Environ. Sci. Technol. 2000, 34, 455-460
Chromium Leaching from a Silicone Foam-Encapsulated Mixed Waste Surrogate CHRISTOPHER M. MILLER* AND STEPHEN E. DUIRK Department of Civil Engineering, University of Akron, Akron, Ohio 44325-3905 KEVIN H. GARDNER Department of Civil Engineering, University of New Hampshire, Durham, New Hampshire 03824
This study assessed chromium leaching from silicone foam-encapsulated salt waste, using a surrogate formulated after Department of Energy complex mixed waste. Two commercial formulations of silicone foam (Wacker ELEKTROGUARD 2100 and General Electric RTV-664) were evaluated as a function of waste load (28-48 wt %). Chromium leaching was formulation specific and increased with increasing waste load as measured by the Toxicity Characteristic Leaching Procedure (TCLP). Chromium release followed transport controlled dissolution at all waste loads under TCLP (cut samples) and Accelerated Leach Test (ALT) (molded samples) conditions. Aqueous and surface complexation modeling was also used to describe reduced chromium effective diffusivity due to iron oxide addition. Comparison of modeling and measured diffusivities as a function of waste load demonstrated that the total available iron surface site concentration increased with increasing waste load, consistent with pore differences measured by image analysis. These results provide a basis for further work on modeling and engineering waste encapsulation using silicone foam.
Introduction The Department of Energy (DOE) will need to manage 1.5 million cubic meters of low-level waste (LLW) over the next 20 years (1). A large fraction is mixed waste (i.e. containing both hazardous components and radioactive isotopes), saltlike materials generated from nuclear weapons production, dismantling activities, and research. Solidification and stabilization (S/S) has been identified as a Best Demonstrated Available Technology (BDAT) for a wide range of wastes including chromium, cadmium, and lead (2). S/S processes are designed to reduce leaching and mobility of contaminants, improve handling, and decrease the available surface area over which contaminant loss can occur (3-5). S/S is typically an ex-situ process in which chemical reagents and binders are mixed with waste to form a solid. Cement-based technologies have been used extensively for heavy metalcontaminated soils and sludge. Cement, however, does not perform well with salt-containing wastes when the waste to binder weight ratio is greater than 1:5 (6). Failure of the concrete to set and poor stability has resulted in leaching of * Corresponding author phone: (330)972-5915; fax: (330)972-6020; e-mail:
[email protected]. 10.1021/es9812958 CCC: $19.00 Published on Web 12/16/1999
2000 American Chemical Society
soluble salts and metals to concentrations in water that exceed regulated values. Polymers have also been used as a S/S technology. Polyethylene encapsulation of 30-70 wt % sodium nitrate has been demonstrated by Franz et al. (7). Durability tests indicated that the encapsulated waste exceeded a target compressive strength value of 50 psi after being immersed in water for 90 days, and leaching indexes (LI), equal to the negative logarithm of the observed diffusivity, ranging from 7.8 to 11.1 were reported. Pilot scale tests were conducted by Kalb et al. (6) with an extruder to encapsulate sodium nitrate waste in a low-density polyethylene and were comparable to laboratory results reported by Franz et al. (7). Polyethylene encapsulation requires a heat source (T > 120 °C) and is limited to ex-situ treatment of waste. One class of polymers that has not been reported in the literature for S/S is silicone foams. Silicone foams have many common applications including cushion seals, sound insulation, and flame retardant. Rigid resin foams which expand and cure at room temperature are commonly know as roomtemperature vulcanizing (RTV) silicone rubbers (8). Silicone foam results from the reaction between a cross linker (SiH), a polysiloxane polymer (SiOH), and a catalyst according to
SiH + SiOH + catalyst f SiOSi + H2(gas)
(1)
Upon mixing of these components, blowing and curing occur. Blowing creates hydrogen gas bubbles (H2) that help the foam develop, and curing results in the formation of the siloxane linkage (SiOSi). The sturdy siloxane linkage allows the silicone foam to be resistant to extreme temperature, pressures, and chemical environments, making it a viable candidate for waste encapsulation. Reactions occur at room temperature but can be accelerated at increased temperatures. Typical catalysts include various oxides and hydrides; however, platinum is most commonly used for flame resistance (8). Additives to silicone foam, called filler, can add stiffness and reduce the overall cost. Inorganic materials and metal oxides, such as silicon dioxide, are the most common filler (9). The objectives of this paper are to characterize chromium leaching from a mixed waste salt surrogate encapsulated by silicone foam and to examine addition of iron oxide to the foam system to reduce leaching. Two commercial silicone foam formulations were examined at variable waste loading (28-48 wt %). Waste loading is a key cost factor in all S/S processes and could impact leaching in a polymer system due to waste-polymer interactions that change the silicone foam curing process. Chromium oxides are typically present in DOE waste solids; therefore elucidation of the mechanism of chromium leaching from silicone foam-encapsulated mixed waste should provide insight into its use and potential as a DOE waste management option.
Experimental Section Silicone Foam, Surrogate Waste, and Chemicals. General Electric silicone compound and catalyst (GE RTV-664a,b) were obtained from General Electric Silicones (Waterford, NY). Wacker ELEKTROGUARD 2100 silicone elastomer and catalyst were obtained from Wacker Silicones Corporation (Adrian, MI). Material properties are shown in Table 1. All chemicals for the nitrate salt surrogate waste were purchased in solid form from Fisher Scientific (Pittsburgh, PA) and mixed in 1 kg batches with the following composition: 530.3 g of NaNO3, 348.3 g of KNO3, 1.7 g of Al2O3, 14.0 g of NaF, 54.0 g of Na2SO4, 31.0 g of NaCl, 16.9 g of Na2HPO4, 1.9 g of CrO3 ([Cr]Total ∼ 1000 mg/kg waste), and 1.9 g of FeCl3. The surrogate VOL. 34, NO. 3, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 1. Silicone Foam Material Propertiesa characteristic viscosity, cP, at 25 °C upper/lower service temperature, °C tensile strength (ASTM D412), psi elongation (ASTM D412), % durometer hardness (ASTM D2240), shore A
General Electric Wacker ELEKTROGE RTV-664 GUARD 2100 110 000 200/-60
15 000 260/-65
800
750
220
130
60
60
a Properties reported are for combination of base and catalyst after mixing at recommended ratios and/or appropriate cure time.
waste was crushed and sieved such that it passed a 0.6 mm sieve and was retained on a 0.3 mm sieve. This waste composition has been determined to be chemically and physically representative of nitrate salts stored at the Idaho National Engineering Laboratory (10). The iron(III) oxide (catalyst grade, 30-50 mesh, FeOOH) was used as obtained from Aldrich (Milwaukee, WI). The specific surface area of this product is reported to be 120 m2/g, with a concentration of replaceable surface hydroxyl groups measured by the fluoride adsorption method of 5 × 10-4 mol/g (11). Barnstead Nanopure water was used for making all reagent stock and batch extraction solutions. Analytical Methods. Analysis of chromium was performed using an inductively coupled plasma atomic emission spectrometer (ICP-AES). All pH measurements were made after two point calibration. Image analysis was used to measure silicone-foam encapsulated waste sample air voids, surface area, and effective length (Lva), which is equal to the sample volume divided by the external surface area. Images were captured with a research stereo microscope. For air void analysis, three replicate samples were obtained from 5.1 cm diameter × 10.2 cm long cylindrical specimens by cutting the top, middle, and bottom third of each cylinder. All foam samples were briefly exposed to water to dissolve waste particles and allowed to air-dry. Optimas (Version 6.1) image analysis software measured the air voids by determining light intensity variation (user defined values) between void and solid foam areas. Surface areas were then determined by expressing each void as an equivalent circle diameter (software determined), allowing for simple calculation of the void surface area contribution to the total external surface area. Silicone Foam Encapsulation Procedure. Depending on the selected waste load, various masses of polymer base material, catalyst, and waste were measured. Manufacturer recommendations regarding catalyst-to-base ratios were followed. Mixing took place in one gallon plastic buckets using a hand-held drill equipped with a stainless steel paddle. Polymer base material and waste were mixed together first for a minimum of 5 min, and then the catalyst was added. For samples prepared with iron oxide, iron was put into the mix with the polymer base material and waste before catalyst addition. The entire mixture was mixed for an additional 5 min and then transferred to plastic cylinder molds (5.1 cm diameter × 10.2 cm). Samples were placed under a hood and allowed to cure for a minimum of 72 h before testing. Toxicity Characteristic Leaching Procedure (TCLP) and Accelerated Leach Test (ASTM C-1308-95). A modified Toxicity Characteristic Leaching Procedure test was performed after guidelines set forth by the USEPA (12). Encapsulated waste samples were cut into square pieces with a knife, such that they passed a 9.42 mm sieve and were retained on a 4.75 mm sieve. Ten grams of cut sample was placed in 456
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FIGURE 1. Chromium TCLP concentration as a function of waste load for Wacker-2100 and GE RTV-664 silicone foam. Initial solution pH ) 2.9 and final pH ) 3.1. a 250 mL high-density polyethylene bottle with 200 mL of pH 2.9 acetic acid extraction fluid and shaken for 18 h with a wrist action shaker set at one (scale of 0-10). Aliquots (5 mL) taken during the test for chromium analysis were replaced with an equal volume of acetic acid extraction fluid, and each test was performed in triplicate. Solution suspensions were then filtered using 0.7 µm borosilicate glass filters and stored in high-density polyethylene vials until analysis. In accordance with ASTM C-1308 (Accelerated Leach Test for Diffusive Releases from Solidified Waste), cylindrical samples (approximate size: diameter-3.0 cm and height-4.0 cm) were suspended from the bottle cap of a 500 mL highdensity polyethylene container with Teflon netting. This method is a short-term test with frequent leachant changes to facilitate the accelerated release of soluble compound (13). The method was modified to include eight leachant changes (instead of 13) and was conducted over a period of 17 days (instead of 11). Deionized water (pH ∼ 6-7) is used as the leachant at a volume (in milliliters) 10 times the surface area (in cm2) of the specimen ((2%).
Results and Discussion Effect of Silicone Foam Formulation on Chromium TCLP. Chromium leaching from silicone foam was formulation specific and increased with increasing waste load under TCLP conditions (Figure 1). Waste encapsulated with Wacker silicone foam exhibited the greatest chromium concentrations, ranging from 8.5 mg/L at 30% waste load to 16.7 mg/L at 50% waste load. These concentrations significantly exceed both the Resource Conservation and Recovery Act (RCRA) and Universal Treatment Standards (UTS) of 5.0 and 0.6 mg/ L, respectively (14). Based on its poor leaching performance and sample deterioration under TCLP conditions, the Wacker formulation was dismissed as a viable material for encapsulation and was not studied further. The GE formulation exhibited similar leaching behavior as Wacker (i.e. increasing Cr with increasing waste load); however, the final TCLP concentrations were significantly lower at all waste loads (70-80% less than Wacker). At the time of this study, the treatment TCLP target value was 5 mg/L, which GE satisfied at all waste loads. Chromium concentrations were, however, greater than the UTS level of 0.6 mg/L at all waste loads. Still, the GE material exhibits the ability to retain chromium that will be quantified and discussed later. It was also noted that the TCLP solution pH was relatively constant over the course of 18 h, indicating that the mechanism for silicone foam-waste encapsulation does not involve pH buffering to reduce chromium solubility. Silicone Foam-Encapsulated Waste Void Area Analysis. Image analysis was performed to evaluate silicone foam
FIGURE 4. Chromium leaching kinetics under TCLP conditions with GE RTV-664 silicone foam and variable waste load (28-48 wt %). Initial solution pH ) 2.9 and final pH ) 3.1.
FIGURE 2. Cut surface images of GE RTV-664: (a, top) no waste and (b, bottom) 28 wt % waste.
Chromium Leaching Kinetics Under TCLP Conditions. During the TCLP test, samples were taken at 0.08 (5 min), 1, 4, 8, and 18 h. Aqueous chromium concentrations were converted to cumulative fraction leached (CFL) for each time interval and plotted versus the square root of time (Figure 4). The first sample (5 min) measures the instantaneous dissolution of the waste exposed by the cut surface. Depending on waste load, approximately 7-9% of the total 15-19% (i.e. CFL) leached in the first 5 min. Therefore, instantaneous dissolution of waste exposed by the cut surface of the TCLP sample accounts for 34-58% of the total chromium concentration measured during the TCLP test, which is a significant fraction of the leachant concentration. Note that the concentration in solution is equal to CFL times the theoretical concentration if all metal leached into solution, which for equivalent CFL, increases with increasing waste load. For example, at 28% waste load, the CFL after 5 min is equal to 0.086, which corresponds to 1.07 mg/L chromium. At 48% waste load, the CFL value is lower at 0.066, but this corresponds to a higher chromium concentration (1.41 mg/ L). For the case of Fickian diffusion-controlled release from a solid in a large bath (concentration in bath is relatively constant) with homogeneous distribution of chromium, the effective diffusion coefficient can be determined using the following expression (15, 16)
( )
Mt 4De ) CFLt ) Mo πLva2
FIGURE 3. Void area of sample cross-section as a function of waste load with GE RTV-664. Dashed lines represent 95% confidence interval. sample curing behavior and void area as a function of waste load (28, 38, and 48 wt %). Figure 2 shows typical images of cut surfaces used for this evaluation. Note that the sample without waste material has circular holes of variable size indicating hydrogen gas capture, whereas samples with waste have voids that consist of both gas pockets and locations where waste was entrapped. Samples exhibited increasing void area with increasing waste load (Figure 3). This trend is expected because increasing waste load results in a higher percentage of volume physically occupied by the waste. Initially, there was concern that waste might reduce the effectiveness of the catalyst resulting in less hydrogen gas formation, but the linearity of Figure 3 suggests that the void area contribution from hydrogen was relatively constant.
0.5
(2)
where Mt is the mass of chromium leached at time t, Mo is the mass of chromium initially in solid, De is the effective diffusivity, and Lva is the ratio of solid volume to external surface area exposed to bath. Based on eq 2, the linearity of Figure 4 indicates transport-controlled dissolution of chromium at all waste loads. Figure 4 also exhibits increasing slope with increasing waste load, potentially indicating increasing diffusivity. The slope, however, is only a bulk measure of De and Lva, so Lva must be determined to calculate De. Image analysis and gravimetric measurement were used to calculate Lva, which, along with regression analysis of Figure 4 data, allowed calculation of De as a function of waste load (Table 2). An increase in De with increasing waste load may indicate changes in the physical attributes of the samples, the implications of which are discussed below. The increase in De observed with increased waste load appears to be a result of the increased pore volume containing waste and the increasingly connected nature of the nearsurface pores (as would be expected with an increase in void VOL. 34, NO. 3, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 2. Chromium Diffusivity as a Function of Waste Load (GE RTV-664 under TCLP Conditions, n ) 3) with and without Iron Oxide Addition (5 wt %) waste load (wt %) 28 38 48
(4De/πLva2)0.5 a (10-4
s-1)
2.67 ( 0.79 3.94 ( 0.87 5.46 ( 0.48
Lvab (10-2
cm) 7.32 ( 1.05 7.08 ( 0.45 6.80 ( 0.50
without iron De (10-10 cm2/s) 3.49 ( 2.18 7.03 ( 4.03 11.20 ( 3.52
waste load
(4De/πLva2)0.5 c
Lvab
with iron De
(wt %) 28 38 48
(10-4 s-1) 1.84 ( 0.32 2.92 ( 0.18 4.19 ( 0.53
(10-2 cm) 6.79 ( 1.50 6.55 ( 0.45 6.29 ( 0.68
(10-10 cm2/s) 0.76 ( 0.26 2.88 ( 0.35 5.53 ( 1.38
a Slope of lines in Figure 4 (based on eq 1). b Calculated by image analysis and gravimetric sample measurement (see Experimental Section). c Slope of lines in Figure 5 (based on eq 1).
is reduced by a factor of between 2.0 at 48% waste load and 4.6 at 28% waste load). To investigate the reduction in diffusive transport further, MINEQL+ (18) was used to conduct aqueous and surface complexation modeling. Constants for surface hydrolysis and chromium complexation were taken from Mesuere and Fish (19). Considering the diffusive flux of mass from within the silicone foam, the equation (in one dimension) that describes the transport with adsorption is (20)
∂C ∂2C ∂S ) De 2 ∂t ∂t ∂x
(3)
where C is the aqueous concentration of solute (chromium) and S is the sorbed chromium concentration. The sorption of chromium to the iron oxide surface has been described in the literature (19). The adsorption reaction dominant at pH ) 3.0 and in the presence of oxygen at atmospheric levels (i.e. TCLP conditions) is as follows
≡SOH + CrO42- + 2H+ ) ≡SHCrO40 + H2O log K ) 18.7 (4) where tS represents the singly protonated iron oxide surface site, and tSHCrO40 is the dominant sorbed species. The Langmuir isotherm can be developed directly from the above expression, resulting in
K′STC 1 ) K′C
S)
(5)
where ST is the total concentration of iron oxide surface sites, C is the total concentration of aqueous (mobile) species of chromium, K′ ) K[H+]2, and [H+] is the hydrogen ion concentration. Substituting the adsorption isotherm into the diffusive transport equation and rearranging gives FIGURE 5. Chromium leaching kinetics under TCLP conditions with GE RTV-664 silicone foam and iron oxide (fixed addition to foam of 5 wt %). Variable waste load (28-48 wt %), initial solution pH ) 2.9, and final pH ) 3.1. volume). While the increased external surface area of the cut samples does not change appreciably (Lva changes by approximately 7% between 28 and 48% waste load, as shown in Table 2), the void area at the surface, and by extrapolation, that near the surface, changes by approximately 39% (Figure 3). Thus, the physical nature of the encapsulating foam changes significantly with waste load, but this is not accounted for by changes in the external surface area of cut samples. This difference could account for the variation in the estimated effective diffusion coefficients and is akin to a decrease in the tortuosity of the samples with higher waste loads. In fact, Shafique et al. (17) postulated that increased porosity or lower tortuosity at the periphery of cementencapsulated waste cylinders could account for increases in the apparent diffusivities of samples with variable content or imperfections. Effect of Iron Oxide Addition on Chromium Leaching. TCLP experiments were also conducted after addition of 5 wt % iron oxide to the waste/foam mixture. Iron oxide was added to investigate the potential of chromium adsorption to the oxide to simultaneously slow the release of chromium and shift its equilibrium distribution between leachant and encapsulating solid. Results are again shown as CFL plotted versus the square root of time in Figure 5. Results from regression analysis of Figure 5 and from image analysis of cut foam samples were used to estimate De and are summarized in Table 2. Again, De is a function of waste load, increasing with increasing waste load, but with lower De values than samples not containing iron oxide (i.e. diffusivity 458
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∂C ) ∂t
(
De
1+
K′ST (1 + K′C)2
)
∂x2
(6)
Defining DR as a retarded diffusion coefficient due to the adsorption process, and R the retardation coefficient, the transport rate of a solute will be reduced (relative to the rate due to effective diffusion alone) by the fraction
R)
DR ) De
1+
1 K′ST
(7)
(1 + K′C)2
It should be noted that the retardation coefficient is not constant using the Langmuir (nonlinear) adsorption isotherm but will be a function, as shown in eq 7, of the total aqueous chromium concentration. For the special case of linear adsorption (when K′C in eq 5 is much less than 1), the righthand side of eq 7 is simplified to a constant value, not dependent on concentration. Under the aqueous conditions present in the experiments chromium adsorbs very strongly to the iron oxide surface; therefore, a linear description is not appropriate. Thus, R will be expected to vary over time and space within the foam pores. Aqueous and surface speciation modeling was conducted to gain insight into how leaching is influenced by the presence of the iron oxide. To model this system, however, it is necessary to know the iron oxide surface site concentration to which the aqueous chromium is exposed because the encapsulation process could reduce the oxide surface available for sorption. The modeling of the system proceeded by answering the following question: given the known surface
FIGURE 6. Retardation coefficient as a function of iron oxide surface site concentration under TCLP conditions for the time-weighted average concentrations within the pore structure of the foam at each waste load (28-48 wt %). Experimentally determined average retardation values provide an estimate of the surface site concentration available at each waste load. and aqueous speciation constants, the total chromium present in the encapsulated waste material, and the observed reduction in diffusive mass transport, what surface site concentrations must be available for sorption to result in the observed behavior. Although it is well established that chromium will sorb strongly to iron oxide under TCLP conditions (e.g. ref 19), it was unclear to what extent the iron oxide surface would be available for sorption reactions. For this analysis, it was assumed that the encapsulated waste was distributed uniformly throughout the pores of the foam material. The total initial concentration of chromium in the pore space of each sample was 0.056, 0.077, and 0.090 M for the 28%, 38%, and 48% waste load, respectively. Over the duration of the TCLP the concentration within the foam decreased as material leached into the bulk solution. For the purposes of modeling, the time-weighted average concentration was used, and it was assumed that the concentration within the foam matrix remained homogeneous (i.e. as mass leached from the surface the remaining mass was redistributed homogeneously throughout the foam). With 5% iron oxide by weight in each sample, and 5 × 10-4 mol/g of surface sites, the maximum possible surface site concentration available in the pore space is 0.26 M. Using the above concentrations for total chromium in the system, the speciation model was run with variable surface site concentrations, and the results of this modeling were used in eq 7 to calculate the retardation coefficient. These results are shown graphically in Figure 6. The results shown in Figure 6 were used to estimate the available surface sites, which are given by the intersection of the average retardation coefficient (observed in the experiments) with the time-weighted average concentration for each waste load. This yields the following estimates: 0.065, 0.079, and 0.097 M sites for the 28%, 38%, and 48% waste load. Expressed as a fraction of the maximum possible surface sites, these are 25%, 30%, and 37%. The increase in available sites is expected, as the void area and porosity of the foam encapsulated waste increase with waste load. The void area of the 38% and 48% waste load samples increased by 24% and 56% over the 28% waste load sample, respectively, while the available surface sites increased 22% and 49% over those available in the 28% waste load sample. These data indicate a linear relationship between the increase in void area and the increase in available surface sites. Further investigation was conducted to assess the error associated with the assumption that the surface site concentration can be estimated using a time-weighted average
FIGURE 7. Retardation coefficient in the 48% waste load foam sample as a function of iron oxide surface site concentration for the pore concentration range during the TCLP test. concentration and that the concentration within the foam matrix remains homogeneous. The concentration profile within the cut cubes of foam used in the TCLP test (assumed an average size of 7 mm on a side) was approximated using an analytical solution to isotropic diffusion in a plane without reaction. The results of this exercise indicated that the region in which concentration was significantly reduced below the initial concentration was limited due to the low diffusion coefficients of this material. For example, after the 18 h TCLP for the 48% waste load sample (which has the largest De), the concentration at 0.08 mm inside the foam surface would be reduced to 83% of the initial, and at 0.04 mm inside the foam surface the concentration would be reduced to 68% of the initial concentration. A final analysis was conducted to investigate the effect of concentration variation that occurs throughout the TCLP on the surface site estimate and the retardation coefficient. This was accomplished by conducting speciation modeling for the range of concentrations present. The results of this for the 48% waste load sample are shown in Figure 7 and demonstrate the extent to which retardation changes during the TCLP. If it is assumed that the surface site concentration remains constant, then the family of curves in Figure 7 show that initially, at the highest concentration during the leaching, there is little retarding influence of the iron oxide (R is approximately 0.95 at the highest concentration with ST ) 9.75 × 10-2 M). By the end of the 18 h of leaching, however, R is reduced to approximately 0.2, giving a more significant reduction in leaching rate. Over a long-term leaching process, such as may be of interest for the environmental behavior of such materials, the iron oxide would be expected to reduce the transport to a much greater extent as the near-surface concentration of chromium in the encapsulating material is decreased. Accelerated Leach Test (ALT) Evaluation. The ALT provides a more accurate representation of the expected behavior of the silicone foam encapsulated waste product (indeed, any encapsulation technique) because leaching is measured for solid cylinders rather than cut samples. The effective diffusion coefficients can be determined as before by linear regression analysis of CFL versus time plots or using the computer program developed for the ALT by Fuhrmann et al. (20). The average (n ) 3) measured effective diffusivities were determined to be De@28% ) 2.20 × 10-10 cm2/s, De@38% ) 1.60 × 10-10 cm2/s, and De@48% ) 1.63 × 10-10 cm2/s, respectively. These results indicate that De is relatively constant during the ALT and exhibits an effective diffusivity similar to that measured during the TCLP. Comparison of chromium concentrations in solution following the ALT to VOL. 34, NO. 3, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TCLP results shows much lower values (0.08, 0.16, and 0.27 mg/L for the three waste loads), all of which fall well below the UTS level of 0.6 mg/L. The measured variation between the two tests can be easily explained by the higher Lva in the ALT (∼0.5 cm), which based on eq 2 would result in lower CFL measurements.
(8) (9) (10)
Acknowledgments This research was supported by Orbit Technologies, Inc. (Carlsbad, CA) and The Department of Energy (Contract No. DE-AC07-94ID13223). Endorsement by granting agencies should not be inferred nor should commercial products. Jennifer Hartong, Tom Quick, Annabelle Foos, and Eric Wagner contributed analytical assistance, and General Electric Silicones (Waterford, NY) and Wacker Silicones Corporation (Adrian, MI) donated silicone materials. Mark Fuhrmann (Brookhaven National Laboratory) provided the computer program for the Accelerated Leach Test, and Steve Prewett and Guy Loomis provided project guidance, particularly regarding DOE waste management concerns.
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Received for review December 14, 1998. Revised manuscript received August 20, 1999. Accepted November 4, 1999. ES9812958
