|Abstract or Summary
- Iodine-129 is a key risk driver at sites where nuclear materials have been fabricated or processed, and it is a predominant isotope of concern in long-term waste storage strategies. I-129 exists primarily as iodate in the subsurface at the Hanford Site in south-central Washington State. Between 15 and 40% of I-129 in the Hanford vadose zone is present in organoiodine form, with the remainder being iodide. Very few alternatives are available for immobilization of dissolved I-129, and the complex biogeochemical behavior of iodine in multiple forms makes it particularly challenging in the context of immobilization. In this work, two Bi-based layered materials for the sequestration of radioactive iodate and iodide were synthesized and characterized. Each material showed remarkable selectivity for iodate, with a maximum K[subscript d] of 2810 mL g observed in groundwater containing high concentrations of carbonate and other competing anions. One of the sorbents displayed nearly quantitative uptake of total iodine, including organoiodine, from Hanford groundwater. Iodide removal was dominated by ion exchange, while iodate immobilization appears to have occurred through a combination of redox and ion-exchange processes. Each sorbent outperformed previously reported non-redox active hydrotalcites in removing iodate from groundwater. Together, these materials comprise a significant step forward for subsurface iodine removal strategies.Another radioactive contaminant of concern present in the Hanford subsurface is strontium-90. Sr-90 is a fission product of uranium, and it has entered the environment as a consequence of nuclear weapons testing, fuel reprocessing activity, and accidental or intentional releases. When ingested, strontium is deposited in bone tissue and teeth. Exposure to Sr-90 increases the risk of leukemia, bone sarcoma, and a number of other health problems. Removing it from contaminated groundwater and preventing its migration in subsurface sediments is of major importance to ongoing remediation efforts at the Hanford Site. Three inorganic sorbent materials--IONSIV™ IE-911, IONSIV™ IE-96, and bone charcoal--were studied in this work. Each sorbent effectively removed strontium from groundwater simulant (GWS) formulated to match groundwater collected at the Hanford Site. Sorption isotherms and kinetics were established for each sorbent in this system, and their maximum sorption capacities for strontium in GWS determined.A third major contaminant at the Hanford Site is uranium, which has entered the vadose zone through multiple routes, including leaks from high-level waste storage tanks and percolation into soil from unlined cribs and retention trenches. Removal of uranium from groundwater simulant by IONSIV™ IE-911, IONSIV™ IE-96, and bone charcoal was evaluated. The impact of uranium on strontium uptake by the sorbents in GWS was quantified, and the influence of uranium on speciation of strontium and calcium was examined. In order to discern actual sorption from precipitation of uranium, the fate of uranium in the contact solutions and sorbents was determined spectrophotometrically. Aqueous speciation of uranium in the GWS was modelled in simulations performed with two geochemical modelling software packages, MINTEQ and PHREEQC. Results of the simulations were compared to experimental data. The formation of aqueous ternary alkaline earth carbonate species of uranium (VI) and mechanisms of uranium sorption via surface complexation were considered.
- Iodine-129 is a key risk driver at sites where nuclear materials have been fabricated or processed