Environ. Sci. Technol. 2005, 39, 7306-7310
Functionalized TiO2 Nanoparticles for Use for in Situ Anion Immobilization SHAS V. MATTIGOD, GLEN E. FRYXELL,* KENTIN ALFORD, TYLER GILMORE, KENT PARKER, JEFF SERNE, AND MARK ENGELHARD Pacific Northwest National Laboratory, Richland, Washington 99352
Anatase particles (40-60 nm) were coated with an organosilane monolayer terminated with an ethylenediamine (EDA) ligand. These functionalized nanoparticles (FNPs) were then treated with an aqueous solution of Cu(II) to create a cationic Cu-EDA complex bound to the nanoparticle surface. Cu(II) and EDA ligand incorporation were confirmed by X-ray photoelectron spectroscopy (XPS) analysis. The Cu(EDA)2 FNP was then studied for its binding affinity for pertechnetate anion from a Hanford groundwater matrix. The Cu(EDA)2 FNP was also evaluated for its injectability into a porous medium for possible application as a subsurface semipermeable reactive barrier. Injection was readily accomplished, and resulted in a highly uniform distribution of the FNP sorbent in the test column.
Introduction Nanomaterials offer many advantages for environmental remediationshigh surface area, high reactivity, ease of modification, and in some cases easy dispersability. Since our first report on self-assembled monolayers on mesoporous supports (1), significant effort has been expended by several groups developing chemically modified nanomaterials to capture toxic heavy metals (2-10) and anions (11-13). In general, these efforts have focused on the use of nanoporous materials due to their exceptionally high surface area, open porosity, and ease of chemical modification. One potential drawback of this approach is that particle size is generally rather large (commonly 10-100 µm). Particles of this size settle rapidly, are abrasive, and are not as easily pumped or injected into subsurface porous media as are submicrometer particles. Many varieties of nanoparticles are commercially available, and in a range of sizes. While these particles do not have the extremely high surface areas of their nanoporous cousins (i.e., larger particles that contain large numbers of nanopores), they do provide adequate surface area for some applications, and offer significant advantages in terms of dispersability, pumpability, and injectability. We have an ongoing effort in the synthesis and design of sorbent materials (1-5, 11-16) that sequester environmentally harmful species. One particular area that needs to be addressed is the design and synthesis of injectable functionalized nanoparticles that can be used as chemically specific semipermeable barriers for in situ remediation of contaminated groundwater plumes. Pertechnetate anions * Corresponding author phone: (509) 375-3856; fax: (509) 3752186; e-mail:
[email protected]. 7306
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tend to interact weakly with soil particles, and as a result tend to be particularly mobile in groundwater plumes (17, 18). We (11) and others (12, 13) have developed cationic transition-metal complexes anchored to nanomaterials for the capture of this anion. In this paper, we describe the use of Cu(EDA) complexes anchored on TiO2 nanoparticles, designed as injectable semipermeable barriers used to clean up contaminated plumes.
Experimental Section Characterization. The Cu-EDA functionalized nanoparticle (FNP) was characterized by XPS, FTIR, 13C NMR, and BET analyses. XPS. XPS measurements were performed using a Physical Electronics Quantum 2000 scanning ESCA microprobe. This system uses a focused monochromatic Al KR X-ray (1486.7 eV) source and a spherical section analyzer. The instrument has a 16-element multichannel detector. The X-ray beam used was a 100 W, 100 µm diameter beam that was rastered over a 1.4 mm by 0.3 mm rectangle on the sample. The X-ray beam was incident normal to the sample, and the photoelectron detector was at 45° off-normal. The narrow scan data were collected using a pass energy of 46.95 eV. For the Ag 3d5/2 line, these conditions produce an fwhm of better than 0.98 eV. Although the binding energy (BE) scale was calibrated using the Cu 2p3/2 feature at 932.62 ( 0.05 eV and Au 4f feature at 83.96 ( 0.05 eV for known standards, lowenergy electrons at ∼1 eV and 21 µA and low-energy Ar+ ions were used to minimize this charging. IR. FTIR spectra were collected on a Nicolet 860 Magna IR instrument, under a dry N2 atmosphere. Sample pellets were prepared using finely ground (mortar and pestle) KBr with approximately 1-2% mass loading, compressed in a hydraulic press at approximately 20000 psi. A minimum of 200 scans were collected. NMR. Solid-state 13C NMR spectra were determined at 25.2458 MHz, using a Chemagnetics CMX-100 NMR quadruple channel spectrometer system. The probe was a 7 mm pencil-type probe with magic angle spinning at 4 kHz. Four microsecond, 90° pulses were utilized for 13C, with the proton decoupling power at 62.5 kHz. Samples were prepared by loading a spinner with a Teflon spacer followed by approximately 65 mg of sample, 15 mg of tetrakis(trimethylsilyl)silane (TTMS), an additional 65 mg of sample, and a sample end spacer. Chemical shifts were referenced to internal TTMS. BET Analyses. BET surface area, porosity, and pore size analyses were directly measured on samples using nitrogen adsorption/desorption collected with a Quantachrome Autosorb 6-B gas sorption system on thoroughly degassed samples. Synthesis. Commercially available “Altair Nano40” TiO2 nanoparticles were chosen as the substrate. This material is composed of 40-60 nm particles of crystalline anatase (99.8% purity), with a surface area of 51.2 m2/g and a density of 3.88 g/cm3. It is available both as a dry powder and as an aqueous suspension, and both forms were tested for functionalization. The best results were obtained with the dry powder. Decant Method. The first method tried involved taking the aqueous suspension of TiO2 nanoparticles (91.863 g; 36.7 g of TiO2), centrifuging them, decanting off the water, washing them with isopropyl alcohol (IPA), centrifuging them again, and decanting off the solvent. The hydrated TiO2 nanoparticles were suspended in toluene and treated with 6.0 mL (27.3 mmol) of [3-(aminoethyl)-2-aminopropyl]trimethoxysilane (“EDA-silane”; this is approximately a 2× excess on the basis of the available surface area). This mixture was 10.1021/es048982l CCC: $30.25
2005 American Chemical Society Published on Web 08/18/2005
heated to reflux for 4 h. After reflux, it was cooled to ambient temperature, and the coated nanoparticles were collected by centrifugation and air-dried. The crude product weighed 45.24 g at this point (23% weight increase), and was offwhite in color. BET showed a surface area of 37 m2/g. This product was treated with 43.5 g of CuCl2 dissolved in 220 mL of water, stirred for 1 h at ambient temperature, filtered, washed with water, and air-dried. The dried product weighed 42.216 g. XPS analysis revealed 21.4 mol % C, 2.3 mol % Cu, and 2.3 mol % N (the very weak XPS cross-section for N makes quantification of the N peak questionable). The observed C/Cu ratio of 9.3 is consistent with the formation of a Cu(EDA)2 complex (the theoretical C/Cu ratio would be 10). FTIR showed peaks at 2930, 2853, 1717, and 1626 cm-1, indicative of aliphatic C-H stretching and CH2 scissoring modes from the EDA-silane. The BET surface area was found to be 43 m2/g. Solid-state 13C NMR results were ambiguous, but clearly showed that some organic material had been lost in the metalation step. The product was Carolina blue, flecked with yellow at this point (note that this material was notably lighter in color than the samples described below). Simple Reflux Method. The second formulation strategy involved taking 10.227 g of the 40% aqueous suspension (4.091 g of TiO2) directly in 50 mL of IPA, treating it with 0.453 g (approximately 20% excess) of EDA-silane, and heating it at reflux for 4 h. The solvent mixture was decanted and the product collected by filtration and air-dried. The product weighed 5.384 g (32% weight increase). BET showed a surface area of 30.5 m2/g. This intermediate was treated with 5 g of CuCl2 in 100 mL of DI water and stirred for 1 h at ambient temperature. The product was collected by filtration, washed with water, and air-dried. The product had both a blue and a yellow hue at this point, so it was taken up in water, washed, filtered, and air-dried. The product was a deep robin’s egg blue color at this point. It weighed 4.597 g. XPS analysis revealed a composition of 20.1 mol % C, 1.6 mol % Cu, and approximately 4.1 mol % N (the very weak XPS cross-section for N makes quantification of the N peak questionable). The observed C/Cu ratio of 12.6 suggests the formation of a Cu(EDA)2 complex (the theoretical C/Cu ratio would be 10). FTIR showed the CH stretches at 2930 and 2883 cm-1, indicative of aliphatic C-H stretching of the EDA-silane. BET showed a surface area of 34.5 m2/g. Solid-state 13C NMR supported the conclusion that organic material had been lost in the metalation step. Powder Method. A 21.613 g sample of dry TiO2 was placed in a 500 mL round-bottom flask with 250 mL of toluene, 0.25 mL of water, and a stir bar. This mixture was stirred vigorously for 2.5 h. A 3.0 mL sample of EDA-silane (13.6 mmol, approximately 50% excess on the basis of the available surface are) was added, and the mixture was heated to reflux for 3 h. It was then allowed to cool to ambient temperature and filtered. The crude product was washed with toluene and IPA and then air-dried. It weighed 23.062 g at this point for a mass increase of 1.449 g (or 6.7%). The surface area of the material at this point was 25.4 m2/g. Assuming a MW of 162.26 for the EDA-silane (19), and that all silanes are in fact anchored to the TiO2 surface (which may not be true, vide infra), this corresponds to 8.9 mmol of silane on the TiO2 surface, or 4.8 silane molecules/nm2, in excellent agreement with previous monolayer population density measurements (11). This sample was then placed in a solution of 1.6 g of CuCl2 dissolved in 150 mL of DI water. This mixture was stirred for 2 h at ambient temperature, filtered, washed copiously with water, and dried in a vacuum desiccator over the weekend. The final product weighed 22.261 g (for a mass loss of 0.801 g, or 3.5%). The surface area of the metalated TiO2-EDA material was 28.5 m2/g.
TABLE 1. Composition of Hanford Groundwater Used as a Test Matrixa
a
cation
concn, mg/l
Ca Mg K Na Cl
49.5 14.6 1.7 13.2 16.4
anion
concn, mg/L
NO3 SO4 silicate CO3
8.6 64.7 16.5 60.8
Spiked with 49.5 pCi/mL Tc-99. pH 8.3 (SU). EC ) 0.47 mS/cm.
For the sake of comparison, similar EDA-silane monolayers prepared on MCM-41 (900 m2/g) have a loading density of approximately 1.2 Cu-EDA complexes/nm2 (11). Summarizing the results described above, it is clear that the EDAsilane loading density (and hence the Cu-EDA complex density) is less than half that obtained on a silica surface. Testing Matrix. To mirror reality as closely as possible, these tests were carried out in Hanford groundwater (the composition of which is summarized in Table 1) spiked with 49.5 pCi/mL Tc-99. The pH of the spiked groundwater was 8.3. The batch contact time was 12 h to ensure that true equilibrium was achieved (in similar sorbent materials, equilibrium is generally reached in a matter of a few minutes). The solution to solids ratio was systematically varied from 25, 50, 100, 500, 1000, 5000, and 10000 mL/g (the “simple reflux” FNPs were used for these tests). After the designated contact time, these suspensions were filtered through 0.45 µm membranes, and the filtrates were analyzed for Tc-99 activity using liquid scintillation counting. Injection Tests. The test column was 1.27 cm in diameter and 100 cm long. It was filled with 20-30 mesh silica sand (Accusand). The packed column had a porosity of 35% and a pore volume of approximately 44 mL. The FNP suspension was approximately 60 wt % sorbent in aqueous suspension, with about 2% added ammonium carboxylate dispersant. The injection rate was approximately 20 mL/min. Approximately 3 pore volumes (130 mL) was injected. The injection pressure was measured, and the anatase distribution was determined at the end of the injection by dissecting the column into 10 cm slices. Each segment was wet sieved through a 325 mesh (44 µm) sieve, the sand and anatase fractions were collected, dried in an oven at 105 °C, and weighed.
Results and Discussion TiO2 was chosen as the foundation for this work because of its greater stability in alkaline environments compared to SiO2. Previous work has shown that Cu-EDA is an effective functional binding site for the selective removal of oxometalate anions (3, 6). Therefore, Cu-EDA was chosen as a model pertechnetate binding site for this study. Regardless of the method chosen to modify the TiO2 nanoparticles (“decant”, “simple reflux”, or “dry powder” method; see the Experimental Section for the details), a general trend was noted in that there was mass lost during the metallization step. If this were a simple metal complexation process, then a mass gain would be predicted. The IR and 13C NMR data (not shown) clearly support the conclusion that organic material is being lost during the metallization step. There are several possible explanations for the loss of organics. First, it could be due to removal of physisorbed polysiloxanes from the surface of the FNP. Second, it might be due to partial (or selective) hydrolysis of the silane monolayer from the surface of the FNP. Third, it might be due to partial dissolution of the TiO2 nanoparticle out from underneath the monolayer. This last possibility is thought to VOL. 39, NO. 18, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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be unlikely given that the stability of TiO2 at near-neutral pH is well established (20). The first two methods (decant and simple reflux) resulted in greater mass gain during the silane deposition step than predicted by theory (unusually excessive silane sorption onto TiO2 surfaces has also been observed by others (21)). This is presumably due to the presence of surplus water during these depositions (and was what led to the use of the dry powder). Thus, the products obtained from the first two runs were clearly wet, and presumably contained some form of excess polysiloxane. After exposure to water, the surface of TiO2 is known to be pitted, and irregularly corrugated (22). The magnitude of mass lost during the metallization step in the two “wet” procedures (i.e., those using the aqueous suspension of the TiO2 as the starting material), as well as the lesser surface area to mass ratio of the intermediate TiO2-EDA, suggests that the most likely explanation for the loss of mass is the loss of physisorbed polysiloxane and excess water. It appears that the polysiloxane is physisorbed to the pitted FNP surface, and subsequently gets washed off the surface once the EDA ligands are complexed to the Cu(II). It is well-known that amines coordinate to the Ti(IV) sites in the defects of the TiO2 surface (23). Presumably this EDA-polysiloxane is similarly coordinated to the available defect sites of the FNP, which may prevent it from being washed off from the FNP during the washing stage. Once the amines are chelated to the Cu(II) ions, however, this binding mechanism is no longer operable, and the polymer is readily released from the surface. In the case of the “dry powder” synthesis, however, the surface population was determined to be 4.8 silane molecules/ nm2, which is in very good agreement with similar EDA monolayers on silica (3). Thus, it appears that a self-assembled monolayer is indeed formed in this silanation procedure. However, mass is still lost in the subsequent metallization step. The TiO2 surface is known to be composed of rows of flat terraces and steps, with rows of reactive bridging oxides. If this surface were uniformly coated with a self-assembled monolayer, then this difference in interfacial reactivity would presumably be reflected in the ease with which the silanes bound to these different centers would be lost to hydrolysis. If this were indeed the case, then it would suggest that the final functional surface of the FNP would look like “cornrows” of parallel ridges of Cu-EDA complexes. The final product would have a surface coverage of approximately half of a typical monolayer coating, exactly as observed. After metallization and washing, XPS confirms that the silane-anchored Cu(EDA) complex is on the nanoparticle surface (see Figures1 and 2). The observed C/Cu ratio suggests a stoichiometry of Cu(EDA)2, presumably with the last two coordination sites being filled with water molecules. This observation falls in line with previous measurements of affinity constants between the Cu(II) cation and the EDA ligand, in which log K1 is 10.75, log K2 is 9.28, and log K3 is
