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Fate and Transport of Contaminants from Acid Mine Drainage

Fate and Transport of Contaminants from Acid Mine Drainage. US EPA Scientist-to-Scientist Meeting Las Vegas, NV June 14-15, 2000 Richard T. Wilkin, Ph.D. National Risk Management Research Laboratory Ada, OK. Fate & Transport Issues.

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Fate and Transport of Contaminants from Acid Mine Drainage

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  1. Fate and Transport of Contaminants from Acid Mine Drainage US EPA Scientist-to-Scientist Meeting Las Vegas, NV June 14-15, 2000 Richard T. Wilkin, Ph.D. National Risk Management Research Laboratory Ada, OK

  2. Fate & Transport Issues • Chemical, Physical, and Biological Processes from Source => • Media Type • Air, Water, Sediment • Metal Type • Geochemical, Toxicity, Ore association

  3. Chemical Processes • Dissolution, sorption, nucleation, growth • Oxidation-Reduction reactions • Acid-Base reactions • Isotope exchange reactions • Modeling exercises • Chemical Speciation • Saturation: DGr = RT lnQ/Keq • Kinetics

  4. Physical & Biological Processes • Transport • Water • Sediment • Wind • Microbial • S-oxidizers, Fe-oxidizers • S-reducers, Fe-reducers • Wetland Plants

  5. Metals Hg, Pb As, Se Cd, Sb, Ag, CN Cu, Zn Pb, U Cr, Fe Hg

  6. Metal Type: Pearson Classification

  7. As Mobility/Speciation: Redox sw gw

  8. Metal Mobility: pH Supersat. solution

  9. Fate & Transport Topics • Kinetics/Mechanisms of S(-II) oxidation • Microbial Processes • Product Transport in Surface Waters • Product Transport/Storage in Sediments • Impact of ARD on Ground Waters • Wetlands • Supergene Processes

  10. Pyrite Oxidation Pyrite Dissolution/Overall Reaction FeS2 + 15/4O2 + 7/2H2O = “Fe(OH)3” + 2H2SO4 Low pH, high acidity Metal rich: As, Sb, Zn, Cu… Fe, Al, Mn rich Sulfate rich

  11. Pyrite Oxidation: II FeS2 + 7/2O2 + H2O = Fe2+ + 2SO42- + 2H+ FeS2 + 14Fe3+ + 8H2O = 15Fe2+ + 2SO42- +16H+ after Stumm and Singer (1980)

  12. Pyrite oxidation kinetics After Langmuir (1996) using rate equations from Williamson & Rimstidt (1994), PyArea=0.05 m2/g

  13. Pyrite Oxidation: III • Chemical • oxygen, Fe(III), water, buffering • Physical • texture, grain size • Ore processing, framboidal pyrite • Biological • Fe- and S-oxidizing bacteria

  14. AMD Prediction(EPA 530-R-4-036, December 1994) • Assessment of Acid-generation and Acid-neutralization capacity (acid, sulfate) • Hydrologic Assessment: Availability of Oxygen and Water (acid, sulfate) • Ore Deposit/Waste rock/Tailings Characterization (metals)

  15. Ore Deposit Types • Volcanic-hosted Massive Sulfides • Sediment-hosted Massive Sulfides • - Shale Type (Rammelsberg) • - Carbonate Type (MVT) • Mafic Intrusive Related (Sudbury, Duluth Complex) • Porphyry Cu-Mo/Skarn • Mesothermal Au • Epithermal Au • Carlin Type Au • Continental Geothermal (Hg, As, Sb) • Coals

  16. Ore Minerals: Metal Mobilization Sources from Metal Sulfides Fe - pyrite, marcasite, pyrrhotite Hg - cinnabar Pb – galena Ag – acanthite, galena As – arsenopyrite, As-rich pyrite, orpiment, tetrahedrite, enargite Ni – pentlandite, millerite Cu – covellite, chalcocite, djurleite, bornite, chalcopyrite, enargite Cd – greenockite Zn – spahlerite Co – cobaltite

  17. Transport of OxidationProducts to Surface Waters Sorption trend onto Fe ppt Pb>Hg>Ag>As>Ni>Cu>Cd>Zn

  18. Wetland Processes Other ORD work at SPRD: T. Canfield et al. Constructed Wetlands

  19. ARD-Ground Water Interactions

  20. ARD-Groundwater Interactions

  21. AMD Related Secondary Precipitates pKsp Alunite KAl3(SO4)2(OH)6 84 Anglesite PbSO4 7.8 Anhydrite CaSO4 4.4 Coquimbite Fe2(SO4)3·9H2O 3.6 Gibbsite Al(OH)3 33.9 Goethite FeOOH 24 Jarosite KFe3(SO4)2(OH)6 95 Melanterite FeSO4·7H2O 2.2 Schwertmannite Fe(III), Fe(II)OH SO4 ? Sulfur S8

  22. Ground Water/Anoxic LimestoneDrains High Fe(II)/Fe(III) pH 2-6, low O2 Al, Metals GW Surface High O2 Fe(II)=>Fe(III) Fe(OH)3 ppt alk takes up acid Limestone Drain • Calcite dissolution • Alkalinity production • Retain Anoxic (FeII/FeIII) • pH increase

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