A model assessing bioavailability of persistent organic pollutants in soil

In this paper a model is proposed for describing persistent organic pollutants (POPs) bloavailability in soil. The model is written in Fortran 90 and describes POPs' behaviour as resulting from four different processes: sorption-desorption equilibrium, slow diffusion (aged fraction), fast irreversible sorption (bound residues) and biodegradation of the bioavailable fraction. The POP sorption to soil surfaces is described assuming a rapid rate of sorption-desorption to and from soil surfaces and a slower rate of diffusion into the internal matrix (aging). Biodegradation is described as resulting from bacterial growth using sigmoidal Monod kinetics for the contaminant dissolved in soil solution (for non-hydrophobic compounds) and first order kinetics for the degradation of the sorbed-available fraction. In the case of hydrophobic compounds, first-order kinetics is employed to describe also the degradation from the soil solution. Sorption and diffusion are approximated by first order kinetics. Finally, the formation of bound residues is described using an exponential saturation equation. The rate constants for the different processes are estimated using linear and non linear first order kinetics approaches. The rate constants of the irreversible processes are estimated from experimental data. Model evaluation was performed using data from previous experiments with phenanthrene as test compound.