Synthesis and Characterization of Functionalized Silica-Based

Mar 11, 2010 - This study investigates the structural evolution of a series of nanohybrid powders and coatings synthesized by direct co-condensation o...
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Synthesis and Characterization of Functionalized Silica-Based Nanohybrid Materials for Oxyanions Adsorption )

Inna Karatchevtseva,*,† Marion Astoux,§ David J. Cassidy,† Patrick Yee,‡ John R. Bartlett, and Christopher S. Griffith‡ † Institute of Materials Engineering, and ‡Minerals, ANSTO, PMB 1, Menai, NSW 2234, Australia, Ecole Nationale Sup erieure de Chimie de Paris, 11, Rue Pierre et Marie Curie, 75005 Paris, France, and School of Natural Sciences, University of Western Sydney, Penrith South DC, NSW 1797, Australia )

§

Received December 17, 2009. Revised Manuscript Received March 2, 2010 This study investigates the structural evolution of a series of nanohybrid powders and coatings synthesized by direct co-condensation of amino-functionalized alkyltrialkoxysilanes and tetraalkoxysilanes with an aromatic carboxylic acid (trimesic acid, TMA) as a structure directing agent. Fourier transform infrared spectroscopy (FTIR) and 13C CP-MAS NMR results have suggested the formation of secondary (-CO-NH-) amide linkages upon interaction of TMA with the amino functionalized silane thus creating a “scaffold” around which the silica network is formed and also assisting in more homogeneous distribution of nitrogen sites within the nanohybrid structure. Functionalized silica powders were investigated for their potential to remove toxic oxyanions from mildly acidic or basic solutions. The uptake of Mo(VI), Se(VI), and Cr(VI) oxyanions was investigated as a function of the nanohybrid composition, oxyanion concentration, and solution pH using laser diffraction particle sizing, gas adsorption, and various spectroscopic techniques. The adsorption data obtained for Mo and Se could be adequately described by Langmuir adsorption isotherms, while the Freundlich isotherm is employed to fit the adsorption data for Cr. An easily accessible processing window (of pH, aging time, etc.) has been identified allowing production of continuous and uniform thin nanohybrid coatings on silicon and glass substrates. These coatings were tested as chemical barriers against Mo leaching from specially prepared Mo-doped glass. Leaching studies were conducted over 200 days in water at 90 °C and the Mo leaching from coated and uncoated samples compared.

Introduction The modification of amorphous silica surfaces by the attachment of organic functionalities is an active area of research that has contributed significantly to the field of organic-inorganic nanohybrids.1 This research has benefited greatly from surfactant templating routes to meso-structured molecular sieves.2-7 The organic template can subsequently be removed by either calcination or solvent extraction, providing materials with exceptionally high surface areas and well-defined pores between 2 and 50 nm. These properties have attracted wide attention for many industrial2,8-10 and environmental applications11,12 and there remains a continued interest in these materials for the separation of *To whom correspondence should be addressed. E-mail: [email protected]. (1) Stein, A.; Melde, B. J.; Schroden, R. C. Adv. Mater. 2000, 12, 1403–1419. (2) Kresge, C. T.; Leonowicz, M. E.; Roth, W. J.; Vartuli, J. C.; Beck, J. S. Nature 1992, 359, 710–712. (3) Monni, A.; Schuth, F.; Huo, Q.; Kumar, D.; Margolese, D.; Maxwell, R. S.; Stucky, G. D.; Krishnamurty, M.; Petroff, P.; Firouzi, A.; Janicke, M.; Chmelka, B. F. Science 1993, 261, 1299–1303. (4) Beck, J. S.; Vartuli, J. C.; Roth, W. J.; Leonowicz, M. E.; Kresge, C. T.; Schmitt, K. D.; Chu, C.T-W.; Olson, D. H.; Sheppard, E. W.; McCullen, S. B.; Higgins, J. B.; Schlenkert, J. L. J. Am. Chem. Soc. 1992, 114, 10834–10843. (5) Øye, G.; Sjoblom, J.; Stocker, M. Adv. Colloid Interface Sci. 2001, 89-90, 439–466. (6) Anwander, R. Chem. Mater. 2001, 13, 4419–4438. (7) Zhao, X. S.; Lu, G. Q.; Millar, G. J. Ind. Eng. Chem. Res. 1996, 35, 2075– 2090. (8) Vartuli, J. C.; Shih, S. S.; Kresge, C. T.; Beck, J. S. Stud. Surf. Sci. Catal. 1998, 117, 13–21. (9) Jones, C. W.; Tsuji, K.; Davis, M. E. Nature 1998, 393, 52–54. (10) Clark, J. H.; Macquarrie, D. J. Chem. Commun. 1998, 8, 853–860. (11) Macquarrie, D. J. Green Chem. 1999, 1, 195–198. (12) Mattigod, S.; Fryxell, G. E.; Feng, X.; Liu, J. In Metal Separation Technologies Beyond 2000: Integrating Novel Chemistry with Processing; Liddell, K. C., Chaiko, D. J., Eds.; The Minerals, Metals, and Materials Society: Warrendale, PA, 1999; p 71.

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various environmental contaminant species.7,13 Oxyanions, such as selenate, chromate, and molybdate, are of particular concern because of their potential toxicity to both wildlife and human health. These elements are often found at elevated concentrations in agricultural drainage and industrial wastewaters creating serious environmental problems throughout the world. Adsorption of these toxic oxyanions by purposely designed sorbents seems to be an effective method for immobilizing the contaminates. It is therefore not surprising that there has been a continuous search for new and/or improved adsorbents for efficient removal of oxyanions from aqueous solutions. Many alternative organic moieties have been used to control the structural evolution of silica-based nanohybrids. This has led to an ability to control the size and spatial orientation of the organic and inorganic domains of these materials.13-16 While numerous studies have been conducting on the cation binding properties of modified silicas,17-22 only limited work has been devoted toward engineering anion binding properties into (13) Hoffmann, F.; Cornelius, M.; Morell, J.; Froba, M. Angew. Chem., Int. Ed. 2006, 45, 3216–3251. (14) Fryxell, G. E. Inorg. Chem. Commun. 2006, 9, 1141–1150. (15) Park, S. S.; Ha, C.-S. Chem. Record. 2006, 6, 32–42. (16) Kickelbick, G. Angew. Chem., Int. Ed. 2004, 43, 3102–3104. (17) Bois, L.; Bonhomme, A.; Ribes, A.; Pais, B.; Raffin, G.; Tessier, F. Colloids Surf., A. 2003, 221, 221–230. (18) Nooney, R. I.; Kalyanaraman, M.; Kennedy, G.; Maginn, E. J. Langmuir 2001, 17, 528–533. (19) Feng, X.; Fryxell, G. E.; Wang, L. Q.; Kim, A. Y.; Liu, J.; Kemner, K. M. Science 1997, 276, 923–926. (20) Liu, J.; Feng, X.; Fryxell, G. E.; Wang, L. Q.; Kim, A. Y.; Gong, M. Chem. Eng. Technol. 1998, 21, 97–100. (21) Mercier, L.; Pinnavaia, T. J. Adv. Mater. 1997, 9, 500–503. (22) Blitz, I. P.; Blitz, J. P.; Gunko, V. M.; Sheeran, D. J. Colloids Surf., A. 2007, 307, 83–92.

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mesoporous silica substrates.1,23,24 The design of anion-selective adsorbents is generally more problematic as many metal oxoanions are of similar size (2.3-2.5 A˚) and shape (e.g., tetrahedral).23 Among the number of organic moieties employed to functionalize silica-based materials, amines17,23,25-28 are the most widely used due to the readily available precursors. Under even mildly acidic conditions, protonation of primary-tertiary amines readily affords potential anion-binding sites in an analogous fashion to traditional organic ion-exchange resins. Grafted amino groups also allow the formation of coordination complexes within the silica framework and these embedded coordination complexes can also act as potential anion exchange sites. An example of this approach has been reported by Fryxell et al.23 where chelated Cu(II) immobilized on mesoporous silica afforded a material with anion exchange properties. The anion exchange mechanism was shown to probably involve initial electrostatic attraction, followed by displacement of one ligand and direct binding with the Cu(II) center. Exchange capacities of greater than 120 mg(anion)/g were measured in this case. Yoshitake et al.25,26 have also investigated the adsorption of chromate and arsenate on mono-, bis-, and tris-amino functionalized mesoporous SBA-1 and MCM-41 silicas. They have shown the dependence of adsorption capacity on the amount of nitrogen in the system, with the tris-amino moieties capturing oxyanions more effectively. Maximum adsorption capacities reported for CrO42- and HAsO4- were 211 and 263 mg/g (or 1.81 and 1.88 mmol/g), respectively. In this study, we intended to investigate (1) the structural evolution of a series of nanohybrid powders and coatings incorporating aromatic carboxylic acid (trimesic acid, TMA), tetraalkoxysilanes, and amino-functionalized alkyltrialkoxysilanes; and (2) to explore their affinity for metal oxyanions from aqueous solutions. One important observation from previous research was that for the material to make an efficient sorbent it is essential that the functional groups are easily accessible.28 Bois et al.17 have suggested that the amino groups could be involved in hydrogen bonding interactions with hydroxyl groups, SiO-...Hþ NH2-, thus making -NH2 less accessible for further adsorption reactions. Furthermore, Yoshitake25 also pointed out strong interactions between the amino groups and TEOS-hydrolyzed products and proposed the electrostatic bonding of amino groups with terminal groups of carboxylic acid or sulfonic acid as a measure to prevent these interactions. Thus, in this work, we anticipated that the introduction of TMA into the hybrid material would not only help to avoid the undesirable interactions between the amino-groups and alkoxysilane hydrolysis products but also assist in more homogeneous distribution of organic fragments within the nanohybrid structure. The growing interests in such structured materials with controlled functionalities has also prompted us to evaluate the possibility of extending these organic-inorganic nanostructures to thin films and evaluate them as a barrier against toxic elements leaching into the environment.

Experimental Section Trimesic acid (benzene-1,3,5-tricarboxylic acid, TMA) and N-[3-(trimethoxysilyl)propyl]-ethylenediamine (TMSEDA), tet(23) Fryxell, G. E.; Liu, J.; Hauser, T. A.; Nie, Z.; Ferris, K. F.; Mattigod, S.; Gong, M.; Hallen, R. T. Chem. Mater. 1999, 11, 2148–2154. (24) Sayari, A; Hamoudi, S. Chem. Mater. 2001, 13, 3151–3168. (25) Yoshitake, H. New J. Chem. 2005, 29, 1107–1117. (26) Yoshitake, H.; Yokoi, T.; Tatsumi, T. Chem. Mater. 2002, 14, 4603–461. (27) Yokoi, T.; Tatsumi, T.; Yoshitake, H. J. Colloid Interface Sci. 2004, 274, 451–457. (28) Burleigh, M. C.; Markowitz, M. A.; Spector, M. S.; Gaber, B. P. Chem. Mater. 2001, 13, 4760–4766.

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ramethyl orthosilicate (TMOS), methanol, and nitric acid (69%) were obtained from Aldrich Chemical Co., Inc. and used without further purification. The sodium salts of molybdate (Na2MoO4), chromate (Na2CrO4), and selenate (Na2SeO4) were also purchased from Aldrich and used as received. All stock solutions were freshly prepared using deionized water and the pH of solutions adjusted by adding HNO3 or NH4OH. Powder Preparation. An extensive parametric study was carried out to produce silica-based hybrid materials of various compositions from the above chemicals. Experimental procedure, including ingredients ratio and sol-gel parameters, was varied to fabricate hybrid materials suitable as adsorbents for oxyanions. It was found that material with a ratio of TMA-TMSEDATMOS = 1:3:20 (designated powder [1-3-20]) afforded a hybrid phase with the maximum adsorption capacity for molybdenum. In this study only best performing compositions are reported. In a typical procedure, TMA (2.10 g, 0.01 mol) was dissolved in methanol (∼30 g) and TMSEDA (6.67 g, 0.03 mol) was added inside a glovebox (O2 and H2O are CdO 3 3 3 3 3 H-N < type hydrogen bonds in the solid state results in a downfield shift of the carbonyl carbon. Supporting evidence for hydrogen bonding between amide segments were also obtained by FTIR (see below). The 13C CP-MAS spectrum shown in Figure 2 also contains resonances at 132.2 and 135.6 ppm, which are due to the TMA aromatic ring. The FTIR spectrum of the parent TMA molecule contains a strong band in the region 1720-1700 cm-1 characteristic of carboxylate (COO-) stretching vibrations36 (Figure 3a). The band observed at about 740 cm-1 is characteristic of the COO(33) Tsai, M.-F.; Lee, Y.-D.; Chen, K.-N. J. Appl. Polym. Sci. 2002, 86, 468–477. (34) Ando, S.; Ando, I.; Shoji, A.; Ozakit, T. J. Am. Chem. Soc. 1988, 110, 3380– 3386. (35) Lumsden, M. D.; Wasylishen, R. E.; Eichele, K.; Schindler, M.; Penner, G. H.; Power, W. P.; Curtist, R. D. J. Am. Chem. Soc. 1994, 116, 1403–1413. (36) Socrates, G. Infrared and Raman characteristic group frequencies: tables and charts, 3rd ed.; Wiley & Sons Ltd.: Chichester, England, 2001.

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Figure 3. FT-IR spectra of (a) TMA, (b) [1-3-20] powder, and (c) TMA-TMSEDA-TMOS film.

in-plane deformation vibration of an aromatic carboxylic acid. A broad and unsymmetrical band around 925 cm-1 has been assigned to the out-of-plane deformation of the carboxylic acid OH 3 3 3 O group.36 The FTIR spectrum of the [1-3-20] powder contained two new bands at 1626 and 1566 cm-1 in place of the single band of the COO- observed in the parent TMA (Figure 3b). These bands have been assigned to the so-called amide I (νCdO) and amide II (δN-H) modes, respectively, and were attributed to a secondary amide structure since 13C CP-MAS NMR results did not indicate the formation of tertiary amides. FTIR results indicated strong hydrogen bonding interactions between the amide groups with the split between the νCdO and δN-H bands found in the range of 60 cm-1, whereas in the absence of hydrogen bonding, differences of 130 to 145 cm-1 are observed.36 This finding supports the 13C NMR data. Furthermore, the band observed at about 740 cm-1 assigned to the COO- in-plane deformation vibration of free carboxylic acid (TMA) disappeared after reaction with TMSEDA and was replaced by a broad peak at about 800 cm-1. This band has been assigned the C-N stretching vibration and the N-H wagging vibration of the amide.36 Additionally, in the FTIR spectra of the [1-3-20] powder (Figure 3b) bands observed at 1200-900 cm-1 have been tentatively assigned to overlapping Si-C stretching bands and silica (Si-O-Si) framework vibrational bands. On the basis of 13C CP-MAS NMR and FTIR results it is believed that upon interaction of TMA with TMSEDA the formation of secondary (-CO-NH-) amide linkages is favored under the chosen experimental conditions. The mechanism of this formation is not well understood at this stage, as it is recognized that the reaction between a carboxylic acid and an amine typically produces an ammonium carboxylate salt. However, it is also known that the salts formed from carboxylic acids and primary and/or secondary amines will undergo thermal dehydration to Langmuir 2010, 26(11), 8327–8335

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Figure 4. Effect of pH on (a) ζ-potential and (b) adsorption of Mo(VI) oxyanions by [1-3-20] powder; [Mo] = 11 ppm, V/m = 20 mL/200 mg. Scheme 1. Possible Representation of the Hybrid Powder [1-3-20] Formation

Figure 5. Equilibrium adsorption isotherms for chromium, selenium, and molybdenum on [1-3-20] powder fitted using n-NLLS regression.

form the corresponding amides. We speculate that the formation of the amide could occur either during the sol-gel reaction of mixed silicon alkoxides, which is the exothermic process, and/or during the heat treatment of powders over long period of time (24 h). Nevertheless, the possibility of the -NH3þ being present in the final powders is not disregarded. On the basis of NMR and FTIR results, the representation of the hybrid powder [1-3-20] formation has been suggested as shown in Scheme 1. This structure indicates three major groups of moieties possibly formed in the powder, that is, (1) secondary amine units, the most likely adsorption sites, (2) secondary amide units, probable additional adsorption centers, and (3) an extensive silica network. ζ-potential measurements were conducted so as to determine at which pH the silica framework carries the highest positive charge and therefore could possibly serve as an adsorbent for oxyanions. Figure 4a shows the effect of pH on ζ-potential of the [1-3-20] powder. The highest ζ-potential values were found in the pH range 2-3. In accord with these results was almost 100% adsorption of molybdate (Figure 4b). Given these results, all adsorption studies concerning Mo, Cr, and Se oxyanions were conducted in this pH range. Langmuir 2010, 26(11), 8327–8335

The equilibrium adsorption isotherms measured for Mo, Se, and Cr on [1-3-20] powder are shown in Figure 5. Despite significant differences in the apparent ultimate levels of adsorption of Mo and Se, both isotherms indicate progressive and rapid loading at low concentrations of each metal species. The adsorption data obtained for Mo and Se were best described by the Langmuir expression. The relevant constants and statistical parameters from the isotherm fits are summarized in Table 1. Removal of molybdenum was found to be facile and an equilibrium capacity of about 1.67 mmol/g (or 160.32 mg/g). For initial concentrations of molybdenum below 500 ppm, the distribution coefficients (Kd) were found to be in excess of 104, indicating practically complete removal of molybdenum. To our knowledge, the adsorption capacity of molybdenum on [1-3-20] powder is some of the highest yet reported for amino-functionalized silicas. Previously Yokoi et al.27 reported the maximum adsorption capacity of approximately 1.29 mmol of molybdenum per gram amino-functionalized MCM-41. The maximum adsorption capacity for Se was estimated at approximately 1.05 mmol/g (or 82.95 mg/g) and shown in Table 1. In all cases, for initial concentration of Se e 250 ppm, the distribution coefficients (Kd) were found to be in the range of DOI: 10.1021/la904747m

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Table 1. The Parameters of Best-Fitted Adsorption Isotherms Calculated for Mo(VI), Se(VI), and Cr(VI) Adsorption Freundlich constants (estimated from Freundlich plot)b

Langmuir parameters (estimated using n-NLLS regression)a

adsorbate

Qmax,mmol/g

b, L/mmol

η*2

Kf

1/n

R2

TMA-TMSEDA-TMOS = 1:3:20 powder Mo (VI) Se (VI) Cr (VI)

1.67 1.05

17.65 6.44

0.999 0.999 0.62

0.69

0.938

TMSEDA-TMOS = 3:20 powder (no TMA) Mo (VI) 1.00 3.92 0.944 a Best-fitted Langmuir linear plots for all powders are shown in Figure S1 and Figure S2, Supporting Information b Freundlich plot is provided in Supporting Information.

103-104, demonstrating comprehensive removal of selenium by [1-3-20] powder in this concentration range. It is also noted that Yokoi et al.27 have previously reported 0.81 mmol/g maximum adsorption capacity of selenate by MCM-41 modified with TMSEDA. As indicated in Table 1, the binding affinity (b) for molybdenum by [1-3-20] powder was almost 2.75 times greater than that of selenium. This observation and the increased equilibrium capacity for molybdenum versus selenium can in part be explained by the existence of different species in solution during the adsorption study. The form of anion that predominates in a solution depends primarily on the pH of the media and the anion concentration. In the case of molybdenum, the solution pH changed during adsorption from 2.1 to approximately 3.3. At molybdenum concentrations below about 1 mmol/L and pH >1, the monomeric molybdate ion predominates.37 Considering that the dissociation constant of molybdate are pK1 = 4.00 and pK2 = 4.24,38 the monovalent molybdenum(VI) species (HMoO4-) are expected to be present under these experimental conditions. Furthermore, according to Mitchell17 and Baes and Mesmer,39 at pH around 3-5 the polymerization condensation of molybdenum species occurs giving at concentration above 1 mmol/L highly anionic clusters, such as hepta- ([Mo7O24]6-) but predominantly octa-molybdate ([Mo8O26]4-) ions. Therefore, in this study various anionic Mo species will be present depending on the experimental conditions. In the case of selenium (pK2 = 1.70), the pH changes from 2.3 to 3.4 during the experiment and, therefore, the predominant species in aqueous solution are the divalent anions (SeO42-). Within these hybrid materials, there are several possible binding sites that could be considered for oxyanions complexation, including the amine and amide nitrogen, CdO, and potentially the residual OH groups hosted on silicon atoms. However, the residual OH groups are more likely to be concealed within the extensive silica network and therefore are the least likely sites for the metal complexes bonding. The amine nitrogen is a lone-pair donor and, under experimental acidic conditions, appears to be a preferential coordination site, or a basic site, for anchoring the (37) Mitchell, P. C. H. In Ullmann’s Encyclopedia of Industrial Chemistry, 5th ed.; Elvers, B., Hawkins, S., Schulz, G., Eds.; VCH: Weinheim, Germany, 1990; Vol. A16, p 675. (38) CRC Handbook of Chemistry and Physics, 83rd ed.; Lide, D. R., Ed.; CRC Press: Boca Raton, FL, 2002. (39) Baes, C. F.; Mesmer, R. E. The hydrolysis of cations; Krieger Publishing Company: Malabar, FL, 1986.

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oxyanions under investigation. Additionally, it is expected that both O and N of amide are also likely to contribute to the oxyanion complexation. On the basis of the Langmuir isotherm assumptions that all adsorption sites are energetically equivalent and there is no interaction between the ions, it is speculated that theoretically, two Nþ sites will be required to accommodate each SeO42- anion, providing Se/N ratio of 1:2. At the same time, theoretical Mo/N ratios will be expected to vary between 1 (1 HMoO4- per 1 Nþ) and 2 (1 [Mo8O26]4- per 4 Nþ). Therefore, the same number of N sites is capable of accommodating more Mo ions then Se. As a result, a greater amount of Mo is adsorbed on [1-3-20] powder in comparison to Se. Both the HMoO4- and the SeO42- oxyanions are more likely to be coordinated to the secondary -NH- amino sites. Predicting the exact nature of the [Mo8O26]4- cluster complexation to the [1-3-20] type of powder is rather difficult, since it forms a large number of complexes with nitrogen-donor ligands, specifically amines.40 However, considering that each [Mo8O26]4- cluster is build up from eight distorted [MoO6] octahedral environments with three types of O atoms found, namely, six O (μ3), six O (μ2) and 14 terminal O (t) atoms, we propose that the complexation of this cluster to the nitrogen sites is more likely to occur via the terminal oxygen atoms. At the saturation level the Mo/N molar ratio is estimated at 1.65, suggesting that [Mo8O26]4- cluster is anchored by 3-4 nitrogen sites. Furthermore, the existence of Mo polyanions (hepta- and octamolybdate ions) in solutions adequately explains the higher affinity of molybdenum for the silica framework, since on the basis of electrostatics, if only monomeric molybdenum (HMoO4-) species were present, Se should have displayed greater affinity. In comparison, the sorption isotherm measured for chromium displayed an almost linear absorption (Figure 5) even up to the highest concentration investigated (∼4 mmol/L). The adsorption data for chromium species could not be modeled well using the Langmuir equation with the correlation parameter (R2) estimated at ∼0.71. This type of isotherm is often indicative of possible interactions between the already adsorbed species or multicomponents adsorption. Therefore the adsorption data for chromium were fitted using the Freundlich equation that allows for this sort of interactions. The relevant constants are shown in Table 1. (Both the Langmuir linear plot and the Freundlich plot for chromium adsorption data are shown in Supporting Information). Hexavalent chromium is highly soluble in wide pH range and exists in anionic form predominantly forming (hydroxy) oxyanions, including hydrogen chromate, HCrO4-; dichromate, Cr2O72-; and chromate, CrO42-. Depending on pH and concentration, the equilibrium 2CrO42- þ 2Hþ T 2HCrO4- T H2O þ Cr2O72- exists between chromate and dichromate ions in an aqueous solution.41 During the chromium adsorption study the pH of solutions has changed from 2.0 to 3.2 at equilibrium, and considering the dissociation constant of chromate pK1 = 0.74 and pK2 = 6.40,38 at lower Cr concentration HCrO4- are believed to be the predominant species. However, at these pH values and in more concentrated solutions the equilibrium will be shifted to the right. The presence of the HCrO4- and Cr2O72- species competing for the same adsorption sites and/or their interaction on adsorption sites, may explain why the Cr adsorption could only be fitted with the Freundlich adsorption isotherm. It is understood (40) Pavani, K.; Lofland, S. E.; Ramanujachary, K. V.; Ramanan, A. Eur. J. Inorg. Chem. 2007, 568–578. (41) Bleam, W. F.; McBridge, M. B. J. Colloid Interface Sci. 1986, 110, 335–346.

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Figure 7. Equilibrium adsorption isotherm for molybdenum on [3-20] powder fitted using n-NLLS method.

Figure 6. Nitrogen adsorption-desorption isotherms.

that Cr/N ratio is more likely to be constant and equal to 1 regardless of the form of ions present in a solution. Similar to molybdate and selenate, chromium species are also expected to be primarily bound to the protonated secondary amine nitrogen sites, and possibly to N and O of amide groups. The value of 1/n being less than 1 (Table 1) is an indication of strong interactions between adsorbate and adsorbent, while higher value of Kf points toward a higher rate of adsorbate removal. Role of TMA and Other Organic Functionalities on Powder Adsorption Capacity. Further modifications to the original [1-320] powder have been made to identify the factors affecting the powder adsorption behavior. First, the original [1-3-20] powder was calcined at 650 °C for 2 h to remove all organic, including nitrogen, and to produce highly porous silica structure; second, [3-20] powder was produced without TMA, while ensuring the number of nitrogen sites in the solid remained comparable with [13-20]. The microanalytical CNH data in fact showed that the amount of nitrogen has been estimated at 4.19 and 4.66 wt % for the as-prepared [1-3-20] and [3-20] powders, respectively. The N2 adsorption-desorption isotherms for both parent and calcined [1-3-20] powders (Figure 6) have been designated as Type II using the IUPAC classification scheme. The plateau normally observed for Type IV adsorption at high relative pressure was not apparent.42 Incomplete pores filling could be associated with the materials containing larger pores that could not be measured by nitrogen adsorption. Both isotherms exhibited a narrow type H3 hysteresis loop over the relative pressure range 0.7-1.0, associated with the capillary condensation. After calcination of the parent [1-3-20] powder at 650 °C for 2 h, BET calculations indicated the development of significant surface area of about 420 m2/g and pore size of around 6 nm in comparison to the 100 m2/g and pore size of about 34 nm for the original powder. This calcined powder was further used to study the removal of molybdenum species from aqueous solutions with initial molybdenum concentration of 467 and 908 ppm. While both the surface (42) Sing, K. S. W.; Everett, D. H.; Haul, R. A. W.; Moscou, L.; Pierotti, R. A.; Rouquerol, J.; Siemieniewska, T. Pure Appl. Chem. 1985, 57, 603–619.

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area and pore size were sufficient enough to accommodate most species, the removal of molybdenum by calcined powder was found to be over 90% lower than by parent [1-3-20] powder in both cases (Figure S4, Supporting Information). These findings are not surprising as it has been reported previously that silica alone does not have an affinity for the ionic species.43 Moreover, changes to the silica framework upon heating (reduced amount of OH, etc.) would be expected to further lower the adsorption capacity of powder. In a separate set of experiments, the solid [3-20] was produced without TMA, and adsorption studies of molybdenum were carried out in a manner similar to [1-3-20] powder. It needs to be pointed out however, that [3-20] powder exhibited a slightly different ζ-potential versus pH profile with the highest positive charge found within the pH range of 1 to 5 (Figure S3, Supporting Information). There was a slight change in the N2 adsorption-desorption isotherm shape observed for the [3-20] powder (Figure 6). With a small plateau recorded at high relative pressures, this isotherm is believed to resemble Type IV adsorption with characteristic H1 hysteresis loop associated with capillary condensation taking place in mesopores.44 The BET surface area of the [3-20] material was measured at approximately 184 m2/g with the average pore size of 17 nm. The equilibrium adsorption isotherm of molybdenum on the [3-20] powder is provided in Figure 7 and Langmuir parameters are summarized in Table 1. It is evident that maximum adsorption capacity of Mo by the [3-20] powder decreased by approximately 40% in comparison with the [1-3-20] powder under similar conditions. The affinity parameter (b) also dropped by almost 32%. Keeping in mind that the surface area of [3-20] powder is almost twice the value of [1-3-20] and that both powders have approximately the same amount of nitrogen sites, it is our hypothesis that these sites are more easily accessible for oxyanions in the [1-3-20] than in the [3-20] sample. To summarize, the apparent increased adsorption capacity in [1-3-20] powder could be explained by a combination of factors, including (1) the number of nitrogen sites incorporated into the powder, (2) their distribution within the silica network, (43) Bruzzoniti, M. C.; Mentasti, E.; Sarzanini, C.; Onida, B.; Bonelli, B.; Garrone, E. Anal. Chim. Acta 2000, 422, 231–238. (44) Dickinson, C. F.; Heal, G. R. Thermochim. Acta 1999, 340-341, 89–103.

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and (3) their accessibility as adsorption sites for oxyanions. These in turn could be the result of a specific structure development in the synthesized hybrid material where the TMA played an essential role. Strong interactions between amino groups and TMOS-hydrolysis products,17,25 together with simultaneous formation of silica network, are more likely to result in nitrogen sites being enveloped by the silicate framework and rendered inaccessible for oxyanions. In the presence of carboxylic acid (TMA) however, these interactions could have been prevented or limited. It is believed that the interaction between TMA and TMSEDA leads to the formation of amide linkages, thus creating a “scaffold”, around which the silica network is formed. Additionally these amide linkages could also assist in more uniform distribution of nitrogen sites, in particular the secondary amine groups, as the most probable adsorption sites for oxyanions within the nanohybrid structure. Furthermore, the degree of the silica network formation could also be responsible for easier access to the adsorption sites. Nanohybrid Coatings on Glass Monoliths. The growing interests in materials with controlled functionalities prompted us to translate the technology developed for powdered adsorbent preparation to thin films and coatings. A particular challenge associated with thin films is to identify the processing window (pH, concentration, aging time, etc.) within which continuous and uniform hybrid coatings can be obtained. By careful manipulation of the sol-gel parameters, namely pH, a precursor solution with a chemical composition similar to [1-3-20] powder, was prepared and used to dip-coat glass substrates doped with molybdenum. The optimum processing conditions are described in detail in the Experimental Section. The coating structure was investigated by FTIR (Figure 3c) and revealed features that were almost identical with the [1-3-20] powder. The bands characteristic of the amide linkage, amide I (νCdO) and amide II (δN-H), were observed at 1615 and 1565 cm-1, respectively. A broadband of significant intensity around 1200-900 cm-1 is assignable to the silica (Si-O-Si) framework vibrational bands. Another broadband at 800 cm-1 has been ascribed to C-N stretching vibrations. Multiple dip-coating of the Mo-doped glass monoliths were possible, however, a two-layer coating was chosen for assessment as a barrier against molybdenum leaching from the glass into water at 90 °C. The measure of molybdenum (as fractional leaching) released from coated and uncoated samples over a period of 200 days are provided in Figure 8. The detection of molybdenum in a deionized water control was typically less than 0.01 ppm. Preliminary data summarized in Figure 8 show that after 200 days the amount of Mo released from the uncoated sample was ostensibly double that the amount released from the coated sample at around 4%. Fractional leaching data are usually fitted using solid-liquid diffusion controlled equations,44 however in this preliminary study the number of data points were insufficient to apply any particular kinetics equation. Nonetheless, as Figure 8 indicates there are several important observations which can be made. First, both systems are still releasing Mo to solution, even after 200 days. Second, there is an induction period of approximately 20 days for the coated glass in comparison to the uncoated substrate where the Mo release starts almost immediately. This clearly demonstrates that the nanohybrid-coating suppresses Mo leaching. Further studies are currently underway to determine the mechanism by which Mo leaching is suppressed, either as (1) a simple barrier coating that prevents diffusion of species to and 8334 DOI: 10.1021/la904747m

Karatchevtseva et al.

Figure 8. Fractional leaching of Mo into water at 90 °C from uncoated and coated Mo-doped glass monoliths.

from the glass surface; (2) as a coating that adsorbs leached Mo from the glass substrate; or (3) as a combination of both these scenarios.

Conclusion This study has systematically investigated the structural characteristics of a series of nanohybrid powders and coatings incorporating aromatic carboxylic acid (trimesic acid, TMA), tetraalkoxysilanes (TMOS), and amine-functionalized alkyltrialkoxysilanes (TMSEDA), and explored their potential use for removing of Mo, Se and Cr from aqueous solutions. 13 C NMR and FTIR results suggest that secondary (-CO-NH-) amide linkages are formed upon interaction of TMA with TMSEDA. It is believed that TMA plays a vital role in a hybrid structure development by possibly preventing or limiting the interaction between amino-groups and TMOS-hydrolyzed products and assisting in more homogeneous distribution of nitrogen sites within the nanohybrid structure. Our results have shown that the carboxylic acid is essential for the synthesis of hybrid materials with affinity for Mo and Se, as without it, the accessibility of grafted amine functionalities seems to be significantly reduced. The adsorption data obtained for Mo and Se were described by Langmuir adsorption isotherms. The maximum adsorption capacity for molybdenum and selenium is estimated at approximately 1.67 and 1.05 mmol/g, respectively. In all cases, at low concentrations (e250 ppm) the Kd values were in the range of 103-104. The Freundlich isotherm, suggestive of potential interactions between the already adsorbed species and/or multicomponent adsorption, was found to be appropriate to describe the adsorption behavior of chromate ions. Finally, the two-layered coatings have been tested as barriers against the molybdenum release from the glass substrate into water at 90 °C. There was an induction period of approximately 20 days for the coated glass in comparison to the uncoated substrate suggesting the barrier effect provided by the two-layered silica-based coating. After 200 days the amount of molybdenum released from the uncoated sample was double the amount released from the coated sample. Acknowledgment. The authors would like to thank Dr. Tamim Darwish for fruitful discussions and assistance with data interpretation. Langmuir 2010, 26(11), 8327–8335

Karatchevtseva et al.

Supporting Information Available: Figure S1. Best-fitted Langmuir linear adsorption isotherms for (a) molybdenum, (b) selenium, and (c) chromium on [1-3-20] powder. Figure S2. Freundlich plot for chromium adsorption on [1-3-20] powder. Figure S3. Best-fitted Langmuir linear adsorption isotherms for molybdenum on [3-20] adsorbent (R2 = 0.993). Figure S4. Effect of pH on ζ-potential for [3-20] powder. Figure S5. Adsorption of molybdenum on [1-3-20]

Langmuir 2010, 26(11), 8327–8335

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powders: as-prepared and calcined at 650 °C for 2 h. Figure S6. FT-IR spectra of [3-20] powder. Figure S7. SEM (a) and TEM (b-d) images of the original [1-3-20] powder. Figure S8. TEM images of the calcined [1-3-20] (a) and original [3-20] (b) powders. Table S1. Qmax values estimated using the n-NLLS and Langmuir linear regressions for adsorption isotherms of Mo(VI) and Se(VI). This material is available free of charge via the Internet at http://pubs.acs.org.

DOI: 10.1021/la904747m

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