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 man-
made 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
• Industrial spills and leaks
• Surface impoundments• Storage tanks and
pipes• Landfills• Burial areas and dumps• Injection wells
ConfiningUnit
Water table
Saline Water
Lateralintrusion of saline water
Ocean
Municipalwater well
Abandonedoil well
DeepAquifer
pond
Infiltration ofpesticides andfertilizers from
farmlands
Brine leakage from ruptured well casing
septic tank leakage
Fresh water
Accidental fuel spill
Municipal landfill
Leakage from hazardous waste site
Contaminated shallow
well
Leakingpetroleum
tank
ConfiningUnit
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________________________________________________________________________________________
Readilydegradable_____________
Somewhatdegradable_____________
Difficult todegrade_____________
Generallyrecalcitrant_____________
fuel oils, gasoline creosote, coaltars
chlorinatedsolvents (TCE)
dioxins
ketones andalcohols
pentachloro-phenol (PCP)
some pesticidesand herbicides
polychlorinatedbiphenyls (PCB)
monocyclicaromatics
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
ATP
ACETATE
CO2
Fe(III)
Fe(II)
Metabolism of a Pollutant-degrading Bacterium
*Benzoate*Benzoate*Toluene*Toluene*Phenol*Phenol*p*p-Cresol-Cresol*Benzene*Benzene
*U(VI)*U(VI)*Co(III)*Co(III)*Cr(VI)*Cr(VI)*Se(VI)*Se(VI)*Pb(II)*Pb(II)*Tc(VII)*Tc(VII)
*CCl*CCl44
*Cl-ethenes*Cl-ethenes*Cl-aromatics*Cl-aromatics*Nitro-aromatics*Nitro-aromatics
Uranium reduction leads to uranium precipitation and immobilization
U6+sol U4+
insol
U6+sol U6+
sol 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) bacterium
cornstarch CO2 + H2O
contaminant
degradationproducts
Hard to degrade contaminants
• Chlorinated hydrocarbons– solvents– lubricants– plasticizers– insulators– herbicides and pesticides.
Chemicals that exist as ring structures, which havechlorine atoms attached are referred to as halogenatedaromatics and are often toxic.
Examples:chlorobenzoates2, -4-dichlorophenoxyacetatechlorobenzenes
Removal of the Cl atom is the rate-limiting step in thedegradation of the halogenated aromatic. Adioxygenase enzyme is used to replace the Cl atomwith an OH group. The process requires the presenceof 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-degrading dmp operon is regulated by DmpR, a NtrC-like positive regulator.
(m o n o o xy g e n a s e )
O H
ph e n o l
O H
O H
c a te c h o l
C O O H
C O O H
c is , c is -m u c o n a te
p h e A
d m p X
p h e n o lh y d ro la s e
p h e B
d m p B
d io xy g e n a s e
s u c c in a te a n da c e ty l C o A
CO2 + H2O
The layout of the genes involved in chlorocatechol-degradation on the plasmid is similar to the layout of the catechol-degrading genes on the chromosome
COOH
Cl
OHOH
OH
COOH
Cl
COOH
Cl
o
C=O
COOH
COOHCOOH
O
COOHCOOH
O
succinate
3-chlorobenzoate 3-chlorocatechol 2-chloro-cis,cis-muconate
dieneiactone
maleylacetate
3-ketoadipate
A B
D
A, pyrocatechase II
B, muconate lactonizing enzyme II
D, hydrolase II
Genetic engineering of bacteria to remove toxic metals from the environment
Hg2+-metallothein
Hg2+→HgoHg2+
New gene/enzyme
New gene/transport proteinsE. coli bacterium
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 DemolitioneXplosive
Stimulates plant growth!
Gene from bacterium moved to plant genome
Careers in Bioremediation
• Outdoor inspection • Lab testing• Administration
Company employeeGovernmentEmployee
Regulatory oversight
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