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Surface Characters and Adsorption Behavior of Pb(II) onto a Mesoporous Titanosilicate Molecular Sieve
Journal of Colloid and Interface Science 209, 380 –385 (1999) Article ID jcis.1998.5885, available online at http://www.idealibrary.com on
Surface Characters and Adsorption Behavior of Pb(II) onto a Mesoporous Titanosilicate Molecular Sieve Ying-Ming Xu,* Rong-Shu Wang,*,1 and Feng Wu† *Department of Chemical Engineering, Tianjin University, Tianjin 300072, People’s Republic of China; and †The Beijing Institute of Technology, Beijing 100081, People’s Republic of China Received June 23, 1998; accepted September 23, 1998
ior of Pb21 ions onto Ti-MCM-41 at different pH values and temperatures. The adsorption dynamics, isotherms, and selectivity for heavy metals are also discussed.
Removal of Pb21 ions from aqueous solution using the adsorption process onto Ti-MCM-41 has been investigated. A simplified surface complexation model was used to calculate the conditional binding constants for surface complexation of Pb21 ions onto Ti-MCM-41. Dynamic modeling of the adsorption showed that the first-order reversible kinetic model held for the adsorption process. The overall rate constant k*, the adsorption rate constant k1, the desorption rate constant k2, and the equilibrium constant Ke for the adsorption process were calculated from the results of the thermodynamic analysis, and standard free energy DG°, standard enthalpy DH°, and standard entropy DS° of the adsorption process were also calculated. Equilibrium modeling of the adsorption showed that the adsorption of Pb21 ions was fitted to a Freundlich isotherm. © 1999 Academic Press Key Words: mesoporous silica; titanium; Ti-MCM-41; adsorption; dynamics; thermodynamics; removal of lead.
MATERIALS AND METHODS
Preparation of Ti-MCM-41
INTRODUCTION
The procedure used for the synthesis of Ti-MCM-41 was as follows: the pH of the aqueous solution of (CTMA)Br was adjusted to 11.0 –11.3 with (TMA) OH and stirred for 10 min. Then TEOS and Ti (OC4H9)4 (dissolved in 5.0 mol ethanol and 0.10 mol isopropyl alcohol) were combined with the resulting solution at room temperature under vigorous stirring. Stirring was maintained for about 2 h, and the reaction mixture was aged at ambient temperature for 24 h. The molar composition of the final gel mixtures was 1.0 TEOS : 0.020 Ti(OC4H9)4 : 0.36 TMAOH : 110 H2O : 0.24 (CTMA)Br. The resultant solid product was filtered, washed several times with distilled water, dried in air at 120°C for 2 h, and finally calcined in air at 650°C for 4 h.
Lead is one of the major heavy metal pollutants in the environmental water system (1). As we know, it is harmful, especially to the central nervous system, liver, and kidneys. Many countries pay serious attention to the improvement of environmental water quality, especially drinking water quality. The allowable concentration of lead in drinking water has been dropped to 0.005 mg/L from the original 0.05 mg/L by the United States Environmental Protection Agency. Precipitation, ion exchange, solvent extraction, and adsorption on activated carbon are the conventional methods for the removal of Pb21 ions from aqueous solution (2–5). However, their industrial and environmental applications as such sorbents are extremely limited. Recently, a new family of mesoporous molecular sieves designated as M41S has been discovered (6); the synthesis of mesoporous silica has greatly expanded the possibilities for the design of open pore structures. Because of their large surface areas and well-defined pore size and pore shape, these materials have great potential in environmental and industrial processes. This paper describes the adsorption behav1
Surface Characters Nitrogen adsorption– desorption measurement was carried out at 77K on a ChemBET-3100 sorptometer using a continuous flow measurement mode. The average pore size was calculated by the method of Horvath and Kawazoe (7). The amount of Ti on the sample was determined by HITACHI 180-80 atomic absorption spectroscopy. The total number of sites available (N s) was determined with the following method: add excessive standard base (B mol/L) to a dispersion system of adsorbent, follow with impregnation and then centrifugation. After that, the excessive base in the supernatant is backtitrated with standard acid ( A mol/L). Then the total number of sites available can be calculated from N s 5 B 2 A.
The surface charge density (s0) on Ti-MCM-41 was defined by
To whom correspondence should be addressed.
0021-9797/99 $30.00 Copyright © 1999 by Academic Press All rights of reproduction in any form reserved.
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ADSORPTION OF Pb(II) ONTO Ti-MCM-41
the net uptake of protons by the surface and was determined by potentiometric titration (8); i.e.,
s 0 5 F~G H1 2 G OH2! 5 F~C A 2 C B 1 @OH2# 2 @H1#!/A, [2] where s0 is in coulombs per square centimeter, A is the total surface area in suspension, C A and C B are the concentrations of acid and base after addition, and F is the Faraday constant. int Intrinsic surface ionization constants, i.e., K int a1 , K a2 , and intrinint sic surface complexation constants, i.e., K Na1, Kint can be NO2 3 obtained by the double extrapolation technique from the data of Ns and s0. To measure the release of protons during adsorption of Pb21 ions, 0.20 L solution of 0.40 g Ti-MCM-41 was prepared at pH 7.0. A stock solution of Pb21 ions was adjusted to the same pH and added to the suspension. As protons were released during the adsorption reaction, 0.1469 M carbonate-free NaOH was titrated into the system to maintain controlled pH (60.10 unit). The titration continued for 2 h, at which time the pH remained stable. The quantity of base added is a direct measure of proton release. A blank titration was performed after stock adsorbate solution was added in the absence of Ti-MCM-41. The blank was less than 2% of the base added. Adsorption All solutions were prepared from AR grade reagents and doubly distilled water. Stock solutions of Pb21, Hg21, Cd21, Zn21, Cu21, Mg21, and Na1 ions were prepared by dissolving Pb(NO3)2, Hg(NO3)2, Cd(NO3)2, Zn(NO3)2 z 6H2O, Cu(NO3)2 z 3H2O, Mg(NO3)2 z 6H2O, and NaNO3 in distilled, deionized water, and pH values of the solutions were adjusted to pH 1.0 by addition of concentrated HNO3. The glassware used in the experiments was carefully treated with dimethyl dichlorosilane and dried before use. The plasticware was cleaned with 2 mol/L HNO3. The ratio of Ti-MCM-41 and solution was 1:500 (g/mL); sodium nitrate was added to
TABLE 1 Pb(II) Hydrolysis Equilibria and Equilibrium Constants at 25°C
Equilibria
Equilibrium constant K
2log K (pK)
Pb21 1 H2O 5 PbOH1 1 H1 PbOH1 1 H2O 5 Pb(OH)2 1 H1 1 Pb(OH)2 1 H2O 5 Pb(OH)2 3 1 H 2 22 Pb(OH)3 1 H2O 5 Pb(OH)4 1 H1 Pb(OH)2(s) 5 Pb21 1 2OH2 1 PbOH1 1 H2O 5 HPbO2 2 1 2H
*k 1 *k 2 *k 3 *k 4 K sp b
7.9 8.3 11.5 13.1 16.1 20.1
achieve an ionic strength of 0.10 M after the addition of all reagents; the flasks containing the adsorbent and lead nitrate, as well as the adsorbent-free flasks, were placed on an orbit shaker and subjected to shaking for 72 h at 200 r/min to achieve equilibrium. The adsorption was carried out at room temperature. The metal concentrations of the solutions before and after Ti-MCM-41 treatments were measured by a HITACHI 180-80 atomic absorption spectrometer. RESULTS AND DISCUSSION
Aquatic Chemistry of Pb(II) The main forms of Pb(II) in the water solution are Pb21, 1 2 22 Pb(OH) , Pb(OH)2, Pb(OH)3 , and Pb(OH)4 , and their respective molar fractions of various species of lead are a0, a1, a2, a3, and a4. They are expressed in terms of the equilibrium equations
a 0 5 @1 1 k 1/~H1! 1 k 1k 2/~H1! 2 1 k 1k 2k 3/ ~H1! 3 1 k 1k 2k 3k 4/~H1! 4# 21
[3]
a 1 5 a 0 z k 1/~H1!
[4]
a 2 5 a 0 z k 1k 2/~H1! 2
[5]
a 3 5 a 0 z k 1k 2k 3/~H1! 3
[6]
a 4 5 a 0 z k 1k 2k 3k 4/~H1! 4,
[7]
where k 1 , k 2 , k 3 , and k 4 are the relevant equilibrium constants. Based on the relevant equilibrium constants (9), their distribution as a function of pH values can be calculated and represented (Fig. 1). It is found that the stability of the Pb(II) solution is rather good at pH 7.0. Pb(II) hydrolysis equilibria and equilibrium constants at 25°C are given in Table 1. Amphoteric Surface of Ti-MCM-41
FIG. 1. Distribution of species of Pb(II) as a function of pH value. ‚, 22 Pb21; 3, PbOH1; 1, Pb(OH)2; F, Pb(OH)2 3 ; E, Pb(OH)4 .
The reaction of Ti-MCM-41 with water results in the formation of surface hydroxyl groups. These surface hydroxyl groups are reactive amphoteric sites that dissociate or adsorb ions. In a dilute solution the dominant reactions of
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XU, WANG, AND WU
TABLE 2 Characteristic Parameters of Surface Complexation of Ti-MCM-41 A a (m2/g)
d b (Å)
N s (2OH/100 Å2)
Ti (wt%)
pHZPCc
pK int a1
pK int a2
int 1 pK Na
pK int NO32
1325
36
8.24
1.86
4.52
2.48
6.56
6.68
14.56
a
Specific surface area. Average pore diameter. c int pHZPC 5 0.5 (pK int a1 1 pK a2 ). b
neutral surface hydroxyls, SOH, are protonation or deprotonation; i.e., 1 SOH1 2 5 SOH 1 H
~K int a1 !
[8]
SOH 5 SO2 1 H1
~K !,
[9]
int a2
2 where SOH1 2 and SO are the surface protonation and deproint tonation species and K int a1 and K a2 are the intrinsic surface ionization constants. In NaNO3 solution, the reaction of SOH with Na1 and NO2 3 ions can be expressed by the equations
SOH 1 Na1 5 SO2 2 Na1 1 H1 1 2 SOH 1 H1 1 NO2 3 5 SOH2 2 NO3
, Kint NO2 3
int ~K Na 1!
[10]
~K NO23 !,
[11]
int
int 1 KNa
are the intrinsic surface complexation conwhere stants; these constants can be calculated from the surface charge density (s0) and the total number of sites available (N s) (Table 2). On the surface of Ti-MCM-41, the total adsorption density of Pb21 ions is a sum of the surface concentrations of [SOPb1], [(SO)2Pb], and [SOPbOH], which are formed through the interaction between Pb21 ions and the neutral
FIG. 2. Pb21 adsorption onto Ti-MCM-41 vs pH at 20°C. 0.10 M NaNO3, 0.10 mg/L (3), 0.05 mg/L (E).
surface hydroxy groups. The reaction and mass law can be expressed by the equations SOH 1 Pb21 5 SOPb1 1 H1 @SO2 2 Pb21# 5
int 21) (*K Pb
@SOH][Pb21# @H 1 # int 21 3 exp@~ec0 2 2ecb!/kT#*KPb
2SOH 1 Pb21 5 ~SO! 2Pb 1 2H1 @~SO! 2Pb21# 5
int 21! ~* b Pb
[13] [14]
@~SOH! 2#@Pb21# @H1# 2 int 21 3 exp@~ec0 2 2ecb !/kT#*bPb
SOH 1 Pb21 1 H2O 5 SOPbOH 1 2H1 @SO2 2 PbOH1# 5
[12]
int 1! ~*KPbOH
[15] [16]
@SOH#@Pb21# @H1]2 int 1, 3 exp@~ec0 2 ecb!/kT#*KPbOH [17]
FIG. 3. Pb21 adsorption onto Ti-MCM-41 vs time at 20°C. 0.20 mg (Pb21)/L, 0.10 M NaNO3, pH 3.40 (3), pH 6.90 (F), pH 9.10 (E).
383
ADSORPTION OF Pb(II) ONTO Ti-MCM-41
TABLE 3 Data for Adsorption of Heavy Metals in Water on Ti-MCM-41a Metal ions
Pb21
Hg21
Cd21
Zn21
Cu21
Mg21
Ca21
Na1
Before (mg/L) After (mg/L) Adsorption capacity (mg/g)
10.00 3.64 3.18
10.00 1.83 4.10
10.00 4.32 2.84
10.00 6.57 1.72
10.00 5.24 2.38
10.00 8.92 0.54
10.00 9.43 0.29
5.00 4.72 0.14
a
pH 6.90.
where SOH represents a protonated surface site and (SO)2Pb and SOPbOH represent surface sites occupied by Pb21 ions; int int int 21, *b *K Pb Pb21, and *K PbOH1 are the intrinsic surface complexation constants; c0 and cb are the mean electrostatic potentials at the surface and inner layer plane, respectively. In the few experimental studies in which adsorption stoichiometry has been characterized, generally more than one proton is released to solution per metal ion adsorbed (10 –12). This phenomenon has been explained by formation of a surface complex with two deprotonated sites (Eq. [14]) (10) or simultaneous adsorption and hydrolysis (Eq. [16]) (13). However, oxide surfaces may have a variety of site types, each with a different characteristic affinity for metal ions and protons; therefore the number of protons that are really released per Pb21 adsorption cannot be consistent with the values of the theoretical calculation (8, 13). The number of protons released when Pb21 ions bind to Ti-MCM-41 was measured. Between 1.5–1.7 protons are released, on the average, per Pb21 ion adsorbed onto Ti-MCM-41. Therefore it is proposed that the adsorption reaction of Pb21 ions should include bidentate surface complexing and surface hydrolysis action with the surface hydroxyl groups. Adsorption of Pb21 Ions onto Ti-MCM-41 The adsorption of Pb21 ions at the concentration 0.050, 0.10 mg/L as a function of the pH values is shown in Fig. 2, which shows that the percentage adsorption of Pb21 ions abruptly increases at pH 4 to 6, and a maximum in removal is achieved at pH . 6. It indicates that this adsorption reaction is a special adsorption of chemistry in the pH range. However, the experimental curve begins to drop at high pH values. One reason for such a phenomenon is that at high pH values the amount of hydrolysis products, such as Pb(OH)1, Pb(OH)2, Pb(OH)2 3, and Pb(OH)22 4 , is increased, while the hydrolysis products with negative charges are difficult to adsorb. The other reason is that with increased concentration, the pH value for Pb21 ions to precipitate is decreased, thus the amount of Pb21 ions that can be adsorbed is decreased at higher pH values. Because of the above two reasons, the experimental curve deviates from the theoretical curve at high pH values, following the model. The effect of contact time on the adsorption of Pb21 ions onto Ti-MCM-41 is illustrated in Fig. 3. The percentage of adsorption of Pb21 ions was more than 80% in only 15 min, and it became constant after about 2 h.
The selectivity for metal ions onto Ti-MCM-41 is given in Table 3. For Hg(II) ions, the main species in solution were Hg21, HgOH1, and Hg(OH)2 at pH 6.90. It was found that the concentration of Hg(II) remaining in aqueous phase at pH 8.0 was 2.4 3 1025 M, a predominant species of adsorption of Hg(II) onto the oxide surface was Hg(OH)2, and no precipitate appeared when solutions contained 5 3 1024 M Hg(II) (14 – 15). Furthermore, titanium oxides exhibit very high sorption selectivity for Hg(II) and Pb(II) ions in nitrate media (16). These results indicate that Hg(II) ions are stable in solution at pH 7.0 and are easily adsorbed by Ti-MCM-41. Table 3 also shows that in a dilute solution the ions’ average charge is decreased with the increased hydrolysis of Pb21, Hg21, Cd21, Zn21, and Cu21 ions, which results in the reduction of dissolving energy and favors the adsorption of these ions onto the surface of Ti-MCM-41 by the coulomb force and the shortdistance force. Thus the adsorption capacity of Pb21, Hg21, Cd21, Zn21, and Cu21 ions will be higher than that of Ca21 and Mg21, which are not easy to hydrolyze in solution. Kinetic Studies Adsorption of Pb21 ions from the liquid phase to the solid phase can be considered as a reversible reaction with an equilibrium being established between the two phases. Therefore, a simple first-order kinetic model is used to establish the rate of reaction (17). The first-order kinetic equation is
FIG. 4. The first-order reversible kinetic fit of Pb21 adsorption onto Ti-MCM-41 at pH 6.90. 0.20 mg (Pb21)/L, 20°C (F), 30°C (3), 45°C (E).
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XU, WANG, AND WU
TABLE 4 Kinetic Parameters of the Adsorption of Pb(II) onto Ti-MCM-41a Temperature (°C)
C Be (mg/L)
C Ae (mg/L)
Ke
k9 3 10 2 (min21)
k 1 3 10 2 (min21)
k 2 3 10 3 (min21)
20 30 45
0.172 0.180 0.164
0.028 0.020 0.036
9.00 5.67 4.56
3.13 2.94 2.70
2.82 2.50 2.26
3.10 4.40 5.00
a
pH 6.90.
ln@1 2 U~t!# 5 2k9t,
[18]
in which k9 is the overall rate constant. Further, k9 5 k 1~1 1 1/K e! 5 k 1 1 k 2,
[19]
where K e is the equilibrium constant and k 1 and k 2 are the first-order forward and reverse rate constants, respectively.
U~t! 5
C0 2 Ct C0 2 Ce
[20]
C 0 , C t , and C e (all in mg/L) are concentration of Pb21 ions in solution initially, at any time t, and at equilibrium, respectively. The plots of ln(C t 2 C e)/(C 0 2 C e) vs time are given for Pb21 ions in Fig. 4. A near-straight line was generally observed, which indicates that the adsorption reaction can be approximated to first-order reversible kinetics. Thermodynamic Studies
Ke 5
C Be , C Ae
[24]
where C Be and C Ae are the equilibrium concentrations of Pb21 ions on the sorbent and solute, respectively. Kinetic parameters of the adsorption are given in Table 4. It can be observed that k 1 (sorption of Pb21 ions from solution onto the sorbent) decreases as the temperature increases, while k 2 (desorption of Pb21 ions from sorbent) increases with increasing temperature. Calculated thermodynamic parameters for the adsorption of Pb21 ions on TiMCM-41 at different temperatures are listed in Table 5. The negative values of these parameters are indicative of the spontaneous nature of the process. Entropy has been defined as the degree of chaos of the system, and the negative value of this parameter found in our investigation reflects the adsorption of Pb21 ions. During the adsorption process, the Pb21 ions become associated on the surface of the adsorbent, resulting in the loss of a degree of freedom; this explains the decrease in the value of this parameter (17). The negative values of DH8 show the exothermic nature of Pb(II) adsorption onto TiMCM-41.
In order to explain the effect of temperature on the adsorption thermodynamic parameters, standard free energy DG8, standard enthalpy DH8, and standard entropy DS8 were determined. To calculate the values of the parameters, the following equations were used.
The adsorption of Pb21 ions onto Ti-MCM-41 can be also interpreted in terms of a Freundlich isotherm (Fig. 5). The total adsorption reaction is
DG8 5 2RT ln K e
1 xH1 SOHx 1 nPb21 5 SOPb~2n21!1 n
DH8 5 R DS8 5
T 2T 1 K e2 ln T 2 2 T 1 K e1
DH8 2 DG8 T
Adsorption Isotherms
[21] [22]
[23]
Here, R is the gas constant and K e, K e1, and K e2 are equilibrium constants at the temperature T, T 1 , and T 2 , respectively. Numerical values of the equilibrium constants were calculated from
[25]
TABLE 5 The Thermodynamic Parameters for the Adsorption of Pb(II) onto Ti-MCM-41 Temperature (°C)
Adsorption (%)
Ke
DG8 (J/mol)
DH8 (J/mol)
DS8 (J/mol)
20 30 45
90 85 82
9.00 5.67 4.56
25352 24371 24016
234103 211635 —
298.13 224.00 —
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ADSORPTION OF Pb(II) ONTO Ti-MCM-41
reacted with one surface hydroxyl group. Rearranging Eq. [28], we have # 5 log K f 1 n log@Pb21#. log@SOPb~2n21!1 n
[30]
Figure 5 shows that log[SOPb(2n21)1 ] is a good linear n relationship with log[Pb21]. The intercept log K f is a function of pH. The slope is about 0.64, and its reciprocal stands for the average number of |SOH groups bound per Pb21 ion. Therefore the adsorption species of Pb21 ions on Ti-MCM-41 involve (SO)2Pb and SOPbOH1. ACKNOWLEDGMENTS
FIG. 5. Adsorption isotherms of Pb21 ions onto Ti-MCM-41 at 20°C. pH 5.50 (E), pH 8.0 (F), pH 9.10 (3), 0.10 M NaNO3.
This project was supported by the National Advanced Materials Committee of China (NAMCC), the National Natural Science Foundation of China (NSFC), and the Doctoral Program Foundation of the Ministry of Education of China.
REFERENCES
@SOPb~2n21!1 #@H1# x n K ad 5 E, @SOHx#@Pb21# n
[26]
where [SOHx] and [SOPb(2n21)1 ] are the concentrations of the n surface adsorption sites and Pb21 ions adsorbed onto the surface, x is the number of protons that are really released per Pb21 ion adsorbed onto the surface, Kad is the surface complex stability constant, and E stands for the electrostatic term. As the effect of E on Kad is very small, it can be presumed K9ad ' Kad/E. In addition, as [SOHT] @ [Pb21]ads, we can replace [SOHx] with the total surface adsorption sites, i.e., [SOHT] ' [SOHx]. K9ad 5 log
#@H1# x @SOPb~2n21!1 n @SOHT#@Pb21# n
# @SOPb~2n21!1 n 5 log K9ad 1 log@SOHT# 1 xpH 21 n @Pb # log K f 5 log K9ad 1 log@SOHT# 1 xpH
[27]
[28] [29]
Here, K f is the Freundlich constant, K9ad is the surface complex condition stability constant. [SOHT] is the total number of sites available on the surface, and n is the number of Pb21 ions
1. Wang, R-S., Hong, C., and Wang, L-P., Sci. China Ser. B 37, 1429 (1994). 2. Gonzales-Davila, M., Santana-Casino, J. M., and Millero, F. J., J. Colloid Interface Sci. 137, 102 (1990). 3. Huang, C. D., and Blankenship, D. W., Water Res. 18, 37 (1984). 4. Naylor, L. M., and Daugue, R. R., J. Am. Water Works Assoc. 67, 560 (1975). 5. Kahashi, Y. Y., and Imai, H., Soil Sci. Plant Nutr. 29(2), 111 (1983). 6. Beck, J. S., Vartuli, J. C., Roth, W. J., Leonowicz, M. E., Kresge, C. T., Schmitt, K. D., Chu, C. T-W., Olson, D. H., Sheppard, E. W., McCullen, S. B., Higgins, J. B., and Schlenker, J. L., J. Am. Chem. Soc. 114, 10834 (1992). 7. Horvath, G., and Kawazoe, K. J., J. Chem. Eng. Jpn. 16, 470 (1983). 8. Davis, J. A., James, R. O., and Leckie, J. O., J. Colloid Interface Sci. 63, 480 (1978). 9. Rodda, D. P., Johnson, B. B., and Wells, J. D., J. Colloid Interface Sci. 161, 57 (1993). 10. Hohl, H., and Stumm, W., J. Colloid Interface Sci. 55, 281 (1976). 11. Davis, J. A., James, R. O., and Leckie, J. O., J. Colloid Interface Sci. 67, 90 (1978). 12. Lauaz, Z. K., and Tang, H. X., Acta Sci. Circumstantiae 14, 129 (1994). 13. Benjamin, M. M., and Leckie, J. O., J. Colloid Interface Sci. 79, 209 (1981). 14. MacNaughton, M. G., and James, R. O., J. Colloid Interface Sci. 47, 431 (1974). 15. Inoue, Y., and Munemori, M., Environ. Sci. Tech. 13, 433 (1979). 16. Abe, M., Wang, P., Chitrakar, R., and Tsuji, M., Analyst 114, 435 (1989). ¨ zel, M. Z., J. Colloid Interface Sci. 187, 17. Bereket, G., Aroguz, A. Z., and O 338 (1997).
Surface Characters and Adsorption Behavior of Pb(II) onto a Mesoporous Titanosilicate Molecular Sieve Ying-Ming Xu,* Rong-Shu Wang,*,1 and Feng Wu† *Department of Chemical Engineering, Tianjin University, Tianjin 300072, People’s Republic of China; and †The Beijing Institute of Technology, Beijing 100081, People’s Republic of China Received June 23, 1998; accepted September 23, 1998
ior of Pb21 ions onto Ti-MCM-41 at different pH values and temperatures. The adsorption dynamics, isotherms, and selectivity for heavy metals are also discussed.
Removal of Pb21 ions from aqueous solution using the adsorption process onto Ti-MCM-41 has been investigated. A simplified surface complexation model was used to calculate the conditional binding constants for surface complexation of Pb21 ions onto Ti-MCM-41. Dynamic modeling of the adsorption showed that the first-order reversible kinetic model held for the adsorption process. The overall rate constant k*, the adsorption rate constant k1, the desorption rate constant k2, and the equilibrium constant Ke for the adsorption process were calculated from the results of the thermodynamic analysis, and standard free energy DG°, standard enthalpy DH°, and standard entropy DS° of the adsorption process were also calculated. Equilibrium modeling of the adsorption showed that the adsorption of Pb21 ions was fitted to a Freundlich isotherm. © 1999 Academic Press Key Words: mesoporous silica; titanium; Ti-MCM-41; adsorption; dynamics; thermodynamics; removal of lead.
MATERIALS AND METHODS
Preparation of Ti-MCM-41
INTRODUCTION
The procedure used for the synthesis of Ti-MCM-41 was as follows: the pH of the aqueous solution of (CTMA)Br was adjusted to 11.0 –11.3 with (TMA) OH and stirred for 10 min. Then TEOS and Ti (OC4H9)4 (dissolved in 5.0 mol ethanol and 0.10 mol isopropyl alcohol) were combined with the resulting solution at room temperature under vigorous stirring. Stirring was maintained for about 2 h, and the reaction mixture was aged at ambient temperature for 24 h. The molar composition of the final gel mixtures was 1.0 TEOS : 0.020 Ti(OC4H9)4 : 0.36 TMAOH : 110 H2O : 0.24 (CTMA)Br. The resultant solid product was filtered, washed several times with distilled water, dried in air at 120°C for 2 h, and finally calcined in air at 650°C for 4 h.
Lead is one of the major heavy metal pollutants in the environmental water system (1). As we know, it is harmful, especially to the central nervous system, liver, and kidneys. Many countries pay serious attention to the improvement of environmental water quality, especially drinking water quality. The allowable concentration of lead in drinking water has been dropped to 0.005 mg/L from the original 0.05 mg/L by the United States Environmental Protection Agency. Precipitation, ion exchange, solvent extraction, and adsorption on activated carbon are the conventional methods for the removal of Pb21 ions from aqueous solution (2–5). However, their industrial and environmental applications as such sorbents are extremely limited. Recently, a new family of mesoporous molecular sieves designated as M41S has been discovered (6); the synthesis of mesoporous silica has greatly expanded the possibilities for the design of open pore structures. Because of their large surface areas and well-defined pore size and pore shape, these materials have great potential in environmental and industrial processes. This paper describes the adsorption behav1
Surface Characters Nitrogen adsorption– desorption measurement was carried out at 77K on a ChemBET-3100 sorptometer using a continuous flow measurement mode. The average pore size was calculated by the method of Horvath and Kawazoe (7). The amount of Ti on the sample was determined by HITACHI 180-80 atomic absorption spectroscopy. The total number of sites available (N s) was determined with the following method: add excessive standard base (B mol/L) to a dispersion system of adsorbent, follow with impregnation and then centrifugation. After that, the excessive base in the supernatant is backtitrated with standard acid ( A mol/L). Then the total number of sites available can be calculated from N s 5 B 2 A.
The surface charge density (s0) on Ti-MCM-41 was defined by
To whom correspondence should be addressed.
0021-9797/99 $30.00 Copyright © 1999 by Academic Press All rights of reproduction in any form reserved.
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ADSORPTION OF Pb(II) ONTO Ti-MCM-41
the net uptake of protons by the surface and was determined by potentiometric titration (8); i.e.,
s 0 5 F~G H1 2 G OH2! 5 F~C A 2 C B 1 @OH2# 2 @H1#!/A, [2] where s0 is in coulombs per square centimeter, A is the total surface area in suspension, C A and C B are the concentrations of acid and base after addition, and F is the Faraday constant. int Intrinsic surface ionization constants, i.e., K int a1 , K a2 , and intrinint sic surface complexation constants, i.e., K Na1, Kint can be NO2 3 obtained by the double extrapolation technique from the data of Ns and s0. To measure the release of protons during adsorption of Pb21 ions, 0.20 L solution of 0.40 g Ti-MCM-41 was prepared at pH 7.0. A stock solution of Pb21 ions was adjusted to the same pH and added to the suspension. As protons were released during the adsorption reaction, 0.1469 M carbonate-free NaOH was titrated into the system to maintain controlled pH (60.10 unit). The titration continued for 2 h, at which time the pH remained stable. The quantity of base added is a direct measure of proton release. A blank titration was performed after stock adsorbate solution was added in the absence of Ti-MCM-41. The blank was less than 2% of the base added. Adsorption All solutions were prepared from AR grade reagents and doubly distilled water. Stock solutions of Pb21, Hg21, Cd21, Zn21, Cu21, Mg21, and Na1 ions were prepared by dissolving Pb(NO3)2, Hg(NO3)2, Cd(NO3)2, Zn(NO3)2 z 6H2O, Cu(NO3)2 z 3H2O, Mg(NO3)2 z 6H2O, and NaNO3 in distilled, deionized water, and pH values of the solutions were adjusted to pH 1.0 by addition of concentrated HNO3. The glassware used in the experiments was carefully treated with dimethyl dichlorosilane and dried before use. The plasticware was cleaned with 2 mol/L HNO3. The ratio of Ti-MCM-41 and solution was 1:500 (g/mL); sodium nitrate was added to
TABLE 1 Pb(II) Hydrolysis Equilibria and Equilibrium Constants at 25°C
Equilibria
Equilibrium constant K
2log K (pK)
Pb21 1 H2O 5 PbOH1 1 H1 PbOH1 1 H2O 5 Pb(OH)2 1 H1 1 Pb(OH)2 1 H2O 5 Pb(OH)2 3 1 H 2 22 Pb(OH)3 1 H2O 5 Pb(OH)4 1 H1 Pb(OH)2(s) 5 Pb21 1 2OH2 1 PbOH1 1 H2O 5 HPbO2 2 1 2H
*k 1 *k 2 *k 3 *k 4 K sp b
7.9 8.3 11.5 13.1 16.1 20.1
achieve an ionic strength of 0.10 M after the addition of all reagents; the flasks containing the adsorbent and lead nitrate, as well as the adsorbent-free flasks, were placed on an orbit shaker and subjected to shaking for 72 h at 200 r/min to achieve equilibrium. The adsorption was carried out at room temperature. The metal concentrations of the solutions before and after Ti-MCM-41 treatments were measured by a HITACHI 180-80 atomic absorption spectrometer. RESULTS AND DISCUSSION
Aquatic Chemistry of Pb(II) The main forms of Pb(II) in the water solution are Pb21, 1 2 22 Pb(OH) , Pb(OH)2, Pb(OH)3 , and Pb(OH)4 , and their respective molar fractions of various species of lead are a0, a1, a2, a3, and a4. They are expressed in terms of the equilibrium equations
a 0 5 @1 1 k 1/~H1! 1 k 1k 2/~H1! 2 1 k 1k 2k 3/ ~H1! 3 1 k 1k 2k 3k 4/~H1! 4# 21
[3]
a 1 5 a 0 z k 1/~H1!
[4]
a 2 5 a 0 z k 1k 2/~H1! 2
[5]
a 3 5 a 0 z k 1k 2k 3/~H1! 3
[6]
a 4 5 a 0 z k 1k 2k 3k 4/~H1! 4,
[7]
where k 1 , k 2 , k 3 , and k 4 are the relevant equilibrium constants. Based on the relevant equilibrium constants (9), their distribution as a function of pH values can be calculated and represented (Fig. 1). It is found that the stability of the Pb(II) solution is rather good at pH 7.0. Pb(II) hydrolysis equilibria and equilibrium constants at 25°C are given in Table 1. Amphoteric Surface of Ti-MCM-41
FIG. 1. Distribution of species of Pb(II) as a function of pH value. ‚, 22 Pb21; 3, PbOH1; 1, Pb(OH)2; F, Pb(OH)2 3 ; E, Pb(OH)4 .
The reaction of Ti-MCM-41 with water results in the formation of surface hydroxyl groups. These surface hydroxyl groups are reactive amphoteric sites that dissociate or adsorb ions. In a dilute solution the dominant reactions of
382
XU, WANG, AND WU
TABLE 2 Characteristic Parameters of Surface Complexation of Ti-MCM-41 A a (m2/g)
d b (Å)
N s (2OH/100 Å2)
Ti (wt%)
pHZPCc
pK int a1
pK int a2
int 1 pK Na
pK int NO32
1325
36
8.24
1.86
4.52
2.48
6.56
6.68
14.56
a
Specific surface area. Average pore diameter. c int pHZPC 5 0.5 (pK int a1 1 pK a2 ). b
neutral surface hydroxyls, SOH, are protonation or deprotonation; i.e., 1 SOH1 2 5 SOH 1 H
~K int a1 !
[8]
SOH 5 SO2 1 H1
~K !,
[9]
int a2
2 where SOH1 2 and SO are the surface protonation and deproint tonation species and K int a1 and K a2 are the intrinsic surface ionization constants. In NaNO3 solution, the reaction of SOH with Na1 and NO2 3 ions can be expressed by the equations
SOH 1 Na1 5 SO2 2 Na1 1 H1 1 2 SOH 1 H1 1 NO2 3 5 SOH2 2 NO3
, Kint NO2 3
int ~K Na 1!
[10]
~K NO23 !,
[11]
int
int 1 KNa
are the intrinsic surface complexation conwhere stants; these constants can be calculated from the surface charge density (s0) and the total number of sites available (N s) (Table 2). On the surface of Ti-MCM-41, the total adsorption density of Pb21 ions is a sum of the surface concentrations of [SOPb1], [(SO)2Pb], and [SOPbOH], which are formed through the interaction between Pb21 ions and the neutral
FIG. 2. Pb21 adsorption onto Ti-MCM-41 vs pH at 20°C. 0.10 M NaNO3, 0.10 mg/L (3), 0.05 mg/L (E).
surface hydroxy groups. The reaction and mass law can be expressed by the equations SOH 1 Pb21 5 SOPb1 1 H1 @SO2 2 Pb21# 5
int 21) (*K Pb
@SOH][Pb21# @H 1 # int 21 3 exp@~ec0 2 2ecb!/kT#*KPb
2SOH 1 Pb21 5 ~SO! 2Pb 1 2H1 @~SO! 2Pb21# 5
int 21! ~* b Pb
[13] [14]
@~SOH! 2#@Pb21# @H1# 2 int 21 3 exp@~ec0 2 2ecb !/kT#*bPb
SOH 1 Pb21 1 H2O 5 SOPbOH 1 2H1 @SO2 2 PbOH1# 5
[12]
int 1! ~*KPbOH
[15] [16]
@SOH#@Pb21# @H1]2 int 1, 3 exp@~ec0 2 ecb!/kT#*KPbOH [17]
FIG. 3. Pb21 adsorption onto Ti-MCM-41 vs time at 20°C. 0.20 mg (Pb21)/L, 0.10 M NaNO3, pH 3.40 (3), pH 6.90 (F), pH 9.10 (E).
383
ADSORPTION OF Pb(II) ONTO Ti-MCM-41
TABLE 3 Data for Adsorption of Heavy Metals in Water on Ti-MCM-41a Metal ions
Pb21
Hg21
Cd21
Zn21
Cu21
Mg21
Ca21
Na1
Before (mg/L) After (mg/L) Adsorption capacity (mg/g)
10.00 3.64 3.18
10.00 1.83 4.10
10.00 4.32 2.84
10.00 6.57 1.72
10.00 5.24 2.38
10.00 8.92 0.54
10.00 9.43 0.29
5.00 4.72 0.14
a
pH 6.90.
where SOH represents a protonated surface site and (SO)2Pb and SOPbOH represent surface sites occupied by Pb21 ions; int int int 21, *b *K Pb Pb21, and *K PbOH1 are the intrinsic surface complexation constants; c0 and cb are the mean electrostatic potentials at the surface and inner layer plane, respectively. In the few experimental studies in which adsorption stoichiometry has been characterized, generally more than one proton is released to solution per metal ion adsorbed (10 –12). This phenomenon has been explained by formation of a surface complex with two deprotonated sites (Eq. [14]) (10) or simultaneous adsorption and hydrolysis (Eq. [16]) (13). However, oxide surfaces may have a variety of site types, each with a different characteristic affinity for metal ions and protons; therefore the number of protons that are really released per Pb21 adsorption cannot be consistent with the values of the theoretical calculation (8, 13). The number of protons released when Pb21 ions bind to Ti-MCM-41 was measured. Between 1.5–1.7 protons are released, on the average, per Pb21 ion adsorbed onto Ti-MCM-41. Therefore it is proposed that the adsorption reaction of Pb21 ions should include bidentate surface complexing and surface hydrolysis action with the surface hydroxyl groups. Adsorption of Pb21 Ions onto Ti-MCM-41 The adsorption of Pb21 ions at the concentration 0.050, 0.10 mg/L as a function of the pH values is shown in Fig. 2, which shows that the percentage adsorption of Pb21 ions abruptly increases at pH 4 to 6, and a maximum in removal is achieved at pH . 6. It indicates that this adsorption reaction is a special adsorption of chemistry in the pH range. However, the experimental curve begins to drop at high pH values. One reason for such a phenomenon is that at high pH values the amount of hydrolysis products, such as Pb(OH)1, Pb(OH)2, Pb(OH)2 3, and Pb(OH)22 4 , is increased, while the hydrolysis products with negative charges are difficult to adsorb. The other reason is that with increased concentration, the pH value for Pb21 ions to precipitate is decreased, thus the amount of Pb21 ions that can be adsorbed is decreased at higher pH values. Because of the above two reasons, the experimental curve deviates from the theoretical curve at high pH values, following the model. The effect of contact time on the adsorption of Pb21 ions onto Ti-MCM-41 is illustrated in Fig. 3. The percentage of adsorption of Pb21 ions was more than 80% in only 15 min, and it became constant after about 2 h.
The selectivity for metal ions onto Ti-MCM-41 is given in Table 3. For Hg(II) ions, the main species in solution were Hg21, HgOH1, and Hg(OH)2 at pH 6.90. It was found that the concentration of Hg(II) remaining in aqueous phase at pH 8.0 was 2.4 3 1025 M, a predominant species of adsorption of Hg(II) onto the oxide surface was Hg(OH)2, and no precipitate appeared when solutions contained 5 3 1024 M Hg(II) (14 – 15). Furthermore, titanium oxides exhibit very high sorption selectivity for Hg(II) and Pb(II) ions in nitrate media (16). These results indicate that Hg(II) ions are stable in solution at pH 7.0 and are easily adsorbed by Ti-MCM-41. Table 3 also shows that in a dilute solution the ions’ average charge is decreased with the increased hydrolysis of Pb21, Hg21, Cd21, Zn21, and Cu21 ions, which results in the reduction of dissolving energy and favors the adsorption of these ions onto the surface of Ti-MCM-41 by the coulomb force and the shortdistance force. Thus the adsorption capacity of Pb21, Hg21, Cd21, Zn21, and Cu21 ions will be higher than that of Ca21 and Mg21, which are not easy to hydrolyze in solution. Kinetic Studies Adsorption of Pb21 ions from the liquid phase to the solid phase can be considered as a reversible reaction with an equilibrium being established between the two phases. Therefore, a simple first-order kinetic model is used to establish the rate of reaction (17). The first-order kinetic equation is
FIG. 4. The first-order reversible kinetic fit of Pb21 adsorption onto Ti-MCM-41 at pH 6.90. 0.20 mg (Pb21)/L, 20°C (F), 30°C (3), 45°C (E).
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XU, WANG, AND WU
TABLE 4 Kinetic Parameters of the Adsorption of Pb(II) onto Ti-MCM-41a Temperature (°C)
C Be (mg/L)
C Ae (mg/L)
Ke
k9 3 10 2 (min21)
k 1 3 10 2 (min21)
k 2 3 10 3 (min21)
20 30 45
0.172 0.180 0.164
0.028 0.020 0.036
9.00 5.67 4.56
3.13 2.94 2.70
2.82 2.50 2.26
3.10 4.40 5.00
a
pH 6.90.
ln@1 2 U~t!# 5 2k9t,
[18]
in which k9 is the overall rate constant. Further, k9 5 k 1~1 1 1/K e! 5 k 1 1 k 2,
[19]
where K e is the equilibrium constant and k 1 and k 2 are the first-order forward and reverse rate constants, respectively.
U~t! 5
C0 2 Ct C0 2 Ce
[20]
C 0 , C t , and C e (all in mg/L) are concentration of Pb21 ions in solution initially, at any time t, and at equilibrium, respectively. The plots of ln(C t 2 C e)/(C 0 2 C e) vs time are given for Pb21 ions in Fig. 4. A near-straight line was generally observed, which indicates that the adsorption reaction can be approximated to first-order reversible kinetics. Thermodynamic Studies
Ke 5
C Be , C Ae
[24]
where C Be and C Ae are the equilibrium concentrations of Pb21 ions on the sorbent and solute, respectively. Kinetic parameters of the adsorption are given in Table 4. It can be observed that k 1 (sorption of Pb21 ions from solution onto the sorbent) decreases as the temperature increases, while k 2 (desorption of Pb21 ions from sorbent) increases with increasing temperature. Calculated thermodynamic parameters for the adsorption of Pb21 ions on TiMCM-41 at different temperatures are listed in Table 5. The negative values of these parameters are indicative of the spontaneous nature of the process. Entropy has been defined as the degree of chaos of the system, and the negative value of this parameter found in our investigation reflects the adsorption of Pb21 ions. During the adsorption process, the Pb21 ions become associated on the surface of the adsorbent, resulting in the loss of a degree of freedom; this explains the decrease in the value of this parameter (17). The negative values of DH8 show the exothermic nature of Pb(II) adsorption onto TiMCM-41.
In order to explain the effect of temperature on the adsorption thermodynamic parameters, standard free energy DG8, standard enthalpy DH8, and standard entropy DS8 were determined. To calculate the values of the parameters, the following equations were used.
The adsorption of Pb21 ions onto Ti-MCM-41 can be also interpreted in terms of a Freundlich isotherm (Fig. 5). The total adsorption reaction is
DG8 5 2RT ln K e
1 xH1 SOHx 1 nPb21 5 SOPb~2n21!1 n
DH8 5 R DS8 5
T 2T 1 K e2 ln T 2 2 T 1 K e1
DH8 2 DG8 T
Adsorption Isotherms
[21] [22]
[23]
Here, R is the gas constant and K e, K e1, and K e2 are equilibrium constants at the temperature T, T 1 , and T 2 , respectively. Numerical values of the equilibrium constants were calculated from
[25]
TABLE 5 The Thermodynamic Parameters for the Adsorption of Pb(II) onto Ti-MCM-41 Temperature (°C)
Adsorption (%)
Ke
DG8 (J/mol)
DH8 (J/mol)
DS8 (J/mol)
20 30 45
90 85 82
9.00 5.67 4.56
25352 24371 24016
234103 211635 —
298.13 224.00 —
385
ADSORPTION OF Pb(II) ONTO Ti-MCM-41
reacted with one surface hydroxyl group. Rearranging Eq. [28], we have # 5 log K f 1 n log@Pb21#. log@SOPb~2n21!1 n
[30]
Figure 5 shows that log[SOPb(2n21)1 ] is a good linear n relationship with log[Pb21]. The intercept log K f is a function of pH. The slope is about 0.64, and its reciprocal stands for the average number of |SOH groups bound per Pb21 ion. Therefore the adsorption species of Pb21 ions on Ti-MCM-41 involve (SO)2Pb and SOPbOH1. ACKNOWLEDGMENTS
FIG. 5. Adsorption isotherms of Pb21 ions onto Ti-MCM-41 at 20°C. pH 5.50 (E), pH 8.0 (F), pH 9.10 (3), 0.10 M NaNO3.
This project was supported by the National Advanced Materials Committee of China (NAMCC), the National Natural Science Foundation of China (NSFC), and the Doctoral Program Foundation of the Ministry of Education of China.
REFERENCES
@SOPb~2n21!1 #@H1# x n K ad 5 E, @SOHx#@Pb21# n
[26]
where [SOHx] and [SOPb(2n21)1 ] are the concentrations of the n surface adsorption sites and Pb21 ions adsorbed onto the surface, x is the number of protons that are really released per Pb21 ion adsorbed onto the surface, Kad is the surface complex stability constant, and E stands for the electrostatic term. As the effect of E on Kad is very small, it can be presumed K9ad ' Kad/E. In addition, as [SOHT] @ [Pb21]ads, we can replace [SOHx] with the total surface adsorption sites, i.e., [SOHT] ' [SOHx]. K9ad 5 log
#@H1# x @SOPb~2n21!1 n @SOHT#@Pb21# n
# @SOPb~2n21!1 n 5 log K9ad 1 log@SOHT# 1 xpH 21 n @Pb # log K f 5 log K9ad 1 log@SOHT# 1 xpH
[27]
[28] [29]
Here, K f is the Freundlich constant, K9ad is the surface complex condition stability constant. [SOHT] is the total number of sites available on the surface, and n is the number of Pb21 ions
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