Adsorption
As species travel through a porous medium they typically attach to (adsorb), and detach (desorb) from the solid phase, which slows chemical transport through the porous medium. Adsorption and desorption respectively reduces or increases species concentrations in the fluid. The adsorption properties vary between chemicals, so a plume containing multiple species can separate into components (Ref. 6). The Adsorption feature includes four predefined and one user defined relationships to predict the solid concentrations, cPi from the concentration in the liquid phase, ci:
(3-18)
The above equations contains the following parameters:
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Freundlich: Freundlich constant KF (SI unit:·mol/kg), Freundlich exponent NF (dimensionless), and reference concentration cref (SI unit: mol/m3).
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Langmuir: Langmuir constant KL (SI unit: m3/mol), and adsorption maximum cPmax (SI unit: mol/kg).
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Toth: Toth constant bT (SI unit: m3/mol), Toth exponent NT (dimensionless), and adsorption maximum cPmax (SI unit: mol/kg).
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BET (Braunauer-Emmett-Teller): BET constant KB (dimensionless), and a monolayer adsorption capacity c0 (SI unit: mol/kg) and a Saturation concentration, cS(SI unit: mol/m3).
These predefined expressions are adsorption isotherms that describe the amount of species sorbed to the solid. Defined at equilibrium, the switch between liquid and solid phases is instantaneous.
Using a Species Source feature, arbitrary expressions can be entered to define, for example, nonequilibrium and temperature-dependent adsorption laws, including those set out by Fetter (Ref. 7) and Bear and Verruijt (Ref. 8).
The retardation factor, RF, describes how adsorption slows the solute velocity, uc, relative to the average linear velocity of the fluid, ua, as in
If the contaminant moves at the average linear velocity of the fluid for RF = 1. For RF > 1, the contaminant velocity is smaller than the fluid velocity owing to residence time on solids.