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Biodegradation Processes for Chlorinated Solvents. 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

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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)

Dehalogenation
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

Dehalorespiration
slide4
CCl =CCl PCE

CHCl=CCl TCE

CHCl=CHCl 1,2 DCE

CH =CHCl VC

H

H

H

Reductive Dechlorination ofChlorinated Ethenes

2

2

2

2

H

Ethylene CH = CH

CO Carbon dioxide

2

2

2

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

Added Danger
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)

Cometabolism
slide7
CCl =CHCl Cl C CHCl CO , Cl ,H O

NADH, O

2

CO , H O Primary Reaction

CH

2

2

4

O

MMO

-

2

2

2

2

NADH, O Secondary Reaction

2

Cometabolic Transformations ofChlorinated Aliphatic Hydrocarbons

(CAHs)

slide8

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
slide10

Natural Attenuation

Will it work for Chlorinated Solvents?

slide11

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
slide12

Natural Attenuation

Not fast enough

Not complete enough

Not frequent enough

to be broadly used for some compounds, especially chlorinated solvents

slide13

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
slide14

Enhanced Bioattenuation of Chlorinated Solvents

  • Inadequate electron donor concentrations
  • Determine methods of adding electron donors
slide15

Electron Donor

Addition

To:

• Treatment

• Treatment / Recycle

• Recycle

Injection

Well

Recovery

Well

Nutrient

Addition

(if necessary)

DNAPL

In Situ Biodegradation Zone

In Situ Biodegradation of Chlorinated Solvents

slide16

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)

slide17

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
slide18

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
slide19

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

slide20

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)
slide21

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 1997 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

slide22

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+

slide23

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
slide24

Blower

Hydrogen gas

Vapor

Treatment

SVE

Well

DNAPL

Tiny

Bubbles

Hydrogen Sparging

Promotes in situ biodegradation - Minimize hydrogen gas entering

unsaturated zone

slide25

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
slide26

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
slide28

HRC Application

  • Delivery Systems - bore-hole backfill or injection via direct-push technologies
  • Designs for Barriers and Source Treatment
    • 1. Upgradient 1 2 3 4
    • barrier
    • 2. Series of
    • barriers
    • 3. Downgradient
    • barrier
    • 4. “Grid” of HRC
    • injection points
slide29

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”

slide30

Substrates for Bioattenuation of CAHs

(Lee et al, 1998)

“No substrate that reliably supports complete dechlorination at all sites has been identified to date.”

limitations for application of bioattenuation technologies
Delivery of materials to the subsurface (contact)

Bioavailability of the contaminants

Toxicity of contaminants

Threshold substrate concentration

Limitations for Applicationof Bioattenuation Technologies
slide32

Methane

Oxygen

Sorption / Desorption

10

1

0.1

Dissolution

Contact in the Subsurface

slide33

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

slide34

Maximum Solvent Concentrations

for Reductive Dechlorination

Solvent Concentration Reference

(mg/L)

PCE 50 Smatlak et al 1996

cis-DCE 8.0 Haston et al 1994

VC 1.9 - 3.8 DiStefano et al 1991

DCM 66 Freedman & Gossett 1991

TCA 100 Galli & McCarty 1989

slide35

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?
slide36

Prognosis?

Electron Donor Technology

for engineered bioattenuation of CAHs will

equal the impact of

Electron Acceptor Technology

on bioremediation of HCs