metals greater solubility usually = greater toxicity chromium (cr) – six oxidation states, +1, +2,...
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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 radionuclidesMetals and radionuclides
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 (depleted uranium)
• 4 oxidation states (+4, +6 most common)• U(VI) water-soluble, U(IV) in-soluble
Metals and radionuclidesMetals and radionuclides
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
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 – using microbes to precipitate heavy metals by changing 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 and precipitation 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 AcceptorsElectron Acceptors
pEpE
AerobicRespiration
AerobicRespiration
O2O2
OrganicsOrganics
O2O2
SOSO44--SOSO44--
SulfateReduction
SulfateReduction
SO4-SO4-
HH22SSHH22SS
MethanogenesisMethanogenesis
CO2CO2
CH4CH4
H2H2
DenitrificationDenitrification
NO3-NO3-
NONO33--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 BiogeochemistryCritical Biogeochemistry
PCE/TCE
Mn (IV)
Cr (VI)U (VI)