Heavy Metal Mobility in Soil: pH-Driven Sorption Equilibrium

Contaminant

Soil Conditions

Soil pore-water pH 6.0

Cation-exchange capacity (organic matter) Medium

Time speed 1.0×

Live Readouts

Bound to colloids

—

Free / bioavailable

—

Partition K_d

—

Root uptake flux

—

Plant / groundwater toxicity risk

Low

How it works

Divalent heavy-metal cations (Pb²⁺, Cd²⁺, Zn²⁺) don't stay dissolved evenly in soil — they constantly exchange between the pore-water solution (mobile, plant-available, leachable to groundwater) and negatively-charged clay/organic-matter surfaces (sorbed, largely immobilised). The equilibrium is set by a pH-dependent distribution coefficient:

K_d(pH) = K_max / (1 + exp(-s·(pH − pH₅₀)))
f_bound = K_d·CEC / (1 + K_d·CEC)   (fraction sorbed at equilibrium)

As pH rises, soil surfaces deprotonate and carry more negative charge, so cation exchange capacity (CEC) sites bind metals more strongly — K_d climbs and the free fraction collapses. Below the metal's characteristic pH₅₀, H⁺ ions outcompete the metal for exchange sites, desorbing it back into solution. This is exactly why acid rain and acidic mine drainage mobilise lead and cadmium that neutral or limed soils hold immobile — each metal has its own pH₅₀ (Pb ≈ 5.5, Cd ≈ 6.2, Zn ≈ 6.8, reflecting real relative affinities of the Irving–Williams-adjacent exchange series).

Root uptake follows Michaelis–Menten saturation kinetics on the free ion concentration only — sorbed metal is not directly available to roots:

U = U_max · C_free / (K_m + C_free)

pH slider — shifts the sorption equilibrium; every particle stochastically re-samples bound/free state at the new rate each step.
CEC slider — soil organic matter / clay content; higher CEC means more exchange sites, pulling more metal onto colloids at any given pH.
Metal selector — swaps K_max and pH₅₀ to the chosen ion's real relative sorption strength.
Pulse button — injects a contamination event (e.g. runoff spike) and shows the system relax back toward equilibrium.

Soil colloid (clay/organic matter) Free ion — bioavailable / mobile Sorbed ion — immobilised

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