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-containingfunctional 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 theAcid blue 25. It can selectively remove Acid blue 25 from a mixture of dyes (i.e., Acid blue 25 and Methyleneblue). The OMS-COOH is a good adsorbent for Methylene blue displaying excellent adsorption capacityand selectivity for the dye. The better selectivity of the OMS-based adsorbents means longer operatinglife and less maintenance. Furthermore, these adsorbents can be regenerated by simple washing withalkaline or acid solution to recover both the adsorbents and the adsorbed dyes.

1. IntroductionThe ordered mesoporous materials (e.g., M41S, FSM,

HMS, and SBA) belong to an important class of molecularsieve materials. Their large surface area, ordered porestructure, and nanometer-sized pores offer a uniqueenvironment for chemical separations1 and reactions.2-4

The concept of “supramolecular templating” has enabledthe design of mesoporous silica with adjustable pore sizesand structures.5,6 Chemical modifications of the pore withmetals,7,8 metal oxides,9,10 and organic moieties1,11,12 aresuccessful in tailoring the physical, chemical, and catalyticproperties of these materials. The two main obstacles forthe widespread industrial application of OMS are its costand stability. In the manufacture of the ordered meso-porous silica, the surfactant template accounts for morethan 80% of the cost. Nondestructive methods for theremoval and recovery of the surfactants and the use ofcheaper polymer substitutes have substantially cut thecost of the OMS.13-15 Better thermal and hydrothermalstability were obtained by postsynthesis treatment of theOMS with organic solvent containing metal alkoxides,16

salt solution,17 and organosilanes.18

The bulk of the adsorption and transport studies onMCM-41 were conducted for gases19-22 with very few works

dealing with the liquid-phase system.23-25 However, themost promising applications of these materials are forliquid-phase separations and reactions. OMS materialscan play a vital role in the “green” production of highvalue fine chemicals and pharmaceuticals.26 The OMShas been used as either substitute catalyst or supportmatrix for homogeneous catalyst, therefore significantlyreducing the needs for these hazardous materials in finechemical production.27 Two other applications involvingliquid-phase systems demonstrate the potential of OMStechnology in separation and pollution problems. It alsoemphasizes the importance of the adsorption and diffusionprocesses in the performance of the OMS. Recent work byHata and co-workers28 has reported that Taxol can beefficiently recovered from the extraction solvent by usingOMS with tailored pore structure. The anticancer drugTaxol was obtained at a high purity. Feng et al.23 haveshown that mercapto-functionalized OMS are effectiveadsorbents for the removal of mercury and other heavymetals from water. Simply changing the pH of the washingsolution allows the total regeneration of the modifiedmesoporous silica adsorbent and full recovery of theadsorbed metals.

Although further cost reduction is needed for theeconomical application of OMS material to generalenvironmental problems, there are many specific caseswhere OMS technology is urgently needed. These include

* To whom correspondence should be addressed. Tel: 852-2358-7123; Fax: 852-2358-0054; E-mail: kekyeung@ust.hk.

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Mater. 1999, 11, 1100.

3019Langmuir 2003, 19, 3019-3024

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

ultrapure water purification and reuse in semiconductorindustry, treatment of pharmaceutical wastewater dis-charge, and recovery of toxic homogeneous catalysts usedin fine chemical production. In most industrial waste-waters, the removal of one or two problematic pollutantsfrom the effluent can mean a significant saving in thetreatment cost. In some instances, their removal permitsthe discharge of the rest of the effluent for municipaltreatment. Dyes used in the textile industry are one suchexample. This work investigates the possibility of design-ing selective adsorbents for the Acid blue 25 and Methyleneblue dyes using ordered mesoporous silica.

2. Experimental Section

2.1 Synthesis and Characterization of MesoporousSilica. Mesoporous MCM-41 has been successfully preparedusing different synthesis procedures and conditions.27 For thisstudy, the MCM-41 powder was crystallized from an alkalinesolution containing tetraethyl orthosilicate (TEOS, 98%, Aldrich),cetyltrimethylammonium bromide (CTABr, 99.3%, Aldrich),ammonium hydroxide (NH4OH, 28-30%, Fisher Scientific), anddeionized water in the mole ratio of 6.45 TEOS: 1 CTABr: 292NH4OH: 2773 H2O. After 16 h of crystallization at roomtemperature, the MCM-41 powder was filtered, washed, and driedbefore it was calcined in a furnace at 823 K for 24 h to removethe organic template. Different chemical moieties could be graftedonto the surface of MCM-41 to alter its surface properties.29-32

Amino- and carboxylic-containing mesoporous silicas wereselected as candidate adsorbents and were prepared using thefollowing procedures. OMS-NH2 was obtained when 2.5 g ofcalcined MCM-41 was refluxed in 250 mL of dry toluene solutioncontaining 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 5min with repeated washing using dry toluene. The preparationof OMS-COOH involves two steps. A CN-containing organicwas first grafted onto the MCM-41 by reflux of 2.5 g powder in250 mL of dry toluene containing 0.1 mole of 3-(triethoxysilyl)-propionitrile (98%, Aldrich) at 381 K for 18 h under nitrogenatmosphere. After recovery by centrifugation, the powder wasdried at 373 K for 24 h to obtain CN-MCM-41. The cyanidegroup was then hydrolyzed to carboxylic acid by reflux with 50%sulfuric acid (95-97%, BDH) at 408 K for 3 h. The sample wasthen collected, washed, and dried to obtain OMS-COOH powder.

The powder morphology and crystal structure of the meso-porous materials were characterized by electron microscopy andX-ray diffraction. For analysis by transmission electron micros-copy (TEM, JEOL JEM-2010), the powder sample was suspendedin methanol and transferred onto a carbon-coated (10 nm) coppergrid. The sample was allowed to dry at room temperature beforeTEM imaging. The particle size was confirmed by scanningelectron microscopy (JEOL JSM-6300). The phase structure andpore size of the mesoporous silica were evaluated from the X-raydiffractionpatternobtained fromPhilip1080X-ray diffractometerequipped with a CuKR radiation source and a graphite mono-chromator. The BET surface area of the mesoporous silica wasmeasured by Coulter SA 3100 nitrogen physi-adsorption ap-paratus. The surface composition and chemistry of MCM-41 wasinvestigated by X-ray photoelectron spectroscopy (PhysicalElectronics PHI 5600) and Fourier transform infrared spectros-copy (Perkin-Elmer model GX 2000). Thermogravimetric anddifferential thermal analyses (TGA/DTA, Setaram 92-18) wereuseful methods for fingerprinting and quantifying the types oforganic moieties present on the MCM-41 surface. In theseexperiments, the powder sample was placed in a platinum holderand heated from 298 to 1073 K at a heating rate of 5 K/min in

air. Alpha alumina powder was used as a standard referencematerial for DTA.

2.2 Liquid-Phase Adsorption Study. The adsorption prop-erties of ordered mesoporous silicas (i.e., OMS, OMS-NH2, andOMS-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 Figure1a and some of their characteristic properties are listed in Table1. The adsorption isotherms were obtained for 0.1 g of adsorbentin 100 mL aqueous solution with dye concentrations of 20-1000mg/L. The solutions were placed in a shaker bath and allowedto equilibrate at constant temperature of 295 ( 2 K for 12 days.The dye concentration was measured by UV/visible spectropho-tometer (Perkin-Elmer 550S). Instead of using the longercharacteristic wavelengths of the dyes at 600-620 nm wherethere is a severe overlap in their signals (Figure 1b), we havechosen the characteristic wavelengths of 257 nm for Acid blue25 and 291 nm for Methylene blue for this study. Both signalsdisplay good linear relationship between intensity and dyeconcentration. The possible recovery and reuse of adsorbent anddye were also examined. In these experiments, 0.1 g of spentadsorbent was washed at different pH (i.e., 10 mL wash solution).The amount of dye recovered was measured by UV/visiblespectrophotometer and the amount of remaining dye in theadsorbent was determined by TGA method.

The adsorption capacity and selectivity of OMS for binary dyemixtures were determined by agitating 0.1 g of adsorbent powderin 100 mL dye solutions containing mole ratios of Acid blue 25to Methylene blue of 0:100, 20:80, 40:60, 60:40, 80:20, and 100:0for 12 days at constant temperature of 295 ( 2 K. The individualdye concentration in the mixture was calculated using the opticaldensities of the dyes at the specified wavelength.

(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 spectraof Acid blue 25 and Methylene blue dyes.

Table 1. Characteristic Properties of Dye Molecules

dyes Acid blue 25 Methylene blue

molecular weight (g/mol) 416.39 319.85molecular size (Å) 14 15characteristic wavelength, λ (nm) 257 291

602-618 622

CA ) (kB2d1 - kB1d2)/(kA1kB2 - kA2kB1) (1)

CB ) (kA1d2 - kA2d1)/(kA1kB2 - kA2kB1) (2)

3020 Langmuir, Vol. 19, No. 7, 2003 Ho et al.

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

3. Results and Discussion3.1 Mesoporous Silica Powders. Mesoporous silica

prepared from alkaline solution has a plate-like morphol-ogy as shown in the SEM picture in Figure 2a. The plateshave an average diameter of 0.9 µm and a thickness ofabout 0.1 µm. TEM analysis indicates that the individualparticles have a hexagonal shape (cf. Figure 2a inset).The mesoporous silica displays the characteristic X-raydiffraction pattern of MCM-4133 as shown in Figure 2b.The presence of both (110) and (200) diffraction peaks areevidence of good crystallinity of the prepared powder. TheDTA plot in Figure 2c shows that the organic templatemolecules have been completely removed during thecalcination. The calcined sample has a BET surface areaof 1071 m2/g and a pore volume of 0.3805 cm3/g. The poresize of the MCM-41 powder can be calculated from theX-ray diffraction interplanar spacing (Figure 2b) usingthe equation,34

where Wd is the pore size, Vp is the primary mesoporousvolume (0.3805 cm3/g from N2 physisorption), F is the porewall density (ca. 2.2 cm3/g for siliceous materials), d is theXRD(100) interplanar spacing (i.e., 35.36 Å for the sample),and c is a constant dependent on the assumed poregeometry and is equal to 1.155 for hexagonal models. Fromthe equation, the pore size of the mesoporous silica is 27.57Å. Routine chemical analysis of the sample by energy-dispersive X-ray spectrometer (EDXS) and XPS detectedonly silicon and oxygen atoms with carbons from adsorbedambient carbon dioxide and hydrocarbons as the mainsurface impurities. The FTIR spectrum of the powderdisplays the characteristic bands for Si-OH groups at3674, 948, and 802 cm-1 (Figure 2d).

Amino and carboxylic functional groups were graftedonto the ordered mesoporous silicas to create adsorbentmaterials that have specific affinity for the target pol-lutants, Acid blue 25 and Methylene blue dyes. The particlemorphology remains unchanged after grafting R-NH2 andR-COOH functional groups onto the mesoporous silica.However, a decrease in the (100) peak’s signal wasobserved after the chemical modification (Figure 2b). Thedecrease in the crystallinity is more likely due to theinherent disorder introduced by the modification processrather than due to the collapse in the pore structure of themesoporous silica. The DTA plots for the mesoporous silicacontaining different chemical moieties display distinctpatterns (Figure 2c). The OMS-NH2 has two sharp peaksat 575 and 602 K and a broad shoulder centered at 816K, whereas OMS-COOH has a small peak at 593 K anda large peak located at 725 K. Table 2 shows that the BETsurface areas of OMS-NH2 and OMS-COOH are smallerthan the precursor powder. Both modified mesoporoussilicas have a smaller pore diameter of about 25.5 Å. TheFourier transform infrared spectrum of OMS-NH2 showsthat the characteristic peaks at 1590, 3300, and 3435 cm-1

for R-NH2 are partially obscured by adsorbed water, butthe 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.

Wd ) cd(FVp/(1 + FVp))1/2 (3)

Figure 2. (a) Scanning electron microscopy picture of MCM-41 adsorbent powder (OMS) after air calcination. Figure insetis a transmission electron microscopy picture of a MCM-41particle. (b) X-ray diffraction pattern for OMS, OMS-NH2, andOMS-COOH adsorbents. (c) Differential thermal analysis plotsfor OMS, OMS-NH2, and OMS-COOH adsorbents. (d) Fouriertransformed infrared spectra for OMS, OMS-NH2, and OMS-COOH adsorbents.

Adsorbents from Ordered Mesoporous Silica Langmuir, Vol. 19, No. 7, 2003 3021

the OMS-COOH sample (Figure 2d). The amount offunctional group grafted onto the mesoporous adsorbentwas calculated from the TGA measurements and is listedin Table 2. The 2.23 mmol/g of RNH2 grafted onto theOMS-NH2 gave a surface concentration of amino groupsof 1.7 per nm2 compared to 1 carboxylic group per nm2 forOMS-COOH.

3.2 Adsorption Properties of Mesoporous SilicaPowders. 3.2.1 Single Dye Adsorption. Figure 3a dis-plays the equilibrium adsorption isotherms for Acid blue25 at 295 K for the OMS, OMS-NH2, and OMS-COOHadsorbent powders. It is clear from the figure that theadsorption capacity of OMS-NH2 for Acid blue 25 issignificantly higher than both the unmodified OMS andOMS-COOH. An adsorption capacity of 250 mg or 0.6mmol of Acid blue 25 per gram of OMS-NH2 was obtained.Although the amount of Acid blue 25 adsorbed is less thanthe amount of available NH2 sites (i.e., 0.6 vs 2.23 mmol/g), calculation indicates that at the maximum adsorptioncapacity there is a monolayer surface coverage. Theadsorption capacity of OMS-NH2 for the acid dye is alsohigher than the reported values for several carbon-basedadsorbents (Table 3).35-38 The initial adsorption rates ofAcid blue 25 (i.e., C0 ) 150 ppm) on OMS, OMS-NH2, andOMS-COOH are 0.4, 66, and 76 ppm dye/h per gram ofadsorbent, respectively. The large adsorption capacity andrelatively rapid adsorption rate of OMS-NH2 makes itan attractive adsorbent for the removal of acid dye

pollutants from effluents, which is an acknowledgedproblem in the textile industry.

Figure 3b shows that Methylene blue dye has goodaffinity for the carboxylic acid group grafted onto OMS-COOH. This can be attributed to the weak Lewis basecharacteristic of Methylene blue and to the cationic-anionic interaction between the dye molecule and thesurface moiety. This adsorbent displays higher adsorptioncapacity for the Methylene blue than both OMS-NH2 andunmodified MCM-41. At the maximum adsorption capac-ity, about 0.3 mmol/g of Methylene blue were adsorbed onOMS-COOH. Although the adsorbates occupy less thana quarter of the available COOH sites, the surface coverageis close to a monolayer. It has been well established thatthe adsorbed Methylene blue on hydroxylated surface (e.g.,MCM-41) is usually coordinated to four surface hydroxylgroups.39 It is possible that the Methylene blue moleculeadsorbed on OMS-COOH is also coordinated to fourCOOH sites. The initial adsorption rates of Methyleneblue (i.e., C0 ) 150 ppm) on OMS, OMS-NH2, and OMS-COOH are 4, 45, and 210 ppm dye/h per gram of adsorbent,respectively. It is clear from Figure 3 that the startingprecursor material MCM-41 is a poor adsorbent for bothdyes. Simply grafting select functional groups (e.g., RNH2and RCOOH) onto MCM-41creates new, high surface areaadsorbents that have selective affinity for Acid blue 25(i.e., OMS-NH2) and Methylene blue dyes (i.e., OMS-COOH). The adsorbent and dyes could be recovered bysimply washing with alkaline or acid solution. Table 4lists the concentration of the recovered dye and thepercentage of dyes removed from the adsorbent after a(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.

(39) Kaewprasit, C.; Hequet, E.; Abidi, N.; Gourlot, J. P. J. CottonSci. 1998, 2, 164.

Table 2. Properties of Ordered Mesoporous Silica Adsorbents

concentration of organic surfacefunctional groupb

maximum adsorption capacity(mg/g, T ) 295 K)

sampleBET

surface area (m2/g) pore sizea (nm) (mmol/g) (groups/nm2) Acid blue 25 Methylene blue

OMS 1071 2.76 0 0 15 54OMS-N H2 774 2.57 2.226 1.7 256 90OMS-C OOH 757 2.55 1.343 1.0 7 113

a The pore size was calculated using eq 3. b The concentration of the surface chemical groups was calculated from the TGA data.

Figure 3. Equilibrium adsorption isotherms of (a) Acid blue25 and (b) Methylene blue dyes on OMS (4), OMS-NH2 (0),and OMS-COOH (O) from single dye solutions.

Table 3. Adsorption Capacity of Carbon-BasedAdsorbents for Acid Blue 25 and Methylene Blue Dyes

maximum adsorptioncapacity (mg/g)

adsorbent Acid blue 25 Methylene blue

bagasse pitha 21.7pithb 17.5activated petroleum cokec 100activated carbon (granular)d 80-300

a Reference 35. b Reference 36. c Reference 37. d Reference 38.

Table 4. Recovery of Spent Adsorbents

pH ofwash

solution

Acid blue25 inwash

solutiona

(ppm)regenerationb (%)

OMS-NH2

Methyleneblue inwash

solutionc

(ppm)regenerationb (%)

OMS-COOH

pH 2 1.82 0.23 25.26 12.14pH 4 7.91 1.02 3.11 1.49pH 7 13.01 1.67 1.34 0.64pH 10 20.87 2.71 1.78 0.85

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

3022 Langmuir, Vol. 19, No. 7, 2003 Ho et al.

single stage washing of the adsorbent using wash solutionswith different pH. Methylene blue dye and MCM-COOHcan be easily recovered and regenerated by acid wash,whereas Acid blue 25 is more strongly adsorbed on MCM-NH2. A single acid wash was able to remove 12% ofadsorbed Methylene blue dyes that were adsorbed onOMS-COOH. Using a more alkaline solution (pH > 10),15% recovery of acid blue 25 can be obtained from singlewashing, but dissolution of the silica matrix is also morelikely to occur at this pH. Table 5 suggests that repeatedwashing is a better alternative and could lead to completeregeneration of adsorbent. These recovery and regenera-tion values are better than for typical carbonaceousadsorbents, which require harsher regeneration conditions(e.g., steam treatments). After washing, there was nodecrease in the number of functional groups in OMS-NH2, but there is a loss of 35% for the OMS-COOH. Thelatter is mainly due to the detachment of the chemicalmoiety from the pore wall during the acid wash.

3.2.2 Binary Dyes Adsorption. The single dye adsorptionexperiments indicate that OMS-NH2 has a higher affinityforAcidblue25thanMethylenebluedye.Similarly,OMS-COOH has a higher adsorption capacity for Methyleneblue than Acid blue 25. To test the performance of thesemesoporous silica adsorbents for selective dye removal,adsorption experiments were conducted using solutionscontaining a mixture of Acid blue 25 and Methylene bluedyes. Figure 4 displays the equilibrium adsorption iso-therms for Acid blue 25 and Methylene blue dyes obtainedfrom these experiments. Figure 4a shows that OMS-NH2remains the best adsorbent for the Acid blue 25. Itsadsorption performance is comparable to that obtainedfrom the single dye adsorption experiment (cf. Figure 3a).On the other hand, the adsorption capacity of the othertwo mesoporous silica adsorbents displays significantlyhigher values for the mixture compared to the single dyeexperiment. This synergistic effect can be more easilyexplained if we also take into account the adsorptionproperties of the adsorbents for Methylene blue (Figure4b). The plots show that the adsorption performance ofOMS-NH2 for Methylene blue is enhanced by the presenceof Acid blue dye in the mixture. Although the competitiveadsorption of Acid blue 25 and Methylene blue dyes onOMS-NH2 could not be ruled out, the adsorption datasuggest that it is more likely that we have a multilayeradsorption. The strong affinity of the Acid blue 25 dye forthe amino groups on the adsorbent means that the firstmonolayer of adsorbed molecules will consist mainly ofAcid blue 25 molecules. This may also explain why theequilibrium adsorption isotherms for Acid blue 25 are thesame for the single and binary dyes mixtures. Theadsorbed dye molecules will change the chemical char-acteristics of the adsorbent and being acidic and anionicin nature, they could further interact with the basic andcationic Methylene blue dyes in the solution leading tomultilayer adsorption and to the observed enhancement

in the adsorption of Methylene blue on OMS-NH2. Aseparate experiment was conducted wherein acid blue 25(C0 ) 2.4 mM) was first adsorbed on OMS-NH2 and thedye-modified OMS was then used as adsorbent forMethylene blue (C0 ) 2.4 mM). 0.69 mmol/g (286 mg/g)of Acid blue 25 was adsorbed on OMS-NH2 and anadditional 0.64 mmol/g (223 mg/g) of Methylene blue wasadsorbed on the dye-modified OMS. This clearly demon-strates that preadsorbed Acid blue 25 on OMS-NH2

changes the chemistry and adsorption properties of theadsorbent and that Methylene blue can be adsorbed ontop of the Acid blue 25 layer.

The same phenomenon could also explain the adsorptionof Acid blue 25 on OMS-COOH (cf. Figure 4a). Theadsorption behavior of Methylene blue dye on OMS-COOH from single and binary dye mixtures are similar.Both reach a maximum adsorption capacity of about 100mg per gram of adsorbent. The single dye experimentclearly shows that the Acid blue dye does not have anyaffinity for the carboxylic acid groups grafted onto OMS-COOH. However, in the presence of Methylene blue, nearly50 mg of Acid blue 25 was adsorbed per gram of theadsorbent. This strongly suggests that the adsorption ofAcid blue 25 onto OMS-COOH is mediated by theadsorbed Methylene blue dyes. This is confirmed in anexperiment wherein Methylene blue was preadsorbed onOMS-COOH (0.38 mmol/g or 123 mg/g) and the dye-modified OMS adsorbent was used to adsorb Acid blue 25from a solution. An additional 0.38 mmol/g (160 mg/g)of Acid blue 25 was adsorbed on the dye-modifiedadsorbent. The adsorption of Acid blue 25 on unmodifiedMCM-41 from the mixture of dyes also displays animprovement compared to the value obtained from thesingle dye experiment (cf. Figures 4a and 3a). However,the adsorption isotherms of the two dyes from the mix-ture suggest that there is competitive adsorption betweenAcid blue 25 and Methylene blue on OMS adsorbent(cf. Figure 4b). This may be because the native hydroxylgroups on MCM-41 do not have a strong affinity for eitherdye.

Table 5. Effect of Repeated Wash on Dye Recovery andAdsorbent Regeneration

no. ofwashing

recovery (%)of Acid blue 25

from OMS-NH2a

recovery (%)of Methylene blue

from OMS-COOHb

1 2.71 12.142 6.17 24.533 8.84 35.594 11.85 46.615 14.99 57.51

a Recovery from pH 10 wash solution and calculated frommaterial balance. b Recovery from pH 2 wash solution and calcu-lated from material balance.

Figure 4. Equilibrium adsorption isotherms of (a) Acid blue25 and (b) Methylene blue dyes on OMS (4), OMS-NH2 (0),and OMS-COOH (O) from binary dye mixtures.

Adsorbents from Ordered Mesoporous Silica Langmuir, Vol. 19, No. 7, 2003 3023

4. Concluding RemarksThis work demonstrates that selective adsorbents could

be prepared from the ordered mesoporous silica (i.e., MCM-41) by grafting chemical moieties that have specific affinityfor the target molecules. The Acid blue 25 represents animportant class of dye pollutants that cannot be effectivelyremoved by traditional carbonaceous adsorbents such asactivated carbons and peat. The amino-modified OMS-NH2 has a strong affinity and adsorption capacity for acidicdye molecules and could efficiently remove Acid blue 25from a mixture. Similarly, OMS containing graftedcarboxylic groups (OMS-COOH) is a good adsorbent forthe removal of Methylene blue dyes and displays com-parable adsorption capacity as the commercial adsorbentsbut with better selectivity. A better selectivity means thatthe OMS-based adsorbents have longer operating life andrequire less maintenance, thus resulting in lower capitaland operating cost. Also unlike the existing adsorbents,the OMS-based adsorbents can be regenerated by a simplewashing procedure.

One additional advantage of using ordered mesoporoussilica is that it provides an ideal material for studyingadsorption and diffusion phenomena. Today, most ad-sorbents possess complex pore structure that is populated

by myriad of chemical groups. It is difficult if not impossibleto accurately describe the different adsorption sites,chemical interactions, and transport processes in thesematerials. The cylindrical pore structure and high degreeof pore symmetry found in OMS are ideal for testingvarious existing adsorption and diffusion models. Thesimple pore geometry is amenable to mathematicaldescription and the amorphous state of the pore wallapproximates an ideal Langmuir surface. The ability tointroduce well-defined functional groups is invaluable fordeveloping mathematical model for surfaces with multipleadsorption sites. This will allow the systematic study ofthe nature of interaction between different surface sitesand lead to the design of better adsorbent materials. Thiswork constitutes the first step toward this goal.

Acknowledgment. The authors would like to grate-fully acknowledge the funding from the Hong KongResearch Grant Councils (grant RGC-HKUST 6037/00P).We also thank the Material Characterization and Prepa-ration Facility at the HKUST for the use of XRD, TEM,SEM, EDXS, XPS, and TGA/DTA equipment.

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3024 Langmuir, Vol. 19, No. 7, 2003 Ho et al.