Rates and Mechanisms of Radionuclide Fixation by Organo-Polyphosphates

C. R. Lemons
University of Tennessee – Knoxville, Oak Ridge National Laboratory

Contaminants in radioactive wastewaters disposed of into the Y-12 Security Complex S-3 ponds have migrated into the surrounding soil and groundwater. This experiment studied the potential role of organo-polyphosphates for uranium and thorium adsorption and immobilization. Phytate is a naturally occurring polyphosphate produced commercially for its utility as a metal chelating and precipitating agent. Polyphosphates can react with contaminants by adsorption, ion exchange, and/or coprecipitation. Forty small (5-10 g) permeable environmental leaching capsules (PELCAPs) were prepared to contain polyacrylamide, alone or in association with soil and polyphosphates. PELCAPs were submerged into contaminated groundwater and then repeatedly retrieved and assayed using x-ray fluorescence spectrometry (XRF). The small uptake of U and Th by polyacrylamide alone reflected the diffusion of dissolved ions into the gel. Results indicate that thorium uptake was initially highest by soil, but the rate of uptake is greatest in those PELCAPs containing polyphosphates. Uranium uptake values were initially highest in soil + Na phytate, but uptake is now dominated by Ca phytate. Future studies will determine the mechanisms of attenuation and degree of leaching of the contaminant-laden PELCAPs. One-half of the PELCAPs will undergo sequential laboratory extractions to determine rates of ion diffusion, ionic exchange, and extraction. The other half of the PELCAPs will be submerged in uncontaminated in situ groundwater. PELCAPs will be analyzed to determine which materials maintain fixation longest relative to the contaminant uptake. These additional analytical methods of laboratory extractions and in situ leaching will assist in determining these mechanisms of radionuclide fixation.