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Oxidizing Pyrite

Oxidizing Pyrite. FeS 2 + 3.5 O 2 + H 2 O  Fe 2+ + 2 SO 4 2- + 2 H + FeS 2 + 14 Fe 3+ + 8 H 2 O  15 Fe 2+ + 2 SO 4 2- + 16 H + 14Fe 2+ + 3.5 O 2 + 14H +  14 Fe 3+ + 7 H 2 O

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Oxidizing Pyrite

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  1. Oxidizing Pyrite • FeS2 + 3.5 O2 + H2O  Fe2+ + 2 SO42- + 2 H+ • FeS2 + 14 Fe3+ + 8 H2O  15 Fe2+ + 2 SO42- + 16 H+ • 14Fe2+ + 3.5 O2 + 14H+ 14 Fe3+ + 7 H2O • Reaction 3 is SLOW at low pH  Traditional view of microbial activity describes how microbes speed that reaction up!

  2. Oxidizing Pyrite • FeS2 + 3.5 O2 + H2O  Fe2+ + 2 SO42- + 2 H+ • FeS2 + 14 Fe3+ + 8 H2O  15 Fe2+ + 2 SO42- + 16 H+ • The oxidation of FeS2 transfers 14 electrons from S22- to 2 SO42- !! • These reactions occur over many steps to develop pathways of oxidation

  3. Data from Williamson and Rimstidt, 1992; Schoonen and Barnes, 1988

  4. Oxidation Kinetics and Microbes • Do microbes couple sulfur oxidation to O2/Fe3+ reduction or is Fe3+ oxidation of those species faster and microbes can only gain energy from Fe2+ oxidation?

  5. Field Site:Iron MountainNorthern CA Iron Mountain = Opportunity to study FeS2 oxidation inside a giant block of FeS2!

  6. Iron Mountain Mine Complex • large complex of several mines operated intermittently between the 1860’s and 1962 for Au, Ag, Cu, and Zn • Became a superfund site in 1983 – millions spent on treatment of effluent • Site of lowest recorded ‘natural’ pH= -3.6 (Nordstrom et al., 2000)

  7. Effluent Geochemistry • pH of majority of the flow 0.6-0.8 • FeT is ~ 0.2 – 0.4 M, SO42- is ~0.6 – 1.1 M • An average of 100,000 moles FeS2/day is oxidized (range ~ 20,000-200,000 mol/day) • ~2 m3 block weighing about a ton per day • Requires 350,000 mol O2 = ~ 8,000 m3 O2 • FeS2 oxidation requires: 30 g/l in water (O2 saturation ~ 3 mg/l) • Water must be re-oxidized thousands of times before exiting, about once per 15-150 cm.

  8. Life at pH 0-1 and lower?? • Significant communities of bacteria, archaea, fungi, and protists!!

  9. Microbes and FeS2 oxidation S anaerobic S aerobic S Fe

  10. Iron Mountain Microbial Metabolisms

  11. Let’s take a closer look at this piece

  12. Tetrathionate had previously been assumed to assumed to oxidize very quickly when formed as a product of pyrite oxidation (Kelsall, 1999; Moses et al., 1987; D.K.K. Nordstrom, personal communication). Results of kinetic experiments show that this assumption has been in error and that the oxidation kinetics of tetrathionate in acidic solutions with ferric iron is quite slow, defined by the rate law at 70º C and pH 1.5: r = 10-6.61±0.3[S4O62-]0.3±0.08[Fe3+]0.06±0.07 • where r is in units of mol L-1 sec-1. The apparent activation energy (EA) for tetrathionate oxidation at pH 1.5 is 105 ± 4 KJ/mol.

  13. Profile – Contrary Creek wetland Contrary Creek, VA • FeS(aq) molecular clusters found as a significant potential substrate for Fe2+ oxidizing microbes

  14. Competition between microbes and abiotic processes • Neutrophilic Iron Oxidizers – cultures of ES-1  what controls the environments where they can eke out a living??

  15. Abiotic-Biotic kinetics

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