What is Acid Mine Drainage (AMD)?
Acid Mine Drainage (AMD) is one of the most severe environmental problems associated with mining. It occurs when sulfide minerals—most notably Pyrite (\(\mathrm{FeS_2}\))—are exposed to oxygen and water, oxidizing to produce highly acidic, metal-rich runoff.
The Chemistry of Pyrite Oxidation
The oxidation of pyrite proceeds in sequential stages:
Stage 1: Direct Oxidation by \(\mathrm{O_2}\)
\[\mathrm{FeS_2 + \frac{7}{2}O_2 + H_2O \rightarrow Fe^{2+} + 2SO_4^{2-} + 2H^+}\]
Stage 2: Oxidation of \(\mathrm{Fe^{2+}}\)
\[\mathrm{Fe^{2+} + \frac{1}{4}O_2 + H^+ \rightarrow Fe^{3+} + \frac{1}{2}H_2O}\]
Stage 3: Accelerated Oxidation by \(\mathrm{Fe^{3+}}\) (Autocatalysis)
\[\mathrm{FeS_2 + 14Fe^{3+} + 8H_2O \rightarrow 15Fe^{2+} + 2SO_4^{2-} + 16H^+}\]
Stage 4: Precipitation of Goethite
\[\mathrm{Fe^{3+} + 2H_2O \rightarrow FeOOH\downarrow + 3H^+}\]
In Stage 3, the generated \(\mathrm{Fe^{3+}}\) acts as an oxidant to further dissolve pyrite. This creates a positive feedback loop: \(\mathrm{FeS_2 \rightarrow Fe^{2+} \rightarrow Fe^{3+} \rightarrow}\) more \(\mathrm{FeS_2}\) oxidized. Once this reaction starts, it accelerates exponentially, which is why AMD is notoriously difficult to stop.
Introducing the REACTION Block
So far, we have used EQUILIBRIUM_PHASES, which instructs PHREEQC to “react until equilibrium is reached.” Today, we introduce the REACTION block. This block is used for irreversible reactions, where a specified amount of a substance is forcibly added to the solution, regardless of equilibrium.
| Feature | EQUILIBRIUM_PHASES | REACTION |
|---|---|---|
| Reaction Type | Reversible (Equilibrium) | Irreversible (Forced Addition) |
| End Condition | When SI = 0 is reached | When specified amount is fully added |
| Common Uses | Mineral dissolution/precipitation | Oxidation, combustion, titration |
| Application to Pyrite | Unsuitable (backward reaction might occur) | ✅ Highly Suitable |
The oxidation of pyrite is thermodynamically irreversible under earth surface conditions—once it dissolves and oxidizes, it does not easily turn back into \(\mathrm{FeS_2}\). Therefore, we use REACTION.
The Role of INCREMENTAL_REACTIONS
The most critical command in this tutorial is INCREMENTAL_REACTIONS true.
Intermediate goethite precipitation is ignored, failing to reproduce the correct reaction pathway.
EQUILIBRIUM_PHASES is evaluated at every step, allowing goethite to precipitate exactly when it becomes supersaturated.
If you include Goethite 0 0 in EQUILIBRIUM_PHASES but forget INCREMENTAL_REACTIONS true, the SI of goethite might remain supersaturated without precipitating across the steps, yielding completely unrealistic results. Always use them together.
Full PHREEQC Code
# ============================================================
# DeepFlow #6 - Pyrite Oxidation and AMD Formation
# Requires INCREMENTAL_REACTIONS true
# ============================================================
SOLUTION 1 "Carbonate Groundwater"
temp 25
pH 7
pe 4
units mol/kgw
Ca 1.5e-2 # 15 mmol/kg (Typical limestone groundwater)
-water 1
EQUILIBRIUM_PHASES 1
CO2(g) -3.5 10 # Equilibrium with atmospheric CO2 (pCO2 = 10^-3.5 atm)
Goethite 0.0 0 # Goethite precipitation sink (Amount=0)
Gypsum 0.0 0 # Gypsum precipitation sink (Amount=0)
REACTION 1
FeS2 4 # Pyrite stoichiometry
O2 15 # Oxygen
# Stepwise addition (moles)
0.0001 0.0003 0.0005 0.001
0.002 0.005 0.010 moles
INCREMENTAL_REACTIONS true # ← Mandatory!
SELECTED_OUTPUT 1
-file pyrite_oxidation.sel
-pH true
-pe true
-totals Fe S Ca
-molalities Fe+2 Fe+3 SO4-2
-saturation_indices Goethite Gypsum Pyrite
-equilibrium_phases Goethite
ENDWe omitted Gypsum 0 0 from this simulation because typical groundwater \(\mathrm{Ca^{2+}}\) concentrations won’t reach gypsum supersaturation here. Gypsum precipitation during AMD is generally only observed in highly concentrated environments where both \(\mathrm{SO_4^{2-}}\) and \(\mathrm{Ca^{2+}}\) are extremely abundant.
Results and Interpretation
Output Data (pyrite_oxidation.sel)
Below is the actual output processed with INCREMENTAL_REACTIONS true.
| Step | pH | pe | Fe (mol/kg) | SO₄ (mol/kg) | SI_Goethite | d_Goethite |
|---|---|---|---|---|---|---|
| 1 | 2.865 | 15.54 | 4.2e-9 | 8.0e-4 | 0.000 | 4.0e-4 Precipitating |
| 2 | 2.291 | 16.35 | 2.3e-7 | 3.2e-3 | 0.000 | 1.2e-3 Precipitating |
| 3 | 1.973 | 16.81 | 3.3e-6 | 7.2e-3 | 0.000 | 2.0e-3 Precipitating |
| 4 | 1.695 | 17.21 | 3.7e-5 | 1.5e-2 | 0.000 | 4.0e-3 Precipitating |
| 5 | 1.440 | 17.59 | 3.5e-4 | 3.1e-2 | 0.000 | 7.6e-3 Precipitating |
| 6 | 1.173 | 17.98 | 3.7e-3 | 7.1e-2 | 0.000 | 1.6e-2 Precipitating |
| 7 | 0.988 | 18.27 | 2.2e-2 | 1.5e-1 | 0.000 | 2.1e-2 pH < 1! High Hazard |
Visualizing the pH Drop
Discussion
1. Mechanism of the pH Drop
The pH plummets from 2.87 to 0.99. This is due to the continuous generation of \(\mathrm{H^+}\) during pyrite oxidation. The drop accelerates after step 4 (cumulative pyrite > 1 mmol), which is a direct signature of \(\mathrm{Fe^{3+}}\) accumulating and triggering the autocatalytic oxidation cycle.
2. Precipitation of Goethite
From Step 1 to the final step, SI_Goethite = 0.000 is maintained, and d_Goethite (delta/amount transferred) is consistently positive. This means goethite is constantly precipitating.
The cycle is relentless: Pyrite Oxidizes \(\rightarrow\) \(\mathrm{Fe^{3+}}\) is generated \(\rightarrow\) It immediately precipitates as Goethite \(\rightarrow\) Releasing even more \(\mathrm{H^+}\). This continuous precipitation of goethite is what dyes AMD rivers their characteristic reddish-brown (often referred to as “yellow boy”).
Notice that dissolved Fe (mol/kg) is exceptionally low (e.g., 4.2e-9 mol/kg in step 1). This is because the generated \(\mathrm{Fe^{3+}}\) precipitates as goethite almost instantaneously. Even at Step 7, where pH drops below 1, dissolved Fe is only 2.2e-2 mol/kg due to this thermodynamic sink.
3. The Rise of ‘pe’
The pe (negative logarithm of electron activity) increases steadily from 15.54 to 18.27. A higher pe indicates a strongly oxidizing environment. Because we are forcibly supplying \(\mathrm{O_2}\) in the REACTION block, the oxidative power of the solution intensifies as the simulation proceeds.
Simulating AMD Neutralization
A common treatment for AMD is neutralization using crushed limestone (calcite) to raise the pH. We can simulate this using the output from Step 7:
# ============================================================
# AMD Neutralization Simulation
# Run this as a continuation of the previous code
# ============================================================
SAVE solution 1 # Save the highly acidic final solution
# *IMPORTANT:* Place the SAVE command BEFORE the first END!
USE solution 1
EQUILIBRIUM_PHASES 2
Calcite 0 10 # Add excess calcite to restore pH
SELECTED_OUTPUT 2
-file amd_neutralization.sel
-pH true
-totals Fe Ca
-saturation_indices Goethite Calcite
ENDBy adding calcite, the pH is neutralized, forcing any remaining dissolved iron to precipitate as goethite. Check amd_neutralization.sel to observe the dramatic pH recovery.
Summary of Key Takeaways
Next, we will explore the Fix_H+ technique within the PHASES block to artificially lock the pH at various values. We will calculate the solubility of aluminum across pH 3 to 14, visualizing the amphoteric nature of Gibbsite in a comprehensive solubility diagram.
References
Other articles in this series:
- #1 Installation and Initial Calculation
- #2 Analyzing Seawater with Speciation
- #3 Mineral Equilibrium and Temperature Effects
- #4 Calcite–CO₂ Interaction (Open vs. Closed Systems)
- #5 Mixing Groundwater and Seawater
- #6 Pyrite Oxidation and AMD Formation (This article)
- #7 Solubility Diagrams (Gibbsite)
DeepFlow | Science beneath the surface