3Department of Civil Engineering, Northwestern University, Evanston, IL 60208-3109

The effects of microbiological activity on actinide speciation are key to understanding their mobilization and immobilization in the subsurface. Increased reliance on natural attenuation as a containment strategy for existing subsurface contamination by the DOE has also increased the need to understand the long-term effects of microbiological activity on actinide speciation.

The effects of microbiological activity on neptunium was investigated because neptunium is a contaminant of interest and an analog for An(V) oxidation states. This was a collaborative effort between researchers at Argonne National Laboratory and Northwestern University. The two key microbiological processes that influence neptunium speciation are bioreduction and biosorption. Under all anaerobic conditions investigated, bioreduction of Np(V) to Np(IV) species occurred. This, along with other geochemical data being obtained in other laboratories, continues to show that Np(IV) has a more important role in defining the subsurface chemistry of neptunium than previously thought. In both anaerobic and aerobic conditions, significant sorption of Np(V) was observed. These are correlated with the functional groups present on the surface of the bacteria investigated and show that significant bioassociation will occur under most subsurface conditions. The results we have obtained in the neptunium systems investigated confirm that biological activity is a key subsurface process in defining neptunium speciation.