One of the most fascinating features of the composites is the complexion characteristic of CTS and SBA-15, encouraging us to explore the utility of these composites in adsorption. To evaluate the adsorption properties of the composites, acid red 18 (AR18) was chosen as the model adsorbate because of its well-known adsorption characteristics [17–22], and batch adsorption experiments as well as the theoretical investigation including adsorption kinetics, equilibrium, and adsorption thermodynamics, were carried out. The adsorption study of AR18 on SBA-15/CTS composites should be useful as the basis for a further extension of such composites to effectively perform their adsorbent function for adsorbing some other adsorbates. 2. Experimental section 2.1. Chemicals and reagents Chitosan (CTS, Mw ffi 1000, deacetylation degree >90%) was purchased from Yuhuan Shell Biological Agents Factories (Zhejiang, China). 3-Isocyanatopropyltriethoxysilane (ICPTES) was obtained from Wuhan University Chemical Factory, (Wuhan, China). Poredirecting agent Pluronic P123 triblock copolymer (EO20PO70EO20, Mav = 5800) was purchased from Sigma–Aldrich. Tetraethyl orthosilicate (TEOS), acid red 18 (AR18), and dimethyl formamide (DMF) were purchased from Sinopharm Chemical Reagent Co., Ltd (Shanghai, China). As shown in Fig. 1, AR18 has a dimensions of 1.42 nm (length)  0.765 nm (width), and the area of AR18

Fig. 1. Preparation scheme of SBA-15/CTS and its application for adsorbing AR18.

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molecule is therefore about 1.09 nm2 [23]. All chemicals were used as received. 2.2. Preparation of SBA-15 and CTS-modified SBA-15 composites As a contrast material, the pure SBA-15 was firstly prepared similar to the procedure proposed by Zhao et al. [24]. In a typical synthesis, 12 g of P123 was dissolved in 90 mL of deionized water. After stirring for 4 h, a clear solution was obtained. Thereafter, 180 mL of 2 mol L1 HCl was added, and the solution was stirred for another 2 h at 40 °C. After the addition of 26.4 g TEOS, the resulting mixture was stirred at 40 °C for 24 h and finally heated to 100 °C for 48 h. The solid products were collected by filtration, washed several times with water/ethanol, and then dried overnight at 100 °C. The template P123 inside the as-synthesized mesoporous sample was removed by Soxhlet extraction with ethanol for 48 h. After being dried at vacuum 80 °C overnight, the white powder, i.e. SBA-15, was obtained. CTS-modified SBA-15 composites were prepared by the synthetic strategy of prehydrolysis–cocondensation (see Fig. 1). The synthesis processes were essentially similar to that of SBA-15, except for the requirements of prehydrolyzing TEOS for 2 h in the initial stage and then adding a certain amount of ICPTES–CTS solution in the reactant composition for cocondensation. The ICPTES–CTS solution was obtained according to the method of Silva with some modification: ICPTES and CTS with predetermined amount were dispersed in DMF and then heated at 105 °C for 48 h to form ICPTES–CTS complex through the highly efficient linkage reaction between NCO group of ICPTES and reactive groups (NH2 or OH groups) of CTS [16]. The experimental synthesis conditions of three CTS-modified SBA-15 composites were summarized in Table 1. Three products were denoted as SBA-15/CTS(5%), SBA-15/ CTS(10%) and SBA-15/CTS(20%), respectively, where the values 5%, 10% and 20% corresponded to the molar percentage of ICPTES/(TEOS + ICPTES). 2.3. Adsorption procedure To evaluate the adsorption properties of SBA-15/CTS(5%), SBA15/CTS(10%) and SBA-15/CTS(20%), a series of adsorption experiments were conducted. Typically, 20 mL of AR18 solution (120 mg L1) were added into a vial that contained 10 mg solid adsorbent, then the mixture was shaken with speed of 120 shakes min1 in a shaker at 303.15 K for 120 min. Then, the AR18-adsorbed solid adsorbent was separated from the solution by centrifugation. The adsorption amount of AR18 was determined by using the UV–Vis spectrometry to monitor the change of AR18 concentration before and after adsorption, and its value was calculated by the following equation:

qe ¼ ðC 0  C e Þ

V m

ð1Þ

where qe is the amount of AR18 adsorbed at equilibrium (mg g1), C0 and Ce are the initial and equilibrium concentrations of AR18 (mg L1), m is the mass of solid adsorbent (g), and V is the volume of solution (L).

Table 1 Reagent dosages used for synthesis of CTS-modified samples. Sample

Molar ratio of ICPTES/ (TEOS + ICPTES) (%)

ICPTES (mL)

CTS (g)

DMF (mL)

P123 (g)

SBA-15 SBA-15/CTS(5%) SBA-15/CTS(10%) SBA-15/CTS(20%)

0 5 10 20

– 1.48 2.96 5.92

– 0.97 1.94 3.88

–

12 12 12 12

9.7 19.4 38.8

17

The effect of solution pH on the adsorption efficiency was investigated in the range of 2–7 through a similar procedure described above, where the solution pH was adjusted by adding a certain amount of 1.0 mol L1 HCl or 0.1 mol L1 NaOH solution. At optimal pH, the effect of ionic strength was also studied by gradually increasing KCl concentration in AR18 solution from 0 to 0.4 mol L1. In order to determine the adsorption rate and kinetic characteristics, three sets of kinetic experiments were conducted by varying the contact time from 2 to 480 min at three different initial AR18 concentrations (120, 240, and 360 mg L1), respectively. To reveal the thermodynamic features, three sets of isothermal adsorption experiments were also carried out at three different temperatures (303.15, 323.15, and 343.15 K) over a range of initial AR18 concentrations from 20 to 360 mg L1. 2.4. Characterization The powder X-ray diffraction (XRD) measurements of mesoporous samples were recorded on an X-ray diffractometer (X’Pert Pro DY2189, PANaytical, B. V., Netherlands) using CuKa radiation with angles of 2h = 0.6–3°. The pore structures of mesoporous samples were observed by transmission electron microscope (TEM) (EDAX9100, Holland), respectively. The surface area, pore size and pore volume of mesoporous samples were analyzed by N2 adsorption/desorption method using automatic surface area analyzer (ASAP2020, US). The surface functional groups of CST-modified SBA-15 composites were detected by Fourier transform infrared (FT-IR) spectroscope (FT-IR-2000, PerkinElmer), where the spectra were performed in KBr pellets and recorded from 4000 to 400 cm1. 29Si MAS NMR experiments were performed on a Varian Infinityplus-300 spectrometer using 7.5 mm probe under magic-angle spinning: the resonance frequency was 79.5 MHz; the 901 pulse width was measured to be 4.8 ms; repetition time of 100 s for single-pulse experiments was used. Thermogravimetric analysis (TGA) was carried out on a PerkinElmer TGA thermal analyzer. The samples were heated from 25 to 1000 °C at 10 °C min1 under air. 3. Results and discussion 3.1. Characterization of materials The physical characterization of adsorbent is crucial to prove that it was well developed. Thus, different techniques were employed to examine and validate the texture and composition of the mesoporous materials involved in this work. Pore structure is essential as identifying characteristics of porous adsorbents since the parameter can determine the surface area and permeability, and thus influence adsorption capacity and adsorption rate. The existence of mesopores of SBA-15, SBA-15/ CTS(5%), SBA-15/CTS(10%), and SBA-15/CTS(20%) can be intuitively observed by TEM technique (see Fig. 2). Obviously, SBA-15 displays a typical hexagonal array of mesopores. With increasing incorporation amount of CTS, the mesopore orderness of CTS-modified SBA15 samples gradually decreases. For the SBA-15/CTS(20%) sample, its mesopores even displays a worm-like structure. The mesopore disordering for CTS-modified samples should result from the interference effect of CTS on the assembly between silicon species and pore-directing agents: the higher the CTS content of mesoporous sample, the greater the disorder of mesopore. XRD patterns of SBA-15-type mesoporous materials are shown in Fig. 3. An intense peak (1 0 0) can be observed for all the samples, being indicative of ordered hexagonal mesoporous structures [25]. Simultaneously, the (1 1 0) and (2 0 0) diffractions are less resolved

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Fig. 2. TEM images of four mesoporous samples: SBA-15 (a); SBA-15/CTS(5%) (b); SBA-15/CTS(10%) (c); and SBA-15/CTS(20%) (d).

Fig. 3. XRD patterns of four mesoporous samples.

with the increase of CTS content in CTS-modified samples, confirming that the mesopore order decreases with the CTS content [25]. N2 adsorption–desorption isotherms of CTS-modified SBA-15 samples are shown in Fig. 4a. All samples possess typical type IV isotherms and H1 hysteresis loops, which are the characteristics of mesoporous solid [24]. Moreover, it can be observed that the pore sizes of SBA-15, SBA-15/CTS(5%), SBA-15/CTS(10%), and SBA15/CTS(20%) are 6.6, 6.5, 6.6, and 6.7 nm, respectively, and all these distributions are very narrow (see Fig. 4b). This result confirms that the pore sizes are almost not affected by the incorporation of CTS, which should thank to the use of cocondensation approach. The detailed texture parameters are collected in Table 2. It can be found

that the CTS incorporation makes surface area and pore volume smaller. In addition to the porous structure, the organic phase of organic-modified adsorbent is also a vital factor that directly influences the adsorption performance of adsorbent. Thus, it is required to further investigate the surface functionality and organic content of CTS-modified SBA-15 materials. 29 Si MAS NMR can provide information about the silicon environment and the degree of functionality of silica-based samples. The 29Si MAS NMR spectra of the tested samples are depicted in Fig. 4c. Further, Table 3 summarizes these observed resonances in 29Si MAS NMR spectra, and the corresponding relative peak areas. The resonance peaks at 110, 100.5, and 91.0 ppm can be attributed to Q4, Q3, and Q2 [Qn = Si(OSi)n(OH)4–n, n = 2–4] silicon species [26]. In addition, the broad peak approximately at – 62.3 ppm should be assigned to T [T = CTS–ICPTES-Si(OSi)3] species [26]. By calculating the values of T/(T + Q2 + Q3 + Q4) [25,26], the degrees of functionalization are found to be 4.6% for SBA-15/ CTS(5%), 9.6% for SBA-15/CTS(10%), and 19.7% for SBA-15/ CTS(20%), respectively. the percentages suggest that the functionalization on SBA-15 is realized as expected. Fig. 4d shows the FT-IR spectra of the prepared materials. The CTS displays its typical peaks such as: (i) the peaks at ca. 1656 and ca. 1590 cm1 correspond to the amide and amino bands, respectively, (ii) the peak at ca. 1381 cm1 is attributed to the stretching vibrations of C–N bond, and (iii) the broad band centered at 3400 cm1 is assigned to the N–H stretch, which overlaps the O–H stretch in the same region [27]. Spectrum of the SBA-15 shows strong peaks in the range between 1250 and 1000 cm1, assigned to SiOSi asymmetric stretching vibration; the range between 820 and 750 cm1, assigned to SiOH bending from the silica; and the peak at ca. 1630 cm1 is possibly due to the OH

Q. Gao et al. / Microporous and Mesoporous Materials 193 (2014) 15–26

Fig. 4. N2 adsorption/desorption isotherms (a), pore size distribution (b),

Table 2 Texture parameters of SBA-15 and CTS-modified samples. Sample

BJH pore size (nm)

BET surface area (m2 g1)

Pore volume(cm3 g1)

SBA-15 SBA-15/CTS(5%) SBA-15/CTS(10%) SBA-15/CTS(20%)

6.6 6.5 6.6 6.7

809.4 748.5 653.9 461.9

1.10 1.07 0.90 0.80

Table 3 Relative peak area (%) in Sample

SBA-15/CTS(5%) SBA-15/CTS(10%) SBA-15/CTS(20%)

29

Si MAS NMR spectra of CTS-modified samples.* 29

Si MAS NMR relative peak area (%)

Q2

Q3

Q4

T

0.95 2.10 2.86

35.6 39.4 30.6

58.9 48.9 46.8

4.6 9.6 19.7

Q2 represents the silicon species of (OH)2Si(–OSi–)2; Q3 represents the silicon species of (OH)Si(–OSi–)3; Q4 represents the silicon species of Si(–OSi–)4; T represents the silicon species of CTS–ICPTES–Si(–OSi–)3.

*

bending vibration from adsorbed water [28]. Spectra of the CTSmodified SBA-15 samples show obvious difference in relation to the number of the peaks when comparing with SBA-15. Some groups from CTS and/or CTS–ICPTES make contributions to the

19

29

Si MAS NMR spectra (c), and FT-IR spectra (d) of samples.

peaks at ca. 2930 and ca. 2865 cm1 (assigned to the symmetric and symmetric of CH2 and CH3) [29], and ca. 1590 cm1 (assigned to NH2 deformation vibration of CTS) [27], which confirms that CTS are successfully immobilized on the surface of SBA-15. TGA curves of the materials under study are shown in Fig. 5. It can be seen that SBA-15 exhibits a major weight loss at a temperature up to ca. 100 °C, which can be attributed to the desorption of physically adsorbed water [26]. An additional weight loss occurs at higher temperatures due to the further condensation of the silicate mesopore walls [26]. In the cases of functionalized samples, the TGA curves exhibit a rapid decline in the temperature range from ca. 200 to 600 °C, which should be mainly ascribed to the removal of organic moieties (CTS and ICPTES). The trend in weight loss is as follows: SBA-15/CTS(20%) > SBA-15/CTS(10%) > SBA-15/CTS(5%), indicating that the grafting amount of organic moieties on SBA15/CTS(20%) is highest followed by SBA-15/CTS(10%) and then SBA-15/CTS(5%).

3.2. Adsorption performance of CTS-modified SBA-15 composites As well known, adsorption characteristics of porous adsorbent are determined by its pore structure (magnitude and distribution of pore) and surface chemistry (kind and quality of surface-bound functional groups) [30]. These CTS-modified SBA-15 composites possess large mesopore diameter (6.6 nm), high surface area (>461.9 m2/g), large pore volume (>0.8 cm3/g), and abundant surface adsorption sites, implying that they might have the potential

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that the AR18 adsorption is positively correlated with the content of CTS (see Fig. 6a).

for being used as highly efficient adsorbents. To confirm this, the adsorption of AR18 onto the composites was intensively investigated. The anionic compound AR18 was chosen as model adsorbate taking into account the Brønsted base character of the composites that would be positively charged under acidic conditions, and the well-known adsorption characteristics of AR18 [17–22].

3.2.2. Effect of ionic strength To further demonstrate the discussion above on the contribution of electrostatic interactions to AR18 adsorption, the effect of ionic strength was also investigated by gradually increasing KCl concentration in AR18 solution from 0 to 0.4 mol L1 at initial pH 2. As shown in Fig. 6b, it can be found that all the adsorption efficiencies of CTS-modified SBA-15 composites are closely related to the ionic strength. For each adsorbate–adsorbent system, the adsorption amount of AR18 decreases gradually as the KCl concentration increases. This phenomenon suggests that the electrostatic interaction should be the main driving force between AR18 and CTS-modified SBA-15 composites. The reduction of adsorption amount in the presence of KCl could be attributed to the electrostatically shielding effect (also known as screening effect) of salt [32]. Nevertheless, it should be also noted that, when the KCl concentration (0.1 mol L1) is about 500 times higher than the initial concentration of AR18 (120 mg L1, also equivalent to 0.198 mmol L1), the CTS-modified SBA-15 composites still showed considerable adsorption capacities for AR18 (see Fig. 6b). The result is of interest, suggesting that the electrostatic interaction between AR18 and CTS-modified SBA-15 was quite strong. Chloride ion did not well compete with AR18 anion for adsorbent.

3.2.1. Effect of pH Considering that the CTS-modified SBA-15 composites are hydrophilic and well dispersed in a strong acidic medium, the adsorption experiments were carried out in the range of pH 2–7, facilitating the separation by centrifugation after adsorption of AR18. The pH dependence of the adsorption performances to AR18 is illustrated in Fig. 3. Clearly, the pure SBA-15 can hardly adsorb AR18 throughout the whole pH range studied, indicating that the surface groups (i.e. Si–OH) of SBA-15 provided negligible interaction ability with AR18. Thus, no further investigation on the adsorption of AR18 onto SBA-15 was carried out in the following work. From Fig. 6a, it is also found that the AR18 adsorption onto SBA-15/CTS(5%), SBA-15/CTS(10%) and SBA-15/CTS(20%) is significantly affected by pH. In more specific terms, the adsorption amount gradually decreases with the increase of pH values, and the maximum adsorption amount of AR18 takes place at pH 2. This observation can be explained by the fact that, decreasing solution pH would result in an increase of positively charged adsorption sites, which favored the adsorption of negatively charged AR18 anions [31]. In addition, it can be seen that at the same pH, SBA-15/ CTS(20%) exhibits the highest adsorption capacity in that order followed by SBA-15/CTS(10%) and then SBA-15/CTS(5%), indicating

3.2.3. Effect of contact time To examine the effect of contact time on the adsorption of AR18, batch adsorption studies were carried out for three CTS-modified SBA-15 adsorbents at different initial concentrations (120, 240, and 360 mg L1). From Fig. 7, it is easily observed that the necessary time to reach equilibrium is varied according to CTS content of adsorbent. For the adsorbents SBA-15/CTS(5%) and SBA-15/ CTS(10%), nearly 20 and 60 min are sufficient for AR18 adsorption to reach equilibrium at all the studied initial AR18 concentrations. As regards the adsorbent SBA-15/CTS(20%), the time of equilibrium adsorption is 100 min at the initial concentration of 120 mg L1 while the time is extended to 120 min at the higher initial concentrations (240 and 360 mg L1). On the whole, the equilibrium time is short (<120 min) [3], indicating that the large-size mesopores of these adsorbents are favorable to fast mass transfer. The decreasing trend of adsorption rate with increasing CTS content of adsorbent can be perhaps explained by the following reasons: Firstly, the AR18 was quickly transported into the pore outlets of adsorbent. When the CTS content of adsorbent was higher, a larger amount of AR18 molecules would be adsorbed onto the pore outlets, which made the subsequent diffusion (i.e. inner diffusion) more difficult. In other words, the increase of CTS content in the

Fig. 5. TGA curves of four mesoporous samples.

Fig. 6. Effect of pH on the AR18 adsorption, using an initial AR18 concentration of 120 mg L1, and the temperature was fixed at 303.15 K (a); and effect of KCl addition on the AR18 adsorption, using an initial AR18 concentration of 120 mg L1, and the pH and the temperature and was fixed at 2 and 303.15 K, respectively (b).

Q. Gao et al. / Microporous and Mesoporous Materials 193 (2014) 15–26

21

Fig. 7. Effect of contact time on AR18 adsorption at different initial AR18 concentrations: 120 mg L1 (a), 240 mg L1 (b), and 360 mg L1 (c), where the pH and the temperature were fixed at 2 and 303.15 K, respectively.

Fig. 8. Effect of temperature on AR18 adsorption onto three different adsorbents: SBA-15/CTS(5%) (a); SBA-15/CTS(10%) (b); and SBA-15/CTS(20%) (c), where the pH and the temperature were fixed at 2 and 303.15 K, respectively.

adsorbent would enhance the diffusion resistance, which resulted in a long equilibrium time. Also from Fig. 7, it is found that the equilibrium time of AR18 adsorption on the adsorbents SBA-15/CTS(5%) and SBA-15/ CTS(10%) is more or less independent of initial AR18 concentration. Possible reason accounting for this behavior was as follows: As revealed by kinetic modeling discussed later, the process of AR18 adsorption onto CTS-modified SBA-15 was predominantly controlled by inner diffusion. In fact, the inner diffusion was mainly determined by the pore structure and surface chemistry of adsorbent [30]. For a given adsorbent, thus, it was believable that the diffusion resistance should be constant under similar solution conditions in which only initial adsorbate concentration varied. Since the uptake amount of AR18 onto SBA-15/CTS(5%) or onto SBA-15/ CTS(10%) almost kept constant among three different initial AR18 concentrations, the contact time to reach adsorption equilibrium should be unchangeable. For the adsorbent SBA-15/CTS(20%), the adsorption capacity was found to be increased with increasing initial AR18 concentration from 120 to 240 mg L1, so the equilibrium time correspondingly increased from 100 to 120 min. Further increase of initial AR18 concentration from 240 to 360 mg L1 did not lead to a visible change in adsorption capacity of SBA-15/ CTS(20%), so the equilibrium time was still kept at about 120 min. 3.2.4. Effect of temperature The effect of temperature on the adsorption of AR18 on CTSmodified SBA-15 composites was investigated by varying the temperature at 303.15, 323.15, and 343.15 K. Fig. 8 shows the bar charts of adsorption capacities versus initial AR18 concentrations (20–360 mg L1) at different temperatures. All the three CTS-modified SBA-15 adsorbents have an identical behavior of decreased uptake of AR18 per unit mass with gradually increasing temperature. The adsorption of adsorbate from the solution phase onto the solid–liquid interface occurs by expelling the solvent molecules (i.e. H2O) from the interfacial region. With a rise in temperature,

the viscosity of solution will decrease, which seems favorable for the transfer and diffusion of adsorbate from bulk solution to adsorbent surface. But some recent studies have indicated that elevating temperature could cause greater mobility of the adsorbate molecules previously adsorbed [33]. Therefore, the phenomenon that the adsorption capacity tended to decrease with temperature might be due to a more serious desorption of AR18 at higher temperature. In order of AR18 adsorption amount, the adsorbents ranked as follows: SBA-15/CTS(5%) < SBA-15/CTS(10%) < SBA-15/CTS(20%). For instance, the maximum adsorption capacities of SBA-15/ CTS(5%), SBA-15/CTS(10%), and SBA-15/CTS(20%) at 323.15 K were 47.1, 98.1, and 232.6 mg g1, respectively (see Fig. 8). When normalized for the surface area, these maximum adsorption capacities at 343.15 K were found to be 0.0630, 0.150, and 0.504 mg m2 for SBA-15/CTS(5%), SBA-15/CTS(10%), and SBA-15/CTS(20%), respectively. The results indicated that the introduction of a higher CTS content into the adsorbent would reduce the surface area to a larger extent, but simultaneously provided the adsorption sites with much higher density to enhance the adsorption of anionic AR18 molecules. 3.3. Kinetic analysis Adsorption kinetics is one of the most important characteristics that represent the adsorption efficiency of the adsorbent and, therefore, largely determines the potential applications of adsorbent. In order to elucidate the adsorption kinetic process, the reaction-based (pseudo-first-order and pseudo-second-order kinetics) and diffusion-based (Weber–Morris diffusion) models were employed. The pseudo-first-order and pseudo-second-order equations are generally expressed as follows [34]:

dqt ¼ k1 ðqe  qt Þ dt

ð2Þ

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dqt ¼ k2 ðqe  qt Þ2 dt

ð3Þ

where qe and qt are the adsorption capacities at equilibrium and at time t, respectively (mg g1), k1 is the rate constant of pseudo-firstorder adsorption (min1), k2 is the rate constant of pseudo-secondorder adsorption (g mg1 min1). Integrating the Eqs. (2) and (3) and applying the boundary conditions, i.e. t = 0 to t = t and qt = 0 to qt = qt, gives linear forms of two kinetic equations:

lnðqe  qt Þ ¼ ln qe  k1 t

ð4Þ

t 1 1 ¼ þ qt j2 q2e qe t

ð5Þ

At the same time, the diffusion-based model was also tested because of reaction-based kinetic model could not give definite mechanism. For an adsorption system using porous solid as adsorbent, the adsorption process can be separated into four steps as follows: (1) migration of the adsorbate from the bulk solution to the hydrodynamic boundary layer surrounding the adsorbent particles (bulk diffusion), (2) diffusion from the boundary layer to the particle external surface or pore outlets (external diffusion or outer diffusion), (3) diffusion of the adsorbent into the pores of adsorbent internal structure (intraparticle diffusion or inner diffusion), and (4) the adsorbate is adsorbed onto the adsorption sites in inner and outer surface of adsorbent (adsorption equilibrium) [35]. Generally, the adsorption rate is controlled by outer or inner diffusion, or both. In order to determine the actual rate-controlling step involved in the AR18 adsorption process, the well-known Weber–Morris equation was applied [36]:

qt ¼ ki t 0:5 þ I

ð6Þ 1

where qt is the adsorption capacity (mg g ) at time t, t is the contact time (min), ki (mg g1 min0.5) is the diffusion coefficient and I (mg g1) is the constant of Weber–Morris kinetic model. If the plot of qt versus t0.5 satisfies the linear relationship with the

experimental data and passes through the origin (i.e. I = 0), then the adsorption process should be controlled by inner diffusion only; if the data exhibit multi-linear plots, then two or more steps have a combined influence on the adsorption process. Fig. 9a–c shows the experimentally generated qt vs. t data of AR18 adsorption onto three adsorbents at three different initial AR18 concentrations and the predicted pseudo-second-order kinetics using the linear regression method. The fitting curves of pseudo-first-order kinetics were not shown in Fig. 9 because they seriously deviated from the experimental data. A glance at Fig. 9a–c can leave one the intuitive impression that the experimental data are well represented by the pseudo-second-order kinetic model. The values of the two kinetic model constants (k1 and k2), the theoretically calculated equilibrium adsorption capacities (qe,calc,1 and qe,calc,2), along with the error function values (R2) are summarized in Table 4. The values of qe,exp (from experiment) and qe,calc,2 (calculated by pseudo-second-order equation) are very close to each other. And, the R2 values (0.9986–0.9999) are closer to unity for pseudo-second-order equation than those (0.2981 < R2 < 0.8834) for pseudo-first-order equation, implying that the AR18 adsorption can be described appropriately by the pseudo-second-order model. This also means that the adsorption rate is proportional to the square of the number of free sites, which corresponds to the term (qe–qt)2 in the pseudo-second-order model [37]. The plots of qt vs. t0.5 are given for AR18 adsorption onto three adsorbents at three different AR18 initial concentrations (see Fig. 9d–f). It can be seen that in all cases the plots have the same general features, consisting of two linear portions followed by a plateau. The first linear portion with a short adsorption period of 0–10 min indicates that the outer diffusion was involved at the early stage of AR18 adsorption [35]. The second linear portion corresponds to inner diffusion and the final plateau represents a state of adsorption equilibrium [38]. As shown in the section marked with yellow background (see Fig. 9d–f), the time taken by inner diffusion is closely related to the kind of adsorbent. It takes a longer inner diffusion time for the mesoporous adsorbent with higher

Fig. 9. Pseudo-second-order kinetic and Weber–Morris diffusion plots for the adsorption of AR18 at different initial AR18 concentrations: 120 mg L1 (a, d), 240 mg L1 (b, e), and 360 mg L1 (c, f).

23

Q. Gao et al. / Microporous and Mesoporous Materials 193 (2014) 15–26 Table 4 Coefficients of pseudo-first-order and pseudo-second-order kinetic models. Initial AR18 concentration (mg L1)

Absorbent

qe,exp (mg g1)

Pseudo-first-order model 1

k1 (min 120

240

360

SBA-15/CTS(5%) SBA-15/CTS(10%) SBA-15/CTS(20%) SBA-15/CTS(5%) SBA-15/CTS(10%) SBA-15/CTS(20%) SBA-15/CTS(5%) SBA-15/CTS(10%) SBA-15/CTS(20%)

42.2 92.6 202.9 45.5 98.4 231.4 46.5 99.4 236.1

0.0058 0.0299 0.0174 0.0030 0.0101 0.0142 0.0010 0.0135 0.0063

)

Pseudo-second-order model 1

qe,cal,1 (mg g

)

R

4.0 59.1 84.5 4.6 27.9 144.9 6.74 41.7 61.5

2

0.4744 0.8834 0.3488 0.3440 0.3486 0.6881 0.3510 0.2981 0.5900

k2 (g mg1 min1)

qe,cal,2 (mg g1)

R2

0.0077 0.0021 0.0010 0.0072 0.0015 0.0007 0.0056 0.0017 0.0010

42.6 93.5 205.8 45.2 100.0 238.1 46.2 100.2 238.7

0.9999 0.9997 0.9988 0.9997 0.9995 0.9989 0.9998 0.9996 0.9986

Table 5 Coefficients of Weber–Morris diffusion models. Initial AR18 concentration (mg L1)

120

240

360

Absorbent

SBA-15/CTS(5%) SBA-15/CTS(10%) SBA-15/CTS(20%) SBA-15/CTS(5%) SBA-15/CTS(10%) SBA-15/CTS(20%) SBA-15/CTS(5%) SBA-15/CTS(10%) SBA-15/CTS(20%)

First stage (outer diffusion)

Second stage (inner diffusion)

ki,1 (mg g1 min0.5)

R2

ki,2 (mg g1 min0.5)

R2

10.74 22.50 52.60 8.87 22.31 56.34 9.82 19.43 50.55

0.9935 0.9899 0.9961 0.9385 0.9901 0.9992 0.9755 0.9935 0.9745

3.36 5.24 7.67 3.67 6.45 9.00 3.63 5.93 8.17

0.9418 0.9542 0.9307 0.9974 0.9696 0.9400 0.9803 0.9501 0.9654

CTS content. For the adsorbents SBA-15/CTS(5%) and SBA-15/ CTS(10%), the time of inner diffusion is 20 and 50 min, respectively, regardless of the initial AR18 concentration applied. Differently, the adsorbent SBA-15/CTS(20%) shows a longer period of inner diffusion, namely 90 min at the initial concentration of 120 mg L1 and 110 min at 240 or 360 mg L1. As we explain previously, this is because the increase of CTS content enhanced the resistance of inner diffusion. For a given adsorbent (i.e. SBA-15/CTS(20%)), the CTS content and thereby the diffusion resistance were fixed, so a higher adsorption amount meant that the inner diffusion would take a longer time. From Fig. 9d–f, it is also easily found that the time taken by inner diffusion is much longer than that taken by outer diffusion. In the experiments on adsorption of AR18 onto SBA-15/CTS(20%) at the initial concentration of 120, 240, and 360 mg L1, the ratios of the time taken by outer diffusion to inner diffusion were about 1:9, 1:11, and 1:11, respectively. So, it can be said that the overall adsorption process was jointly controlled by outer diffusion and inner diffusion, but inner diffusion should be predominated over the outer mass transfer. The slope values of qt vs. t0.5 lines, along with R2 values are listed in Table 5. Obviously, all the R2 values (>0.9307) are close to unity, confirming the applicability of the Weber– Morris model. 3.4. Equilibrium adsorption analysis Equilibrium adsorption isotherm is of fundamental importance in optimizing the use of adsorbents, and the analysis of isotherm data by fitting them to different isotherm models is an important step to find the suitable model that can be used for design of adsorption system. In this study, Langmuir and Freundlich models were used to determine the adsorption equilibrium between these mesoporous adsorbents and AR18 molecules. The Langmuir equation is based on the assumption that the adsorption site is homogeneous in which each site accommodates one adsorbate molecule or ion; the adsorption is monolayer coverage, and there is no interaction between adsorbed molecules or

ions [39]. The Freundlich equation is an empirical equation explored for heterogeneous systems and is not restricted to the formation of a monolayer [39]. The linear forms of Langmuir, and Freundlich isotherm equations can be expressed as follows:

1 1 1 1 ¼ þ qe qm K L qm C e ln qe ¼ lnK F þ

1 ln C e n

ð7Þ

ð8Þ

where Ce is the equilibrium concentration of AR18 (mg L1), qe is the amount of AR18 adsorbed at equilibrium (mg g1), qm is the maximum adsorption capacity (mg g1), and KL (L g1) is the Langmuir isotherm constant; KF (mg11/n L1/n g1) and 1/n (dimensionless) are Freundlich constants representing adsorption capacity and adsorption intensity (level of favorability), respectively. When 1/n < 0.01, the adsorption is pseudo-irreversible; when 0.01 < 1/ n < 0.1, strong favorable; when 0.1 < 1/n < 0.5, favorable; when 0.5 < 1/n < 1, pseudo-linear; when 1/n = 1, linear; when 1/n > 1, unfavorable [40]. The validity of Langmuir and Freundlich models was checked. Fig. 10 shows the resulting plots of Freundlich model by constructing linear plots of lnqe vs. lnCe, which are associated with experimental data. It should be pointed out that the fitting curves of Langmuir model were not shown in Fig. 10 because they seriously deviated from the experimental data. The calculated values of isotherm parameters (qm, KL, KF, and n) along coefficient of determination R2 values are summarized in Table 6. In all cases, the Freundlich isotherm exhibits higher correlation coefficient (0.9410 < R2 < 0.9934) than Langmuir isotherm (0.4642 < R2 < 0.9015), indicating that the Freundlich isotherm is an appropriate description of the data for AR18 adsorption. As shown in Table 6, when the temperatures increase in the range of 303.15–343.15 K, the constant KF that is related to the adsorption capacity has values of 23.98–30.72, 67.97–76.40, and 163.86–168.06 mg11/n L1/n g1 for SBA-15/CTS(5%), SBA-15/

24

Q. Gao et al. / Microporous and Mesoporous Materials 193 (2014) 15–26

Fig. 10. Linear fitted isotherms for the adsorption of AR18 onto three different adsorbents: SBA-15/CTS(5%) (a); SBA-15/CTS(10%) (b); and SBA-15/CTS(20%) (c).

Table 6 Adsorption isotherm constants for AR18 adsorption onto three adsorbents. Absorbent

T (K)

Langmuir isotherm constants 1

qm (mg g SBA-15/CTS(5%)

SBA-15/CTS(10%)

SBA-15/CTS(20%)

303.15 323.15 343.15 303.15 323.15 343.15 303.15 323.15 343.15

40.3 39.8 31.8 96.6 96.9 89.2 201.2 211.9 218.3

)

Freundlich isotherm constants 1

KL (L g

)

3.07 1.09 17.01 1.92 1.27 0.85 0.012 0.051 0.035

CTS(10%), and SBA-15/CTS(20%), respectively. The results indicate that the adsorption of AR18 on CTS-modified adsorbents is favored at high CTS content. The constant 1/n values of SBA-15/CTS(5%) at 303.15–343.15 K are 0.1038–0.1189, which are in the range of 0.1– 0.5, representing favorable adsorption. At the same temperature range, the 1/n values of SBA-15/CTS(10%) and SBA-15/CTS(20%) are 0.062–0.067 and 0.052–0.053, respectively. Both of them are less than 0.1, implying that there are high affinities (strong favorability) between AR18 and each of the two adsorbents. The decreasing trend of 1/n values with increasing CTS content indicate that more CTS in adsorbent is capable of making stronger interactions between AR18 and adsorbent. In addition, the value of 1/n is also indicative of the heterogeneity of the adsorbent surface, with 1/n closer to 0 implying a heterogeneous surface (see Table 6).

2

R

KF (mg11/n L1/n g1)

1/n

R2

0.8111 0.7558 0.6907 0.8552 0.8677 0.9015 0.5200 0.4642 0.6779

30.72 27.31 23.98 76.40 74.07 67.97 168.06 167.63 163.86

0.104 0.112 0.119 0.062 0.063 0.067 0.053 0.052 0.052

0.9525 0.9531 0.9410 0.9557 0.9781 0.9531 0.9648 0.9745 0.9934

K0 ¼

C ad;e Ce

ð10Þ

where Cad,e is the amount of AR18 (mg) adsorbed on the adsorbent per liter of the solution at equilibrium, and Ce is the equilibrium concentration (mg L1) of AR18 in solution. Values of K0 can be obtained by plotting ln(Cad,e/Ce) versus Ce based on a least–squares analysis and extrapolating Ce to zero [41]. The intersection with the vertical axis gives the value of K0. Subsequently, the values of DG0 can be obtained from Eq. (9). Then, the values of DG0 obtained were further used for the evaluation of DH0 and DS0 using the following relationship [32]:

DG0 ¼ DH0  T DS0

ð11Þ 0

3.5. Thermodynamic analysis Thermodynamic considerations of an adsorption process can give very valuable insight into nature of the uptake process such as its spontaneity, randomness, endothermicity or exothermicity etc. The Gibb’s free energy change, DG0, is the fundamental criterion of spontaneity. Adsorption can occur spontaneously at a given temperature if DG0 is negative, and vice versa. A negative value of enthalpy change (DH0) suggests that the adsorption phenomenon is exothermic while a positive value implies that the adsorption process is endothermic. The magnitude and sign of entropy change (DS0) can reflect the degree of disorder or randomness of adsorption process. As well known, DG0 is related to the equilibrium constant by the classical Van’t Hoff equation [32]:

DG0 ¼ RTlnK 0

Finally, the values of DH and DS can be obtained from the slope of the plot of DG0 versus T. The values of DG0, DH0, and DS0 were listed in Table 7. In all cases, the DG0 shows a negative value (4.28  11.23 kJ mol1) confirm the feasibility of the process and spontaneous adsorption of AR18 on CTS-modified SBA-15 adsorbents. Moreover, for any one of the CTS-modified SBA-15 adsorbents, the increase in negative value of DG0 with the decrease in temperature indicates that

Table 7 Thermodynamic parameters for AR18 adsorption onto three adsorbents. Absorbent

T (K)

DG (kJ mol1)

DH (kJ mol1)

DS (J mol1 K1)

SBA-15/CTS(5%)

303.15 323.15 343.15 303.15 323.15 343.15 303.15 323.15 343.15

7.56 5.92 4.28 8.03 7.17 6.31 11.23 10.78 10.33

32.40

81.96

24.50

48.20

18.05

22.51

SBA-15/CTS(10%)

ð9Þ

where R is universal gas constant (8.314 J mol1 K1), T is the absolute temperature (K). K0 is the thermodynamic equilibrium constant, which can be calculated by the following equation:

0

SBA-15/CTS(20%)

25

Q. Gao et al. / Microporous and Mesoporous Materials 193 (2014) 15–26 Table 8 Comparison of AR18 adsorption capacity onto various adsorbents. Adsorbent

Maximum adsorption capacity (mg g1)

Conditions (pH value and temperature)

KF (mg11/n L1/n g1)

1/n

Ref.

MWCNTs Activated carbon from poplar wood Chitosan films Bentonite based composite Acidic treated pumice Copper(II) complex of dithiocarbamate-modified starch SBA-15/CTS(20%)

166.67 3.91 187.5–239 69.8 29.7 99.14 232.6

pH pH pH – pH pH pH

42.27 1.51 60–134.1 43.81 12.17 105.79 168.06

0.32 0.22 0.29–0.47 0.127 0.185 0.049 0.053

[17] [18] [19] [20] [21] [22] This work

the adsorption process becomes more favorable at lower temperatures. As we discussed in the previous section, this is possibly because the increase of temperature enhanced the mobility of AR18 molecules previously adsorbed, which generated a trend of desorbing AR18 from adsorbent surface. The magnitude of DG0 may also give an idea about the type of adsorption. Generally, the magnitude of DG0 for physical adsorption is smaller than that of chemisorption. The former ranges from 20 to 0 kJ mol1, and the latter ranges from 80 to 400 kJ mol1 [42]. So, it seems that adsorption of AR18 onto CTS-modified SBA-15 was a physical process. Negative values of DH0 (18.05  32.40 kJ mol1) shown in Table 7 suggest that the interaction of AR18 adsorbed by CTSmodified SBA-15 composites is exothermic, which is supported by the increasing adsorption of AR18 with the decrease in temperature. From Table 7, it can be also found that the DS0 shows negative values of 22.51  81.96 J mol1 K1, which suggests the randomness at the solid–solution interface in the adsorption system decreases during the adsorption process. In all cases, the decrease in entropy change might be attributed to the fact that the adsorbate molecules lose at least one degree of freedom when adsorbed [43]. The data given in Table 7 shows that |DH0| > |TDS0| for three adsorbents at all temperatures. This indicates that the adsorption processes were dominated by enthalpic rather than entropic changes [44].

7; 298.15 K 7; 293.15 K 3.5; 298–328 K 3.5; 293.15 K 3; 298.15–318.15 K 2.0; 343.15 K

at the initial stage followed by the inner diffusion during the subsequent period. Adsorption isotherms indicated that the AR18 adsorption obeyed Freundlich model better than Langmuir model. Thermodynamic analysis suggested the AR18 adsorption was a spontaneous, endothermic, and entropy-reduction process. While more works are still required to determine the adsorption performance of CTS-modified SBA-15 for other adsorbates such as heavy metal ions (e.g., Ag+, Cu2+, etc.), biomolecules (e.g., amino acids, proteins and enzymes), and drug molecules (e.g., ibuprofen, aspirin, etc.), our work have disclosed the potential of these mesoporous composites for such applications. Acknowledgements The authors acknowledge the research Grant provided by the Fundamental Research Funds for the Central Universities, China University of Geosciences (Wuhan) (No. CUG120115), Special Fund for Basic Scientific Research of Central Colleges, China University of Geosciences (Wuhan) (No. CUGL090305), and Land Resources Geology Survey Projects of China (Grant No. 12120113015300), and National Natural Science Foundation of China (Grant No. 21303170) References

3.6. Comparison of various adsorbents The adsorption capacities of AR18 by SBA-15/CTS(20%) and other adsorbents were summarized in Table 8. It is difficult to compare SBA-15/CTS(20%) directly with other adsorbents because of the different applied experimental conditions. Nevertheless, the AR18 uptake value obtained in this study is apparently higher than those of most of other adsorbents, somewhat indicating the SBA15/CTS(20%) has outstanding adsorption performance for AR18. In addition, the values of the Freundlich parameters KF and 1/n for the adsorption of AR18 in different adsorbents used in the literatures with SBA-15/CTS(20%) are also summarized in Table 8. It is found that the SBA-15/CTS(20%) has a highest KF value (168.06 mg11/n L1/n g1) but a very low 1/n value (0.053), further confirming the extraordinary adsorption characteristics of SBA15/CTS(20%). 4. Conclusion We have successfully fabricated a series of novel composites of natural polymer chitosan (CTS) and siliceous mesoporous SBA-15 with varying amounts of CTS. The adsorption properties of these mesoporous composites for AR18 were evaluated. The results showed that AR18 adsorption could achieve equilibrium quickly (<120 min) and the experimental maximum adsorption capacity could be up to 236.2 mg g1. All the adsorption processes could be better described using pseudo-second-order kinetics, and the adsorption rates were found to be controlled by outer diffusion

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