Adsorption processes, which include its inverse (desorption) are generated due to the physical or chemical binding of tracer particles to the surface of the grains of the aquifer.

While the chemical bond to the matrix is usually irreversible, a physical bond is usually reversible, that is, depending on the relation between adsorption and transport velocity an equilibrium between adsorption and desorption appears.

Such equilibria can be described for constant temperature conditions through so-called adsorption isotherms with both linear (Henry isotherm) and non-linear character (Freundlich and Langmuir isotherms) and considered in the mass transport calculation.

 

Comparison of the different adsorption isotherms

 

The adsorption rate is described by a function c_s = f(c_w):

with:

c_s = concentration of substances adsorbed to the matrix

c (or c_w ) = concentration of substances dissolved in the fluid.

 

All three approaches (Henry, Freundlich and Langmuir) assume a constant fluid density

.

 

Linear adsorption based on Henry