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Bioremediation
Chapter 8
What is bioremediation?
The use of bacteria and fungi and
plants to break down or degrade toxic
chemical compounds that have
accumulated in the environment
What are environmental
contaminants?
• Pollutants
– naturally-occurring
compounds in the
environment that are
present in unnaturally high
concentrations.
– Examples:
•
•
•
•
crude oil
refined oil
phosphates
heavy metals
• Xenobiotics
– chemically synthesized
compounds that have
never occurred in nature.
– Examples:
• pesticides
• herbicides
• plastics
Early examples of bioremediation
• Outhouse→Centralized engineered
wastewater treatment systems
– Microorganisms oxidize organic waste
molecules to carbon dioxide and water
– Why do we want to use engineered manmade for this?
More recent examples
• By 1970s it became apparent that we were
polluting the environment faster than the natural
microbial processes could degrade the
pollutants
– Congress established the Environmental Protection
Agency
• Identified “Superfund Sites” that had priority over other
polluted systems for special funding and cleanup in 1980
– 1 in 5 Americans lives within 3-4 miles of a polluted site treated
by the EPA
– Not much progress has been made even though $billions has
been spent
Sources of Contamination
Munic ipal
w ater w ell
pond
Le aking
pe troleum
tank
Ocea n
Wate r table
Fre sh
w ater
Saline
Wate r
se ptic tank
le akage
La teral
intrusion of
sa line wa ter
Infiltration of
pe sticides and
fertilize rs from
farmlands
Brine lea kage from
rupture d w ell casing
Munic ipal
la ndfill
Contaminated
shallow
w ell
Acc idental
fue l spill
Confining
Unit
Confining
Unit
Aba ndoned
oil w ell
Le akage from
ha zardous
w aste site
Dee p
Aquifer
• Industrial spills and
leaks
• Surface
impoundments
• Storage tanks and
pipes
• Landfills
• Burial areas and
dumps
• Injection wells
Groundwater contamination
• Groundwater constitutes 96% of
available freshwater in U.S.
• 95% of potable water in rural areas of
U.S. comes from groundwater
• In 1988, EPA confirmed that 26 states
had various amounts of 44 different
pesticides in their groundwater
• Cost of cleanup is in the $ trillions
• Issues that are still hotly debated
– How clean is clean?
Most recent
• National Institute of Environmental Health
Sciences established the Environmental
Genome Project
– Study impact of environmental chemicals on
human disease
• Identify genes and their products that are sensitive
to toxic chemicals in the environment
• Identify genes that encode for products that
detoxify the chemicals
What types of treatment technologies are in
use to remove contaminants from the
environment?
•
•
•
•
•
•
•
•
Soil vapor extraction
air sparging
bioremediation
thermal desorption
soil washing
chemical dehalogenation
soil extraction
in situ soil flushing
What Makes Bioremediation a Promising
Approach?
• permanence
– contaminant is degraded
• potentially low cost
– 60-90% less than other technologies
Why use Bioremediation?
• No additional disposal
costs
• Low maintenance
• Does not create an
eyesore
• Capable of impacting
source zones and thus,
decreasing site clean-up
time
Economics of in-situ vs. ex-situ
remediation of contaminated soils
• Cost of treating contaminated soil in place $80$100 per ton
• Cost of excavating and trucking contaminated
soil off for incineration is $400 per ton.
• Over 90% of the chemical substances classified
as hazardous today can be biodegraded.
Fundamentals of
Biodegradation
• All organics are biodegradable,
BUT biodegradation requires
specific conditions
• There is no Superbug
• Contaminants must be bioavailable
• Biodegradation rate and extent is
controlled by a “limiting factor”
Contaminants Potentially Amenable to Bioremediation
____________________________________________
Readily
degradable
____________
_
Somewhat
Difficult to
Generally
degradable
degrade
recalcitrant
_____________ _____________ _____________
fuel oils, gasoline creosote, coal
tars
chlorinated
solvents (TCE)
dioxins
ketones and
alcohols
pentachlorophenol (PCP)
some pesticides
and herbicides
polychlorinated
biphenyls (PCB)
monocyclic
aromatics
bicyclic aromatics
(naphthalene)
What challenges exist for bioremediation of
pollutants and xenobiotics?
• Pollutants
– may exist at high, toxic
concentrations
– degradation may
depend on another
nutrient that is in
limiting supply
• Xenobiotics
– microbes may not yet
have evolved
biochemical pathways
to degrade
compounds
– may require a
consortium of
microbial populations
Fundamentals of cleanup reactions
• Aerobic metabolism
– Microbes use O2 in their metabolism to
degrade contaminants
• Anaerobic metabolism
– Microbes substitute another chemical for O2 to
degrade contaminants
• Nitrate, iron, sulfate, carbon dioxide, uranium,
technicium, perchlorate
Metabolism of a Pollutant-degrading Bacterium
Fe(III)
ACETATE
*Benzoate
*Toluene
*Phenol
*p-Cresol
*Benzene
ATP
*U(VI)
*Co(III)
*Cr(VI)
*Se(VI)
*Pb(II)
*Tc(VII)
CO2
Fe(II)
*CCl4
*Cl-ethenes
*Cl-aromatics
*Nitro-aromatics
Uranium reduction leads to uranium
precipitation and immobilization
U6+sol
U6+sol
U6+sol
U4+insol
U4+insol
Volatile organic compounds (VOC)
• These are major contributors to air
pollution
– Paint industry
– Pharmaceutical industry
– bakeries
– printers
– dry cleaners
– auto body shops
Cometabolism
• Bacterium uses some
other carbon and
energy source to
partially degrade
contaminant (organic
aromatic ring
compound)
contaminant
degradation
products
bacterium
corn
starch
CO2 + H2O
Hard to degrade contaminants
• Chlorinated hydrocarbons
– solvents
– lubricants
– plasticizers
– insulators
– herbicides and pesticides.
Chemicals that exist as ring structures, which have
chlorine atoms attached are referred to as halogenated
aromatics and are often toxic.
Examples:
chlorobenzoates
2, -4-dichlorophenoxyacetate
chlorobenzenes
Removal of the Cl atom is the rate-limiting step in the
degradation of the halogenated aromatic. A
dioxygenase enzyme is used to replace the Cl atom
with an OH group. The process requires the presence
of oxygen.
Degradation of chlorinated
hydrocarbons
• Degradation of organic toxins requires the
participation of entire biochemical
pathways involving many enzymes coded
for by many genes.
• Some of the genes exist on the
chromosome while other genes reside on
plasmids.
phenol
OH
cis, cis-muconate
catechol
pheA
dmpX
phenol
hydrolase
OH
pheB
OH dmpB
dioxygenase
COOH
COOH
succinate and
acetyl CoA
CO2 + H2O
(monooxygenase)
• Phenol-degrading dmp operon is regulated by
DmpR, a NtrC-like positive regulator.
The layout of the genes involved in chlorocatecholdegradation on the plasmid is similar to the layout of
the catechol-degrading genes on the chromosome
3-chlorobenzoate
COOH
3-chlorocatechol
OH
OH
A
2-chloro-cis,cismuconate
COOH
C=O
B
COOH
Cl
OH
o
dieneiactone
Cl
Cl
COOH
COOH
succinate
3-ketoadipate
A, pyrocatechase II
B, muconate lactonizing enzyme II
D
COOH
COOH
O
O
D, hydrolase II
COOH
maleylacetate
Genetic engineering of bacteria to remove
toxic metals from the environment
New gene/transport proteins
Hg2+
E. coli bacterium
Hg2+-metallothein
Hg2+→Hgo
New gene/enzyme
Hgo (less toxic form of metal)
Phytoremediation
• ≈350 plant species naturally take up toxic
materials
– Sunflowers used to remove radioactive
cesium and strontium from Chrenobyl site
– Water hyacinths used to remove arsenic from
water supplies in Bangladesh, India
Phytoremediation
• Drawbacks
– Only surface soil (root zone) can be treated
– Cleanup takes several years
Transgenic plants
Royal
Demolition
eXplosive
Gene from
bacterium
moved to
plant genome
Stimulates plant growth!
Careers in Bioremediation
• Outdoor inspection
• Lab testing
• Administration
Government
Employee
Regulatory oversight
Company employee
Summary
• Many factors control biodegradability of
a contaminant in the environment
• Before attempting to employ
bioremediation technology, one needs
to conduct a thorough characterization
of the environment where the
contaminant exists, including the
microbiology, geochemistry, mineralogy,
geophysics, and hydrology of the
system