Selective Adsorbents from Ordered Mesoporous Silica - Langmuir

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Langmuir 2003, 19, 3019-3024

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Selective Adsorbents from Ordered Mesoporous Silica Ka Yee Ho, Gordon McKay, and King Lun Yeung* Department of Chemical Engineering, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, SAR-P.R. China Received October 17, 2002. In Final Form: January 6, 2003 Ordered mesoporous silica adsorbents were prepared by grafting amino- and carboxylic-containing functional groups onto MCM-41 for the removal of Acid blue 25 and Methylene blue dyes from wastewater. The amino-containing OMS-NH2 adsorbent has a large adsorption capacity and a strong affinity for the Acid blue 25. It can selectively remove Acid blue 25 from a mixture of dyes (i.e., Acid blue 25 and Methylene blue). The OMS-COOH is a good adsorbent for Methylene blue displaying excellent adsorption capacity and selectivity for the dye. The better selectivity of the OMS-based adsorbents means longer operating life and less maintenance. Furthermore, these adsorbents can be regenerated by simple washing with alkaline or acid solution to recover both the adsorbents and the adsorbed dyes.

1. Introduction The ordered mesoporous materials (e.g., M41S, FSM, HMS, and SBA) belong to an important class of molecular sieve materials. Their large surface area, ordered pore structure, and nanometer-sized pores offer a unique environment for chemical separations1 and reactions.2-4 The concept of “supramolecular templating” has enabled the design of mesoporous silica with adjustable pore sizes and structures.5,6 Chemical modifications of the pore with metals,7,8 metal oxides,9,10 and organic moieties1,11,12 are successful in tailoring the physical, chemical, and catalytic properties of these materials. The two main obstacles for the widespread industrial application of OMS are its cost and stability. In the manufacture of the ordered mesoporous silica, the surfactant template accounts for more than 80% of the cost. Nondestructive methods for the removal and recovery of the surfactants and the use of cheaper polymer substitutes have substantially cut the cost of the OMS.13-15 Better thermal and hydrothermal stability were obtained by postsynthesis treatment of the OMS with organic solvent containing metal alkoxides,16 salt solution,17 and organosilanes.18 The bulk of the adsorption and transport studies on MCM-41 were conducted for gases19-22 with very few works * To whom correspondence should be addressed. Tel: 8522358-7123; Fax: 852-2358-0054; E-mail: [email protected]. (1) Thomas, J. M. Nature 1994, 368, 289. (2) Corma, A.; Navarro, M. T.; Pariente, J. P. J. Chem. Soc., Chem. Commun. 1994, 147. (3) Shinoda, T.; Izumi, Y.; Onaka, M. J. Chem. Soc., Chem. Commun. 1995, 1801. (4) Kloetstra, K. R.; Vandenbroek, J.; van Bekkum, H. Catal. Lett. 1997, 47, 235. (5) Vartuli, J. C.; Schmitt, K. D.; Kresge, C. T.; Roth, W. J.; Leonowicz, M. E.; McCullen, S. B.; Hellring, S. D.; Beck, J. S.; Schlenker, J. L.; Olson, D. H.; Sheppard, E. W. Chem. Mater. 1994, 6, 2317. (6) Biz, S.; Occelli, M. L. Catal. Rev.sSci. Eng. 1998, 40, 329. (7) Okumura, M.; Tsubota, S.; Iwamoto, M.; Haruta, M. Chem. Lett. 1998, 315. (8) Long R. Q.; Yang, R. T. Catal. Lett. 1998, 52, 9. (9) Mulukutla, R. S.; Asakura, K.; Namba, S.; Iwasawa, Y. J. Chem. Soc., Chem. Commun. 1998, 14, 1425. (10) Grubert, G.; Rathousky, J.; Schulzekloff, G.; Wark, M.; Zukal, A. Microporous Mesoporous Mater. 1998, 22, 225. (11) Van Rhijn, W.; De Vos, D.; Bossaert, W.; Bullen, J.; Woulters, B.; Grobet, P.; Jacob, P. Stud. Surf. Sci. Catal. 1998, 117, 183. (12) Koyano, K. A.; Tatsumi, T.; Tanaka, Y.; Nakata, S. J. Phys. Chem. B 1997, 101, 9436. (13) Kawi, S.; Lai, M. W. CHEMTECH 1998, 28, 26. (14) Chen, C. Y.; Li, H. A.; Davis, M. E. Microporous Mater. 1993, 2, 17. (15) Whitehurst, D. D. U.S. Patent 5,143,879, 1992.

dealing with the liquid-phase system.23-25 However, the most promising applications of these materials are for liquid-phase separations and reactions. OMS materials can play a vital role in the “green” production of high value fine chemicals and pharmaceuticals.26 The OMS has been used as either substitute catalyst or support matrix for homogeneous catalyst, therefore significantly reducing the needs for these hazardous materials in fine chemical production.27 Two other applications involving liquid-phase systems demonstrate the potential of OMS technology in separation and pollution problems. It also emphasizes the importance of the adsorption and diffusion processes in the performance of the OMS. Recent work by Hata and co-workers28 has reported that Taxol can be efficiently recovered from the extraction solvent by using OMS with tailored pore structure. The anticancer drug Taxol was obtained at a high purity. Feng et al.23 have shown that mercapto-functionalized OMS are effective adsorbents for the removal of mercury and other heavy metals from water. Simply changing the pH of the washing solution allows the total regeneration of the modified mesoporous silica adsorbent and full recovery of the adsorbed metals. Although further cost reduction is needed for the economical application of OMS material to general environmental problems, there are many specific cases where OMS technology is urgently needed. These include (16) McCullen S. B.; Vartuli, J. C. U.S. Patent 5,156,829, 1992. (17) Ryoo, R.; Jun, S. J. Phys. Chem. B 1997, 101, 317. (18) Tatsumi, T.; Koyano, K. A.; Tanaka, Y.; Nakata, S. Stud. Surf. Sci. Catal. 1998, 117, 143. (19) Rathousky, J.; Zukai, A.; Franke, O.; Schulz-Ekloff, G. Faraday Trans. 1995, 91, 937. (20) Llewellyn, P. C.; Schu¨th, F.; Grillet, Y.; Rouqoerol, F.; Rouquerol, J.; Unger, K. K. Langmuir 1995, 11, 574. (21) Branton, P. J.; Hall, P. G.; Sing, K. S. W.; Reichert, H.; Schu¨th, F.; Unger, K. K. Faraday Trans. 1994, 90, 2965. (22) Branton, P. J.; Hall, P. G.; Treguer, M.; Sing, K. S. W. Faraday Trans. 1995, 91, 2041. (23) Feng, X.; Fryxell, G. E.; Wang, L. Q.; Kim, A. Y.; Liu, J.; Kemner, K. M. Science 1997, 276, 923. (24) Derylo-Marczewska, A.; Goworek, J.; Borowka, A.; Kusak, R. Proceedings of the Second Pacific Basin Conference on Adsorption Science and Technology; Brisbane, Australia, 2000. (25) Nooney, R. I.; Kalyanaraman, M.; Kennedy, G.; Maginn, E. J. Langmuir 2001, 17, 528. (26) Biz, S.; Occelli, M. L. Catal. Rev.sSci. Eng. 1998, 40, 329. (27) Zhao, X. S.; Lu, G. Q.; Millar, G. J. Ind. Eng. Chem. Res. 1996, 35, 2087. (28) Hata, H.; Saeki, S.; Kimura, T.; Sugahara, Y.; Kuroda, K. Chem. Mater. 1999, 11, 1100.

10.1021/la0267084 CCC: $25.00 © 2003 American Chemical Society Published on Web 02/14/2003

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ultrapure water purification and reuse in semiconductor industry, treatment of pharmaceutical wastewater discharge, and recovery of toxic homogeneous catalysts used in fine chemical production. In most industrial wastewaters, the removal of one or two problematic pollutants from the effluent can mean a significant saving in the treatment cost. In some instances, their removal permits the discharge of the rest of the effluent for municipal treatment. Dyes used in the textile industry are one such example. This work investigates the possibility of designing selective adsorbents for the Acid blue 25 and Methylene blue dyes using ordered mesoporous silica. 2. Experimental Section 2.1 Synthesis and Characterization of Mesoporous Silica. Mesoporous MCM-41 has been successfully prepared using different synthesis procedures and conditions.27 For this study, the MCM-41 powder was crystallized from an alkaline solution containing tetraethyl orthosilicate (TEOS, 98%, Aldrich), cetyltrimethylammonium bromide (CTABr, 99.3%, Aldrich), ammonium hydroxide (NH4OH, 28-30%, Fisher Scientific), and deionized water in the mole ratio of 6.45 TEOS: 1 CTABr: 292 NH4OH: 2773 H2O. After 16 h of crystallization at room temperature, the MCM-41 powder was filtered, washed, and dried before it was calcined in a furnace at 823 K for 24 h to remove the organic template. Different chemical moieties could be grafted onto the surface of MCM-41 to alter its surface properties.29-32 Amino- and carboxylic-containing mesoporous silicas were selected as candidate adsorbents and were prepared using the following procedures. OMS-NH2 was obtained when 2.5 g of calcined MCM-41 was refluxed in 250 mL of dry toluene solution containing 0.1 mole of 3-aminopropyltrimethoxysilane (97%, Aldrich) at 383 K for 18 h. After cooling to room temperature, the powder was collected by centrifugation at 10 000 rpm for 5 min with repeated washing using dry toluene. The preparation of OMS-COOH involves two steps. A CN-containing organic was first grafted onto the MCM-41 by reflux of 2.5 g powder in 250 mL of dry toluene containing 0.1 mole of 3-(triethoxysilyl)propionitrile (98%, Aldrich) at 381 K for 18 h under nitrogen atmosphere. After recovery by centrifugation, the powder was dried at 373 K for 24 h to obtain CN-MCM-41. The cyanide group was then hydrolyzed to carboxylic acid by reflux with 50% sulfuric acid (95-97%, BDH) at 408 K for 3 h. The sample was then collected, washed, and dried to obtain OMS-COOH powder. The powder morphology and crystal structure of the mesoporous materials were characterized by electron microscopy and X-ray diffraction. For analysis by transmission electron microscopy (TEM, JEOL JEM-2010), the powder sample was suspended in methanol and transferred onto a carbon-coated (10 nm) copper grid. The sample was allowed to dry at room temperature before TEM imaging. The particle size was confirmed by scanning electron microscopy (JEOL JSM-6300). The phase structure and pore size of the mesoporous silica were evaluated from the X-ray diffraction pattern obtained from Philip 1080 X-ray diffractometer equipped with a CuKR radiation source and a graphite monochromator. The BET surface area of the mesoporous silica was measured by Coulter SA 3100 nitrogen physi-adsorption apparatus. The surface composition and chemistry of MCM-41 was investigated by X-ray photoelectron spectroscopy (Physical Electronics PHI 5600) and Fourier transform infrared spectroscopy (Perkin-Elmer model GX 2000). Thermogravimetric and differential thermal analyses (TGA/DTA, Setaram 92-18) were useful methods for fingerprinting and quantifying the types of organic moieties present on the MCM-41 surface. In these experiments, the powder sample was placed in a platinum holder and heated from 298 to 1073 K at a heating rate of 5 K/min in (29) Macquarrie, Duican J. J. Chem. Soc., Chem. Commun. 1996, 1961. (30) Butterworth, A. J.; Clark, J. H.; Walton, P. H.; Barlow, S. J. Chem. Soc., Chem. Commun. 1996, 1859. (31) Elings, J. A.; Ait-Meddour, R.; Clark, J. H.; Macquarrie, D. J. J. Chem. Soc., Chem. Commun. 1998, 2707. (32) Jaroniec, C. P.; Kruk, M.; Jaroniec, M. J. Phys. Chem. B 2001, 105, 5503.

Figure 1. (a) Molecular formula and (b) UV-visible spectra of Acid blue 25 and Methylene blue dyes. Table 1. Characteristic Properties of Dye Molecules dyes molecular weight (g/mol) molecular size (Å) characteristic wavelength, λ (nm)

Acid blue 25 Methylene blue 416.39 14 257 602-618

319.85 15 291 622

air. Alpha alumina powder was used as a standard reference material for DTA. 2.2 Liquid-Phase Adsorption Study. The adsorption properties of ordered mesoporous silicas (i.e., OMS, OMS-NH2, and OMS-COOH) were investigated for two different organic dyes, the Acid blue 25 (45%, Aldrich) and Methylene blue (Aldrich). The molecular structures of the two dyes are shown in Figure 1a and some of their characteristic properties are listed in Table 1. The adsorption isotherms were obtained for 0.1 g of adsorbent in 100 mL aqueous solution with dye concentrations of 20-1000 mg/L. The solutions were placed in a shaker bath and allowed to equilibrate at constant temperature of 295 ( 2 K for 12 days. The dye concentration was measured by UV/visible spectrophotometer (Perkin-Elmer 550S). Instead of using the longer characteristic wavelengths of the dyes at 600-620 nm where there is a severe overlap in their signals (Figure 1b), we have chosen the characteristic wavelengths of 257 nm for Acid blue 25 and 291 nm for Methylene blue for this study. Both signals display good linear relationship between intensity and dye concentration. The possible recovery and reuse of adsorbent and dye were also examined. In these experiments, 0.1 g of spent adsorbent was washed at different pH (i.e., 10 mL wash solution). The amount of dye recovered was measured by UV/visible spectrophotometer and the amount of remaining dye in the adsorbent was determined by TGA method. The adsorption capacity and selectivity of OMS for binary dye mixtures were determined by agitating 0.1 g of adsorbent powder in 100 mL dye solutions containing mole ratios of Acid blue 25 to Methylene blue of 0:100, 20:80, 40:60, 60:40, 80:20, and 100:0 for 12 days at constant temperature of 295 ( 2 K. The individual dye concentration in the mixture was calculated using the optical densities of the dyes at the specified wavelength.

CA ) (kB2d1 - kB1d2)/(kA1kB2 - kA2kB1)

(1)

CB ) (kA1d2 - kA2d1)/(kA1kB2 - kA2kB1)

(2)

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where CA is the concentration of Acid blue 25; CB is the concentration of Methylene blue; kA1, kA2, kB1, and kB2 are the calibration constants for the dyes at their characteristic absorption wavelength (i.e., λ1 and λ2); d1 and d2 are the optical densities at the two wavelengths λ1 and λ2.

3. Results and Discussion 3.1 Mesoporous Silica Powders. Mesoporous silica prepared from alkaline solution has a plate-like morphology as shown in the SEM picture in Figure 2a. The plates have an average diameter of 0.9 µm and a thickness of about 0.1 µm. TEM analysis indicates that the individual particles have a hexagonal shape (cf. Figure 2a inset). The mesoporous silica displays the characteristic X-ray diffraction pattern of MCM-4133 as shown in Figure 2b. The presence of both (110) and (200) diffraction peaks are evidence of good crystallinity of the prepared powder. The DTA plot in Figure 2c shows that the organic template molecules have been completely removed during the calcination. The calcined sample has a BET surface area of 1071 m2/g and a pore volume of 0.3805 cm3/g. The pore size of the MCM-41 powder can be calculated from the X-ray diffraction interplanar spacing (Figure 2b) using the equation,34

Wd ) cd(FVp/(1 + FVp))1/2

(3)

where Wd is the pore size, Vp is the primary mesoporous volume (0.3805 cm3/g from N2 physisorption), F is the pore wall density (ca. 2.2 cm3/g for siliceous materials), d is the XRD(100) interplanar spacing (i.e., 35.36 Å for the sample), and c is a constant dependent on the assumed pore geometry and is equal to 1.155 for hexagonal models. From the equation, the pore size of the mesoporous silica is 27.57 Å. Routine chemical analysis of the sample by energydispersive X-ray spectrometer (EDXS) and XPS detected only silicon and oxygen atoms with carbons from adsorbed ambient carbon dioxide and hydrocarbons as the main surface impurities. The FTIR spectrum of the powder displays the characteristic bands for Si-OH groups at 3674, 948, and 802 cm-1 (Figure 2d). Amino and carboxylic functional groups were grafted onto the ordered mesoporous silicas to create adsorbent materials that have specific affinity for the target pollutants, Acid blue 25 and Methylene blue dyes. The particle morphology remains unchanged after grafting R-NH2 and R-COOH functional groups onto the mesoporous silica. However, a decrease in the (100) peak’s signal was observed after the chemical modification (Figure 2b). The decrease in the crystallinity is more likely due to the inherent disorder introduced by the modification process rather than due to the collapse in the pore structure of the mesoporous silica. The DTA plots for the mesoporous silica containing different chemical moieties display distinct patterns (Figure 2c). The OMS-NH2 has two sharp peaks at 575 and 602 K and a broad shoulder centered at 816 K, whereas OMS-COOH has a small peak at 593 K and a large peak located at 725 K. Table 2 shows that the BET surface areas of OMS-NH2 and OMS-COOH are smaller than the precursor powder. Both modified mesoporous silicas have a smaller pore diameter of about 25.5 Å. The Fourier transform infrared spectrum of OMS-NH2 shows that the characteristic peaks at 1590, 3300, and 3435 cm-1 for R-NH2 are partially obscured by adsorbed water, but the CO stretching band at 1718 cm-1 is clearly evident in (33) Zhao, X. S.; Lu, G. Q.; Whittaker, A. K.; Millar, G. J.; Zhu, H. Y. J. Phys. Chem. B 1997, 101, 6525. (34) Jaroniec, M.; Kruk, M.; Sayari, A. Mesoporous Mol. Sieves 1998, 117, 325.

Figure 2. (a) Scanning electron microscopy picture of MCM41 adsorbent powder (OMS) after air calcination. Figure inset is a transmission electron microscopy picture of a MCM-41 particle. (b) X-ray diffraction pattern for OMS, OMS-NH2, and OMS-COOH adsorbents. (c) Differential thermal analysis plots for OMS, OMS-NH2, and OMS-COOH adsorbents. (d) Fourier transformed infrared spectra for OMS, OMS-NH2, and OMSCOOH adsorbents.

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Table 2. Properties of Ordered Mesoporous Silica Adsorbents

sample

BET surface area (m2/g)

pore sizea (nm)

OMS OMS-N H2 OMS-C OOH

1071 774 757

2.76 2.57 2.55

a

concentration of organic surface functional groupb (mmol/g) (groups/nm2) 0 2.226 1.343

maximum adsorption capacity (mg/g, T ) 295 K) Acid blue 25 Methylene blue

0 1.7 1.0

15 256 7

54 90 113

The pore size was calculated using eq 3. b The concentration of the surface chemical groups was calculated from the TGA data. Table 3. Adsorption Capacity of Carbon-Based Adsorbents for Acid Blue 25 and Methylene Blue Dyes maximum adsorption capacity (mg/g) adsorbent

Acid blue 25

bagasse pitha pithb activated petroleum cokec activated carbon (granular)d

21.7 17.5

a

Methylene blue

100 80-300

Reference 35. b Reference 36. c Reference 37.

d

Reference 38.

Table 4. Recovery of Spent Adsorbents Methylene Acid blue blue in 25 in wash pH of wash a b wash solution regeneration (%) solutionc regenerationb (%) solution (ppm) OMS-NH2 OMS-COOH (ppm) pH 2 pH 4 pH 7 pH 10

Figure 3. Equilibrium adsorption isotherms of (a) Acid blue 25 and (b) Methylene blue dyes on OMS (4), OMS-NH2 (0), and OMS-COOH (O) from single dye solutions.

the OMS-COOH sample (Figure 2d). The amount of functional group grafted onto the mesoporous adsorbent was calculated from the TGA measurements and is listed in Table 2. The 2.23 mmol/g of RNH2 grafted onto the OMS-NH2 gave a surface concentration of amino groups of 1.7 per nm2 compared to 1 carboxylic group per nm2 for OMS-COOH. 3.2 Adsorption Properties of Mesoporous Silica Powders. 3.2.1 Single Dye Adsorption. Figure 3a displays the equilibrium adsorption isotherms for Acid blue 25 at 295 K for the OMS, OMS-NH2, and OMS-COOH adsorbent powders. It is clear from the figure that the adsorption capacity of OMS-NH2 for Acid blue 25 is significantly higher than both the unmodified OMS and OMS-COOH. An adsorption capacity of 250 mg or 0.6 mmol of Acid blue 25 per gram of OMS-NH2 was obtained. Although the amount of Acid blue 25 adsorbed is less than the amount of available NH2 sites (i.e., 0.6 vs 2.23 mmol/ g), calculation indicates that at the maximum adsorption capacity there is a monolayer surface coverage. The adsorption capacity of OMS-NH2 for the acid dye is also higher than the reported values for several carbon-based adsorbents (Table 3).35-38 The initial adsorption rates of Acid blue 25 (i.e., C0 ) 150 ppm) on OMS, OMS-NH2, and OMS-COOH are 0.4, 66, and 76 ppm dye/h per gram of adsorbent, respectively. The large adsorption capacity and relatively rapid adsorption rate of OMS-NH2 makes it an attractive adsorbent for the removal of acid dye (35) McKay, G.; Geundi, M. EL; Nassar, M. M. Trans. I Chem. E 1996, 74, 277. (36) Chen, B.; Hui, C. W.; Mckay, G. Chem. Eng. J. 2001, 84, 77. (37) Shawwa, R.; Smith, W.; Sego, C. Water Res. 2001, 35, 745. (38) Potgieter, J. H. J. Chem. Educ. 1991, 68, 349.

1.82 7.91 13.01 20.87

0.23 1.02 1.67 2.71

25.26 3.11 1.34 1.78

12.14 1.49 0.64 0.85

a Concentration of Acid blue 25 dye in 10 mL of wash solution used for regenerating OMS-NH2 adsorbent. b Amount of dye removed from the adsorbent based on TGA measurement. c Concentration of Methylene blue dye in 10 mL of wash solution used for regenerating OMS-COOH adsorbent.

pollutants from effluents, which is an acknowledged problem in the textile industry. Figure 3b shows that Methylene blue dye has good affinity for the carboxylic acid group grafted onto OMSCOOH. This can be attributed to the weak Lewis base characteristic of Methylene blue and to the cationicanionic interaction between the dye molecule and the surface moiety. This adsorbent displays higher adsorption capacity for the Methylene blue than both OMS-NH2 and unmodified MCM-41. At the maximum adsorption capacity, about 0.3 mmol/g of Methylene blue were adsorbed on OMS-COOH. Although the adsorbates occupy less than a quarter of the available COOH sites, the surface coverage is close to a monolayer. It has been well established that the adsorbed Methylene blue on hydroxylated surface (e.g., MCM-41) is usually coordinated to four surface hydroxyl groups.39 It is possible that the Methylene blue molecule adsorbed on OMS-COOH is also coordinated to four COOH sites. The initial adsorption rates of Methylene blue (i.e., C0 ) 150 ppm) on OMS, OMS-NH2, and OMSCOOH are 4, 45, and 210 ppm dye/h per gram of adsorbent, respectively. It is clear from Figure 3 that the starting precursor material MCM-41 is a poor adsorbent for both dyes. Simply grafting select functional groups (e.g., RNH2 and RCOOH) onto MCM-41creates new, high surface area adsorbents that have selective affinity for Acid blue 25 (i.e., OMS-NH2) and Methylene blue dyes (i.e., OMSCOOH). The adsorbent and dyes could be recovered by simply washing with alkaline or acid solution. Table 4 lists the concentration of the recovered dye and the percentage of dyes removed from the adsorbent after a (39) Kaewprasit, C.; Hequet, E.; Abidi, N.; Gourlot, J. P. J. Cotton Sci. 1998, 2, 164.

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Table 5. Effect of Repeated Wash on Dye Recovery and Adsorbent Regeneration no. of washing

recovery (%) of Acid blue 25 from OMS-NH2a

recovery (%) of Methylene blue from OMS-COOHb

1 2 3 4 5

2.71 6.17 8.84 11.85 14.99

12.14 24.53 35.59 46.61 57.51

a Recovery from pH 10 wash solution and calculated from material balance. b Recovery from pH 2 wash solution and calculated from material balance.

single stage washing of the adsorbent using wash solutions with different pH. Methylene blue dye and MCM-COOH can be easily recovered and regenerated by acid wash, whereas Acid blue 25 is more strongly adsorbed on MCMNH2. A single acid wash was able to remove 12% of adsorbed Methylene blue dyes that were adsorbed on OMS-COOH. Using a more alkaline solution (pH > 10), 15% recovery of acid blue 25 can be obtained from single washing, but dissolution of the silica matrix is also more likely to occur at this pH. Table 5 suggests that repeated washing is a better alternative and could lead to complete regeneration of adsorbent. These recovery and regeneration values are better than for typical carbonaceous adsorbents, which require harsher regeneration conditions (e.g., steam treatments). After washing, there was no decrease in the number of functional groups in OMSNH2, but there is a loss of 35% for the OMS-COOH. The latter is mainly due to the detachment of the chemical moiety from the pore wall during the acid wash. 3.2.2 Binary Dyes Adsorption. The single dye adsorption experiments indicate that OMS-NH2 has a higher affinity for Acid blue 25 than Methylene blue dye. Similarly, OMSCOOH has a higher adsorption capacity for Methylene blue than Acid blue 25. To test the performance of these mesoporous silica adsorbents for selective dye removal, adsorption experiments were conducted using solutions containing a mixture of Acid blue 25 and Methylene blue dyes. Figure 4 displays the equilibrium adsorption isotherms for Acid blue 25 and Methylene blue dyes obtained from these experiments. Figure 4a shows that OMS-NH2 remains the best adsorbent for the Acid blue 25. Its adsorption performance is comparable to that obtained from the single dye adsorption experiment (cf. Figure 3a). On the other hand, the adsorption capacity of the other two mesoporous silica adsorbents displays significantly higher values for the mixture compared to the single dye experiment. This synergistic effect can be more easily explained if we also take into account the adsorption properties of the adsorbents for Methylene blue (Figure 4b). The plots show that the adsorption performance of OMS-NH2 for Methylene blue is enhanced by the presence of Acid blue dye in the mixture. Although the competitive adsorption of Acid blue 25 and Methylene blue dyes on OMS-NH2 could not be ruled out, the adsorption data suggest that it is more likely that we have a multilayer adsorption. The strong affinity of the Acid blue 25 dye for the amino groups on the adsorbent means that the first monolayer of adsorbed molecules will consist mainly of Acid blue 25 molecules. This may also explain why the equilibrium adsorption isotherms for Acid blue 25 are the same for the single and binary dyes mixtures. The adsorbed dye molecules will change the chemical characteristics of the adsorbent and being acidic and anionic in nature, they could further interact with the basic and cationic Methylene blue dyes in the solution leading to multilayer adsorption and to the observed enhancement

Figure 4. Equilibrium adsorption isotherms of (a) Acid blue 25 and (b) Methylene blue dyes on OMS (4), OMS-NH2 (0), and OMS-COOH (O) from binary dye mixtures.

in the adsorption of Methylene blue on OMS-NH2. A separate experiment was conducted wherein acid blue 25 (C0 ) 2.4 mM) was first adsorbed on OMS-NH2 and the dye-modified OMS was then used as adsorbent for Methylene blue (C0 ) 2.4 mM). 0.69 mmol/g (286 mg/g) of Acid blue 25 was adsorbed on OMS-NH2 and an additional 0.64 mmol/g (223 mg/g) of Methylene blue was adsorbed on the dye-modified OMS. This clearly demonstrates that preadsorbed Acid blue 25 on OMS-NH2 changes the chemistry and adsorption properties of the adsorbent and that Methylene blue can be adsorbed on top of the Acid blue 25 layer. The same phenomenon could also explain the adsorption of Acid blue 25 on OMS-COOH (cf. Figure 4a). The adsorption behavior of Methylene blue dye on OMSCOOH from single and binary dye mixtures are similar. Both reach a maximum adsorption capacity of about 100 mg per gram of adsorbent. The single dye experiment clearly shows that the Acid blue dye does not have any affinity for the carboxylic acid groups grafted onto OMSCOOH. However, in the presence of Methylene blue, nearly 50 mg of Acid blue 25 was adsorbed per gram of the adsorbent. This strongly suggests that the adsorption of Acid blue 25 onto OMS-COOH is mediated by the adsorbed Methylene blue dyes. This is confirmed in an experiment wherein Methylene blue was preadsorbed on OMS-COOH (0.38 mmol/g or 123 mg/g) and the dyemodified OMS adsorbent was used to adsorb Acid blue 25 from a solution. An additional 0.38 mmol/g (160 mg/g) of Acid blue 25 was adsorbed on the dye-modified adsorbent. The adsorption of Acid blue 25 on unmodified MCM-41 from the mixture of dyes also displays an improvement compared to the value obtained from the single dye experiment (cf. Figures 4a and 3a). However, the adsorption isotherms of the two dyes from the mixture suggest that there is competitive adsorption between Acid blue 25 and Methylene blue on OMS adsorbent (cf. Figure 4b). This may be because the native hydroxyl groups on MCM-41 do not have a strong affinity for either dye.

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4. Concluding Remarks This work demonstrates that selective adsorbents could be prepared from the ordered mesoporous silica (i.e., MCM41) by grafting chemical moieties that have specific affinity for the target molecules. The Acid blue 25 represents an important class of dye pollutants that cannot be effectively removed by traditional carbonaceous adsorbents such as activated carbons and peat. The amino-modified OMSNH2 has a strong affinity and adsorption capacity for acidic dye molecules and could efficiently remove Acid blue 25 from a mixture. Similarly, OMS containing grafted carboxylic groups (OMS-COOH) is a good adsorbent for the removal of Methylene blue dyes and displays comparable adsorption capacity as the commercial adsorbents but with better selectivity. A better selectivity means that the OMS-based adsorbents have longer operating life and require less maintenance, thus resulting in lower capital and operating cost. Also unlike the existing adsorbents, the OMS-based adsorbents can be regenerated by a simple washing procedure. One additional advantage of using ordered mesoporous silica is that it provides an ideal material for studying adsorption and diffusion phenomena. Today, most adsorbents possess complex pore structure that is populated

Ho et al.

by myriad of chemical groups. It is difficult if not impossible to accurately describe the different adsorption sites, chemical interactions, and transport processes in these materials. The cylindrical pore structure and high degree of pore symmetry found in OMS are ideal for testing various existing adsorption and diffusion models. The simple pore geometry is amenable to mathematical description and the amorphous state of the pore wall approximates an ideal Langmuir surface. The ability to introduce well-defined functional groups is invaluable for developing mathematical model for surfaces with multiple adsorption sites. This will allow the systematic study of the nature of interaction between different surface sites and lead to the design of better adsorbent materials. This work constitutes the first step toward this goal. Acknowledgment. The authors would like to gratefully acknowledge the funding from the Hong Kong Research Grant Councils (grant RGC-HKUST 6037/00P). We also thank the Material Characterization and Preparation Facility at the HKUST for the use of XRD, TEM, SEM, EDXS, XPS, and TGA/DTA equipment. LA0267084