Effect of Humic Substances on the Trapping and Transformations of U(VI) by Ferrihydrite

The Old Rifle DOE site in Colorado was a major site for milling uranium ore. U concentrations up to 1.8 uM persist in the Rifle aquifer, even after 'cleaning' the waste source of contaminations [1]. Understanding the behavior of U(VI) in this anthropogenically perturbed system is crucial for controlling the level of U contamination. Direct investigations of U speciation at this site have shown that U is associated with a wide variety of minerals as well as with natural organic matter (NOM) [2]. NOM has multiple functional groups which can be highly reactive with respect to aqueous metal ions, including actinides. Such interactions result in the formation of organo-mineral-metal (ternary) complexes and catalyze redox transformations; in addition, they can enhance mineral dissolution and metal transport [3,4,5]. In the complex soil/sediment system, aqueous, mineral, and organic phases are intimately mixed and their interactions are difficult to characterize by direct investigation [1]. The nanoparticulate iron hydroxide ferrihydrite (Fh), which is ubiquitous in many natural soils and highly reactive toward metal ions, is expected to significantly influence the fate of U in natural soils and is abundant in the subsurface at the Rifle site. NOM is also abundant at this site; however, little is known about the effect of NOM associated with ferrihydrite on the fate of U in such subsurface environments. To date, simple model systems composed mainly of two components (Fh and NOM) [6], (U and NOM or simple organic molecules) [7], or (Fh and U) [8,9], and more rarely composed of three components [10,11] have been studied in an effort to understand interactions among these components. In order to extend this earlier work to ternary systems, we have carried out batch reactions of U, a humic acid standard - Eliott soil humic acid (ESHA), and Fh under conditions that mimic those in the subsurface at Rifle. We have used U L3- and Fe K-edge XANES and EXAFS spectroscopy coupled with macroscopic uptake measurements to study the effect of this model humic acid on U uptake by Fh in these batch experimental systems. Results from this study will be presented and discussed mainly in terms of the affinity of U towards ESHA vs. Fh under the different conditions tested. [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] Gu B. et al. (2005) 39, 5268-5275 [6] Stewart B.D. et al. (2009) Environmental Science & Technology 43, 4922-4927 [7] Sachs S. and Bernhard G. (2011) Journal of Radioanalytical and Nuclear Chemistry 290, 17-29 [8] Nico P.S. et al. (2009) Environmental Science & Technology 43, 7391-7396 [9] Boland D.D. et al. (2011) Environmental Science & Technology 45, 1327-1333 [10] Gustafsson J.P. et al. (2009) Applied Geochemistry 24, 454-462 [11] Payne T.E. et al. (1996) Radiochimica Acta 74, 239-243