University of Delaware (Di Toro)

Summary

The goal of this research is to develop models for predicting the toxicity and mobilization of individual metals and metal mixtures in sediments. These predictions are critical in evaluating the risk associated with contaminated sediments at Superfund sites. In past Superfund basic research projects, project investigators have developed methods of assessing the toxicity of individual metals - Cd, Cu, Ni, Pb and Zn - for sediments with excess acid volatile sulfides (AVS). The researchers are developing a model that predicts the toxicity of metals in sediments with little or no AVS. This requires a prediction of the partitioning of sediment metals to the other important sediment phases - sediment organic carbon if it is in sufficient supply, and then the other phases (e.g. iron oxyhydroxides) that are important in the aerobic layer of sediments. Project researchers also propose to develop the next generation of models for the prediction of mobilization of metals from sediments that explicitly include the mechanisms of mobilization. They model the mechanisms of metal sulfide oxidation directly and relate them to the cycles of manganese and iron in sediments. The researchers have included such a mechanism involving the Fe(ll)-facilitated oxidation of arsenic in their present project. They propose to extend this work to the oxidation of metal sulfides.

The project objectives are:

1. For cationic metals, to extend the sediment Biotic Ligand Model (BLM) so that it can predict the toxicity of metals and metal mixtures by considering competitive interactions of metals and major ions to the organic carbon, (hydrous) oxide surfaces, clays and to uptake sites on benthic organisms.
2. For metals that form insoluble metal sulfides in sediments (Cd, Cu, Ni, Pb, Zn), to determine the rates of metal sulfide oxidation and their dependence on solution parameters for inclusion in metal  mobilization models.
3. For arsenic, to investigate the effects of inorganic and organic ligands present in sediments that affect the rate of Fe(ll)-catalyzed oxidation of arsenite and ultimately the rates of arsenic mobilization from sediments.
4. For chromium, to investigate the effect of natural organic matter on the rates of chromium(lll) oxidation by manganese oxide in sediments, and the subsequent release of chromium(VI) to the pore water and the overlying water.
5. For the metals discussed above, to construct an integrated model that combines the metal mobilization and toxicity mechanisms, together with a model of the seasonal cycling of redox-sensitive species including oxygen, organic matter, iron, manganese and sulfur in sediments.