Radionuclide Reaction Chemistry as a Function of Temperature at the Cheshire Site

Abstract

The goals of this task were to evaluate the availability of published temperature-dependent thermodynamic data for radionuclides and sorbing minerals and to evaluate the applicability of published estimation methods for temperature-dependent aqueous complexation, radionuclide mineral precipitation, and sorption. This task fills a gap in the hydrologic source term (HST) modeling approach, which, with few exceptions, has neglected the effects of temperature on radionuclide aqueous complexation, using 25 C complexation data for all temperatures without evaluating the consequences of this assumption. In this task, we have compiled thermodynamic data available in the literature and evaluated the options and benefits of applying temperature-dependent radionuclide speciation to future HST modeling. We use the recent experience of HST modeling at Cheshire (Pawloski et al., 2001) to focus our evaluation. Our literature search revealed that few thermodynamic data or extrapolation methods could be used to define the temperature-dependent speciation of key HST radionuclides Np, Pu, Am, and U, particularly for the higher valence-state (e.g., 5+ and 6+), the oxidation states most pertinent to NTS groundwater conditions at Cheshire. This suggests that using 25 C data for all temperatures may be the best modeling approach currently available. We tested established estimation techniques such as the Criss-Cobblemore » method and other correlation algorithms to calculate thermodynamic parameters needed to extrapolate aqueous complexation data to higher temperatures. For some reactions, the isocoulombic method does allow calculation of free energy data and equilibrium values at higher temperatures. Limitations in algorithms and input data for pentavalent and hexavalent cations prevent extending temperature ranges for reactions involving radionuclides in these oxidation states and their complexes. In addition, for many of the radionuclides of interest, carbonate complexes appear to be the dominant complexes formed in NTS groundwaters, and data for these types of complexes are lacking for radionuclides as well as analog species. For the few species where enough data are available, the effect of temperature on radionuclide aqueous complexation has been calculated. These calculations allow partial estimation of the potential error that may be involved in ignoring speciation changes as a function of temperature, as was done in the Cheshire HST model (Pawloski et al., 2001). In some cases, differences between the most recent 25 C data available in the literature and data used in Pawloski et al. (2001) were more significant than calculated speciation changes as a function of temperature. To incorporate radionuclide speciation as a function of temperature, a robust set of temperature-dependent reaction constants is necessary. Based on our literature search and the few reactions that could be extrapolated to higher temperatures, the change in dominant complexes with temperature cannot be adequately addressed at this time. However, the effect of temperature on speciation can be qualitatively examined. In general, the log K values for radionuclide complexation reactions considered here increase with increasing temperature, suggesting that increasing temperature may enhance radionuclide aqueous complexation. However, complexation reactions often involve H{sup +} and reactant species such as carbonate which exhibit their own temperature-dependent speciation. Thus, any change in the value of a radionuclide complexation log K may be offset or enhanced by temperature effects on pH and carbonate speciation. In addition, sorption processes that involve surface complexation change with increasing temperature, and these reactions may enhance or negate the mobility effects of any increase in aqueous complexation with temperature. While increasing temperature may increase complexation, it also may reduce or increase ligand concentrations through shifts in speciation. Similarly, higher temperatures may favor or reduce sorption and/or co-precipitation in mineral phases. Consequently, the net effect on radionuclide mobility of increasing temperature depends on the effects of temperature on a number of geochemical processes. Thus, it is even difficult to make qualitative assumptions about the direction much less the magnitude of temperature effects on radionuclide mobility. Until sufficient data become available in the literature to precisely capture the effects of temperature on radionuclide complexation, it appears unwarranted to invest in complex estimation techniques based on extrapolations from available data.« less