Effect of Redox Conditions on Actinide Speciation and Partitioning with Colloidal Matter

Abstract

Colloid-facilitated transport of actinides in subsurface environments depends on various geochemical and geological properties among which the redox conditions are very important. In reducing conditions, redox-sensitive actinides are stabilized in low valence states and could potentially form intrinsic colloids while in oxidizing conditions, they form pseudocolloids by sorption or coprecipitation onto natural aquatic colloids. The difference in actinides’ behavior associated with colloids at two sites in Russia under different redox conditions is discussed in this chapter. Successive micro- and ultrafiltrations of groundwater samples were performed by electron microscopic methods, nanoSIMS and sequential extraction for partitioning. Under oxidizing conditions of the PA “Mayak” site (Southern Urals, Russia), U and Np are present in the higher oxidation states while Pu is mostly found in the tetravalent state. Under these conditions, actinides are bound to amorphous hydrous ferric oxide (HFO) and Mn oxides, forming pseudocolloids as demonstrated by nanoSIMS. According to nanoSIMS, at the Tomsk site (Siberian Chemical Combine, Russia) oxidizing waste solutions were mixed with reducing groundwater resulting in reduction of U(VI) and formation of intrinsic U(IV) hydroxocolloids. Most of the U and Pu in a reducing environment are present in a residual fraction of low-mobility that favors slow migration rates.