Transcript The fate of Xenobiotics in Wetlands

Institute of Food and Agricultural Sciences (IFAS)
Biogeochemistry of Wetlands
Science and Applications
June 23-26, 2008
Topic: Toxic Organic Compounds
(Xenobiotics)
Wetland Biogeochemistry Laboratory
Soil and Water Science Department
University of Florida
Instructor:
Todd Z. Osborne
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Biogeochemistry of Wetlands
Science and Applications
Topic: Toxic Organic Compounds (Xenobiotics)
Learning Objectives
Define xenobiotics
 Ecological significance of xenobiotics
 Sources and common classifications
 Environmental fate of xenobiotics
 Abiotic pathways
 Biotic pathways

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What is a Xenobiotic?
 Xeno means foreign, Bios means life:
 Xenobiotic is in essence a compound that is foreign to
life
 Synonyms
 Toxics, toxic [organic] substances, priority organic
pollutants [POPs], endocrine disruptors
 Examples:
 Pesticides, fungicides, herbicides, industrial toxins,
petroleum products, landfill leachate
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Are Xenobiotics an issue in
Wetlands?
Wetlands are often the receiving bodies of
Agricultural and urban drainage
Extent of xenobiotic contamination in
wetlands
Approximately 5000 wetlands and aquatic
systems impacted by pesticides
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Are Xenobiotics an issue in
Wetlands?
Wetlands may be excellent pollutant
removers (aerobic - anaerobic interfaces)
Wetlands are not in the spot light !
No wetland superfund site, etc.
Upland (aerobic) and aquifier (anaerobic) soil
contamination and remediation is the driving
force in our current know-how
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Ecological Significance
• Lethal toxicity to biota
• Non-lethal toxicity to biota
– Endocrine disruptors
– Hormone mimicry
– Reproductive disorders
– Harmful mutation - DNA damage
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Types of Xenobiotics
 Petroleum products (BTEX, MTBE)
 Pesticides (DDT, DDE…..)
 Herbicides (2,4D, Atrazine)
 Industrial wastes (PCB’s, aromatics)
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Aromatic Compounds
CH3
Napthalene
Benzene
Toluene
OH
Naphthol
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OH
Phenol
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Biphenyl
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Halogenated Compounds
Carbon tetrachloride
Chloroform
Vinyl chloride
1,2-Dichloroethane
Trichloroethylene
Tetrachloroethylene
Benzoates
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Halogenated Aromatic Compounds
 Polychlorinated Biphenyls
 PCBs
 Organochlorine Insecticides
 DDT, Toxaphene, …
 Chlorinated Herbicides
 2,4-D, 2,4,5-T, Atrazine…..
 Chlorinated Phenols
 Pentachlorophenol
 2,4-dichlorophenol…2,4-D
 2,4,5-trichlorophenol…. 2,4,5-T
 2,3,and 4-Nitrophenol
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Halogenated Aromatic Compounds
CCl3
Cl
DDT
CH
Cl
Cl
Cl
Chlorobenzene
CCl2
C
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
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CCl2
DDE
Cl
CH
1,3-dichlorobenzene
PCB
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Cl
DDD
Cl
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Halogenated Aromatic Compounds
O-CH2COOH
Chlorinated Herbicides
Cl
2,4-Dichlorophenoxyacetic Acid
[2,4-D]
Cl
Chlorinated Phenols
Pentachlorophenol
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OH
Cl
Cl
Cl
Cl
Cl
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Halogenated Aliphatic Compounds
Cl
Cl
C
Cl
C
Br
Cl
Trichloroethylene [TCE]
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H
H
C
C
H
H
Br
Ethylene Dibromide [EDB]
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Sources of Xenobiotics
Wetlands can receive:
- Drainage from agricultural land [pesticides,
herbicides]
- Drainage from urban areas
- Discharge from industrial facilities
- Landfill leachates
- Undetonated military explosives [TNT, HMX,
RDX, etc.] in war zones and training bases
- Spills [fuels, etc.] due to transportation
accidents
- Atmospheric deposition
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Environmental Fate of Xenobiotics
• Need to know constants
• Fugacity Modeling
– KOW
– KH
– Ka
– Kd
– Kr
– MW
– Sw
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Octanol – water partition coefficient
Henry’s Law constant
Dissociation constant
Partition [sorption] coefficient
Reaction rate constants
Molecular weight
Solubility in water
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Predicting the Fate of Xenobiotics
• Need to know the biogeochemical /
environmental conditions in the wetland
soils
– Microbial consortia
– Redox potential
– Salinity
– C content
– Other e- acceptors
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- pH
- Temperature
- N, P availability
- Oxygen status
- Vegetation type
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Environmental Fate of Xenobiotics
Abiotic Pathways
Sorption
Photolysis
Volatilization
Export
Leaching / surface run-off
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Abiotic Pathway: Sorption
S
Soil Particle
Organic Matter
[mg/kg]
Desorption
Adsorption
Chemical
in Solution
C
[mg/L]
Products of
Biodegradation
Partition coefficient (L/kg) Kd = S (mg/kg)/C (mg/L)
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Abiotic Pathway: Sorption
 For organic chemicals not adsorbed by soils,
Kd is equal to zero
 For a given organic chemical, sorption (Kd) is
greater in soils with larger organic matter
content.. These chemicals move slowly in soils
 For a given soil, organic chemicals with smaller
Kd values are sorbed to lesser extent… and
highly mobile
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Abiotic Pathway: Sorption
 Bioavailability of xenobiotics to degradation is
strongly influenced by sorption
 Chemicals with low sorption coefficients are
generally more soluble, and are more readily
degraded
 Sorption of chemicals increases with amount
soil organic matter
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Abiotic Pathway: Sorption
Sorption may protect biota from toxic
levels of chemicals
High levels of DOM may increase the
mobility of chemicals
Chemicals with high sorption coefficients
are generally less mobile
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Environmental Fate of Xenobiotics
Biotic pathways
Extracellular enzyme hydrolysis
Microbial degradation
Plant and microbial uptake
Bioaccumulation / magnification
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Biotic Pathways:
Microbial ecology: why do microbes degrade
Xenobiotics?
1) Derive energy
i) Electron acceptor
ii) Electron donor
2) A source of Carbon
3) Substitution for a similar “natural” compound:
Cometabolism.
Cometabolism: organisms mediating the mineralization of a
certain compound obtain no apparent benefit from the
process
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Energetics of Xenobiotic
Biodegradation
Energetics of aerobic and anaerobic
benzoate degradation
Reaction
G (kJ)
Benzoate + 7.5O2  2CO2
-3175
Benzoate + 6NO3-  3N2 +7CO2
-2977
Benzoate + 8NO3-  14NH4+ + 7CO2
-1864
Benzoate + 30 Fe3+  30 Fe2+ + 7 CO2
-303
Benzoate + SO42-  7CO2 + 3.75HS-
-185
Benzoate + So  7CO2 + 15HS16/07/2015
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Thauer et al. 1977
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Biodegradation
 Hydrolysis [ + H2O ]
 Oxidation [ + O2 ]
 Reduction [ + e- ]
 Synthesis [ + Functional Groups]
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Biotic Pathway: Hydrolysis
 Extracellular, possibly not compound specific
 Ether hydrolysis
 R-C-O-C-R + H2O
R-C-OH + HO-C-R
 Ester hydrolysis (Chlorpropham)
 R-C-O-C=O + H2O
R-C-OH + HO-C=O
 Phosphate ester hydrolysis (Parathion)
 R-C-O-P=O + H2O
R-C-OH + HO-P=O
 Amide hydrolysis (Propanil)
 R-N-C=O + H2O
O=C-OH + H-N-R
 Hydrolytic dehalogenation (PCP)
 R-C-CL + H2O
-C-OH + HCl
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Biotic Pathway: Oxidation
Key to xenobiotic detoxification and
subsequent mineralization through
oxidation:
Presence of molecular oxygen
Presence of selected aerobic or facultative
aerobic microbial groups (fungi or bacteria)
Aromatic rings without functional groups
Benzene, toluene, naphthalene
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Biotic Pathway: Oxidation
Benzene
H2O
+ O2
OH
H2O
OH
OH
+ O2
Monooxygenase
Monooxygenase
+ O2
Dioxygenase
CO2 + H2O
COOH
COOH
Muconic Acid
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Biotic Pathway: Reduction
 Reductive dechlorination
 (TCE, PCB, PCP)
 Reduction of the aromatic ring
 (BTEX)
6 H + + 6 e Reduction of the Nitro group
 (Parathion)
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C-C-NH2
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Reductive Dechlorination
OH
Cl
Cl
Cl
Cl
Cl
H++
2e-
OH
H++
2e-
OH
Cl
H
H
H
Cl- Cl
Cl
Cl- Cl
Cl
Cl
Cl
 Sequential replacement of Cl- ions with H atoms
 Usually results in accumulation of toxic intermediates
 Promoted under highly reducing conditions (low redox
potential) and high microbial activity
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Acetate Oxidation with Different
Electron Acceptors
Go’
Electron acceptor
ATP
(kJ/ mol Ac) (mol/mol Ac)
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O2 / H2O
-858
28
PCP / TeCP
-557
18
NO3 - / NO2-
-556
18
SO4 2- / HS-
-56
2
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Reductive Dechlorination of PCP in
Methanogenic Everglades Soils
PCP
345 TCP
35 DCP
1
0.8
345 TCP
0.6
PCP
35 DCP
0.4
0.2
0
0
20
40
60
80
Time, days
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Reductive Dechlorination by
Anaerobic Microorganisms
[Polychlorinated Biphenyls (PCBs)]
Van Dort and Bedard, 1991; Appl. Environ. Micro. 57:1576-1578
Relative Mole Fraction
100
80
235-CB
60
26-CB
40
20
0
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2356-CB
25-CB
18 20 22 24 26 28 30 32 34 36 38
Incubation time (weeks)
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Biotic Pathway: Reduction
NH2
NO2
+ 6 e- + 6 H+
+ H2O
OH
OH
p-nitrophenol
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p-aminophenol
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Anaerobic Degradation of
2,4,6-trinitrotoluene [TNT]
Boopathy et al. 1993 Water Environ. Res. 65:272-275
TNT (ppm)
120
100
No electron acceptors
80
Sulfate Reducing
60
H2 : CO2
40
20
Nitrate Reducing
0
0
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10
20
30
Time (days)
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50
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Fate Processes of
Chlorophenols in Soil
Microbial transformations
Reductive dechlorination
Aerobic catabolism
Sorption
Sorption/
Desorption
Aerobic
Catabolism
PCPaq
PCP s
CO2
Reductive
Dechlorination
CPaq
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Aerobic
Catabolism
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Coupled Anaerobic-Aerobic
PCP Degradation
OH
Cl
OH
Cl
Cl
Anaerobic:
Cl-
Cl
Cl
H
H
H
Cl
removal
Cl
PCP
DCP
OH
H
H
CO2 + H2O + 2Cl-
+ O2
Aerobic: Cl- removal and ring cleavage
H
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Cl
Cl
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Biotic Pathway: Polymerization
 Oxidative coupling under aerobic conditions
Recalcitrant humic-like polymers. Example TNT
CH3
NO2
toxic, mutagenic
NO2
NO2
NO2
N
NO2
2,4,6-trinitrotoluene
NO2
N
NO2
2,2’,6,6’-tetranitro-4.4’-azoxytoluene
Field et al. 1995. Antonie van Leeuwenhoek 67:47-77
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Biotic Pathway: Polymerization
OH
OH
Cl
Cl
OH
O
Cl
O
Cl
2, 4-dichlorophenol
Cl
Cl
2,3,7,8-dibenzo-p-dioxin
Field et al. 1995. Antonie van Leeuwenhoek 67:47-77
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Case Study: Lake Apopka
• 1940’s marshes of lake Apopka drained
for agricultural use (19,000 acres)
• 1950-1990 extensive eutrophication and
numerous fish / alligator kills
• 1992 alligator / turtle population crash
– Reproduction problems, gender defintion
– 1980 Dicofol spill (90% gator die-off)
Lake Apopka
Marsh
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Case Study: Lake Apopka
• 1997 muck farm buy out by state ($100m)
• 1998 marsh restoration and reflooding
begins (July)
• November 1998 massive wading bird kill
on Apopka with dispersion (est. 1000+
birds)
• Necropsy found DDT, Diedrin, Toxaphene
Contaminant Exposures and Potential Effects on
Health and Endocrine Status for Alligators in the
Greater Everglades Ecosystem
Source: Wiebe et al (2003)
Xenobiotics in Wetlands
 Sources and examples
 Aerobic-Anaerobic interfaces
 Fate dictated by partitioning
 Abiotic pathways
Sorption, photolysis, volatilization
 Biotic pathways
Hydrolosis, Oxidation, Reduction, Synthesis
Mediated by microbial consortia
Biogeohemical controls
Environmental controls
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