Microfluidic devices provide ideal environments to study solvent extraction. When droplets form and generate plug flow down the microfluidic channel, the device acts as a microreactor in which the kinetics of chemical reactions and interfacial transfer can be examined. Here, we present a methodology that combines chemometric analysis with online micro-Raman spectroscopy to monitor biphasic extractions within a microfluidic device. Among the many benefits of microreactors is the ability to maintain small sample volumes, which is especially important when studying solvent extraction in harsh environments, such as in separations related to the nuclear fuel cycle. In solvent extraction, the efficiency of the process depends on complex formation and rates of transfer in biphasic systems. Thus, it is important to understand the kinetic parameters in an extraction system to maintain a high efficiency and effectivity of the process. This monitoring provided concentration measurements in both organic and aqueous plugs as they were pumped through the microfluidic channel. The biphasic system studied was comprised of HNO as the aqueous phase and 30% (v/v) tributyl phosphate in n-dodecane comprised the organic phase, which simulated the plutonium uranium reduction extraction (PUREX) process. Using pre-equilibrated solutions (post extraction), the validity of the technique and methodology is illustrated. Following this validation, solutions that were not equilibrated were examined and the kinetics of interfacial mass transfer within the biphasic system were established. Kinetic results of extraction were compared to kinetics already determined on a macro scale to prove the efficacy of the technique.

On-line monitoring based on optical spectroscopy provides unprecedented insight into the chemical composition of process streams or batches. Amplifying this approach through utilizing multiple forms of optical spectroscopy in sensor fusion can greatly expand the number and type of chemical species that can be identified and quantified. This is demonstrated herein, on the analysis of used nuclear fuel recycling streams: highly complex processes with multiple target and interfering analytes. The optical techniques of visible absorbance, near-infrared absorbance, and Raman spectroscopy were combined to quantify plutonium(III, IV, VI), uranium(IV, VI), neptunium(IV, V, VI), and nitric acid. Chemometric modeling was used to quantify analytes in process streams in real time, and results were successfully used to enable immediate process control and generation of a product stream at a set composition ratio. This represents a significant step forward in the ability to monitor and control complex chemical processes occurring in harsh chemical environments.

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