Environ. Sci. Technol. 2002, 36, 603-607
Interaction of Methyl Bromide with Soil TING TAO AND GARY E. MACIEL* Department of Chemistry, Colorado State University, Fort Collins, Colorado 80523-1872
Because methyl bromide (CH3Br) is a widely used agricultural fumigant for soil disinfection, it is important to know the chemical behavior and fate of CH3Br as a result of its use for soil treatment. A solid-state 13C NMR study of 13CH3Br-treated soil and soil-component samples shows that methylation of soil organic matter may be the major pathway for degradation of CH3Br in soils. Adsorption of CH3Br on a dried clay like Ca-montmorillonite or kaolinite does not contribute directly to the degradation of CH3Br. The results are interpreted in terms of the chemical structures of separated soil fractions and the nature of the separation procedure.
Introduction Methyl bromide (CH3Br) is the most widely used fumigant in the world for the control of insects, nematodes, fungi, and plant diseases in soils (1). However, CH3Br is recognized as a potentially significant source of atmospheric bromine radicals, which destroy stratospheric ozone (2, 3). This raises a concern with regard to a possible need to phase out CH3Br in order to protect the environment, yet many uncertainties remain. There is no single alternative to CH3Br in its wide spectrum of applications, and existing alternatives for specific applications are typically not as efficient as CH3Br or as costeffective. Although a reduction of CH3Br emissions after soilfumigation would be at least a partial or temporary solution for this aspect of ozone-layer protection, CH3Br is mainly produced from natural sources (3). Emissions from biomass burning and automobiles are two potentially significant sources of atmospheric CH3Br (4). It is very important to understand the geochemical cycle of CH3Br and similar compounds, even if CH3Br is on an irreversible course for phase-out. Soils and oceans have recently been identified as two main sinks of CH3Br (5, 6). Several studies, supported by indirect experimental evidence, have suggested that CH3Br can react with nucleophilic sites in soil organic matter (-SH, -NH2, -OH, -COO-), resulting in the methylation of soil organic matter (7-9). The fact that relevant scientific data available on this subject are limited in terms of direct experimental evidence hinders attempts to establish the importance of this sink (10). High-resolution solid-state NMR techniques (11) have begun to emerge among the host of investigative methods used in elucidating the interactions of organic pollutants with soil or its major components (12-15). The orientation of the study reported here is to understand the intrinsic chemical transformation, if any, of CH3Br in soils. The major focus of this work was to study the chemical behavior of CH3Br in whole soils and in such soil components as humic materials and clay. We believe that the results reported here * Corresponding author phone: (970)491-6480; fax: (970)491-1801; e-mail:
[email protected]. 10.1021/es010943b CCC: $22.00 Published on Web 01/03/2002
2002 American Chemical Society
can provide direct evidence on important aspects of the qualitative chemical behavior (e.g., methylation of soil organic matter) of a CH3Br-treated site. Because of practical considerations resulting from the relatively low detection sensitivity of NMR, adsorbate concentrations in this study are far greater than would be encountered in actual field applications of methyl bromide. Nevertheless, the results of this study can provide qualitative guidelines regarding the kinds of reactions and interactions that can occur in actual field applications. CH3Br that was used in this study contains 99% 13C in order to partially overcome the relatively low inherent NMR sensitivity of 13C in its natural abundance (1.1%).
Experimental Section Materials. 13C-Labeled methyl bromide (13CH3Br) was obtained from Cambridge Isotope Laboratories. The soil sample was collected from the Uncompahgre National Forest of southwestern Colorado, about 4 in. below the “leaf litter” (16). Humic acid, fulvic acid, and humin were fractionated from the corresponding Uncompahgre soil via a classic acidbase separation procedure (17, 18), as described in detail elsewhere (19), and briefly below. Analysis results of the whole soil and its separated components are summarized in Table 1. Ca-montmorillonite (STx-1) and kaolinite (KGa-1b) were purchased from the Clay Minerals Repository of the University of Missouri at Columbia. The water contents of the Camontmorillonite (STx-1) and kaolinite (KGa-1b) employed were 15% and 10%, respectively, determined by weight loss upon dehydration at 150 °C under vacuum (10-3 Torr). The Clay Minerals Repository provided the following information on the clays: The Ca-montmorillonite has a surface area of 83.8 m2/g with 200-300 mesh particle size, and the kaolinite has a surface area of 10.1 m2/g with 200-300 mesh particle size. Sample Preparation. Isolation of Soil Components (19). A water slurry of the Uncompahgre soil, originally with pH ) 5.1, was treated with NaOH to bring the pH to 11.8. After filtration (H2O-washed residue: humin fraction), the humic acid was obtained by acidification of the filtrate to pH ) 2.1, and centrifugation, with the fulvic acid obtained by rotovap concentration of the resulting supernatant solution, followed by treatment with an ion-exchange resin and rotary evaporation. Interaction of CH3Br with Soil. In a vacuum line, 3.014 g of soil in a 50 mL flask was evacuated for 8 h at 3 × 10-3 Torr and room temperature, and 2.210 g of dried soil was thereby obtained. It can be argued that such a dry soil might not represent well the situation in the field. However, a sample without vacuum-drying was prepared for comparison, and almost no difference was observed between the 13C NMR spectra of treatment products obtained from the dry and “wet” soil samples. Drying of the soil and soil-related samples was used for preventing hydrolysis of 13C-labeled methyl bromide to methanol in the 13CH3Br/substrate samples. A 0.5 g portion of 13CH3Br(g) in a 500 mL flask at room temperature was attached to another port in the same vacuum line, and the flask was immersed in a liquid-nitrogen bath to solidify 13CH3Br. After leaving both ports open to the vacuum line at 3 × 10-3 Torr for 5 min, the corresponding portion of the vacuum line was closed off and isolated. Then, after removal of the liquid nitrogen bath, the isolated portion of the vacuum system was allowed to return to roomtemperature gradually, and the final pressure in the isolated region of the vacuum line reached 373 mmHg. A liquid VOL. 36, NO. 4, 2002 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 1. Summary of Carbon and Ash Contents in Uncompahgre Soil and Soil Components soil component
wt % of soil
% C from elemental analysis
% ash
whole soil humic acid fulvic acid humin
100.00 8.92 3.66 87.42
16.07 43.91 21.84 11.86
70.3 15.8 47.4 76.8
nitrogen bath was then placed in contact with the flask containing the soil; white solid 13CH3Br appeared inside the flask. After removal of the liquid nitrogen bath, the system was allowed to reach equilibrium for 5 h at room temperature, and the 13CH3Br-treated soil was found to weigh 2.299 g. It was stored in a refrigerator for 3 days before NMR measurements. The residual 13CH3Br remaining in the vacuum line was recovered for further use. Interactions of CH3Br with Soil Component. Procedures similar to that described above were used to prepare 13CH Br-treated humic acid, fulvic acid, humin, Ca-mont3 morillonite, and kaolinite. NMR Measurements. Probably the most common solidstate NMR experiment in use is a 13C NMR measurement using cross polarization (CP) and magic-angle spinning (CPMAS). During cross polarization, magnetization is transferred from a more abundant nuclide (such as 1H in organic solids) to an insensitive nucleus, such as 13C. The net 13C magnetization experiences a CP signal enhancement that is less than or equal to the ratio of the magnetogyric ratios or roughly a factor of 4 for 1H f 13C polarization transfer. Magic-angle spinning (MAS) is maintained throughout the entire experiment to average the chemical shift anisotropy (CSA). Another very common experiment uses direct polarization (DP), based on 13C spin-lattice relaxation; this is also known as the single pulse or Bloch decay experiment. In the DP experiment, the net magnetization of nuclei to be observed is tilted from the axis of the static magnetic field by a short, powerful RF pulse. For many, if not most, chemical situations, 1H-13C heteronuclear dipolar interactions are too large to be averaged by MAS. This necessitates the addition of strong proton decoupling during the data acquisition phase of the experiment. All solid-state 13C MAS NMR spectra shown in this paper were obtained at 22.6 MHz at room temperature on a Chemagnetics M-90S spectrometer. A Chemagnetics 14 mm PENCIL rotor was used at a spinning speed of about 3.5 kHz. The following cross-polarization conditions were employed: 1H 90° pulse length, 5.5 µs; 1H decoupling, 45 kHz; CP contact time, 1 ms; delay between scans, 1 s. For the DP experiment, the following parameters were used: 13C 90° pulse length, 6.0 µs; 1H decoupling, 45 kHz; delay between scans, 5 s. 13C spectra were externally referenced to liquid tetramethylsilane (TMS) on the basis of substitution of the secondary reference, solid hexamethylbenzene (peaks at 16.9 and 132.3 ppm, relative to TMS).
Results and Discussion The solid-state 13C NMR spectrum of the 13CH3Br-treated soil sample (Figure 1A) shows some significant changes in the aliphatic carbon region, when compared with the 13C NMR spectrum of the untreated soil sample (Figure 1B). Figure 1C, having two major peaks at 10 and 55 ppm, was generated from the difference between parts A and B of Figure 1. From literature data on liquid solutions (20), we know that the 10 ppm peak is due to absorbed or physically adsorbed 13CH Br in the soil matrix; this portion of the initial 13CH Br 3 3 apparently has experienced no chemical transformation in the soil. The 10 ppm peak gradually decreases when the 604
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FIGURE 1. Solid-state 13C CP-MAS NMR spectra of (A) 13CH3Brtreated soil, (B) whole soil, and (C) the difference between A and B, 10 000 scans. 13CH
3Br-treated soil sample is placed in ambient conditions for several days and eventually disappears after a week. The peak at about 55 ppm in Figure 1 presumably results from the reaction of 13CH3Br with oxygen nucleophiles (-O-) in soil, producing methoxy (13CH3O-) groups. This peak remained in the spectrum (not shown here) when the sample was stirred in water for 24 h at room temperature. This persistence removes any suspicion that the peak at about 55 ppm was due to some small molecules, such as methanol or dimethyl ether, generated from the reaction of 13CH3Br with moisture in soil. Some broad, weak signal intensity between 50 and 20 ppm might correspond to 13CH3N< moieties, resulting from the reaction of 13CH3Br with nitrogen nucleophiles, e.g., amine or amide structures. In any case, it is clear that, in the treatment of soil with CH3Br, some of reagent experiences a chemical transformation with soil, i.e., formation of -OCH3 moieties. However, since the difference spectrum (Figure 1C) shows only the 55 and 10 ppm peaks, it appears that treatment with CH3Br brings about no more dramatic (e.g., framework altering) transformation in the chemical structure of the organic soil material. For the purpose of understanding in more detail the chemical transformation(s) of CH3Br in soil, some principal soil components were individually treated with 13CH3Br, and the resulting samples were also studied by solid-state NMR. Figure 2 shows solid-state 13C CP-MAS NMR spectra of the humic acid before and after treatment with 13CH3Br and the difference spectrum. The humic acid used here was separated from the same soil sample represented in Figure 1. One can see from Figure 2 that the difference between 13CH3Br-treated humic acid and untreated humic acid is again physical adsorption (and/or absorption) of 13CH3Br (peak at 10 ppm) and some formation of methoxy groups (peak at 55 ppm with very small intensity). Clearly, the fraction of organic matter that was perturbed by 13CH3Br was much smaller for the humic acid than for the raw soil (vide infra). Solid-state 13C CP-MAS NMR spectra of the fulvic acid before and after treatment with 13CH3Br, and their difference, are presented in Figure 3; no signal of substantial intensity appears in the difference spectrum (Figure 3C), i.e., no CH3Br-generated peaks. This indicates that neither physical adsorption/absorption nor chemical reaction occurs when fulvic acid is treated with CH3Br. Humin is a soil component that does not dissolve in strong aqueous base (pH > 11) during the separation procedure and contains a substantial inorganic component of the soil (e.g., clays or silicates). When humin was treated with 13CH Br, the 13C CP-MAS NMR spectrum shown in Figure 4A 3
FIGURE 2. Solid-state 13C CP-MAS NMR spectra of (A) 13CH3Brtreated humic acid, (B) humic acid only, and (C) the difference between A and B, 10 000 scans.
FIGURE 3. Solid-state 13C CP-MAS NMR spectra of (A) 13CH3Brtreated fulvic acid, (B) fulvic acid, and (C) the difference between A and B, 5000 scans. was obtained. By comparing this spectrum to the spectrum of untreated humin (Figure 4B), one obtains the difference spectrum (Figure 4C), which shows that the major change was due to the formation of methoxy (13CH3-O-) linkages. It is interesting that the incorporation of 13CH3 groups into the organic soil matter occurs via both physical adsorption/absorption and formation of CH3O- moieties in the humic acid and humin fractions and not in the fulvic acid fraction. It is nontrivial to offer a convincing hypothesis for the apparent absence of physically adsorbed/absorbed 13CH Br in the fulvic material, relative to the humin, and 3 especially relative to the humic acid. Perhaps this pattern of physical adsorption/absorption is related to the fact that the overall aromatic content (represented by 13C NMR intensity from roughly 160-110 ppm) is greatest for the humic acid and smallest for the fulvic acid, among these three soil components. If the physisorption of CH3Br is more favored (e.g., by dispersion forces) for aromatic structures than for aliphatic structures, then one could rationalize the apparent absence of 13CH3Br physisorption in the fulvic acid fraction. Nevertheless one must still confront the fact that the 13C
FIGURE 4. Solid-state 13C CP-MAS NMR spectra of (A) 13CH3Brtreated humin, (B) humin only, and (C) the difference between A and B, 10 000 scans. NMR intensities associated with physically adsorbed/ absorbed 13CH3Br in the three soil components treated by 13CH Br (Figures 2C, 3C, and 4C) are smaller than the 3 corresponding intensity of the 13CH3Br-treated whole soil (Figure 1C), as discussed below. The fact that a substantial quantity of 13CH3O- moieties is absent from the 13CH3Br-treated fulvic acid, in comparison to the humic acid and humin cases, may also be related to the distribution of functional groups in the three soil components. Comparison of the 13C CP-MAS spectra of the untreated humic acid (Figure 2B), humin (Figure 4B), and fulvic acid (Figure 3B) reveals that much stronger relative intensities due to -OH substituted phenolic carbons (at about 155 ppm) are present in the humic acid and humin spectra than in the fulvic acid spectrum, suggesting that the formation of 13CH3OAr (Ar ≡ aromatic) linkages may be an important 13CH Br reaction route for the former two components and 3 largely absent in the fulvic acid case. An analogous, but somewhat less convincing, argument can also be made in terms of the peak at about 60 ppm, which is likely due, at least partly, to >CHOH moieties, especially in the humic acid and humin samples. An interesting question that arises from the data presented in Figures 1-4 is the following: Why does the whole, untreated soil have a larger 13CH3Br capacity than the weighted combination of individual soil components (humin, humic acid, and fulvic acid), especially since the humin fraction contains much of the mineral matter (clays, silicates, etc.) originally present in the whole soil? Perhaps the answer lies the presence of a clay-humic complex (21-23), in which both mineral and organic components are present together, at least in part, in the form of an intimate interaction that enhances the 13CH3Br capacity of one or both of the components beyond its own individual capacity. The interaction of organic substances with clay has a multitude of consequences that are reflected in physical and chemical properties of the soil matrix. Another possible answer to the above question could be based on the possible presence of highly nucleophilic species in the original soil (e.g., -COO- and ArO- anions) that would be expected to react readily with 13CH3Br; such species would be converted to less nucleophilic species (e.g., -COOH and ArOH) by the acid treatment that precedes the isolation of humic acid and fulvic acids. Such lower-nucleophilicity species would then be less reactive toward 13CH3Br. This scenario would also account for the fact that the generation VOL. 36, NO. 4, 2002 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 5. Solid-state 13C CP-MAS NMR spectra of (A) 13CH3Brtreated Ca-montmorillonite, (B) Ca-montmorillonite, and (C) the difference between A and B, 5000 scans.
FIGURE 6. Solid-state 13C CP-MAS NMR spectra of (A) 13CH3Brtreated kaolinite, (B) kaolinite, and (C) the difference between A and B, 5000 scans.
of -O13CH3 moieties by 13CH3Br treatment is more extensive for the humin than for the fulvic acid (for which no methoxy generation is observed) or for humic acid (where a small amount of methoxy generation is observed). Montmorillonite is an important 2:1 clay mineral that consists of parallel stacked sheets, each composed of two tetrahedral layers (primarily silicon-centered) and one octahedral layer (primarily aluminum centered) (24), which are charge balanced with cations (e.g., Na+, Ca2+, Mg2+, etc.). The high surface area and cation exchange capabilities of montmorillonites allow a wide variety of chemically active forms (e.g., containing acidic cations, metal complexes, adsorption sites, etc.). Figure 5A shows the 13C CP-MAS NMR spectrum of a 13CH3Br-treated Ca-montmorillonite sample, in which a physically adsorbed/absorbed 13CH3Br resonance appears at 12 ppm. It is not surprising that the 13C CP-MAS NMR spectrum of Ca-montmorillonite itself (Figure 5B) gives no carbon signal, because montmorillonite should contain little, if any, organic matter. Although the 13CH3Br-treated Ca-montmorillonite sample does not result in chemical transformation, such as methylation, it is somewhat surprising that physical adsorption/ absorption of the low molecular weight organic compound, 13CH Br (bp 4 °C), results in a relatively strong cross3 polarization (CP) NMR signal at room temperature, when Ca-montmorillonite is treated with this compound. Highly mobile species do not generate CP signals. Thus, the mobility of 13CH3Br that is physically adsorbed/absorbed on a specific clay mineral is expected to determine the efficiency of the cross-polarization process in that system. During cross polarization, magnetization is transferred from protons to 13C with a rate governed by a time constant, T , that depends CH on a static component of a 1H-13C magnetic dipole-dipole interaction. An increase in mobility yields a larger TC-H value. If the sample is liquidlike, this kind of CP is not observed at all. The observation of a 13CH3Br CP signal indicates that the mobility of 13CH3Br in the 13CH3Br/Ca-montmorillonite sample is restricted and, therefore, that the interaction between 13CH3Br and Ca-montmorillonite must be very strong (e.g., in a “complex”, like 13CH3BrfM, where M represents some cationic site(s) in Ca-montmorillonite). In many cases, interlayer cations in montmorillonite give rise to strong absorption sites for small molecules (25-27). When the 13CH3Br-treated Ca-montmorillonite sample was opened to ambient air in a laboratory hood for 2 days, the peak at 12 ppm disappeared from the 13C CP-MAS NMR spectrum. The desorption of CH3Br upon exposure to ambient air may be explainable on the basis of the effect of moisture on adsorption (28, 29). This is the same behavior as shown by the 13CH3Br-treated whole soil sample, which may imply that
moisture in air saturates the clay surface, allowing the release of the physically adsorbed/absorbed 13CH3Br. Kaolinite is an abundant, naturally occurring clay, often found in relatively pure deposits; this clay has a relatively low porosity and cation exchange capability (13). Figure 6B,A presents the 13C CP-MAS NMR spectra of the kaolinite before and after 13CH3Br treatment. No significant 13C signal was obtained from the 13CH3Br-treated kaolinite sample; this shows that no chemical transformation has occurred in this sample. It also indicates that, if any 13CH3Br is physically adsorbed/absorbed in this sample, the 13CH3Br must be too mobile to be detected by 13C CP-MAS NMR. A DP-MAS 13C NMR spectrum (not shown here) was also obtained from the 13CH Br-treated kaolinite sample; no 13C NMR signal was 3 detected, nor were significant 13C NMR signals detected in DP-MAS experiments on any of the other 13CH3Br/adsorbent systems studied. In a DP (direct polarization) experiment, 13C spin polarization is generated directly by 13C spin-lattice relaxation, with no CP involved. Highly mobile (e.g., liquidlike) species are active in this mode. Hence, the combination of CP-MAS and DP-MAS 13C NMR experiments shows that there is very little, if any, 13CH3Br that is physically adsorbed/ absorbed in kaolinite under the current preparation conditions. Kaolinite is a 1:1 clay composed of one tetrahedral and one octahedral layer. Cohesion in kaolinite is ensured by its hydrogen-bond network rather than interlayer cations. Therefore, it is not surprising that kaolinite, with its low content of exchangeable cations, has low adsorption capacity. Solid-state NMR can provide important structural and dynamical information on the interactions between organic pollutants and soils at a molecular level of detail. 13C CP-MAS NMR spectra have demonstrated that 13CH3Brtreatment of soil results in the formation of 13CH3O moieties in the organic components in soil, which could permanently modify the properties of the soil and might have significant impact on the performance of soil in the ecosystem. Physical adsorption of 13CH3Br in the soil matrix was also detected by the NMR technique. The adsorption of 13CH3Br in soil or soil components can be modulated when the adsorption conditions are changed. NMR evidence collected from 13CH3Brtreated soil components illustrate that the well-established differences in chemical structures of the individual soil components can lead to different behaviors when they are treated with 13CH3Br.
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Acknowledgments This work was supported by NSF Grant DEB-9707296 and DOE Grant DE-FG03-95ER14558. The authors are grateful to C. Keeler for the soil and humic materials and Drs. I.-S. Chuang and H. Lock for technical assistance. Thanks are due to the research groups of Professor I. Burke at Colorado
State University and Professor D. Johnson at Desert Research Institute, Nevada, for their cooperation in this project.
Literature Cited (1) Morse, P. Chem. Eng. News 1998, 76, 46. (2) Sauvegrain, R. Acta Hortic. 1995, 382, 51. (3) Yagi, K.; Williams, J.; Wang, N.-Y.; Cicerone, R. J. Science 1995, 267, 1979. (4) Mano¨, S.; Andreae, M. O. Science 1994, 263, 1255. (5) Serca, D.; Guenther, A.; Klinger, L.; Helmig, D.; Hereid, D.; Zimmerman, P. Atmos. Environ. 1998, 32, 1581. (6) Lobert, J. M. Science 1995, 267, 1002. (7) Papiernik, S. K.; Gan, J.; Yates, S. R. J. Environ. Qual. 2000, 29, 1322. (8) Gan, J.; Yates, S. R.; Anderson, M. A.; Spencer, W. F.; Ernst, F. F.; Yates, M. V. Chemosphere 1994, 29, 2685. (9) Arvieu, J. C. Acta Hortic. 1983, 152, 267. (10) Shorter, J. H.; Kolb, C. E.; Harris, R. C. Nature 1995, 377, 717. (11) Maciel, G. E. Science 1984, 226, 282. (12) Tao, T.; Maciel, G. E. Environ. Sci. Technol. 1998, 32, 350. (13) Urkiewicz, A.; Maciel, G. E. Sci. Total Environ. 1995, 164, 195. (14) Hinedi, Z. R.; Johnson, C. T.; Erickson, C. Clays Clay Miner. 1993, 41, 87. (15) Tao, T.; Yang, J. J.; Maciel, G. E. Environ. Sci. Technol. 1999, 33, 74. (16) Keeler, C.; Maciel, G. E. J. Mol. Struct. 2000, 550-551, 297. (17) Hayes, M. H. B. Humic Substances in Soil, Sediment and Water; Aiken, G. R., Ed.; Wiley: New York, 1985; pp 329-362. (18) Schnitzer, M. Organic Matter Characterization. In Methods of Soil Analysis, Part 2. Chemical and Microbiological Properties,
(19) (20) (21) (22) (23)
(24) (25) (26) (27) (28) (29)
2nd ed.; American Society of Agronomy and Soil Science Society of America: Madison, WI, 1982; pp 581-594. Keeler, C. Ph.D. Dissertation, Colorado State University, 2001. Silverstein, R. M.; Bassler, G. C.; Morrill, T. C. Spectrometric Identification of Organic Compounds, 5th ed.; John Wiley & Sons: New York, 1991; p 243. Stevenson, F. J. Humus Chemistry, 2nd ed.; John Wiley & Sons: New York, 1994; pp 429-452. Huang, P. M.; Wang, T. S. C.; Wang, M. K.; Wu, M. H.; Hsu, N. W. Soil Sci. 1977, 123, 213. Stevenson, F. J.; Ardakani, M. S. Organic Matter Reactions involving Micronutrients in Soils. In Micronutrients in Agriculture; Mortvedt, J. J., Giordano, P. M., Lindsay, W. L., Eds.; American Society of Agronomy: Madison, 1972; pp 79-144. Pinnavaia, T. J. Science 1983, 220, 365. Rytwo, G.; Nir, S.; Crespin, M.; Margulies, L. J. Colloid Interface Sci. 2000, 222, 12. Kowalska, M.; Gueler, H.; Cocke, D. L. Sci. Total Environ. 1994, 141, 223. Tennakoon, D. T. B.; Thomas, J. M.; Tricker, M. J.; Williams, J. O. J. Chem. Soc., Dalton Trans. 1974, 20, 2207. Matsumoto, M.; Suzuki, M.; Takahashi, H.; Saito, Y. Bull. Chem. Soc. Jpn. 1985, 58, 1. Low, P. F. Prog. Colloid Polym. Sci. 1994, 95, 98.
Received for review May 4, 2001. Revised manuscript received October 22, 2001. Accepted October 23, 2001. ES010943B
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