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MetalsGreater 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 Metals and radionuclidesradionuclides
RadionuclidesRadiotoxicity – from emitted particles – DNA damageChemotoxicity – 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 (depleteduranium)
• 4 oxidation states (+4, +6 most common)• U(VI) water-soluble, U(IV) in-soluble
Metals and Metals and radionuclidesradionuclides
Bioremediation
Bioremediation strategies for metals andradionuclides are extremely varied andinclude both above ground and in situtreatments and many of the treatmentsalready perfected for organicbiodegradation.
– Above ground strategies include bioreactors,biosorption, prepared beds, biopiles,bioleaching, phytoremediation, etc.
– In situ strategies include bioimmobilization,biomobilization, soil washing, infiltrationgalleries, bioventing, etc.
Bioimmobilization
U(VI), Cr(VI) – soluble, toxic and mobile
U(IV), Cr(III) – insoluble, less toxic, immobile
e-
Electrons from organic C (lactate, acetate, ethanol) or H2
Metal reducing bacterium
Direct reductive precipitation – usingmicrobes to precipitate heavy metals bychanging their valency
Useful in above-ground and in situ treatments
Oxidation
Reduction
Bioimmobilization
Fe(III), SO4-
Fe(II), H2S
e-
Electrons from organic C (lactate, acetate, ethanol) or H2
Metal reducing bacterium
Indirect reductive precipitation -microbial reduction of other TEAs (Fe3+
or SO4-) results in abiotic reduction andprecipitation of heavy metals
Useful in above-ground and in situ treatments
U(VI), Cr(VI)solublemobiletoxic
U(IV), Cr(III)insoluble,immobile less toxic
Ox
Red Ox Red
TimeTime
Distance from SourceDistance from Source
Dominant Terminal Electron Accepting ProcessDominant Terminal Electron Accepting Process
+10+10
00
-10-10
Electron AcceptorsElectron AcceptorsElectron Acceptors
pE
pE
AerobicRespiration
AerobicRespiration
O2O2
OrganicsOrganics
O2O2
SO4-SOSO44--
SulfateReduction
SulfateReduction
SO4-SO4-
H2SHH22SS
MethanogenesisMethanogenesis
CO2CO2
CH4CH4
H2H2
DenitrificationDenitrification
NO3-NO3-
NO3-NONO33--
Iron (III)ReductionIron (III)
Reduction
Fe (III)Fe (III)
Fe (II)Fe (II)
Chemical SpeciesChemical Species
Eq
uiv
alen
tsE
qu
ival
ents
Critical BiogeochemistryCritical BiogeochemistryCritical Biogeochemistry
PCE/TCE
Mn (IV)
Cr (VI)U (VI)