Download Microbially influenced METAL corrosion

Consequences of Biogeochemical Cycling………..
Dr. Diby Paul
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Microbially influenced METAL corrosion
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Microbially influenced METAL corrosion
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Microbially influenced corrosion
Damage occurs to metals structures submerged in water – hulls of ships,
pipe lines that carry water/oil
Sulfate reducing bacteria are involved
SRB are active under strictly anaerobic conditions – utilize Sulfate as TEA
When BIOFILMS form on metal surfaces, anaerobic microsites develop
within biofilm
SRB are common constituents of natural environments, it easily gets in to
oil/water pipes
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ROLE OF ‘SRB’ IN CORROSION PROCESS???
BIOFILM help to create an anaerobic environment
SRB utilize H2 as an electron donor, thereby removing it from the environment
and providing a driving force for the electrochemical reactions
Step-1 Fe2+ + 2H2O
Step-2 4H2 + SO42-
Fe(OH)2
+ H2 (spontaneous)
H2S + 2OH- + 2H2O (SRB, eg. Desulfovibrio desulfuricans)
Step-3 H2S + Fe2+
Overall: 2Fe2+ + SO42-
FeS
+ H2 (spontaneous)
+ 2H2
FeS + Fe(OH)2
+ 2OH6
How to control???
Coat the metal surfaces with bactericidal chemicals
Mechanical and chemical disruption of biofilms
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CONCRETE CORROSION
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Microbes produce ACID METABOLITES
The binding component of concrete are acid sensitive
Fermentative bacteria produce ORGANIC ACIDS
Chemoautotrophic SULFUR OXIDISING BACTERIA – sulfuric acid
NITRIFYING BACTERIA – nitric acid
Corrosion of concrete sanitary sewer pipes have been well documented
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Concrete pipe
Head space
Anaerobic sulfide oxidation
H2SO4
H2SO4
H2S
Sewage pipe
H2S
Anaerobic sulfate reduction
Liquid sewage
Corrosion of concrete sanitary sewer pipe
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In the liquid environment, anaerobic condition are created by microbial activity
in the organic rich sewage
Under anaerobic condition, SRB generate sulfide which is converted to the
volatile H2S form and exchanged across liquid headspace interface
In the aerobic and moist environment of the head space, sulfur oxidizing
bacteria on the concrete wall, oxidize H2S to sulfuric acid
Sulfuric acid react with calcium hydroxide binder in the concrete to form
calcium sulfate with no binding capability
H2SO4 + Ca(OH)2
CaSO4 + 2H2O
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Over a 12 yr period of use, the thickness of pipe reduce from 88mm to 32mm
CONTROL MEASURES???
Treat sewage liquid to prevent production of H2S – inhibit microbial growth
Neutralize the surface of the concrete above the liquid level, by spraying
with high pH solution.
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ACID MINE DRAINAGE
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PYRITE (FeS2) is a major source of sulfur in the lithosphere
During mining of ore deposits, pyrite is exposed to oxygen and moisture and
becomes the source of an acidic iron-rich leachate known as ACID MINE
DRAINAGE
The pH can be less than 2 & can seriously impair the quality of receiving waters
4FeS2 + 15O2 + 14H2O
4Fe(OH)3- + 8H2SO4
This process is initiated spontaneously, but as the pH drops, a sulfur and iron
oxidizing bacterium, Thiobacillus ferroxidans begins to participate in the
reaction
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T. ferrooxidans attach directly to the pyrite crystal lattice and utilize FeS2 as
an electron donor
The autooxidation of Fe2+ is a very slow process
The bacteria enhances the oxidation process aiding in an increase in the
following reaction:
4FeS2 + 15O2 + 14H2O
4Fe(OH)3- + 8H2SO4
This leachate is highly acidic
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Microbial desulfurization of coal
Coal reserves can contain high levels of sulfur compounds (mainly as pyrite)
Industrial burning of coal releases sulfur compounds as SO2
In the atm, SO2 combines with water to form H2SO4 – acid rain
Most important S-compound in coal is FeS2
4FeS2 (pyrite) + 15O2 + 14H2O
4Fe(OH)3- + 8H2SO4
This reaction, aided by T. ferrooxidans removes S compounds from coal
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METAL RECOVERY
Microbially catalyzed oxidation of minerals is used in the commercial
recovery of copper, uranium and gold from low grade ores
As of 1989, more than 30% US copper and uranium production was
microbially mediated
Energy is provided to the microbe by the oxidation of Cu+ to Cu2+
2Cu2S
Chalcocite
+
O2
+
4H+
2CuS +
2Cu2+
+ 2H2O
leached copper
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Biomethylation of metals
Some of the metals are environmental pollutants and can be toxic to higher animals
Methylation occurs extensively in nature, Hg, being most extensively studied
Microbes can also bind long carbon chains to metal – alkylation
Methylation alters the toxicity of the element
The most common form of mercury in the env is Hg2+
Microbes methyl;ate mercury in aerobic and anaerobic environments
Sulfate reducing bacteria are major players in methylation
The intra cellular agent of mercury ,Methylation is methylcobalamine (CH3CoB12)
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CH3CoB12 + Hg2+ + H2O
Methylcobalamine
CH3Hg+ + H2OCoB12
methylmercury
CH3CoB12 + CH3Hg + + H2O
Methylcobalamine
(CH3)2 Hg+ + H2OCoB12
dimethylmercury
Mercury poisoning- minamata disease / cat’s dance disease
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To atmosphere
Hgo
reduction
Bioaccumulation
in food chain
methylation
Hg2+
oxydation
demethylation
To atmosphere
methylation
CH3Hg
(CH3)2Hg
demethylation
Spontaneous reaction
HgS
Microbially mediated reactions with Hg2+ in the env.
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