A conceptual phenanthrene bioavailability model applied to rhizosphere environment

A conceptual model is proposed that describes the phenanthrene bioavailability in soil. The model describes the substrate 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 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 and first order kinetics for the degradation of the sorbed-available fraction. Sorption and diffusion are approximated by first order kinetics. Finally, the formation of bound residues is described using an exponential saturation equation. The model was evaluated using phenanthrene bioavailability and aging data obtained with cyclodextrines and methanol extractions in sterilised and non sterilised soil microcosms. Monod kinetics parameters were estimated using microbial growth and biodegradation data .