Rates and Mechanisms of Radioncuclide Fixation by Organo-Polyphosphates

Contaminants in radioactive wastewaters disposed of into the Y-12 Security Complex S-3 ponds for 32 years 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 in contaminated groundwater then repeatedly retrieved and assayed using x-ray fluorescence spectrometry (XRF). Uranium and Th levels were compared to other elements (Sr, Rb, Fe, Mn, and Zr) which remained relatively constant. The small uptake of U and Th by polyacrylamide alone reflected the diffusion of dissolved ions into the gel. Thorium uptake was initially highest by soil only, but the final values of soil and soil + phytates were relatively similar. Uranium uptake values were initially highest in soil + Na phytate, but uptake was quickly dominated by soil + 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 by deionized water, weak ionic exchange by dissolved salts, and extraction of adsorbed soil oxides by dilute acid. The other half of the PELCAPs will be submerged in uncontaminated in situ groundwater. PELCAPs will be analyzed to determine which materials maintain radionuclide fixation longest relative to the contaminant uptake. As XRF data are insufficient to conclude the specific means of attenuation, the additional analytical methods of laboratory extractions and in situ leaching will assist in determining these mechanisms of radionuclide fixation.