Scale‐dependent desorption of uranium from contaminated subsurface sediments

Abstract

[1] Column experiments were performed to investigate the scale-dependent desorption of uranyl [U(VI)] from a contaminated sediment collected from the Hanford 300 Area at the U.S. Department of Energy (DOE) Hanford Site, Washington. The sediment was a coarse-textured alluvial flood deposit containing significant mass percentage of river cobble. U(VI) was, however, only associated with its minor fine-grained (<2 mm) mass fraction. U(VI) desorption was investigated both from the field-textured sediment using a large column (80 cm length by 15 cm inner diameter) and from its <2 mm U(VI)-associated mass fraction using a small column (10 cm length by 3.4 cm inner diameter). Dynamic advection conditions with intermittent flow and stop-flow events of variable durations were employed to investigate U(VI) desorption kinetics and its scale dependence. A multicomponent kinetic model that integrated a distributed rate of mass transfer with surface complexation reactions successfully described U(VI) release from the fine-grained U(VI)-associated materials. The field-textured sediment in the large column displayed dual-domain tracer-dependent mass transfer properties that affected the breakthrough curves of bromide, pentafluorobenzoic acid (PFBA), and tritium. The tritium breakthrough curve showed stronger nonequilibrium behavior than did PFBA and bromide and required a larger immobile porosity to describe. The dual-domain mass transfer properties were then used to scale the kinetic model of U(VI) desorption developed for the fine-grained materials to describe U(VI) release and reactive transport in the field-textured sediment. Numerical simulations indicated that the kinetic model that was integrated with the dual-domain properties determined from tracer PFBA and Br best described the experimental results. The kinetic model without consideration of the dual-domain properties overpredicted effluent U(VI) concentrations, while the model based on tritium mass transfer underpredicted the rate of U(VI) release. Overall, our results indicated that the kinetics of U(VI) release from the field-textured sediment were different from that of its fine-grained U(VI)-associated mass fraction. However, the desorption kinetics measured on the U(VI)-containing mass fraction could be scaled to describe U(VI) reactive transport in the contaminated field-textured sediment after proper consideration of the physical transport properties of the sediment. The research also demonstrated a modeling approach to integrate geochemical processes into field-scale reactive transport models.Citation: Liu, C., J. M. Zachara, N. P. Qafoku, and Z. Wang (2008), Scale-dependent desorption of uranium from contaminated subsurface sediments, Water Resour. Res., 44, W08413,

Multi-Scale Mass Transfer Processes Controlling Natural Attenuation and Engineered Remediation: An IFRC Focused on Hanford?s 300 Area Uranium Plume January 2010 to January 2011

“…A four-fold increase in alkalinity resulted in a 2.5-fold increase in the initial rate of release of U(VI) after each of the stop flow events. This is in good agreement with the 2.2-fold increase predicted by the multi-rate SCM model (Liu et al, 2008;Yin et al, 2008). Approximately 80% of the labile U(VI) was extracted with AGW containing 1 meq/L alkalinity and ~95% using an AGW with 4 meq/L (Figure V-3).…”

“…Over the following pore volumes, changes in major ion concentrations steadily approached those expected based on changes in the influent concentrations. Trends in U(VI) desorption in column #1 were similar to those reported previously for vadose-zone U(VI)-contaminated sediments from the site (e.g., Qafoku et al, 2005;Liu et al, 2008), with an initial rapid decline in U(VI) followed by a long tail ( Figure V-7a). Trends in U(VI) desorption in column #2 were generally consistent with imposed water composition changes ( Figure V-7b).…”

“…Rate-limited U(VI) desorption was observed in all five stop-flow events ( Figure V-7a,b), indicating that the mass transfer of U(VI) is not in equilibrium with the aqueous water chemistry. During stop-flow, the sites at the faster end of the rate-coefficient distribution become populated with the U(VI) released from sites at the slower end of the distribution (Liu et al, 2008). The increased U(VI) concentration after each stop-flow is attributed to this redistribution.…”

“…Previous studies demonstrated reactivity of bulk sediments from the Hanford formation is dominated by the < 2 millimeter size fraction (Liu et al, 2008). Six of the composites came from specific locations (boreholes for IFRC wells 2-26, 2-27, 2-30, 2-31, 3-31, and 3-32).…”

Rates and mechanisms of uranyl oxyhydroxide mineral dissolution

“…This experimental approach, however, has the potential disadvantages of preferential flow paths, transport-limited rather than surface-limited reactions, and non-uniform contact of the reactant solution with surfaces (Steefel and Maher, 2009;Zhu, 2009). Column experiments have been used to study the release of U from contaminated soils and sediments under different environmental conditions (Qafoku et al, 2005;Liu et al, 2008;Eagling et al, 2013). However, no studies have reported dissolution rates of uranyl oxyhydroxide minerals under thermodynamically undersaturated and/or dynamic conditions of flow within porous media to simulate subsurface conditions.…”