A kind of biocomposite was prepared by the intercalation of chitosan in bentonite and the cross-linking reaction of chitosan with glutaraldehyde, which was referred to as cross-linked chitosan/bentonite (CCS/BT) composite. Adsorptive removal of methyl orange (MO) from aqueous solutions was investigated by batch method. The adsorption of MO onto CCS/BT composite was affected by the ratio of chitosan to BT and contact time. pH value had only a minor impact on MO adsorption in a wide pH range. Adsorption kinetics was mainly controlled by the pseudo-second-order kinetic model. The adsorption of MO onto CCS/BT composite followed the Langmuir isotherm model, and the maximum adsorption capacity of CCS/BT composite calculated by the Langmuir model was 224.8 mg/g. Experimental results indicated that this adsorbent had a potential for the removal of MO from aqueous solutions.

INTRODUCTION

Synthetic dyestuffs widely exist in the effluents of industries such as textile, rubber, leather, and plastics. Discharging even a small amount of dyes into water resources can destroy the biotic environment of ecosystems. Therefore, several techniques are applied to remove dyes from wastewater, such as membrane filtration, oxidation, advanced oxidation, coagulation, biological treatment, electrochemical process, and adsorption Among these methods, adsorption can be thought to be the most efficient process for the treatment of wastewater owing to high removal efficiency for different types of effluents, ease of operation, availability of a variety of cheap adsorbents, and the absence of sludge and harmful by-product formation (Hena 2010). At present, many types of adsorbents like alum sludge (Tripathy et al. 2006), chitosan (Bansiwal et al. 2009; Pitakpoolsil & Hunsom 2013), clays (Sun et al. 2013; Zha et al. 2013; Fan et al. 2014), biocomposite (Saravanan et al. 2013; Nair et al. 2014), and agricultural waste (Nguyen et al. 2013; Cao et al. 2014) have been tried. Among these materials, biocomposite has attracted great attention because of its high adsorption capacity and good mechanical strength. Chitosan, a natural polymer, is an excellent biosorbent for the removal of metal ions and dyes from wastewater due to its non-toxicity, biocompatibility, and biodegradability. Also, chitosan possesses various functional groups in its structure such as amine, hydroxyl, and acetamide groups that can be involved in the adsorption process (Wan Ngah et al. 2011). However, chitosan still has some drawbacks such as poor mechanical strength, easy agglomeration, and its solubility in dilute acids. To overcome these problems, the synthesis of biocomposite has presented a novel approach. Recently composites of chitosan such as chitosan–zeolite (Wan Ngah et al. 2012, 2013; Nesic et al. 2013), chitosan–fly ash (Wen et al. 2011), and chitosan–montmorillonite (Nesic et al. 2012; Pereira et al. 2013) were prepared in various ways in order to remove environmental pollutants from wastewater. Clay materials are made of hydrous aluminosilicates, which have high cation exchange capacity and large-specific surface areas. Moreover, clay minerals are abundant, chemically and mechanically stable, and low-cost, making it an attractive immobilization material for chitosan (Nesic et al. 2012). Among the mineral clays, bentonite has been highlighted for the preparation of chitosan composites because of its high cationic exchange capacity and the possibility of lamellar expansion (Anirudhan et al. 2010; Anirudhan & Rijith 2012), which provides greater versatility in the interaction and intercalation of bulky molecules such as natural polymers. Therefore, in this work, a combination of physical and chemical modifications was applied in the preparation of cross-linked chitosan/bentonite (CCS/BT) composite, where chitosan would intercalate in bentonite and be cross-linked with glutaraldehyde (GLA). Methyl orange (MO) was chosen as a model dye in this study, because it is a common water-soluble azo dye, which is widely used in chemical, textile, and paper industries, and it is a serious hazard to the environment. The color index number of MO is 13025. Removal of MO from aqueous solutions was investigated by batch method. The factors affecting the adsorption of MO onto CCS/BT composite including the ratio of chitosan to bentonite, initial pH value of MO solution, and contact time were investigated in detail.

MATERIALS AND METHODS

Materials

Chitosan was purchased from Sinopharm Group Chemical Reagent Company Limited (China). Bentonite powder with a particle size of 200 mesh was acquired from the chemical factory of Shentai, Xinyang, Henan, China. MO was supplied by Sigma Chemical Company, and used as adsorbate in this study. MO concentrations were measured using a UV–Vis spectrometer at a wavelength corresponding to the maximum absorbance for MO: 464 nm. All other reagents were of analytical grade. The pH value of MO solutions was adjusted by adding 0.1 M HCl or NaOH solutions.

Preparation of CCS/BT composite

CCS/BT composite was prepared according to the references (Tirtom et al. 2012; Pereira et al. 2013) with some modifications. Chitosan powder (2 g) was dissolved in 100 mL of 2% (wt/v) acetic acid solution. About 2 g of bentonite was added and then mixed for 2 h at 20 °C. GLA solution (25 wt%) was added into the mixed solution and the ratio of GLA to chitosan was approximately 5 mL/g of chitosan. Cross-linking reaction occurred for 23 h at 60 °C and then the CCS/BT composite was washed using distilled water to remove any free GLA and dried in an oven at 60 °C for 24 h. After drying, this composite was ground and sieved. The composite with 200 mesh particle size was collected and utilized for the batch experiments. The resultant composite was referred to as CCS/BT composite.

Characterization of CCS/BT composite

X-ray diffraction (XRD) patterns were obtained for natural bentonite and CCS/BT composite using a Shimadzu XD3A diffractometer equipped with a monochromatic Cu Ka source operating at 40 kV and 30 mA. The diffraction patterns were recorded from 3 ° to 55 ° with a scan rate of 0.02 °/s.

Adsorption of MO onto CCS/BT composite

Batch experiments were performed in a set of 250 mL flasks that contained 50 mL MO solutions of different initial concentrations and 0.03 g adsorbent. The flasks were then agitated at 200 rpm on a thermostated shaker. After a defined time, the supernatant was filtered through filter paper to determine the residual concentrations. The adsorbate concentrations in the initial and residual aqueous solutions were measured to calculate the removal efficiency and adsorption capacity. To study the effect of the ratio of chitosan to bentonite on MO adsorption, adsorption experiments were conducted at 293 K by using 50 mL of solution with MO concentration of 100 mg/L and 0.03 g of adsorbent dosage. To study the effect of initial pH value on MO adsorption by CCS/BT composite, adsorption experiments were performed at 293 K by using 50 mL of solution with MO concentrations of 50 and 100 mg/L and 0.03 g of adsorbent dosage. Effect of contact time was studied at 293 K, with initial MO concentrations of 100, 150, 200, and 400 mg/L, by varying contact time from 5 to 120 min. All experimental data were the average of triplicate determinations and the relative errors were about 5%.

Adsorption kinetics

Experimental data generated from MO adsorption tests using CCS/BT composite were evaluated by pseudo-first- and second-order kinetic, Elovich kinetic and intraparticle diffusion models to understand the dynamics of the adsorption process. The pseudo-first-order kinetic model, also known as the Lagergren kinetic equation as expressed in Equation (1), is widely employed to understand the kinetic behavior of the adsorption reactions (Ho 2004) 
formula
1
where qe and qt are the amounts of adsorbate adsorbed (mg/g) at equilibrium and at contact time t (min), respectively, and k1 is the rate constant of the pseudo-first-order adsorption (min−1). The pseudo-second-order kinetic model is given by Equation (2) (Repo et al. 2013) 
formula
2
where k2 is the rate constant of the pseudo-second-order adsorption (g/(mg min)). In reactions involving chemisorption of adsorbates on solid surface, with no desorption products, the sorption process may be described by the Elovich equation, given by Equation (3) (Kellner et al. 1998) 
formula
3
where α is the initial adsorption rate (mg/(g min)) and β is related to the extent of surface coverage and activation energy for chemisorption (g/mg). The pseudo-first-order and pseudo-second-order kinetic models cannot identify the diffusion mechanism, so kinetic results need to be analyzed by the intraparticle diffusion model. The intraparticle diffusion was explored by using Equation (4) (Weber & Morris 1964) 
formula
4
where kp is the rate constant of the intraparticle diffusion (mg/(g min0.5)) and B is a constant that gives an idea about the thickness of the boundary layer, i.e., the larger the value of B, the greater is the boundary layer effect.

Isothermal study

Adsorption isotherms were used to evaluate the equilibrium data. Langmuir and Freundlich models were often used to analyze the data in solid–liquid adsorption systems. The Langmuir adsorption isotherm is based on the assumption that the adsorbate molecules form a monolayer on the surface of the adsorbent. Adjacent adsorbed molecules do not interact; i.e., the adsorption of adsorbate at a particular site is independent whether or not the neighboring sites are adsorbed. The Langmuir isotherm can be represented as Equation (5) (Harmoudi et al. 2014) 
formula
5
where Ce is the MO concentration at equilibrium (mg/L), qe is the amount of MO adsorbed per unit of adsorbent at equilibrium (mg/g), Q is the maximum amount of adsorption with complete monolayer coverage on the adsorbent surface (mg/g), and b is the Langmuir constant, which is related to the energy of adsorption (L/mg). The Langmuir constant can be estimated from the linear plot of Ce/qe versus Ce. The Freundlich isotherm is applicable for heterogeneous adsorption reactions and involves the formation of a multilayer. The Freundlich isotherm can be represented as Equation (6) (Li et al. 2012): 
formula
6
where Kf ((mg/g) (L/mg)1/n) and n are Freundlich constants related to adsorption capacity and heterogeneity factor, respectively. The values of Kf and n can be obtained from the slope and intercept of the plot of logqe versus logCe.

RESULTS AND DISCUSSION

Characterization of CCS/BT composite

Figure 1 shows the XRD patterns of natural bentonite and CCS/BT composite. XRD analysis can tell whether chitosan entered into the interlayer spacing of bentonite. The XRD pattern for bentonite exhibited a typical reflection of montmorillonite at 2θ = 6.56 °. After bentonite interacted with chitosan, this peak shifted to a lower diffraction angle, suggesting chitosan intercalated in bentonite. Also, a decrease in the intensity of this peak was observed. This decrease may be attributed to a slight distortion of the intrinsic lattice arrangement of the silicate layers, and a decrease in the crystallinity caused by the interaction of bentonite with chitosan. These results indicated the incorporation of chitosan into the interlayer spacing of bentonite. A similar result was also observed by Pereira et al. (2013) for the intercalation of chitosan into the interlayer spacing of KSF-Na montmorillonite. This increase in interlayer spacing would facilitate the adsorption of adsorbate onto adsorbent.

Figure 1

XRD patterns of natural bentonite (a) and cross-linked chitosan/bentonite composite (b).

Figure 1

XRD patterns of natural bentonite (a) and cross-linked chitosan/bentonite composite (b).

Effect of the ratio of chitosan to bentonite on MO adsorption

Figure 2 shows the effect of the ratio of chitosan to bentonite in CCS/BT composite on MO adsorption. The composite allowed higher removal efficiencies toward MO as compared with both cross-linked chitosan and bentonite. The maximum removal efficiency was observed at the ratio of 2/2. This dramatically high removal efficiency may be due to the big interlayer spacing resulting from the intercalation of chitosan in bentonite and the strong interaction between this composite and dye molecules. However, the removal efficiency toward MO reduced with a decrease in this ratio, suggesting that increasing content of bentonite in CCS/BT composite was adverse to the adsorption of MO onto CCS/BT composite. Therefore, the ratio of chitosan to bentonite in CCS/BT composite was controlled at 2/2 in this study.

Figure 2

Effect of ratio of chitosan to bentonite on MO adsorption.

Figure 2

Effect of ratio of chitosan to bentonite on MO adsorption.

Effect of pH value of MO solution

The initial pH value of dye solution is recognized as an important operational parameter which can significantly affect the adsorption mechanism between the dye molecules and the adsorbent. The effect of pH value of MO solutions on MO adsorption is shown in Figure 3. The results revealed that the changes of pH values appeared to have little impact on MO adsorption using CCS/BT composite. Over 95 and 92% removal efficiencies for MO solutions with 50 and 100 mg/L were obtained at pH 3–11. Only a slight decrease in MO adsorption was observed as the pH increased over 10. Two possible mechanisms of MO adsorption on CCS/BT composite may be occurring: (a) electrostatic interactions between amine groups from chitosan and anionic dye and (b) chemical reaction through various reactive groups from MO and adsorbent: i.e., MO can form hydrogen bonds with the OH groups of chitosan owing to the high electronegativity of its O, N, and S atoms. In acidic medium, the NH2 groups of chitosan were protonated into −NH3+. The strong electrostatic interaction between positively charged −NH3+ and anion groups of dye resulted in high removal efficiency. In alkaline solution, the NH2 groups of chitosan were deprotonated and abundant OH ions created a competitive environment with anion groups of MO for the adsorption sites; thus the removal efficiency decreased. Yao et al. (2011) also reported a similar result about the MO adsorption onto multi-walled carbon nanotubes. However, more than 90% MO can be removed by CCS/BT composite at natural pH. Therefore, the pH value of MO solutions was unadjusted in later adsorption experiments.

Figure 3

Effect of initial pH of MO solution on MO adsorption.

Figure 3

Effect of initial pH of MO solution on MO adsorption.

Effect of contact time

Equilibrium time is one of the most important parameters in the design of economical wastewater treatment systems (Chen et al. 2010). To investigate the kinetics of adsorption, four different initial concentrations of MO were chosen, 100, 150, 200, and 400 mg/L. Figure 4 shows these plots of adsorption capacity (qt) of MO onto CCS/BT composite versus contact time for different initial MO concentrations. As seen in Figure 4, qt value at equilibrium time increased evidently from 156.8 to 219.3 mg/g with the increase in initial concentration from 100 to 400 mg/L. The initial concentration played an important role in the adsorption capacity of MO on CCS/BT composite. An increase in the adsorption capacity of MO with increasing initial MO concentration might be attributed to an increase in the driving force of concentration gradient with the increase in the initial concentration (Wu et al. 2001). The adsorption was initially rapid, and then slowed, perhaps because a large number of vacant surface sites were available for adsorption during the initial stage, and then the remaining vacant surface sites were difficult to occupy because of the repulsive forces between the dye molecules on the CCS/BT composite and the bulk phase. For the sake of the attainment of equilibrium, an equilibrium time of 100 min was considered to be optimum in further experiments.

Figure 4

Effect of contact time on MO adsorption.

Figure 4

Effect of contact time on MO adsorption.

Adsorption kinetics

The kinetics of MO adsorption on CCS/BT composite was studied at natural pH and 293 K for several initial concentrations from 100 to 400 mg/L under predetermined time intervals. Kinetic models were applied on the experimental data to investigate the potential rate-determining step of the adsorption process. Table 1 presents the parameters for pseudo-first-order, pseudo-second-order, and Elovich kinetic models of MO adsorption on CCS/BT composite, while in Figure 5 are shown all the investigated kinetic models. Data presented in Table 1 show that the adsorption process of MO on CCS/BT composite followed the pseudo-second-order rather than pseudo-first-order model. The experimentally calculated values of qe at various concentrations of MO were in a good agreement with theoretical calculated values. Further, the values of correlation coefficients (R2) for the pseudo-first-order kinetic model were lower than the ones for the pseudo-second-order kinetic model. Also, relatively high values of correlation coefficients for the Elovich equation were observed, indicating that the chemisorption involving valence forces through sharing or exchange of electrons between MO and active sites on the surface of CCS/BT composite occurred. Also, it was not difficult to find that these plots of qt versus the square root of time obtained were not linear over the whole time range, indicating that more than one step was involved in the adsorption of MO. If intraparticle diffusion is the only rate-controlling step, the plot should pass through the origin, otherwise the boundary layer diffusion affects the adsorption to some degree. These plots were not passing through the origin, indicating that intraparticle diffusion was not the only rate-determining factor, and more than one step was involved in the adsorption of MO. Table 2 presents a comparison of intraparticle diffusion rate parameters for the adsorption of MO onto CCS/BT composite. The results indicated kp1 > kp2 > kp3. This was not surprising because the concentration of MO left in the solutions gradually decreased with increasing adsorption time.

Table 1

Pseudo-first-order, pseudo-second-order and Elovich kinetic models parameters for the adsorption of MO onto cross-linked chitosan/bentonite composite

C0/mg/L qe,exp/mg/g Pseudo-first-order model
 
Pseudo-second-order model
 
Elovich model
 
R2 k1/min−1 qe,cal/mg/g R2 k2/g/(mg min) qe,cal/mg/g R2 β/g/mg α/mg/(g min) 
100 161.3 0.9455 1.8571 148.7 0.9998 0.0022 163.9 0.9701 0.0674 6404 
150 177.9 0.9869 2.3422 167.3 0.9998 0.0017 182.8 0.9767 0.0600 8052 
200 184.2 0.9884 2.2481 175.2 0.9999 0.0018 188.7 0.9645 0.0607 14086 
400 226.3 0.8526 1.3478 217.4 0.9996 0.0013 231.5 0.9709 0.0608 135578 
C0/mg/L qe,exp/mg/g Pseudo-first-order model
 
Pseudo-second-order model
 
Elovich model
 
R2 k1/min−1 qe,cal/mg/g R2 k2/g/(mg min) qe,cal/mg/g R2 β/g/mg α/mg/(g min) 
100 161.3 0.9455 1.8571 148.7 0.9998 0.0022 163.9 0.9701 0.0674 6404 
150 177.9 0.9869 2.3422 167.3 0.9998 0.0017 182.8 0.9767 0.0600 8052 
200 184.2 0.9884 2.2481 175.2 0.9999 0.0018 188.7 0.9645 0.0607 14086 
400 226.3 0.8526 1.3478 217.4 0.9996 0.0013 231.5 0.9709 0.0608 135578 
Table 2

Intraparticle diffusion parameters for the adsorption of MO onto cross-linked chitosan/bentonite composite

C0/mg/L Kp1/mg/(g min0.5Kp2/mg/(g min0.5Kp3/mg/(g min0.5B1/mg/g B2/mg/g B3/mg/g R12 R22 R32 
100 9.48 1.21 0.11 98.56 140.4 159.1 0.9909 0.9455 0.9237 
150 18.45 4.71 0.82 82.18 134.9 186.1 0.9886 0.9871 0.8958 
200 15.81 3.66 0.53 102.0 160.3 183.7 0.9667 0.9321 0.8847 
400 9.87 2.36 0.04 157.3 172.8 232.8 0.9547 0.9975 0.9186 
C0/mg/L Kp1/mg/(g min0.5Kp2/mg/(g min0.5Kp3/mg/(g min0.5B1/mg/g B2/mg/g B3/mg/g R12 R22 R32 
100 9.48 1.21 0.11 98.56 140.4 159.1 0.9909 0.9455 0.9237 
150 18.45 4.71 0.82 82.18 134.9 186.1 0.9886 0.9871 0.8958 
200 15.81 3.66 0.53 102.0 160.3 183.7 0.9667 0.9321 0.8847 
400 9.87 2.36 0.04 157.3 172.8 232.8 0.9547 0.9975 0.9186 
Figure 5

Adsorption kinetic models of MO by cross-linked chitosan/bentonite composite: (a) pseudo-first-order kinetic model; (b) pseudo-second-order kinetic model; (c) Elovich model; (d) intraparticle diffusion model.

Figure 5

Adsorption kinetic models of MO by cross-linked chitosan/bentonite composite: (a) pseudo-first-order kinetic model; (b) pseudo-second-order kinetic model; (c) Elovich model; (d) intraparticle diffusion model.

Adsorption isotherm

The isotherm study was carried out by varying the initial concentration (100–400 mg/L) at 293 K and natural pH. Isotherm models, namely Langmuir model and Freundlich model, were used to describe the interaction between the solute and the adsorbent. The Langmuir and Freundlich isotherm parameters and their correlation coefficients (R2) are shown in Table 3, while the plots for both isotherm models are presented in Figure 6. According to the results, the Langmuir model seemed to describe adsorption better than the Freundlich model isotherm in respect to these correlation coefficients obtained for both models.

Table 3

Langmuir and Freundlich isotherm parameters of MO adsorption onto cross-linked chitosan/bentonite composite

Isotherms Parameters 293 K 303 K 313 K 323 K 
Langmuir isotherm 224.8 230.9 211.4 215.0 
b 0.0936 0.1090 0.6123 0.1575 
R2 0.9942 0.9970 0.9998 0.9991 
χ2 0.1457 1.7689 0.5585 1.2679 
Freundlich isotherm Kf 143.6 144.8 139.1 150.0 
n 14.48 14.76 11.80 14.84 
R2 0.9930 0.9293 0.9837 0.9665 
χ2 1.696 6.221 3.077 2.534 
Isotherms Parameters 293 K 303 K 313 K 323 K 
Langmuir isotherm 224.8 230.9 211.4 215.0 
b 0.0936 0.1090 0.6123 0.1575 
R2 0.9942 0.9970 0.9998 0.9991 
χ2 0.1457 1.7689 0.5585 1.2679 
Freundlich isotherm Kf 143.6 144.8 139.1 150.0 
n 14.48 14.76 11.80 14.84 
R2 0.9930 0.9293 0.9837 0.9665 
χ2 1.696 6.221 3.077 2.534 
Figure 6

Adsorption isotherm models of MO onto cross-linked chitosan/bentonite composite: Langmuir (a) and Freundlich (b).

Figure 6

Adsorption isotherm models of MO onto cross-linked chitosan/bentonite composite: Langmuir (a) and Freundlich (b).

To identify a suitable isotherm model for the adsorption of MO onto CCS/BT composite, chi-square analysis was carried out. The equivalent mathematical statement is presented as follows (Subramanyam & Das 2009): 
formula
7
where qe,cal is equilibrium capacity obtained by calculating from the model (mg/g) and qe is the experimental data of the equilibrium capacity(mg/g). If data from the model are similar to the experimental data, χ2 will be a small number, while if they differ, χ2 will be a bigger value. The results of chi-squared analysis are presented in Table 3. The lower χ2 values of the Langmuir isotherm than the Freundlich further confirmed the applicability of the Langmuir model for the sorption of MO on CCS/BT composite. The Langmuir model predicts the formation of a monolayer of the adsorbate on the adsorbent surface (Vasconcelos et al. 2008). The maximum monolayer adsorption (Qmax) of MO obtained by CCS/BT composite was 224.8 mg/g. In the present study, the adsorption capacity of CCS/BT was compared to other adsorbents in terms of MO removal. From Table 4, CCS/BT has a higher adsorption capacity, suggesting that it was a promising adsorbent for the adsorption of MO from aqueous solutions.
Table 4

Comparison of the maximum monolayer adsorption capacities of MO onto various adsorbents

Adsorbent Adsorption capacity (mg/g) Reference 
Mesoporous Y-Fe2O3/SiO2 nanocomposites 476 Deligeer et al. (2011)  
Activated carbon modified by silver nanoparticles 0.69 Pal et al. (2013)  
Calcined Lapindo volcanic mud 333.3 Jalil et al. (2010)  
Bentonite 33.8 Leodopoulos et al. (2012)  
Maghemite/chitosan films 29 Jiang et al. (2012)  
Chitosan/alumina composite 33 Zhang et al. (2011)  
Chitosan 34.83 Saha et al. (2010)  
Acid-modified carbon-coated monolith 147.1 Cheah et al. (2013)  
Cross-linked chitosan/bentonite composite 224.8 This study 
Adsorbent Adsorption capacity (mg/g) Reference 
Mesoporous Y-Fe2O3/SiO2 nanocomposites 476 Deligeer et al. (2011)  
Activated carbon modified by silver nanoparticles 0.69 Pal et al. (2013)  
Calcined Lapindo volcanic mud 333.3 Jalil et al. (2010)  
Bentonite 33.8 Leodopoulos et al. (2012)  
Maghemite/chitosan films 29 Jiang et al. (2012)  
Chitosan/alumina composite 33 Zhang et al. (2011)  
Chitosan 34.83 Saha et al. (2010)  
Acid-modified carbon-coated monolith 147.1 Cheah et al. (2013)  
Cross-linked chitosan/bentonite composite 224.8 This study 

CONCLUSIONS

The aim of this study was to prepare and characterize CCS/BT composite, and investigate its ability to be used for the adsorption of MO. The result from XRD analysis showed CCS intercalated in bentonite. The adsorption capacity of MO onto CCS/BT composite was much higher than that of cross-linked chitosan or bentonite. Also, the adsorption of MO onto CCS/BT composite was dependent on the ratio of chitosan to bentonite and contact time. The pH value had a minor impact on MO adsorption in a wide pH range. The adsorption kinetics obeyed the pseudo-second-order model, and the isotherm followed the Langmuir model. The adsorption capacity calculated by the Langmuir model was 224.8 mg/g. The cost of adsorbent was reduced by compositing bentonite with chitosan, and its adsorption capacity improved due to the intercalation of chitosan in bentonite. Therefore, CCS/BT composite might be a promising adsorbent for the removal of MO from aqueous solutions.

ACKNOWLEDGEMENT

The financial support extended to the projects by the National Natural Science Foundation of China (Grant No. 51003086) is gratefully acknowledged.

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