Fate and Transport of Contaminants from Acid Mine Drainage US EPA Scientist-to-Scientist Meeting Las Vegas, NV June 14-15, 2000 Richard T. Wilkin, Ph.D.

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Presentation transcript:

Fate and Transport of Contaminants from Acid Mine Drainage US EPA Scientist-to-Scientist Meeting Las Vegas, NV June 14-15, 2000 Richard T. Wilkin, Ph.D. National Risk Management Research Laboratory Ada, OK

Fate & Transport Issues u Chemical, Physical, and Biological Processes from Source => u Media Type –Air, Water, Sediment u Metal Type –Geochemical, Toxicity, Ore association

Chemical Processes u Dissolution, sorption, nucleation, growth u Oxidation-Reduction reactions u Acid-Base reactions u Isotope exchange reactions u Modeling exercises –Chemical Speciation –Saturation:  G r = RT lnQ/K eq –Kinetics

Physical & Biological Processes u Transport –Water –Sediment –Wind u Microbial –S-oxidizers, Fe-oxidizers –S-reducers, Fe-reducers u Wetland Plants

Hg, Pb As, Se Cd, Sb, Ag, CN Cu, Zn Pb, U Cr, Fe Hg Metals

Metal Type: Pearson Classification

As Mobility/Speciation: Redox sw gw

Metal Mobility: pH solution Supersat.

Fate & Transport Topics u Kinetics/Mechanisms of S(-II) oxidation u Microbial Processes u Product Transport in Surface Waters u Product Transport/Storage in Sediments u Impact of ARD on Ground Waters u Wetlands u Supergene Processes

Pyrite Oxidation Pyrite Dissolution/Overall Reaction FeS /4O 2 + 7/2H 2 O = “Fe(OH) 3 ” + 2H 2 SO 4 Low pH, high acidity Metal rich: As, Sb, Zn, Cu… Fe, Al, Mn rich Sulfate rich

Pyrite Oxidation: II FeS 2 + 7/2O 2 + H 2 O = Fe SO H + FeS Fe H 2 O = 15Fe SO H + after Stumm and Singer (1980)

Pyrite oxidation kinetics After Langmuir (1996) using rate equations from Williamson & Rimstidt (1994), Py Area =0.05 m 2 /g

Pyrite Oxidation: III u Chemical –oxygen, Fe(III), water, buffering u Physical –texture, grain size v Ore processing, framboidal pyrite u Biological –Fe- and S-oxidizing bacteria

AMD Prediction (EPA 530-R-4-036, December 1994) u Assessment of Acid-generation and Acid-neutralization capacity (acid, sulfate) u Hydrologic Assessment: Availability of Oxygen and Water (acid, sulfate) u Ore Deposit/Waste rock/Tailings Characterization (metals)

Volcanic-hosted Massive Sulfides Sediment-hosted Massive Sulfides - Shale Type (Rammelsberg) - Carbonate Type (MVT) Mafic Intrusive Related (Sudbury, Duluth Complex) Porphyry Cu-Mo/Skarn Mesothermal Au Epithermal Au Carlin Type Au Continental Geothermal (Hg, As, Sb) Coals Ore Deposit Types

Ore Minerals: Metal Mobilization Sources from Metal Sulfides Fe - pyrite, marcasite, pyrrhotite Hg - cinnabar Pb – galena Ag – acanthite, galena As – arsenopyrite, As-rich pyrite, orpiment, tetrahedrite, enargite Ni – pentlandite, millerite Cu – covellite, chalcocite, djurleite, bornite, chalcopyrite, enargite Cd – greenockite Zn – spahlerite Co – cobaltite

Transport of Oxidation Products to Surface Waters Sorption trend onto Fe ppt Pb>Hg>Ag>As>Ni>Cu>Cd>Zn

Wetland Processes Other ORD work at SPRD: T. Canfield et al. Constructed Wetlands

ARD-Ground Water Interactions

ARD-Groundwater Interactions

AMD Related Secondary Precipitates AluniteKAl 3 (SO 4 ) 2 (OH) 6 84 AnglesitePbSO AnhydriteCaSO CoquimbiteFe 2 (SO4) 3 ·9H 2 O3.6 GibbsiteAl(OH) GoethiteFeOOH24 JarositeKFe 3 (SO 4 ) 2 (OH) 6 95 MelanteriteFeSO 4 ·7H 2 O2.2 SchwertmanniteFe(III), Fe(II)OH SO 4 ? Sulfur S 8 pK sp

Ground Water/Anoxic Limestone Drains High Fe(II)/Fe(III) pH 2-6, low O 2 Al, Metals GW Limestone Drain Calcite dissolution Alkalinity production Retain Anoxic (FeII/FeIII) pH increase High O 2 Fe(II)=>Fe(III) Fe(OH) 3 ppt alk takes up acid Surface