A Non-Electrostatic Surface Complexation Approach to Modeling Radionuclide Migration at the Nevada Test Site: II. Aluminosilicates

Technical Report · Thu Dec 16 00:00:00 EST 2004

DOI:https://doi.org/10.2172/15015931· OSTI ID:15015931

Zavarin, M; Bruton, C J

Reliable quantitative prediction of contaminant transport in subsurface environments is critical to evaluating the risks associated with radionuclide migration. As part of the Underground Test Area (UGTA) program, radionuclide transport away from selected underground nuclear tests conducted in the saturated zone at the Nevada Test Site (NTS) is being examined. In the near-field environment, reactive transport simulations must account for changes in water chemistry and mineralogy as a function of time and their effect on radionuclide migration. Unlike the Kd approach, surface complexation reactions, in conjunction with ion exchange and precipitation, can be used to describe radionuclide reactive transport as a function of changing environmental conditions. They provide a more robust basis for describing radionuclide retardation in geochemically dynamic environments. In a companion report (Zavarin and Bruton, 2004), a database of radionuclide surface complexation reactions for calcite and iron oxide minerals was developed. In this report, a second set of reactions is developed: surface complexation (SC) and ion exchange (IE) to aluminosilicate minerals. The most simplified surface complexation model, the one-site non-electrostatic model (NEM), and the Vanselow IE model were used to fit a large number of published sorption data and a reaction constant database was developed. Surface complexation of Am(III), Eu(III), Np(V), Pu(IV), Pu(V), and U(VI) to aluminum oxide, silica, and aluminosilicate minerals was modeled using a generalized approach in which surface complexation to aluminosilicate >SiOH or >AlOH reactive sites was considered equivalent to the reactivity of aluminum oxide and silica reactive sites. Ion exchange was allowed to be mineral-dependent. The generalized NEM approach, in conjunction with Vanselow IE, was able to fit most published sorption data well. Fitting results indicate that surface complexation will dominate over ion exchange at pH >7 for the rare earth and actinide ions examined here. Ion exchange is effectively suppressed due to aqueous speciation at high pH which tends to result in neutral or negatively charged aqueous species that are less likely to undergo ion exchange. The resulting set of average NEM and Vanselow IE constants provides a consistent set of constants for use in reactive transport simulations. The average NEM and Vanselow IE constants were used to predict single-mineral K{sub d}s under conditions similar to K{sub d} measurements reported by the Yucca Mountain site characterization program. In most cases, predicted Kds were consistent with measured K{sub d}s. In some cases, differences could be explained by surface area, mineralogy, or redox state. The NEM and Vanselow IE constants described here are an attempt to arrive at a consistent simplified database of reaction constants to be used in reactive transport simulations in chemically and mineralogically heterogeneous environments. The accuracy of these reaction constants is limited by the quality and quantity of available sorption data and the limitations of the NEM and Vanselow IE approach used. The reactivity and accessibility of natural minerals is complicated and cannot be assumed to behave ideally. Thus, the validity of the NEM and Vanselow IE constants must always be examined for the sediment of interest. For example, Triay et al. (1997) suggested that the weak sorption of Np(V) on tuff containing small amounts of hematite may indicate that the iron oxide mineral is passivated. Thus, the reactive surface area of hematite in these samples may be lower than expected. On the other hand, a limited comparison of NEM and Vanselow IE constants determined here and K{sub d}s reported by Wolfsberg (1978) for alluvium from Frenchman Flat, NTS, suggests that the reaction constants and reactive surface areas developed here would provide a conservative estimate of radionuclide retardation in Frenchman Flat alluvium.

• Research Organization: Lawrence Livermore National Lab. (LLNL), Livermore, CA (United States)
• Sponsoring Organization: US Department of Energy (US)
• DOE Contract Number: W-7405-ENG-48
• OSTI ID: 15015931
• Report Number(s): UCRL-TR-208672; TRN: US0501668
• Resource Relation: Other Information: PBD: 16 Dec 2004
• Country of Publication: United States
• Language: English

Related Subjects

12 MANAGEMENT OF RADIOACTIVE WASTES, AND NON-RADIOACTIVE WASTES FROM NUCLEAR FACILITIES
58 GEOSCIENCES
54 ENVIRONMENTAL SCIENCES
ACTINIDES
ALUMINIUM
FORECASTING
HEMATITE
ION EXCHANGE
IRON OXIDES
NEVADA TEST SITE
RADIOISOTOPES
RADIONUCLIDE MIGRATION
RARE EARTHS
SILICA
SITE CHARACTERIZATION
SURFACE AREA
WATER CHEMISTRY
YUCCA MOUNTAIN