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Adsorption of nitrobenzene from aqueous solution by MCM-41
Journal of Colloid and Interface Science 315 (2007) 80–86 www.elsevier.com/locate/jcis
Adsorption of nitrobenzene from aqueous solution by MCM-41 Qingdong Qin, Jun Ma ∗ , Ke Liu School of Municipal and Environmental Engineering, Harbin Institute of Technology, P.O. Box 2627, 202 Haihe Road, Harbin 150090, People’s Republic of China Received 24 April 2007; accepted 24 June 2007 Available online 30 June 2007
Abstract Adsorption of nitrobenzene onto mesoporous molecular sieves (MCM-41) from aqueous solution has been investigated systematically using batch experiments in this study. Results indicate that nitrobenzene adsorption is initially rapid and the adsorption process reaches a steady state after 1 min. The adsorption isotherms are well described by the Langmuir and the Freundlich models, the former being found to provide the better fit with the experimental data. The effects of temperature, pH, ionic strength, humic acid, and the presence of solvent on adsorption processes are also examined. According to the experimental results, the amount of nitrobenzene adsorbed decreases with an increase of temperature from 278 to 308 K, pH from 1.0 to 11.0, and ionic strength from 0.001 to 0.1 mol/L. However, the amount of nitrobenzene adsorbed onto MCM-41 does not show notable difference in the presence of humic acid. The presence of organic solvent results in a decrease in nitrobenzene adsorption. The desorption process shows a reversibility of nitrobenzene adsorption onto MCM-41. Thermodynamic parameters such as Gibbs free energy are calculated from the experimental data at different temperatures. Based on the results, it suggests that the adsorption is primarily brought about by hydrophobic interaction between nitrobenzene and MCM-41 surface. © 2007 Elsevier Inc. All rights reserved. Keywords: Nitrobenzene; Adsorption; MCM-41; Isotherm; Desorption
1. Introduction Nitrobenzene has been widely adopted in the manufacture of dyes, plastic, pesticides, explosives, pharmaceuticals, and intermediates in chemical synthesis industries for years. After use, nitrobenzene in solution is generally discharged to wastewater treatment plants where a larger proportion of it cannot be removed and finally is discharged into the surrounding aquatic environment, which tends to persist in the environment and poses a potential toxic threat to ecological and human health [1,2]. Therefore, a variety of possible treatment technologies such as adsorption, ozonation, and advanced oxidation processes have been taken into account for the purification of nitrobenzenecontaminated water [3–6]. Particularly, adsorption with granular activated carbon (GAC) materials is considered to be one of the most economical and efficient methods for controlling the nitrobenzene in water [7]. However, GAC is relatively flam-
* Corresponding author. Fax: +86 451 82368074.
E-mail address: [email protected] (J. Ma). 0021-9797/$ – see front matter © 2007 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2007.06.060
mable and difficult to regenerate. Thus, an alternative adsorbent for nitrobenzene removal is required. Adsorption of organic compounds onto siliceous materials has been investigated intensively by many researchers [8–11]. Siliceous materials such as zeolite, silica, and clay are found widespread in the environment and they are environmentfriendly materials for environmental contaminant remediation. In order to enhance the adsorption capacity, modifications of siliceous materials by cationic surfactants have been made in previous studies [12,13]. The modified materials exhibit excellent adsorption properties for organic pollutants like perchloroethylene, atrazine, lindane, and diazinone. On the other hand, the synthesis of ordered mesoporous silica with high surface area and high hydrophobicity has attracted much attention in several areas because of its possible industrial application as reaction catalyst, catalyst support, chemical sensor, adsorbent for environmentally hazardous chemicals, and electrical and optical devices [14,15]. MCM-41, one member of the mesoporous molecular sieve M41S family, possesses a high specific surface and a regular hexagonal array of cylindrical pores, which is largely used in shape-selective catalysis, selective adsorp-
Q. Qin et al. / Journal of Colloid and Interface Science 315 (2007) 80–86
tion and separation processes, and chemical sensors, as well as nanotechnology [15]. Such material is characterized by large surface areas, narrow pore size distribution, and moderate hydrophobic character. Recently, Cooper and Burch reported that M41S possessed large adsorption capacities for efficient elimination of both cyanuric acid and p-chlorophenol from aqueous solution and the adsorption capacity can be regenerated by ozonation [16]. Additionally, Wang et al. pointed out that MCM-22 could be an effective adsorbent for the removal of methylene blue, crystal violet, and rhodamine B from aqueous solution [17]. Nevertheless, there are very scarce reports on the use of M41S for nitrobenzene removal from water and more effort is required in this area in order for such adsorbent to be practically applied. Therefore, the purpose of the present research effort is to investigate the adsorption characteristics of nitrobenzene from aqueous solutions onto MCM-41 material. As a first step in the adsorption of nitrobenzene, it is necessary to quantify the equilibrium time during adsorption. In turn, adsorption isotherms are conducted at batch experiments under different temperatures and then thermodynamic parameters are calculated. Influencing parameters such as pH, ionic strength, humic acid, and the presence of solvent are evaluated to characterize the extent of nitrobenzene adsorption. Finally, the desorption of nitrobenzene by MCM-41 has been studied to determine the reversibility of adsorption. 2. Materials and methods 2.1. Materials Nitrobenzene (98%; without further purification) was purchased from Tianjin Third Chemical Factory. Cetyltrimethylammonium bromide (CTMAB) was obtained from Xilong Chemical. Tetramethyl orthosilicate (TMOS) was purchased from Tianjin Kermel Chemical. All solvents used here were pure analytical HPLC grade solvents. Humic acid was purchased from Shanghai Chemical. The initial pH of background solution was adjusted by introducing appropriate amounts of acid (HCl) or base (NaOH) solutions. Deionized distilled water was purified by Millipore Milli-Q system and the ionic strength (I ) was adjusted by NaCl solution. Unless otherwise stated, all reagents used in this study were analytical grade.
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surfactant molecules occluded in the pore system of the mesoporous silica. It was heated to 813 K at a rate of 1 K/min and kept for 5 h. The BET surface area of calcined MCM-41 was 712 m2 /g and the pHzpc (pH of zero point of charge) was 2.5. 2.3. Adsorption experiments The effect of contact time on nitrobenzene adsorption onto MCM-41 was studied based on the nitrobenzene concentration in the range from 2 to 16 µmol/L. The adsorption experiments were carried out by mixing 1.0 g MCM-41 samples with a 200ml aqueous solution in a 500-ml stirred flask at temperature of 298 K. Samples were taken out and filtrated by a glass fiber with 0.7-µm pore size at different times. Then the residual concentration of nitrobenzene was determined. The adsorption isotherm experiments of nitrobenzene onto MCM-41 were performed on the basis of a batch experiment. A given amount of adsorbent (0.05 g) was placed in a 50-ml flask, into which 10 ml of a nitrobenzene solution with varying concentrations was added. The experiments were performed in a temperature-controlled water bath shaker for 4 h at a mixing speed of 180 rpm. After the adsorption reached equilibrium, the solutions were filtered and analyzed for the remaining concentration of nitrobenzene. Prior to all experiments, nitrogen was purged into solution to determine the effect of dissolved oxygen (DO) on nitrobenzene adsorption, showing that DO had no influence on the adsorption. Then flowing experiments were undertaken under common conditions. The amount of nitrobenzene adsorbed onto MCM-41 was calculated from the mass balance equation as qe =
(C0 − Ce )V , M
(1)
where qe (µmol/g) is the amount of nitrobenzene adsorbed per gram of MCM-41 at equilibrium; C0 (µmol/L) and Ce (µmol/L) are the initial and equilibrium liquid-phase concentration of nitrobenzene, respectively; V (L) is the volume of nitrobenzene solution, and M (g) is the mass of MCM-41 used. To check reproducibility, nitrobenzene adsorption was carried out in duplicate. The relative deviations met with the requirement of less than 5%.
2.2. Preparation of MCM-41
2.4. Desorption experiments
The MCM-41 was synthesized in an acidic medium using conventional literature recipes [14]. Typically, CTMAB was dissolved in HCl solution, into which a TMOS solution was then added dropwise. The molar gel composition was as the following: 3.5 CTMAB:30 SiO2 :26.9 H+ :3800 H2 O. The white precipitate was homogenized by vigorous stirring for 2 h at ambient temperature and then heated at 373 K in a Teflon-coated stainless-steel autoclave for 48 h under autogenous pressure. Subsequently, the white precipitate was filtered, washed with distilled water, and dried at 353 K overnight. Finally, the assynthesized sample was calcined in air in order to remove the
Desorption isotherms were obtained from the adsorption samples in equilibrium. Solution containing MCM-41 was transferred into centrifuge tubes that were centrifuged at 4200g for 20 min with the centrifuge temperature being set at the incubation temperature. Then, 5 ml of the supernatant was removed and compensated for sampling by adding 5 ml of pure water or 0.1 mol/L solution of NaCl. The tubes were placed on a reciprocating shaker for 1 h at 298 K. Preliminary kinetic studies showed that desorption could reach equilibrium state within a time of 1 min. After shaking, the suspensions were centrifuged and 1 ml of the supernatant was collected for analysis.
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Fig. 1. Effect of contact time on nitrobenzene adsorption by MCM-41 (adsorption conditions: adsorbent dosage = 5 g/L, pH 5.8, and temperature = 298 K).
Fig. 2. Adsorption isotherm of nitrobenzene onto MCM-41 at different temperatures (adsorption conditions: adsorbent dosage = 5 g/L and pH 5.8).
2.5. Analytical method
equilibrium concentration of adsorbate, are usually represented by adsorption isotherms, which is of importance in the design of adsorption systems. The adsorption isotherms of nitrobenzene onto MCM-41 at temperatures of 278, 288, 298, and 308 K are shown in Fig. 2. All adsorption isotherms are nonlinear with curvatures concave to the abscissa. The adsorption of nitrobenzene onto MCM-41 decreases from 2.018 µmol/g (57.7% removal) to 1.092 µmol/g (31.2% removal) when temperature is increased from 278 to 308 K at an initial concentration of 17.5 µmol/L. The decrease in the equilibrium adsorption of nitrobenzene with temperature demonstrates that nitrobenzene removal by adsorption onto MCM-41 favors a low temperature. In order to describe the adsorption isotherm, two important isotherms are selected in this study, the Langmuir and Freundlich isotherms,
The concentration of nitrobenzene was determined by high performance liquid chromatography (Waters) equipped with UV-visible detection at a wavelength of 263 nm, using a symmetry C-18 column (4.6 × 150 mm, 5-µm spheres, Waters). The injection volume was 20 µl and the mobile phase was a mixture of methanol–water (60:40 v/v) with a rate of 1 ml/min. Under these conditions, the retention time for nitrobenzene in the column was 5.2 min. 3. Results and discussion 3.1. Effect of contact time on nitrobenzene adsorption As is illustrated by Fig. 1, an apparent adsorption equilibrium is generally obtained within 1 min for the initial nitrobenzene concentration of 2, 4, and 16 µmol/L, respectively. Thereafter, no detectable concentration changes occur (<30 min) after adsorption equilibrium (1 min) and the average removal efficiency of nitrobenzene reaches about 72.6, 64.5, and 56.1% when the initial concentrations of nitrobenzene are 2, 4, and 16 µmol/L, respectively, during this period. It is assumed here that the adsorption takes place primarily at easily accessible surface sites, requiring no diffusion into micropores. And a hydrophobic interaction between adsorbent and organic compounds may be attributed to the rapid adsorption rate [18]. Additionally, Fig. 1 also illustrates that the removal of nitrobenzene at adsorption equilibrium decreases with increasing initial nitrobenzene concentration. The reason is the limited number of adsorption sites available for the uptake of nitrobenzene at a fixed adsorbent dosage. 3.2. Adsorption isotherms Adsorption equilibrium data, expressed by the mass of adsorbate adsorbed per unit weight of adsorbent and liquid phase
qe =
Q0 KL Ce , 1 + KL Ce 1/n
qe = KF Ce ,
(2) (3)
where qe (µmol/g) is the amount of nitrobenzene adsorbed per gram of MCM-41 at equilibrium; Ce (µmol/L) the equilibrium concentration of nitrobenzene in solution; Q0 (µmol/g) the maximum monolayer adsorption capacity; KL (L/µmol) the constant related to the free energy of adsorption; KF [µmol/g(L/µmol)1/n ], a Freundlich isotherm constant for the system and the slope 1/n, ranging between 0 and 1, indicative of the degree of nonlinearity between solution concentration and adsorption. The isotherm parameters and linear regression obtained from the fitting curves by Langmuir and Freundlich models are given in Table 1. It is clear that adsorption isotherms at different temperatures can be fitted well using two isotherm models (evidenced from the correlation coefficients, >0.990). However, the Langmuir model is more suitable than the Freundlich model to describe the adsorption isotherm, as reflected with correlation coefficients. It suggests that the adsorbent is homogeneous, and the adsorption film is monomolecular. Then the following equations use Langmuir constants to calculate the
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Table 1 Parameters of adsorption isotherms of nitrobenzene onto MCM-41 at different temperatures Temperature (K)
Langmuir model Q0 (µmol/g)
KL (L/µmol)
R2
Freundlich model KF [µmol/g (L/µmol)1/n ]
1/n
R2
278 288 298 308
3.705 2.682 2.142 1.841
0.143 0.126 0.124 0.117
0.9988 0.9995 0.9993 0.9998
0.463 0.317 0.255 0.217
0.75 0.71 0.69 0.67
0.9980 0.9958 0.9950 0.9914
thermodynamic parameters. On the other hand, the maximum monolayer adsorption capacity, Q0 , defined the total capacity of MCM-41 for nitrobenzene adsorption and decreases with increasing temperature. Its maximum value is determined as 3.705 µmol/g at 278 K. A change in the maximum monolayer adsorption capacity with temperature is possibly due to the low hydrothermal stability of MCM-41 in water. At high temperatures, the structure of MCM-41 could collapse by mechanical compression through the hydrolysis of siloxane bonds in the presence of adsorbed water, which may decrease the amount of active sites [15]. Moreover, when the interaction is exothermic, the adsorbate has a tendency to escape from the solid phase to the solution. Thus, rise in temperature and excess energy supply will promote desorption. The free energy (G0 ) of nitrobenzene adsorption onto MCM-41 at different temperatures is calculated based on the adsorption isotherm by [19] G0 = −RT ln KL ,
(4)
where R (J/mol K) is the universal gas law constant; T (K) the absolute temperature of solution, and KL (L/mol) the Langmuir constant. The values of G0 are calculated as from −27.4 to −29.9 kJ/mol. It is noted that the G0 values are all negative, which indicate the feasibility and spontaneous adsorption of nitrobenzene onto MCM-41. The enthalpy change (H 0 ) and entropy change (S 0 ) could not be established because a linear relationship of ln KL and 1/T in the van’t Hoff equation could not be established. 3.3. Effect of pH on nitrobenzene adsorption The pH of solution is one of the most important parameters affecting the adsorption process. In order to determine the effect of pH on adsorption capacity of MCM-41, solutions were prepared at different pH values ranging from 1.0 to 11.0. The dependence of pH on the adsorption of nitrobenzene at an initial concentration of 17.5 µmol/L onto MCM-41 is illustrated in Fig. 3. Obviously, the amount of adsorbed nitrobenzene (qe ) is decreased by increasing the pH value. At pH 1.0, the uptake of nitrobenzene is 54.3% (1.90 µmol/g), while at pH 11.0, the uptake of nitrobenzene is only 18.1% (0.63 µmol/g). It is apparent that using solutions at pH value lower than 1.0 gives the highest qe value. For adsorption onto a solid surface, six adsorption mechanisms are believed to exist (i.e., electrostatic interaction, ion ex-
Fig. 3. Effect of solution pH on the adsorption of nitrobenzene onto MCM-41 (adsorption conditions: adsorbent dosage = 5 g/L, temperature = 298 K, initial nitrobenzene concentration = 17.5 µmol/L, and ionic strength = 0.1 mol/L).
change, ion–dipole interactions, coordination by surface metal cations, hydrogen bonding, and hydrophobic interaction [20]). Since nitrobenzene is an unionizable compound, electrostatic interaction and ion-exchange mechanisms are negligible. The ion–dipole interactions between the charged surface and the nonionic nitrobenzene are also expected to be negligible in this experiment. Coordination by surface metal cations is important only when the organic ligand is a good electron donor relative to water. Therefore, the adsorption mechanisms can be contributed by hydrophobic interaction and hydrogen bonding. Nitrobenzene is an unionizable compound. Thus, the effect of pH on the adsorption of nitrobenzene onto MCM-41 may result from a changed MCM-41 surface. Based on the conclusions of Lu et al. [20], the maximum extent of nitrobenzene adsorption is at a pH value close to its pHzpc (2.5) if hydrogen bond interaction is the dominant mechanism for nitrobenzene adsorption. However, it is noteworthy that the maximum extent of adsorption (Fig. 3) is not at a pH value around its pHzpc in this study. Hence, it can be inferred that a specific interaction like hydrophobic interaction at the interface occurs between nitrobenzene and MCM-41. The possible explanation for the sharp decrease in nitrobenzene adsorption with the increase of pH could be that MCM-41 material is relatively stable with high acid resistance, whereas it degrades readily in basic solution that will decrease the hydrophobic character and destroy the structure of MCM-41 [15]. 3.4. Effect of ionic strength and humic acid on nitrobenzene adsorption Since NaCl is often used as a stimulator in adsorption processes, the effect of ionic strength (adjusted by NaCl) on nitrobenzene adsorption has been determined and the results are presented in Fig. 4. It indicates that the amount of adsorbed nitrobenzene onto MCM-41 decreases with the increase of NaCl concentration of the solution. The Langmuir constants
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Fig. 4. Effect of ionic strength on the adsorption of nitrobenzene onto MCM-41 (adsorption conditions: adsorbent dosage = 5 g/L, pH 5.8, and temperature = 298 K).
Fig. 5. Effect of humic acid on the adsorption of nitrobenzene onto MCM-41 (adsorption conditions: adsorbent dosage = 5 g/L, pH 3.0, ionic strength = 0.1 mol/L, and temperature = 298 K).
(KL ) at NaCl concentrations of 0.001, 0.01, and 0.1 mol/L are 0.128, 0.127, and 0.116 L/µmol, respectively. As nitrobenzene is a nonpolar hydrophobic chemical, increasing ionic strength will lead to a decrease of the solubility of nitrobenzene in water. Therefore, for nitrobenzene, increasing ionic strength should usually be accompanied by an increase in partition into MCM-41. However, the amount of adsorbed nitrobenzene onto MCM-41 decreases with the increase of ionic strength. Given the pHzpc value, the surface of MCM-41 is expected to be dominantly negatively charged at the experimental pH of 5.8. It is beneficial for Na+ to be adsorbed onto MCM-41 by electrostatic interaction. It infers that the additive (NaCl) prefers to occupy the strongest sites on MCM-41, and competes with nitrobenzene for space in MCM-41 due to the surface heterogeneity, which leads to the decrease in the amount of nitrobenzene adsorption. Humic acid exists extensively in surface water and it interferes with the adsorption of trace organic compounds on porous adsorbents such as powdered activated carbon (PAC) by pore blockage and direct competition for adsorption sites [21]. The effect of humic acid on nitrobenzene adsorption onto MCM41 is shown in Fig. 5. The results suggest that the existence of humic acid at a concentration of 10 and 50 mg/L does not significantly affect the extent of nitrobenzene adsorption. The amount of humic acid adsorbed at concentrations of 10 and 50 mg/L is about 0.15 and 0.26 mg/g, respectively. This observation can be explained by the large humic acid molecule. As the molecule of humic acid is larger than the pore diameter (4.3 nm) of MCM-41, it would not be able to penetrate the pore spaces of MCM-41 to form pore blockage and compete for adsorption sites. Hence, it does not reduce the adsorption efficiency for nitrobenzene. Similar results were observed by other researchers who examined the adsorption of Geosmin and 2-Methylisoborneol by ultrastable zeolite-Y in the presence of humic acid [22].
Fig. 6. Effect of solvent on the adsorption of nitrobenzene onto MCM-41 (adsorption conditions: adsorbent dosage = 5 g/L, pH 5.8, and temperature = 298 K).
3.5. Adsorption of nitrobenzene from solvent–water solutions Adsorption of nitrobenzene from aqueous solutions containing 10% methanol or 10% acetone was studied. Methanol is a protic solvent with hydrogen bonding character. Acetone, less polar than methanol, is a dipolar aprotic solvent that cannot act as a hydrogen bond donor. The adsorption isotherm from solvent–water solutions is illustrated in Fig. 6. A decrease in the adsorption is evident when organic solvent is added in water solution. The maximum monolayer adsorption capacities (Q0 ) in water solution, methanol–water solution, and acetone– water solution are 2.14, 1.88, and 0.63 µmol/g, respectively. It also indicates that the presence of acetone appears to inhibit nitrobenzene adsorption to an extent greater than the presence of methanol.
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4. Conclusions
Fig. 7. Adsorption–desorption isotherms of nitrobenzene (adsorption conditions: adsorbent dosage = 5 g/L, pH 5.8, and temperature = 298 K).
The addition of organic solvent to water enhanced the interaction between the nitrobenzene and the solvent, resulting in a decrease in adsorption capacity. This could be attributed to the difference in proton affinity and solvation of these solvents. Thus, the inhibitory effect in less polar solution for nitrobenzene adsorption is greater than that in polar solution due to hydrophobic driving force being the dominant mechanism for nitrobenzene adsorption, where less polar solvent interacts with nitrobenzene more strongly than polar solvent. In other words, increasing nitrobenzene solubility in the mixed solvent results in a reduced driving force for nitrobenzene adsorption. Acetone interacts with nitrobenzene more strongly than methanol. Consequently, the hydrophobic driving force could be responsible for the adsorption of nitrobenzene onto MCM-41. 3.6. Desorption of nitrobenzene by MCM-41 Desorption is one of the key processes affecting the ultimate fate of contaminants in solids. In order to evaluate the reversibility of nitrobenzene adsorption onto MCM-41, desorption characteristics were also determined. Desorption isotherms of nitrobenzene adsorbed onto MCM-41 are shown in Fig. 7. There is no hysteresis in Milli-Q water between adsorption and desorption. It suggests that the adsorption and desorption are reversible and the exchange of nitrobenzene between bulk water and MCM-41 can be realized with a similar mechanism. Thus, desorption could be predicted on the basis of adsorption isotherms. However, since adsorption of nitrobenzene onto MCM-41 is ionic strength dependent, a change of suspension ionic strength would, of course, influence the desorption of adsorbed nitrobenzene. The observed desorption isotherm in 0.1 mol/L NaCl is similar to the adsorption isotherm obtained in 0.1 mol/L NaCl. It seems that after adsorption, there are no strong bonding forces at the interface between nitrobenzene and MCM-41. Hydrophobic interaction may be the significant bonding force in adsorption of nitrobenzene onto MCM-41.
Synthetic MCM-41 by a hydrothermal method was demonstrated to adsorb nitrobenzene from aqueous solution. The results showed that nitrobenzene could be adsorbed onto MCM-41 quickly. The data obtained from adsorption isotherms at different temperatures were fit to the Langmuir model. The maximum monolayer adsorption capacity for nitrobenzene decreased from 3.705 to 1.841 µmol/g with an increase of temperature from 278 to 308 K at pH 5.8. It was found that increasing pH from 1.0 to 11.0 decreased the adsorption capacities from 54.3 to 18.1%. The result was attributed to destroying the hydrophobic characteristic of MCM-41 in basic solutions. Increasing ionic strength from 0.001 to 0.1 mol/L decreased the extent of nitrobenzene adsorption from 2.12 to 1.81 µmol/g for ions mainly occupied the strongest sites on MCM-41 and led to a steric hindrance. The presence of humic acid had no influence on nitrobenzene adsorption and the presence of cosolvent (methanol and acetone) affected the adsorption process by a cosolvent effect. Desorption isotherms showed that adsorption and desorption of nitrobenzene were reversible. Combined with pH effects, it was concluded that hydrophobic interaction was the main force to adsorb nitrobenzene from aqueous solution. Further research is needed to improve the stability of MCM-41 in aqueous solution. Acknowledgment The authors are grateful for the financial support provided by the National Natural Science Foundation of China (NSFC, Project 50578051). References [1] S.B. Haderlein, R.P. Schwarzenbach, Environ. Sci. Technol. 27 (1993) 316. [2] X.K. Zhao, G.P. Yang, X.C. Gao, Chemosphere 52 (2003) 917. [3] S.A. Boyd, G.Y. Sheng, B.J. Teppen, C.T. Johnston, Environ. Sci. Technol. 35 (2001) 4227. [4] O.V. Makarova, T. Rajh, M.C. Thurnauer, A. Martin, P.A. Kemme, D. Cropek, Environ. Sci. Technol. 34 (2000) 4797. [5] A. Latifoglu, M.D. Gurol, Water Res. 37 (2003) 1879. [6] F.J. Beltrán, J.M. Encinar, M.A. Alonso, Ind. Eng. Chem. Res. 37 (1998) 25. [7] M. Franz, H.A. Arafat, N.G. Pinto, Carbon 38 (2000) 1807. [8] M.A. Anderson, Environ. Sci. Technol. 34 (2000) 725. [9] H.T. Shu, D.Y. Li, A.A. Scala, Y.H. Ma, Sep. Purif. Technol. 11 (1997) 27. [10] C.Y. Chang, W.T. Tsai, C.H. Ing, C.F. Chang, J. Colloid Interface Sci. 260 (2003) 273. [11] C.F. Chang, C.Y. Chang, K.H. Chen, W.T. Tsai, J.L. Shie, Y.H. Chen, J. Colloid Interface Sci. 277 (2004) 29. [12] Z.H. Li, R.S. Bowman, Environ. Sci. Technol. 32 (1998) 2278. ˇ [13] J. Lemi´c, D. Kovaˇcevi´c, M. Tomaševi´c-Canovi´ c, D. Kovaˇcevi´c, T. Stani´c, R. Pfend, Water Res. 40 (2006) 1079. [14] X.S. Zhao, G.Q. Lu, G.J. Millar, Ind. Eng. Chem. Res. 35 (1996) 2075. [15] P. Selvam, S.K. Bhatia, C.G. Sonwane, Ind. Eng. Chem. Res. 40 (2001) 3237. [16] C. Cooper, R. Burch, Water Res. 33 (1999) 3689. [17] S.B. Wang, H.T. Li, L.Y. Xu, J. Colloid Interface Sci. 295 (2006) 71.
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Adsorption of nitrobenzene from aqueous solution by MCM-41 Qingdong Qin, Jun Ma ∗ , Ke Liu School of Municipal and Environmental Engineering, Harbin Institute of Technology, P.O. Box 2627, 202 Haihe Road, Harbin 150090, People’s Republic of China Received 24 April 2007; accepted 24 June 2007 Available online 30 June 2007
Abstract Adsorption of nitrobenzene onto mesoporous molecular sieves (MCM-41) from aqueous solution has been investigated systematically using batch experiments in this study. Results indicate that nitrobenzene adsorption is initially rapid and the adsorption process reaches a steady state after 1 min. The adsorption isotherms are well described by the Langmuir and the Freundlich models, the former being found to provide the better fit with the experimental data. The effects of temperature, pH, ionic strength, humic acid, and the presence of solvent on adsorption processes are also examined. According to the experimental results, the amount of nitrobenzene adsorbed decreases with an increase of temperature from 278 to 308 K, pH from 1.0 to 11.0, and ionic strength from 0.001 to 0.1 mol/L. However, the amount of nitrobenzene adsorbed onto MCM-41 does not show notable difference in the presence of humic acid. The presence of organic solvent results in a decrease in nitrobenzene adsorption. The desorption process shows a reversibility of nitrobenzene adsorption onto MCM-41. Thermodynamic parameters such as Gibbs free energy are calculated from the experimental data at different temperatures. Based on the results, it suggests that the adsorption is primarily brought about by hydrophobic interaction between nitrobenzene and MCM-41 surface. © 2007 Elsevier Inc. All rights reserved. Keywords: Nitrobenzene; Adsorption; MCM-41; Isotherm; Desorption
1. Introduction Nitrobenzene has been widely adopted in the manufacture of dyes, plastic, pesticides, explosives, pharmaceuticals, and intermediates in chemical synthesis industries for years. After use, nitrobenzene in solution is generally discharged to wastewater treatment plants where a larger proportion of it cannot be removed and finally is discharged into the surrounding aquatic environment, which tends to persist in the environment and poses a potential toxic threat to ecological and human health [1,2]. Therefore, a variety of possible treatment technologies such as adsorption, ozonation, and advanced oxidation processes have been taken into account for the purification of nitrobenzenecontaminated water [3–6]. Particularly, adsorption with granular activated carbon (GAC) materials is considered to be one of the most economical and efficient methods for controlling the nitrobenzene in water [7]. However, GAC is relatively flam-
* Corresponding author. Fax: +86 451 82368074.
E-mail address: [email protected] (J. Ma). 0021-9797/$ – see front matter © 2007 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2007.06.060
mable and difficult to regenerate. Thus, an alternative adsorbent for nitrobenzene removal is required. Adsorption of organic compounds onto siliceous materials has been investigated intensively by many researchers [8–11]. Siliceous materials such as zeolite, silica, and clay are found widespread in the environment and they are environmentfriendly materials for environmental contaminant remediation. In order to enhance the adsorption capacity, modifications of siliceous materials by cationic surfactants have been made in previous studies [12,13]. The modified materials exhibit excellent adsorption properties for organic pollutants like perchloroethylene, atrazine, lindane, and diazinone. On the other hand, the synthesis of ordered mesoporous silica with high surface area and high hydrophobicity has attracted much attention in several areas because of its possible industrial application as reaction catalyst, catalyst support, chemical sensor, adsorbent for environmentally hazardous chemicals, and electrical and optical devices [14,15]. MCM-41, one member of the mesoporous molecular sieve M41S family, possesses a high specific surface and a regular hexagonal array of cylindrical pores, which is largely used in shape-selective catalysis, selective adsorp-
Q. Qin et al. / Journal of Colloid and Interface Science 315 (2007) 80–86
tion and separation processes, and chemical sensors, as well as nanotechnology [15]. Such material is characterized by large surface areas, narrow pore size distribution, and moderate hydrophobic character. Recently, Cooper and Burch reported that M41S possessed large adsorption capacities for efficient elimination of both cyanuric acid and p-chlorophenol from aqueous solution and the adsorption capacity can be regenerated by ozonation [16]. Additionally, Wang et al. pointed out that MCM-22 could be an effective adsorbent for the removal of methylene blue, crystal violet, and rhodamine B from aqueous solution [17]. Nevertheless, there are very scarce reports on the use of M41S for nitrobenzene removal from water and more effort is required in this area in order for such adsorbent to be practically applied. Therefore, the purpose of the present research effort is to investigate the adsorption characteristics of nitrobenzene from aqueous solutions onto MCM-41 material. As a first step in the adsorption of nitrobenzene, it is necessary to quantify the equilibrium time during adsorption. In turn, adsorption isotherms are conducted at batch experiments under different temperatures and then thermodynamic parameters are calculated. Influencing parameters such as pH, ionic strength, humic acid, and the presence of solvent are evaluated to characterize the extent of nitrobenzene adsorption. Finally, the desorption of nitrobenzene by MCM-41 has been studied to determine the reversibility of adsorption. 2. Materials and methods 2.1. Materials Nitrobenzene (98%; without further purification) was purchased from Tianjin Third Chemical Factory. Cetyltrimethylammonium bromide (CTMAB) was obtained from Xilong Chemical. Tetramethyl orthosilicate (TMOS) was purchased from Tianjin Kermel Chemical. All solvents used here were pure analytical HPLC grade solvents. Humic acid was purchased from Shanghai Chemical. The initial pH of background solution was adjusted by introducing appropriate amounts of acid (HCl) or base (NaOH) solutions. Deionized distilled water was purified by Millipore Milli-Q system and the ionic strength (I ) was adjusted by NaCl solution. Unless otherwise stated, all reagents used in this study were analytical grade.
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surfactant molecules occluded in the pore system of the mesoporous silica. It was heated to 813 K at a rate of 1 K/min and kept for 5 h. The BET surface area of calcined MCM-41 was 712 m2 /g and the pHzpc (pH of zero point of charge) was 2.5. 2.3. Adsorption experiments The effect of contact time on nitrobenzene adsorption onto MCM-41 was studied based on the nitrobenzene concentration in the range from 2 to 16 µmol/L. The adsorption experiments were carried out by mixing 1.0 g MCM-41 samples with a 200ml aqueous solution in a 500-ml stirred flask at temperature of 298 K. Samples were taken out and filtrated by a glass fiber with 0.7-µm pore size at different times. Then the residual concentration of nitrobenzene was determined. The adsorption isotherm experiments of nitrobenzene onto MCM-41 were performed on the basis of a batch experiment. A given amount of adsorbent (0.05 g) was placed in a 50-ml flask, into which 10 ml of a nitrobenzene solution with varying concentrations was added. The experiments were performed in a temperature-controlled water bath shaker for 4 h at a mixing speed of 180 rpm. After the adsorption reached equilibrium, the solutions were filtered and analyzed for the remaining concentration of nitrobenzene. Prior to all experiments, nitrogen was purged into solution to determine the effect of dissolved oxygen (DO) on nitrobenzene adsorption, showing that DO had no influence on the adsorption. Then flowing experiments were undertaken under common conditions. The amount of nitrobenzene adsorbed onto MCM-41 was calculated from the mass balance equation as qe =
(C0 − Ce )V , M
(1)
where qe (µmol/g) is the amount of nitrobenzene adsorbed per gram of MCM-41 at equilibrium; C0 (µmol/L) and Ce (µmol/L) are the initial and equilibrium liquid-phase concentration of nitrobenzene, respectively; V (L) is the volume of nitrobenzene solution, and M (g) is the mass of MCM-41 used. To check reproducibility, nitrobenzene adsorption was carried out in duplicate. The relative deviations met with the requirement of less than 5%.
2.2. Preparation of MCM-41
2.4. Desorption experiments
The MCM-41 was synthesized in an acidic medium using conventional literature recipes [14]. Typically, CTMAB was dissolved in HCl solution, into which a TMOS solution was then added dropwise. The molar gel composition was as the following: 3.5 CTMAB:30 SiO2 :26.9 H+ :3800 H2 O. The white precipitate was homogenized by vigorous stirring for 2 h at ambient temperature and then heated at 373 K in a Teflon-coated stainless-steel autoclave for 48 h under autogenous pressure. Subsequently, the white precipitate was filtered, washed with distilled water, and dried at 353 K overnight. Finally, the assynthesized sample was calcined in air in order to remove the
Desorption isotherms were obtained from the adsorption samples in equilibrium. Solution containing MCM-41 was transferred into centrifuge tubes that were centrifuged at 4200g for 20 min with the centrifuge temperature being set at the incubation temperature. Then, 5 ml of the supernatant was removed and compensated for sampling by adding 5 ml of pure water or 0.1 mol/L solution of NaCl. The tubes were placed on a reciprocating shaker for 1 h at 298 K. Preliminary kinetic studies showed that desorption could reach equilibrium state within a time of 1 min. After shaking, the suspensions were centrifuged and 1 ml of the supernatant was collected for analysis.
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Fig. 1. Effect of contact time on nitrobenzene adsorption by MCM-41 (adsorption conditions: adsorbent dosage = 5 g/L, pH 5.8, and temperature = 298 K).
Fig. 2. Adsorption isotherm of nitrobenzene onto MCM-41 at different temperatures (adsorption conditions: adsorbent dosage = 5 g/L and pH 5.8).
2.5. Analytical method
equilibrium concentration of adsorbate, are usually represented by adsorption isotherms, which is of importance in the design of adsorption systems. The adsorption isotherms of nitrobenzene onto MCM-41 at temperatures of 278, 288, 298, and 308 K are shown in Fig. 2. All adsorption isotherms are nonlinear with curvatures concave to the abscissa. The adsorption of nitrobenzene onto MCM-41 decreases from 2.018 µmol/g (57.7% removal) to 1.092 µmol/g (31.2% removal) when temperature is increased from 278 to 308 K at an initial concentration of 17.5 µmol/L. The decrease in the equilibrium adsorption of nitrobenzene with temperature demonstrates that nitrobenzene removal by adsorption onto MCM-41 favors a low temperature. In order to describe the adsorption isotherm, two important isotherms are selected in this study, the Langmuir and Freundlich isotherms,
The concentration of nitrobenzene was determined by high performance liquid chromatography (Waters) equipped with UV-visible detection at a wavelength of 263 nm, using a symmetry C-18 column (4.6 × 150 mm, 5-µm spheres, Waters). The injection volume was 20 µl and the mobile phase was a mixture of methanol–water (60:40 v/v) with a rate of 1 ml/min. Under these conditions, the retention time for nitrobenzene in the column was 5.2 min. 3. Results and discussion 3.1. Effect of contact time on nitrobenzene adsorption As is illustrated by Fig. 1, an apparent adsorption equilibrium is generally obtained within 1 min for the initial nitrobenzene concentration of 2, 4, and 16 µmol/L, respectively. Thereafter, no detectable concentration changes occur (<30 min) after adsorption equilibrium (1 min) and the average removal efficiency of nitrobenzene reaches about 72.6, 64.5, and 56.1% when the initial concentrations of nitrobenzene are 2, 4, and 16 µmol/L, respectively, during this period. It is assumed here that the adsorption takes place primarily at easily accessible surface sites, requiring no diffusion into micropores. And a hydrophobic interaction between adsorbent and organic compounds may be attributed to the rapid adsorption rate [18]. Additionally, Fig. 1 also illustrates that the removal of nitrobenzene at adsorption equilibrium decreases with increasing initial nitrobenzene concentration. The reason is the limited number of adsorption sites available for the uptake of nitrobenzene at a fixed adsorbent dosage. 3.2. Adsorption isotherms Adsorption equilibrium data, expressed by the mass of adsorbate adsorbed per unit weight of adsorbent and liquid phase
qe =
Q0 KL Ce , 1 + KL Ce 1/n
qe = KF Ce ,
(2) (3)
where qe (µmol/g) is the amount of nitrobenzene adsorbed per gram of MCM-41 at equilibrium; Ce (µmol/L) the equilibrium concentration of nitrobenzene in solution; Q0 (µmol/g) the maximum monolayer adsorption capacity; KL (L/µmol) the constant related to the free energy of adsorption; KF [µmol/g(L/µmol)1/n ], a Freundlich isotherm constant for the system and the slope 1/n, ranging between 0 and 1, indicative of the degree of nonlinearity between solution concentration and adsorption. The isotherm parameters and linear regression obtained from the fitting curves by Langmuir and Freundlich models are given in Table 1. It is clear that adsorption isotherms at different temperatures can be fitted well using two isotherm models (evidenced from the correlation coefficients, >0.990). However, the Langmuir model is more suitable than the Freundlich model to describe the adsorption isotherm, as reflected with correlation coefficients. It suggests that the adsorbent is homogeneous, and the adsorption film is monomolecular. Then the following equations use Langmuir constants to calculate the
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Table 1 Parameters of adsorption isotherms of nitrobenzene onto MCM-41 at different temperatures Temperature (K)
Langmuir model Q0 (µmol/g)
KL (L/µmol)
R2
Freundlich model KF [µmol/g (L/µmol)1/n ]
1/n
R2
278 288 298 308
3.705 2.682 2.142 1.841
0.143 0.126 0.124 0.117
0.9988 0.9995 0.9993 0.9998
0.463 0.317 0.255 0.217
0.75 0.71 0.69 0.67
0.9980 0.9958 0.9950 0.9914
thermodynamic parameters. On the other hand, the maximum monolayer adsorption capacity, Q0 , defined the total capacity of MCM-41 for nitrobenzene adsorption and decreases with increasing temperature. Its maximum value is determined as 3.705 µmol/g at 278 K. A change in the maximum monolayer adsorption capacity with temperature is possibly due to the low hydrothermal stability of MCM-41 in water. At high temperatures, the structure of MCM-41 could collapse by mechanical compression through the hydrolysis of siloxane bonds in the presence of adsorbed water, which may decrease the amount of active sites [15]. Moreover, when the interaction is exothermic, the adsorbate has a tendency to escape from the solid phase to the solution. Thus, rise in temperature and excess energy supply will promote desorption. The free energy (G0 ) of nitrobenzene adsorption onto MCM-41 at different temperatures is calculated based on the adsorption isotherm by [19] G0 = −RT ln KL ,
(4)
where R (J/mol K) is the universal gas law constant; T (K) the absolute temperature of solution, and KL (L/mol) the Langmuir constant. The values of G0 are calculated as from −27.4 to −29.9 kJ/mol. It is noted that the G0 values are all negative, which indicate the feasibility and spontaneous adsorption of nitrobenzene onto MCM-41. The enthalpy change (H 0 ) and entropy change (S 0 ) could not be established because a linear relationship of ln KL and 1/T in the van’t Hoff equation could not be established. 3.3. Effect of pH on nitrobenzene adsorption The pH of solution is one of the most important parameters affecting the adsorption process. In order to determine the effect of pH on adsorption capacity of MCM-41, solutions were prepared at different pH values ranging from 1.0 to 11.0. The dependence of pH on the adsorption of nitrobenzene at an initial concentration of 17.5 µmol/L onto MCM-41 is illustrated in Fig. 3. Obviously, the amount of adsorbed nitrobenzene (qe ) is decreased by increasing the pH value. At pH 1.0, the uptake of nitrobenzene is 54.3% (1.90 µmol/g), while at pH 11.0, the uptake of nitrobenzene is only 18.1% (0.63 µmol/g). It is apparent that using solutions at pH value lower than 1.0 gives the highest qe value. For adsorption onto a solid surface, six adsorption mechanisms are believed to exist (i.e., electrostatic interaction, ion ex-
Fig. 3. Effect of solution pH on the adsorption of nitrobenzene onto MCM-41 (adsorption conditions: adsorbent dosage = 5 g/L, temperature = 298 K, initial nitrobenzene concentration = 17.5 µmol/L, and ionic strength = 0.1 mol/L).
change, ion–dipole interactions, coordination by surface metal cations, hydrogen bonding, and hydrophobic interaction [20]). Since nitrobenzene is an unionizable compound, electrostatic interaction and ion-exchange mechanisms are negligible. The ion–dipole interactions between the charged surface and the nonionic nitrobenzene are also expected to be negligible in this experiment. Coordination by surface metal cations is important only when the organic ligand is a good electron donor relative to water. Therefore, the adsorption mechanisms can be contributed by hydrophobic interaction and hydrogen bonding. Nitrobenzene is an unionizable compound. Thus, the effect of pH on the adsorption of nitrobenzene onto MCM-41 may result from a changed MCM-41 surface. Based on the conclusions of Lu et al. [20], the maximum extent of nitrobenzene adsorption is at a pH value close to its pHzpc (2.5) if hydrogen bond interaction is the dominant mechanism for nitrobenzene adsorption. However, it is noteworthy that the maximum extent of adsorption (Fig. 3) is not at a pH value around its pHzpc in this study. Hence, it can be inferred that a specific interaction like hydrophobic interaction at the interface occurs between nitrobenzene and MCM-41. The possible explanation for the sharp decrease in nitrobenzene adsorption with the increase of pH could be that MCM-41 material is relatively stable with high acid resistance, whereas it degrades readily in basic solution that will decrease the hydrophobic character and destroy the structure of MCM-41 [15]. 3.4. Effect of ionic strength and humic acid on nitrobenzene adsorption Since NaCl is often used as a stimulator in adsorption processes, the effect of ionic strength (adjusted by NaCl) on nitrobenzene adsorption has been determined and the results are presented in Fig. 4. It indicates that the amount of adsorbed nitrobenzene onto MCM-41 decreases with the increase of NaCl concentration of the solution. The Langmuir constants
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Fig. 4. Effect of ionic strength on the adsorption of nitrobenzene onto MCM-41 (adsorption conditions: adsorbent dosage = 5 g/L, pH 5.8, and temperature = 298 K).
Fig. 5. Effect of humic acid on the adsorption of nitrobenzene onto MCM-41 (adsorption conditions: adsorbent dosage = 5 g/L, pH 3.0, ionic strength = 0.1 mol/L, and temperature = 298 K).
(KL ) at NaCl concentrations of 0.001, 0.01, and 0.1 mol/L are 0.128, 0.127, and 0.116 L/µmol, respectively. As nitrobenzene is a nonpolar hydrophobic chemical, increasing ionic strength will lead to a decrease of the solubility of nitrobenzene in water. Therefore, for nitrobenzene, increasing ionic strength should usually be accompanied by an increase in partition into MCM-41. However, the amount of adsorbed nitrobenzene onto MCM-41 decreases with the increase of ionic strength. Given the pHzpc value, the surface of MCM-41 is expected to be dominantly negatively charged at the experimental pH of 5.8. It is beneficial for Na+ to be adsorbed onto MCM-41 by electrostatic interaction. It infers that the additive (NaCl) prefers to occupy the strongest sites on MCM-41, and competes with nitrobenzene for space in MCM-41 due to the surface heterogeneity, which leads to the decrease in the amount of nitrobenzene adsorption. Humic acid exists extensively in surface water and it interferes with the adsorption of trace organic compounds on porous adsorbents such as powdered activated carbon (PAC) by pore blockage and direct competition for adsorption sites [21]. The effect of humic acid on nitrobenzene adsorption onto MCM41 is shown in Fig. 5. The results suggest that the existence of humic acid at a concentration of 10 and 50 mg/L does not significantly affect the extent of nitrobenzene adsorption. The amount of humic acid adsorbed at concentrations of 10 and 50 mg/L is about 0.15 and 0.26 mg/g, respectively. This observation can be explained by the large humic acid molecule. As the molecule of humic acid is larger than the pore diameter (4.3 nm) of MCM-41, it would not be able to penetrate the pore spaces of MCM-41 to form pore blockage and compete for adsorption sites. Hence, it does not reduce the adsorption efficiency for nitrobenzene. Similar results were observed by other researchers who examined the adsorption of Geosmin and 2-Methylisoborneol by ultrastable zeolite-Y in the presence of humic acid [22].
Fig. 6. Effect of solvent on the adsorption of nitrobenzene onto MCM-41 (adsorption conditions: adsorbent dosage = 5 g/L, pH 5.8, and temperature = 298 K).
3.5. Adsorption of nitrobenzene from solvent–water solutions Adsorption of nitrobenzene from aqueous solutions containing 10% methanol or 10% acetone was studied. Methanol is a protic solvent with hydrogen bonding character. Acetone, less polar than methanol, is a dipolar aprotic solvent that cannot act as a hydrogen bond donor. The adsorption isotherm from solvent–water solutions is illustrated in Fig. 6. A decrease in the adsorption is evident when organic solvent is added in water solution. The maximum monolayer adsorption capacities (Q0 ) in water solution, methanol–water solution, and acetone– water solution are 2.14, 1.88, and 0.63 µmol/g, respectively. It also indicates that the presence of acetone appears to inhibit nitrobenzene adsorption to an extent greater than the presence of methanol.
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4. Conclusions
Fig. 7. Adsorption–desorption isotherms of nitrobenzene (adsorption conditions: adsorbent dosage = 5 g/L, pH 5.8, and temperature = 298 K).
The addition of organic solvent to water enhanced the interaction between the nitrobenzene and the solvent, resulting in a decrease in adsorption capacity. This could be attributed to the difference in proton affinity and solvation of these solvents. Thus, the inhibitory effect in less polar solution for nitrobenzene adsorption is greater than that in polar solution due to hydrophobic driving force being the dominant mechanism for nitrobenzene adsorption, where less polar solvent interacts with nitrobenzene more strongly than polar solvent. In other words, increasing nitrobenzene solubility in the mixed solvent results in a reduced driving force for nitrobenzene adsorption. Acetone interacts with nitrobenzene more strongly than methanol. Consequently, the hydrophobic driving force could be responsible for the adsorption of nitrobenzene onto MCM-41. 3.6. Desorption of nitrobenzene by MCM-41 Desorption is one of the key processes affecting the ultimate fate of contaminants in solids. In order to evaluate the reversibility of nitrobenzene adsorption onto MCM-41, desorption characteristics were also determined. Desorption isotherms of nitrobenzene adsorbed onto MCM-41 are shown in Fig. 7. There is no hysteresis in Milli-Q water between adsorption and desorption. It suggests that the adsorption and desorption are reversible and the exchange of nitrobenzene between bulk water and MCM-41 can be realized with a similar mechanism. Thus, desorption could be predicted on the basis of adsorption isotherms. However, since adsorption of nitrobenzene onto MCM-41 is ionic strength dependent, a change of suspension ionic strength would, of course, influence the desorption of adsorbed nitrobenzene. The observed desorption isotherm in 0.1 mol/L NaCl is similar to the adsorption isotherm obtained in 0.1 mol/L NaCl. It seems that after adsorption, there are no strong bonding forces at the interface between nitrobenzene and MCM-41. Hydrophobic interaction may be the significant bonding force in adsorption of nitrobenzene onto MCM-41.
Synthetic MCM-41 by a hydrothermal method was demonstrated to adsorb nitrobenzene from aqueous solution. The results showed that nitrobenzene could be adsorbed onto MCM-41 quickly. The data obtained from adsorption isotherms at different temperatures were fit to the Langmuir model. The maximum monolayer adsorption capacity for nitrobenzene decreased from 3.705 to 1.841 µmol/g with an increase of temperature from 278 to 308 K at pH 5.8. It was found that increasing pH from 1.0 to 11.0 decreased the adsorption capacities from 54.3 to 18.1%. The result was attributed to destroying the hydrophobic characteristic of MCM-41 in basic solutions. Increasing ionic strength from 0.001 to 0.1 mol/L decreased the extent of nitrobenzene adsorption from 2.12 to 1.81 µmol/g for ions mainly occupied the strongest sites on MCM-41 and led to a steric hindrance. The presence of humic acid had no influence on nitrobenzene adsorption and the presence of cosolvent (methanol and acetone) affected the adsorption process by a cosolvent effect. Desorption isotherms showed that adsorption and desorption of nitrobenzene were reversible. Combined with pH effects, it was concluded that hydrophobic interaction was the main force to adsorb nitrobenzene from aqueous solution. Further research is needed to improve the stability of MCM-41 in aqueous solution. Acknowledgment The authors are grateful for the financial support provided by the National Natural Science Foundation of China (NSFC, Project 50578051). References [1] S.B. Haderlein, R.P. Schwarzenbach, Environ. Sci. Technol. 27 (1993) 316. [2] X.K. Zhao, G.P. Yang, X.C. Gao, Chemosphere 52 (2003) 917. [3] S.A. Boyd, G.Y. Sheng, B.J. Teppen, C.T. Johnston, Environ. Sci. Technol. 35 (2001) 4227. [4] O.V. Makarova, T. Rajh, M.C. Thurnauer, A. Martin, P.A. Kemme, D. Cropek, Environ. Sci. Technol. 34 (2000) 4797. [5] A. Latifoglu, M.D. Gurol, Water Res. 37 (2003) 1879. [6] F.J. Beltrán, J.M. Encinar, M.A. Alonso, Ind. Eng. Chem. Res. 37 (1998) 25. [7] M. Franz, H.A. Arafat, N.G. Pinto, Carbon 38 (2000) 1807. [8] M.A. Anderson, Environ. Sci. Technol. 34 (2000) 725. [9] H.T. Shu, D.Y. Li, A.A. Scala, Y.H. Ma, Sep. Purif. Technol. 11 (1997) 27. [10] C.Y. Chang, W.T. Tsai, C.H. Ing, C.F. Chang, J. Colloid Interface Sci. 260 (2003) 273. [11] C.F. Chang, C.Y. Chang, K.H. Chen, W.T. Tsai, J.L. Shie, Y.H. Chen, J. Colloid Interface Sci. 277 (2004) 29. [12] Z.H. Li, R.S. Bowman, Environ. Sci. Technol. 32 (1998) 2278. ˇ [13] J. Lemi´c, D. Kovaˇcevi´c, M. Tomaševi´c-Canovi´ c, D. Kovaˇcevi´c, T. Stani´c, R. Pfend, Water Res. 40 (2006) 1079. [14] X.S. Zhao, G.Q. Lu, G.J. Millar, Ind. Eng. Chem. Res. 35 (1996) 2075. [15] P. Selvam, S.K. Bhatia, C.G. Sonwane, Ind. Eng. Chem. Res. 40 (2001) 3237. [16] C. Cooper, R. Burch, Water Res. 33 (1999) 3689. [17] S.B. Wang, H.T. Li, L.Y. Xu, J. Colloid Interface Sci. 295 (2006) 71.
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