3.4 The Speciation of Lanthanide Ions in Solution Systems by an NMR Measurement of Molecular Motion

Fig. 3-7 Relationships between the temperature factor and the reciprocal of longitudinal relaxation time of 139La in nitrate solutions of various concentrations
In the unit for temperature factor, cP represents centipoise for viscosity and K absolute temperature.
[La (NO3) 3] = 0.024 mol/l ; temperature = 274-343 K

The different slopes of the straight lines in the figure reflect changes in the electronic state and hence in the complex structure around the lanthanum nucleus.


magnified picture
Fig. 3-8 Structure of a lanthanum ion
in nitrate and chloride solution systems
In a nitrate ion system, hydrated lanthanum is incorporated in water networks, while it is isolated in a chloride ion system.

How do you imagine the microstructure of fission-produced tervalent lanthanides and actinides in nitric acid solution during nuclear fuel reprocessing? We obtained information on the chemical species of aqueous lanthanum ion in solution systems by 139La nuclear magnetic resonance (139La NMR). NMR is a magnetic resonance phenomenon of the central nucleus of an atom, as it is, and it reflects the electronic symmetry surrounding the nucleus. One of the experimental results, Fig. 3-7, shows the relation between the temperature factor and the reciprocal of longitudinal relaxation time (T1, the recovering period to the initial state from the radiowave-induced resonance state). The relaxation time is determined by the electronic structure surrounding the nucleus as mentioned above, and the mobility of the ion in the solution system. As the ion mobility is changed with temperature, the slopes of the straight lines in Fig. 3-7 indicate the symmetry of the electrons surrounding the nucleus. Relaxation occurs slower in a state of higher symmetry of electrons surrounding the nucleus and in such a more symmetric structure, the slope is lower. Conversely high slope indicates lower symmetry. From Fig. 3-7, it can be seen that the slope of the straight line becomes steeper with increasing concentration of nitrate solution.
Considering the activation energies of rotation in the nitrate and chloride ion solution system (obtained by the NMR method), together with the results in Fig. 3-7, it is shown that, as in Fig. 3-8 La3+ forms LaNO32+ in 0.05-3 M nitrate solution, while, in the chloride solution, La3+ is directly surrounded by no chloride ions but by water molecules. Furthermore, over 0.6 mol/l chloride solution, the hydrated La ion becomes isolated from the three-dimensional water network by the effect of the chloride ions breaking the network. The NMR method allows deeper understanding of the solution systems and is a way to understand the process chemistry and is useful for the safe and high performance of reprocessing as well.

Reference
T. Yaita et al., 139La NMR Relaxation and Chemical Shift Studies in the Aqueous Nitrate and Chloride Solution, J. Phys. Chem. B., 102 (20), 3886 (1998).

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