Stanford

Understanding the behaviour of U(VI) in soils and aquifers is crucial for controlling the impact of anthropogenic U contamination. For example, U concentrations up to 1.8 μM persist in the aquifer of the Old Rifle DOE site in Colorado, even after “cleaning” the waste source of contaminations [1]. Direct investigations of U speciation at this site have shown that U is associated with a wide variety of mineral phases as well as with natural organic matter (NOM) [2]. In addition, NOM is known to form organo-mineral-metal (ternary) complexes and affect the partitioning of actinides, in particular at low pH when mineral surface is positively charged [3,4,5]. However, the mechanism by which NOM affects this partitioning is not well known; in particular it is not clear if U binds directly to the minerals or through NOM bridges. This information is important for predicting the stability of U in the solid phase. In order to understand the effect(s) of NOM on the fate of uranium in soils, and to identify the conditions that enhance or retard them, U sorption rates and speciation were investigated in the batch ternary system ferrihydrite-humic acids-U(VI). Nanoparticulate ferrihydrite (Fh) was chosen because it is abundant in the subsurface at the Rifle site and ubiquitous in soils. In addition, it is highly reactive to metal ions and is expected to significantly influence the fate of U. Our results show that humic acids affect, albeit slightly, the proportion of uranium retained by ferrihydrite, and it increases the proportion of easily exchangeable uranium. These effects depend on the proportion of humic acids and on the sequence in which humics and uranium enter the system. Results from U LIII edge EXAFS spectroscopy suggest the partitioning and exchange reactions are linked to variations in the proportion of uranium associated with ferrihydrite versus humic acids.

[1] Campbell K.M. et al. (2012) Applied Geochemistry 27, 1499–1511

[2] Maher K. et al. (2012) Inorganic Chemistry 52, 3510−3532.

[3] Means J.L. and Crerar D.A. (1978) Science 200, 1477-1481

[4] Ludwig C. et al. (1995) Nature 375, 44-47

[5] Payne et al. (1996) Radiochimica Acta 74, 239-243