Metals and radionuclides

1
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
• PGE used it as rust inhibitor
• Migration into water supply

2
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

3
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.

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Bioimmobilization
Direct reductive precipitation using microbes
to precipitate heavy metals by changing their
valency
Electrons from organic C (lactate, acetate,
ethanol) or H2
e-
Oxidation
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
5
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), SO4-
U(IV), Cr(III) insoluble, immobile less toxic
Red
Ox
Red
Metal reducing bacterium
U(VI), Cr(VI) soluble mobile toxic
Fe(II), H2S
Useful in above-ground and in situ treatments
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Critical Biogeochemistry
Chemical Species
Equivalents
10
Electron Acceptors
0
pE
-10
Dominant Terminal Electron Accepting Process
Time
Distance from Source