Thermodynamics of water sorption on PuO2: Consequences for oxide storage and solubility

Gibbs energies for sorption of water on plutonium dioxide are derived from equilibrium pressure-surface composition data at 298-358 K. Sorption occurs in well-defined initial steps that form variable-composition surface phases by successive addition of H2O layers and ultimately in a continuum. ΔG0 values vary from −24.6 to −16.1 kJ per mol H2O as y in the initial PuO2−y(OH)2y product increases from 0 to 1 and from −16.1 to −11.0 kJ per mol H2O during formation of PuO(OH)2 · nH2O (n = 0 to 1) in the second step. These results are consistent with dissociative chemisorption at 298 K and with desorption near 575 K. Gibbs energies for higher sorption steps at 298 K asymptotically approach ΔG0 for condensation of water (−8.56 kJ mol−1) and are consistent with reversible physisorption. Respective ΔGf2980 and ΔHf2980 are −1247 and −1338 kJ mol−1 for PuO(OH)2; corresponding values are −1488 and −1608 kJ mol−1 for PuO(OH)2 · H2O. Results do not support claims that all water is reversibly desorbed at 350-500 K or that dioxide solubility is enhanced by formation of Pu(OH)4(am) on the surface.