Modeling desorption kinetics of a persistent organic pollutant from field aged sediment using a bi-disperse particle size distribution

Abstract

With the predicted climate change, it is expected that the chances of river flooding increase. During flood events, sediments will resuspend and when sediments are polluted, contaminants can be transferred to the surrounding water. In this paper we discuss a numerical intraparticle diffusion model that simulates desorption of dieldrin from a suspension of contaminated porous sediment particles with a well-characterized particle size distribution. The objective of this study was to understand the desorption rate (flux) of dieldrin from a suspension of field-aged sediment at different hydraulic retention times (HRT) of the aqueous phase and to elaborate the effect of particle-size distribution on mass transfer. Desorption kinetics of dieldrin, a persistent organic pollutant (POP), were experimentally measured and described in a separate paper using field-contaminated sediment. A radial diffusion model, accommodating intraparticle reversible sorption kinetics, aqueous phase pore diffusion, and a sink term for bulk aqueous phase refreshment was used to describe the experimental data. We observed rapid equilibrium of contaminants between small particles (10 μm) and the surrounding water even though the sorption affinity of dieldrin towards organic matter was high. On the contrary, for the larger particles (84 μm), calculations show that desorption was limited by intraparticle diffusion. Combining small and larger particles in our radial diffusion model resulted in the biphasic desorption behavior often observed even when using a linear isotherm. Flood events will result in an increase of desorption rate of POPs from sediments to the surrounding water. HRT and the particle-size distribution determine the desorption rate. We conclude that nonstationary diffusion within organic matter is the main process of mass transfer. Particle size distributions are very valuable to understand the phenomenology related to mass transfer limitations often described as limited bioavailability and can be used as basis to develop engineering options to limit contaminant mass fluxes into the environment.