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)