Radionuclide Immobilization in Reducing Cementitious Materials
EMSL Project ID
48824
Abstract
Reducing cementitious materials are used as waste forms and engineered barriers for low-level radioactive waste disposal in the vadose zone. These materials are specifically formulated to contain elevated concentrations of sulfides and ferrous iron to promote the reductive precipitation of redox sensitive radionuclides, especially for the key risk-driver 99Tc. The hypothesis of this study is that the capacity of reducing cementitious materials to immobilize redox-sensitive radionuclides changes as the material ages and as infiltrating porewater passes through it. This hypothesis is based on the observations that: (1) the mineralogy of traditional non-reducing cementitious materials changes as it undergoes natural aging processes, such as hydration, calcification, and carbonation, (2) sulfur and to a lesser extent iron leaches from reducing cementitious materials with infiltrating porewater, and (3) reductants (Fe2+ and S2-) in reducing cementitious materials become oxidized by infiltrating oxygenated groundwater. The concept of an evolving mineralogy and chemistry in traditional cementitious waste forms is well developed through research directed at predicting the longevity of cementitious materials for the construction industry. However an understanding of redox chemistry and its effects on redox sensitive Tc has not been well studied. The paucity of information on this subject can be attributed to this relatively novel and esoteric use of cementitious materials, namely to immobilize radioactive waste. To date, reduction capacity in cementitious waste form is described as a static condition that eventually gets consumed with infiltrating oxygenating groundwater (i.e., the Shrinking Core Model). The proposed research challenges this conceptual model. The objective of this study is to characterize the speciation of the reductant, the transitory nature of the reductant as the reducing cementitious material ages, and finally, the effect of these aging processes on Tc and Cr immobilization. Chromium(III/VI), which exists at 100s of mg/kg concentrations in waste forms, will also be studied because it provides an upper limit to the Tc(IV/VII) redox couple. EMSL resources will be used to evaluate: the relative contribution of Fe2+ and S2- to the reduction capacity, the types of Fe and S phases present, Tc and Cr speciation, and the relative roles that Fe-oxide, Fe-sulfides, and/or Fe/S phases contribute to Tc and Cr immobilization. Twelve reducing cementitious materials from 4-yr old samples recovered from the SRS Saltstone Disposal Facility (radionuclide containing) and some non-radiological simulant samples will be used in column experiments leached with simulated cementitious-impacted groundwater for varying lengths of time and then characterized for iron mineralogy (M?ssbauer), sulfur (and Cr) oxidation state chemistry (XPS) and elemental (Fe/S/Tc/Cr) mapping (EMPA). The non-rad samples will be examined the first year and the radioactive samples will be analyzed the second year. The EMSL data will be combined with column flow data, aqueous chemistry (Tc, Cr, pH, Eh, S (VI/0/-II), Fe(II/III)), and S- and Fe-XAS data (two beam times are scheduled at APS for 2015). Kinetic and thermodynamic modelling (Geochemists’ Workbench) will be conducted to evaluate the phases controlling solubility and the reducing conditions. The expected outcome of this study is an improved understanding of the transient mineralogical and chemical conditions responsible for reducing conditions, and to provide greater insight into how Tc is immobilized in this material.
Project Details
Project type
Large-Scale EMSL Research
Start Date
2015-10-01
End Date
2017-09-30
Status
Closed
Released Data Link
Team
Principal Investigator
Team Members
Related Publications
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Rod K.A., M. Nguyen, M. Elbakhshwan, S. Gill, B. Kutchko, T. Varga, and A.L. McKinney, et al. 2019. "Insights into the physical and chemical properties of a cement-polymer composite developed for geothermal wellbore applications." Cement and Concrete Composites 97. PNNL-SA-131466. doi:10.1016/j.cemconcomp.2018.12.022
Saslow SA, W Um, CI Pearce, MH Engelhard, ME Bowden, WW Lukens, II Leavy, BJ Riley, DS Kim, MJ Schweiger, and AA Kruger. 2017. "Reduction and Simultaneous Removal of 99Tc and Cr by Fe(OH)2(s) Mineral Transformation." Environmental Science & Technology 51(15):8635–8642. doi:10.1021/acs.est.7b02278
Wang G, W Um, Z Wang, E Reinoso Maset, NM Washton, KL Mueller, N Perdrial, PA O'Day, and JD Chorover. 2017. "Uranium Release from Acidic Weathered Hanford Sediments: Single-Pass Flow-Through and Column Experiments." Environmental Science & Technology 51(19):11011-11019. doi:10.1021/acs.est.7b03475