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

Abstract

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 Ammore » 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.« less

Authors:

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

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+ Show Author Affiliations

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

Publication Date:

Mon Jul 13 00:00:00 EDT 2015

Research Org.:

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

Sponsoring Org.:

USDOE Office of Nuclear Energy (NE)

OSTI Identifier:

1247312

Alternate Identifier(s):

OSTI ID: 1249612

Report Number(s):

LA-UR-15-25173
Journal ID: ISSN 0265-931X; PII: S0265931X15300436

Grant/Contract Number:  

AC52-06NA25396

Resource Type:

Accepted Manuscript

Journal Name:

Journal of Environmental Radioactivity

Additional Journal Information:

Journal Volume: 148; Journal Issue: C; Journal ID: ISSN 0265-931X

Publisher:

Elsevier

Country of Publication:

United States

Language:

English

Subject:

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