Transcript Definition of Natural Attenuation

Biodegradation Processes for
Chlorinated Solvents
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)
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 hydrogenproducing bacteria to produce H2, which is the
preferred primary substrate
Reductive Dechlorination of
Chlorinated Ethenes
H
H
H
H
CCl =CCl
PCE
CHCl=CCl 2
TCE
CHCl=CHCl
1,2 DCE
CH 2 =CHCl
VC
2
Ethylene CH2 = CH 2
2
CO2
Carbon dioxide
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
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)
Cometabolic Transformations of
Chlorinated Aliphatic Hydrocarbons
(CAHs)
NADH, O 2
CO , H O
CH4
2
MMO
CCl2 =CHCl
NADH, O 2
2
Primary Reaction
O
Cl2C
CHCl
-
CO2 , Cl ,H2 O
Secondary Reaction
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
Dechlorination Zones
C hlor ina ted Sol ven ts
C o-d isp ose d w ith
su bs trates ( e.g ., BTEX,
is opr opa no l, ac eto ne,
etc.)
Va do se
Zo ne
Type III Zone:
No Subs trates Low Dec hlori nation
Rates
Fl ow
Type II Zone:
Natural Subs trates Moderate to Low
Dec hl ori nation
Rates
Type I Zone:
Added Substrates High Dec hl orination
Rates
Natural Attenuation
Will it work for Chlorinated
Solvents?
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
Natural Attenuation
Not fast enough
Not complete enough
Not frequent enough
to be broadly used for some compounds,
especially chlorinated solvents
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
Enhanced Bioattenuation
of Chlorinated Solvents
• Inadequate electron donor concentrations
• Determine methods of adding electron donors
In Situ Biodegradation of
Chlorinated Solvents
Electron Donor
Addition
To:
• Treatment
• Treatment / Recycle
• Recycle
Injection
Well
Recovery
Well
Soil Rinsing
w/ Nutrients
Nutrient
Addition
(if necessary)
DNAPL
In Situ Biodegradation Zone
Enhanced Bioattenuation
Petroleum
Hydrocarbons
Chlorinated
Solvents
(e– acceptor)
(e– donor)
Liquid Delivery
Oxygen
Nitrate
Sulfate
Benzoate
Lactate
Molasses
Carbohydrates
Biosparge
Air (oxygen)
Ammonia
Hydrogen
Propane
Slow-release
Oxygen
(ORC)
Hydrogen
(HRC)
Technology
Selective Enhancement of Reductive
Dechlorination
• Competition for available H2 in subsurface
• Dechlorinators can utilize H2 at lower
concentrations than methanogens or sulfatereducers
• Addition of more complex substrates that
can only be fermented at low H2 partial
pressures may provide competitive
advantage to dechlorinators
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
Electron Donors
Acetate
Acetic acid
Benzoate
Butyrate
Cheese whey
Chicken manure
Corn steep liquor
Ethanol
Glucose
Hydrocarbon
contaminants
Hydrogen biochemical
electrochemical
gas sparge
Humic acids naturally occurring
Isopropanol
Lactate
Lactic acid
Methanol
Molasses
Mulch
Pickle liquor
Polylactate esters
Propionate
Propionic acid
Sucrose
Surfactants Terigitol5-S-12
Witconol 2722
Tetraalkoxsilanes
Wastewater
Yeast extract
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)
Electron Donor Technology in Field-Scale Pilots
Electron
Electron
Donor
Acceptor
Benzoate
Site
CO2
Victoria, TX
Reference
Beeman et al 1994
Beeman 1994
Acetate
Yeast Extract
Methanol /
Sucrose
Tergito15-S-12
NO3
Moffett Air Field, CA
Semprini et al 1992
Schoolcraft, MI
Dybas et al 1997
Niagara Falls, NY
Buchanan et al 1995
FAA facility, OK
Christopher et al 1997
SO 4
Corpus Christie, TX
Lee et al 1995
?
Breda, Netherlands
Spuij et al 1997
SO /CO
4
?
2
Witconol 2722
Methanol
Electron Donor Technology in Field-Scale Pilots
Electron
Electron
Donor
Acceptor
Lactic acid
Reference
Watertown, MA
ABB Environmental
Dover AFB, DE
Grindstaff 1998
?
Pinellas, Fl
US DOE 1998
Molasses
?
Eastern PA
Nyer et al 1998
Molasses
?
Williamsport, PA
Nyer & Suthersan 1996
Lactate
Benzoate / Lactate /
?
Site
Fe
3+
Methanol
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
Hydrogen Sparging
Blower
Hydrogen gas
Vapor
Treatment
SVE
Well
DNAPL
Tiny
Bubbles
Promotes in situ biodegradation - Minimize hydrogen gas entering
unsaturated zone
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
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
Hydrogen Releasing Compound
®
(HRC )
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
barrier
4. “Grid” of HRC
injection points
1
2
3
4
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”
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
Contact in the Subsurface
Methane
Oxygen
Sorption / Desorption
10
1
0.1
Dissolution
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
Maximum Solvent Concentrations
for Reductive Dechlorination
Solvent
Concentration
Reference
(mg/L)
PCE
cis-DCE
VC
50
8.0
1.9 - 3.8
DCM
66
TCA
100
Smatlak et al 1996
Haston et al 1994
DiStefano et al 1991
Freedman & Gossett 1991
Galli & McCarty 1989
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?
Prognosis?
Electron Donor Technology
for engineered bioattenuation of CAHs will
equal the impact of
Electron Acceptor Technology
on bioremediation of HCs