Systematic Evolution from Uranyl(VI) Phosphites to Uranium(IV) Phosphates

Six new uranium phosphites, phosphates, and mixed phosphate–phosphite
compounds were hydrothermally synthesized, with an additional uranyl
phosphite synthesized at room temperature. These compounds can contain
UVI or UIV, and two are mixed-valent UVI/UIV compounds. There appears to be a strong correlation
between the starting pH and reaction duration and the products that
form. In general, phosphites are more likely to form at shorter reaction
times, while phosphates form at extended reaction times. Additionally,
reduction of uranium from UVI to UIV happens
much more readily at lower pH and can be slowed with an increase in
the initial pH of the reaction mixture. Here we explore the in situ
hydrothermal redox reactions of uranyl nitrate with phosphorous acid
and alkali-metal carbonates. The resulting products reveal the evolution
of compounds formed as these hydrothermal redox reactions proceed
forward with time.