The temperature-dependent reaction properties of actinide
elements
are of particular interest in the safety assessment of high-level
radioactive waste (HLRW) disposal systems. In this study, the hydrolysis
of Pu(III) and the solubility of Pu(OH)3(am) were investigated
at various temperatures (10–40 °C) in 0.1 M NaClO4. A strong reducing condition for maintaining the oxidation
state of Pu(III) while slowly increasing the pH of the solution was
realized by electrolysis. The formation constants of the first hydrolysis
species, log
*
β1
′, and the solubility products
of Pu(OH)3(am), log
*
K
s,0
′, at 10, 17, and 40 °C were experimentally determined using
spectrophotometry, laser-induced breakdown detection, and radiometry.
The enthalpy and entropy changes for these reactions were estimated
using the van’t Hoff equation. The first hydrolysis of Pu(III)
is endothermic (Δr
H
m
° = 34.10 ±
4.48 kJ mol–1), and the dissolution of Pu(OH)3(am) is exothermic (Δr
H
m
° = −294.29
± 23.05 kJ mol–1) with negative entropy changes.
These thermodynamic data will contribute to improving the reliability
of the safety assessment of HLRW disposal facilities and understanding
the geochemical behavior of Pu under reducing or anoxic aqueous conditions
at elevated temperatures.