Desorption of dieldrin from field aged sediments: simulating flood events

Background, Aim and Scope With the predicted climate change, it is expected that the chances of flooding may increase. During flood events, sediments will resuspend and when the sediments are polluted, contaminants can be transferred to the surrounding water. Mass transfer of organic compounds like Persistent Organic Pollutants (POPs) from soils and sediments to the surrounding aqueous phase are essential regarding fate and transport of these chemicals in the aqueous environment. The distribution of POPs between sorbed and aqueous phases and the time needed to obtain equilibrium are required to calculate the exposure to potential receptors. A reactor was designed in which the water flow is controlled and low POP concentrations could be measured by tenax extraction outside the reactor vessel. This reactor design named SPEED (Solid Phase Extraction with External Desorption) was used to study desorption from aged contaminated sediment in relation to sediment particle size. Materials and Methods In the newly developed SPEED (Solid Phase Extraction with External Desorption) reactor, the water flow rate was set and controlled, and low aqueous POP concentrations were measured by sorption to Tenax® outside the reaction vessel. The effect of particle size on desorption rate was studied using a widely used Tenax® solid phase extraction method. Results The experiments, by specific measurement of the aqueous dieldrin concentration at different HRT, show that desorption of dieldrin in time is faster when short HRTs were applied. However, the mass of dieldrin desorbed per liter refreshed water is higher for longer HRTs. Therefore, the mass transfer of dieldrin within the sediment particles is the rate determining process in contaminant desorption. This observation was confirmed by Tenax® solid phase extractions which were applied for different particle size fractions. Desorption rates of POPs from the sediment fraction with small particles were faster than desorption rates from the sediment fraction with large particles. Organic matter was present as separate particles in the sediment sample. All experiments demonstrated biphasic desorption. The fluxes calculated for both phases are supportive of non-stationary diffusion as the main process of mass transfer. Discussion In the literature, the relation between particle size and desorption of organic contaminants from soils and sediments is contradictory. Most often this seems to be due to overlooking the spatial configuration of organic matter in the soils and sediments. In several papers the presence of organic matter as a thin coating around mineral particles has been overlooked. There-fore, milling had no effect on desorption behavior of contaminants, as the diffusion length will not be affected. In our opinion, both the particle size and spatial configuration of organic matter are rate determining parameters of the desorption process. Conclusions Flood events will result in an increase of desorption rate of POPs from sediments to the surrounding water. HRT and particle size determine the concentration gradient and, thereby, the desorption rate. Furthermore, the diffusion length will be smaller when sediment particles are suspended and more water is present to decrease the aqueous concentration. We conclude that non-stationary diffusion within organic matter is the main process of mass transfer. The combination of simulated in-situ measurements of desorption from sediments with generic measurable parameters like flow rate and particle size distribution results in a quantitative measurable flux of contaminants, which resembles the in-situ (bio)availability as the result of dynamic processes in the sediment/water system. Recommendations and Perspectives The results obtained provided a sound basis for mechanistic modeling of POP mass transfer from sediment to water. The modeling results will be presented in a separate paper. Besides the HRT, also mixing conditions can be changed to assess the desorption from sediment layers. The possibility to combine flow rate and mixing intensity enables the study of the effect of hydraulically different river systems on desorption of contaminants. In a long term perspective we foresee a link with hydrology and sediment transport with desorption in water bodies