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Fundamentals of Mine-Drainage Formation and Chemistry

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Fundamentals of Mine-Drainage Formation and Chemistry Fundamentals of Mine-Drainage Formation and Chemistry Kathleen S. Smith and Thomas R. Wildeman Billings Symposium / ASMR Annual Meeting Assessing the Toxicity Potential of Mine-Waste Piles Workshop June 1, 2003 U.S. Department of the Interior Colorado School of Mines U.S. Geological Survey Acidity pH is a function of: ¾ Balance between acid-generating and acid-consuming reactions ¾ Relative rates of these reactions ¾ Accessibility of minerals that contribute to these reactions Acid-Generating Reactions ¾ Oxidation of pyrite and some other sulfide minerals ¾ Hydrolysis of metal cations – especially Fe and Al ¾ Precipitation of hydrous metal-oxide minerals – such as iron and aluminum oxides Oxidation Reactions (from Singer and Stumm, 1970; Forstner and Wittmann, 1979) Initiator Reaction: 2+ 2- + FeS2 + 3.5 O2 + H2O Fe + 2 SO4 + 2 H Pyrite Acidity Propagation Cycle: Bacteria 2+ + 3+ Fe + 0.25 O2 + H 0.5 H2O + Fe 3+ 14 Fe + FeS2 + 8 H2O 2+ 2- + 15 Fe + 2 SO4 + 16 H Acidity Oxidation Reactions Pyrite (FeS2) Pyrite oxidation by oxygen: 2+ 2- + FeS2 + 3.5 O2 + H2O Fe + 2 SO4 + 2 H Pyrite oxidation by ferric iron (Fe3+): 3+ 2+ 2- + FeS2 + 14 Fe + 8 H2O 15 Fe + 2 SO4 + 16 H Oxidation Reactions Enargite (Cu3AsS4) Enargite oxidation by oxygen: Cu3AsS4 + 8.75 O2 + 2.5 H2O 2+ 2- 2- + 3 Cu + HAsO4 + 4 SO4 + 4 H Enargite oxidation by ferric iron (Fe3+): 3+ Cu3AsS4 + 35 Fe + 20 H2O 2+ 2+ 2- 2- + 3 Cu + 35 Fe + HAsO4 + 4 SO4 + 39 H Sources of Fe3+ ¾ Microbially catalyzed oxidation of Fe(II) – 106 times faster than abiotic (Singer and Stumm, 1970) – See Schrenk et al., 1998 ¾ Secondary iron-sulfate soluble salts . – e.g., Coquimbite [Fe2(SO4)3 9H2O] – Form from acid mine drainage Soluble Salts ¾ Soluble salts may form upon evaporation of acidic waters ¾ These salts store acid and metals until released by rainfall or snowmelt Acid-Generating Reactions Hydrolysis of metal cations 3+ + Fe + 3 H2O Fe(OH)3 + 3 H 3+ + Al + 3 H2O Al(OH)3 + 3 H Acid-Generating Reactions Precipitation of hydrous metal-oxide minerals Acid-Consuming Reactions ¾ Dissolution of carbonate o minerals releases Ca, Mg, H2CO3 - 2- or HCO3 or CO3 ¾ Dissolution of aluminosilicate minerals releases Al, Ca, Fe, K, Mg, Mn, Na, Si ¾ Dissolution of hydrous Fe and Al oxide minerals releases Fe, Al, sorbed elements ¾ Sorption of H+ onto mineral surfaces releases sorbed elements Acid-Consuming Reactions Most weathering reactions consume acid + 2+ CaCO3 + 2 H Ca + H2CO3 calcite + KAl3Si3O10(OH)2 + H + 9 H2O muscovite + K + 3 H4SiO4 + 3 Al(OH)3 The Carbonate System CO2 (g) + H2O (l) H2CO3 (aq) - + H2CO3 HCO3 + H - 2- + HCO3 CO3 + H ¾ Important pH buffering system ¾ Usually responsible for alkalinity Carbonate Buffering 100 - HCO3 H CO o CO 2- 50 2 3 3 0 4 5 6 7 8 9 10 11 12 13 % Total Inorganic Carbon pH Alkalinity ¾ Capability of water to neutralize acid ¾ Important to aquatic life ¾ Buffers against rapid pH change ¾ Often related to hardness ¾ Hardness is the concentration of divalent cations (e.g., Ca2+, Mg2+) ¾ Water-quality criteria for some metals are hardness-dependent ¾ In mining impacted systems, Ca2+, Mg2+, etc. can originate from the weathering of a variety of minerals Chemical Constituents Concentration of a chemical element is a function of: ¾ Presence and concentration of that element in the ore or host-rock minerals ¾ Accessibility of these minerals (mining method, porosity, grain size, climatic conditions, etc.) ¾ Susceptibility of these minerals to weathering ¾ Mobility of the element Geoavailability “…that portion of a chemical element’s or a compound’s total content in an earth material that can be liberated to the surficial or near-surface environment (or biosphere) through mechanical, chemical, or biological processes” Smith and Huyck (1999) More Total Metal Source Geoavailability: Access to Weathering Susceptibility to Weathering Metal Concentration Transport Physical Chemical Dispersivity Mobility Plants Animals Intake Fate Bioavailability Less Toxicity after Smith and Huyck (1999) Trace Elements in Natural Waters Regardless of their source, high concentrations of dissolved trace elements often do not persist as they are transported through aqueous systems due to: ¾ Precipitation reactions ¾ Sorption reactions ¾ Biological uptake and transformation ¾ Dilution ¾ Some exceptions include Mn and Zn Metal Hydroxides Metal Sulfides Sorbed Metals n io t e c s a u e d r c re n i increase in e t H a f p l h e u t oxidation of particle concentrationg s s ic n a n e re organic matter or o tr c i s e increase in alkalinity d Metal ExchangeableMetals Dissolved e Carbonates Metals M s O a re N c in n o i t a Smith and Huyck (1999) l biotransformation umu Organically-bound c c Metals a o i b Volatilized or Mineralized or Chelated Metals Metals in Biota.
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