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Heavy Metal Remediation Using Functionalized Mesoporous Silicas with Controlled Macrostructure Robert I. Nooney,† Mohan Kalyanaraman,‡ Gordon Kennedy,§ and Edward J. Maginn*,† Department of Chemical Engineering and Center for Molecularly Engineered Materials, University of Notre Dame, Notre Dame, Indiana 46556, Materials Characterization and Catalysis Technology Group, ExxonMobil Research and Engineering, PO Box 480, Paulsboro, New Jersey 08066, and Corporate Strategic Research, ExxonMobil Research and Engineering Company, 1545 Route 22 East, Annandale, New Jersey 08801 Received May 25, 2000. In Final Form: November 1, 2000 A synthesis procedure is described for making functionalized mesoporous silica macrostructures that can serve as self-supporting adsorbents for environmental remediation and other separations applications. The material, whose mesopores were functionalized with 3-mercaptopropyltrimethoxysilane ligands, can be made into spheres, irregular particles, and truncated cones having diameters from 1 to 15 mm through a one-step emulsion synthesis procedure. Other shapes such as pellets can be formed by molding the precursor gel. The macro- and mesomorphology of the materials were characterized by X-ray diffraction, transmission electron microscopy, scanning electron microscopy, and 29Si MAS NMR. Physisorption properties were investigated using nitrogen adsorption measurements. The surface area of these materials determined via the BET method ranged from 864 to 1184 m2 g-1. The materials are extremely effective in the removal of mercury and silver ions from aqueous solutions. The amount of mercury adsorbed ranged from 0.24 to 1.26 mmol g-1, depending on the degree of functionalization. Silver is less strongly adsorbed than mercury, with a maximum loading of 0.89 mmol g-1. In binary adsorption of mercury and silver ion mixtures, the selectivity for mercury ranged from 1.39 to 2.24. The adsorption capacity of the functionalized materials for nitrogen is comparable to that of unfunctionalized materials.
Introduction There is growing interest in the use of “hybrid” organic/ inorganic materials1,2 for a range of applications including catalysis3,4 and adsorption-based separations.5 The versatility of these hybrid materials stems from the range of properties that can be exploited through a combination of inorganic and organic constituents. One approach to synthesizing these materials involves the chemical grafting of ligands (typically an organosilane) onto mesoporous silica.6,7 Feng et al. grafted 3-mercaptopropyltrimethoxysilane (MPTS) onto MCM-41 and found that the material was effective for the removal of mercury ions from an aqueous stream.8,9 The material was highly selective for mercury and other heavy metals. MPTS was an effective ligand because sulfur, a soft nonmetal, reacts readily to form a covalent bond with soft metals on the right of the d-block. They found that the functionalized material had †
University of Notre Dame. Materials Characterization and Catalysis Technology Group, ExxonMobil Research and Engineering. § Corporate Strategic Research, ExxonMobil Research and Engineering. ‡
(1) Klein, L. C.; Francis, L. F.; Guire, M. R. D.; Mark, J. E. Organic/ Inorganic Hybrid Materials II; Proceedings of the Materials Research Society Symposium, Warrendale, PA, 1999; Vol. 576. (2) Ozin, G. A. Chem. Commun. 2000, 419-432. (3) Vartuli, J. C.; Shih, S. S.; Kresge, C. T.; Beck, J. S. Mesoporous Mol. Sieves 1998, 117, 13-21. (4) Jones, C. W.; Tsuji, K.; Davis, M. E. Nature 1998, 393, 52-54. (5) Herbst, J. A.; Kresge, C. T.; Olson, D. H.; Schmitt, K. D.; Vartuli, J. C.; Wang, D. I. U.S. Patent No. 5,378,440, 1995. (6) Moller, K.; Bein, T. Mesoporous Mol. Sieves 1998, 117, 53-64. (7) Asefa, T.; Maclachlan, M. J.; Coombs, N.; Ozin, G. A. Nature 1999, 402, 867-871. (8) Feng, X.; Fryxell, G. E.; Wang, L.-Q.; Kim, A. Y.; Liu, J.; Kemner, K. M. Science 1997, 276, 923-6. (9) Liu, J.; Feng, X.; Fryxell, G. E.; Wang, L.-Q.; Kim, A. Y.; Gong, M. Chem. Eng. Technol. 1998, 21, 97-100.
a large distribution coefficient as well as a high sorption capacity. The high capacity of these materials is thought to be due to the large surface area of MCM-41 and the ability of MPTS to from a “close-packed” monolayer on the surface at a high density. The regular pore network also enabled mercury to diffuse throughout the entire structure without pore blockage. An alternative synthesis procedure to the “postsynthesis” functionalization method described above involves a “one-step” approach, in which the mesoporous silica is formed and functionalized all at the same time. Mann et al. have used this method to synthesize mesoporous silica functionalized with MPTS as well as a variety of other alkyl and vinyl groups.10 The organosilane molecules bind to the silica pore walls through multiple siloxane bonds, which greatly enhances stability. The structure-directing molecules can be removed via solvent extraction without significant removal of the functional group. One practical problem with both synthesis procedures is that the resulting functionalized material is typically comprised of micron-sized particles. These particles are quite difficult to handle and must be bound to an inert material such as kaolin clay for use in packed beds. This leads to increased bed volumes and can cause a reduction in mass transfer rates. It would be desirable to develop hybrid materials that are self-supporting, while still retaining their other favorable properties. In this work, a method is described in which mesoporous silica particles of macroscopic dimensions and functionalized with organic ligands can be made in a one-step synthesis. The materials are self-supporting and easy to handle and have high adsorption capacity for mercury and silver when a sulfur-bearing ligand is used. Unlike (10) Fowler, C. E.; Burkett, S. L.; Mann, S. Chem. Commun. 1997, 1769-1770.
10.1021/la000720j CCC: $20.00 © 2001 American Chemical Society Published on Web 12/16/2000
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Figure 1. Photographs of the different macrostructures made: (A) small spheres; (B) agglomeration of spheres; (C) large sphere; (D) truncated cone; (E) irregular particles (“blocks”); (F) molded pellet (end on and side view). Scale is in centimeters.
materials formed using postsynthesis functionalization, which typically have a reduced adsorption capacity compared to that of unfunctionalized materials, the current materials retain their high adsorption capacity upon functionlization. The approach used here combines the one-step functionalization method described above10 with an emulsion process that has been used recently to form mesoporous silica-based materials with varying size and shape.11 It has been shown previously that, through careful control of the emulsion chemistry and hydrodynamics, a variety of mesoporous silica morphologies can be formed, including hollow spheres,12 solid spheres,13 fibers,14 bubbles,15 thin films,16,17 and hollow heliocoids.18 In the present work, functionalized materials having various shapes, including spheres, truncated cones, and pellets, have been made with dimensions ranging from 1 to 15 mm. The adsorption characteristics of these materials are measured and compared with hexagonal mesoporous silica19 (HMS), postsynthesis functionalized HMS,20 and postsynthesis functionalized amorphous silica. Synthesis Details To form small, functionalized spheres such as those shown in Figure 1a, MPTS and tetrabutyl orthosilicate (TBOS) are mixed (11) Zhao, D.; Yang, P.; Huo, Q.; Chmelka, B. F.; Stucky, G. D. Curr. Opin. Solid State Mater. 1998, 3, 111-121. (12) Schacht, S.; Huo, Q.; Voigt-Martin, I. G.; Stucky, G. D.; Schuth, F. Science 1996, 273, 768-771. (13) Huo, Q.; Feng, J.; Schuth, F.; Stucky, G. D. Chem. Mater. 1997, 9, 14-17. (14) Huo, Q.; Zhao, D.; Feng, J.; Weston, K.; Buratto, S.; Stucky, G. D.; Schacht, S.; Schuth, F. Adv. Mater. 1997, 9, 974-978. (15) Ogawa, M.; Yamamoto, N. Langmuir 1999, 15, 2227-2229. (16) Yang, H.; Coombs, N.; Sokolov, I.; Ozin, G. A. Nature 1996, 381, 589-592. (17) Yang, H.; Kuperman, A.; Coombs, N.; Mamiche-Afara, S.; Ozin, G. A. Nature 1996, 379, 703-705. (18) Yang, S. M.; Sokolov, I.; Coombs, N.; Kresge, C. T.; Ozin, G. A. Adv. Mater. 1999, 11, 1427-1433. (19) Tanev, P. T.; Pinnavaia, T. J. Science 1995, 267, 865-867. (20) Mercier, L.; Pinnavaia, T. J. Adv. Mater. 1997, 9, 500-3.
together and added to a basic aqueous phase containing cetyltrimethylammonium bromide (CTAB). The mesostructures form at the interface between the TBOS-rich hydrophobic phase and the aqueous phase containing CTAB. There is no need for an auxiliary organic phase, because TBOS forms an emulsion in water. The interface is stabilized by the amphiphilic CTAB. Functionalized macrostructures were successfully made using mole percentages of 5.2, 10.1, 15.1, 17.3, and 20.4 MPTS in TBOS. The resulting weight percentages of sulfur, as determined by elemental analysis using a Leco model CS444 analyzer, were 1.65, 2.79, 3.46, 6.62, and 7.36. To distinguish between materials functionalized at different MPTS levels, we will refer to the initial nominal percentage of MPTS used (i.e., 5% MPTS, 10% MPTS, etc.). To synthesize 10% MPTS spheres, 0.63 g of CTAB was dissolved in 35 g of deionized water. A 2.70 g sample of 2 M NaOH was then added. The initial pH was 13.3. A prehomogenized mixture containing 4.04 g of TBOS and 0.275 g of MPTS was then added with stirring at approximately 200 rpm using a 4.5 cm stir bar. The mixture was aged under agitation for 30 h in a 100 mL round-bottom flask, after which a collection of hard, transparent spheres was filtered and then dried under a humid environment for 5 days. The CTAB template molecules were then removed by mixing 1 g of solid product with 100 mL of 1 M HCl in ethanol under reflux for 24 h.21 To ensure complete removal of unbound MPTS, the product was Soxhlet extracted with ethanol for an additional 24 h. We took this conservative approach to ensure quantitative results. Methods to improve template extraction efficiency will be developed in the future. The macromorphology of the resulting material is sensitive to the synthesis conditions. Upon raising the initial pH by approximately 0.1, the small spheres readily agglomerate to form irregular shapes (Figure 1b) or larger, stable spheres up to 1 cm in diameter (Figure 1c). By controlling the hydrodynamics of the synthesis liquid, different shapes can be formed. Figure 1d shows a truncated cone that formed at the interface created by the vortex in the flask. A gel forms at lower stirring rates, which when left undisturbed agglomerates to form irregular-shaped “blocks” (Figure 1e). The gel can also be removed from the flask (21) Chen, C. Y.; Li, H. X.; Davis, M. E. Microporous Mater. 1993, 2, 17-26.
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Figure 2. SEM micrographs of spherical particles and interior surfaces of an unfunctionalized material (A-C) and a material functionalized with MPTS ligands (D-F). and molded into pellets of any size (Figure 1f). In general, materials formed from a gel are less stable than those formed directly in solution.
Characterization Figure 2 shows scanning electron microscopy (SEM) images of unfunctionalized (Figure 2a-c) and functionalized (Figure 2d-f) spheres. The micrographs were obtained on a Philips XL40 SEM. Samples were placed on a double-sided carbon tape on a polished aluminum stub. To reduce charging effects, samples were coated with Au/ Pd prior to imaging. A typical siliceous sphere is transparent in the as-synthesized state and after drying but becomes opaque on removal of the template. As can be seen in Figure 2a, there are several cracks on the surface of the spheres. Despite this, the dried materials are quite tough and can be handled without any special precautions. The surface appears relatively smooth at the micron level. At the submicron level (Figure 2b), fibers having a diameter of roughly 0.2 µm and several microns in length can be seen. The fibers are made up of an agglomeration of tiny particles approximately 25 nm in diameter (inset, Figure 2b). The interior surface of a siliceous sphere appears smooth at the micron scale (Figure 2c). Figure 2d is a micrograph of a 10% MPTS sphere. At the micron scale the morphology appears similar to that of the unfunctionalized sample, although the MPTS does impact the surface morphology. Fibers similar in size to that seen with unfunctionalized materials are observed, but they are comprised of a collection of helical kinks (Figure 2e). The kinks are similar to those observed previously14 and are likely due to the slower alkoxysilane hydrolysis rates that occur under the slightly less basic conditions of the functionalized material synthesis relative to that used in synthesizing the unfunctionalized materials. A pockmarked surface formed at the interface between two agglomerated spheres (Figure 2f). This is likely the result of hydrophilic species becoming trapped between the spheres during polymerization. To examine the size and ordering of the mesopores, transmission electron microscopy (TEM) images were taken of the samples. To do this, the macrostructures were crushed to form powders and then dispersed in isopropyl alcohol. Some samples were also embedded in LR White embedding medium and cured prior to microtoming into 70 nm thick samples. Samples were picked up on a carboncoated copper grid for examination in the TEM. The
Figure 3. A TEM micrograph of a disordered array of dispersed mesopores in a 10 mol % MPTS sphere.
instrument was a JEOL 2010 operated at 200 kV. Images were obtained using a bottom-mounted Gatan 794 CCD camera. The images indicate that the mesopores are dispersed in size with random orientation for all functionalized materials. Figure 3 shows one such image for a 10% MPTS sphere. Unfunctionalized materials show regions of ordered and disordered pores. Powder X-ray diffraction (XRD) patterns were collected on a Scintag XDS 2000 defractometer with Cu KR radiation and equipped with a diffracted beam monochromator. Figure 4 shows diffraction patterns of unfunctionalized spheres, 10% MPTS spheres, and 20% MPTS blocks. In all cases only one peak was observed, with maximum d spacings of 32.8 (+5.1, -4.0), 27.5 (+10.3, -3.9), and 28.2 (+7.7, -2.9) Å for the unfunctionalized spheres, 10% MPTS spheres, and 20% MPTS blocks, respectively. The crystallites were of reasonable size to discount the possibility of line broadening. Hence, the broad range in d spacing indicates a wide range of pore diameters. The asymmetry between left and right full width half-maximum values arises because of pronounced skewness of peaks at low 2θ values. The d spacings are consistent with the pore diameters measured using TEM. Liquid nitrogen adsorption isotherms were measured using a Quantachrome Autosorb-1. Unfunctionalized
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Figure 4. X-ray diffraction patterns of three macrostructures.
Figure 5. Nitrogen isotherms at -196 °C of mesoporous spheres and HMS powder. The effects of in-situ and postsynthesis grafting of MPTS are compared. Adsorption (ADS) and desorption points (DES) are the open and filled symbols, respectively.
materials were activated under vacuum at 150 °C for 4 h. Functionalized materials were activated under vacuum at 80 °C for 4 h to avoid decomposing the ligands. Figure 5 shows the isotherms measured for unfunctionalized spheres, 10% MPTS spheres, an unfunctionalized HMS powder, and a postsynthesis functionalized HMS powder. At 0.9 P/P0, the total capacity of each material was 22.2, 23.4, 22.8, and 13.5 mmol g-1, respectively. Both unfunctionalized materials exhibited type IV isotherms, although the isotherm for the spheres showed hysteresis, while the isotherm for the HMS powder showed no hysteresis. In agreement with previous work,22 postsynthesis grafting lowered the total adsorption capacity and BET surface area of the HMS powder. This is believed to be due to the fact that the functional groups bind on the inner surface of the pores, thereby reducing total pore volume. The isotherm is type I in character, which would indicate that the material is microporous. Interestingly, functionalization of the macrostructured material does not reduce adsorption capacity, nor does it appear to reduce mesoporosity. The 10% MPTS spheres actually exhibit slightly higher nitrogen adsorption than the unfunctionalized spheres at all pressures below 0.8 P/P0, and the isotherm remained type IV. The slight hysteresis in the desorption trace at P/P0 > 0.5 for both the functionalized and unfunctionalized spheres indicates the presence of some mesoporous domains in addition to the pores that formed (22) Mercier, L.; Pinnavaia, T. J. Environ. Sci. Technol. 1998, 32, 2749-2754.
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around the template. This mesoporosity is not eliminated by one-step functionalization. By applying the Kelvin equation to the capillary desorption branch of these materials and assuming the pore walls are approximately 1 nm thick,22 the corresponding mesopore center-to-center distance is estimated to be 5 nm. This is significantly larger than the regular pore sizes determined by TEM and XRD analysis. It is not clear what the origin of this additional mesoporous domain is. It may be that the macrostructures contain large domains made up of fused nanoparticles, such as those seen in Figure 2b. Secondary adsorption and capillary condensation could then occur in the interstices between fused nanoparticles. The adsorption capacity of the functionalized macrostructures depends a great deal on the synthesis and aging procedure that is used. For example, samples synthesized with nominally the same 10% MPTS level had nitrogen capacities at P/P0 ) 0.9 ranging from 12 to 43.4 mmol g-1, with a typical value being about 24 mmol g-1. It is not clear what the source of the variability in capacity is, although we are studying this to see whether high-capacity materials may be made on a more consistent basis. Table 1 provides a summary of the BET surface area of each material along with sulfur content as determined by elemental analysis. The amount of sulfur incorporated ranged from 1.65 to 7.36 wt %. As a means of comparing the effectiveness of these materials in subsequent adsorption experiments, amorphous silica was also functionalized with excess MPTS solution. Regardless of the initial amount of MPTS used, the sulfur loading on amorphous silica was very low (less than 1 wt %). NMR data were recorded with a Bruker AMX-360 widebore NMR spectrometer equipped with a solids accessory. The 71.55 MHz 29Si MAS NMR spectra were obtained using single pulse Bloch decay and cross-polarization techniques with a Bruker 7 mm probe using 3.5 kHz spinning, high-power H decoupling, and a 25 kHz spectral window. The single-pulse experiment was conducted with a 4 µs 90° 29Si pulse and 60 s recycle delay, and 1136 transients were collected. The cross-polarization experiment was conducted with a 5 µs 90° 1H pulse, 1.5 ms contact time, and 1.5 s recycle delay, and 1200 transients were collected. Shown in Figure 6 are the 29Si MAS NMR spectra of the 10% MPTS mesospheres obtained with Bloch decay (A) and cross-polarization (B) conditions. Peaks at approximately -90, -100, and -110 ppm from TMS correspond to Si(OSi)2(OX)2, Si(OSi)3(OX), and Si(OSi)4 species, respectively, in the bulk (where X ) H or R). Peaks at approximately -50, -57, and -64 ppm from TMS in each spectrum correspond to isolated (i.e., monosiloxane), terminal (i.e., disiloxane), and internal (i.e., trisiloxane) mercaptopropylsilyl groups, respectively. Silicons in close proximity to hydrogens are preferentially enhanced in the cross-polarization experiment. These spectra confirm that the mercaptopropylsilyl functionalities are covalently incorporated and not simply occluded. The relative peak areas in the Bloch decay spectrum are quantitative and indicate that 9% of total Si is associated with surface functionalization. This suggests that ∼90% of the MPTS has reacted in the synthesis of these mesospheres. The distribution of surface species (in relative percent) obtained by deconvolution of the -40 to -70 ppm region of the spectrum are 14% isolated, 37% terminal, and 49% internal. Mercury and Silver Liquid Adsorption Liquid-phase adsorption experiments were conducted on solutions containing varying levels of Hg2+ and Ag+ as
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Table 1. Surface Area, Sulfur Incorporation from MPTS Functionalization, and Heavy Metal Adsorption Data of Selected Mesoporous Adsorbents and Amorphous Silica
adsorbent
BET surface area (m2 g-1)
0 mol % MPTS sphere 5 mol % MPTS blocks 10 mol % MPTS sphere 15 mol % MPTS powder 17 mol % MPTS powder 20 mol % MPTS blocks MPTS-amorphous silica powder HMS powder HMS powder with grafted MPTS
1047 874.3 1375 883.1 949.3 350-410a 1406 825
20 mol % MPTS blocks 10 mol % MPTS spheres
613.7 -
a
sulfur wt % in adsorbent
max metal loading/adsorbent (mmol/g)
max metal loading/adsorbent (g/g)
max metal uptake/sulfur (mol/mol)
Mercury Adsorption 0 1.65 2.79 3.46 6.62 7.36 0.78 0 -
0 0.31 0.42 0.88 0.86 1.26 0.28 0 1.04
0 0.063 0.084 0.17 0.18 0.25 0.056 0 0.202
0 0.61 0.48 0.82 0.42 0.55 0.98 0 -
Silver Adsorption 7.36 2.79
0.89 0.23
0.096 0.028
0.39 0.30
Surface area of nonfunctionalized material.
Figure 7. Liquid Hg2+ adsorption isotherms at 25 °C for macrostructures functionalized with varying levels of MPTS. The solid line indicates maximum theoretical loading. Figure 6. 29Si MAS NMR spectra of 10% MPTS mesospheres obtained with Bloch decay (A) and cross-polarization (B) conditions.
well as mixtures containing both metal ions. Different quantities of adsorbent material (from 5 to 50 mg) were weighed out and mixed with either a 500 ppm of Hg(II) (NO3)2 or 500 ppm of Ag(I) (NO3)2 solution in 10 mL of deionized water. For binary adsorption experiments, 10 mL of 500 ppm solutions were prepared with a 2:1 mole ratio of Ag+ to Hg2+. The samples were agitated and allowed to come to equilibrium at room temperature. A typical experiment took 24 h. The final ion concentration in solution was measured using a Perkin-Elmer model 3300 XL inductively coupled plasma-optical emission spectroscope. The detector was set to record emission spectra at 253 nm for Hg2+ and at 328 nm for Ag+. The amount of metal adsorbed was determined by mass balance. Figure 7 shows the isotherms measured for solutions containing only Hg2+ ions as a function of elemental sulfur content in the adsorbent. As the sulfur content increases, the capacity of the material for Hg2+ also increases, as expected. The unfunctionalized material adsorbs a negligible amount of Hg2+. Table 1 lists the adsorption capacities of the various materials for both Hg2+ and Ag+ ions. Qualitative agreement was found between the total amount of single component (Hg2+ or Ag+) adsorbed and the amount of sulfur grafted onto the pores. For the highest grafting level (7.36 wt % S), 1.26 mmol of Hg2+ per gram of adsorbent was removed from solution. Although considerable, this capacity is less than that found in thiol functionalized mesoporous silica powders characterized
previously. In postsynthesis grafting of HMS22 and MCM419 powder samples adsorption capacities of 1.5 and 3.0 mmol g-1 of Hg2+ were achieved, respectively. In a onestep functionalization of MCM-41 powder Lim et al.23 adsorbed 2.1 mmol g-1 of Hg2+. However, it should be noted that, in practical applications of these materials, powder samples must be bound to an inert support in order to minimize pressure drop. Thus, the effective capacity of the powder samples will be lower than the value reported in the literature. The materials synthesized in the present work are self-supporting so the capacity reported here is the same as what one would obtain under operating conditions. The moles of metal adsorbed per mole of sulfur ranged between 0.42 and 0.61 for Hg2+ and from 0.30 to 0.39 for Ag+ (see Table 1). In all cases more Hg2+ than Ag+ was adsorbed. Given that the theoretical maximum is 1.0 mol of metal per mole of sulfur, these results show that some of the MPTS ligands are either inaccessible to the metal ions or are not active. This could be due to a number of factors, including pore blockage or steric hindrance effects resulting from residual template, framework silica, adjacent ligands, or prior bound metal. The BET surface area of 20 mol % MPTS blocks was reduced by about a third upon adsorption of Ag+ (see Table 1), which clearly indicates that the metal ions block regions of the pores. Interestingly, the functionalized amorphous silica adsorbed 0.98 mol of Hg2+ per mole of sulfur, which means that nearly all the ligands are accessible and active for this material. It is likely that most of the functional groups are located on the outer surface of the amorphous (23) Lim, M. H.; Blanford, C. F.; Stein, A. Chem. Mater. 1998, 10, 467-470.
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2 in the adsorbed phase and y1 and y2 are the molar fractions of adsorptives in the liquid phase. The selective adsorption for Hg2+ over Ag+ is not easily explained. Both material are relatively soft, and their metal sulfur covalent bond strengths are approximately equal. Feng et al. studied the bonding of Hg2+ to MPTS by X-ray adsorption fine structure analysis.9 They postulated that bivalent Hg2+ bound to the thiol group and then bound to another mercury via a bridging oxygen atom. This added stability could be the reason for the selectivity. Conclusions
Figure 8. Competitive adsorption results at 30 °C from an aqueous solution containing 500 ppm metal ions. The initial Ag+/Hg2+ ratio was 2:1. The solid line indicates maximum theoretical loading.
material and hence are easily accessible. Despite the high “binding efficiency” of amorphous silica, the functionalized mesoporous materials have a much greater total capacity for adsorption, due to the higher MPTS grafting densities that can be attained. For example, the 20% MPTS material had a mercury adsorption capacity that was over 5 times greater than that of the functionalized amorphous silica. Competitive adsorption results for a binary aqueous mixture of Hg2+ and Ag+ on 20% MPTS material are shown in Figure 8. The binary selectivity for Hg2+ ranged between 1.4 and 2.2. The binary selectivity for component 2 is defined as
S2 )
x2/x1 y2/y1
(1)
where x1 and x2 are the mole fractions of adsorbates 1 and
A one-step method for the preparation of functionalized mesoporous silica of macroscale dimensions has been reported. These materials can be prepared as spheres, irregular particles, truncated cones or pellets in a range of sizes from 1 mm to 1.5 cm. The mesostructure is composed of dispersed pores in disordered arrays. Functional ligands can be added to the materials to alter the adsorption properties. It was shown that MPTS ligands enable these materials to function as selective adsorbents for the removal of heavy metals from aqueous streams. The presence of the ligands does not reduce the nitrogen adsorption capacity of these materials relative to comparable unfunctionalized materials. Because the materials are macroscopic in size, they function as self-supporting adsorbents and can be used directly in practical applications. By adding other ligands, these materials can be tailored for a wide range of selective separations or catalytic applications. Acknowledgment. Funding for this work was provided by the National Science Foundation under Grant CTS-9701470. LA000720J
