Environ. Sci. Technol. 2006, 40, 2384-2389
Remediation of Elemental Mercury Using in Situ Thermal Desorption (ISTD) ANNA M. KUNKEL, JEREMY J. SEIBERT, LUCAS J. ELLIOTT, RICCI (LAMBERT) KELLEY, LYNN E. KATZ,* AND GARY A. POPE Department of Civil, Architectural, and Environmental Engineering, ECJ 8.6, University of Texas, Austin, Texas 78712
In situ thermal desorption (ISTD) is a soil heating method that simultaneously applies heat and vacuum to the subsurface at temperatures up to 600 °C. As the soil is heated, the vapor pressure of the contaminant increases allowing mass transfer to the gas phase and extraction from the soil using vacuum wells. The overall goal of this research is to assess the feasibility of using ISTD to remove elemental mercury from soils. The initial phase of research included design of a laboratory soil column apparatus and preliminary soil column experiments with surrogate nonaqueous phase liquids (perfluorocarbons) to test the apparatus and investigate the effects of air flow rate and temperature on the ISTD process. Following the preliminary experiments, a mercury off-gas treatment system was added and mercury experiments were conducted. Experiments performed using elemental mercury showed greater than 99.8% removal of the mercury from Ottawa sand. These results show that ISTD can remove mercury from soil at temperatures well below its boiling point and that perfluorodecalin can be used as a surrogate for elemental mercury in laboratory experiments. A flow and transport simulator was used to model the results from both the perfluorocarbon and the mercury experiments.
Introduction Mercury is one of the most common metals identified at Superfund sites (1, 2). In February 2004, the USEPA reported that approximately 35% of Superfund sites on the National Priority List had mercury contamination (3). Elemental mercury (Hg(0)) has been widely used in this country with applications ranging from dental amalgams to nuclear weapons production. Sznopek and Goonan (4) reported that the chlor-alkali industry released 8 metric tons in 1996 with 101 metric tons unaccounted for on the basis of their analysis of the reported data. Other research indicated that soils surrounding chlor-alkali plants contain Hg(0) concentrations up to 75 times background levels (5). Mercury use for nuclear weapons production at the US DOE Oak Ridge Y-12 Plant during the 1950s and 1960s created one of the most publicized mercury contaminated sites in the US. An estimated 700 000 pounds of mercury were lost to the environment and 1.3 million pounds were unaccounted for through bookkeeping errors (6). Current remediation techniques for Hg(0), including soil excavation followed by ex situ thermal treatment, involve risk of atmospheric transport and human exposure (7-11). * Corresponding author phone: (512)471-4244; fax: (512)4715870; e-mail:
[email protected]. 2384
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In situ thermal desorption (ISTD) (12) is being investigated as an alternative method to remediate Hg(0) contaminated soils (13-16). In ISTD, heat is supplied to the subsurface by passing electrical current through heating elements suspended in wells, with vacuum applied to withdraw the contaminants (12). There is no practical limit as to the treatment depths because the heaters were originally designed for use at depths reaching 2000 feet. ISTD has many advantages (12, 17-25) over other thermal remediation options such as electrical resistance heating (ERH). ISTD differs from ERH in that the ERH heating capacity is limited to the boiling point of water (100 °C) because soil porewater is required for conductance of the electrical energy through the soil. Once the soil water has been boiled off, the electrical conductivity of the soil becomes negligible and the soil can be only minimally heated. By contrast, the soil heating in ISTD occurs via thermal conduction through the soil from electrical resistance heaters placed in wells rather than by electrical conduction through the soil porewater. Therefore, the soil temperature can be increased through thermal conduction, if desired, to values on the order of 600 °C, far exceeding the boiling point of water. Another significant advantage is that the effectiveness of ISTD is significantly less sensitive to subsurface heterogeneities compared to other methods because ISTD depends primarily on thermal conduction to transport heat through the subsurface (12, 18, 19). Compared to fluid permeabilities, thermal conductivity is much less variable, changing by only a factor of approximately 4 from clay to sand. The design of the ISTD contaminant recovery system is dependent on the type of contaminant (21, 25). For metals such as mercury, which are not subject to decomposition reactions, contaminant recovery is essential. Other concerns with respect to the ISTD process include high cost, site accessibility for well installation, and the potential need for control of water recharge for contaminated zones located below the water table. The main objective of this research was to evaluate the potential of the ISTD process for removal of elemental mercury from contaminated soil. A laboratory apparatus was designed and used to conduct soil column experiments to accomplish this objective. Preliminary experiments with perfluorocarbon were used to determine the general trends associated with the impact of parameters such as soil column temperature on contaminant removal, and then similar experiments were performed with Hg(0) in the soil column. The results of the experiments were modeled using a flow and transport simulator. The simulator chosen for this study was the Steam and Thermal Advanced Reservoir Simulator (STARS) developed by Computer Modeling Group (CMG) Ltd. (26). A mechanistic simulator such as STARS can be used to help interpret laboratory experiments and then subsequently applied to model field applications under different site conditions once they have been validated by the laboratory experiments. STARS has been used to study the remediation of organic contaminants using ISTD technology (25, 27, 28).
Materials and Methods The perfluorocarbon used in this research was 98+% pure perfluorodecalin, C10F18 (Alfa Aesar, Ward Hill, MA, and ABCR Chemicals, Karlsruhe, Germany). The perfluorodecalin was evaluated as a surrogate for Hg(0) in laboratory soil column tests since it is also a dense nonaqueous phase liquid (DNAPL) and is nontoxic and easy to detect using gas chromatography (GC). The hypothesis tested was that the effluent profile could 10.1021/es0503581 CCC: $33.50
2006 American Chemical Society Published on Web 02/28/2006
FIGURE 1. Vapor pressure curves for perfluorodecalin, C10F18, and mercury. be scaled directly as a function of vapor pressure. Figure 1 shows the vapor pressure of both C10F18 and Hg(0). The vapor pressure of perfluorodecalin at 70 °C (0.084 atm) is approximately the same as Hg(0) at 245 °C (0.085 atm). Therefore, the Hg(0) experiments were conducted at a soil column temperature of about 245 °C. Table 1 provides a comparison of several other physical properties of mercury and perfluorodecalin. Two different experimental apparati were used in this research for the two DNAPLs, perfluorocarbon and mercury (Figure 2). Both experimental apparati utilized a 5 cm diameter, stainless steel soil column packed with Ottawa F-95 silica sand (US Silica, Berkeley Springs, WV), which has an average grain size of 0.16 mm (29). The sand pack within the column was approximately 25 cm long. The soil porosity was determined using a conservative pulse tracer technique for each packed column and ranged from 0.31 to 0.38. The tracer was injected as a pulse into the air flow, and then the effluent concentration of the tracer was monitored over time using gas chromatography. Using the known air flow rate, the pore volume of the column and the porosity were determined from the tracer breakthrough profile (29). For each experiment, the sand was first heated to the desired temperature and then a known mass of the DNAPL was manually injected as a liquid through the top of the column over a period of 5 s using a 5 mL syringe containing a side hole needle to promote outward migration and minimize downward migration of the DNAPL. After the perfluorocarbon was injected into the soil column, pressurized air flowed through an influent mass flow meter under ambient conditions (ca. 22 °C) and then into the soil column. Pressurized house air that had passed through columns of granular activated carbon and Dri-Rite (Blue Island, IL) was used in lieu of vacuum to simplify the experimental system. The pressures applied to our columns were only 0.001 bar or less above atmospheric pressure. In the ISTD process, the typical vacuum applied is on the order of 0.02 bar. Because the mole fraction of the contaminant in the air (the vapor pressure divided by the pressure) is the controlling factor and its value is almost identical under either very low pressure or low vacuum (either 1.001 or 0.98 bar), there is no significant difference between the application of such a low pressure or a low vacuum. The mass flow meter controlling the flow to the column was calibrated using a water displacement technique, and the flow was found to vary less than 2%. Two electric band heaters connected to temperature controllers (Ogden Manufacturing ETR-404-03, Arlington Heights, IL) and thermocouples placed within column thermowells were used to maintain the column temperature at the setpoint. An Insulfrax R 1800 thermal insulation blanket composed of a calcium-magnesium-silica material rated
up to 1000 °C was used to minimize heat loss. The variation in the column temperature was less than 5 °C once the column was equilibrated. Condensation in the effluent tubing was minimized by heating the tubing above the sand column temperature. Rope tape heaters were used on the effluent lines. All temperatures were recorded using an Omega HH611PL4C temperature logger. Effluent samples were collected for the perfluorocarbon experiments by routing the effluent flow stream into a sample loop every 2 min and analyzed using a gas chromatograph equipped with a thermal conductivity detector. The detection limit for the perfluorocarbon was estimated to be 5.2 mg/L. As shown in Figure 2b, the ISTD apparatus was modified for Hg(0) experiments by installing an effluent sampling line to collect discrete mercury samples, a condenser to recover the majority of the mercury from the effluent vapor, and an offgas treatment system to capture any remaining elemental mercury vapor. Effluent mercury samples were collected in 50 mL plastic vials containing 25 mL of a 5% HNO3/10% H2O2 solution and 20 mL of 3 mm glass beads. All chemicals used in the effluent system were reagent grade (Fischer Scientific, Fairlawn, NJ). Total mercury was analyzed using inductively coupled plasma-atomic emission spectroscopy (ICP-AES) using EPA method 101-ALT 006 (34). The method detection limit was determined to be 346 ppb. The off gas treatment system contained three impingers, two containing 100 mL of a 4% potassium permanganate, 10% sulfuric acid mixture and one containing a 10% hydrogen peroxide, 5% nitric acid mixture. This off gas system was designed in accordance with USEPA Method 29 and ASTM Method D6784 (Ontario Hydro Method) (35, 36). The final mass of mercury remaining in the soil after treatment was measured by digesting the soil using a modified version of EPA Method 7471A (37) to account for the low concentrations of mercury remaining in our systems. The oxidizing solution was a mixture of 150 mL of deionized water, 75 mL of 36 N sulfuric acid, 37.5 mL of 15 N nitric acid, and 225 mL of a 5 wt % potassium permanganate solution. Preliminary studies conducted by spiking uncontaminated soil yielded recoveries of approximately 65%. Soil concentrations were determined to be similar for atomic absorption and ICP-AES analyses of duplicate samples of treated soil.
Results and Discussion Perfluorocarbon Experiments. Perfluorodecalin was used to determine the effect of varying the air flow rate and the column temperature. Table 2 summarizes the results from all of the C10F18 experiments. As expected, the perfluorocarbon is removed more rapidly at higher temperature due to the increase in its vapor pressure. Figure 3 shows the results of representative experiments. As shown in Figure 3a, the C10F18 concentration in the effluent air increased rapidly to a plateau value followed by a gradual decrease until the concentration was below the GC detection limit. The plateau concentration was approximately 740 mg/L for both experiments PFD2 and PFD4, which were performed at 70 °C with different air flow rates. This effluent concentration is a little more than half of the equilibrium concentration of 1382 mg/L on the basis of the vapor pressure and Raoult’s law. Since at 70 °C the results are nearly the same at both flow rates, concentrations less than the maximum theoretical value are likely due to dilution caused by air bypassing the contaminant zone rather than by rate-limited mass transfer. Indeed, the volume of the liquid perfluorocarbon is very small compared to the pore volume of the sand (about 180 mL), so some of the air flowing in the sand column does not contact it. Modeling results were found to be consistent with this interpretation as discussed in the next section. Figure 3b shows the cumulative recovery of C10F18 for four experiments at different flow rates and temperatures. VOL. 40, NO. 7, 2006 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 1. Properties of Perfluorodecalin and Mercury (30-33) substance
boiling pt, Tb (K)
crit temp, Tc (K)
crit pressure, Pc (bars)
density at 25 °C (g/mL)
surf tension at 25 °C (dyn/cm)
contact angle (deg)
perfluorodecalin, C10F18 mercury, Hg
414 629.9
558.5 1750
16.05 172.0
1.908 13.59
19.1 485.5
not available 140
FIGURE 2. Apparatus for (a) perfluorocarbon experiments and (b) elemental mercury experiments. A small effect of flow rate is seen at the lower temperature of 48 °C. Simulation of Perfluorocarbon Experiments. The STARS simulator with a three-dimensional Cartesian grid was used to simulate and help interpret the column experiments. Air was injected at the bottom and produced at the top of the vertical sand column model. A proportional heating technique was used to maintain the simulated column at a 2386
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constant temperature. While this was not necessary for the current isothermal simulation, future simulations of ISTD experiments will require this level of complexity since current ISTD field operations involve multiple heating stages. The simulated column was 0.176 m × 0.048m × 0.048 m yielding a pore volume of 126 mL. The simulated sand pack was 4.8 cm on a side and 17.8 cm in length. Preliminary experiments showed that the length of the uncontaminated zone did not
TABLE 2. Summary of Perfluorodecalin (PFD) ISTD Experiments
name
ambient air flow rate (mL/min)
sand temp (°C)
vapor pressure (atm)
mass injected (g)
max effluent concntrated (mg/L)
av effluent concntrated for plateau region (mg/L)
dilutn factor
time of recovery (min)
cumulative vol of air (L)
PFD1 PFD2 PFD3 PFD4 PFD5 PFD6 PFD7 PFD8 PFD9 PFD10 PFD11 PFD12
66.2 67.0 65.3 12.7 62.0 11.3 65.5 76.7 73.5 71.4 71.4 13.4
69.9 70.3 70.8 70.3 48.4 48.2 69.5 70.5 70.6 69.9 69.7 69.4
0.084 0.085 0.087 0.085 0.030 0.030 0.082 0.086 0.086 0.084 0.083 0.082
2.45 2.44 2.44 2.42 2.46 2.45 2.47 2.42 2.49 15.17 15.19 15.12
1376.4 1399.0 1427.6 1399.0 529.6 524.5 1354.2 1410.4 1416.1 1376.4 1365.3 1348.7
762.1 738.0 738.2 752.4 415.2 488.5 724.1 737.0 769.3 833.7 803.0 748.8
1.81 1.90 1.93 1.85 1.28 1.07 1.87 1.91 1.84 1.65 1.70 1.80
49 46 54 304 117 620 54.5 52 54 248 242 1311
3.24 3.11 3.54 3.88 7.25 6.98 3.57 4.03 4.00 17.7 17.3 17.6
FIGURE 4. Distribution of NAPL used in the simulator for (a) the perfluorocarbon experiment and (b) the mercury experiment.
FIGURE 3. (a) Effluent perfluorodecalin concentration profiles and (b) cumulative mass fraction removal curves for selected C10F18 experiments. impact remediation because downward migration was not significant. Thus, the reduction in the length of the column used in the simulation did not impact the results. The grid was 8 × 8 × 24 as shown in Figure 4. Further refinement of the grid did not alter the results. The porosity was 0.31. The initial volume of the contaminant was specified by the initial saturation of the DNAPL in each grid block containing DNAPL. The sum of the DNAPL volume in all of the grid blocks equaled the desired total DNAPL volume corresponding to the column experiment. The experimental results suggest that vapor pressure is the key physical property affecting the removal of the contaminant. For modeling the C10F18 experiments, the vapor pressure correlation in Varushchenko (30) was used as shown in Figure 2 and calculated from the following equation:
ln Pvap ) 205.68 - 11915.05/T - 30.80 ln T + 0.03208T
In this equation, the temperature is in degrees kelvin and the vapor pressure is in kPa. Assuming Raoult’s law applies, the mole fraction of C10F18 in the vapor phase can be calculated by dividing Pvap by the total column pressure for any column temperature. Pvap was also used to perform the same calculation in STARS in each grid block at each time step assuming local equilibrium in each grid block. The time required to remove the contaminant depends on its distribution in the sand. Figure 4a shows the distribution used to simulate experiment PFD5 at 48 °C and 62 mL/min. The initial DNAPL saturation was 10% in each of the shaded grid blocks shown in Figure 4a. In each of the four contaminated layers, 44 grid blocks had this initial saturation of 10% for a total of 176 contaminated grid blocks (a total DNAPL volume of 1.3 mL). Figure 5 compares the cumulative mass fraction in the effluent from experiment PFD5 (48 °C) with a simulation using the DNAPL distribution shown in Figure 4a. Although there are small differences, clearly the simulator has captured the general trend of the data as the time required for complete removal of the contaminant is essentially equivalent. Figure 5 also shows a predictive simulation of experiment PFD2 (70 °C) using the same DNAPL distribution (Figure 4a) as used to model experiment PFD5. This level of agreement between the simulations and experiments supports the interpretation that local equilibrium is a good approximation under the experimental conditions. A second simulation of experiment PFD5 was performed using an equally plausible DNAPL distribution in terms of mass, and the results were found to be sensitive to the DNAPL distribution. Thus, additional laboratory experiments are needed to determine the actual DNAPL distribution within the sand column. Mercury Experiments. Upon completion of the perfluorocarbon investigation, the ISTD apparatus was tested with VOL. 40, NO. 7, 2006 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 5. Comparison of the experimental and model calibration results for C10F18 experiment PFD2 (calibration involved modifying the contaminant distribution within the grid) and experimental and model prediction results for experiment PFD5 (same contaminant distribution used in this simulation).
TABLE 3. Summary of Mercury ISTD Experiments
name
ambient air flow rate (mL/min)
sand temp (°C)
vapor pressure (atm)
time of recovery (min)
cumulative vol of air (L)
Hg1 Hg2 Hg3
73.5 69.5 63.6
250.4 258.8 243.5
0.099 0.123 0.082
705 695 785
51.16 48.00 49.96
elemental mercury. Table 3 lists the parameters and results of three experiments operated at air flow rates between 64 and 74 mL/min and temperatures between 243 and 259 °C. For each experiment, approximately 15 g of mercury was injected into the heated sand. The mass of mercury in the soil was reduced from approximately 15 g to less than 0.025 g after treatment with recovery times between 11 h, 35 min, to 13 h, 5 min. The cumulative volume of air needed to achieve these removals ranged from 48.0 to 51.2 L (267 to 284 pore volumes). These experiments showed that more than 99.8% of the elemental mercury was removed from the soil at temperatures ranging from 244 to 259 °C, far below its boiling point of 356.5 °C and in a very short period of time. It should be noted that recovery times in our system were obtained using a low organic content soil in the absence of water. The presence of water will increase energy consumption and recovery time. Nonetheless, our results are consistent with earlier studies on thermal desorption of PCBs and PAHs, which showed that low residual contaminant concentrations in soil could be achieved at temperatures below the contaminant’s boiling point provided the residence time at temperature was sufficient (22, 38). The perfluorodecalin and mercury experiments performed at the same vapor pressures test the hypothesis that the effluent concentrations can be scaled as a function of contaminant vapor pressure. The hypothesis assumes that for the same molar mass of contaminant in the sand and the same flow rates and vapor pressure, the contaminants should be removed at the same rate; i.e., the time of recovery, the cumulative volume of air needed, and the effluent concentrations should be nearly the same for the perfluorodecalin and mercury. If the cumulative volume is normalized by the molar mass, then the data can be compared as shown in Figure 6a. Comparison of these normalized values shows good agreement between the mercury and perfluorodecalin experiments as well as between perfluorodecalin experiments with different molar masses initially in the sand. For example, the normalized cumulative volume of air is 0.65 L/mmol for experiment Hg3 and 0.67 L/mmol for experiment PFD7. Thus, 2388
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FIGURE 6. (a) Comparison of mercury experiment Hg3 with experiments PFD2 and PFD7 as a function of normalized cumulative volume of air and (b) comparison of mercury experiment Hg3 with simulation. these results support the hypothesis that effluent concentrations were primarily a function of vapor pressure. The soil extractions showed that 99.87% of the mercury was removed from the soil and the overall recovery in the effluent samples, tubing, off-gas treatment system, and condenser was 103%. However, only 30% of the mercury removed from the column was collected in the effluent samples during the experiment, while over 70% was collected in the effluent condenser and tubing after the experiment. Therefore, Figure 6a was normalized by the mercury collected in the samples rather than the total mercury collected. Simulation of Mercury Experiment. The results of mercury experiment Hg3, operated at a soil temperature of 244 °C and an air flow rate of 64 mL/min, were compared to the results of a simulation using STARS. The mercury vapor pressure data shown in Figure 2 was used in these simulations. The mercury distribution shown in Figure 4b provided the best fit to the cumulative removal data from experiment Hg3 (Figure 6b). In this distribution, 150 grid blocks were given 10% DNAPL saturation with three layers each containing 50 blocks adding up to a total DNAPL volume of 1.1 mL. The distribution required to match the mercury experiment is similar to the distribution used to match the perfluorodecalin experiment. The distributions would be expected to be the same if the ratio of the heads to the entry capillary pressures were the same for each fluid. A simple calculation using the data for density and interfacial tension given in Table 1 showed that this ratio was of the same order of magnitude assuming equal contact angles, so perfluorodecalin can be used as a surrogate for mercury. Using perfluorodecalin as a surrogate has very significant advantages since much larger volumes can be used safely in both column and tank experiments.
Acknowledgments The funding for this research is from the royalty income from TerraTherm, which licensed the patented ISTD technology from the University of Texas at Austin after Shell’s donation of the ISTD patents to the University of Texas at Austin. The authors thank George Stegemeier and Harold Vinegar for their pioneering development of ISTD while working at Shell Oil Co. We also thank Computer Modeling Group Ltd. for the use of the STARS simulator.
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Received for review February 22, 2005. Revised manuscript received August 19, 2005. Accepted August 23, 2005. ES0503581 VOL. 40, NO. 7, 2006 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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