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

Abstract 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 H 2 O layers and ultimately in a continuum. Δ G 0 values vary from −24.6 to −16.1 kJ per mol H 2 O as y in the initial PuO 2− y (OH) 2 y product increases from 0 to 1 and from −16.1 to −11.0 kJ per mol H 2 O during formation of PuO(OH) 2 · n H 2 O ( 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 Δ G 0 for condensation of water (−8.56 kJ mol −1 ) and are consistent with reversible physisorption. Respective Δ G f 298 0 and Δ H f 298 0 are −1247 and −1338 kJ mol −1 for PuO(OH) 2 ; corresponding values are −1488 and −1608 kJ mol −1 for PuO(OH) 2 · H 2 O. 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.