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

Ground Water/Anoxic LimestoneDrains

High Fe(II)/Fe(III)pH 2-6, low O2

Al, MetalsGW

Limestone Drain

•Calcite dissolution•Alkalinity production•Retain Anoxic (FeII/FeIII)•pH increase

High O2

Fe(II)=>Fe(III)Fe(OH)3 ppt

alk takes up acid

Surface