Thermodynamic studies of actinide complexation at elevated temperature


Sue B. Clark and Alena Paulenova. Department of Chemistry, Washington State University, POBox 4630, Pullman, WA 99164-4630
Organic ligands used in separation processes or generated in radioactive waste can have a significant impact on chemical behavior and solubility of actinides (An), and little is currently known about the impact of temperature on these processes. A thermodynamic model describing actinide speciation at high temperature is under development for the processing and vitrification of high-level waste. In support of this model development, complexation equilibria for Anz+(aq) + Ln-(aq)=AnL(z-n)+(aq) have been studied over the temperature interval from 10-75oC, pH 5 - 6. Apparent stability constants with EDTA, NTA, acetate, oxalate, gluconate and citrate have been measured using solvent extraction at ionic strength 0.1 M NaClO4. Values for the apparent stability constants have been extrapolated to zero ionic strength using SIT parameters. Complexation enthalpies and entropies were calculated from temperature dependences of stability constants. The impact of temperature will be discussed in the context of effective cationic charge for the actinide ions.