Solubility of plutonium hydroxides/hydrous oxides under reducing conditions and in the presence of oxygen

The present paper analyzes solubility data reported for plutonium hydroxides or hydrous oxides under different redox conditions (in the presence of reducing chemicals, in solutions without reducing or oxidising agents under Ar atmosphere and under air). Special attention is dedicated to the solubility controlling solid phases, total Pu concentrations, oxidation-state distributions and simultaneously measured redox potentials. Thermodynamic calculations and experimental data show that Pu(OH)(3)(S) is not stable. Under reducing conditions in the stability field of water it converts into PUO2(s,hyd). The solubility is given by the equilibrium concentrations of aqueous Pu(III) and Pu(IV) species. In the absence of reducing and oxidising agents but in the presence of traces of O-2(g), the total Pu concentration at pH > 3 is dominated by Pu(V) and the solubility is controlled by PUO2+x(s,hyd), mixed valent (PUV)(2x)(PUIV)(1-2x)O2+x(s,hyd). Small Pu(IV) colloids/polymers present in neutral to alkaline solutions at a constant level of log[Pu(IV)]coll = -8.3 +/- 1.0 play an important role for the redox potentials in these systems. Including these species, the experimental results in the system Pu/e(-)/H+/OH-/NaCIO4 or NaCl/H2O (25 degrees C) can be described in terms of equilibrium thermodynamics.