Laboratory investigation of the role of desorption kinetics on americium transport associated with bentonite colloids

Journal Article · Mon Jul 13 00:00:00 EDT 2015 · Journal of Environmental Radioactivity

DOI:https://doi.org/10.1016/j.jenvrad.2015.07.001· OSTI ID:1247312

Dittrich, Timothy Mark ^[1]; Boukhalfa, Hakim ^[1]; Ware, Stuart Douglas ^[1]; Reimus, Paul William ^[1]

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1. Los Alamos National Lab. (LANL), Los Alamos, NM (United States)

Understanding the parameters that control colloid-mediated transport of radionuclides is important for the safe disposal of used nuclear fuel. We report an experimental and reactive transport modeling examination of americium transport in a groundwater–bentonite–fracture fill material system. A series of batch sorption and column transport experiments were conducted to determine the role of desorption kinetics from bentonite colloids in the transport of americium through fracture materials. We used fracture fill material from a shear zone in altered granodiorite collected from the Grimsel Test Site (GTS) in Switzerland and colloidal suspensions generated from FEBEX bentonite, a potential repository backfill material. The colloidal suspension (100 mg L^–1) was prepared in synthetic groundwater that matched the natural water chemistry at GTS and was spiked with 5.5 × 10^–10 M²⁴¹Am. Batch characterizations indicated that 97% of the americium in the stock suspension was adsorbed to the colloids. Breakthrough experiments conducted by injecting the americium colloidal suspension through three identical columns in series, each with mean residence times of 6 h, show that more than 95% of the bentonite colloids were transported through each of the columns, with modeled colloid filtration rates (k_f) of 0.01–0.02 h^–1. Am recoveries in each column were 55–60%, and Am desorption rate constants from the colloids, determined from 1-D transport modeling, were 0.96, 0.98, and 0.91 h^–1 in the three columns, respectively. The consistency in Am recoveries and desorption rate constants in each column indicates that the Am was not associated with binding sites of widely-varying strengths on the colloids, as one binding site with fast kinetics represented the system accurately for all three sequential columns. As a result, our data suggest that colloid-mediated transport of Am in a bentonite-fracture fill material system is unlikely to result in transport over long distance scales because of the ability of the fracture materials to rapidly strip Am from the bentonite colloids and the apparent lack of a strong binding site that would keep a fraction of the Am strongly-associated with the colloids.

• Research Organization: Los Alamos National Lab. (LANL), Los Alamos, NM (United States)
• Sponsoring Organization: USDOE Office of Nuclear Energy (NE)
• Grant/Contract Number: AC52-06NA25396
• OSTI ID: 1247312
• Alternate ID(s): OSTI ID: 1249612
• Report Number(s): LA-UR-15-25173; PII: S0265931X15300436
• Journal Information: Journal of Environmental Radioactivity, Vol. 148, Issue C; ISSN 0265-931X
• Publisher: Elsevier
• Country of Publication: United States
• Language: English

Related Subjects

42 ENGINEERING
54 ENVIRONMENTAL SCIENCES
58 GEOSCIENCES
12 MANAGEMENT OF RADIOACTIVE AND NON-RADIOACTIVE WASTES FROM NUCLEAR FACILITIES
americium
granite/granodiorite
reactive transport
desorption kinetics
column experiments
bentonite colloids