1. Biodegradation Processes for Chlorinated Solvents

2. Dehalogenation
Stripping halogens (generally Chlorine) from an organic molecule
Generally an anaerobic process, and is often referred to as reductive dechlorination
R–Cl + 2e– + H+ ––> R–H + Cl–
Can occur via
Dehalorespiration (anaerobic)
Cometabolism (aerobic)

3. Dehalorespiration
Certain chlorinated organics can serve as a terminal electron acceptor, rather than as a donor
Confirmed only for chlorinated ethenes
Rapid, compared to cometabolism
High percentage of electron donor goes toward dechlorination
Dehalorespiring bacteria depend on hydrogen-producing bacteria to produce H2, which is the preferred primary substrate

4. Reductive Dechlorination of Chlorinated Ethenes
CCl =CCl PCE
CHCl=CCl TCE
CHCl=CHCl 1,2 DCE
CH =CHCl VC H 2 2 H 2 H H 2
Ethylene CH = CH
CO Carbon dioxide 2 2 2

5. Added Danger
Dechlorination of PCE and TCE should be encouraged, but monitored closely
The dechlorination products of PCE are more hazardous than the parent compound
DCE is 50 times more hazardous than TCE
Vinyl Chloride is a known carcinogen

6. Cometabolism
Fortuitous transformation of a compound by a microbe relying on some other primary substrate
Generally a slow process - Chlorinated solvents don’t provide much energy to the microbe
Most oxidation is of primary substrate, with only a few percent of the electron donor consumption going toward dechlorination of the contaminant
Not all chlorinated solvents susceptible to cometabolism (e.g., PCE and carbon tetrachloride)

7. Cometabolic Transformations of Chlorinated Aliphatic Hydrocarbons
(CAHs) 2 -
MMO
NADH, O Secondary Reaction O CH 4
NADH, O
CO , H O Primary Reaction
CCl =CHCl Cl C CHCl CO , Cl ,H O

8. Classification System for Chlorinated
Solvent Plumes
Type 1 : Anaerobic due to anthropogenic carbon
Type 2 : Anaerobic due to naturally occurring carbon
Type 3 : Aerobic due to no fermentation substrates

9. Dechlorination Zones

10. Will it work for Chlorinated Solvents?
Natural Attenuation
Will it work for Chlorinated Solvents?

11. Natural Reductive Dechlorination
Natural dechlorination of solvents in aquifers with rich organic load and low redox potential
Not frequently found
Many chlorinated solvent plumes located in low organic load, aerobic aquifers

12. to be broadly used for some compounds, especially chlorinated solvents
Natural Attenuation
Not fast enough
Not complete enough
Not frequent enough
to be broadly used for some compounds, especially chlorinated solvents

13. Enhanced Bioattenuation
Engineered system to increase the intrinsic biodegradation rate to reduce contaminant mass
Usually addition of electron acceptors (oxygen, nitrate, sulfate) or electron donors (organic carbon, hydrogen)
Could involve bioaugmentation - adding the catalyst for bioattenuation

14. Enhanced Bioattenuation of Chlorinated Solvents
Inadequate electron donor concentrations
Determine methods of adding electron donors

15. In Situ Biodegradation of Chlorinated Solvents
Electron Donor
Addition
Nutrient
(if necessary)
Injection
Well
To:
• Treatment
• Treatment / Recycle
• Recycle
DNAPL
Recovery
In Situ Biodegradation Zone

16. Enhanced Bioattenuation
Petroleum Chlorinated
Technology Hydrocarbons Solvents
(e– acceptor) (e– donor)
Liquid Delivery Oxygen Benzoate
Nitrate Lactate
Sulfate Molasses
Carbohydrates
Biosparge Air (oxygen) Ammonia
Hydrogen
Propane
Slow-release Oxygen Hydrogen
(ORC) (HRC)

17. Selective Enhancement of Reductive
Dechlorination
Competition for available H2 in subsurface
Dechlorinators can utilize H2 at lower concentrations than methanogens or sulfate-reducers
Addition of more complex substrates that can only be fermented at low H2 partial pressures may provide competitive advantage to dechlorinators

18. Electron Donors Alcohols and acids
Almost any common fermentable compound
Hydrogen apparently universal electron donor, but no universal substrate
Laboratory or small-scale field studies required to determine if particular substrate will support dechlorination at particular site

19. Electron Donors Acetate Hydrogen - Pickle liquor
Acetic acid biochemical Polylactate esters
Benzoate electrochemical Propionate
Butyrate gas sparge Propionic acid
Cheese whey Humic acids - Sucrose
Chicken manure naturally occurring Surfactants -
Corn steep liquor Isopropanol Terigitol5-S-12
Ethanol Lactate Witconol 2722
Glucose Lactic acid Tetraalkoxsilanes
Hydrocarbon Methanol Wastewater
contaminants Molasses Yeast extract
Mulch

20. Electron Donor Demand Theoretical demand for 1 g PCE = 0.4 g COD
Must use many times more substrate due to competition for electron donors
Minimum of 60 mg/L TOC to support dechlorination beyond DCE in microcosm studies in Victoria, TX soils (Lee et al., 1997)

21. Electron Donor Technology in Field-Scale Pilots
Electron Electron Site Reference
Donor Acceptor
Benzoate CO Victoria, TX Beeman et al 1994
Beeman 1994
Acetate NO Moffett Air Field, CA Semprini et al 1992
Schoolcraft, MI Dybas et al 1997
Yeast Extract SO /CO Niagara Falls, NY Buchanan et al 1995
Methanol / ? FAA facility, OK Christopher et al Sucrose
Tergito15-S-12 SO Corpus Christie, TX Lee et al 1995
Witconol 2722
Methanol ? Breda, Netherlands Spuij et al 1997 2 3 4 2 4

22. Electron Donor Technology in Field-Scale Pilots
Electron Electron Site Reference
Donor Acceptor
Lactic acid ? Watertown, MA ABB Environmental
Lactate Fe Dover AFB, DE Grindstaff 1998
Benzoate / Lactate / ? Pinellas, Fl US DOE 1998
Methanol
Molasses ? Eastern PA Nyer et al 1998
Molasses ? Williamsport, PA Nyer & Suthersan 1996 3+

23. Engineered Delivery Systems
Air injection into vadose zone - venting / bioventing
Air injection into ground water - air sparging / biosparging
Gas, other than air, injection into ground water - ammonia, hydrogen, propane
Slow release into ground water - ORC, HRC
Liquid addition - infiltration or injection wells, surfactant / cosolvent flush
Recirculation - extraction / reinjection systems, UVB wells, pump and treat

24. Promotes in situ biodegradation - Minimize hydrogen gas entering
Hydrogen Sparging
Hydrogen gas
Blower
Vapor
Treatment
Tiny
Bubbles
DNAPL
SVE
Well
Promotes in situ biodegradation - Minimize hydrogen gas entering
unsaturated zone

25. Hydrogen Releasing Compound (HRC )
A food grade polylactate ester slowly degraded to lactic acid
Lactic acid metabolized to acetic acid with production of hydrogen
Hydrogen drives reductive dechlorination

26. Hydrogen Releasing Compound (HRC )
A moderately flowable, injectable material
Facilitates passive barrier designs
Slow hydrolysis rate of lactic acid from ester keeps hydrogen concentration low, may favor reductive dechlorination over methanogenesis

27. Hydrogen Releasing Compound (HRC )

28. HRC Application
Delivery Systems - bore-hole backfill or injection via direct-push technologies
Designs for Barriers and Source Treatment
1. Upgradient
barrier
2. Series of
barriers
3. Downgradient
4. “Grid” of HRC
injection points

29. Substrates for Bioattenuation of CAHs
(Lee et al, 1997)
“any substrate that will yield hydrogen under fermentative and/or methanogenic conditions will ... support dechlorination of PCE to DCE if the microbial population is capable of ... the dechlorination reaction”
“biotransformation of DCE to VC and ethene ... not ... universal and may require specific substrates or enrichment strategies”

30. Substrates for Bioattenuation of CAHs
(Lee et al, 1998)
“No substrate that reliably supports complete dechlorination at all sites has been identified to date.”

31. Limitations for Application of Bioattenuation Technologies
Delivery of materials to the subsurface (contact)
Bioavailability of the contaminants
Toxicity of contaminants
Threshold substrate concentration

32. Contact in the Subsurface10 1
0.1
Methane
Oxygen
Dissolution
Sorption / Desorption

33. Toxicity of Trichloroethylene
Air or water in contact with oily phase may exceed toxic limit for microorganisms
TCE:
> 6 mg/L in water (30% reduction = 1.8 mg/L; Moffett field)
> 2 mg/L in air

34. Maximum Solvent Concentrations for Reductive Dechlorination
Solvent Concentration Reference
(mg/L)
PCE Smatlak et al 1996
cis-DCE Haston et al 1994
VC DiStefano et al 1991
DCM Freedman & Gossett 1991
TCA 100 Galli & McCarty 1989

35. What We Don’t Know
Should you use a slow, controlled release or large/small periodic dosing of electron donor?
Is it redox reduction or electron donor addition that triggers reductive dechlorination?
Under field conditions, does competion for hydrogen exist between dechlorinators, methanogens, and sulfate reducers? Does it matter?

36. Prognosis? Electron Donor Technology
for engineered bioattenuation of CAHs will
equal the impact of
Electron Acceptor Technology
on bioremediation of HCs