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• OK, so whats the speed of dark?

When everything is coming your way, you're
obviously in the wrong lane
Who laughs last, thinks slowest
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(No Transcript)
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U6220 Environmental Chem. Tox.Thursday, June
30 2005

1. NOM Power of ecosystems - redox chain
2. Oxido-Reduction Environmental speciation and
   remediation
3. Metals in the environment some case studies

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Central Park Lake
Massive fluxes of soot 30 fold higher than other
urban lakes
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Speciation The role of Ecosystems

• Ecosystem conditions controls speciation
• Speciation controls mobility and toxicity

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Fate of contaminants Speciation
Metals
Metals do not change per se ? speciate
Single variable diagram pH
What is the most abundant species of iron in
natural waters?
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• Geochemical controls of As cycling
• Fe/Mn oxyhydroxides

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Single Variable Diagrams pH
What is the most abundant species of arsenic in
natural waters?
How does pH influence As distribution?
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Chemical Reactions
Oxidation-Reduction (Redox)
The redox state of an element can be of
considerable interest, because it often
determines the chemical and biological behavior,
including toxicity, of that element as well as
its mobility in the environment CrO42-
Cr3 (mobile and very toxic) (less
solube and toxic) CrO42- 3e- 8H ?? Cr3
4H2O
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Two Variable Diagrams pE-pH
As a general rule, most reactions that involve
electrons also involve protons. Oxidation usually
releases protons or acidity (basic cause for acid
mine drainage). Conversely, reduction usually
consumes protons, and the pH rises Fe2 3H2O ?
Fe(OH)3 3H e-
The reaction affects the pH of the medium
(solution) and vice versa ? the pH of the
environment affects the redox potential
established by Fe3 (and other species).
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Natural Organic Matter Power of ecosystems

• Photosynthesis
• 6H2O 6CO2 E (h?) ? C6H12O6 3O2
• Respiration
• C6H12O6 3O2 ? 6H2O 6CO2 E

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NOM Power of Ecosystems

• Oxidation-Reduction
• CH2O 1/4H2O ? 1/4CO2 e- H
• 1/4O2 e- H ? 1/2H2O
• CH2O 1/4O2 ? 1/4CO2 1/4H2O
• DGº -29.9 kcal/mol
• However
• DGt DGº RT lnQ
• And
• Q (PCO2)1/4/((PO2)1/4CH2O)
• When you solve for Q
• DGt -29.8 kcal/mol

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NOM Power of Ecosystems

• Oxidation-Reduction Anoxic biodegradation (lack
  of molecular O2)
• 1) Nitrate reduction
• 1/4 CH2O 1/5NO3- ? 1/10 N2 1/4 CO2 7/20 H2O
  
• DGº -30.3 kcal/mol
• However
• DGt -27.5 kcal/mol
• 2) Iron hydroxide reduction
• 1/2 CH2O Fe(OH)3 2H ? Fe2 1/2 CO2 11/4
  H2O
• DGº -24 kcal/mol
• However
• DGt -12 kcal/mol

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NOM Power of Ecosystems

• Oxidation-Reduction Anoxic biodegradation (lack
  of molecular O2)
• 3) Manganese oxide reduction
• 1/2 CH2O MnO2 2H ? Mn2 1/2 CO2 11/4 H2O
  
• DGt -24.3 kcal/mol
• 4) Sulfate reduction
• 1/2 CH2O 1/8 SO42- 1/8 H ? 1/8 HS- 1/4 CO2
  1/4 H2O
• DGt -7.4 kcal/mol
• 5) Methanogenesis (fermentation)
• 1/4 CH2O ? 1/8 CO2 1/8 CH4
• DGt -5.5 kcal/mol

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NOM Power of Ecosystems
- The ecological redox scale
Change in oxidant concentrations with respect to
time in a flooded soil
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NOM Power of Ecosystems

• The ecological redox scale Change in oxidant
  concentrations with respect to distance in
  groundwater
• 1/2 CH2O Fe(OH)3 2H ? Fe2 1/4 CO2 11/4
  H2O
• 1/4 CH2O 1/2 MnO2 H ? 1/2 Mn2 1/4 CO2
  3/4 H2O
• 1/2 CH2O 1/8 SO42- 1/8 H ? 1/8 HS- 1/4 CO2
  1/4 H2O

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Two Variable Diagrams pE-pH

• Lets consider the reaction
• Fe2 3H2O ? Fe(OH)3 3H e-

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• As desorption and dissolution due to changes in
  reducing conditions

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Two Variable Diagrams pE-pH
What is the most abundant species of arsenic in
natural waters?

1. H3AsO4 ? H2AsO4- H
2. H2AsO4- 3H 2e- ? H3AsO3 H2O

• Speciation is important because it often
  determines
• Mobility (solubility)
• Toxicity
• i.e. arsenite (III) is about 60 times more toxic
  than arsenate (IV)

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Redox Potential - Acid Mine Drainage
Sulfate reduction SO42- 10H 8e- ? HS-
4H2O CH2O H2O ? CO2 4H 4e- SO42- 2CH2O
2H ? H2S 2H2O 2CO2 With the presence of
Fe2 Fe2 H2S ? FeS 2H And FeS S ? FeS2
FeS2 H2O 7/2O2 ? Fe2 2SO42- 2H And FeS2
14Fe3 8H2O ? 15Fe2 8H2SO4 Later 4Fe2
O2 10H2O ? 4Fe(OH)3 8H
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Redox potential and Speciation of Environmental
Contaminants
Chromium (tanning processes). Small scale
tanneries produce approx 0.4 kg of Cr(III) waste
per 100 kg of treated hide. 2Cr3 ? 2Cr6
6e- oxidation of Cr(III) to Cr(VI) 2Cr3 7 H2O
? Cr2O72- 14H 6e- 3/2 O2 6H 6e- ? 3
H2O 2Cr3 4 H2O 3/2 O2 ? Cr2O72-
8H However, in anaerobic systems CrO42
3Fe2 8 H2O ? Cr(OH)3 3Fe(OH)3 4H
Redox Potential - Acid Mine Waters
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O2 solubility and ventilation

• O2 solubility is dependent on water temperature
• Usually oscillates between 6-14 mg/L in aerated
  natural waters. O2 diffusion in surface waters is
  a slow process aided by turbulent mixing of water
  (and cold temperatures)

• How much O2 do aquatic organisms need?
• 8-15 mg/L Excellent
• 6-8 mg/L OK
• 4-6 mg/L Stressed
• 2-4 mg/L Critical
• lt2 mg/L Hypoxia

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Density change and turnover (ventilation)

• Fresh water maximum density at 4?C ? Seasonal
  inversion and stratification

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Seasonal mixing and Dissolved O2

• Strong seasonal dependence on ventilation and
  nutrient-oxygen mixing

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Saturated zones
- The ecological redox scale Change in oxidant
concentrations with respect to distance in
groundwater flow
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Arsenic in Texas Drinking Water
What is the environmental legacy of U mining in
South Texas...
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Finalized EPA Drinking Standard for Arsenic

• The Safe Drinking Water Act, as amended in 1996,
  requires EPA to revise the existing drinking
  water standard for arsenic.
• EPA reduced the maximum level of arsenic allowed
  in drinking water that reduces the maximum level
  allowed from 50 parts per billion (ppb) to 10
  ppb.
• This was challenged by the Bush Administration
• New standard will be applied to all community
  water systems (serving 254 million people)
• 12 of these systems will likely have to take
  corrective action
• Estimated National Cost
• 3 ppb 645 M, 5 ppb 379 M
• 10 ppb 166 M, 20 ppb 65 M

Fallonites, Don Cooper, 82, and wife Norma, 81,
raise a toast to Nevada's arsenic-rich homebrew
on the outskirts of town. Concentrations in
drinking water are approximaately 100
ppb. Outside magazine, February 2001
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Data map 31,350 ground-water arsenic samples
collected in 1973-2001 Ryker, S.J., Nov. 2001,
Mapping arsenic in groundwater Geotimes v.46
no.11, p.34-36.
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Arsenicin Texas Groundwater
TWDB and NURE Data Sets
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Molybdenumin Texas Groundwater
TWDB and NURE Data Sets
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Geogenic Source of Metals
Catahoula formation, an oxidized volcanic ash is
a source of U, As, Mo and other trace metals
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Metal cycling and groundwater redox A case of
chromatographic separation
Adapted from Devoto (1978)
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South Texas Uranium Roll Front
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Uranium cycling - A proxy for nuclear waste?
Fe(III) are generally the most important
potential sorbents for U (with organic matter).
If reduction doesnt follow adsorption, uranyl
can be desorbed by an increase in alkalinity or
increase in pH (low sorption capacity for
carbonate complexes!)
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Texas Uranium History

• Oxidized uranium ores were open-pit mined from
  sandstone-hosted roll-front deposits (1960 -
  1983)
• Open pit mining feasible because of shallow depth
  to ore (lt300 feet) and the poorly cemented nature
  of overburden
• Voluminous spoils stockpiled near pits. Two
  processing mills in western Karnes Co. generated
  large tailings piles

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U Mining in the Nueces and San Antonio River
Basin
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Surface Exposure of Protore
Reduced sediments near the uranium ore were
enriched in As, Mo, Se and radionuclides. Termed
protore (proto ore), this material was placed
on the top of spoil piles where it was most
readily eroded.
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Stratigraphic Inversion
Oxidized overburden (upper strata) were placed at
the bottom of spoil Deeper strata enriched in
trace elements was placed on top of spoil.
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Eroded Spoil at the Haase Moy Wiatrek Mine
Gonzales County
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South Texas Ecological Impacts of Metals
Molybdenosis in Black Angus Cattle, South Texas
Arsenic exposure to wildlife at ground water
seeps in the Nueces River watershed