Redox and Sorption Reactions of Iodine and Cesium During Transport Through Aquifer Sediments (NUREG/CR-6977)
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Manuscript Completed: May 2008
Date Published: March 2009
J.A. Davis and P.M. Fox
U.S. Geological Survey
Menlo Park, CA; 94025
M. Fuhrmann, NRC Project Manager
Office of Nuclear Regulatory Research
U.S. Nuclear Regulatory Commission
Washington DC 20555-0001
NRC Job Code Y6462
Radioactive isotopes of iodine (131I and 129I) and cesium (137Cs) are important contaminants present in nuclear waste. These radioisotopes have been introduced into the environment through nuclear weapons tests as well as nuclear accidents such as Chernobyl. Although iodine is commonly found as iodide (I-), which is generally considered to behave conservatively, it has been proposed that iodide can be oxidized to elemental iodine (I2) or iodate (IO3-) by manganese oxides or nitrate, which may behave less conservatively in sediments due to uptake by organic matter or adsorption onto mineral surfaces. Cesium is generally present as a cation (Cs+) and can be strongly adsorbed by sediments.
In order to further our understanding of the chemical behavior of I and Cs in groundwater systems, a series of laboratory and field experiments were undertaken. The kinetics of I- oxidation by the manganese oxide, birnessite, was investigated under a variety of geochemical conditions. In order to determine Cs and I sorption and I oxidation, batch experiments with aquifer sediments and with binary sediment-Mn oxide systems were performed. Iodide transport was studied in a column filled with aquifer sediments. Three field tracer test experiments were performed to elucidate the redox chemistry and transport of I and Cs in an aquifer characterized by distinct geochemical zones: (1) injection of CsI into a well oxygenated zone of the aquifer, (2) injection of CsIO3 into a well oxygenated zone, and (3) injection of CsIO3 into a zone of the aquifer characterized by active Fe(III) reduction (but not sulfate reduction).
In laboratory experiments, birnessite oxidized I- to I2 and IO3- in a two-step process. The oxidation of I- proceeded according to first order kinetics with respect to initial I- concentration, pH, and birnessite concentration. I2 sorption to birnessite was high (up to 0.25 mmol/g), while IO3- sorption to birnessite was an order of magnitude lower (up to 0.024 mmol/g). Uptake of I- in batch experiments by sediments was fairly low at pH 4.8 or above, as was I- retardation in column experiments at this pH. In column experiments at pH 4.50, the results suggested some oxidation of I- occurred due to a 7% loss of iodine mass exiting the column, presumably due to volatilization of elemental I2. IO3- uptake in batch experiments with sediments was higher than that of I-, reaching up to 12% adsorbed. Cs also adsorbed to aquifer sediments, with up to 22% removed from solution after 24 hours.
Results from the field tracer tests show that I- was oxidized to both I2 (up to 46%) and IO3- (up to 6%) in the oxic zone, with the extent of oxidation increasing with transport. A pulse of dissolved Mn was liberated from the sediments, providing evidence that Mn oxides were responsible for I- oxidation. Iodate was retarded relative to a conservative tracer (Br), arriving 5 days later at 3.9 m downgradient. In the Fe-reducing zone, IO3- was quickly reduced to I- without any observed production of I2 intermediate. About 60% of the iodate was reduced to I- in 1 m of transport, with complete reduction occurring after 3 m of transport. Cesium transport was retarded relative to the conservative tracer in all three tracer tests. In the oxic zone of the aquifer, the peak Cs concentration arrived 3.9 m downgradient after 35 days of transport, whereas the Br peak arrived at 8 days. Cs was so attenuated during the first 3.9 m of transport that the maximum Cs concentration reached was only 6% of the injected concentration.
While the Cs and I concentrations used in these experiments was much higher than would be relevant for concentrations of radioactive isotopes of these elements, the studies are relevant for revealing reaction mechanisms that affect the transport of these radionuclides in the environment. The results of these experiments demonstrate that not only can redox transformations of iodine easily occur in groundwater systems, but also that I2, IO3-, and Cs behave non-conservatively by adsorbing to sediments and minerals. The results indicate the importance of considering the complex redox and sorption chemistry of iodine when predicting its transport in waste plumes.