Environ. Sci. Technol. 2000, 34, 2557-2563
Removal of Halogenated Organic Compounds in Landfill Gas by Top Covers Containing Zero-Valent Iron CHARLOTTE SCHEUTZ, KLAUS WINTHER, AND PETER KJELDSEN* Department of Environmental Science & Engineering, Building 115, Technical University of Denmark, DK-2800 Lyngby, Denmark
Transformation of gaseous CCl3F and CCl4 by zero-valent iron was studied in systems unsaturated with water under anaerobic conditionssin an N2 gas and in a landfill gas atmosphere. The transformation was studied in batch as well as flow-through column tests. In both systems, the transformation process of the compounds was pseudo-firstorder. Transformation rate constants, referring to the water phase and normalized to 1 m2 iron surface/mL, of up to 1100 mL m-2 h-1 (batch) and 200 mL m-2 h-1 (flowthrough) were found. The transformation was strongly dependent on pH and the presence of oxygen. During continuous aerobic conditions, the transformation of CCl3F decreased toward zero. Model calculations show that use of zero-valent iron in landfill top covers is a potential treatment technology for emission reduction of halogenated trace compounds from landfills.
Introduction Waste deposited in landfills will undergo anaerobic decomposition resulting in generation of landfill gas (LFG). The main components in LFG are methane (55-60 vol %) and carbon dioxide (40-45 vol %) (1). Beside the main components, LFG also contains trace compounds up to a few volume percentages as the maximum (2, 3). Halogenated organic compounds (such as PCE, TCE, and vinyl chloride) are often found in LFG originating from residues of solvents disposed of at the landfill. Typical concentrations are in the range of 1-250 mg m-3 (3). CFCs, especially CCl3F (CFC-11) and CCl2F2 (CFC-12), are also observed (3) and are mainly originating from insulation foams and operating media in refrigerating equipment disposed of at the landfill (4). Typical concentrations are in the range of 10-500 mg m-3 (3). The CFC in insulation foams is released very slowly to the surrounding gas phase due to small diffusivities in the polyurethane foam (5, 6). The LFG produced in landfills is extracted in some cases for utilization or on-site flaring, but even in these cases a significant amount is emitted. The LFG, which is not extracted, is due to pressure and concentration gradients transported through soil top covers of landfills causing emission of trace compounds into the atmosphere. Emission of chlorinated organic compounds such as TCE or vinyl chloride can be a threat to workers and local inhabitants due * Corresponding author present address (until July 2000): North Carolina State University, Department of Civil Engineering, Campus Box 7908, Raleigh, NC 27695-7908; telephone: (919)515-7631; fax: (919)515-7908; e-mail:
[email protected]. 10.1021/es991301f CCC: $19.00 Published on Web 05/05/2000
2000 American Chemical Society
to the toxicity of the compounds (7). The CFCs may contribute to depletion of the ozone layer (8). The U.S. Clean Air Act (9) regulates the emissions of non-methane volatile organic compounds at all landfills of significant size. Several authors have recognized the significance of insulation foams in global CFC balances. Khalil and Rasmussen (10) state that after production of CFC-11 has been terminated (i.e., after 1996), future atmospheric concentrations will mainly depend on the continued release from polyurethane foams. They estimated that of the total mass of CFC-11 that has ever been produced, about 10% is still contained in insulation foam. Due to the limited lifetime of products containing insulation foam, the waste stream is containing considerable amounts of CFCs, which in many countries ends up in landfills. The attenuation of methane in landfill top covers by methane oxidation and ways to enhance the attenuation has been the subject to several studies (11-15). These studies showed that top covers often consist of a lower anaerobic zone where pore gas merely consists of LFG and an upper zone characterized by a pore gas mixture of LFG and atmospheric air diffusing into the top cover from the atmosphere. Typical depths of the aerobic zone are 20-50 cm, leaving an anaerobic depth of 50-80 cm in a 1-m top cover (16). Kjeldsen and co-workers (17) showed that the trace organics, especially aromatic hydrocarbons, potentially are degraded in the aerobic zone. Also the chlorinated compounds were degraded due to co-metabolic reactions in the methanothrophic zone of the top cover. However, the rates of degradation of the chlorinated compounds studied were low. Abiotic transformation (reductive dehalogenation) of halogenated organic compounds by zero-valent iron is a promising technology for remediating polluted groundwater (18). The process is working under anaerobic conditions (18). The abiotic transformation is driven by anaerobic corrosion of the zero-valent iron, which produces electrons used for reducing the halogenated organics (19, 20). The potential for an effective removal of halogenated compounds in LFG while passively emitting through the lower anaerobic zone of top covers containing granular iron thus exists. However, the process has so far not been studied with the objectives of treating gaseous halogenated compounds, nor has the transformation of CFCs been evaluated. The objective of this study was to evaluate the potential of degrading halogenated organic compounds in waterunsaturated landfill top covers containing granular zerovalent iron. The trichlorofluoromethane (CCl3F) is probably the most abundant of the halogenated organic compounds in waste received at municipal landfills due to the high content in insulation foams. Carbon tetrachloride (CCl4) was used as a reference compound since many studies of CCl4 degradation by zero-valent iron have been reported. Transformation kinetics and different governing factors (oxygen, methane and carbon dioxide, water content) influencing the transformation process were examined.
Experimental Section Reagents and Materials. Halogenated solvents were obtained in high purity. A stock solution of CCl3F was prepared by dissolving CCl3F [obtained in methanol (5000 µg mL-1) from Supelco, Bellefonte, PA] in ethylene acetate. To avoid the influence of solvents on the transformation experiments, CCl3F was added to some of the experiments in gas form. CCl4 (>99.8%) was obtained from Merck, Darmstadt, Germany. VOL. 34, NO. 12, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 1. Overview of the Four Series of Batch Experiments series
atmosphere
compounds
iron used
descriptionc
replicates
controls
1 2 3 4
N2 (anaerobic) 80% N2, 20% O2 (aerobic) 60% CH4, 35% CO2, 5% N2 (landfill gas) 80% N2, 20% CO2
CCl3F + CCl4 CCl3F CCl3F
CIF,a MBIFb CIF,a MBIFb MBIFb MBIFb
WS + WUS WUS WUS WUS
2, 6d 3, 4d 3 2
2 2 1 2
a CIF, coarse iron filings. under WUS.
b
MBIF, Master Builder iron filings. c WS, water saturated; WUS, water unsaturated.
The sand was oven-dried quartz with a density of 1.65 g cm-3 and a porosity of 40% (vol). Experiments were conducted using two different types of iron metal: Coarse iron filings (BDH Laboratory Supplies, Poole, England) and MB iron filings [a commercial source of iron (metal cuttings) from Master Builder, Cleveland, OH]. The coarse iron filings vary in particle size from 425 to 850 µm with a surface area of 0.0461 m2 g-1. The MB iron filings vary in particle size from 600 to 2000 µm with a surface area of 1.1 m2 g-1. Batch Experiments. To evaluate the potential of using iron metal in treating gaseous halogenated organic compounds, batch experiments were conducted under waterunsaturated conditions. The batch contained a mixture of iron, sand, water, and gas, simulating the unsaturated zone in a soil profile. As a reference to results reported in the literature (21, 22), tests were also conducted under watersaturated conditions in batches containing iron, water, and sand. The transformation of CCl3F and CCl4 was carried out using coarse iron filings under anaerobic conditions. In total, four series of batch experiments were carried out. Table 1 gives an overview of the batch experiments. The transformation of CCl3F and CCl4 was examined in glass bottles (117 mL in total volume) equipped with Mininert (VICI AG, Schenkon, Switzerland) sampling valves made of Teflon. The valves enabled gas to be injected or sampled by a hypodermic needle and a syringe. The transformation of CCl4 and CCl3F was examined under water-saturated and water-unsaturated conditions (batch series 1). The watersaturated experiments were conducted in a batch containing 37 g of water and 1 g of iron. In general, the water-unsaturated conditions were achieved by adding 22.4 g of sand, 3.7 g of water, and 1 g of iron. The batch containers were flushed with N2 to obtain anaerobic conditions. The water was deoxygenated by sparging the water with N2 gas. After the batches were sealed, the specific compound to be degraded was added with a syringe. The total amount of CCl3F and CCl4 added to each batch container varied between 2 and 6 µg. Between sampling, the bottles were placed in darkness in a rotary box (1 min-1). For analysis of the test compounds, gas samples were taken and analyzed by gas chromatography by direct gas injection. Control batches without iron were run in parallel with the transformation experiments. Each batch was set up in no less than duplicate (confer Table 1). At the end of the batch experiments, pH was measured. In unsaturated batch experiments, measurements of pH were conducted by adding a fixed amount of deoxygenated distilled water containing NaCl to stabilize the pH measurements. While measuring pH, the inlet of the batch was gently flushed with N2 to avoid exchange of gases. Transformation of CCl3F was also studied under aerobic conditions using coarse iron filing and MB iron filing (batch series 2). Information on replication and controls is given in Table 1. In the aerobic batch experiments, the atmosphere consisted of atmospheric air. The transformation was also examined under conditions resembling landfill gas where the gas phase consisted of approximately 35% CO2, 60% CH4, and 5% N2 (batch series 3) and also in a 80% N2/20% CO2 atmosphere (batch series 4) to further study the iron corrosion process under the presence of CO2. In the latter series, 2558
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d
Experiments with CCl3F and CIF
degradation of CCl3F was not studied. Techniques similar to the above-mentioned were used in the latter experiments. Information on replication and controls is given in Table 1. Column Experiments. Column experiments were carried out with CCl3F in order to examine the transformation process in a dynamic system. The column experiment was conducted to simulate a landfill top cover. Gas containing CCl3F was pumped through a column containing a quartz sand/iron/ water mixture. Changing the conditions in the column from anaerobic to aerobic for a period of 6 days simulated air intrusion in a landfill top cover due to an increase in barometric pressure. Thereafter the column was again made anaerobic. The column was made of rigid PVC, 100 cm long by 8 cm (i.d.). Sampling ports were located along the column length at intervals of 10 cm with the first port positioned 5 cm from the inlet. The sample ports were equipped with Teflon-coated silicone septum, which enabled gas samples to be taken with a syringe needle. A total of 3 mL was drawn from the column at each sampling. The test compound was extracted in 500 µL of hexane before analysis on a gas chromatograph. To measure the main components (O2, CO2, CH4) in the gas, 3 mL of gas was transferred to vacuumed serum glasses (Venoject, Terumo Europe n.v., Belgium) and analyzed by gas chromatography. Between the two sampling rounds, a period of minimum one residence time elapsed. The column packed with coarse iron filings contained a mixture of sand, iron, and water in a 22.4/1/2.6 g ratio, equivalent to approximately 4% (w/w) iron filings and 6% (vol) water. The column packed with MB iron filings contained sand, iron, and water in a 22.4/5/2.6 g ratio, equivalent to approximately 22% (w/w) iron filings and 6% (vol) water. To ensure anaerobic conditions, the column was flushed with several pore volumes of N2 gas before adding the halogenated organic compound to the gas. The gas containing CCl3F was kept in a Tedlar bag (SKC Inc., Eighty Four, PA) equipped with a sampling port and fed to the inlet of the column by a gas-tight piston pump (FMI Lab Pump, model QG, Fluid Metering Inc., Syosset, NY). The initial gas concentration was approximately 25 µg L-1, which is within the normal concentration range found in LFG. The flow through the column was measured at the outlet and varied between 0.1 and 0.8 m3 m-2 d-1. Transformation of CCl3F was studied under both anaerobic and aerobic conditions. Under anaerobic conditions the gas consisted of N2, while under aerobic conditions the gas was atmospheric air. A control column without iron metal was run in parallel. Analyses. The transformation kinetics of CCl3F and CCl4 were determined by periodic sampling of the gas phase and analysis by gas chromatography. Samples (20-100 µL) were injected for analysis via an on-column inlet to a HP 5890 series II gas chromatograph equipped with a HP 19233 electron capture detector. The gas chromatograph contained two columns connected to the same detector through a glass T-gate. Samples containing CCl3F were injected on a (encapsulated) PLOT fused-silica (capillary) column (25 m × 0.53 mm CP-PoraPLOT Q) with an isothermal oven temperature at 170 °C. Samples containing CCl4 were injected on a CB5 Poraplot (25 m × 0.53 mm) with an isothermal
oven temperature at 120 °C. Analysis for oxygen, carbon dioxide, and methane were performed on a transportable CP-2002 P Chrompack micro gas chromatograph (Chrompack International BV, EA Middelburg, The Netherlands) equipped with a thermal conductivity detector. The gas chromatograph contained two parallel columns: a Molsieve 5A (4 m) (Snr. DG 175) and a Poraplot Q (10 m) (Snr. DG 207). Calibration standards were prepared by adding a specific mass of CCl3F and CCl4 to a known volume of pentane. Calibration curves were prepared from standards at no fewer than four concentration levels. The standard curves were linear within the concentration range of interest. Sample concentration was determined by comparison with the standard curve. The detection limit for CCl3F and CCl4 was 1.2 × 10-2 and 6.3 × 10-2 µg/L, respectively, by injection of 100 µL of gas. Iron surface area was determined by gas adsorption (BET analysis) using a standard procedure (23). The pH measurements were conducted using a HI 1940B electrode. Data Evaluation. From the measured gas concentration, the total amount (µg) of test compound in the batch was determined by phase distribution calculations. Experiments (results not shown) showed that sorption to sand and iron of the halogenated compounds tested was negligible, meaning that the phase distribution calculations only include the air and water phases. Assuming steady state, the total concentration of the organic compound can be expressed as a function of the measured gas concentration using Henry’s law. The dimensionless Henrys law constant, KH, used in calculations was 4.534 and 1.007 for CCl3F and CCl4, respectively (24). The kinetics of transformation were examined by plotting the total concentration of halogenated compound versus time. The transformation of CCl4 and CCl3F is characterized by a pseudo-first-order reaction with respect to the total amount of the halogenated compound and is then described by a first-order rate constant, k, and a half-life constant, t1/2. The rate constant, k, obtained from the pseudo-first order model describes the total disappearance of halogenated compound in the system. For transformation, the halogenated compound must diffuse into the water phase; therefore, the rate constant for the pseudo-first-order reaction is modified to a first-order rate constant referring only to the water phase, kwater:
(
)
KHg kwater ) k 1 + w
(1)
where g is the gas-filled porosity and w is the water-filled porosity. The reduction of halogenated compounds in an iron/ water system is first-order in respect not only to the concentration of the halogenated compound but also to the concentration of iron surface, which can serve as a reductant (25). To compare results obtained from the different systems in this study and to compare them with studies reported in the literature, the rate constant for the water phase is normalized to 1 m2 of iron surface/mL of water, kN:
kN )
kwater as Fm
(2)
where as is the specific surface area of iron and Fm is the mass concentration of iron. In the column experiments, transformation rates were calculated daily by fitting measured CCl3F concentration profiles through the column (concentration versus depth) with a model expressing the total concentration of halogen-
FIGURE 1. CCl3F concentrations relative to initial concentration in batches using coarse iron filings under water-saturated (a) and unsaturated conditions (b). b, control; 9, iron containing. ated organic compound as a function of the distance x from the gas inlet to the column:
(
CT(x) ) CT,0 exp -
κ x Rg vg
)
(3)
where
w + g KH
(4)
g k KH water
(5)
Rg ) κ)
and CT,0 is the total concentration of halogenated compound at the inlet to the iron-containing zone and vg is the pore gas velocity in the column.
Results and Discussion Transformation Kinetics under Water-Unsaturated Conditions. Transformation of CCl3F and CCl4 was observed in water-unsaturated as well as water-saturated batches. The transformation of the halogenated organic compound followed first-order kinetic. Figure 1 shows the transformation of CCl3F under water-saturated and -unsaturated conditions. Table 2 shows the first-order rate constants together with the regression coefficients (r 2) obtained from fitting the experimental data with a first-order model equation. For both compounds, normalization of the transformation rates shows that transformation is faster for the unsaturated systems. For experiments carried out with CCl4, the normalized transformation rate appears to be approximately 8 times lower in the water-saturated systems, probably due to transport limitation caused by slower diffusion of the organic compounds in water as compared to air. VOL. 34, NO. 12, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 2. Rate Constants for Transformation of CCl3F and CCl4 Using Coarse Iron Filings under Water-Saturated and Water-Unsaturated Conditions compd
test conditions
k a (h-1)
kwater (h-1)
t1/2water (h)
kN,water (mL m-2 h-1)
t1/2N,water (h)
r2b
CCl3F CCl3F CCl4 CCl4
saturated unsaturated saturated unsaturated
0.003 (0.003, 0.003) 0.023 (0.028, 0.029, 0.021, 0.020, 0.012, 0.030) 0.262 (0.219, 0.305) 0.256 (0.191, 0.321)
0.032 2.57 0.802 6.626
21.8 0.30 0.86 0.11
10.3 833 130 1075
0.067 0.005 0.005 0.0006
0.90-0.96 0.97-0.99 0.90-0.91 0.93-0.97
a The numbers in parentheses are the actual calculated rate for each replicate. decay equation to the experimental data.
FIGURE 2. CCl3F and O2 concentrations relative to initial concentration in batches using coarse iron filing under unsaturated and (initially) aerobic conditions. b, oxygen; 9, CCl3F. For the experiments with CCl3F, the difference in the normalized transformation rates for the two systems approximates 80. Besides transport limitation in the saturated systems, the difference is believed to rely on the fact that these batches contain ethylene acetate, which in aqueous solution acts as a buffer toward a rise in pH. Studies have shown that rates are increasing with decreasing pH (20). The concentration of ethylene acetate in the water phase is 10 times higher in the unsaturated systems leading to a greater buffer capacity of this system, accounting for the higher transformation rates observed in the unsaturated systems. For both systems, the control batches showed no decrease in concentration over the experimental time. Gillham and O’Hannesin (21) examined the transformation of CCl4 in water-filled 40-mL hypovials containing 10 g of iron powder and found half-lives (t1/2,water) of 0.25 h. Matheson and Tratnyek (22) report half-lives of 0.26 h for transformation of CCl4 in 60-mL serum bottles containing 10 g of iron powder and no headspace. In this study, the half-life (t1/2,water) for transformation of CCl4 was found to be 0.86 h under water-saturated conditions. Considering the different types of iron used, the transformation rates obtained in this study are in good agreement with the half-life reported in the literature. Transformation of CCl3F under Aerobic Conditions. CCl3F was transformed in all batches, and the transformation approximated first-order kinetics. Transformation of CCl3F occurred simultaneously with aerobic iron corrosion. Figure 2 shows concentration profiles of O2 and CCl3F resulting from one of the aerobic tests. Some of the concentration profiles showed a tendency of a lag period of 20-40 h. The transformation of CCl3F occurred at a rate almost similar to or higher than parallel experiments carried out under anaerobic conditions. The results do not indicate a competition between CCl3F and oxygen for reactive sites on the iron surface. Table 3 contains the rate constants obtained from the experiments. For comparison, the results from the anaerobic batch experiments are also shown in Table 3. Our study agrees with the results of Gotpagar and coworkers (26), who studied the transformation of trichloro2560
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b
Regression coefficients (r 2) obtained from fitting the first-order
ethylene (TCE) using zero-valent iron in the presence of dissolved oxygen in 40-mL hypovials filled with TCE solution. Their conclusion was that the rate of dechlorination of TCE was not affected by the presence of dissolved oxygen in the solution. However, our study does not agree with the results by Helland and co-workers (27), who studied reductive dechlorination of carbon tetrachloride with iron powder. They reported that dechlorination occurred under oxic conditions (1 g of Fe0 in 265 mL of solution, oxygen concentration of 7.4 mg/L), although the rates were significantly slower as compared to anoxic systems. Helland and co-workers (27) concluded that the slower rate under oxic condition was due to a competition between CCl4 and oxygen for electrons from iron. In the aerobic batch tests, pH was measured to about 8.7 at the end of the experiments. At this pH level one must expect precipitation of different iron hydroxides, which will keep the pH constant. Precipitation of iron hydroxides was observed within the first two days of the experiments, as the color of the iron/sand mixture changed to reddish brown. The corrosion of iron results in production of aqueous ferrous iron and hydrogen gas, which both are potential reductants for reductive dehalogenation and thereby could affect the transformation rate. In the presence of a suitable catalyst, such as impurities or inhomogenities in the iron surface or other solid phases present in solution, hydrogen gas might cause reductive dehalogenation (20). Iron-free batch experiments containing Fe2+ and CCl3F showed no transformation of CCl3F within 5 days (results not shown), which is in agreement with Helland and co-workers (27), who reported no significant removal of CCl4 in iron-free batch tests containing H2 and Fe2+ within 3 days. The high transformation rate observed in the aerobic systems may be a result of complex formation with both Fe2+ and Fe3+ in solid or solute phases that have a dramatic effect on their redox potentials creating more reducing species (28) as well as an increase in surface area or creation of new surface reaction sites due to pitting of the iron surface (22, 27). Transformation of CCl3F in LFG Environment. LFG contains mainly CH4 and CO2, which may influence the dehalogenation process of halogenated trace compounds with iron in a landfill top cover. The possible influence of CO2 and CH4 on the dehalogenation process of CCl3F was investigated in a batch experiment with MB iron filing in an atmosphere resembling landfill gas (35% CO2, 60% CH4 and 5% N2). Figure 3 shows the transformation curve for the experiment. As shown in Figure 3, transformation of CCl3F approximates first-order kinetics very well. Control batches, not containing MB iron filing, did not show any decrease in CCl3F concentration during the experiment. Characteristic parameters for the first-order transformation are shown in Table 4. Comparing the results shown in Table 4 for transformation of CCl3F in an atmosphere of artificial landfill gas with the results obtained from transformation of CCl3F in an N2
TABLE 3. Rate Constants for Transformation of CCl3F by Coarse Iron Filings (1 g) and MB Iron Filings (1 g) under Anaerobic and Aerobic Conditions iron type
atmosphere
k (h-1)a
kwater (h-1)
kN,water (h-1)
r 2b
coarse iron filing
anaerobic aerobic anaerobic aerobic
0.023 (0.028, 0.029, 0.021, 0.020, 0.012, 0.030) 0.025 (0.022, 0.018, 0.030, 0.030) 0.004 (0.003, 0.005, 0.006) 0.017 (0.016, 0.018, 0.017)
2.566 2.868 0.535 1.917
833 931 1.79 6.45
0.90-0.96 0.79-0.95 0.90-0.97 0.98-0.99
MB iron filing
a The numbers in parentheses are the actual calculated rate for each replicate. decay equation to the experimental data.
b
Regression coefficients (r 2) obtained from fitting the first-order
TABLE 5. Initial and Final Gas Composition in Batch Experiments under CH4/CO2/N2 and CO2/N2 Atmosphere gas concn (% vol) initial atmosphere
CH4
CO2
N2
final O2
CH4
CO2
N2
O2
CH4/CO2/N2a
FIGURE 3. CCl3F concentrations relative to initial concentration in batches with MB iron filings under water-unsaturated conditions in an atmosphere resembling landfill gas. b, control; 9, iron containing.
control batch 1 batch 2 batch 3
59 59 61 61
37 34 35 36
3 6 3 3
0.4 0.6 0.15 0.15
53 46 45 56
31 0.03 0.01 0.01
14 47 51 32
3 0.6 0.8 0.5
control control batch 1 batch 2
∼0 ∼0 ∼0 ∼0
19 19 19 19
83 83 84 83
CO2/N2b 0.08 0.1 0.02 0.10
∼0 ∼0 ∼0 ∼0
15 14 0.08 0.04
82 82 101 101
4 3 0.6 1
a
Final composition after 20 days.
b
Final composition after 1 day.
TABLE 4. Parameters for First-Order Transformation of CCl3F with MB Iron Filings (5 g) in an Atmosphere Resembling Landfill Gas atmosphere
k (h-1)a
kwater kN,water (h-1) (h-1)
CH4, CO2, N2 0.021 (0.023, 0.024, 0.016) 2.34
1.57
r 2b 0.98-0.99
a
The numbers in parentheses are the actual calculated rate for each replicate. b Regression coefficients (r 2) obtained from fitting the firstorder decay equation to the experimental data.
atmosphere reveals that the transformation proceeds at a similar rate under both conditions. The batches containing CH4 and CO2 are not buffered by ethylene acetate, which means that lower transformation rates are to be expected. Thus, the transformation of CCl3F appears to be enhanced by the presence of CO2 and CH4. The increased transformation rate is probably a result of dissolved CO2 in the aqueous phase increasing the buffer capacity. During the experiment, a decrease in CO2 concentration in the iron-containing batches was actually observed as shown in Table 5. At the end of the experiment, the concentration of CO2 thus equaled zero in the active batches, whereas the concentration in the controls was only reduced by approximately 15%, probably solely due to exchange of the water phase. Another study has shown that CO2 was transformed on the surfaces of zerovalent iron by a reaction similar to a Fischer-Tropsch synthesis (29). The batch experiments with zero-valent iron in an N2/CO2 atmosphere showed that the gas phase in the batches consisted of 100% N2 after 1 day (refer to Table 5). This indicated that the CO2 is transformed to a nongaseous product by the anaerobic iron corrosion. However in the batches containing N2/CH4/CO2, the gas composition at the end of the experiment did not add up to 100%, indicating a production of a gaseous product. The process of iron corrosion in LFG-affected environments needs more investigation. Transformation under Dynamic Situation. The column experiments showed that CCl3F was degraded in the presence
FIGURE 4. Relative total concentration versus distance along the column containing coarse iron filings (9) and the control column (b) for a representative day. The measured data for the coarse iron column is fitted using a transformation model (eq 3). of zero-valent iron under anaerobic conditions. The transformation followed first-order kinetics (r 2 > 0.92 in all cases). Figure 4 shows a representative transformation profile for the first anaerobic period for the column containing coarse iron filings. In the same figure are also shown results from the control column where no loss of CCl3F was observed. To give an overview of the column experiment, the first-order transformation constants obtained from fitting the experimental data are shown in Figure 5 for each day through the experiment for both columns. During the experiment, a tendency of a decline in the rate constants was observed within the first anaerobic period for both columns. As the water phase in the columns is not replaced during the experiment, one can expect accumulation of reaction products, increasing pH, inactivation of the iron surface, or other changes in the reaction conditions. In a field situation, the water phase will be replaced by water infiltrating the top cover. The first day after changing the column to aerobic conditions, CCl3F was degraded in both columns despite the VOL. 34, NO. 12, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 5. Calculated rate constants for transformation of CCl3F using coarse iron filing (a) and MB iron filings (b) as a function of experimental time. presence of oxygen in the column. Analogue to the batch experiments, transformation of CCl3F, and aerobic iron corrosion occurred simultaneously. During the aerobic period, a reduction in the ability of iron metal to reduce oxygen was observed. However, during the rest of the aerobic period, no reduction in CCl3F concentration was observed in any column, which indicates that CCl3F is not degraded under continuous aerobic conditions. In a transition period from anaerobic to aerobic conditions or in shorter periods with low oxygen concentration, transformation of CCl3F is however still possible. During the aerobic period, the iron surface in the columns was visibly rusted in the whole length of the column. When changing the conditions from aerobic back to anaerobic, a small increase in the transformation rate was observed for the column containing coarse iron filings as shown in Figure 5. This might be explained by an increase in surface area or the creation of new reactive sites due to corrosion of the iron surface. However, for the column containing MB iron filings, the transformation never came back after the return of anaerobic conditions, which was expected based on the results from the batch experiments. The reason for these contradicting results has not been found. The average transformation rate constant for the first anaerobic period normalized to 1 m2/mL is 67 mL m-2 h-1. The normalized average transformation rate constant obtained in batch experiments conducted with coarse iron filings under anaerobic conditions is 833 mL m-2 h-1 (see Table 3). In the batch experiment, the CFC was used in the ethylene acetate matrix giving lower pH values and, by this, higher transformation rates. Practical Aspects. The first-order transformation model (eq 3) is used for describing steady-state conditions in a landfill top cover in order to evaluate the potential for using zero-valent iron in a treatment technology toward emission of halogenated organic trace compounds from landfills. The 2562
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TABLE 6. Calculation of Required Iron for 90% Removal of CCl3F under Transport through Top Cover system
column
(h-1)
72a
kN,water flow (m3 m-2 yr-1) iron (kg m-2)
6 14
batch 833b
60 140
6 1.2
60 12
a Normalized transformation constant for the column experiment carried out under nonbuffered conditions b Normalized transformation constant for the batch experiment buffered by ethyl acetate.
transformation rates obtained in the batch experiment and the column experiment are used in the calculations. It is assumed that a sandy loam is used as a natural top cover with typical values for total porosity of 40% (vol) and water content at field capacity [30% (vol)], respectively (30). Gas fluxes of 6-60 m3 yr-1 m-2 resembling a specific LFG production rate of 0.3-3 m3 of LFG (m of landfilled waste)-3 year-1 for a landfill with an average waste height of 20 m is assumed. The specific LFG production rates used are typical for landfills in the stable methane production phase and are based on data from a high number of LFG utilization projects (31). Table 6 shows the required iron content of the top cover for a 90% reduction of the CCl3F concentration in landfill gas passing through the top cover. It is observed that the calculated iron requirements are much lower using the batch data where the pH of the system was controlled by the presence of ethyl acetate. For the low flow rate of 6 m of LFG3 m-2 yr-1, the iron required is 1.2 kg m-2 using batch data, which seems to be a realistic amount. For the high LFG flow rate, however, considerable amounts of iron are needed. The reductions in Table 6 are achieved for anaerobic conditions where the halogenated trace compounds are present in an inert atmosphere, which is not the case in top
covers. The presence of a wide variety of compounds in landfill gas and the changing redox conditions in top covers will presumably effect the transformation rates. CO2 however seems to have a positive effect on the transformation rate apparently through buffering of the water phase. Likewise, the activity of the iron seems, at least in some cases, to be resistant to periodical oxygen intrusion as could be the case during periods with increased barometric pressure. Other circumstances such as the dynamics in top covers (infiltration/evaporation of water) will potentially influence the transformation rates as well as biotic processes (including anaerobic processes, co-metabolistic, methanothrophic processes, and aerobic microbial processes). Some evidence exist in the literature that CFCs are degradabale under anaerobic conditions (32, 33). The rates seem, however, to be much lower than the abiotic transformation rates with zero-valent iron obtained in this study. According to our knowledge, no evidence exists that CFCs are degradable under methanotrophic conditions. On the basis of the findings, the use of iron in landfill top covers seems to be a promising treatment technology for reduction of emission of halogenated trace compounds from landfills. For phases where the gas production is high, the costs for constructing an iron barrier over the entire surface area of the landfill is probably too costly. In that situation, extraction of the LFG with following energy utilization in combustion engines, where the CFCs will be destroyed, is probably a better solution. The iron barrier technology may however be attractive when LFG utilization is not economically feasible. Here treatment reactors with a limited surface area and volume may be constructed over the gas extraction wells. Gas-tight composite top covers, which in the United States often are in place, will effectively impede gas emission through surface areas outside the reactor area. The reactors could be relying on zero-valent iron processes and natural biological processes in combination. Due to the expected slow release of CFC-11 from the waste, significant concentrations of CFC-11 are expected in the LFG after utilization becomes economical infeasible because of too low methane production rates. By constraining the emission of LFG to a limited area, anaerobic conditions in the lower part of the reactor can be maintained. Emission reduction of the methane may be obtained in the upper part of the reactor due to methane oxidation driven by oxygen diffusing in from the atmosphere. These reactors may be a cost-effective alternative to LFG flaring. However, this study is the first of its kind, and more experiments addressing daughter product formation, longterm aspects, and use of alternative iron sources such as scrap iron received as waste on the landfill or as the ferrous fraction of MSW incineration residues need to be performed to verify the true potential of this treatment technology. Also pilot-scale studies are to be performed.
Acknowledgments We thank Stephanie O’Hannesin, EnvironMetal Inc., Guelph, Canada, for helping us out with the analysis of specific surfaces of the iron materials used in this research.
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Received for review November 18, 1999. Revised manuscript received March 17, 2000. Accepted March 29, 2000. ES991301F
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