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.
AAPG Search and Discover Article #90087 © 2008 AAPG/SEG Student Expo, Houston, Texas