Delayed immobilization of heavy metals in soils and sediments under reducing and anaerobic conditions; consequences for flooding and storage

The consequences of permanent inundation and storage of flood plain soils and aquatic sediments on metal mobility were studied. The main goal was to quantify kinetic mobilisation processes in order to pinpoint the conditions that pose emission risks to groundwater and surface waters. Anaerobic pore water compositions of six existing depots, containing sediments from either aquatic or terrestrial origin, were measured to obtain reliable field references. Reduction experiments were performed with SOFIE cells, in which time-dynamic measurements of reducing pore waters were carried out at the reigning redox conditions. A parallel experiment tested the possibility if sulfide-deficiencies could be compensated by the reduction of added gypsum, thus increasing the available pool of reactive S. Model calculations were performed to distinguish between the thermodynamic and kinetic processes. Reduction of flood plain soils showed that dissolved organic matter (DOM) concentrations increased up to 7-fold over time, as a result of nutrient-mediated metabolic boost of organic matter degrading microorganisms. The association of metals to DOM increased significantly due to the loss of reactive Mn-oxide and Fe-oxyhydroxide sorption phases during reduction. DOM released metals only slowly, therefore kinetically hindering the phase shifting to stable metal-sulfide precipitates. The observed effects lasted at least 10 months. In aquatic sediments, reduction rates of sulphates were six times faster, and the release of DOM occurred in lower amounts than in soils from terrestrial origin. It is shown that the addition of gypsum stimulates the formation of sulfides, thereby decreasing dissolved concentrations of most metals. On the short term however, the addition of gypsum leads to elevated concentrations of Zn and Cd due to increased electrolyte strength and subsequent Ca-metal-competition for sorption sites. This effect may be eliminated by not only adding an electron acceptor to the sediment, but also an effective donor, e.g., degradable organic matter. For depots containing flood plain soils, the same kinetic mechanisms were observed as in the reduction experiments (elevated production of DOM and associated metals). It was concluded that reduction of flood plain soils may not necessarily result in efficient metal trapping. When storage is considered, a distinction should be made between sediments of terrestrial and aquatic origin.