In situ Raman spectroscopy of uranyl peroxide nanoscale cage clusters under hydrothermal conditions.

ABSTRACT

Aqueous solutions containing the nanoscale uranyl peroxide cage clusters U60, [(UO2)(O2)(OH)]6060-, and U60Ox30, [{(UO2)(O2)}60(C2O4)30]60-, were monitored by in situ Raman spectroscopy during stepwise heating to 180 °C. In solutions containing U60, clusters persist to 120 °C, although conversion of U60 to U24, [(UO2)(O2)(OH)]2424-, occurs above 100 °C. U60Ox30 persisted in solutions heated to 150 °C, although partial conversion to smaller uranyl peroxide clusters species was observed beginning at 100 °C. Upon breakdown of the uranyl peroxide cage clusters, uranium precipitated as a compreignacite-like phase, K2[(UO2)3O2(OH)3]2(H2O)7, and metaschoepite, [(UO2)8O2(OH)12](H2O)10. The role of the countercations, oxalate bridge, and solution pH are examined in order to better understand the mobility of these species at elevated temperatures.