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“OK, so what’s the speed of dark?”

“When everything is coming your way, you're obviously in the wrong lane”. “Who laughs last, thinks slowest”. “OK, so what’s the speed of dark?”. U6220: Environmental Chem. & Tox. Thursday, June 30 2005. NOM: Power of ecosystems - redox chain

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“OK, so what’s the speed of dark?”

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  1. “When everything is coming your way, you're obviously in the wrong lane” “Who laughs last, thinks slowest” “OK, so what’s the speed of dark?”

  2. U6220: Environmental Chem. & Tox.Thursday, June 30 2005 NOM: Power of ecosystems - redox chain Oxido-Reduction: Environmental speciation and remediation Metals in the environment: some case studies

  3. Central Park Lake Massive fluxes of soot: 30 fold higher than other urban lakes

  4. Speciation: The role of Ecosystems • Ecosystem conditions controls speciation • Speciation controls mobility and toxicity

  5. 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?

  6. Geochemical controls of As cycling: • Fe/Mn oxyhydroxides

  7. Single Variable Diagrams: pH What is the most abundant species of arsenic in natural waters? How does pH influence As distribution?

  8. 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

  9. 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).

  10. Natural Organic Matter: Power of ecosystems Photosynthesis: 6H2O + 6CO2 + E (h)  C6H12O6 + 3O2 Respiration: C6H12O6 + 3O2 6H2O + 6CO2 + E

  11. 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)1/4[CH2O]) • When you solve for Q: • DGt = -29.8 kcal/mol

  12. 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

  13. 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

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

  15. 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

  16. Two Variable Diagrams: pE-pH • Let’s consider the reaction: • Fe2+ + 3H2O  Fe(OH)3 + 3H+ + e-

  17. As desorption and dissolution due to changes in reducing conditions

  18. Two Variable Diagrams: pE-pH What is the most abundant species of arsenic in natural waters? H3AsO4  H2AsO4- + H+ 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)

  19. 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+

  20. 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

  21. 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 • <2 mg/L: Hypoxia

  22. Density change and turnover (ventilation) Fresh water maximum density at ~4C  Seasonal inversion and stratification

  23. Seasonal mixing and Dissolved O2 Strong seasonal dependence on ventilation and nutrient-oxygen mixing

  24. Saturated zones - The ecological redox scale: Change in oxidant concentrations with respect to distance in groundwater flow

  25. Arsenic in Texas Drinking Water What is the environmental legacy of U mining in South Texas…...

  26. 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

  27. 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.

  28. Arsenicin Texas Groundwater TWDB and NURE Data Sets

  29. Molybdenumin Texas Groundwater TWDB and NURE Data Sets

  30. Geogenic Source of Metals Catahoula formation, an oxidized volcanic ash is a source of U, As, Mo and other trace metals

  31. Regionally Reduced Oxidized Se U Mo As Metal cycling and groundwater redox: A case of “chromatographic separation” Adapted from Devoto (1978)

  32. South Texas Uranium Roll Front

  33. Uranium cycling - A proxy for nuclear waste? Fe(III) are generally the most important potential sorbents for U (with organic matter). If reduction doesn’t follow adsorption, uranyl can be desorbed by an increase in alkalinity or increase in pH (low sorption capacity for carbonate complexes!)

  34. 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 (<300 feet) and the poorly cemented nature of overburden • Voluminous spoils stockpiled near pits. Two processing mills in western Karnes Co. generated large tailings piles

  35. San Antonio River Nueces River U Mining in the Nueces and San Antonio River Basin

  36. 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.

  37. 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.

  38. Eroded Spoil at the Haase Moy Wiatrek Mine Gonzales County

  39. 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

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