Thermodynamics and Kinetics of Advanced Separations Systems ? FY 2010 Summary Report

This report presents a summary of the work performed in the area of thermodynamics and kinetics of advanced separations systems under the Fuel Cycle Research and Development (FCR&D) program during FY 2010. Thermodynamic investigations into metal extraction dependencies on lactate and HDEHP have been performed. These metal distribution studies indicate a substantial deviation from the expected behavior at conditions that are typical of TALSPEAK process operational platform. These studies also identify that no thermodynamically stable mixed complexes exist in the aqueous solutions and increasing the complexity of the organic medium appears to influence the observed deviations. Following on from this, the first calorimetric measurement of the heat of extraction of americium across a liquid-liquid boundary was performed

Toward understanding the thermodynamics of TALSPEAK process. Medium effects on actinide complexation

The ingenious combination of lactate and diethylenetriamine-N,N,N’,N”,N”-pentaacetic acid (DTPA) as an aqueous actinide-complexing medium forms the basis of the successful separation of americium and curium from lanthanides known as the TALSPEAK process. While numerous reports in the prior literature have focused on the optimization of this solvent extraction system, considerably less attention has been devoted to the understanding of the basic thermodynamic features of the complex fluids responsible for the separation. The available thermochemical information of both lactate and DTPA protonation and metal complexation reactions are representative of the behavior of these ions under idealized conditions. Our previous studies of medium effects on lactate protonation suggest that significant departures from the speciation predicted based on reported thermodynamic values should be expected in the TALSPEAK aqueous environment. Thermodynamic parameters describing the separation chemistry of this process thus require further examination at conditions significantly removed from conventional ideal systems commonly employed in fundamental solution chemistry. Such thermodynamic characterization is the key to predictive modelling of TALSPEAK. Improved understanding will, in principle, allow process technologists to more efficiently respond to off-normal conditions during large scale process operation. In this report, the results of calorimetric and potentiometric investigations of the effects of aqueous electrolytes on the thermodynamic parameters for lactate protonation and lactate complexation of americium and neodymium will be presented. Studies on the lactate protonation equilibrium will clearly illustrate distinct thermodynamic variations between strong electrolyte aqueous systems and buffered lactate environment

Understanding the solution behavior of minor actinides in the presence of EDTA(4-), carbonate, and hydroxide ligands

Understanding the solution behavior of minor actinides in the presence of EDTA(4-), carbonate, and hydroxide ligand

CONTRIBUTING TO THE DISCUSSIONS ON THE FUNDAMENTAL ASPECTS AND COMPLEXITIES OF TALSPEAK CHEMISTRY

When liquid-liquid distribution of lanthanides was monitored at Talspeak-related conditions a characteristic drop in the extraction efficiency was observed at high lactate concentrations. The lactate dependency trend also appears to be directly affected by the increasing complexity of the non-aqueous environment. Some considerations of the non-ideal solution behavior in aqueous and organic environment are presented here in an attempt to explain the observed metal partitioning trends. While the mechanism of metal ion phase transfer appears to adhere to the conventional thermodynamic struggle between HDEHP and DTPA, the diminished metal distribution and suppressed slopes for the extractant dependencies suggest further build-up in the complexity of the non-aqueous environment in Talspeak systems

First calorimetric determination of heat of extraction of 248Cm in a bi-phasic system

This report presents a summary of the work performed to meet FCR&D level 2 milestone M21SW050201, 'Complete the first calorimetric determination of heat of extraction of 248Cm in a bi-phasic system'. This work was carried out under the auspices of the Thermodynamics and Kinetics FCR&D work package. To complement previous work undertaken under this work package we have extended out heat of extraction studies by di-2-ethyl-hexyl-phosphoric acid to curium. This report also details the heat of extraction of samarium in the same system. This work was performed to not only test the methodology but also to check for consistency with the heats of extraction obtained with those in the prior literature. The heat of extraction for samarium that was obtained in this study was -9.6 kJ mol-1, which is in reasonable agreement with the previously obtained value of -10.9 kJ mol-1. The curium heat of extraction was performed under two sets of conditions and the obtained heats of extraction were in reasonable agreement with each other at -16.0 {+-} 1.1 and -16.8 {+-} 1.5 kJ mol-1

Thermodynamics and Kinetics of Advanced Separations Systems – FY 2010 Summary Report

This report presents a summary of the work performed in the area of thermodynamics and kinetics of advanced separations systems under the Fuel Cycle Research and Development (FCR&D) program during FY 2010. Thermodynamic investigations into metal extraction dependencies on lactate and HDEHP have been performed. These metal distribution studies indicate a substantial deviation from the expected behavior at conditions that are typical of TALSPEAK process operational platform. These studies also identify that no thermodynamically stable mixed complexes exist in the aqueous solutions and increasing the complexity of the organic medium appears to influence the observed deviations. Following on from this, the first calorimetric measurement of the heat of extraction of americium across a liquid-liquid boundary was performed

Ion Interaction Models and Measurements of Eu3+ Complexation: DTPA in Aqueous Solutions at 25 °C Containing 1:1 Na+ Salts and Malonate pH Buffer

The separation of lanthanides from actinides in the TALSPEAK liquid–liquid distribution process is accomplished using an aminopolycarboxylate complexing agent, for example diethylenetriamine-N,N,N′,N″,N″-pentaacetic acid (DTPA, CAS Reg. No. 67-43-6), in a low pH buffered aqueous phase in contact with an organic phase containing an extractant such as di(2-ethylhexyl)phosphoric acid (HDEHP, CAS Reg. No. 298-07-7). Literature measurements show that the partitioning of lanthanides to the organic phase falls with rising pH whereas thermodynamic equilibrium models suggest that, at pH above approximately 3.5, the partitioning should increase. In this study, the partitioning of Eu3+ between an aqueous phase (with NaNO3 background electrolyte, malonate buffer, and DTPA complexing agent), and an organic phase (HDEHP in n-dodecane) is measured from pH 2 to 4.5 and for ionic strengths from 0.25 to 1.0 mol kg–1. The measurements include systems with reduced (by 10×) concentrations of buffer, DTPA, and Eu3+. A Pitzer activity coefficient model of the aqueous mixture is developed based upon available osmotic and activity coefficient data, and stoichiometric equilibrium constants in different 1:1 electrolyte media over a range of ionic strengths. This enables the DTPA and buffer speciation, and complexation of Eu3+ by both DTPA and malonate, to be calculated for different solution compositions and pH. The measured distribution coefficients are consistent with model predictions up to pH 3.5 and, below this pH, vary little with ionic strength. At higher pH, the distribution coefficients at different ionic strengths deviate both from the model and each other, consistent with other reactions occurring in the organic phase than the simple exchange of lanthanide and H+ embodied in the TALSPEAK phase transfer reaction

Fiscal Year 2011

This report presents a summary of the work performed in the area of thermodynamics and kinetics of advanced separations systems under the Fuel Cycle Research and Development (FCR&D) program during FY 2011 at the INL. On the thermodynamic front, investigations of liquid-liquid distribution of lanthanides at TALSPEAK-related conditions continued in FY11. It has been determined that a classical ion-exchanging phase transfer mechanism, where three HDEHP dimers solvate the metal ion in the organic phase, dominates metal extraction for systems that contain up to 0.1 M free lactate in solution. The correct graphical interpretation of the observed data in those regions relied on incorporating corrections for non-ideal behavior of HDEHP dimer in aliphatic diluents as well as sodium extraction equilibria. When aqueous conditions enter the complex regions of high lactate concentrations, slope analysis is no longer possible. When normalized metal distribution ratios were studied along the increasing concentration of free lactate, a slope of -1 was apparent. Such dependency either indicates aqueous complexing competition from lactate, or, a more likely scenario, a participation of lactate in the extracted metal complex. This finding agrees with our initial assessment of postulated changes in the extraction mechanism as a source of the lactate-mediated loss of extraction efficiency. The observed shape in the lanthanide distribution curve in our studies of TALSPEAK systems was the same for solutions containing no lactate or 2.3 M lactate. As such we may conclude that the mechanism of phase transfer is not altered dramatically and remains similarly sensitive to effective charge density of the metal ion. In addition to these thermodynamics studies, this report also summarizes the first calorimetric determination of heat of extraction of 248Cm in a bi-phasic system. The heat of extraction measured by isothermal titration calorimetry is compared to that determined using van't Hoff calculations. Further investigations on the kinetics of extraction in TALSPEAK with macro quantities of lanthanides present in the initial aqueous phase composition have been performed. These results have been summarized and compared to previous work performed for FCR&D. In addition, the effects of HDEHP concentration in the organic phase on europium extraction have been studied

Ion Interaction Models and Measurements of Eu3+ Complexation: HEDTA in Aqueous Solutions at 25 °c Containing 1:1 Na+ Salts and Citrate pH Buffer

In the TALSPEAK liquid–liquid distribution process, dissolved lanthanides can be separated from actinides using a complexing agent such as N-(2-hydroxyethyl)ethylenediamine-N,N′,N′-triacetic acid (HEDTA, CAS Reg. No. 150-39-0) in a low pH buffered aqueous phase in contact with an organic phase containing a suitable extractant. This study focuses on the chemical speciation of HEDTA, citrate pH buffer, and Eu3+ in aqueous solutions of 1:1 Na+ salts (mainly NaNO3) as a function of ionic strength and pH. New measurements of stoichiometric protonation constants of HEDTA, and the HEDTA complex of Eu3+, in aqueous NaNO3 are reported for ionic strengths from 0.5 to 4.0 M at 25 °C. A Pitzer activity coefficient model of the aqueous mixture has been developed based upon these measurements, available osmotic and activity coefficient data, and stoichiometric equilibrium constants in different 1:1 electrolyte media over a range of ionic strengths. This enables the HEDTA and buffer speciation, and complexation of Eu3+ by both HEDTA and citrate, to be calculated for different solution compositions and pH values. The model of the citrate buffer, which is based on an extensive range of data for NaCl and NaNO3 media, should also be useful in other practical applications