Environ. Sci. Technol. 2008, 42, 8908–8915
Implementing Heterogeneous Catalytic Dechlorination Technology for Remediating TCE-Contaminated Groundwater MATTHEW G. DAVIE,† HEFA CHENG,† GARY D. HOPKINS,† CARMEN A. LEBRON,‡ AND M A R T I N R E I N H A R D * ,† Department of Civil and Environmental Engineering, Stanford University, Stanford, California 94305-4020, and Naval Facilities Engineering Command, Engineering Service Center, 1100 23rd Avenue ESC-411, Port Hueneme, California 93043
Received May 29, 2008. Revised manuscript received September 7, 2008. Accepted September 10, 2008.
To transition catalytic reductive dechlorination (CRD) into practice, it is necessary to demonstrate the effectiveness, robustness, and economic competitiveness of CRD-based treatment systems. A CRD system scaled up from previous laboratory studies was tested for remediating groundwater contaminated with 500-1200 µg L-1 trichloroethylene (TCE) at Edwards Air Force Base (AFB), California. Groundwater was pumped from a treatment well at 2 gal min-1, amended with hydrogen to 0.35 mg L-1 and contacted for 2.3 min with 20 kg eggshell-coated Pd on alumina beads (2% Pd by wt) packed in a fixed-bed reactor, and then returned to the aquifer. Operation was continuous for 23 h followed a 1 h regeneration cycle. After regeneration, TCE removal was 99.8% for 4 to 9 h and then declined to 98.3% due to catalyst deactivation. The observed catalyst deactivation was tentatively attributed to formation of sulfidic compounds; modeling of catalyst deactivation kinetics suggests the presence of sulfidic species equivalent to 2-4 mg L-1 hydrogen sulfide in the reactor water. Over the more than 100 day demonstration period, TCE concentrations in the treated groundwater were reduced by >99% to an average concentration of 4.1 µg L-1. The results demonstrate CRD as a viable treatment alternative technically and economically competitive with activated carbon adsorption and other conventional physicochemical treatment technologies.
Introduction Chlorinated solvents, especially chlorinated ethylenes, occur widely in contaminated groundwater aquifers in the United States (1-3). There is a significant need for treatment technologies that destroy chlorinated solvents dissolved in groundwater without producing harmful byproducts or generating hazardous secondary waste streams. Catalytic reductive dechlorination (CRD) of chlorinated ethyleness including tetrachloroethylene (PCE), trichloroethylene (TCE), dichloroethylene (DCE) and vinyl chloride (VC)susing supported palladium (Pd) as the catalyst and dissolved hydrogen * Corresponding author phone: 650-723-0308; fax: 650-723-7058; e-mail:
[email protected]. † Stanford University. ‡ Naval Facilities Engineering Command. 8908
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as the reductant is facile (4-16). Pd catalyzes the substitution of chlorine with hydrogen atoms and saturation of the double bond, forming ethane and hydrochloric acid (reaction a, Table 1) (5, 7). Given excess hydrogen, the reduction rate is typically directly proportional to the amount of Pd present, and the substrate removal follows pseudo-first-order kinetics. For TCE hydrodechlorination, the rate law can be described as dCTCE ) -k1[Pd]CTCE dt
(1)
where CTCE represents the aqueous TCE concentration, [Pd] is the concentration of active Pd (gsurface Pd Lwater-1) and k1 is the observed pseudo-first-order reaction rate constant (Lwater gsurface Pd-1 min-1). The activation energy for dihydrogen chemisorption is close to zero, i.e., monomolecular hydrogen formation at the surface of Pd is essentially instantaneous (9). In the presence of excess hydrogen (e.g., 10 times the stoichiometric amount required for complete TCE reduction, which is the case in this study), hydrogen concentration variation through the course of the reaction is negligible and does not affect the reaction rate. Under such conditions, the formation of ethane is quantitative, while the formation of dechlorinated byproduct, such as vinyl chloride, is insignificant (7-9, 13, 14, 16-18). Although the research effort to understand the mechanism of CRD is steadily expanding (10) and its technological potential is recognized, transitioning the process into practice requires the documentation of implementation issues and analysis of the economic competitiveness of the process. Catalyst fouling, biological growth control, and regeneration have been studied at the laboratory scale (8, 10, 12, 13, 19). However, to date only a few successful field-scale CRD applications have been documented (14, 16). A potential issue is the presence of sulfide and sulfur compounds, which can poison the catalyst (8, 13, 14, 16). Sulfate-reducing bacteria utilize sulfate as the electron acceptor, producing sulfide: SO24 + 4H2 f HS + OH + 3H2O
(2)
Therefore, design and operation of CRD reactors must take into consideration the possibility of fouling by sulfatereducing bacteria. Munakata and Reinhard (12) studied the deactivation of Pd catalyst by sulfide and developed a mathematical model to describe catalyst deactivation kinetics caused by sulfide. Based on the reaction schemes summarized in Table 1, the concentration of active Pd ([Pd]) at a given time (t) can be written as (12) [Pd] )
k3[OH-]p k2a[H2S]m + k2b[HS-]n - k3[OH-]p k3[OH-]p
k2a[H2S]m + k2b[HS-]n - k3[OH-]p
)
(
PdT + Pd0 -
PdT exp[-(k2a[H2S]m +
k2b[HS-]n - k3[OH-]p)t] (3) where PdT and Pd0 are the total surface Pd concentration and the initial level of [Pd], respectively (Pd0 ) PdT after fully regenerating the catalyst activity). The empirical reaction rate coefficients (with forward rate coefficients k2a and k2b and reverse rate coefficient k3) and exponents (m for H2S, n for HS-, and p for OH-) derived from laboratory column experiments (12) are summarized in Table 1. The concentrations of H2S, HS-, and OH- are functions of the total sulfide species concentration and solution pH. 10.1021/es8014919 CCC: $40.75
2008 American Chemical Society
Published on Web 11/06/2008
TABLE 1. Values of Reaction Rate Constants and Stoichiometric Coefficients for Catalyst Deactivation Derived from Laboratory Column Experiments (Adapted from Ref 12) reaction
parameter
optimal value
parameter range
unit
(a)
k1
0.10
0.09–0.11
Lwater gsurface Pd–1 min–1
(b)
k2a
12.4
10.2–14.7
mMH2S–m d–1
(c)
k2b
7.5
5.5–9.1
mMHS-–n d–1
(d)
k3
0.13
0.11–0.14
mMOH-–p d–1
m n p
0.69 0.68 0.19
0.64–0.73 0.59–0.74 0.16–0.21
– – –
Pd ⁄ k1
Cl2C ) CHCl + 4H2 f H3C - CH3 + 3HCl
k2a
Pd + aH2S f Pd * Sa + aH2(aq)
k2b
– Pd + bHS(aq) + bH2O f Pd * Sb + – bH2(aq) + bOH(aq)
k3
– Pd * Sb + bH2(aq) + bOH(aq) f Pd + – + bH2O bHS(aq)
TABLE 2. Operational Conditions for Laboratory and Edwards AFB Reactor Columns parameter diameter
laboratorya
1.5 cm (outer) 1.2 cm (inner) length 9.8 cm column gross volume 10.5 mL catalyst used 0.5% Pd by wt net mass of catalyst 1.0 g flowrate 0.5 mL min-1 residence time 1.75 min influent TCE 1000 µg L-1 concentration effluent TCE 400 µg L-1 concentration TCE removal 60% a
Edwards AFB 15 cm (outer) 14 cm (inner) 3.05 m 25 L 2% Pd by wt 20 kg 7.6 L min-1 (2 gal min-1) 2.3 min 500-1200 µg L-1 5 µg L-1 99.2%
Taken from ref 13.
Laboratory and field studies also showed that oxidizing agents such as air, hypochlorite, and hydrogen peroxide can be used to act as regenerants and biocides, reversing or preventing catalyst deactivation (8, 14, 16, 19). Hypochlorite was found to be most practical and effective for regenerating the activity of deactivated Pd catalyst although Schu ¨ th et al. (16) showed hydrogen peroxide is also applicable. Exposure of the deactivated Pd catalyst to chlorinated water results in oxidation of the surface sulfide to sulfate and Pd to Pd(II) (8, 13, 19). To restore catalyst activity, the oxidized surface Pd(II) must be subsequently reduced to Pd(0) by flushing the column with hydrogen-containing water (13). Transitioning CRD from laboratory to practice requires demonstrating that treatment objectives can be met economically, safely, and reliably for extended periods of time. To demonstrate the feasibility and economic viability of the technology, a CRD reactor was installed at a groundwater site contaminated with approximately 500-1200 µg L-1 TCE. To allow as much as possible for unattended operation, the reactor was equipped with an automated catalyst regeneration and fouling control system. Performance and cost analysis demonstrate that Pd catalysis can be competitive with conventional remediation technologies for TCE removal.
Site Characterization The demonstration was executed at Edwards Air Force Base (AFB), located in the Mojave Desert of Southern California. This site was selected because TCE was the only contaminant of concern and amenable to complete remediation by hydrodechlorination. Additionally, site infrastructure and characterization were available from a previous study (20). The aquifer materials consisted mostly of unconsolidated alluvial sediments (fine to medium sand with some silt) overlying granitic bedrock; the fraction of organic carbon was low, about 0.01-0.4%. The groundwater at the site was about 9 m below ground surface, hydraulic conductivities ranged from 1.5 to 5.5 × 10-3 cm s-1 with an average of 3.4 × 10-3 cm s-1 (20). TCE concentrations in the groundwater varied between 500 and 1,200 µg L-1, with an average concentration of approximately 750 µg L-1. The pH of the groundwater was around 7.6, and sulfate concentration was as high as 710 mg L-1 (20). The groundwater at the site was borderline anoxic with 29 ng L-1).
Results and Discussion 1. Optimization of Catalyst Regeneration. The conditions of catalyst bleaching and reactivation were optimized to maximize throughput while maintaining the catalyst activity and meeting treatment goals. A series of tests varying bleaching and reactivation periods and hydrogen gas flow rate was conducted to determine the most effective regeneration protocol. Figure 2 shows the percentage of catalyst activity recovered under different regeneration schemes calculated from the TCE removal immediately following catalyst regeneration. The high hydrogen demand for regeneration was tentatively attributed to residual hypochlorite solution. As shown, 35 min of bleaching followed by 20 min reactivation with 30 mL min-1 hydrogen flow yielded no significant recovery in catalyst activity. Increasing the reactivation period and the hydrogen flow improved recovery of catalyst activity. Partial recovery was observed by increasing the reactivation period to 30 min and the hydrogen flow rate to 60 mL min-1, and 100% activity recovery was achieved by 30 min reactivation with 75 mL min-1 hydrogen flow. These results suggest that a minimum amount of hydrogen and a
minimum reactivation time were necessary to achieve catalyst activity recovery after bleaching. With a strong preference to minimize system down time for regeneration, we adopted 20 min bleaching followed by 30 min regeneration with 75 mL min-1 hydrogen amendment as the standard catalyst regeneration procedure. Figure 3 shows the performance of the CRD reactor using the optimized regeneration scheme. The average influent and effluent TCE concentrations were 600 µg L-1 and 4.1 µg L-1, respectively, resulting in an overall system efficiency of 99.3% removal. As indicated by the near stoichiometric conversion of TCE to ethane in Figure 3b, the TCE in the contaminated groundwater was completely reduced to ethane in the CRD reactor, which is consistent with results of laboratory experiments under excess hydrogen conditions (7, 13). The difference in the influent TCE (4.58 ( 0.56 µmol L-1) and effluent ethane (4.98 ( 0.44 µmol L-1) is statistically insignificant and attributed to instrument calibration errors and temperature effects. 2. Kinetics of Catalyst Deactivation. Figure 4 shows the observed deactivation curves of Pd catalyst (indicated by the relative TCE concentration) for two major fouling events. Also shown are model predictions for total sulfide concentrations of 2.0, 3.0, and 4.0 mg L-1. The model parameters used were developed previously with the same type of catalyst except that in the field study reported here the Pd content was 2%, whereas in the laboratory study it was 1% (12). The catalyst activity recovered fully after each regeneration cycle. Deactivation typically became noticeable after a lag period of 4 to 9 h. (The data for this lag phase are not shown.) Thereafter, deactivation increased, causing concentration to increase following a sigmoid curve, as predicted by the model. However, since hydrogen sulfide was not detected in the influent, we conclude that sulfide and/or other sulfidic foulants were formed within the CRD reactor after hydrogen addition. Laboratory data suggest that the process is biological in nature although we cannot exclude with certainty abiotic processes. Abiotic reduction of sulfate to sulfide was not observed in the CRD process (8). In the column effluent, hydrogen sulfide was detected by odor (>29 ng L-1) in some instances after major fouling events but never exceeded the 0.1 µg L-1 analytical detection limit. Sulfide formation during fouling events is consistent with laboratory observations made with groundwater from the site (11). The fact that the sulfide concentration in the effluent was below the model prediction is unexplained. It is possible that stronger foulants than hydrogen sulfide are formed (such VOL. 42, NO. 23, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 3. Performance of the CRD reactor under the optimized catalyst regeneration scheme: (a) TCE concentrations in the reactor influent and effluent and (b) molar concentrations of TCE in reactor influent and ethane in reactor effluent. Between day 6 and day 10, the system was not in operation due to a power failure.
FIGURE 4. TCE relative concentration in the reactor effluent as a function of time following two fouling events. The time was set to zero when TCE began to breakthrough, i.e., at the end of the lag phase (4-9 h after regeneration cycle). The fitted breakthrough curves correspond to the presence of 2 to 4 mg L-1 hydrogen sulfide in the reactor water. as polysulfides or organic sulfides) or that hydrogen sulfide was retained in the reactor. The fact that bleaching effectively 8912
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regenerated the catalyst is consistent with laboratory data that demonstrated that adsorbed sulfidic species can be
FIGURE 5. Performance of the CRD reactor: (a) daily variation in TCE concentration in the reactor effluent due to catalyst deactvation and regeneration. The reactor was operated under the optimized daily regeneration scheme (20 min bleach recycle, 30 min reactivation at 75 mL min-1 hydrogen gas flow), with the start of each regeneration cycle denoted by the arrow. (b) Long-term performance of Pd column over a 100-day demonstration period. Data acquired during periods of mechanical and/or electrical malfunctions are not shown. oxidatively removed (8, 13, 19). Reactivating the catalyst with hydrogen-containing water (12, 13) reduces the oxidized surface Pd(II) back to Pd(0). However, more data are needed to more fully evaluate the chemical basis and the applicability of the deactivation model. The data in Figure 4 indicate that breakthrough was rapid once it became apparent. To prevent contaminant breakthrough and severe fouling (due to prolonged hydrogen sulfide exposure), it was necessary to initiate catalyst regeneration when fouling was still minimal. It has been reported that sulfide penetrates 1-2 nm into Pd after 1 day of exposure to a 10 mM sodium sulfide solution (32 mg L-1 sulfide) at 25 °C (24), which translates to an estimated diffusivity of 1-4 × 10-18 m2 d-1 (13). Hence, greater penetration depth is expected for longer exposure, which would require longer bleach cycles to regenerate. 3. System Overall Performance. Figure 5a shows a 7-day TCE concentration history in the CRD reactor effluent under the optimized daily regeneration scheme. After each regeneration cycle, catalyst activity was restored to nearly 100%, and effluent TCE concentrations remained near or below the detection limit (1.0 µg L-1) for 4 to 9 h. Over the next 14 to 19 h, the catalyst slowly lost activity, shown by the gradual increase in effluent TCE concentrations from 1 to close to 10 µg L-1 (TCE removal decreased from 99.8% to 98.3%). The time period to significant deactivation likely reflects the lag period required for the production of sulfidic species (via biological and/or abiotic processes) in the reactor after the regeneration. In the later stage of each operation cycle,
effluent TCE concentrations began to exceed the MCL of 5 µg L-1 and reached near 10 µg L-1 by the end. Averaged over the entire operation period, the effluent TCE concentration was 4.1 µg L-1, which is below the MCL of TCE. Figure 5b shows 100-day performance of the CRD reactor during the demonstration period. Daily regeneration effectively maintained catalyst reactivity. Occasional interruption of data collection, reactor down time due to operational problems, and power failures left gaps in the data set (actual data for these periods not shown). The overall performance of the CRD reactor was stable throughout the demonstration: long-term TCE removal during treatment was >99% and the MCL was met on average. There were multiple events when the catalyst reactor was severely deactivated by prolonged exposure (up to weeks) to presumably high concentrations of sulfide or other sulfidic species. To restore catalyst activity after such severe deactivation episodes, bleach was applied at 500 mg L-1 (as free chlorine) for 7 days. This treatment recovered catalyst activity nearly to the initial levels. Whether regeneration by strong bleaching damaged the catalyst is unknown. Laboratory studies show Pd catalyst is stable against repeated bleaching and under acidic (pH 4.8) and basic (10.4) conditions (13), but Pd dissolution becomes significant below pH 4 (11). Considering both, ours and the experiences at LLNL (14), the estimated lifetime of the Pd catalyst at Edwards AFB can be expected to exceed 5 years, even if several major fouling events should occur. 4. Technical and Economic Comparison with Competing Technologies. Air stripping, granular activated carbon VOL. 42, NO. 23, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 3. Cost Comparison of Competing Technologies for Groundwater Remediation technology
TCE [mg L-1]
site
Air Stripping effective at removing all chlorinated ethylenes but generates secondary waste stream and inefficient at high concentrations Granular Activated Carbon (GAC) ineffective at high concentrations, cannot handle hydrophilic contaminants, produces secondary waste stream
Gold Coast, FLb Des Moines, IAb La Salle, IL
b
13.3
Old Mill, OHb LLNL, CAc Commencement Bay, WAb Permeable Reactive Barrier (PRB) effective at destroying chlorinated ethylenes but only efficient in shallow aquifers Palladium Reductive Catalysis effective at destroying chlorinated ethylenes, faster kinetics than Fe, applicable at high concentrations, no secondary waste stream d
a All costs amortized for 10 y operation. Data not given.
b
Moffett, CAb Intersil, CAb
9
6.1 3 0.13 20 13
Edwards AFB, CA
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cost per 1000 gala [$]
99 96
7 1
96
250
75 99 98
336 83 10
-d 98
547 228
99.6
Data taken from EPA report no. 542-R-99-006 (29).
(GAC) adsorption, and permeable reactive barrier (PRB) are the physicochemical remediation technologies commonly used to remove TCE and other chlorinated volatile organic compounds from contaminated groundwater. In Table 3, some of the advantages and limitations of these competing technologies are briefly compared with those of CRD. Cost analysis of CRD and the competing technologies for groundwater remediation was conducted using the net present value (NPV) approach (25), with details summarized elsewhere (26). The results of cost analysis under 5% annual inflation and 8% interest rate are shown in Table 3. Assuming a total operation period of 10 years at Edwards AFB (with catalyst replacement after Year 5), the cost for treating the contaminated groundwater plume is expected to be $8 per 1000 gal. At this level, CRD technology is cost competitive with GAC and PRB at the field sites listed. Air stripping is more costeffective at low TCE concentrations, but if TCE is present above 1 mg L-1, Pd catalysis becomes more competitive. There are many compounds, including DCE and VC, which adsorb weakly on GAC and for which GAC is relatively inefficient. In cases where the groundwater is contaminated with multiple chlorinated ethylenes, the costs of GAC adsorption (because of poor absorbability) and air stripping (because of low volatility) increase significantly. The efficiency of Pd catalysis, in contrast, is equally efficient for all chlorinated ethylenes independent of the degree of chlorination (based on laboratory rates (7)) and therefore should offer advantages when chlorinated ethylenes are present as mixtures. Aerobic cometabolic biodegradation of TCE at Edwards AFB was previously conducted, which reduced the TCE concentration in the treatment zone from about 1000 µg L-1 to an average of 18 to 24 µg L-1 (20). The obvious advantages of CRD over bioremediation include complete destruction of TCE without production of toxic byproducts (e.g., VC) and rapid reaction rate. Additionally, CRD has better treatment efficiency when operated reliably under optimal conditions and avoids the problem of pump and mechanical clogging caused by microbial growth. Furthermore, this technology is applicable for site remediation and source control where biological processes may be susceptible to toxic effects, for instance because of elevated contaminant concentrations or toxic cocontaminants. Results of this study demonstrate that CRD is a competitive treatment technology for remediation of groundwater con8914
0.45 0.045
removal [%]
c
8
Data taken from ref 14.
taminated with chlorinated solvents compared to conventional approaches. In Pd catalysis, the reductant (hydrogen) is low cost, and the catalysts are commercially available, making this technology accessible to industrial and commercial applications. The system can be automated and remotely operated, making it possible for installation and operation in remote locations. Implementation of CRD reactors requires adjustment of the process to local groundwater conditions. The control system should feature automated safe shutdown and restart procedures in case of power failures. To prevent the effluent TCE concentration from exceeding the MCL, either of two approaches may be applicable: increasing the frequency of the regeneration cycle to every 10 h or incorporating a dual-column system with alternating operation/regeneration cycles, at the cost of reducing system throughput or increasing system cost. With recent developments in bimetallic catalysts (27, 28), catalyst activity and robustness should continue to improve and make the technology applicable to a wider range of contaminants.
Acknowledgments The authors would like to thank the Environmental Security Technology Certification Program (ESTCP) for the support in completing this field demonstration project. Additional funding and support were provided by the U.S. Environmental Protection Agency under agreements R-825421 and R-815738-01, the National Science Foundation under agreement CTS-0120978, and the Environmental Management Restoration Branch at Edwards AFB. Support from the Environmental Management Office at Edwards AFB, EPA Region 9, the California Department of Toxic Substances Control, the California State Water Resources Control Board, and the Lahontan Regional Water Quality Control Board is also acknowledged.
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