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Metals and radionuclides

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

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Metals and radionuclides

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  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 • PG&E 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.

  4. 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

  6. H2 Fe (II) Organics SO4- H2S O2 NO3- CH4 O2 NO3- Mn (IV) U (VI) Cr (VI) Fe (III) SO4- CO2 PCE/TCE Aerobic Respiration Iron (III) Reduction Sulfate Reduction Denitrification Methanogenesis Critical Biogeochemistry Chemical Species Equivalents +10 Electron Acceptors 0 pE -10 Dominant Terminal Electron Accepting Process Time Distance from Source

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