Langmuir 2003, 19, 1977-1979
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Unusual Adsorption Properties of Microporous Titanosilicate ETS-10 toward Heavy Metal Lead George X. S. Zhao,* Jia L. Lee, and Phai A. Chia Department of Chemical and Environmental Engineering, National University of Singapore, 10 Kent Ridge Crescent, Singapore 119260 Received August 29, 2002. In Final Form: November 18, 2002 Microporous titanosilicate ETS-10 was synthesized by using TiF4 as the Ti source and characterized by using X-ray diffraction, Fourier transform infrared and Raman spectroscopies, and nitrogen adsorption. The adsorption properties of heavy metal ion Pb2+ on the ETS-10 sample were studied by measuring the adsorption kinetics and equilibria using a batch-type method. It has been observed that the adsorption rate of Pb2+ on ETS-10 is extremely rapid: less than 5 s is required to attain maximum adsorption capacity in a 10 mmol/L solution with a batch factor of 200 mL/g. The kinetic data can be fitted very well by a pseudo-second-order model, whereas the equilibrium data are better fitted to the Langmuir isotherm than to the Freundlich isotherm. The maximum adsorption capacity of Pb2+ on ETS-10 as predicted by the Langmuir equation is 1.12 mmol/g. This is the highest adsorption capacity of Pb2+ on zeolites that has been observed so far.
Introduction Heavy metals such as lead (Pb), cadmium (Cd), and cupper (Cu) are common groundwater contaminants that must be controlled to an acceptable level according to the increasingly stringent environmental regulations. The heavy metals, especially Pb present in drinking water, are extremely detrimental to human beings. Depending on the existing form of the metals in solution, they can be removed by using different technologies that are currently available, such as chemical precipitation, membrane filtration, ion exchange, and carbon adsorption.1 Unfortunately, none of these technologies can afford reducing the heavy metals to an extremely low level at a minimal contact time with a high adsorption capacity, which is of significance in the treatment of water, especially in purification of drinking water using filters. ETS-10, a microporous titanosilicate ETS-10 discovered by Engelhard in 1989,2 is a large-pore zeolite material with a pore-opening size of 0.8 nm.2-4 The basic anhydrous formula of an as-synthesized ETS-10 is Na1.5K0.5TiSi5O13. Unlike conventional zeolites, the framework of ETS-10 is constituted from SiO4 tetrahedra and TiO6 octahedra by corner-sharing oxygen atoms.3 The presence of each tetravalent Ti atom in an octahedron generates two negative charges, which are balanced by exchangeable cations 1.5Na+ and 0.5K+. Such a unique framework property manifests itself as a promising potential ion exchanger for many cationic metal ions that are present in waters such as Pb2+, Cd2+, Cu2+, Zn2+, and so forth. However, adsorption data of heavy metals on ETS-10 are hardly available.5,6 Al-Attar and Blackburn compared the uptake properties of uranium on ETS-10 materials synthesized with different Ti sources and noted that the method of ETS-10 preparation has a considerable effect * Corresponding author. E-mail:
[email protected]. Phone: 65-68744727. Fax: 65-67791936. (1) Bailey, S. E.; Olin, T. J.; Bricka, R. M.; Adrian, D. D. Water Res. 1992, 33, 2469-2479. (2) Kuznicki, S. M. U.S. Patent 4,853,202, 1989. (3) Anderson, M. W.; Terasaki, O.; Ohsuna, T.; Philippou, A.; MacKay, S. P.; Ferreira, A.; Rocha, J.; Lidin, S. Nature 1994, 367, 347-351. (4) Rocha, J.; Anderson, M. W. Eur. J. Inorg. Chem. 2000, 801-818. (5) Al-Attar, L.; Dyer, A.; Blackburn, R. J. Radioanal. Nucl. Chem. 2000, 246, 451-455. (6) Kuznicki, S. M.; Thrush, K. A. U.S. Patent 4,994,191, 1991.
on the uptakes of uranium at shorter time periods.5 In a U.S. patent,6 Kuznicki and Thrush observed that ETS-10 and ETAS-10 (aluminum-substituted ETS-10) displayed an extraordinarily rapid adsorption rate toward Pb2+. The concentration of Pb2+ was reduced to a negligible amount from 2000 ppm in a very short contact time at a liquidto-solid ratio of 100:2.4 (g/g). Unfortunately, adsorption equilibrium data have not been available. Motivated by the work of Kuznicki and Thrush,6 we have carried out a systematic study on the adsorption equilibria and kinetics of several heavy metal ions including Pb2+, Cd2+, Cu2+, Zn2+, and Ni2+ on ETS-10 using a batch-type technique. Our observations have not only confirmed that ETS-10 does exhibit a remarkable adsorption rate toward heavy metal ions but also demonstrated that the maximum adsorption capacity of Pb2+ on ETS-10 is as high as 1.12 mmol/g as predicted by the Langmuir model. This is the highest uptake that has been observed on zeolite materials.1 In this letter, we briefly report the unusual adsorption properties of ETS-10 toward Pb2+. Experimental Section The ETS-10 sample used in the study was synthesized in the absence of template or seeds according to Yang and co-workers.7 The molar composition of the synthesis system was 8NaOH/ 2KOH/TiF4/5.7SiO2/350H2O. TiF4 (Aldrich) and sodium silicate solution (Merck) were used as the Ti and Si sources, respectively. All chemicals were used as received. The ETS-10 sample was characterized by using X-ray diffraction (XRD) on a Shimadzu XRD-6000 diffractometer (Cu KR radiation), nitrogen adsorption on a Quantachrome NOVA 1000, Fourier transform infrared (FTIR) spectroscopy on a Biorad spectrometer with the KBr technique, and Raman spectroscopy on a Bruker FRA 106/S FT-Raman spectrometer. Adsorption of heavy metal ion Pb2+ on the solid ETS-10 sample was conducted using a batch-type method at room temperature (23 °C). For kinetic measurement, 1 g of air-dried ETS-10 was added to 100 mL of solution preacidified by nitric acid under shaking so as to generate a solution of pH ) 5.8 according to Kuznicki and Thrush.6 Then, 100 mL of 20 mmol/L Pb(NO3)2 solution was added to obtain a mixture with an initial Pb2+ concentration of approximately 10 mmol/L, a pH value of about 5.0, and a batch factor (ratio of liquid volume to solid mass) of (7) Yang, X.; Paillaud, J.-L.; van Breukelen, H. F. W. J.; Kessler, H.; Duprey, E. Microporous Mesoporous Mater. 2001, 46, 1-11.
10.1021/la026490l CCC: $25.00 © 2003 American Chemical Society Published on Web 02/12/2003
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Langmuir, Vol. 19, No. 6, 2003
Figure 1. XRD pattern and Raman (left-hand insert) and FTIR (right-hand insert) spectra of the ETS-10 sample used in this study. about 0.2 L/g. Five milliliters of the mixture was withdrawn at an appropriate time interval by using a 5 mL syringe and rapidly filtered through a 0.2 µm nylon membrane filter. The filtrate was collected in a sample valve and analyzed for Pb, Na, and K concentrations using an atomic absorption spectrometer (PerkinElmer Analyst 300). The amount of metal adsorbed at time t (s), qt (mmol/g), was deduced from the mass balance between the initial concentration and the concentration at time t. The experimental data were fitted to the linear form of the pseudosecond-order equation (t/qt ) 1/v0 + t/qe),8 where qt and qe in mmol/g are the amount of metal adsorbed at time t and at equilibrium, respectively, v0 (mmol/g/s) is the initial adsorption rate which is kqe2, and k (g/mmol/s) is the adsorption rate constant. The values of v0 and qe can be obtained experimentally by plotting t/qt versus t. Adsorption equilibrium data were collected in a similar way as the kinetic measurement. The equilibrium time was 10 min, which, according to the kinetics data, was found to be sufficiently long to attain adsorption equilibrium. The experimental data were fitted to both the Langmuir isotherm [qe ) qmbCe/(1 + bCe)], where qm (mmol/g) is the maximum adsorption capacity, Ce (mmol/L) is the equilibrium concentration of the heavy metal ion in solution, and b (L/mmol) is the Langmuir constant, and the Freundlich isotherm (qe ) KCe1/n), where K (mmol1-1/n L1/n g-1) and n (dimensionless) are constants.
Results and Discussion Figure 1 depicts the XRD pattern and Raman and FTIR spectra of the ETS-10 sample used in this study. The XRD pattern is identical to that of ETS-10 materials reported previously,2,7 showing that the sample is a pure ETS-10 phase without the presence of ETS-4 impurity (it has been shown that ETS-4 is a thermodynamically more stable phase than ETS-10 and it is normally present in an ETS10 product.9) The Raman spectrum (left-hand insert) further confirms the purity of the sample. A most intense peak at 728 cm-1, assigned to Ti-O-Ti stretching in corner-shared TiO6 chains,10 and a small band at 305 cm-1, attributed to Ti-O-Si bending,11 can be seen. The absence of any peak above 800 cm-1 on the Raman spectrum further confirms the inexistence of ETS-4 in this sample.10,11 The observation of a main band at about 1024 cm-1 due to Si-O stretching and a few small bands at 668, 570, and 434 cm-1 because of Ti-O stretching, Si-O rocking and O-Ti-O bending, and O-Si-O and O-Ti-O bending (8) Ho, Y. S.; McKay, G. Water Res. 2000, 34, 735-742. (9) Xu, H.; Zhang, Y.; Navrotsky, A. Microporous Mesoporous Mater. 2001, 47, 285-291. (10) Kim, W. H.; Lee, M. C.; Yoo, J. C.; Hayhurst, D. T. Microporous Mesoporous Mater. 2000, 41, 79-88. (11) Su, Y.; Balmer, M. L.; Bunker, B. C. J. Phys. Chem. B 2000, 104, 8160-8169.
Letters
Figure 2. Adsorption kinetics of Pb2+ on ETS-10 (C0 ) 10 mmol/L, V/m ) 0.2 L/g).
Figure 3. Adsorption isotherm of Pb2+ on ETS-10. Table 1. Langmuir and Freundlich Parameters for Pb2+ Adsorption on ETS-10 Freundlich model K (mmol1-1/n L1/n g-1)
n
1.50
0.102
Langmuir model R2
qm (mmol g-1)
b (L mmol-1)
R2
0.932
1.12
480
0.986
and Ti-O rocking, respectively, on the FTIR spectrum (right-hand insert) is consistent with the literature data of ETS-10.12 The specific surface area calculated from the Brunauer-Emmett-Teller (BET) equation at the relative pressure range of 0.05-0.15 was about 258 m2/g. Figure 2 shows the adsorption kinetics of Pb2+ on ETS10 together with the pseudo-second-order kinetic curve. It is seen that the adsorption rate is extremely fast. Under the experimental conditions, less than 10 s was required to attain saturation adsorption. When the concentration of Pb was about 2.5 mmol/L, Pb2+ was not detected after 5 s. This extremely rapid adsorptive behavior of ETS-10 toward Pb is of interest and significance in purification of drinking water as Kuznicki and Thrush suggested.6 It is also seen that the experimental data fit well to the pseudo-second-order equation. Figure 3 shows the experimental adsorption isotherm of Pb2+ on ETS-10. The experimental data were fitted to both Langmuir and Freundlich isotherms, and the results are included in Figure 2 as well. The parameters derived (12) Mihailova, B.; Valtchev, V.; Mintova, S.; Konstantinov, L. Zeolites 1996, 16, 22-24.
Letters
Figure 4. Adsorption kinetic curve of Pb2+ on ETS-10 and desorption kinetic curves of both Na+ and K+ (b, adsorption of Pb2+; 0, desorption of Na+; 4, desorption of K+).
from the two models are summarized in Table 1. As can be seen, the Langmuir isotherm predicts the experimental data much better than the Freundlich isotherm. The maximum adsorption capacity of Pb2+ on ETS-10 as predicted by the Langmuir isotherm is 1.12 mmol/g or about 232 mg/g. Such a high adsorption capacity of Pb2+ on zeolite materials had never been observed.1 The adsorption of Pb2+ on a commercial zeolite NaY sample (Si/Al ) 2.45) was measured as well, and the results showed that its maximum adsorption capacity toward Pb2+ was about 56.3 mg/g, much less than ETS-10. The adsorption of other heavy metals including Cd2+, Cu2+, and Zn2+ on ETS-10 and zeolite NaY was also studied and compared. A similar adsorption behavior was observed; namely, the adsorption rate of these metal ions on ETS10 was extremely fast and the adsorption capacity was much higher on ETS-10 than on zeolite NaY. The maximum adsorption capacities of Cd, Cu, and Zn on ETS-
Langmuir, Vol. 19, No. 6, 2003 1979
10 were found to be all around 0.5 mmol/g, while they became about 0.2 mmol/g on zeolite NaY. During the adsorption measurements, the concentrations of both Na+ and K+ were monitored as well. Figure 4 shows the concentration profiles of Pb2+, Na+, and K+ as a function of time (t). The initial concentration of Pb2+ was 0.485 mmol/L, and the batch factor was 200 mL/g. It is seen that the increase in Na+ and K+ concentrations is at the expense of the decrease in Pb+ concentration. In addition, the concentration of K+ in the solution is always 1 /3 of that of Na+, indicating the equal opportunity of ion exchange of Pb2+ with the two alkali metal ions, which is consistent with the chemical formula of ETS-10. Furthermore, the concentration sum of Na+ and K+ exactly doubles the concentration of Pb2+ at any time, suggesting that each Pb2+ ion can replace “1.5Na+ + 0.5K+” ions. Conclusion In conclusion, we have demonstrated the unusual adsorption behaviors of microporous titanosilicate ETS10. Heavy metal ions adsorb on ETS-10 at an extremely fast rate and in a large adsorption capacity, in particular Pb2+. The adsorption is most likely an ion exchange process. The remarkably rapid adsorption rate and the huge adsorption amount of ETS-10 toward heavy metal ions make it a promising sorbent for water and wastewater treatment. We believe that the unique framework structure of ETS-10 determines its adsorption properties in solution. Acknowledgment. We thank the National University of Singapore for financial support, Mr. Qin Zhen for AAS measurement, and Mr. Wei Xuming for XRD measurement. LA026490L
