NUREG/CR-6977, "Redox and Sorption Reactions of Iodine and Cesium During Transport Through Aquifer Sediments"

Radioactive isotopes of iodine (1311 and 1291) 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 (F), which is generally considered to behave conservatively, it has been proposed that iodide can be oxidized to elemental iodine (12) or iodate (103-) 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 F 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 Cs1O 3 into a well oxygenated zone, and (3) injection of CSIO 3 into a zone of the aquifer characterized by active Fe(III) reduction (but not sulfate reduction). In laboratory experiments, birnessite oxidized F to 12 and 103" in a two-step process. The oxidation of F proceeded according to first order kinetics with respect to initial F concentration, pH, and birnessite concentration. 12 sorption to birnessite was high (up to 0.25 mmol/g), while 103 sorption to birnessite was an order of magnitude lower (up to 0.024 mmol/g). Uptake of F in batch experiments by sediments was fairly low at pH 4.8 or above, as was F retardation in column experiments at this pH. In column experiments at pH 4.50, the results suggested some oxidation of F occurred due to a 7% loss of iodine mass exiting the column, presumably due to volatilization of elemental 12. 103uptake in