1. Reactions of Carbonate Minerals with Acid Mine Drainage: Is Dolostone or Limestone Better?

2. Ashley McKendreeUniversity of Georgia Tammie GerkeUniversity of Cincinnati/ USEPA Paul BehmOSM-MCR Barry MaynardUniversity of CIncinnati Supported by SMART grants from the Indiana Division of Reclamation

3. Acidic, metal-laden drainage from mines is a serious water pollution problem world-wide Sulfuriic acid leach in Mt Gunson Cu mine, Australia

4. Sources include  Drainage from the mine  Drainage through waste rock piles  Drainage through tailings piles

5. The tailings breach at Aznalcollar, Spain

7. Formation of Acid Mine Drainage by Oxidation of Pyrite: FeS 2 + 7/2O 2 + H 2 O Fe 2+ + 2SO 4 2- + 2H + Fe 2+ + H + 1/4O 2 Fe 3+ + 1/2H 2 O Fe 3+ + 3H 2 O Fe(OH) 3 + 3H +

8. A popular remediation strategy is constructed wetlands, but these fail if alkalinity is not kept high Tecumseh 2000

9. Anoxic limestone drains are a way to boost input alkalinity ahead of open wetland cells

10. Limestone (Calcite) dissolution H + + CaCO 3  Ca 2+ + HCO 3 - Gypsum formation (log Ksp = -4.85) SO 4 2- + Ca 2+  CaSO 4 ·2H 2 O Important reactions in an anoxic drain filled with limestone

11. Limestone (Calcite) dissolution H + + CaCO 3  Ca 2+ + HCO 3 - Limestone (or marble) dissolves readily in contact with acid

12. Gypsum formation (log Ksp = -4.85) SO 4 2- + Ca 2+  CaSO 4 ·2H 2 O However, so much SO 4 is released in acid mine drainage that gypsum can form

13. Water exiting anoxic limestone drains tends to be strongly supersaturated with respect to gypsum (data from Flege et al 2012 – sampled spring and fall 2000) Drain exits to wetland Flow

14. Limestone grains cemented with gypsum – Tecumseh wetland Gypsum Limestone Porosity

15. Dolostone (Dolomite) dissolution: 2H + + CaMg(CO 3 ) 2  Ca 2+ + Mg 2+ + 2HCO 3 - Epsomite (MgSO 4 ) formation does not occur (log Ksp = -2.14) Could dolostone be substituted as the fill material to alleviate gypsum clogging?

16. Karstified Precambrian dolomite, S Africa Dissolving dolomite, Missouri

17. Batch Experiments MgCa

18. Final result is neutral pH, but LS gets there faster

19. Faster stirring reduces the lag between LS and DS

20. Conclusion 1 the initial reaction rate of dolostone reaction with H 2 SO 4 is slower than limestone, but catches up after a few hours. on the time scale of retention in ALDs, (15-30 hours) the reaction rates are equivalent

21. Vertical Column Experiments

22. Influent Concentrations 1. SO 4 2500 ppm, pH 1.5 2. SO 4 2500 ppm ; Al 20 ; pH 1.5 3. SO 4 2500 ppm; Fe 500 ; pH 1.5 4. SO 4 2500 ppm; Al 20 ; Fe 500 ; pH 1.5

23. LS column fails at 86 days; DS continues to function

24. LS behaves better initially, but column still fails

27. Amounts of sulfate retained in column experiments ExperimentLimestoneDolostone SO 4 2- accumulated (mg/g fill) H 2 SO 4, pH 1.556.812.5 H 2 SO 4 + SO 4 + Al78.36.1 H 2 SO 4 + SO 4 + Fe86.512.2 H 2 SO 4 + SO 4 + Fe + Al120.536.3 H 2 SO 4 + SO 4 133.157.2

28. Limestone experiments LS: SO 4 Gypsum cement bridging porosity between limestone particles

29. Limestone experiments LS: SO 4 + Al Gypsum cement subsequently covered by amorphous Al(OH) 3

30. Limestone experiments LS: SO 4 + Fe Iron hydroxide coating subsequently covered by gypsum cement

31. Limestone experiments LS: SO 4 + Al + Fe Iron hydroxide coating subsequently covered by gypsum cement. No apparent Al(OH) 3

32. Dolostone experiments DS: pH 1.5 H 2 SO 4 Minor gypsum cement

33. Dolostone experiments DS: SO 4 + Al Rim of amorphous Al(OH) 3 coating grains

34. Dolostone experiments DS: SO 4 + Fe Iron hydroxide coating subsequently covered partially by gypsum

35. Dolostone experiments DS: SO 4 + Al + Fe Skeletal remnants of iron hydroxide coating from which remaining dolomite has been dissolved; some subsequent gypsum, but no apparent Al(OH) 3

36. Mineral saturations: Limestone column

37. Mineral saturations :Dolostone column

38. Conclusion 2: Dolostone works better at bench scale. Is it better in field applications? Simco. Flooded by beaver dam

39. Branam & Harper 2010 The Enos Reclamation Project Limestone Dolostone

41. Conclusion: If you think about it, dolostone should work better - and it does.