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Metal and Radionuclide Bioremediation

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Metals and radionuclides
Metals
Greater solubility usually = greater toxicity
Chromium (Cr) – Six oxidation states, +1, +2, +3, +4, +5, +6
• +3, +6 most common
• used to prevent corrosion (stainless steel)
• Cr(VI) is toxic (skin, lungs, kidneys) and carcinogenic
• Hexavalent Cr(VI) is most toxic and most soluble
• Induces free-radical formation
• oxidative stress – DNA/membrane damage
Erin Brockovich
• PG&E used it as rust inhibitor
• Migration into water supply
Metals and radionuclides
Radionuclides
Radiotoxicity – from emitted particles – DNA damage
Chemotoxicity – oxidative stress – DNA/membrane damage
Uranium – heaviest natural element - 17 isotopes
• Natural form % = U-238 (99.27), U-235 (0.72), U-234 (0.006)
• U-238 (t1/2 = 4.5 billion years), U-235 (t1/2 = 7 million years)
• Emit alpha, beta and gamma
• Used in nuclear fuel – U-235 (readily fissionable)
• Used in nuclear and conventional weapons
• Uranium enrichment (increase percentage of U-235)
• U-238 used as a precursor of Pu-239
• U-238 used to strengthen ammunition (depleted
uranium)
• 4 oxidation states (+4, +6 most common)
• U(VI) water-soluble, U(IV) in-soluble
Bioremediation
Bioremediation strategies for metals and
radionuclides are extremely varied and
include both above ground and in situ
treatments and many of the treatments
already perfected for organic
biodegradation.
– Above ground strategies include bioreactors,
biosorption, prepared beds, biopiles,
bioleaching, phytoremediation, etc.
– In situ strategies include bioimmobilization,
biomobilization, soil washing, infiltration
galleries, bioventing, etc.
Bioimmobilization
Direct reductive precipitation – using
microbes to precipitate heavy metals by
changing their valency
Electrons from organic C (lactate, acetate, ethanol) or H2
eOxidation
U(VI), Cr(VI) – soluble, toxic and mobile
Reduction
Metal reducing bacterium
U(IV), Cr(III) – insoluble, less toxic, immobile
Useful in above-ground and in situ treatments
Bioimmobilization
Indirect reductive precipitation microbial reduction of other TEAs (Fe3+
or SO4-) results in abiotic reduction and
precipitation of heavy metals
Electrons from organic C (lactate, acetate, ethanol) or H2
e- Ox
Fe(III), SO4U(IV), Cr(III)
insoluble,
immobile
Red
Ox Red less toxic
Metal reducing bacterium
Fe(II), H2S
U(VI), Cr(VI)
soluble
mobile
toxic
Useful in above-ground and in situ treatments
Critical Biogeochemistry
H2
Equivalents
Chemical Species
Fe (II)
Organics
SO4-
H2S
NO3-
O2
O2
CH4
NO3-
+10
Mn (IV)
U (VI)
Cr (VI)
0
Electron Acceptors
Fe (III)
SO4-
CO2
PCE/TCE
-10
Dominant Terminal Electron Accepting Process
Aerobic
Respiration
Denitrification
Iron (III)
Reduction
Sulfate
Methanogenesis
Reduction
Time
Distance from Source
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