Desorption kinetics in modeling of toxic chemicals

Two models describing the sorption of hydrophobic organic pollutants are compared. The models describe interactions in the water column between the fraction adsorbed on suspended sediments and the fraction in solution. They differ in that one considers desorption kinetics while the other assumes the adsorbed and dissolved fractions are always in equilibrium. The study shows that as the time constant of the system expressing the relative importance of sorption and elimination rate increases, the results of the models converge to the same value. The octanol-water partition coefficient has a strong and positive correlation with this ratio. This means that for organic compounds with a high octanol-water partition coefficient, equilibrium models will probably be more accurate than for compounds with a low octanol-water partition coefficient. Relationships are developed whereby a modeler may determine if the kinetic approach is necessary or if the equilibrium approach is satisfactory.