Biosorption isotherms describe how adsorbate interacts with biosorbents and equilibrium is established between adsorbed metal ions on the biosorbent and the residual metal ions in the
solution during the surface biosorption. Equilibrium isotherms are measured to determine the capacity of the biosorbent for metal ions. The most common types of models describing this type of
system are the Langmuir and Freundlich isotherms.