Anal. Chem. 2004, 76, 2893-2901
Characterization of Interlayer Cs+ in Clay Samples Using Secondary Ion Mass Spectrometry with Laser Sample Modification G. S. Groenewold,*,† R. Avci,*,‡ C. Karahan,‡ K. Lefebre,‡ R. V. Fox,† M. M. Cortez,† A. K. Gianotto,† J. Sunner,§ and W. L. Manner|
Idaho National Engineering and Environmental Laboratory, Idaho Falls, Idaho 83415, Image and Chemical Analysis Laboratory, and Department of Chemistry, Montana State University, Bozeman, Montana 59717, and Brigham Young UniversitysIdaho, Rexburg, Idaho 83460
Ultraviolet laser irradiation was used to greatly enhance the secondary ion mass spectrometry (SIMS) detection of Cs+ adsorbed to soil consisting of clay and quartz. Imaging SIMS showed that the enhancement of the Cs+ signal was spatially heterogeneous: the intensity of the Cs+ peak was increased by factors up to 100 for some particles but not at all for others. Analysis of standard clay samples exposed to Cs+ showed a variable response to laser irradiation depending on the type of clay analyzed. The Cs+ abundance was significantly enhanced when Cs+exposed montmorillonite was irradiated and then analyzed using SIMS, which contrasted with the behavior of Cs+exposed kaolinite, which displayed no Cs+ enhancement. Exposed illitic clays displayed modest enhancement of Cs+ upon laser irradiation, intermediate between that of kaolinite and montmorillonite. The results for Cs+ were rationalized in terms of adsorption to interlayer sites within the montmorillonite, which is an expandable phyllosilicate. In these locations, Cs+ was not initially detectable using SIMS. Upon irradiation, Cs+ was thermally redistributed, which enabled detection using SIMS. Since neither the illite nor the kaolinite is an expandable clay, adsorption to inner-layer sites does not occur, and either modest or no laser enhancement of the Cs+ signal is observed. Laser irradiation also produced unexpected enhancement of Ti+ from illite and kaolinite clays that contained small quantities of Ti, which indicates the presence of microscopic titanium oxide phases in the clay materials. Contaminant radionuclides such as 137Cs are in contact with mineral surfaces common to the geosphere as a result of waste disposal activities, leakage of liquid radioactive waste, and fallout from atmospheric releases.1-3 In all of these release scenarios, a * To whom correspondence should be addressed. E-mail:
[email protected]. † Idaho National Engineering and Environmental Laboratory. ‡ Image and Chemical Analysis Laboratory, Montana State University. § Department of Chemistry, Montana State University. | Brigham Young UniversitysIdaho. (1) Eisenbud, M.; Gesell, T. Environmental Radioactivity From Natural, Industrial and Military Sources, 4th ed.; Academic Press: San Diego, CA, 1997. 10.1021/ac035400u CCC: $27.50 Published on Web 04/10/2004
© 2004 American Chemical Society
question of critical importance is whether the radiocesium will be transported or will remain sequestered as a result of strong adsorption to an immobile mineral matrix. In fact, adsorption comprises a strategy that has been used to manage radiocesium released in the environment;4,5 the approach is viable because of the presence of strong adsorption occurring between cesium and clay minerals. Cesium adsorption to clay minerals occurs by reaction with multiple adsorptive sites, which exist because clays have sheetlike morphology and hence are termed phyllosilicates.6 Frayed edge sites participate in what is effectively multidentate cesium bonding that results in practically irreversible adsorption, but these edge sites account for only a small fraction of the total number of Cs+ adsorption sites.7-12 Sites on the surfaces of the clay particles account for a much larger fraction of the Cs+ adsorptive sites, where isomorphic substitution of a lower valence metal has resulted in a fixed negative charge.6,8,10,11,13,14 The binding of Cs+ to these sites is much less aggressive than the frayed edge sites, and desorption leading to transport is a possibility. Since many clays are 2:1 silicate-aluminate-layered phyllosilicates that stack in layers, adsorption can occur between the 2:1 layers6,8,14-17 in cases where the layers can expand.18 Montmorillonite and other (2) Brookins, D. G. Geochemical Aspects of Radioactive Waste Disposal; SpringerVerlag: New York, 1984. (3) Long, J. C. S., Ed. Research Needs in Subsurface Science; National Academy Press: Washington, DC, 2000. (4) Erten, H. N.; Aksoyoglu, S.; Gokturk, H. Sci. Total Environ. 1988, 69, 269296. (5) Krumhansl, J. L.; Brady, P. V.; Anderson, H. L. J. Contam. Hydrol. 2001, 47, 233-240. (6) Krauskopf, K. B.; Bird, D. K. Introduction to Geochemistry, 3rd ed.; WCB McGraw-Hill: Boston, MA, 1995. (7) Zachara, J. M.; Smith, S. C.; Liu, C.; McKinley, J. P.; Serne, R. J.; Gassman, P. L. Geochim. Cosmochim. Acta 2002, 66, 193-211. (8) Bostick, B. C.; Vairavamurthy, M. A.; Karthikeyan, K. G.; Chorover, J. Environ. Sci. Technol. 2002, 36, 2670-2676. (9) Desmet, G. M.; Van Loon, L. R.; Howard, B. J. Sci. Total Environ. 1991, 100, 105-124. (10) Jeong, C.-H.; Kim, C.-S.; Kim, S.-J.; Park, S.-W. J. Environ. Sci. Health, Part A 1996, A31, 2173-2192. (11) Poinssot, C.; Baeyens, B.; Bradbury, M. H. Geochim. Cosmochim. Acta 1999, 63, 3217-3227. (12) Wauters, J.; Vidal, M.; Elsen, A.; Cremers, A. Appl. Geochem. 1996, 11, 595-599. (13) Doppelman, M. H.; Dillard, J. G. ACS Symp. Ser. 1975, No. 18, 186-201. (14) Kim, Y.; Cygan, R. T.; Kirkpatrick, R. J. Geochim. Cosmochim. Acta 1996, 60, 1041-1052.
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smectite clays are in this category and are encountered practically everywhere. Cesium bound to interlayer sites will be more strongly bound than Cs on the outer clay surface, and hence, desorption processes will be slower from the interlayer sites.15-17 Interlayer Cs can account for a significant fraction of the total adsorbed metal,15 and its desorption kinetics will likely be different from Cs adsorbed to outer layer sites, which in turn would influence solubilization and subsequent transport in the environment. For this reason, distinction between forms of Cs+ bound to clay minerals constitutes an important analytical goal. Classically, this has been handled using the method of sequential aqueous extractions, in which metal is desorbed from the host matrix using a series of increasingly harsh chemical extractants.19-25 This approach has been applied specifically to Cs+-contaminated soils by a number of researchers;16,26-33 of these studies, the recent paper by Mincher and co-workers28 is most relevant, because it examined Cs+ speciation in soils common to the radioactive waste disposal areas at the Idaho National Engineering and Environmental Laboratory (INEEL), which is the subject matrix of this report. Their conclusion was that the majority of the Cs+ resided in a residual fraction that strongly resisted solubilization. Spectroscopic approaches have also been employed for improving understanding of Cs+ speciation on soils. The coincidence of environmental radionuclide contamination and synchrotron facilities within the United States Department of Energy complex has resulted in elegant speciation determinations for clay samples by Bostick and co-workers, who were able to identify both innersphere and outer-sphere complexes of Cs adsorbed to montmorillonite.8 More conventional analytical approaches such as XRD,7,34,35 (15) Evans, D. W.; Alberts, J. J.; Clark, R. A. I. Geochim. Cosmochim. Acta 1983, 47, 1041-1049. (16) von Gunten, H. R.; Benes, P. Radiochim. Acta 1995, 69, 1-29. (17) Comans, R. N. J.; Hockley, D. E. Geochim. Cosmochim. Acta 1992, 56, 11571164. (18) Hensen, E. J. M.; Smit, B. J. Phys. Chem. B 2002, 106, 12664-12667. (19) Tessier, A.; Campbell, P. G. C.; Bisson, M. Anal. Chem. 1979, 51, 844851. (20) Litaor, M. I.; Ibrahim, S. A. J. Environ. Qual. 1996, 25, 1144-1152. (21) Parat, C.; Leveque, J.; Dousset, S.; Chaussod, R.; Andreux, F. Anal. Bioanal. Chem. 2003, 376, 243-247. (22) Schultz, M. K.; Burnett, W.; Inn, K. G. W.; Smith, G. J. Radioan. Nucl. Chem. 1998, 234, 251-256. (23) Loyland, S. M.; LaMont, S. P.; Herbison, S. E.; Clark, S. B. Radiochim. Acta 2000, 88, 793-798. (24) Loyland Asbury, S. M.; Lamont, S. P.; Clark, S. B. Environ. Sci. Technol. 2001, 35, 2295-2300. (25) Lo, I. M.-C.; Yang, X.-Y. Waste Manage. 1998, 18, 1-7. (26) Vidal, M.; Tent, J.; Llaurado, M.; Rauret, G. J. Radioecol. 1993, 1, 49-55. (27) Fuhrmann, M.; Zhou, H.; Nieiheisel, J.; Schoonen, M. A. A.; Dyer, R. Sci. Total Environ. 1997, 202, 5-24. (28) Mincher, B. J.; Fox, R. V.; Riddle, C. L.; Cooper, D. C.; Groenewold, G. S. Radiochim. Acta 2003, 91, 1-7. (29) Carbol, P.; Skarnemark, G.; Skalberg, M. Sci. Total Environ. 1993, 130, 129-137. (30) Park, C. K.; Woo, S. I.; Tanaka, T.; Kamiyama, H. J. Nucl. Sci. Technol. 1992, 29, 1184-1193. (31) Vidal, M.; Roig, M.; Rigol, A.; Llaurado, M.; Rauret, G.; Wauters, J.; Elsen, A.; Cremers, A. Analyst 1995, 120, 1785-1790. (32) Marin, B.; Valladon, M.; Polve, M.; Monaco, A. Anal. Chim. Acta 1997, 342, 91-112. (33) Hinton, T. G.; Malek, M. A.; Sherony, C.; Clark, S. B. J. Radioanal. Nucl. Chem. 1998, 235, 185-190. (34) Shahwan, T.; Sayan, S.; Erten, H. N.; Black, L.; Hallam, K. R.; Allen, G. C. Radiochim. Acta 2000, 89, 681-686. (35) Shahwan, T.; Erten, H. N. Radiochim. Acta 2001, 89, 799.
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Table 1. Average RWMC Soil Particle Size Distribution and Surface Area < 2 µm (clay) 2-50 µm (silt) > 53 µm (sands) specific surface average pore size
14 ( 1 wt % 60 ( 2 wt % 26 ( 1 wt % 32.6 ( 0.2 m2/g 26 ( 1 Å
XPS,14,34,36-38 EDX,7 and secondary ion mass spectrometry (SIMS)34,35,39-41 have also been employed: in general, these are capable of providing a wealth of information regarding the nature of adsorbed Cs. Yet, identification of interlayer contamination remains a challenging task. The approach taken in the present article involves using a nitrogen UV laser to provide modest heating to the mineral sample, which causes the contaminant metal to be redistributed to the surface where it can be sensitively detected using SIMS. SIMS has been broadly used for analysis of particulates,42-44 and the combination of SIMS with laser surface irradiation has been used by Kyser for the measurement of isotope ratios of lighter elements.45 The present study however, is focused on analysis of clay-type materials contaminated with Cs+. Herein the influence of laser irradiation on subsequent SIMS analysis is evaluated with specific emphasis on the importance of Cs+ occupying interlayer sites on expandable phyllosilicate clays. EXPERIMENTAL SECTION Cs+ Exposure of Soil Samples and Clay Separates. Soil samples were collected from a 20-ft-deep pit that was located right next to the subsurface disposal area of the Radioactive Waste Management Complex (RWMC) at the INEEL. The RWMC was dedicated to permanent shallow-land disposal of solid, low-level transuranic waste that was generated by national defense programs from 1954 to 1970; however, the soil samples used in the present study were uncontaminated. The RWMC soil is a silty loam, where the major portion of the soil is contained in the silt and clay size fractions (Table 1). The bulk of the soil particles resided in the 2-50-µm range, with the remainder split between finer and coarser particles. Chemical analysis of the soil samples identified the soil samples as predominantly quartzic, with substantial aluminosilicates, and variable concentrations of carbonates that resulted in a slightly alkaline soil pH (7.5-8.0). The elemental and mineralogical XRD analyses indicated that the soil consisted predominantly of clay minerals (illite, smectite, mixed illite-smectite 2:1 clays, and some kaolinite). The cation exchange (36) Ebina, T.; Iwasaki, T.; Onodera, Y.; Chatterjee, A. Comput. Mater. Sci. 1999, 14, 254-260. (37) Onodera, Y.; Iwasaki, T.; Ebina, T.; Hayashi, H.; Torii, K.; Chatterjee, A.; Mimura, H. J. Contam. Hydrol. 1998, 35, 131-140. (38) Shahwan, T.; Suzer, S.; Erten, H. N. Appl. Radiat. Isot. 1998, 49, 915-921. (39) Groenewold, G. S.; Ingram, J. C.; McLing, T.; Gianotto, A. K.; Avci, R. Anal. Chem. 1998, 70, 534-539. (40) Tait, J. C.; Hocking, W. H.; Betteridge, J. S.; Bart, G. Adv. Ceram. 1986, 20, 559-565. (41) Shahwan, T.; Erten, H. N.; Black, L.; Allen, G. C. Sci. Total Environ. 1999, 226, 255-260. (42) Van Vaeck, L.; Adriaens, A.; Gijbels, R. Mass Spectrom. Rev. 1999, 18, 1-47. (43) Van Grieken, R.; Xhoffer, C.; Wouters, L.; Artaxo, P. Anal. Sci. 1991, 7, 1117-1122. (44) Adriaens, A.; Van Vaeck, L. Mass Spectrom. Rev. 1999, 18, 48-81. (45) Kyser, T. K. Can. Mineral. 1995, 33, 261-278.
Table 2. Source Clay Composition composition (%) clay
interlayer charge
SiO2
Al2O3
TiO2
Fe2O3
FeO
MgO
CaO
Na2O
K2O
kaolin KGa-1, low defect (Georgia) illite IMt-1 and IMt-2 (Montana) illite-smectite mixed layer (Czech) Na-montmorillonite SWy-1 (Wyoming)
-0.06
44.2
39.7
1.39
0.13
0.08
0.03
0.00
0.01
0.05
-1.68
49.3
24.2
.55
7.32
0.55
2.56
0.00
0.00
7.83
-1.29
51.6
25.6
0.04
1.11
0.10
2.46
0.67
0.32
5.36
-0.55
62.9
19.6
0.09
3.35
0.32
3.05
1.68
1.53
0.53
capacity (using Ba2+ and NH4+) was relatively low ( Na+ > K+. Ten-gram soil samples were exposed to 100 mL of CsNO3 solutions. Generally, the exposure concentration was 1 mM, although in some experiments 0.1, 10, and 100 mM exposure concentrations were used. Most of the Cs+ partitioned onto the soil samples (distribution coefficients ranged from 300 ( 16 to 8000 ( 370 mL g-1 at pH 8.0 over an exposure concentration range of 0.6-1.0 mM Cs+).28 After an 18-h exposure, samples were decanted, washed with distilled deionized water, and dried at room temperature prior to analysis. Clay separates were also analyzed: these were obtained from The Clay Minerals Society, Aurora, CO. The samples were kaolinite, illite, mixed illite-smectite, and montmorillonite (Table 2). The samples were exposed to 1 mM CsNO3 solutions as described above. Imaging Time-of-Flight Secondary Ion Mass Spectrometry (TOF-SIMS). Analyses were performed using a triple focusing time-of-flight secondary ion mass spectrometer (TRIFT TOF-SIMS, Physical Electronics, Eden Prairie, MN),46,47 located at the Image and Chemical Analysis Laboratory, Montana State University. Clay particles were placed into squares of indium foil (∼1 cm2); the amount used was substantially less than 1 mg and visually consisted of several grains or clumps held between the tines of sharp forceps. Once in place, the soil sample was pressed into the foil48 using a glass microscope slide. This had the effect of spreading out the particle aggregates into an area a small fraction of 1 mm across. The indium-mounted samples were then mounted on the sample holder. Typically, a 100 µm × 100 µm area was scatter rastered using the primary ion beam in a pulsed fashion. The temporal width of a single primary ion pulse was 14 ns, and the repetition rate was 10 kHz. Every fifth pulse was used to fire a pulsed electron gun, which compensated for electrical charging of the sample. The primary ion source was a microfocused Ga+ gun which operated at 1.3 nA dc, at +15 keV relative to ground. The target stage was biased nominally at +3.0 keV for analysis, optimized for emission of secondary ions. Thus, primary ion impact energy was 12 keV. The flux density was calculated at 2.2 × 1010 ions/s cm2. Particles were typically analyzed for 1 min with a total dose imparted to the samples of 1.3 × 1012 ions/cm2. (46) Schueler, B.; Sander, P.; Reed, D. A. Vacuum 1990, 41, 1661-1664. (47) Schueler, B. W. Microsc. Microanal. Microstruct. 1992, 3, 119-139. (48) Reich, D. F. In ToF-SIMS Surface Analysis by Mass Spectrometry; Vickerman, J. C., Briggs, D., Eds.; IM Publications and SurfaceSpectra Limited: Chichester, U.K., 2001; p 123.
The imaging TOF-SIMS instrument was a combined ion microprobe/ion microscope,46,47 which operated at an effective spatial resolution on the order of 1 µm for the analysis of the soil and clay separates. This feature enabled the SIMS spectra of specific “regions of interest” (roi) to be retrospectively separated from spectral features arising from other regions. An example of a roi would be the laser-irradiated area or areas of high cation abundance within the irradiated area. The TOF-SIMS was fitted with a 337-nm UV nitrogen laser (Laser Science, Inc., Franklin, MA, model VSL-337ND-S) that was directed onto the sample stage using a combination of mirrors and lenses. Each laser pulse has ∼4-ns pulse width and ∼10-µJ average pulse energy spread over a 30-mm2 area. The focused laser beam was carefully directed through a glass viewing port to irradiate the sample stage of the TOF-SIMS instrument. Coalignment of the laser focus and the primary ion beam was achieved using a silicon target covered with ∼10-nm gold film on which a 100-µm square was etched using the Ga+ beam. Since the goldcoated silicon underwent discoloration when irradiated with the laser, the laser focus could be moved to the center of the etched square by watching the target using a microscope with a video. The laser was focused so that a spot ∼30 µm in diameter was irradiated. With a focal area of ∼1000 µm2, the fluence is calculated to be ∼1 J/cm2. Pulsed laser irradiation and imaging SIMS analyses were performed in an alternating fashion, which enabled the effect of the laser irradiation to be evaluated. It was found that enhancement of metal secondary ions increased sharply with increasing number of laser pulses until ∼12 pulses had been delivered. After this point, further enhancement was modest. Hence, we adopted 12 pulses as our standard photon dose for comparing the effects of laser irradiation. RESULTS AND DISCUSSION A heterogeneous soil, which had been collected from an area near a nuclear waste burial site (RWMC, located at the INEEL, a U.S. Department of Energy site) was examined in the present study. The site has historically been used as a burial ground for radionuclide-bearing wastes,1,3 and hence, there is strong motivation for understanding contaminant partitioning and speciation. Mineralogical analysis showed that the soil was predominantly quartzic in nature (60-80%), with the balance containing substantial quantities of clay materials.28,49,50 An XRD analysis indicated (49) Mark, L. E.; Thackray, G. D. In Geology, Hydrogeology, and Environmental Remediation: Idaho National Engineering and Environmental Laboratory, Eastern Snake River Plain, Idaho; Link, P. K., Mink, L. L., Eds.; Special Paper 353; Geological Society of America: Boulder, CO, 2002; p 92.
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Figure 1. The 100-µm images of a soil aggregate exposed to 0.1 mM CsNO3 and then washed. A “thermal” color scale is used, in which intensity increases on going from white to yellow to red. Black represents zero counts. (a) Total ion image prior to laser irradiation. (b) Total ion image of the same area, acquired after laser irradiation; the area of irradiation is shown in the upper left-hand corner of the image. (c) m/z 118 image (an organic corresponding to C6H16NO+) after irradiation, one convolve step. (d) K+ image after irradiation. (e) Cs+ image after irradiation. (f) Ti+ image after irradiation, one convolve step.
the presence of kaolinite, illite, smectite, and mixed illite-smectite clays. The presence of this variety of clays is important because, while they are all sheet aluminosilicates, differences in their structure result in large differences in their adsorptive behavior. Kaolinite is a 1:1 silicon-aluminum sheet clay, having a low cation exchange capacity. Illite is a 2:1 aluminosilicate, as is smectite; however, the latter displays a pronounced tendency to swell, which means that the aluminosilicate layers separate, allowing solution(50) Mincher, B. J.; Fox, R. V.; Cooper, D. C.; Groenewold, G. S. Radiochim. Acta 2003, 91, 397-401.
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phase cations to occupy interlayer sites. This property usually manifests itself in terms of a higher cation exchange capacity for smectite clays. The SIMS images were acquired over a 100 µm × 100 µm area. They were, with the exception of K+, highly homogeneous. The total ion image (Figure 1a) is representative of the homogeneity, and other individual ion images were similar in appearance. The single exception was K+, which did display modest heterogeneity; an example of this can be seen in the lower left-hand portion of the K+ image (Figure 1d).
Figure 2. Region of interest spectra from roi 1 (see Figure 1e). (a) Prior to irradiation. (b) After irradiation.
Figure 3. Region of interest spectra from roi 2 (see Figure 1b,d). (a) Prior to irradiation. (b) After irradiation.
An examination of the mass spectra prior to irradiation showed abundant Al and Si, consistent with the aluminosilicate nature of the material. Lower abundance cations corresponding to Na+, Mg+, K+, Ca+, and Fe+ were observed, which were either adsorbed onto the surface or incorporated into the aluminosilicate matrix. Figures 2a, 3a, and 4a are typical; these were collected from different roi within the overall secondary ion image. These roi were selected because, after laser irradiation, substantially different spectra were observed from these regions. A close examination of the spectra also showed the presence of organic adsorbates revealed by the presence of background organic ions typically observed in static SIMS; these ions were reported in the first static SIMS analyses published by Benninghoven and co-workers.51,52 Relatively abundant examples are observed at m/z 41 and 43; in other instances, notably m/z 27 and 39, the organic ions are isobaric with the mineral-derived ions of interest. However, the mass resolution of the TOF analyzer (51) Benninghoven, A.; Muller, A. Surf. Sci. 1973, 39, 416-426. (52) Benninghoven, A.; Gudenauer, F. G.; Werner, H. W. Secondary Ion Mass Spectrometry; John Wiley & Sons: New York, 1987.
Figure 4. Region of interest (roi) spectra from roi 3 (see Figure 1f). (a) Prior to irradiation. (b) After irradiation.
(m/∆m 3000) was sufficient to differentiate the organic-derived from the mineral-derived ions. Some intact organics could also be observed at higher mass; the most salient example is m/z 118, (Figure 4a), which had an elemental composition of C6H16NO. It has been speculated that this ion is derived from an adsorbed hydroxyaliphatic amine.53,54 Similarly, an ion at m/z 100 has been shown to be derived from adsorbed cyclohexylamine.55 These organic ions, while of low abundance, have diagnostic value because they are removed from the surface by the laser pulses. In instances where the region of laser impact was not visible on the monitor, the organic image could be used as a “negative” for identification of the irradiated area. Soil samples that had been exposed to 0.10 mM CsNO3 were then examined. Because the distribution coefficients for Cs in contact with INEEL soils are strongly skewed toward adsorption,28 virtually all of the Cs+ was adsorbed to the soil under the conditions used in this experiment; this would yield a bulk concentration on the order of 130 ppm (mass/mass). Surprisingly, only very low abundances Cs+ were recorded in the SIMS spectra from these soils. Some modest evidence for cation exchange was observed, but by and large, the mass spectra were identical to those recorded for soil aggregates that had not been exposed to Cs+. The secondary ion images for the Cs+-exposed soils were also highly homogeneous; the total ion image shown in Figure 1a is representative. A 337-nm UV laser was targeted on the soil samples such that the area irradiated overlapped that rastered by the Ga+ primary ion beam, as described in the Experimental Section. Following irradiation, the area was reanalyzed using the Ga+ ion gun, whereupon more intense secondary ion emission was observed from the irradiated area. This is recorded as a brown-red region in Figure 1b, which is a total ion image of the same area that was imaged in Figure 1a. The laser-irradiated area displayed inhomogeneous secondary ion intensity, which could be due to variable laser density; however, analysis of spatially resolved mass spectra (53) Silva, P. J.; Prather, K. A. Anal. Chem. 2000, 72, 3553-3562. (54) Murphy, D. M.; Thompson, D. S. J. Geophys. Res. 1997, 102, 6341-6352. (55) Groenewold, G. S.; Ingram, J. C.; Gianotto, A. K.; Appelhans, A. D.; Delmore, J. E. J. Am. Soc. Mass Spectrom. 1996, 7, 168-172.
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suggested that spatial variations in the composition of the soil were responsible. The extent of the area irradiated was further substantiated by examining the image of the m/z 118 ion, which arises from an hydroxy C6 amine (Figure 1c); in this image, the irradiated area appeared as an intensity “negative”, indicating that the organic adsorbate had been removed, and outlined the area of laser irradiation. Substantial heterogeneity was observed when the individual ion images were examined. The K+ image (Figure 1d) showed a number of irregularly spaced “bright” regions. Bright regions are also seen in the Cs+ image (Figure 1e), and in the Ti+ image (Figure 1f). Surprisingly, the locations of the bright regions for these three ions did not show a one-to-one correlation with one another: for example, while enhanced Cs+ does coincide with enhanced K+, the converse is not always true, and the Ti+ bright spots do not correlate with the locations of the alkali metals. Secondary ion mass spectra were generated (offline, retrospectively) from the bright roi observed in the single ion images. Spectra from a given roi acquired after irradiation were compared with those from the same roi prior to irradiation. Primary ion doses were kept the same for both analyses (before and after irradiation), so that ion abundances could be directly compared without resorting to normalization. Normalization to a matrix ion or to the sum of the ions was not valid in these analyses because the laser increased the abundance of practically all inorganic ions and decreased the abundance of the organic ions. There was no evidence for the laser removing the top layer of the mineral surface. If this is occurring, it is less than the resolution of our atomic force microscope (i.e., few nanometers); in experiments performed on flat surfaces, depressions caused by the laser could not be observed. The SIMS spectrum from roi 1 (Figure 2b, from a bright Cs region in Figure 1e) showed a very abundant Cs+, which was remarkable since no Cs+ was detected from this region prior to irradiation (Figure 2a). The abundance of K+ was also dramatically enhanced, by >10 times. The abundance of the majority of the other metal ions was also augmented, by anywhere from 1.5 to 3 times, and the abundance of Si+ was notably increased. In contrast, the abundance of the organic ions was decreased, which suggested that laser irradiation removed the organics, which promoted increased metal and Si+ abundance. Similar behavior was observed in spectra from roi corresponding to the other bright Cs+ regions (Figure 1e); the magnitude of Cs enhancement was >10 times in all cases. Most of the irradiated area, however, did not produce spectra enhanced in Cs+, but rather enhanced in K+. This behavior is exemplified by roi 2 (Figure 1b,d), which is particularly bright, but is typical of many of the regions within the irradiated area (for example, in Figure 1d, the yellow area just right of roi 2). The mass spectrum from roi 2 after irradiation (Figure 3b) showed large enhancement of K+ and Cs+, and Fe+ and Mg+ were also disproportionately enhanced. As in the case of roi 1, more modest enhancement of Al+ and Si+ was also observed. High Ti abundance was observed in roi 3 after irradiation (Figure 1f). This region did not correlate spatially with any of the other bright regions in the single ion images. An examination of the mass spectrum (Figure 4b) showed a surprising amount of 2898
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Ti+ with markedly enhanced K+, suggestive of kaolinite or illite (see below). Soils were also exposed to 1.0, 10, and 100 mM Cs+ solutions, allowed to equilibrate, and then analyzed. The Cs+ ions were observable in all spectra and abundance increased with increasing exposure concentration; however. above 10 mmol, the increase was marginal, which indicated that the surface of the soil particles was saturated with Cs+. Images of these samples acquired before irradiation were remarkably homogeneous in all cases. After irradiation, the soils exposed to 1 mM Cs+ did display notable heterogeneity, principally in the Cs+ ion image. Spectra from bright Cs+ regions contained Cs+ abundances that were enhanced 10-50 times compared with spectra from the same regions prior to irradiation. These values were generally lower than those recorded for the soils exposed to 0.10 mM Cs+, which was in large part due to the fact that Cs+ was either very low abundance or not observed at all in these spectra prior to irradiation. For the soils exposed to 10 or 100 mM, only modest Cs enhancement factors of 2-3 times were noted upon laser irradiation; this extent of laser enhancement of the secondary ion signals was on the order of that observed for Si+, Al+, and the other metal cations. The sum of these observations on the inhomogeneous soil system suggested several explanations. For soil exposed to low Cs+ concentrations, laser irradiation clearly causes redistribution of some elements, Cs+ in particular, such that detection using SIMS becomes facile. This phenomenon is clearly spatially heterogeneous, which is likely due to variation in the chemistry of soil aggregates, and is consistent with known composition of the soil. The analyses show that heterogeneity is observable on a micrometer scale. An attractive explanation is that the expandable smectite clays are responsible for selectively adsorbing low concentrations of Cs+ and that adsorption occurs at interlayer sites that are not accessible to the surface interrogation using the Ga+ primary ion beam. Other clay materials illite, kaolinite, and quartz are not competitive with the interlayer sites of the smectites for adsorption of Cs+. The mechanism by which laser irradiation redistributes Cs+ from the interlayer sites to the surface is unknown; however, short-duration intense heating by the laser may well serve to mobilize more volatile cations from the interlayer sites. Some of these will be deposited on the surface where detection by SIMS is readily accomplished. A similar explanation may hold for K+ produced from illite clays, although the magnitude of the effect may be smaller since the illites would tend to bind the K+ more strongly than would the smectites. An alternative to redistribution of Cs+ ions by laser irradiation is that the laser pulses are merely removing the topmost layer of the aluminosilicate particles, exposing underlying countercations in the clay interior. This would require removal of both adsorbed organics and at least one 2:1 Si/Al/Si oxide layer. While we cannot rule this possibility out from the present data, we believe that this is unlikely. The main reason is the observation that the Cs+ ion signal, from clays that had been exposed to high Cs+ ion concentrations, did not decrease as a result of laser illumination. Had the interior of the clay materials been exposed by laser ablation, the Cs+ signal should have decreased. The decreasing laser enhancement with increasing Cs+ concentration is consistent with the explanation of Cs+ ion redistribution by the laser pulses and with expected solution adsorption
Figure 5. Spectra of irradiated roi of montmorillonite exposed to 1 mM Cs. (a) Prior to irradiation. Note that the abundance scale (y axis) is ∼4 times less than after irradiation. (b) After irradiation.
behavior. As all of the interlayer sites are occupied, more surfaceaccessible cation exchange sites begin to be occupied with Cs+. At these locations, Cs+ is detected directly using SIMS without laser enhancement. Irradiation certainly redistributes the cations, but the surface analysis does not appear substantially different afterward. The implication of the smectite clays in the laser enhancement of Cs+ was evaluated by analyzing clay separates that were exposed to Cs+: standard kaolinite, illite, mixed illite-smectite, and montmorillonite were exposed to 1 mM Cs and then analyzed using SIMS. Montmorillonite is a type of smectite, an expandable 2:1 sheet aluminosilicate with a large cation exchange capacity and substantial interlayer adsorptive sites. Analysis of Cs+-exposed montmorillonite prior to irradiation resulted in a mass spectrum in which Cs+ was barely noticeable above background (Figure 5a). As in the case of the soils, the spectrum was dominated by Al+ and Si+, with abundant Na+, Mg+, and Ca+. The nonobservation of abundant Cs+ was surprising because, at this exposure concentration, the Cs+ concentration17 in the soil should be on the order of 1000 ppm, well above the static SIMS detection limit. Upon laser irradiation, Cs+ became clearly evident above background (Figure 5b). What was surprising was that the ion abundances of every other metal and Si+ were also markedly enhanced. As noted previously, this latter effect is partly due to removal of the organics; however, the magnitude of the effect suggests other factors are in play. This is substantiated by comparing the laser enhancement factors of cesiated versus noncesiated soils. For montmorillonite, enhancements of inorganic ions (other than Cs+) were typically 3-6 times for the cesiated sample (Figure 6b), but only 1-2 times for unmodified samples (Figure 6a). We suspect that redistribution of the Cs+ to the surface lowers the work function; however, this should decrease production of secondary ions.56 More research will be required to quantify the magnitude and understand the origin of the effect. The enhancement of Cs+ in the laser-irradiated montmorillonites ranged from 10 to >100 times, depending upon the sample (56) Wirtz, T.; Duez, B.; Migeon, H.-N.; Scherrer, H. Int. J. Mass Spectrom. 2001, 209, 57-67.
Figure 6. Comparison of average enhancement factors for clay separates. Enhancement factors were computed as the ratio of ion abundances from SIMS analyses performed before and after laser irradiation. Note the breaks in the y axes, which were utilized to accommodate large factors for Ti+ and Cs+. (a) Unmodified clays. (b) Clay separates exposed to 1 mM Cs+.
and the area examined; the extent of enhancement was similar to that observed for the high-Cs+ spots in the Cs+-exposed soils and hence supports the idea that the bright Cs+ spots observed in the laser-irradiated soils are regions where smectite clays are aggregated. The group I and II cations in the mass spectra of illite were substantially different compared with those observed in montmorillonite. The dominant counterion in the SIMS spectrum is K+, which is consistent with the composition of the clay; the fact that this is enhanced by laser irradiation suggests that a portion occupies interlayer sites. Interlayer K+ is a well-known attribute of illite clays. Notable Mg and Fe may occupy either lattice sites, where they may substitute for Al3+ and Si3+, respectively, or may be present as adsorbed or precipitated species. The Ca2+ is not known to substitute into the aluminosilicate lattice of illite and hence must be surface adsorbed. The Cs+ ion was clearly observed prior to irradiation of illite and is only modestly enhanced by the laser (Figure 7). This behavior contrasts significantly with that of montmorillonite, where at the same exposure concentration Cs+ was not observed, and dramatic enhancement was observed upon irradiation. Interlayer binding of Cs+ is unlikely with illite, which is consistent with the idea that strong K+ inner-sphere complexes bind the 2:1 aluminosilicate layers together6 such that larger ionic radius Cs+ cannot penetrate between the layers. The dramatic increase in Ti+ abundance was surprising since only very low abundance Ti was observed in the spectrum before irradiation and since bulk analysis of the clay indicated that it only contained 0.55% TiO2 (Table 2). The fact that it was not detected in abundance prior to irradiation, but was enhanced by 30-100 times after irradiation, suggested that Ti did not originally occupy surface sites, but was redistributed to the surface by laser irradiation. The exact nature of Ti in this clay is not known; however, it is not known to substitute into the illite lattice; therefore, it may be present as microscopic titanium oxide phases, which are somehow made detectable by the laser irradiation. Analytical Chemistry, Vol. 76, No. 10, May 15, 2004
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Figure 7. Spectra of irradiated roi of illite exposed to 1 mM Cs. (a) Prior to irradiation. (b) After irradiation.
Figure 8. Spectra of irradiated roi of mixed illite-smectite exposed to 1 mM Cs. (a) Prior to irradiation. (b) After irradiation.
Migration of Ti upon laser irradiation seems less likely because of the highly refractory nature of titanium oxide minerals. On the other hand, titanium oxides are well-known photocatalysts, which if present near the surface may be capable of more efficient absorption of photon energy. This might enable redistribution of Ti cations from the otherwise refractory material. Cs+ was observed prior to irradiation in the spectra of mixed illite-smectite exposed to 1 mM Cs+, although abundance was low (Figure 8). Irradiation resulted in a modest enhancement (4-5 times); while these values were not near those measured for montmorillonite, they were modestly greater than laser enhancement noted for either Cs+-exposed illite or kaolinite. This is consistent with the idea that a fraction of this material is an expandable smectite. The significant augmentation in K+ abundance is consistent with the fact that this is primarily an illite clay. No Ti+ was observed in this sample either before or after laser irradiation, and none was present in the bulk analysis. Kaolinite was the final clay separate to be examined using laserassisted SIMS. Other than Al+ and Si+, very few cations were present in abundance, which is consistent with few ion exchange 2900 Analytical Chemistry, Vol. 76, No. 10, May 15, 2004
Figure 9. Spectra of irradiated roi of kaolinite exposed to 1 mM Cs. (a) Prior to irradiation. (b) After irradiation.
sites and limited lattice substitution anticipated for kaolinite. Exposure to 1 mM Cs+ resulted in an abundant Cs+ secondary ion, which indicated replacement of most of the exchangeable cations (Figure 9a). However, Cs+ abundance was not influenced at all by laser irradiation, pointing to the fact that there are no interior adsorptive sites that the Cs+ can bind to. The surprising observation in the analysis of the kaolinite was the remarkable laser enhancement of Ti+, which could only be observed at low abundance in the spectra acquired prior to irradiation. The Titanium oxide accounts for 1.39% of the kaolinite bulk separate; however, the spectrum after irradiation contains an abundant Ti+ out of proportion to its abundance in the bulk sample. Unlike illite, Ti can substitute for lattice sites in kaolinite; however, whether Ti exists as a submicrometer TiO2 phase or as an isomorphic substitution is unknown. The observation of augmented Ti in the soil aggregate analyses (Figure 1f) indicates that this minor phase is either kaolinite or illite and is not involved in significant Cs+ adsorption since abundant Cs+ was not observed at this location. CONCLUSIONS The analysis of complex soil mixtures can be highly challenging because of the varied mineral types that can be encountered. A surface analysis method such as static SIMS has value for quickly and easily identifying the elemental constituents present on the top monolayer of the sample surface, which is important because these are the ions most likely to be solubilized and transported through the geosphere. However, there are also cations adsorbed to interlayer positions found within expandable clays such as smectite. Contaminant metals occupying interlayer positions can be transported after a delayed release or can be transported as clay colloids; thus, interlayer contaminants can still pose a threat despite the fact that they are more strongly bound. Interlayer contaminant metals are difficult to unequivocally speciate. The present work indicates that clay-interlayer Cs+ can be qualitatively identified by combining laser irradiation with subsequent imaging static SIMS analyses. Using SIMS, Cs+ could only be directly detected at very low abundance in soils exposed to
low concentrations. Reanalysis after laser irradiation showed the presence of Cs+-, K+-, and Ti+-enhanced regions that were inhomogeneously distributed within the laser-irradiated area. Analysis of Cs+-exposed clay separates indicated that regions that displayed large laser enhancements of Cs+ likely consisted of aggregated smectite clays. This conclusion implied that, within the soil samples, clays tend to aggregate by type into micrometersized clumps. Further implied is that, at low exposure concentrations, Cs+ will preferentially adsorb to interlayer sites of smectites and, finally, that Cs+ thus attached can be redistributed by a laser irradiation. The idea that Cs+ will preferentially occupy interlayer sites is consistent with the fact that cations having a large ionic radius, such as Cs+, can be more strongly bound in interlayer sites, where multidentate coordination of the metal or its hydrates can occur. Redistribution upon laser irradiation is less well understood; examination of enhancement factors shows that dramatic enhancements were only seen for Cs+ and Ti+; hence, it cannot yet be concluded that laser modification will be general for enhancing detection of a broad range of cations. On the other hand, other cations have not been explicitly evaluated. The mechanism by which the cations are redistributed is also an open question at this time. Enhanced ionic conductivity with increasing temperature from irradiation could be responsible; however, one might expect larger enhancement factors from cations in addition to Cs+. Yet another possibility is laser ablation of the surface, which would
merely remove surface material and would expose the underlying adsorbed Cs+. However this is inconsistent with our experience with laser ablation, which suggests that the amount of power applied was not sufficient to pit the surface. The phenomenon of Ti+ redistribution is also subject to multiple explanations. We would expect titanium oxide species to resist redistribution processes that are merely thermally driven. Instead, Ti may be selectively redistributed or decomposed because it more efficiently absorbs the 337-nm light compared with the surrounding phases. This is consistent with the wellknown properties of titanium used as a photocatalyst. ACKNOWLEDGMENT The funding support of the Inland Northwest Research Alliance Grant 1-1-05 and the United States Department of Energy, Environmental Systems Research program, under Contract DEAC-07-99ID13727 BBWI, is gratefully acknowledged. The support of the staff at the Image and Chemical Analysis Laboratory, Montana State University, is also gratefully acknowledged. The authors thank Prof. David Mogk and Dr. Jim Delmore for helpful discussions and Kristi Bailey for assistance in manuscript production. Received for review November 26, 2003. Accepted March 1, 2004. AC035400U
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