Environ. Sci. Technol. 2000, 34, 4822-4827
Surfactant-Templated Mesoporous Silicate Materials as Sorbents for Organic Pollutants in Water H O N G T I N G Z H A O , † K A T H R Y N L . N A G Y , * ,† JACOB S. WAPLES,† AND GEORGE F. VANCE‡ Department of Geological Sciences, University of Colorado, Boulder, Colorado 80309-0399, and Soil and Environmental Chemistry Group, Department of Renewable Resources, University of Wyoming, Laramie, Wyoming 82071-3354
Hexagonal (MCM-41) mesoporous materials were synthesized at 23 °C using hexadecyltrimethylammonium bromide (HDTMA) and tetramethyl-orthosilicate (TMOS) or Nasilicate. Products with (as-synthesized and dried at 23 or 70 °C) HDTMA removed significant amounts of trichlorethylene and tetrachloroethylene from water, similar to the behavior of organic-cation-exchanged smectites. Products without HDTMA (calcined) were weaker sorbents. Structural Al increased the sorption capacity of as-synthesized products but decreased that of calcined products. Structural Al and synthesis using TMOS both increased the stability of as-synthesized materials in 0.005 M CaCl2. All assynthesized materials have apparent Si solubilities between those of quartz and amorphous silica. Si dissolution rates for as-synthesized (using Na-silicate) products at pH ∼7-8 are 2-3 orders of magnitude higher than those of quartz and 1 order of magnitude higher than that of glass. Rates show little ionic strength dependence in up to 0.1 M CaCl2 at Si concentrations up to three times quartz solubility. HDTMA leaching increased slightly with decreasing ionic strength; however only 1.5% is removed after exchange with 1050 pore volumes of H2O. The ease of synthesis, environmental stability, and significant sorption capacity indicate that as-synthesized MCM-41 materials could be used as sorbents for organic pollutants in water and as components of contaminant barriers.
Introduction Organic contaminants that seriously threaten the health of humans and the environment are nonionic hydrophobic organic compounds (HOCs), such as trichloroethylene (TCE), characteristic of dense nonaqueous phase liquid (DNAPL) contamination across the country (1). HOCs with relatively high water solubility (e.g., TCE or benzene) are mobile in low organic matter soils or sediments and are frequently found in groundwater. Great effort has been expended to develop new sorbents and stabilizers (e.g., organoclay, activated carbon, surfactant-modified zeolites) for HOCs and to immobilize HOCs in situ (2, 3). Here, we report on the use of as-synthesized mesoporous silicates as HOC sorbents with the intent of exploring their applications in water treatment, * Corresponding author phone: (303)492-6187; fax: (303)492-2606; e-mail:
[email protected]. † University of Colorado. ‡ University of Wyoming. 4822
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as barrier liners, and as in situ precipitates in barrier construction. Hexagonally ordered mesoporous silica materials were created and developed as molecular sieves exhibiting a wide range (∼13 to 100 Å) of pore size (4, 5). These materials are synthesized via a liquid-crystal templating mechanism with subsequent removal of the organic template by calcination or acid dissolution. The structures are defined by the ordered arrays of templating surfactant molecules, and the pore size is adjusted by changing the hydrocarbon chain length of the surfactants (6, 7). Varying the surfactant/Si ratio and synthesis conditions produces solids with uniform hexagonally ordered cylindrical pores (MCM-41), layered materials (MCM-50), and materials with a three-dimensional pore system and proposed cubic symmetry (MCM-48) as well as other structures (8, 10). Sorption behavior of mesoporous silicates has been investigated with respect to various gas molecules and aqueous ions. Most studies have used materials from which the organic template had been removed and which also may have been functionalized with sorptive molecules or ligands specific to certain ions, such as mercury (11) or arsenate and chromate (12). Enzymes were sorbed from water by a calcined MCM-41 material, resulting in enhanced stabilities with no loss in activity (13). Calcined and phenyl-modified MCM-41 materials have shown great affinity for benzene (14), as has calcined MCM-41 for cyclohexane (15) from the vapor phase. Both calcined pure-Si and aluminosilicate MCM-41 have a high sorption capacity for cyclohexane (>0.4 g g-1 dry solid) and a low sorption capacity for water (90%), and the data were not adjusted for these recoveries. Solubility/Stability Experiments. Two types of stability studies were carried out in duplicate. In the first, 500 ( 0.1 mg of sample were placed into 125 mL transparent plastic bottles containing 0.005 M CaCl2, chosen to simulate the ionic strength of typical groundwater and soil solutions. Samples were shaken periodically every 8 h for 124 days. Four milliliter aliquots were removed at intervals, filtered with a 0.2 µm PTFE filter, and analyzed for pH, dissolved Si, and total organic carbon (TOC). In the second, 100 mg of sample was placed in 25 mL of pure water, 0.005 M CaCl2,
FIGURE 1. Representative XRD patterns: (a) as-synthesized MCM41(∞); (b) calcined MCM-41(∞); (c) as-synthesized MCM-41(∞)-IN; (d) as-synthesized MCM-41(40); (e) calcined MCM-41(40); and (f) as-synthesized MCM-41(40)-IN. or 0.1 M CaCl2. Samples were rotated end over end for 2 days and centrifuged at 1200 rpm, and 15 mL of supernatant was removed for pH, TOC, and dissolved Si analyses. Fifteen milliliters of fresh solution was added, and the procedure was repeated six times. TOC was assayed with a Model 700 TOC analyzer (O.L. Corp., College Station, TX), and Si was analyzed colorimetrically using the molybdate blue method (23). Precision of the Si analyses is 3%, while precision of the TOC analyses is 24% in the first set of experiments and 3% in the second set. The second set of experiments enabled estimation of the dissolution rate of the Si-framework under flow conditions and at Si concentrations typical of groundwater as well as the leaching rate of HDTMA.
Results and Discussion Product Characterization. XRD patterns of MCM-41 materials are similar to those reported previously (21, 22) (Figure 1). Both as-synthesized and calcined MCM-41s exhibit at least three diffraction peaks (100, 110, 200), indicating uniform pore channels in a regular hexagonal array (Table 1). Calcination resulted in greater lattice contraction of the pores as shown by the shift in the (100) peak, due to the condensation of framework silanol groups and the formation of siloxane bonds at high temperature (5). Aluminum reduced the pore size dimension Ro and crystallinity in calcined materials (24) (Table 1). Addition of Al also decreased Ro and the amount of surfactant (O.C.%) retained within the pores in the as-synthesized materials prepared with TMOS. However, Al had little effect on Ro of inorganically synthesized air-dried materials, although it decreased the amount of surfactant retained. No significant differences were found in XRD patterns of inorganically synthesized samples dried in air and at 23 and 70 °C (data not shown). A stronger condensation reaction likely arises from the acid addition in the inorganic synthesis that makes effects of Al content and heating immeasurable. VOL. 34, NO. 22, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 1. Selected Characteristics of As-Synthesized and Calcined MCM-41 Materials sample
Si/Ala
O.C.%b
d100 (Å)
aoc (Å)
MCM-41(∞) MCM-41(40) MCM-41(30) MCM-41(20) MCM-41(∞)-IN MCM-41(40)-IN
∞ 40 30 20 ∞ 40
32.2 29.2 30.4 30.5 30.1 26.4
40.5 (35.3d) 35.9 (34.1) 35.8 (35.1) 36.4 (32.4) 40.3 41.0
46.8 (40.8d) 41.5 (39.4) 41.3 (40.5) 42.0 (37.4) 46.5 47.3
a Al/Si molar ratio used in the reactants. b Organic carbon content %. c a0 ) 2d100/x3, which is the spacing between the centers of adjacent pore channels. For an ideal regular system the spacing is the sum of the internal pore diameter and the pore wall thickness. d Numbers in parentheses are data for calcined products.
FIGURE 2. SEM photomicrographs of (a) as-synthesized MCM-41(∞) and (b) MCM-41(40). The characteristic XRD peaks of MCM-41 broaden and weaken with increasing Al content (Figure 1), indicating decreased grain size and/or crystallinity, a term describing channel regularity, the only element of order in MCM-41 materials observed using Si29- and Al27-NMR (7). Distortion of long-range ordering and presence of defective hexagonal arrays (24) with increasing Al could facilitate diffusion of sorbate molecules into the solid and enhance sorption capacity. MCM-41(∞) consisted of aggregated 0.3-1 µm spherical or ellipsoidal particles (Figure 2); however, in materials containing Al, the number of smaller particles that occurred in aggregates increased, consistent with the XRD results. Pore-wall thickness is generally reported to be 1015 Å for MCM-41, depending on synthesis conditions, and incorporation of Al generally increases wall thickness (8, 15). Using reported pore wall thicknesses yields pore diameters of 26-37 Å (Table 1). Sorption by As-Synthesized MCM-41. Sorption isotherms for TCE and PCE from water by as-synthesized MCM-41s are of two types (Figures 3,4). Sorption of TCE yielded curvilinear isotherms (Figure 3a), whereas sorption of PCE resulted in linear isotherms (Figure 4a). Curvilinear isotherms have been explained by simultaneous partitioning and cosorption of organic molecules by HDTMA-exchanged clays (25, 26). It was thought that sorption of TCE increased the organic matter content of the sorbent, which promoted additional TCE sorption (25, 26). Linear isotherms suggest only a partitioning sorption mechanism (27). The linearity of PCE sorption also may be a function of the lower concentration range investigated. In comparison, Langmuir isotherms were observed for solution concentrations of up to 84 mg C L-1 for sorption of 3-chlorophenol on HDTMA-MCM-41 (17). Incorporation of Al clearly increased the sorption of both TCE and PCE; however, no significant difference was found among the aluminosilicate MCM-41s synthesized with TMOS. The enhanced sorption demonstrated by Al-containing 4824
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FIGURE 3. Sorption isotherms for trichloroethylene at 25 ( 1 °C by (a) as-synthesized and (b) calcined MCM-41 materials.
FIGURE 4. Sorption isotherms for tetrachloroethylene at 25 ( 1 °C by (a) as-synthesized and (b) calcined MCM-41 materials. products was attributed to two factors: (1) increased stability (described below) and (2) a larger number of smaller grains (Figure 2) which would facilitate access of HOCs to the HDTMA-filled channels. TCE and PCE isotherms were linearly regressed (R > 0.984) in order to calculate sorption coefficients
TABLE 2. Selected Parameters for the Sorption of Organic Compounds by MCM-41s log Komb organic compd trichloroethylene tetrachloroethylene
aqueous
solubilitya 1155 150
(mg
L-1)
log
Kowa
2.61 3.4
MCM-41(∞)
MCM-41c
MCM-41(∞)-IN
MCM-41(40)-IN
2.18 2.72
2.34 ( 0.01 2.83 ( 0.01
2.27 2.87
2.40 2.99
a The aqueous solubilities and log K ow values were adapted from refs 19 and 20 and references therein. Log Kow ) the log of the partition coefficient value between octanol and water. b Kom ) Kd [100/(O.C.% × f)], where f equals the molecular weight of the organic cation divided by the weight of C in the organic cation. c Average of MCM-41(40), MCM-41(30), and MCM-41(20).
(Kd) that correspond to the ratio of the amount of the sorbed chemical by MCM-41 (mg kg-1) to its equilibrium solution concentration in water (mg L-1). Kd values were normalized on an organic-matter basis to obtain Kom values (Table 2). As-synthesized MCM-41s have significantly higher sorption affinity for PCE than TCE, reflecting the higher log Kow value of the former. It is noteworthy that the inorganically synthesized MCM-41 has higher sorption affinity for both organic substances than organically synthesized MCM-41. For comparison, HDTMA-exchanged Wyoming smectite (SWy-1) had a log Kom value of 2.30 for TCE (2), and HDTMAmodified zeolite exhibited a log Kom value of 2.82 for PCE (28), demonstrating that the sorption properties of assynthesized aluminosilicate MCM-41s are comparable to those of HDTMA-modified clays. Sorption by Calcined MCM-41. Calcination removed all organic carbon (O.C.% < 0.1) and caused significant reduction in TCE and PCE sorption from water compared to the assynthesized materials (Figures 3b and 4b). In addition, the sorption affinities of calcined materials in water are much less than those reported in the vapor phase (15), which has been explained by siloxane surface characteristics of the pore walls, particularly hydrophobicity. Generally, at the same concentration range, calcined MCM-41 has a slightly higher sorption affinity for PCE than TCE, indicating the role of hydrophobic attraction. In contrast to the as-synthesized materials, sorption decreased with increasing Al content for both TCE and PCE. This suggests that pore wall hydrophobicity was reduced by Al incorporation. The hydrophobic nature of siloxane surfaces plays a welldocumented role in the uptake of HOCs by clay minerals (29, 30). However, pore walls of calcined MCM-41 also contain hydrophilic patches (31, 32). Other factors contributing to the uptake of vapor phase organics by calcined MCM-41 include high surface areas (up to 1200 m2 g-1) and large pore volume (4). As determined by XRD, calcined samples were stable after immersion in distilled water for 2 weeks, indicating no collapse of the pore structure during the sorption experiments. Based on the critical molecular dimensions (6.7 Å × 6.5 Å × 3.7 Å) (19, 20) of TCE and PCE, there would be no steric inhibition to entrance of these compounds into pores. Therefore, the low sorption affinity in water must be due simply to the presence of water molecules hindering access to the pores. Stability of MCM-41s in Aqueous Solutions. Dissolution of air-dried as-synthesized MCM-41 materials in 0.005 M CaCl2 solutions over 124 days indicates that constant values of dissolved Si, TOC, and pH were obtained after approximately 20 days (Figure 5). After 124 days, XRD analysis showed no change in the solid structures. TOC concentrations were on the order of 10-12 ppm, well below HDTMA’s critical micelle concentration of 205 ppm TOC (33), and accounting for dissolution of only ∼1% of the sample. Higher initial TOC and dissolved Si values suggest that finer-grained and/or less well-crystallized material dissolved rapidly. The decrease in TOC and Si with time indicates readsorption or reprecipitation. The decrease in Si with time also indicates that the measured concentrations after 124 days may reflect
FIGURE 5. Stability of as-synthesized MCM-41 materials in 0.005 M CaCl2 batch experiments. supersaturation rather than the attainment of equilibrium with the solids. Apparent solubilities of the MCM-41 materials were 3448 mg L-1 Si (73-103 ppm SiO2) at pH values of 8.04 ( 0.02 (synthesis with Na-silicate) to 8.83 ( 0.04 (synthesis with TMOS). The solubilities of quartz and amorphous silica at these conditions are 5 and 54 mg L-1 Si (11 and 116 ppm SiO2), respectively (34). Although well above quartz solubility, a concentration of 34 mg L-1 Si falls within the range of those reported for soil and groundwaters (35), particularly soil moisture in the vadose zone and waters in soils developed on rock containing unstable mafic minerals or feldspars. It is worth noting that the amorphous nature of the Si framework (5, 16) and higher enthalpies of formation of MCM-41 materials relative to quartz and pure Si glass (36) do not result in solubilities above that of amorphous silica. Al incorporation improves stability; however, organically synthesized materials are more stable than inorganically synthesized materials. This is presumably because in the organic synthesis (1) a better-ordered structure was formed, leading to less Si dissolved or (2) some Si reprecipitated with Ca at the higher pH. As-synthesized materials (using Na-silicate) had a mean initial dissolution rate of 5.8 ( 0.5 × 10-5 mg Si gsolid-1 s-1 in pure water, 0.005 M CaCl2, and 0.1 M CaCl2 (Figure 6a). Drying temperature did not affect rates (data not shown); however, pure Si samples showed a greater increase in initial disVOL. 34, NO. 22, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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contain 2-3 and 4-5 times the amount of organic carbon as organic-cation-exchanged clays and surfactant-modified zeolites, respectively. The fact that this organic carbon is strongly retained by inorganically prepared MCM-41s during contact with aqueous solutions suggests that the cost of synthesis per pound (∼1.5× that of HDTMA-clay and ∼0.6× that of SMZ), which is dominated by the cost of the surfactant, is economic.
Acknowledgments K. L. Nagy appreciates research support from the U.S. Department of Defense through the Strategic Environmental Research and Development Program (SERDP). We thank Dr. Steve Boese, University of Wyoming, for the organic carbon analyses. Comments from three anonymous reviewers were useful in revising the manuscript.
Literature Cited
FIGURE 6. Si released to solution and HDTMA remaining on assynthesized air-dried (23 °C) solids after seven washing cycles. solution rate with increasing ionic strength than samples with Si/Al ) 40 (40% vs 10%). After the first washing cycle the mean rate (corrected for Si mass loss) decreased by more than 50% for all samples at all ionic strengths, due to the presence of Si in solution from the previous washing cycle (12-22 mg L-1), which increased the initial saturation state. Using a maximum external BET surface area reported for as-synthesized MCM-41s of 10 m2 g-1 (10), the mean initial dissolution rate after 2 days for all samples is 2.5 ( 0.2 × 10-10 mol Si m-2 s-1. Surface area-normalized dissolution rates are 2-3 orders of magnitude faster than those of quartz (37) and 1 order of magnitude faster than that reported for a borosilicate glass (38). At the initial dissolution rates, 1 g of material would dissolve in 583 da if flushed with pore water at a rate of ∼1 × 10-4 cm3 s-1. In pore waters containing up to three times the amount of Si equal to quartz solubility, 1 g of material would dissolve in 1165 da. HDTMA leaching rates were much slower than the Si dissolution rates (Figure 6b). Whereas ∼6.5% of the Si in the solids had dissolved after flushing with 1050 pore volumes, less than 1.5% of the HDTMA had been removed in water and 0.005 M CaCl2, and this amount decreased to ∼0.1% in 0.1 M CaCl2. (The number of pore volumes is a minimum and was estimated using a measured bulk density of 1 g cm-3 without any adjustment for aggregate porosity.) These results agree with HDTMA-leaching rates from organic-exchanged smectites (39) and surfactant-modified zeolites (40) with respect to the trend with ionic strength, explained by increased expansion of the electric double layer at low ionic strength (40). However, the HDTMA-leaching rates are much slower than those reported for other HDTMA-containing materials. The slower leaching rates may be related to the structural location of HDTMA in the mesoporous materials (i.e., interior to the Si framework). In this case the excess Si release may be due to an excess of truly amorphous silica (i.e. not formed around the HDTMA-templating structure), in agreement with the high initial Si release observed in the batch experiments, or to HDTMA being readsorbed into the structure over time. As-synthesized MCM-41 materials 4826
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(1) Lerner, D. W.; Walton, N. R. G. Contaminated Land and Groundwater: Future Directions; The Geological Society: London, Engineering Geology Spec. Pub., No. 14, 1998; 248p. (2) Boyd, S. A.; Lee, J.-F.; Mortland, M. M. Nature 1988, 6171, 345347. (3) Xu, S., Sheng, G.; Boyd, S. A. Advances Agronomy 1997, 59, 2562. (4) Kresge, C. T.; Leonowicz, M. E.; Roth, W. J.; Vartuli, J. C.; Beck, J. S. Nature 1992, 359, 710-712. (5) Beck, J. S.; Vartuli, J. C.; Roth, W. J.; Leonowicz, M. E.; Kresge, C. T.; Schmitt, K. D.; Chu, C. T-W.; Olson, K. H.; Sheppard, E. W.; McCullen, S. B.; Higgins, J. B.; Schlenker, J. L. J. Am. Chem. Soc. 1992, 114, 10834-10843. (6) Stucky, G. D.; Monnier, A.; Schu ¨ th, F.; Huo, Q.; Margolese, D.; Kumar, D.; Krishnamurty, M.; Petroff, P.; Firouzi, A.; Janicke, M.; Chmelka, B. F. Mol. Cryst. Liq. Cryst. 1994, 240, 187-200. (7) Cheng, C.-F.; Zhou, W. Z.; Park, D. H.; Klinowski, J.; Hargreaves, M.; Gladden, L. F. J. Chem. Soc., Faraday Trans. 1997, 93, 359363. (8) Monnier, A.; Schuth, F.; Huo, Q.; Dumar, K.; Margolese, K.; Maxwell, R. S.; Stucky, G. D.; Krishnamurty, M.; Petroff, P.; Firouzi, A.; Janicke, M.; Chmelka, B. F. Science 1993, 261, 317321. (9) Schacht, S.; Huo, Q.; Voigt-Martin, I. G.; Stucky, G. D.; Schu ¨ th, F. Science 1996, 273, 767-771. (10) Voegtlin, A. C.; Matijasic, C.; Patarin, J.; Sauerland, C.; Grillet, Y.; Huve, L. Microporous Mater. 1997, 10, 137-147. (11) Mercier, L.; Pinnavaia, T. J. Environ. Sci. Technol. 1998, 32, 27492754. (12) Fryxell, G. E.; Liu, J.; Hauser, T. A.; Nie, Z. M.; Ferris, K. F.; Mattigod, S.; Gong, M. L.; Hallen, R. T. Chem. Mater. 1999, 11, 2148-2154. (13) Diaz, J. F.; Balkus, K. J. J. Mol. Catal. B-Enzymatic 1996, 2, 115126. (14) Bambrough, C. M.; Slade, R. T. C.; Williams, R. T.; Burkett, S. L.; Sims, S. D.; Mann, S. J. Colloid Interface Sci. 1998, 201, 220222. (15) Chen, C.-Y.; Li, H.-X.; Davis, M. E. Microporous Mater. 1993, 2, 17-26. (16) Tanev, P. T.; Pinnavaia, T. J. In Access in Nanoporous Materials; Pinnavaia, T. J., Thorpe, M. F., Eds.; Plenum Press: New York, 1995; pp 13-27. (17) Denoyel, R.; Rey, E. S. Langmuir 1998, 14, 7321-7323. (18) Zhao, H. T.; Vance, G. F. Water Res. 1998a, 32, 3710-3716. (19) Zhao, H. T.; Vance, G. F. J. Incl. Phenom. Mol. Recognit. Chem. 1998b, 31, 305-317. (20) Zhao, H. T.; Vance, G. F. Clays Clay Miner. 1998c, 46, 712-718. (21) Anderson, M. T.; Martin, J. E.; Odinek, J.; Newcomer, P. In Access in Nanoporous Materials; Pinnavaia, T. J., Thorpe, M. F., Eds.; Plenum Press: New York, 1995; pp 29-37. (22) Edler, K. J.; White, J. W. J. Chem. Soc., Chem. Commun. 1995, 155-156. (23) Grasshoff, K. Methods of Seawater Analysis; Springer-Verlag: New York, 1976; 315p. (24) Lin, H.-P.; Mou, C.-Y. Chem. Mater. 1998, 10, 581-589. (25) Jaynes, W. F.; Vance, G. F. Soil Sci. Soc. Am. J. 1996, 60, 17421749. (26) Sheng, G.; Xu, S.; Boyd, S. A. Environ. Sci. Technol. 1996, 30, 1553-1557.
(27) Chiou, C. T.; Peter, L. J.; Freed, V. H. Science 1979, 206, 831832. (28) Li, Z.; Bowman, R. T. Environ. Sci. Technol. 1998, 32, 2278-228. (29) Jaynes, W. F.; Boyd, S. A. Clays Clay Miner. 1991, 4, 428-436. (30) Laird, D. A.; Fleming, P. D. Environ. Toxicol. Chem. 1999, 18, 1668-1672. (31) Cauvel, A.; Brunel, D.; Di Renzo, F. Langmuir 1997, 13, 27732778. (32) Llewelln, P. L.; Schu ¨ th, F.; Grillet, Y.; Rouquerol, F.; Rouquerol, J.; Unger, K. K. Langmuir 1995, 11, 574-577. (33) Hendra, P. J.; Passingham, C.; Warnes, G. M.; Burch, R.; Rawlence, D. J. Chem. Phys. Lett. 1989, 164, 178-184. (34) Rimstidt, J. D. Geochim. Cosmochim. Acta 1997, 61, 2553-2558. (35) Langmuir, D. Aqueous Environmental Geochemistry: PrenticeHall: New Jersey, 1997; 600p.
(36) Navrotsky, A. In Reviews in Mineral; Heaney P. J., Prewitt C. T., Gibbs, G. V., Eds.; Min. Soc. Amer: Washington, D.C, 1994; Vol. 29, pp 309-329. (37) Dove, P. M.; Elston, S. F. Geochim. Cosmochim. Acta 1992, 56, 4147-4156. (38) Knauss, K. G.; Bourcier, W. L.; McKeegan, K. D.; Merzbacher, C. I.; Nguyen, S. N.; Ryerson, F. J.; Smith, D. K.; Weed, H. C.; Newton, L. Mater. Res. Soc. Symp. Proc. 1990, 176, 371-381. (39) Xu, S.; Boyd, S. A. Environ. Sci. Technol. 1995, 29, 312-320. (40) Li, Z.; Roy, S. J.; Zou, Y.; Bowman, R. S. Environ. Sci. Technol. 1998, 32, 2628-2632.
Received for review February 9, 2000. Revised manuscript received August 10, 2000. Accepted August 24, 2000. ES000990O
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