Title: AEROBIC AND ANAEROBIC BIODEGRADATION
1- AEROBIC AND ANAEROBIC BIODEGRADATION
2Bioavailability
Not accessible
Accessible
3Biodegradation and Mineralization
Biodegradation a biological process of reducing
a compound complexity. Mineralization a
degradation process of organic compounds into
inorganic one.
4Biodegradation and Biotransformation
- Conversion of contaminants to mineralized (e.g.
CO2, H2O, and salts) end-products via biological
mechanisms - Biotransformation refers to a biological process
where the end-products are not minerals (e.g.,
transforming TCE to DCE) - Involves the process of extracting energy from
organic chemicals via oxidation of the organic
chemicals
5Biodegradation
- Aerobic and anaerobic degradation
- Reduces aqueous concentrations of contaminant
- Reduction of contaminant mass
- Most significant process resulting in reduction
of contaminant mass in a system
6Fundamentals of Biodegradation
- All organics are biodegradable, BUT
biodegradation requires specific conditions - There is no Superbug - not Volkswagon
- Contaminants must be bioavailable
- Biodegradation rate and extent is controlled by a
limiting factor
7Biodegradation
GROWTH - CELL DIVISION INCREASE IN BIOMASS
O2 consumption
ORGANIC POLLUTANT AND NUTRIENTS (C,P,N,O,Fe,S)
CO2 evolved
Controlled release of energy Slow Burning!
8Microbes Role on Biodegradation
Biodegradation uses naturally occurring
microbes. Microbes use the organic compounds for
their carbon and sometimes energy sources to grow.
9What do Microbes Need to Grow?
- Like all living things, microorganisms need
- Food
- Supply carbon
- Supply energy
- Respiratory substrate
- Something to breathe
- Some use oxygen as the electron acceptor,
others can use alternatives including chlorinated
solvents. Otherwise there should be donor
electron as energy source
10Oxygen and Electron Acceptors Crucial for
biodegradation reactions in the environment
11Terminology and Definitions
- Electron donor
- A compound that donates electrons during its
oxidation - Simple organic compounds such as sugars,
alcohols, or methane can be oxidized to carbon
dioxide (CO2) - Electron acceptor
- A compound that accepts electrons during its
reduction - Inorganic compounds like oxygen, nitrate,
sulfate, oxidized metals, or CO2 can be reduced
to water, dinitrogen gas, hydrogen sulfide,
dissolved metals, or methane, respectively
12Electron Acceptor Use Preferred Order
High
O2 NO3- Fe (III) Mn (IV) SO42- CO2, Organics
Energy Yield
Low
13Role of electron acceptors rate of biodegradation
O2
NO3-
SO42-
Fe3
NO2- N2
H2O
H2S
Fe2
0.814V
-0.214V
-0.185V
0.741V
FAST GROWTH
SLOW GROWTH
14Aerobic vs Anaerobic Biodegradation (A matter of
terminal electron acceptor)
- If oxygen is the terminal electron acceptor, the
process is called aerobic biodegradation - All other biological degradation processes are
classified as anaerobic biodegradation - In most cases, bacteria can only use one terminal
electron acceptor - Facultative aerobes use oxygen, but can switch to
nitrate in the absence of oxygen
15Electron Acceptor Zone Formation
16Microbes
- Obligate aerobes - Microbes for which the
presence of oxygen is essential. Oxygen is the
only electron acceptor that these species can
employ. - Facultative anaerobes - Can use oxygen if it is
available but are able to switch to alternate
electron acceptors when oxygen is depleted. - Obligate anaerobes - Use alternate electron
acceptors exclusively. Oxygen is toxic.
17Electron Exchange
18Metabolism and Oxidation
19Biotransformation of Organic Substances
- If carbon is in oxidized form (positive
valencies), biotranformation by reduction is more
important - If carbon is reduced (negative valencies),
biotranformation by oxidation is more efficient
perchloroethene (PCE) C(II)
carbon tetrachloride (CT) C(IV)
vinyl chloride C(-I)
20Aerobic Oxidation
21Oxidation and Extraction of Energy
- Oxidation of organic matter provides energy for
living organisms because such reactions are
thermodynamically favored - ¼ CH2O ½ O2 ? ¼ CO2 ¼ H2O
- ?G? -119.98 kJ/electron equivalent
- Microbes employ catalyzing enzymes to surmount
kinetic barriers. - Enzymes function by forming a complex with the
reactants, bringing them in close contact.
22Microbiology of Aerobic Oxidation
- A wide variety of microorganisms can carry out
these oxidation processes - Activity is believed to be ubiquitous
bioaugmentation is not likely to be required - Activity can be stimulated by oxygen addition
- Oxygen solubility is limited so the treatable
concentrations are low
23Oxygen Utilization of Substrates
- Benzene C6H6 7.5O2 gt 6CO2 3H2O
- Stoichiometric ratio (F) of oxygen to benzene
- Each mg/L of benzene consumes 3.07 mg/L of O2
24Fixation of oxygen as a first step in
biodegradation
Cell membrane
Cell Biomass
Further degradation
CO2
25Cl
Cl
Cl
BphD
BphC
BphA
BphB
OH
O
COOH
COOH
OH
OH
Degradation pathway of biphenyl/PCBs by
Pseudomonas KF707
26Biodegradation of Pentachlorophenol
27Aerobic Oxidation
Cl-
R-Cl
CO2 H2O
O2
28Cometabolism (Aerobic)
- Fortuitous transformation of a compound by a
microbe relying on some other primary substrate - Generally a slow process - Chlorinated solvents
dont 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)
29Methanotrophs, example of cometabolism
- Use methane as the primary substrate, but
cometabolize chlorinated solvent compounds. - They oxidize methane to methanol using methane
monooxygenase (MMO). - MMO is non-specific, and cometabolizes
trichloroethene (TCE) to TCE epoxide - This eventually degrades to CO2, Cl- and H2O.
30Cometabolic Transformations ofChlorinated
Aliphatic Hydrocarbons (CAHs)
NADH, O2
Primary Reaction
CO2 , H2O
CH4
MMO
O
- CCl2CHCl Cl2C CHCl CO2,
Cl , H2O
-
Secondary Reaction
NADH, O
2
31Aerobic Co-metabolism
32Anaerobic Co-metabolism
Cl-
R-Cl
O2
Mn4 NO3- Fe3 SO42- CO2 H2
Mn2 N2 Fe2 S2- CH4
33Dehalogenation
- Dehalogenation refers to the process of stripping
halogens (generally Chlorine) from an organic
molecule - Dehalogenation is generally an anaerobic process,
and is often referred to as reductive
dechlorination - RCl 2e H gt RH Cl
- Can occur via dehalorespiration or cometabolism
- Some rare cases show cometabolic dechlorination
in an aerobic environment
34Dehalogenation of PCE
- PCE (perchloroethylene or tetrachloroethylene)
- TCE (trichloroethylene)
- DCE (cis-, trans-, and 1,1-dichloroethylene
- VC (vinyl chloride)
35Anaerobic Transformation
- Reductive dechlorination - Most common anaerobic
process. A cometabolic process in which the
solvent is reduced by the replacement of a
chlorine atom with a hydrogen atom. - Compounds that contain more than 1 Cl atom
dechlorinate in a series of steps, each involving
loss of a single Cl atom. - Carbon tetrachloride degrades to chloroform via
reductive dechlorination. The latter is more
resistant and accumulates. - PCE and TCE degrade with cis-1,2-DCE and vinyl
chloride as intermediates. - VC is highly mobile and toxic.
36Anaerobic biotransformation pathways of
chlorinated aliphatic hydrocarbons (From Vogel et
al., 1987). a abiotic pathway.
37Reductive dechlorination of TCE under anaerobic
conditions (from Vogel, 1994).
38Dehalorespiration (Anaerobic)
- 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
hydrogen-producing bacteria to produce H2, which
is the preferred primary substrate
39Dependence on Redox Condition
1. Highly biodegradable
2. Moderately biodegradable
3. Slow biodegradation
4. Not biodegraded
40Biodegradation of Chlorinated Organics
- More resistant to biodegradation than aromatic
hydrocarbons. - Bacteria cannot use most of these compounds as a
substrate. - Most biodegradation occurs via cometabolism.
- Cometabolism is slower than heterotrophic
metabolism and requires the presence of suitable
primary substrates.
41Stoichiometry
- Electron Donor to Electron acceptor ratios
- Hydrocarbon requirements for electron acceptor
are well defined - Electron donor requirements for dechlorination
are poorly defined - Cometabolic processes are not predictable
- Each Electron Donor/Electron Acceptor pair has a
unique stoichiometric ratio