PNNL-13501 - Pacific Northwest National Laboratory

Thermochemical Methods for Measuring Ligand-Actinide Ion Binding Strength
Study Control Number: PN00087/1494
Gregg J. Lumetta, Richard L. Sell, Bruce K. McNamara
Safe management and disposal of highly radioactive spent fuel is a major problem threatening the future viability of
nuclear energy. Chemical processing to separate and remove long-lived radionuclides before disposing of the spent
nuclear fuel in geologic repositories would significantly improve the post-closure performance of these repositories. The
results of this project expand the tools available for developing highly specific separating agents for actinide ions.
Project Description
Thermochemical methods are being employed to
quantitatively determine the binding enthalpy for MLn
complexes, where M is a lanthanide or actinide element
and L is a ligand of the type that might form the basis of
an extractant (an amide). The key tools employed in this
study are differential thermal analysis and
thermogravimetric analysis. Complementary
spectroscopic measurements are conducted to fully
characterize the chemical systems being examined.
Introduction
Actinide separations is an important scientific and
technological research area because spent nuclear fuel
contains dangerous levels of long-lived radionuclides.
Coordination chemistry forms the basis for the
development of efficient metal ion separations, and
quantum leaps in efficient separation will hinge on
understanding structure-function relationships central to
coordination chemistry. This is especially true for the
actinide ions. The results of this project expand the tools
available for developing highly specific separating agents
for actinide ions. The work has focused on applying
direct thermochemical methods for measuring the
thermodynamics of binding for ligands to actinide and
other metal ions.
Approach
In this work, we applied thermochemical methods to
quantitatively determine the binding enthalpy for MLn
complexes, where M is a lanthanide or actinide element,
and L is a ligand of the type that might form the basis of
an extractant (an amide). The enthalpy of the following
type of reaction is measured.
MXzLn → MXz + nL
436 FY 2000 Laboratory Directed Research and Development Annual Report
The key tools employed in this study have been
differential thermal analysis and thermogravimetric
analysis (TGA). Complementary spectroscopic
measurements have been be conducted to fully
characterize the chemical systems being examined.
Results and Accomplishments
The decomposition of NdCl3L (L = N,Ndimethylformamide
[DMF] or N,N-dimethylacetamide
[DMA]) compounds has been investigated by
thermogravimetric analysis and differential thermal
analysis, coupled with Fourier transform infrared (FTIR)
spectroscopy.
When heated in air, the NdCl3⋅L (L = DMF or DMA)
compounds decompose by a mechanism involving the
oxidation of the amide ligand. Figure 1 presents the TGA
data for the thermal decomposition of NdCl3·DMF in air
and the species identified by Fourier transform infrared in
the off-gas. When NdCl3·DMF is heated from 80 to
150°C, TGA indicates a complex series of weight losses.
Fourier transform infrared analysis of the off-gas
indicates that these mass losses are from the release of
water and methanol. Release of DMF becomes evident at
~155°C, although the DMF release is gradual from 155 to
270°C. From 270 to 340°C, a more rapid mass loss
occurs. All of the DMF is released by the time the sample
is heated to 340°C. Gas evolution becomes more
complex during the accelerated mass loss between 270
and 340°C. In addition to DMF, HCl and CO2 are evident
in the off-gas, indicating combustion of DMF.
Similar to NdCl3·DMF, water and methanol are released
in a complicated series of steps when NdCl3·DMA is
heated to 165°C in air. DMA is first evident in the offgas
at about 200°C. The DMA loss occurs primarily in
two regimes. The first occurs from 270 to 310°C. In this
regime, free DMA and a small amount of CO2 and H2O
are evident in the off-gas. However, as the temperature is