Oxidizing Pyrite

1. FeS2 + 3.5 O2 + H2O Fe2+ + 2 SO42- + 2 H+

2. FeS2 + 14 Fe3+ + 8 H2O 15 Fe2+ + 2 SO42- + 16 H+

3. 14Fe2+ + 3.5 O2 + 14H+ 14 Fe3+ + 7 H2O

Reaction 3 is SLOW at low pH Traditional view of microbial activity describes how microbes speed that reaction up!

Oxidizing Pyrite

• FeS2 + 3.5 O2 + H2O Fe2+ + 2 SO42- + 2 H+

• FeS2 + 14 Fe3+ + 8 H2O 15 Fe2+ + 2 SO42- + 16 H+

The oxidation of FeS2 transfers 14 electrons from S22-

to 2 SO42- !!

These reactions occur over many steps to develop pathways of oxidation

species pK1 pK2

H2S 7.05 18.5

H2S2O3 1.73

H2S4O6 -2

H2SO3 1.85 7.2

H2SO4 -6 1.99

Data from Williamson and Rimstidt, 1992; Schoonen and Barnes, 1988

Oxidation Kinetics and Microbes

• Do microbes couple sulfur oxidation to O2/Fe3+ reduction or is Fe3+ oxidation of those species faster and microbes can only gain energy from Fe2+ oxidation?

Field Site:Iron MountainNorthern CA

Iron Mountain = Opportunity to study FeS2 oxidation inside a

giant block of FeS2!

Iron Mountain Mine Complex

• large complex of several mines operated intermittently between the 1860’s and 1962 for Au, Ag, Cu, and Zn

• Became a superfund site in 1983 – millions spent on treatment of effluent

• Site of lowest recorded ‘natural’ pH= -3.6 (Nordstrom et al., 2000)

• pH of majority of the flow 0.6-0.8• FeT is ~ 0.2 – 0.4 M, SO4

2- is ~0.6 – 1.1 M• An average of 100,000 moles FeS2/day is

oxidized (range ~ 20,000-200,000 mol/day)– ~2 m3 block weighing about a ton per day– Requires 350,000 mol O2 = ~ 8,000 m3 O2

• FeS2 oxidation requires: 30 g/l in water (O2 saturation ~ 3 mg/l)

• Water must be re-oxidized thousands of times before exiting, about once per 15-150 cm.

Effluent Geochemistry

Life at pH 0-1 and lower??

• Significant communities of bacteria, archaea, fungi, and protists!!

Microbes and FeS2 oxidation

S

S

SFe

aerobicanaerobic

Iron Mountain Microbial Metabolisms

Organism ## Org C/ O2 Org C /Fe3+

Fe2+/O2 SxOyn-/

O2

SxOyn-/

Fe3+

Ferromicrobium sp.

Acidimicrobium sp.Few Yes Yes Yes Yes

Sulfobacillus spp. Few Yes Yes Yes Yes Yes

Thermoplasma sp. Few Yes No NoFungi, protists Some Yes No No No No

Ferroplasma acidarmanus

Lots Yes Yes Yes No No

Leptospirillum spp. Lots No No Yes No No

Let’s take a closer look at this piece

Tetrathionate had previously been assumed to assumed to oxidize very quickly when formed as a product of pyrite oxidation (Kelsall, 1999; Moses et al., 1987; D.K.K. Nordstrom, personal communication). Results of kinetic experiments show that this assumption has been in error and that the oxidation kinetics of tetrathionate in acidic solutions with ferric iron is quite slow, defined by the rate law at 70º C and pH 1.5:

r = 10-6.61±0.3[S4O62-]0.3±0.08[Fe3+]0.06±0.07

• where r is in units of mol L-1 sec-1. The apparent activation energy (EA) for tetrathionate oxidation at pH 1.5 is 105 ± 4 KJ/mol.

Contrary Creek, VA

• FeS(aq) molecular clusters found as a significant potential substrate for Fe2+ oxidizing microbes

Profile 3 - Contrary Creek 08-2003

0

1

2

3

4

5

0 10 20 30 40 50 60

Current (nA)

Dep

th (

mm

)

O2

Mn2+

Fe2+

FeS(aq)

FeS+Mn(aq)

Fe3+

Profile – Contrary Creek wetland

ES1_2D Profile

0

2

4

6

8

10

12

14

16

18

20

0 50 100 150 200 250 300Concentration (uM)

Dep

th (

mm

)

O2

Fe(II)

FeS (nA*10)

Fe(III) (nA*10)

Competition between microbes and abiotic processes

• Neutrophilic Iron Oxidizers – cultures of ES-1 what controls the environments where they can eke out a living??

ES1_2DCT 03-11-03

0

5

10

15

20

0 50 100 150 200 250 300

Concentration (uM)

dept

h (m

m)

O2

Fe(II)

FeS (nA*10)

Fe(III) (nA*10)

Abiotic-Biotic kineticsExperiment 1 - 10 mM O2

-4.5

-4.4

-4.3

-4.2

-4.1

-4.0

0 5 10 15 20time (minutes)

log

Fe(

II)

(M)

Biotic

Azidecontrol

Pasteurizedcontrol

Linear(Azidecontrol)Linear(Biotic)

Linear(Pasteurized control)

Experiment 4 - 25 mM O2

-5.20

-5.00

-4.80

-4.60

-4.40

-4.20

-4.00

0 5 10

time (minutes)

log

Fe(

II) (

M)

biotic

Azidecontrol

Linear(Azidecontrol)Linear(biotic)

Experiment 5 - 45 mM O2

-4.4

-4.3

-4.2

-4.1

-4.0

0 5 10 15

time (minutes)

log

Fe(

II) (M

)

Biotic

AzideControl

Linear(AzideControl)Linear(Biotic)

Exp 9 - 275 mM O2

y = -0.0818x - 4.7255

R2 = 0.9601

-6.0

-5.8

-5.6

-5.4

-5.2

-5.0

-4.8

-4.6

-4.4

0 5 10 15

time (min)

log

Fe(

II)

(M)

Biotic

Azidecontrol

Linear(Biotic)

Linear(Azidecontrol)

Biotic-Abiotic Kinetic Competition

0.0

0.2

0.4

0.6

0.8

1.0

1.2

0 50 100 150 200 250 300

O2 ( M)

Ab

ioti

c:B

ioti

c ra

te r

atio

rat

io