OPTIMIZING
REDUCTIVE DECHLORINATION IN A HIGH-SULFATE
IN-SITU
BIOREMEDIATION SYSTEM
Margaret Findlay, Ph.D. (MFindlay@bciLabs.com),
Samuel Fogel, Ph.D., Donna Smoler,
(Bioremediation Consulting Inc. Watertown, MA, USA)
Bradley F. Droy, Ph.D., Frank Manale, and Peikang Jin, Ph.D.,
(Toxicological and Environmental Associates, Baton Rouge, LA, USA)
Catherine Creber, (The Dow Chemical Company, Sarnia, Ontario, Canada)
Gary Klecka, Ph.D., (The Dow Chemical Company, Midland MI, USA)
ABSTRACT: The Dow Chemical Company is conducting an in-situ anaerobic remediation of 200 acres at an active chemical manufacturing facility in Pittsburg, CA, using a 3,800-foot in-situ bioactive zone created by 39 wells to inject and mix organic acids and accomplish intra-site bioaugmentation. Sulfate at this site ranges from 400 to 900 mg/L, and microcosm studies have shown that DCE (dichloroethene) dechlorination by un-acclimated site bacteria may be delayed due to the presence of sulfate. Since optimal remediation requires maintainance of vigorous dechlorination in the bio-active zone while allowing re-introduction of high-sulfate groundwater from up-gradient, microcosm tests were conducted to determine whether re-addition of sulfate would inhibit on-going dechlorination. Two microcosms were constructed from site soil and groundwater, amended with organic acids, and allowed to reduce their native 700 mg/L sulfate, then were spiked with cDCE. During vigorous DCE-dechlorination, one microcosm was spiked with sulfate and the other allowed to continue dechlorination in the absence of sulfate. With excess dissolved H2, vigorous on-going dechlorination of cDCE to ethene at 12 mM /day was not inhibited by the addition of 400 mg/L sulfate, which was simultaneously reduced at 85 mg SO4/L/day
INTRODUCTION
The Dow Chemical Company is conducting an in-situ anaerobic bioremediation for 200 acres of their operating chemical plant in Pittsburg, CA to treat tetrachloroethene (PCE) and daughter products, as well as carbon tetrachloride (CT) and daughter products. Microcosm tests have shown that the site contains bacteria capable of dechlorinating dichloroethene (DCE) completely to ethene, and others that dechlorinate CT. These studies also demonstrated that electron donors were limiting in situ, and that degradation results could be enhanced when exogenous donors were added. The remedial system is an in-situ biologically active treatment zone, extending 3,800 feet across the plume, created by 39 alternating injection and extraction wells which inject and mix organic acid donors. Sulfate at this site range from 400 to 900 mg /L, and previous microcosm studies indicated that un-acclimated site bacteria may experience delays in DCE-dechlorination due to the presence of sulfate. This study was undertaken to determine whether re-addition of sulfate from up-gradient would inhibit on-going dechlorination in the bio-active zone. It was concluded that under conditions of excess dissolved H2, vigorous on-going dechlorination of cDCE to ethene was not inhibited by the addition of sulfate.
Samples were obtained from location PZ004B at the Dow Pittsburg site. Bottles for ground water were pre-filled with anoxic gas (Argon), and provided with FeS reducing agent (to give 0.4 mM) to react with traces of oxygen that might enter during sampling. Soil Cores were obtained in 6” brass cylinders, capped and taped immediately, and shipped to the Bioremediation Consulting laboratory, where they were transferred to anaerobic chambers maintained in anoxic atmosphere (96 % argon, 4 % H2).
To create microcosms, 160 ml serum bottles were flushed with Argon, stoppered, and placed in a glove box with the soil core, and the glove box was flushed with 95% Argon-4% H2 in the presence of catalyst to provide an oxygen-free atmosphere. The soil, 30 grams, was placed in each bottle, and the bottles stoppered. Bottles were then removed from the glove box and given 90 ml of ground water using anoxic transfer procedures involving continuous flushing of the groundwater sample and of the microcosm bottle with Argon. Microcosms were then sealed with Teflon-lined butyl rubber stoppers, crimped, and overpressurized. Each microcosm was amended to give 180 mg/L each of formate and lactate, 30 mg/L ammonium-nitrogen, 60 mg/L phosphate, and given 5 mg yeast extract and 5 mg vitamin B12. Microcosms were sparged with ultra-pure Argon to remove chlorinated solvents, then briefly with 80% N2 /20% CO2 to replace Argon. Additional lactate and formate were added on ten occassions during the 237 day test period, to maintain high dissolved H2 concentrations. Additional B12 was added on six occassions. Amendment solutions were prepared using anoxic procedures, and additions were made using anaerobic technique. Microcosms were maintained in darkness, inverted, at 22 ± 1 oC , and shaken briefly 3 times per week.
After sulfate-reducing bacteria had completely reduced the native 700 ppm sulfate (day 16), the microcosms were given 40 mM PCE or 50 mM cDCE, which were completely dechlorinated to ethene (day 50). On day 89, both microcosms received 53 mM cDCE, and on day 91, only microcosm A received 180 mg/L sulfate. Sulfate in bottle A was completely reduced, and cDCE was completely dechlorinated in both bottles by day 106. On day 136 bottles were sparged to remove ethene, methane, and H2S, and 62 mM cDCE was added to both bottles. On day 139, sulfate, 200 mg/L, was added only to bottle B. By day 148, cDCE had been completely dechlorinated to ethene in both bottles, and sulfate had been completely reduced in bottle B. On day 217, the bottles were sparged, and both bottles received 78 mM cDCE. On day 220, only bottle B received 423 mg/L sulfate. By day 237, cDCE in both bottles had been completely dechlorinated to ethene, and the sulfate in bottle B had been completely reduced.
RESULTS AND DISCUSSION
Figure 1 shows
the concentrations of cDCE and SO4 for each event in which one
microcosm is challenged with sulfate addition during DCE dechlorination, and
the other acts as a control.
Figure 1. Effect of adding
sulfate during dechlorination of cDCE.
Left column,
microcosm A. Right column,
microcosm B.
Table 1 shows the results of the first challenge, in which both microcosms were spiked with 53 mM cDCE on day 89, then two days later, Microcosm A was spiked with 180 mg/L sulfate. Prior to the sulfate spike, both microcosms were dechlorinating DCE at approximately 6 mM DCE/day. For three days after the sulfate addition to Microcosm A, sulfate was not reduced, and no difference was seen in the rate of DCE dechlorination in the two microcosms (about 11 mM /day). On day 93, sulfate reduction was initiated in Microcosm A, but the rate of DCE dechlorination remained the same in the two microcosms, about 6 mM /day. During this time period, days 89 to 96, the dissolved H2 remained generally above 1000 nM. It is concluded that the addition of sulfate did not effect the rate of dechlorination in A, either during the lag prior to sulfate reduction, nor during active sulfate reduction.
TABLE 1. Effect of sulfate added during dechlorination, day 91.
|
|
Challenged Microcosm (A) |
Control, no SO4 |
|
|
days |
DCE Dechlorination mM DCE / day |
SO4 Reduction ppm/day |
DCE Dechlorination mM DCE / day |
Add DCE day 89 |
89 - 91 |
6 |
|
7 |
Add SO4 day 91 |
91 - 93 |
11 |
0 |
10 |
|
93 - 96 |
6 |
45 |
5 |
Table 2 shows the results of spiking both microcosms with 62 mM DCE on day 136, then challenging Microcosm B with 200 mg/L sulfate on day 139 while observing Microcosm A as a control. Immediately after DCE was added, Microcosm B had a slightly higher rate of DCE dechlorination than Microcosm A. After sulfate was added to Microcosm B, sulfate was not degraded for four days and dechlorination in B remained faster than that in A. (During this time, PCE was present in both microcosms, but did not degrade until after day 145.) Between day 139 and day 148, the dissolved H2 was generally above 23 nM in B and above 7 nM in A. It is concluded that the addition of sulfate did not inhibit the on-going dechlorination in B.
TABLE 2. Effect of sulfate added during dechlorination, day 139.
|
|
Challenged Microcosm (B) |
Control, no SO4 |
|
|
days |
DCE Dechlorination mM DCE / day |
SO4 Reduction ppm/day |
DCE Dechlorination mM DCE / day |
Add DCE day 136 |
136 - 139 |
8 |
|
3 |
Add SO4 day 139 |
139 - 143 |
10 |
0 |
8 |
|
143 - 148 |
|
32 |
9 |
Table 3 shows the results of spiking both microcosms with 77 mM DCE on day 217, then challenging Microcosm B with 420 mg/L sulfate on day 220 while observing Microcosm A as a control. Prior to sulfate addition, A and B had the same rate of dechlorination, approximately 7 mM/day. After sulfate addition on day 220, Microcosm B continued to dechlorinate at the same rate as A, while also supporting sulfate reduction at the rate of 85 mg SO4/L/day. (During the time from day 217 to day 225, dissolved H2 concentrations were maintained between 3 and 1000 nM in both microcosms.)
TABLE 3. Effect of sulfate added during dechlorination, day 220.
|
|
Challenged Microcosm (B) |
Control, no SO4 |
|
|
days |
DCE Dechlorination mM DCE /day |
SO4 Reduction ppm/day |
DCE Dechlorination mM DCE / day |
Add DCE day 217 |
217 - 220 |
7 |
|
6 |
Add SO4 day 220 |
220 - 225 |
12 |
85 |
12 |
On-going dechlorination was not inhibited by the addition of over 400 mg/L sulfate during dechlorination. The observation that competition between sulfate reduction and dechlorination can be overcome by acclimation of the microbial community and by maintaining high dissolved H2 should be useful in remediating other sites containing cDCE.
.