Acid Mine Drainage & Lime Neutralization: pH Recovery Lab (2D)

Reaction Conditions

Initial water pH 2.80

Pyrite oxidation rate (k) 0.150 /hr

Aeration (Fe²⁺→Fe³⁺) 0.30 /hr

Treatment

Lime dosing rate (Ca(OH)₂) 0.120 mol/L/hr

Live Readout

Elapsed time

0.0 hr

2.80

Dissolved Fe

0.0 mg/L

Precipitated Fe

0.0 mg/L

How it works

Pyrite in the exposed rock oxidizes in contact with water and air, releasing acid and ferrous iron — the primary acid-mine-drainage (AMD) reaction:

FeS2 + 7/2 O2 + H2O → Fe2+ + 2 SO4^2- + 2 H+

The pyrite pool decays first-order (dP/dt = −k·P), so acid and Fe2+ generation taper as the reactive surface depletes — the same shrinking-core behaviour used across this site's ore-kinetics sims. Aeration then oxidizes some of that Fe2+ to Fe3+ (Fe2+ + 1/4 O2 + H+ → Fe3+ + 1/2 H2O); this matters because the two ions have wildly different hydroxide solubility products (Ksp): Fe(OH)2 ≈ 4.9×10⁻¹⁷ needs the water near pH 9 to drop below 1 mg/L, while Fe(OH)3 ≈ 2.8×10⁻³⁹ is essentially insoluble by pH 3–4 — which is why real treatment plants aerate before liming.

Lime (Ca(OH)2) neutralizes acid directly — Ca(OH)2 + 2 H+ → Ca2+ + 2 H2O — raising pH. At every step, dissolved [Fe2+] and [Fe3+] are capped at their solubility ceiling Ksp/[OH-]ⁿ; whatever iron exceeds the ceiling precipitates into the sludge layer, exactly mirroring metal-hydroxide precipitation in a real neutralization pond.

pH chart — pH over simulated time; the shaded band marks the ~6–9 typical discharge-compliance range.
Metal chart (log scale) — dissolved Fe concentration; a steep drop marks the pH crossing the Ksp solubility ceiling.
Reactor — water colour darkens with dissolved iron and clears as it precipitates into the sludge layer at the bottom.

pH (compliance band 6–9) Dissolved Fe (log mg/L) Sludge / precipitate