Biogeochemistry - Modelling and applications

What is Acid Mine Drainage (AMD)?

Acidic water draining from sulfide-bearing rocks exposed to oxygen and water.

What causes red water in AMD streams?

Dissolved Fe(III) in very acidic water.

What causes orange/yellow colours downstream?

Precipitation of Fe(III) minerals as pH increases.

What mineral drives AMD?

Pyrite (FeS2).

Why is AMD dangerous?

It produces acid and mobilises toxic metals, harming ecosystems and infrastructure.

Key fast reaction in AMD:

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

Why is microbial activity essential for AMD?

Abiotic Fe(II) oxidation is extremely slow; bacteria accelerate it by 5–6 orders of magnitude.

What is the Indirect microbial mechanism?

Bacteria oxidise Fe2+ to Fe3+, which then attacks pyrite.

What is the direct microbial mechanism?

Microbes attach to pyrite and oxidise Fe(II) at the mineral surface.

Why can microbes attach to pyrite at low pH?

EPS becomes protonated (+), allowing adsorption to negatively charged pyrite.

Key AMD microorganisms:

Acidothiobacillus ferrooxidans; Leptospirillum ferrooxidans; Ferroplasma acidarmanus.

Why does AMD accelerate over time?

Fe3+ is regenerated by bacteria, creating a self-sustaining oxidation cycle.

Environmental impacts of AMD:

Acidification, toxic metals, Fe(III) precipitates - smothering life, long-distance contamination.

Prevention strategies of AMD:

• Alkalinity →Add limestone or Ca(OH)2 to neutralise acid before pH drops too low.

• Phosphate → Add phosphate to inhibit pyrite oxidation.

• Capping → Cover waste piles to block oxygen and water.

• Flooding → Induce anoxia to stop Fe(II) oxidation.

Downstream treatment of AMD:

• bases → Add CaCO3, CaO, or NaOH to raise pH and precipitate metals.

• SRB → Promote sulfate-reducing bacteria to form metal sulfides.

• wetlands → Use constructed wetlands for long-term passive treatment.

What is bioremediation?

Use of microorganisms to transform contaminants into less harmful or less mobile forms.

What determines remediation strategy?

Environmental conditions and contaminant type.

Goal for organic contaminants:

Degrade (oxidise) them to CO2 or harmless intermediates.

Goal for inorganic contaminants:

Immobilise them to prevent spreading.

What is biostimulation?

Adding nutrients, PEDs, TEAs, or surfactants to stimulate native microbes.

What is bioaugmentation?

Adding selected or engineered microbial species.

What is monitored natural attenuation?

No intervention; monitor natural degradation.

Aerobic bioremediation strategy:

Aeration (bioventing or air sparging) to stimulate aerobic chemoheterotrophs.

Anaerobic bioremediation strategy:

Use anaerobic chemoheterotrophs; may require electron shuttles or nutrients.

What is the role of surfactants?

Help microbes attach to or disperse in hydrophobic contaminants like oil.

Categories of inorganic contaminants:

Metals, metalloids, radionuclides, nutrients.

Strategies for bioremediation:

• Bioreduction → Change oxidation state to a less mobile form (e.g., U(VI) → U(IV)).

• Biomineralisation → Induce precipitation of a mineral without changing oxidation state.

• Biosorption → Passive binding of contaminants to microbial surfaces.

• Bioaccumulation ^→ Uptake of contaminants into cells.

What are the limitations of bioreduction?

Competition from NO3-, Fe(III), and SO42- as TEAs.

What are the advantages of biosorption?

Dead cells can be used; works for low–medium metal concentrations.

What are the limitations of biosorption?

Reversible, competitive, and surface sites saturate quickly.

What is the biomineralisation of U(VI)?

Microbes release PO43- which forms U-phosphate minerals.

What are the problems with biomineralisation?

PO43- reacts with other cations, causes clogging, and may re-dissolve.