Environ. Sci. Technol. 2005, 39, 5787-5795
Redox Processes and Release of Organic Matter after Thermal Treatment of a TCE-Contaminated Aquifer A . K . F R I I S , * ,‡ H . - J . A L B R E C H T S E N , ‡ G. HERON,† AND P. L. BJERG‡ Institute of Environment & Resources, Technical University of Denmark, building 115, Bygningstorvet, DK-2800 Kgs. Lyngby, Denmark
Redox conditions in heated and unheated microcosm experiments were studied to evaluate the effect of thermal remediation treatment on biogeochemical processes in subsurface environments. The results were compared to fieldscale observations from thermal treatments of contaminated sites. Trichloroethene-contaminated aquifer material and groundwater from Ft. Lewis, WA were incubated for 200 days at ambient temperature (i.e., 10 °C) or heated to 100 °C for 10 days and cooled slowly over a period of 150 days to mimic a thermal treatment. Increases of up to 14 mM dissolved organic carbon were observed in the aqueous phase after heating. Redox conditions did generally not change during heating in the laboratory experiment, and only minor changes occurred as an effect of heat treatment in the field. The conditions were slightly manganese/ironreducing in two sediments and possibly sulfate-reducing in the third sediment based on production of up to 0.20 mM dissolved iron and 0.15 mM dissolved manganese and consumption of 0.08 mM sulfate. The calculated energy gain of less than -20 kJ/mol H2 for iron and sulfate reduction as well as methane production indicated that these processes were thermodynamically favorable. Sulfate reduction and methane production occurred in the unheated microcosms upon lactate amendment. Little or no reduction of the redox level was identified in heated lactateamended microcosms, possibly because of limited microbial activity. Because the redox conditions, pH, and alkalinity remained within normal aquifer levels upon heating, bioaugmentation may be feasible for stimulating anaerobic dechlorination in heated samples or in future field applications.
Introduction Contamination of aquifers by chlorinated solvents represents one of the greatest threats to our drinking water supply (1, 2) due to the chemicals’ carcinogenic and mutagenic potential as well as their neurotoxicity (3, 4). For many sites, the risks can only be controlled by effective remediation of the source zones. Thermal treatment has proven to be an efficient in situ remediation technology for chlorinated solvents. The most commonly applied methods are electrical resistive * Corresponding author email:
[email protected]; phone: +45 4525 1600; fax: +45 4593 2850. † TerraTherm, Inc. 10554 Round Mt. Rd. Bakersfield, CA 93308. ‡ Technical University of Denmark. 10.1021/es048322g CCC: $30.25 Published on Web 06/22/2005
2005 American Chemical Society
heating, steam-enhanced extraction, and thermal conductive heating (5). For treatment of volatile contaminants, the subsurface is heated to temperatures of 100-120 °C (6), which increases the contaminants’ mobility and volatility (7, 8) and thereby facilitates extraction of the contaminants. When implemented successfully, thermal remediation removes all nonaqueous phase liquid (NAPL) from the source zone (9). However, residual contamination can remain dissolved in the water or sorbed to the sediment (8). The residues may be removed by less aggressive methods, such as bioremediation. After thermal treatment, the temperature can remain elevated for a period of months or years (10). The elevated temperature can also increase the contaminant solubility and bioavailability (11) as well as the metabolic activity of specific degraders. If the optimal conditions for bioremediation are known, such biopolishing may involve manipulation of the redox conditions and the subsurface temperature. During thermal treatment in aquifers, conditions similar to those in an autoclave with temperatures of 121 °C and 2 atm pressure are reached approximately 10 m below the groundwater table (6). The subsurface can potentially be negatively affected by heating resulting in a decreased microbial activity (10, 12, 13), pathogen blooming (14, 15), or release of metals. However, some subsurface microorganisms are present after such harsh treatment and are able to degrade various hydrocarbons (16). Characterization of post-thermal redox conditions is essential when determining the potential for subsequent degradation of chlorinated ethenes, for example, by reductive dechlorination, the most common degradation pathway (17) in nature. In this process, the chlorinated compounds act as electron acceptors in competition with other electron acceptors (i.e., NO3-, Fe/Mn, SO42-, and fermented organic matter) for the electron donor (commonly H2). During reductive dechlorination, chlorinated ethenes become more reduced with less chloride atoms, and trichloroethene (TCE) to cis-dichloroethene (cDCE) dechlorination can occur under Fe-reducing or more reduced conditions, whereas cDCE to vinyl chloride (VC) dechlorination requires SO42--reducing or more reduced conditions (18, 19). The final step with VC to ethene production commonly occurs at significant rates under methanogenic environments (20); however, ethene production has been observed without methanogenesis (21). Reductive dechlorination is often limited by a lack of sufficient electron donors and/or specific dechlorinating microorganisms. The effect of heating on the abundance and character of electron acceptors and organic matter has not been explored previously and is the first step in determining the potential for anaerobic dechlorination after a thermal treatment. The purposes of this study were to (i) determine the effect of thermal treatment with focus on release of organic matter and redox conditions; (ii) identify the effects of electrondonor (lactate) amendment to unheated and heated samples; and (iii) relate field observations of redox conditions to the laboratory investigation. The effect of heating on contaminant degradation and microbial activity will be discussed in following papers.
Materials and Methods Site Description and Sampling at Ft. Lewis, East Gate Disposal Yard (EGDY), WA. Three source areas have been designated for thermal treatment using electrical resistance heating (ERH). Aquifer material was retrieved from the first treatment area, a source primarily contaminated with TCE, VOL. 39, NO. 15, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 1. Site geology and sampling of aquifer sediment. The figure is not in scale (24).
TABLE 1. Aqueous Concentrations at Start in Bioassaysa O2 NO3Mndiss (mM) Fediss (mM) SO42- (mM) CH4 DOC (mM) pH alkalinity (meq/L) Cl- (mM) a
E07
F12
J10
< < 0.05 0.03 0.09 < 0.41 5.96 4.3 0.10
< < 0.01 0.002 0.08 < 0.48 5.50 2.1 0.07
< < 0.06 0.07 0.08 < 0.44 6.12 5.0 0.12
< indicates less than 0.01 mM.
cDCE, and oils. The samples were collected prior to the initiation of ERH. Aquifer material was collected from three locations: sediment E07 (5.8-6.7 mbs, brown/gray sand with clayey silt), sediment F12 (13.3-15 mbs, brown sandy gravel with silt and cobbles), and sediment J10 (8.3-9.2 mbs, gray sandy silt with sparse cobbles) (Figure 1, Table 1). Cores were collected with a Roto Sonic Drill Rig within an approximately 15-cm polyethylene (PE) sleeve and immediately transferred to a sterilized field-portable glovebox with argon (Ar) atmosphere to avoid contact with O2 and introduction of nonindigenous microorganisms. The outer 2-5 cm of the sediment material was removed, and the pristine sediment was transferred into diffusion-proof aluminum bags with Teflon coating on the inside. The sediment was collected with minimum headspace and sealed with aluminum tape. The collected sediments contained dissolved-phase contaminant and no NAPL, as confirmed by PID and UV fluorescence. Ethanol was detected in the sediments prior to microcosm setup and can either be a cocontaminant from the aquifer or stem from sediment collection in the field. Groundwater was sampled at one location (B09) at 6.3-8.0 mbs using a peristaltic Masterflex 5788
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7529-60 pump with a Phar-med Masterflex tubing 6402-88 and a PE tube lowered into the well. Groundwater was transferred into 1-L presterilized and argon-flushed serum bottles. The bottles were filled to a little over capacity and tightly capped with PVC lids. Both the sediment and groundwater samples were kept at approximately 4 °C up to 6 days (maximally 10 °C for a few hours) during transport to the laboratory. Microcosms were set up within 6 days of sampling. Microcosm Setup and Incubation Conditions. Inside an anaerobic Coy glovebox, microcosms were established by transferring 100 g (wet wt) of the well-mixed aquifer material into 500-mL sterile glass bottles and closed with 1-cm butyl rubber stoppers. The headspace was flushed for 2-3 min with N2-CO2 (always 80-20%) to remove excess H2 from the glovebox atmosphere. Groundwater was sterile filtered with 0.2-µm nitrocellulose filters and purged with N2-CO2 to strip off the remaining O2. Then 0.2 L of groundwater was added to each batch while purging. Subsequently, the flasks were capped and sealed with screw caps. Unheated (two unamended, four lactate-amended, two inhibited controls) and heated (four unamended, six lactateamended, three inhibited controls) microcosms were used for each sediment. For a subset of the microcosms, a lactic acid sodium salt solution (Fluka, 50% in water) was added with sterile syringes after heating, resulting in a concentration of 5 mM lactate. For inhibited controls, HgCl2 was added to a final concentration of 15 mM. The microcosms were kept inverted in a water bath in the dark between sampling. Prior to sampling, the bottles were shaken to mix the gas and water and thereafter laid horizontally for approximately 30 min to obtain a clear aqueous phase. During sampling, microcosms were also kept in water baths or incubators to avoid larger temperature variations. The pressure in the microcosms was sustained by injecting N2 in an equal volume to the sampled gas/water. The aqueous samples were withdrawn from the microcosms using disposable sterile needles (0.60 mm) and syringes. The syringe needle was never inserted twice in the same hole in the stopper. The gases (H2 and CH4) were sampled from the headspace using gastight glass syringes and sterile needles (0.40 mm). The syringes were rinsed three times in N2-CO2 between each sample. Testing showed that there was no effect of preheating the sampling equipment on gas concentrations (data not shown), therefore unheated equipment was used. Incubation Temperature and Cooling History. The incubation temperature and cooling history in the laboratory experiment was selected based on field experiences. In general, the cooling period after a thermal treatment follows an exponential decline (as depicted in Figure 2). Post-thermal temperatures are influenced by the groundwater flow, the properties of the porous media (especially water content), the size of the treated area, and the depth below ground surface. Field experiments have demonstrated that groundwater can remain at 45-75 °C up to two years following heating (10). However, when cooling is applied (e.g., by extracting groundwater or vacuum ventilation) temperatures of approximately 30 °C can be obtained within two to four months (22, 23). A cooling period of five months was selected for the experiments based on the high groundwater flow at the field site (24). The microcosms were heated by linearly increasing the temperature from 10 °C to 100 °C over a 10day period, maintaining it at 100 °C for a 10-day period, and subsequent logarithmic cooling to 10 °C for five months (Figure 2). The temperature was controlled in incubators and monitored using a Grant 1200 Series Squirrel temperature logger every 10th minute. Analytical Methods Dissolved iron (Fe2+) was measured immediately in filtered samples (0.45 µm) using the ferrozine method (25). Dissolved manganese (Mndiss) was analyzed in
FIGURE 2. Experimental setup. Unheated and heated microcosms were either unamended or lactate-amended. filtered samples (0.45 µm) preserved with concentrated HNO3- on a Perkin-Elmer Instruments Analyst 200 Atomic Absorption Spectrometer 5000 with flame detection (279.5 nm) and a detection limit of 0.01 mM. Anions (SO42-, NO3-, Cl-) were analyzed using a DIONEX DX-120 ion chromatograph (26) with a detection limit below 0.003 mM. HS- was analyzed immediately on a Merck Spectroquant Nova 60 with Merck S14799 hydrogen sulfide field test kit following the producer’s instructions. The detection limit was 0.003 mM. Dissolved organic carbon (DOC) was determined using a Shimadzu TOC 5000 analyzer on filtered samples (27). Ethanol was detected by gas chromatography (GC, Agilent 6890N) using a mass selective detector (Agilent 5973). Separation was performed in a 25 m × 320 µm × 1 µm capillary column (J&W GSQ) with helium as a carrier gas. The pH was analyzed within 2 min of sampling using a microelectrode with temperature correction. The alkalinity was determined by Gran titration (28), and the sediment total organic carbon (TOC) was determined using a LECOanalyzer on acidified samples (29). H2 was analyzed using a Trace Analytical RGD2 Reduction Gas Detector GC with two columns in series: a 6′ Carbosieve used as a precolumn with back-flush and a 3′ 13 molieve column. CH4 was analyzed in 0.2-mL samples on a GC Shimadzu GC-14A with a flame ionization detector. The column was a 3% SP1500, Carbopack B 1-m packed column analyzing at 100 °C for 0.9 min with a detection limit of 0.003 mM. The headspace concentrations were converted to aqueous concentrations by using Henry’s law with temperature dependency (30).
Results and Discussion Redox Condition Changes in Unamended Microcosms. Figure 3 shows the concentration of redox-sensitive parameters in unamended samples during the 200-day incubation period. The conditions were generally Mn/Fe reducing and showed little or no change with time. The NO3- concentrations were below 0.1 mM for all of the sediments at all of the sampling points (not shown), and O2 was eliminated during setup as described above. The concentration of Fe2+ and Mndiss increased to maximally 0.10 mM, and the SO42concentration did not decrease in any heated sediments. In unheated sediments, Fe2+ and Mndiss increased to maximally 0.2 mM and SO42- did not decrease except in sediment J10. This indicates that the redox processes were slightly influ-
enced by heating and that the conditions were most reduced in sediment J10. The increase in Fe and Mn concentrations were presumably not a result of dissolution of authigenic or allochtonous minerals because the then expected decrease in the release rate with decreasing temperature was not observed. Observations from field sites indicate that minor changes in redox conditions occur as an effect of heat treatment (Table 2). At Hedehusene, Denmark, where steam was injected for a period of several months, the redox conditions changed from oxic to oxic/NO3- reducing during remediation of a PCE source. However, groundwater was probably also influenced by mixing with tap water from a leaking pipeline, indicated by a SO42- concentration decrease in the presence of NO3-. Therefore, it is not clear whether the change in redox condition was caused solely by the heating. At Cape Canaveral, where a source area was heated by ERH without the injection of electron acceptors, the aquifer remained anaerobic during electrical heating (Table 2). This agrees with the findings in this study for aquifer materials E07 and F12. However, it is opposed to the results from sediment J10 where SO42- reducing conditions were present in unheated samples and Mn/Fe reducing in the heated samples, indicating more microbial activity in the unheated samples. pH and Alkalinity. The pH increased with time and ranged from 5.5 to 6.5 in all of the sediments (Figure 3). Both the pH and the alkalinity decreased in the heated compared to the unheated microcosms for sediments E07 and J10. No effect of heating was observed in sediment F12. Previous studies have reported autoclaving of subsurface sediments resulting in both a pH drop and an increase (31). Heating of hemipelagic sediment demonstrated a pH drop (32, 33) or constant pH (32). It was suggested that dissolution of acidic species (e.g., from released organic matter) during heating result in a pH decrease. This corresponds with our findings of constant or decreasing pH as a result of heating. Release of Organic Carbon. The release of organic carbon from the sediment to the aqueous phase during heating was observed for all three sediments (Figure 4). The concentration of dissolved organic carbon increased by 12.6 mM in E07, 0.84 mM in F12, and 1.63 mM in J10. Unexpectedly, the released organic carbon in sediment E07 was as high as what is commonly added to aquifers during biostimulation (34). The large release of organic carbon in sediment E07 corresponded with a measured fraction of sediment-bound organic carbon of 0.46%. The organic carbon was lower in sediments F12 (0.09%) and J10 (0.06%). The increase of organic carbon in the aqueous phase corresponded to between 1 and 8% of the sediment-bound organic carbon. Data from field sites show similar increases in dissolved organic matter concentrations. The reported increases were the following: 2.5 mM at Hedehusene, 11.7 mM at Alameda Point, and 10.8 mM at Cape Canaveral (Table 2). It is demonstrated that the thermal decomposition of organic matter becomes important at temperatures significantly greater than 60 °C and that the kinetics of this process are strongly temperature-dependent (33). Laboratory studies with heating of hemipelagic sediments to 500 °C and 500 bar also documented the release of organic matter (32, 33), which was identified as complex long-chain fatty acids (32). This corresponds with our laboratory data, in which released organic carbon could not be identified as lactate, acetate, formate, propionate, ethanol, or CH4. The exception was sediment E07, where up to 7% of the released organic carbon was propionate or formate (data not shown). H2 Levels. The concentration of dissolved hydrogen in the groundwater relates to the energy available for redox reactions and can be used as an indicator for determining the dominant redox processes in an environment (35). The concentrations of H2 did not change significantly after heating VOL. 39, NO. 15, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 3. Aqueous concentration of redox-sensitive parameters in unamended samples. All of the data points represent average values from triplicate bioassays by including standard deviations. If no error bars are shown, then the standard deviations were too small to be illustrated, except for SO42-, where only one assay is shown. (Figure 3). However, in the unheated microcosms, the H2 levels decreased with time and reached a nearly constant level after approximately 50 days. This level of 0.1-1 nM H2 corresponds to those in pristine anaerobic aquifer sediments (36). Our data indicates that microbially mediated redox reactions occur in the unheated sediments and not to the same extent or not at all in the heated sediments. The initial H2 concentration in the experiments were approximately 5 orders of magnitude above those commonly found in groundwater environments (36). The excess H2 may stem from borehole construction (37) or from the anaerobic glovebox used during setup, which uses an atmosphere of nitrogen and hydrogen gas (>95/
