Behavior of iodate substituted ettringite during aqueous leaching

摘要

Ettringite [Ca6Al2(SO4)(3)(OH)(12)center dot 26H(2)O] found in cementitious waste forms (CWFs) is known to immobilize iodate (IO3-) via sulfate (SO42-) substitution; however, limited studies have investigated the effects of leaching on the long-term fate and behavior of IO3- retained in ettringites. In this study, ettringite mineral transformation and iodine release from IO3- incorporated ettringite ((IO3-)-Ett) were investigated using batch experiments to determine the mechanisms likely to drive IO3- release/retention within ettringite in environmentally relevant solutions of varying composition, pH, and alkalinity. (IO3-)-Ett was leached in six simulated solutions relevant to the Hanford Site (Washington State, USA) for a period of 56 days: double deionized water, 24 ppm CO32-, 1400 ppm SO42-, Hanford groundwater, Hanford vadose zone porewater and a cement-equilibrated solution. By monitoring IO3- uptake and removal and characterizing changes in the solid material, incongruent dissolution of ettringite to calcite and oxyanion substitution into the ettringite structure (e.g., SO42- or CO32-) were identified as contributing IO3- release mechanisms. However, the formation of calcium carbonates, e.g., calcite, may re-incorporate IO3- (pH < 10) or prevent the release of IO3- when formed on solution-exposed surfaces of ettringite. These results provide valuable insights into iodine release from CWFs that may be used to improve current CWF formulations for the treatment of iodine-containing nuclear waste.