Desorption during resuspension events: Kinetic v. equilibrium model

Abstract

The spills and release of hydrophobic organic chemicals (HOCs) into waterbodies have resulted in the contamination of bottom sediments. When these sediments are resuspended by runoff events or by dredging, particulate-phase contaminants desorb to the water phase. Equilibrium between the particulate phase and the dissolved phase is usually assumed for most modelling applications regardless of time scale. For well mixed systems in which chemical transport is not limited by mass transfer between the bulk water phase and sediment aggregates, an intraparticle diffusion model can be applied to estimate the time required for various HOCs to reach an equilibrium state between dissolved and particulate-phase concentrations. Calculations from this model show that total equilibrium time scales cover a wide range, from less than a day to a few hundred days. This study compares predicted suspension times for sediments of various sizes with expected equilibration times for desorption. This comparison indicates that the equilibrium assumption is not valid for a wide range of parameter values typical of natural systems. In particular, compounds with high partition coefficients, greater than about 104 mL g-1, will have minimum equilibration times of at least one to ten days. This is likely to be greater than the expected resuspension time.