Ettringite [Ca6Al2(SO4)3(OH)12●26H2O] formation in cementitious waste forms (CWFs) is a valuable resource for immobilizing contaminant anions, as these anions may substitute for structural sulfate sites during formation. This mineral incorporation mechanism is well documented for the oxidized species of radionuclide iodine-129, iodate (IO3-). Considering iodine-129 is a major contaminant of concern in nuclear waste at the Hanford Site (Washington State, USA), due to its long half-life and high environmental mobility, ettringite incorporation of IO3- could improve long-term CWF iodine retention. Yet, poor iodine retention in CWFs remains a common challenge, even for those CWFs with significant ettringite content. In this study, IO3- leachability and ettringite mineral transformation due to changes in environmentally relevant aqueous conditions were used to (i) evaluate the mechanism for IO3- retention and/or release from ettringite and (ii) identify aqueous conditions that dictate IO3- release from CWFs. Leach tests were performed using IO3--incorporated ettringite (I-Ett) suspended in synthetic Hanford Site relevant solutions of varying ionic strengths. Changes in the initial and final solid products were determined by X-ray diffraction (XRD) and scanning electron microscopy/energy dispersive X-ray spectroscopy. Using this approach, IO3- release from I-Ett over 56 days was observed to range from ~80% in vadose zone pore water (VZPW) to 42% in groundwater (HGW). This observation is attributed to higher CO32- and SO42- concentrations in the VZPW compared to HGW. Furthermore, XRD shows that the ettringite content in I-Ett is reduced significantly over 56 days in both solutions and increased levels of calcite in VZPW solids suggest ettringite dissolution/transformation processes may play a role in IO3- release. These results and those of ongoing tests will allow us to better understand the release of iodine from CWFs, identify parameters contributing to its release, and improve upon current cementitious formulations for the treatment of iodine-containing nuclear waste streams.

Impact of ettringite stability and transformation on iodine retention in cementitious waste forms

Macromolecular Chemistry: The Second Century

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ACS Spring 2020 National Meeting & Expo

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