Lecture Notes - University of Bristol

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Environmental Geochemistry
DM Sherman, University of Bristol
Organic Geochemistry
DM Sherman, Environmental Geochemistry
University of Bristol
Composition of Living Matter
Proteins
Carbohydrates
Lipids
Lignin
Vascular plants
7
50
10
33
Phytoplankton
23
66
11
0
Diatoms
29
63
8
0
Zooplankton
60
22
18
0
Page 1
Molecular Components of Biomass
Lipids
Carbohydrates and Polysaccharides
Molecular Components of Biomass
Lignin
Page 2
Hydrolysis Products of Biomass
Fatty Acids (Hydrophyllic Acids)
Sugars
Amino Acids
Biological Breakdown of Biomolecules
Amino Acids
Hydrolysis
Fatty Acids
Sugars
Respiration
CO2
Humic/Fulvic Acids
Proteins
Lipids
Fermentation
Carbohydrates
Extremely anaerobic
conditions
Lignin
Acetate
Alcohols
Methanogenesis
CH4
Page 3
Abiotic Breakdown Products of Biomass
Fulvic Acids
Abiotic Breakdown Products of Biomass
Humic Acid
Page 4
Complexation of Metals by Humic/Fulvic
Acids
Some metals (Cu, Hg) are
complexed by HA and FA.
This may play a role in oreforming processes.
Organic Carbon in Terrestrial Aquatic
systems
Page 5
Nature of Humic/Fulvic Acids and Humin
• Major form of organic matter in soils and freshwater.
• Origins are still not fully understood but main
hypothesis is that humic/fulvic acids are derived from
the breakdown of lignin.
• Humic Acid = soluble at high pH only
• Fulvic Acid = soluble over all pH values
• Humin = not soluble at any pH
Organic Carbon in Aquatic systems
Page 6
Organic Carbon in Sediments
• Sediments contain < 1% organic C.
• Nearly all DOC and POC in the water column is
oxidized by respiration.
• For organic C to accumulate in sediments, the flux of
POC must be high and the redox conditions must be
anaerobic.
Diagenesis and Kerogen Formation
• Hydrolysis of complex organics
• Functional groups are removed.
• Double bonds are hydrogenated to give saturated
hydrocarbons
• Aromatic compounds increase relative to aliphatic
• Condensation of molecular fragments to complex
macromolecules
• End product is kerogen a mixture of complex organic
compounds that dominate organic matter in sediments.
Page 7
Catagenesis of Kerogen to Oil and Gas
Diagenesis: methanogens
produce methane. Organic
matter converted to kerogen.
Catagenesis: breakdown
of kerogen
Metagenesis: breakdown
of oil into gas + graphite
Methane Hydrates
At the pressures and temperatures
of the ocean bottom, biogenic
methane is trapped as clathrates.
Page 8
Methane Hydrates
The stability field of
methane hydrates is
limited to shallow sediment
depths.
Methane is an extremely
potent greenhouse gas
and release from
sediments could greatly
accelerate global
warming.
Carbon Isotopic Fractionation
Page 9
Carbon Isotopic Fractionation
Pollution by Organic Compounds
Page 10
Organic Pollutants: BTEX Compounds
Biodegradation of Organics (Cont.)"
C6H6 + 7.5 O2 ! 6CO2 + 3H2O
+
! 6CO2 + 18H2O + 15Mn2+
C6H6 + 6NO3- + H+ ! 6CO2 + 3N2 +6H2O
C6H6 + 30 FeOOH + 60 H
+
! 6CO2 + 48 H2O + 30Fe2+
Decreasing Eh
C6H6 + 15MnO2 + 30H
C6H6 + 3.75SO42- + 7.5H+ ! 6CO2 + 3.75H2S + 3H2O
C6H6 + 4.5H2O ! 2.25CO2 + 3.75CH4
Page 11
Spatial Distribution of electron acceptors after
biodegradation progresses in soil
PAH (Polycyclic Aromatic Hydrocarbons)
in Creosote"
OH
phenol
OH
CH3
napthalene
Benzo-[a]-pyrene
cresol
anthracene
Creosote is a common wood preservative but
is now banned because it contains PAHs
(carcinogenic). These are slow to degrade in
soil.
Page 12
Synthetic Chlorohydrocarbons"
Organohalide Breakdown by
Methanogens"
(1) CCl2=CCl2 (PCE) + H2 ! CCl2=CClH (TCE)+ HCl
(2) CCl2=CClH (TCE) + H2 ! CCl2=CH2 (DCE) + HCl
(3) CCl2=CH2 (DCE) + H2 ! CH2=CHCl (VC) + HCl
Breakdown of vinyl chloride (CH2=CHCl) requires
aerobic conditions:
CH2=CHCl (VC) + 5/2O2 ! 2CO2 + H2O + HCl
Page 13
Summary: Abiotic Breakdown of
Biomolecules
Lignin, Carbohydrates, Proteins, Lipids
Fulvic, Humic acids, Humin
Kerogen
Hydrocarbons
Methane
Graphite
Page 14

Lecture Notes - University of Bristol

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