Although pollutant removal by a solid inert surface (substratum) in a bioreactor is normally modelled using adsorption isotherms (Langmuir/Freundlich) due to it being considered a very simple compartment, with nothing entering or leaving the substratum and no transformations taking place. In the case of a non-inert permeable/semi-permeable surface membranes (i.e. plant roots), supplying the biofilm with gaseous or dissolved substrates, how would you model the pollutant removal capabilities (Nutrients and Heavy Metals) of such a reactive boundary?