Physicochemical and Biogeochemical Stabilization of Uranium in a Low Level Radioactive Waste Disposal Cell

Abstract

An on-site disposal cell is being evaluated for the long-term disposition of materials generated by CERCLA (Comprehensive Environmental Response Compensation and Liability Act of 1980) remediation activities on the U.S. Department of Energy Oak Ridge Reservation (ORR), Oak Ridge, Tennessee. The leaching characteristics of the expected waste materials indicates that uranium concentrations in the leachate could exceed 30 mg/L. Addition of a geochemical barrier to the disposal cell design, or the application of chemical amendments to the waste materials, have been suggested as methods to reduce uranium levels in leachate leaving the disposal cell. Three promising approaches were identified: a) stabilization of mobile uranium by sulfate-reducing bacteria; b) stabilization of uranium as low solubility oxides by hydrated lime addition; and c) sorption of mobile uranium by a peat moss-based geochemical barrier in the disposal cell. Bench scale tests were conducted to evaluate the relative effectiveness of these three approaches. The results suggest that: a) a peat moss-based barrier would be effective in removing uranium initially, but long-term exposure to an alkaline leachate would result in the resolubilization and transport of uranium previously sorbed onto the peat moss; b) even large applications of hydrated lime to uranium-contaminated soils would not stabilize uranium in a high alkalinity, high pH environment; and c) mixed cultures of sulfate-reducing bacteria will greatly attenuate uranium levels (even in alkaline solutions) by precipitating it in the reduced form as UO 2 . Consequently, a geochemical barrier containing sulfate-reducing bacteria should be more effective than one using peat moss or hydrated lime.