Identification of dissipation pathways for pharmaceuticals in soils – a modelling approach†

Concentrations of pollutants like pharmaceuticals in soils typically decrease over time, though it often remains unclear whether this dissipation is caused by the transformation of the pollutant or a decreasing extractability. We developed a mathematical model that (1) explores the plausibility of different dissipation pathways, and (2) allows the quantification of concentration differences between aqueous soil extracts and soil solution. The model considers soil particles as uniform spheres, kinetic sorption towards an equilibrium (Freundlich model), and two dissipation pathways, i.e. irreversible transformation and mineralization (following 1^st order kinetics) as well as the formation of non-extractable residues via intraparticle diffusion. Applying the model to 19 published datasets (eight pharmaceuticals, three soils) showed that intraparticle diffusion accounts for approximately two thirds of all simulated mass fluxes. Reversible formation of non-extractable residues could play a major role for dissipation, while irreversible transformation and mineralization cannot be excluded either based on the available datasets. The difference between concentrations in aqueous extracts and soil solution quantified by the model is below typical model and measurement uncertainties for most of the investigated pharmaceuticals. Larger differences might be linked to a positive ionic charge of the pollutant. The model helps in disentangling different dissipation pathways in soils and optimizing experiments elucidating the long-term fate of pollutants in soils.