SEQUESTRATION OF CS BY NA-BIRNESSITE FROM pH 3 TO 13 AS MEASURED WITH TIME-RESOLVED SYNCHROTRON X-RAY DIFFRACTION

Low-level nuclear waste storage tanks at the DOE Hanford site in Richland, Washington have leaked more than 1 million gallons of solution that is extremely basic (pH > 13) with high ionic strength and concentrated in radioactive Cs-137 (2 x 10¹⁰ Bq/L, equivalent to 0.04 mmol/L). The underlying Ringold Formation consists of poorly consolidated clays, silts, and sands rich in Fe and Mn oxides, including the phyllomanganate birnessite. Interlayer cations in birnessite are highly exchangeable, and Lopano et al. (2009) have demonstrated that millimolar concentrations of aqueous Cs⁺ will rapidly exchange for Na⁺ in the birnessite interlayer in neutral solutions. For the first time, we have explored diadochic substitution of Cs⁺ for Na⁺ in birnessite over a full pH range. We characterized the cation exchange products of in situ reactions using time-resolved synchrotron X-ray diffraction at pH values ranging from 3 to 13. These data demonstrate that Cs exchange rates are slowest at neutral pH and fastest at pH 9. No Cs cation exchange was observed at pH 3. Instead, the birnessite structure transformed from triclinic to hexagonal symmetry, indicating that aqueous H⁺ outcompeted Cs⁺ for partitioning into the solid.