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 % Total Inorganic 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) Source Transport Fate Mobility Chemical Intake Animals Toxicity oavailability Total Metal Bi ty i Geoavailability: v i Access to Weathering Plants Susceptibility to Weathering Physical Dispers after Smith and Huyck (1999) e r Metal Concentration Less Mo 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 Metal Sulfides Carbonates or ox i n Metal n g i c a da io re ni t Hydroxides a ti c s c o u e m n d in a o tt f re a e e lk r t a or a l f ini l se Sorbed u ea ty s cr in Metals pH Volatilized or increase in biotransformation Mineralized or Dissolved particle concentration Chelated Metals Metals n ionic tio la strength umu NOM decrease cc oa increase bi Exchangeable Metals Organically- Metals in Biota bound Metals
Smith and Huyck (1999)