Microscopic reactive diffusion of uranium in the contaminated sediments at Hanford, United States

Abstract

[1] Microscopic and spectroscopic analyses of uranium-contaminated sediments from select locations at the U.S. Department of Energy (DOE) Hanford site have revealed that sorbed uranium (U) often exists as uranyl precipitates associated with intragrain fractures of granitic clasts. The release of U to contacting fluids appears to be controlled by intragrain ion diffusion coupled with the dissolution kinetics of the precipitates that exist in the form of Na-boltwoodite. Here we present a coupled microscopic reactive diffusion model for the contaminated sediment on the basis of experimental measurements of intragrain diffusivity in the granitic lithic fragments and the dissolution kinetics of synthetic Na-boltwoodite. Nuclear magnetic resonance, pulse gradient spin echo measurements showed that the intragrain fractures of the granitic clasts isolated from the sediment contained two domains with distinct diffusivities. The fast diffusion domain had an apparent tortuosity of 1.5, while that of the slow region was two orders of magnitude larger. A two-domain diffusion model was assembled and used to infer the geochemical conditions that led to intragrain uranyl precipitation during waste-sediment interaction. Rapid precipitation of Na-boltwoodite was simulated with an alkaline U-containing, high-carbonate tank waste solution that diffused into intragrain fractures, which originally contained Si-rich pore water in equilibrium with feldspar grains in the lithic fragments. The model was also used to simulate uranyl dissolution and release from contaminated sediment to recharge waters. With independently characterized parameters for Na-boltwoodite dissolution, the model simulations demonstrated that diffusion could significantly decrease the rates of intragrain uranyl mineral dissolution due to diffusion-induced local solubility limitation with respect to Na-boltwoodite.

Distribution of Microbial Biomass and Potential for Anaerobic Respiration in Hanford Site 300 Area Subsurface Sediment

“…These sediments are physically and mineralogically heterogeneous and could contain microsites where advection forces are low (29) and hot spots (33) where electron donors are relatively concentrated. This circumstance would generate microenvironments of O 2 depletion where anaerobic respiration could occur.…”

Upscaling of Long-Term U9VI) Desorption from Pore Scale Kinetics to Field-Scale Reactive Transport Models

“…White and Brantley [2003], Maher et al [2006]). In contrast to the 1-D study of Liu et al[2006], 2-D pore scale modeling efforts , ) found that spatial distributions of reactivity under advective conditions can lead to different observed macroscopic rate behavior even though the total reactivity between simulations was the same. In column experiments pore scale water velocities caused by particle scale heterogeneities have been linked with observed reaction rates ), although the cause of this link has yet to be delineated (Zhang et al [2008], Chen and Wagenet [1995], which also include data at: 1) batch to column scales, 2) bench (batch or column scales)…”

“…In Liu et al [2006] a combined experimental and modeling study was completed to explore chemical reactivity and intragranular diffusion. Through extensive characterization of micro-scale chemical heterogeneities (Liu et al [2004]) a conceptual model was created to explain the slow release of uranium from micro-precipitates present in fractures deep within the porous media.…”

DNA Profile Analysis and Interpretation

“…At shallower depths, EXAFS analyses suggest that the major uranium-controlling phase was meta-torbernite, Cu(UO 2 ) 2 (PO 4 ) 2 • xH 2 O (Catalano et al 2006b;Arai et al 2007) with uranyl-carbonate precipitates, predominantly consistent with uranium-rich calcite (Catalano, personal communication). Until recently, it had been speculated that the uranyl-silicate phase present in the 300 Area sediments was either Na-boltwoodite, Na(UO 2 )(SiO 3 OH) • xH 2 O, or uranophane, Ca(UO 2 ) 2 [SiO 3 (OH)] 2 • xH 2 O-both minerals have the uranophane group structure (Liu et al 2004;Liu et al 2006;Zachara et al 2005;McKinley et al 2006;McKinley et al 2007). The minute inclusions, which prevent bulk analyses, and similarities between the uranophane group structures had precluded conclusive identification of the uranyl-silicate phase.…”