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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 that contribute to these reactions Acid-Generating Reactions

¾ Oxidation of pyrite and some other minerals ¾ Hydrolysis of metal cations – especially Fe and Al ¾ Precipitation of hydrous metal- minerals – such as and aluminum 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 :

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 (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- soluble salts . – e.g., Coquimbite [Fe2(SO4)3 9H2O] – Form from 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 surfaces releases sorbed elements Acid-Consuming Reactions

Most reactions consume acid

+ 2+ CaCO3 + 2 H Ca + H2CO3

+ 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 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 Carbonates or ox i n Metal n g i c a da 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)