Sensor development for the nuclear fuel cycle: electrochemistry, spectroelectrochemistry, spectroscopy, and chemometric analysis
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Fast, robust, and cost-effective means of detecting various species in complex solution environments are needed throughout the nuclear fuel cycle. Several techniques have the potential to meet this need and this manuscript will cover two spectroscopic based methods for accomplishing this. Spectroelectrochemistry will be the first method discussed, and is a technique that can specifically quantify lanthanides and transition metals by simultaneously monitoring at least two physio-chemical properties. Application of this technique can be limited by both redox chemistry and spectral characteristics of analytes of interest; which is particularly apparent in species like the lanthanides and some free transition metals which have very weak spectral signatures. It is possible to circumvent these limitations and successfully apply spectroelectrochemistry to the analysis of these hard-to-detect species by capturing them in complexes with improved spectral characteristics. This is demonstrated with europium and ruthenium; these elements were chosen due to their spectroscopic and electrochemical characteristics as well as their relevance within the fuel cycle and industrial fields. The electrochemical and the spectroelectrochemical characteristics of Eu(bpy)2 type complexes will be discussed. As will the in situ electrochemical generation of Ru(bpy)3 complexes and their subsequent spectroelectrochemical sensing within a singular spectroelectrochemical sensor device. The second method discussed will be Raman spectroscopy utilized in tandem with chemometric analysis. A novel micro-Raman probe was developed and tested to monitor streams within microfluidic cells, allowing for characterization of small sample sizes either in-line or through grab samples. This system was tested on simple and complex systems containing HNO3, NaNO3, and/or UO2(NO3)2. Chemometric modeling has been paired with this to build predictive models capable of identifying and quantifying these species based on Raman signatures. Initial testing on larger cell path lengths was successful and translates well to preliminary studies with a 250 µm path length microfluidic device. Overall, these two methods have been used to successfully characterize and quantify examples of transition metals (Ru), lanthanides (Eu), and actinides (U) and have significant potential to be applied to other species of interest.