ADSORPTION: MT3D allows 3 options for adsorption isotherms and uses them to reduce the velocity term in the solution through the retardation factor as follows:

Linear Isotherm

is the sorbed concentration

= dry bulk density

= effective porosity

Freundlich isotherm:








Langmuir isotherm:

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To define adsorption in MT3D the user specifies:
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= dry bulk density
= effective porosity
S₁ and S₂

For the Linear Isotherm, S₁ = K_d and S₂is not used:


For the Freundlich Isotherm, S₁ = K_f and S₂is a, the Freundlich exponent:



For the Langmuir Isotherm, S₁ = K_l and S₂is the total concentration of sorption sites:



Non-equilibrium Sorption is represented as a first-order reversible kinetic reaction:

= first-order mass transfer rate between dissolved and sorbed phases
K_d = distribution coefficient for the sorbed phase

As increases, the mass transfer rate increases and approaches the equilibrium-controlled linear sorption