To understand the behavior of radionuclides during alkaline pretreatment of tank sludges, simulants were prepared according to the BiPO4, Redox and PUREX processes and incorporated with U-233, U-238, Am-243 or Sr-90. The simulated sludges were washed with solutions of various compositions - acidic, alkaline, or with chelating agents. Spectroscopic techniques including UV/Vis, fluorescence, NMR and EXAFS provided additional information on the speciation. A combination of selective leaching of uranium in the presence of oxalate and malonate and NMR experiments demonstrated the competition between complexation and hydrolysis. Limited experiments with Co-60 radiation were performed to evaluate the effect of radiolysis on the leaching. Preliminary experiments were conducted to study the oxidative leaching of americium from the sludge, using S2O82- as the oxidant. These data help to predict the behavior of radionuclides in sludge washing and improve the washing strategies.

HEDPA (1-hydroxyethane-1,1-diphosphonic acid) and other diphosphonic acids are remarkably strong chelating agents in moderately acidic solutions and have found applications in separations and HEDPA has also been identified as a promising candidate for the decorporation of uranium from the human body. Consequently, the coordination chemistry of the lanthanides and actinides with HEDPA has been studied extensively in acidic solutions, and to a lesser extent at physiological pH. However, no studies have been carried out on the complexation of HEDPA with uranyl(VI) at higher pH. In this work, potentiometry and calorimetry were used to determine the thermodynamic parameters of the complexation of uranyl(VI) with HEDPA over a wide pH range. Spectroscopic techniques, including NMR, Laser-induced fluorescence and EXAFS, were used to obtain insight into the nature of the complexes.This work is supported by the Office of Science, Biological and Environmental Research, U.S. Department of Energy under Contract No. DE-AC03-76SF0098 at Lawrence Berkeley National Laboratory and Contract No. W-31-109-ENG-38 at Argonne National Laboratory.

Diphosphonic acid ligands such as HEDPA (1-hydroxyethane-1,1-diphosphonic acid) have been found to have superior complexation properties in acidic media over carboxylic acids. Consequently, they have been incorporated in a number of actinide separation processes: for example, surface decontamination and solvent extraction. Though these systems are well understood in the acidic region, relatively little work has been carried out with regards to the interaction between these ligands and metals in neutral and alkaline solutions. Recently we have started studies on the coordination of HEDPA with uranyl(VI), with emphasis on the complexation in neutral to basic solutions. Thermodynamic results from potentiometry and calorimetry and structural information from spectroscopic techniques are presented.