Acid Mine Drainage

Question 1

What do dark red and orange waters indicate in terms of iron?

Answer 1

Dark red water:
- Fe(III) in solution

Orange water:
- Fe(III) ppt. out of solution

Question 2

Where does AMD generally occur?

Answer 2

Metal or coal mines.

But can occur anywhere sulphide rock is exposed to water and oxygen.

Question 3

What is the overall reaction of pyrite being oxidised to ferrihydrite?

Answer 3

FeS2 + 3.75O2 + 3.5H2O&raquo_space; Fe(OH)3 + 2H2SO4

Fe(II) oxidised to Fe(III)
Sulphide in pyrite is also oxidised to sulphate in sulphuric acid.

Question 4

What is the abiotic reaction of AMD?

From pyrite, so both iron and sulphide reactions

Answer 4

Iron:

1) Fe2+ reacted with O2 to oxidise to Fe3+
2) Fe3+ reacted with water to precipitate Fe(OH)3
- Ferrihydrite as Fe3+

Sulphide:
S2OH- reacted with water and oxygen to produce sulphate.
- S is oxidised to sulphate, SO42-

Both reaction produce protons (acid)

Question 5

What pH does Fe(III) in solution ppt. out as ferrihydrite?

Answer 5

pH 4.5 +

Below 4.5, Fe(III) remains dissolved in soltuion

Question 6

What is the reaction of Fe(III) oxidising pyrite?

Answer 6

14Fe3+ + FeS2 + 8H2O&raquo_space; 15Fe2+ + 2SO42- + 16H+

Question 7

What happens to pyrite abiotically?

Answer 7

Pyrite can reduced Fe(III) to Fe(II) faster than oxygen can oxidise Fe(II) to Fe(III).

Means all the Fe(III) will be reduced and the reaction stops.

Question 8

How do Fe(II)-oxidising acidophillic bacteria help pyrite dissolution in AMD?

Answer 8

Fe(II)-oxidising acidophillic bacteria (Chemolithoautotrophs) oxidise Fe(II) to Fe(III) 5-6x faster than abiotic reactions.

Pyrite dissolution can continue causing acid formation

Question 9

What are the indirect AMD mechanisms in terms of microorganisms?

Answer 9

• No microbial attachment

- Microorganisms generate Fe(III) from dissolved Fe(II)

Question 10

What are the direct AMD mechanisms in terms of microorganisms?

Answer 10

• Microorganisms attach to pyrite
• Pyrites surface charge is is -ve above pH2.5 but EPS functional groups are protonated in acidic conditions (make surface +ve)
• Negative microorganisms can adsorb to pyrite surface and accumulate Fe(III)

Question 11

What are two of the main microbes involved in AMD?

Answer 11

Acidothiobacillus ferrooxidans (pH 1.8-2.5)
L. ferrooxidans (pH < 1.8)

Generally Chemolithoautotrophs which produce organic carbon heterotrophs can live off.

TEA = O2
PED = Fe(II)

Question 12

What are some environmental impacts with AMD?

Answer 12

• Degrade water quality
• Toxic to aquatic life
• Acid corrodes infrastructure
• iron-hydroxides and iron-sulphate minerals smother photosynthesisers
• Acidity change may impair autotrophs

Question 13

What are the two main AMD remediations strategies?

Answer 13

1. Prevention at the source

2. Treatment downstream

Question 14

What does prevention at the source AMD remediation involve?

Answer 14

• Add alkalinity before pH drops below 4.5
• Add phosphate to inhibit pyrite oxidation
• Cap piles to block O2 and H2O
• Add vegetation/microbes to tailing piles to consume H2O and O2

Question 15

What are the problems with prevention at the source AMD remediation?

Answer 15

Excluding O2 doesn’t work:

• A.ferrooxidans is also a facultative anaerobe
• can use Fe(III) as TEA rather than O2, PED = reduced sulphur
• Will produce sulphuric acid so still acidic.

Question 16

What does treatment downstream AMD remediation involve?

Answer 16

• Divert AMD to water treatment plant/pond
• Divert to artificial wetlands
• Add base to raise pH and ppt. metal oxides
• Add biomass to absorb metals and recover
• Add organic substrates (PED) to promote SRB (FeS will be precipitated)