Near-infrared spectra and molar absorption coefficients of trivalent lanthanides dissolved in molten LiCl-KCl eutectic

Determining the concentration of the dissolved lanthanide species in LiCl-KCl eutectic salt is important to the development of pyrochemical reprocessing of used nuclear fuel. In this process, lanthanide fission products are found dissolved in the electrorefiner electrolyte in their trivalent oxidation state. The presence of dissolved trivalent lanthanides increases the liquidus temperature of the electrolyte mixture and can lead to the formation of insoluble oxide or oxychloride phases and must therefore be continuously monitored and controlled during the operation. Absorbance spectroscopy is a promising method for continuous measurement of the concentration of lanthanides and other elements dissolved in the electrolyte. The absorption of light by elements is linearly proportional to the concentration of the element for relatively dilute solutions according to the Beer-Lambert law. Although measurement of the absorption of ultraviolet and visible range light by lanthanides in LiCl-KCl eutectic molten salt have been explored previously, near infrared (NIR) absorption spectroscopy has received far less attention. It may, however, provide a better analytical signal when insoluble phases are present due to less Raleigh scattering compared to shorter wavelength radiation. Additionally, it may allow for concentration determination for certain elements using NIR ab-sorption features where UV and visible range features are overlapping with features from other species. In this study, we report the UV-Vis-NIR spectra of the trivalent lanthanide chlorides of neodymium, samarium, and dysprosium in LiCl-KCl eutectic. Molar absorption coefficients are reported for analytically useful absorption maxima, with a focus on the molar absorption coefficients for NIR absorption maxima which have not been reported previously. Additionally, we observe a NIR-range absorption band of Nd3+ which was previously predicted but never experimentally observed. We compare the calculated crystal field levels to the newly observed absorbance band and find them to be in good agreement with previous predictions.

Automated summary

This study investigates the UV-Vis-NIR spectra of lanthanide chlorides in LiCl-KCl eutectic salt and reports the molar absorption coefficients for analytically useful absorption maxima. The results show that NIR absorption spectroscopy may provide a better analytical signal when insoluble phases are present and can be used for concentration determination of certain elements.