The chemistry and partitioning of uranium in freshwaters: application of speciation modelling

Uranium mining, processing, use and disposal can result in contamination of freshwater with this toxic element. Predicting the behaviour of uranium in the freshwater environment is thus key in understanding its behaviour, fate, bioaccumulation and toxicity. Speciation modelling, including prediction of uranium partitioning between solution and suspended sediment, clearly has a potentially important role to play. Compared to other cationic metals, uranium (in the +VI oxidation state as uranyl) presents a relatively complex solution and surface chemistry with formation of mixed complexes, for example with alkaline earth metals and carbonate, being important under natural conditions. Similar considerations exist when considering uranyl binding to mineral surfaces such as iron(III) oxyhydroxides, with formation of ternary complexes involving carbonate and phosphate indicated by laboratory studies. Despite considerable efforts into developing databases of solution uranyl binding constants for use in speciation modelling, considerable differences among model databases remain. This study will present a brief review of existing databases for solution and surface complexation, and for uranyl binding to humic substances, and will outline efforts to develop an up to date set of binding constants describing solution, surface and humic complexation for the WHAM7 model. Application of the model to predict uranyl partitioning in freshwater systems will be presented and the results discussed in the context of goodness-of-fit and the predicted speciation in the solid phase and in solution. Current gaps in binding data will be highlighted and recommendations for further research will be made