Modelling uptake into roots and subsequent translocation of neutral and ionisable organic compounds

A study on uptake of neutral and dissociating organic compounds from soil solution into roots, and their subsequent translocation, was undertaken using model simulations. The model approach combines the processes of lipophilic sorption, electrochemical interactions, ion trap, advection in xylem and dilution by growth. It needs as input data, apart fromplant properties, log KOW, pKa and the valency number of the compound, and pH and chemical concentration in the soil solution. Equilibrium and dynamic (steady-state) models were tested against measured data from several authors, including non-electrolytes as well as weakly acidic and weakly basic compounds. Deviations from the measured values led to further development of the model approach: sorption in the central cylinder may explain the small transpiration stream concentration factor of lipophilic compounds. For non-electrolytes, the model predicted uptake and translocation with high accuracy. For acids and bases, the tendency of the results was satisfactory. The dynamic model and the equilibrium approach gave similar results for the root concentration factor. The calculation of the transpiration stream concentration factor was more accurate with the dynamic model, but still gave deviations up to factor of ten or more. The dominating process for monovalent weak electrolytes was found to be the ion trap effect. C 2000 Society of Chemical Industry