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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
Chemical, Physical, and Biological Processes from Source =>
Media Type– Air, Water, Sediment
Metal Type– Geochemical, Toxicity, Ore association
Chemical Processes Dissolution, sorption, nucleation, growth Oxidation-Reduction reactions Acid-Base reactions Isotope exchange reactions Modeling exercises
– Chemical Speciation
– Saturation: Gr = RT lnQ/Keq
– Kinetics
Physical & Biological Processes
Transport– Water– Sediment– Wind
Microbial– S-oxidizers, Fe-oxidizers– S-reducers, Fe-reducers
Wetland Plants
S urface W ate r
G round W ater
S edim en t/so il
A ir
MediaSource
Sm elters/ore processing
Tailings
Undergroundworkings
Waste rock
Heap leachpads
Pit lakes
Hg, Pb
As, SeCd, Sb,Ag, CNCu, ZnPb, UCr, Fe
Hg
Metals
Metal Type: Pearson ClassificationA-type Tweeners B-type“Hard” acids (filled shells) transition metals “Soft”Low polarazability Highly PolarizedIonic BondingNa+, K+, Ca2+, Mg2+ Mn2+, Co2+, Fe2+ Cu+, Ag+, Au+
Si4+, Al3+ Ni2+, Cu2+, Fe3+ Zn2+, Hg2+, Cd2+, Pb2+
OH-, F-, Cl- HS-, CN-, Sx2-,
SO42-, CO3
2- CO
Others:U(VI) (UO2
2+, UO2CO3°) MCL:U(V) (UO2
+) <10 ppbAs (V, III, Org. As), Sb 10-100 ppbSe (VI, IV, 0, -II) >100 ppb
As Mobility/Speciation: Redox
-1.00
-0.75
-0.50
-0.25
0.00
0.25
0.50
0.75
1.00
0 2 4 6 8 10 12 14
H AsO3 4
H AsO3 3
AsS 2
-
H AsO2 3
-
HAsO 3
2-
H AsO2 4
-
HAsO 4
2-
AsO 4
3-
AsSAs S2 3
As
p H
Eh
25 °CS= 10As= 10
-3
-5
sw
gw
Metal Mobility: pH
-16
-14
-12
-10
-8
-6
-4
-2
0
0 2 4 6 8 1 0 12 14
log
A
l, F
e, o
r S
i
pH
Hematite
G ibbsite
Amorph. Fe(O
H) (pK=37)
3
Q uartz
solutionS
upersat.
Fate & Transport Topics
Kinetics/Mechanisms of S(-II) oxidation Microbial Processes Product Transport in Surface Waters Product Transport/Storage in Sediments Impact of ARD on Ground Waters Wetlands Supergene Processes
Pyrite OxidationPyrite Dissolution/Overall Reaction
FeS2 + 15/4O2 + 7/2H2O = “Fe(OH)3” + 2H2SO4
Low pH, high acidityMetal rich: As, Sb, Zn, Cu…Fe, Al, Mn richSulfate rich
Pyrite Oxidation: IIFeS2 + 7/2O2 + H2O = Fe2+ + 2SO4
2- + 2H+
FeS2 + 14Fe3+ + 8H2O = 15Fe2+ + 2SO42- +16H+
FeS (s) + O2 2
Fe(II) + S 2
2-
+ O 2
Fe (II) + SO 4
2-
+ FeS (s)2
fast
fastm icrobia l
+ O 2
slowinorg.
Fe(III) = Fe(O H ) (s)3
afterStumm and Singer (1980)
Pyrite oxidation kinetics
Assumed Conditions/Predicted Half Times
Oxidants pH log[O2] log[Fe3+] log[Fe2+] t1/2
________________________________________________________O2 2.0 -7.0 -- -- 780y
-3.6 -- -- 16y
Fe3+ 2.0 -- -2.0 -4.0 4.4d-2.0 -2.0 150d
After Langmuir (1996) using rate equations fromWilliamson & Rimstidt (1994), PyArea=0.05 m2/g
Pyrite Oxidation: III
Chemical– oxygen, Fe(III), water, buffering
Physical– texture, grain size
Ore processing, framboidal pyrite
Biological– Fe- and S-oxidizing bacteria
AMD Prediction(EPA 530-R-4-036, December 1994)
Assessment of Acid-generation and Acid-neutralization capacity (acid, sulfate)
Hydrologic Assessment: Availability of Oxygen and Water (acid, sulfate)
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 SulfidesFe - pyrite, marcasite, pyrrhotiteHg - cinnabarPb – galenaAg – acanthite, galenaAs – arsenopyrite, As-rich pyrite, orpiment, tetrahedrite, enargiteNi – pentlandite, milleriteCu – covellite, chalcocite, djurleite, bornite, chalcopyrite, enargiteCd – greenockiteZn – spahleriteCo – cobaltite
Transport of OxidationProducts to Surface Waters
D rainage S urface W ater
H , Fe, SOM n, e
+ 2-
4
MM ixing /d ilu tion
pH increaseppt o f “Fe(O H ) ”, “A l(O H ) ”su lfate d ilu ted/sorbedM etals sorbed/co-ppt w ith Fe and A l
3 3
Sorption trend onto Fe pptPb>Hg>Ag>As>Ni>Cu>Cd>Zn
Wetland Processes
Inpu tFe , SO , H , Me3+ 2- +
4 O utput=C leaner
P lan t uptake
R educ ingS O to H Sppt (F e ,M e)S
4 2
2-
O xid iz ing ppt o f Fe -O H -O -S Oadsorption
4
[inc. pH]
See Kwong & Stem pvoort (1994)(M t. Washington, B .C .)
Other ORD work at SPRD: T. Canfield et al.Constructed Wetlands
ARD-Ground Water Interactions
O 2H O2
Fe 2+SO 4
2-
Fe(OH)3
Al(OH)3
FeCO 3 CaCO 3after Ptacek and B lowes (1994)Other authigenic ppts: Goethite , jarosite , schwertm annite
Mine Tailings
ARD-Groundwater Interactions
-0 .75
-0.50
-0.25
0.00
0.25
0.50
0.75
1.00
1.25
1 2 3 4 5 6 7 8
Fe 2+
FeCO (s)3
Fe(O H ) (s)3
Fe 3+
Stream s
G rou ndw a ters
after G ang and Langm uir (1974), coal strip-m in ing in PA10 °C , Fe 10 , C=10 -2 .75 -3.4
pH
Eh (V )
AMD Related Secondary Precipitates
Alunite KAl3(SO4)2(OH)6 84Anglesite PbSO4 7.8Anhydrite CaSO4 4.4Coquimbite Fe2(SO4)3·9H2O 3.6Gibbsite Al(OH)3 33.9Goethite FeOOH 24Jarosite KFe3(SO4)2(OH)6 95Melanterite FeSO4·7H2O 2.2Schwertmannite Fe(III), Fe(II)OH SO4 ?Sulfur S8
pKsp