Development of Online pH Monitoring for Lactic, Malonic, Citric, and Oxalic Acids Based on Raman Spectroscopy Using Hierarchical Chemometric Modeling.

Online spectroscopic measurements can be used to provide unique insight into complex chemical systems, enabling new understanding and optimization of chemical processes. A key example of this is discussed here with the monitoring of pH of various acid systems in real-time. In this work the acids used in multiple chemical separations processes, such as TALSPEAK (Trivalent Actinide-Lanthanide Separation by Phosphorus reagent Extraction from Aqueous Komplexes) and oxalate precipitation, were characterized. Raman spectroscopy, a robust optical approach that can be integrated in corrosive processes, was used to follow the unique fingerprints of the various protonated and deprotonated acid species. This data was analyzed using a hierarchical modeling approach to build a consolidated model scheme using optical fingerprints from all weak acids to measure pH associated with any of the weak acid systems studied here. Validation of system performance included utilizing Raman spectroscopy under dynamic flow conditions to monitor solution pH under changing process conditions in-line. Overall, the Raman based approach provided accurate analysis of weak acid solution pH.

Measurement of local magnetic fields in actinide tetrafluorides.

Fluorine-19 magnetic shielding tensors have been measured in a series of actinide tetrafluorides (AnF4) by solid state nuclear magnetic resonance spectroscopy. Tetravalent actinide centers with 0-8 valence electrons can form tetrafluorides with the same monoclinic structure type, making these compounds an attractive choice for a systematic study of the variation in the electronic structure across the 5f row of the Periodic Table. Pronounced deviations from predictions based on localized valence electron models have been detected by these experiments, which suggests that this approach may be used as a quantitative probe of electronic correlations.

Mechanisms of Plutonium Redox Reactions in Nitric Acid Solutions.

Plutonium (Pu) exhibits a complex redox behavior in aqueous solutions. This is due to the ability of the element to adapt a wide range of oxidation states typically from +3 to +6 and the tendency for dynamic interconversion between the oxidation states that primarily depend upon acid concentration and presence of coordinating ligands. This work interrogates the Pu redox behavior in aqueous nitric acid via a combination of voltammetry and in situ vis-NIR spectroelectrochemistry under controlled potentials to map the interconversion between the various Pu oxidation states. The NIR-spectroelectrochemistry studies used to complement the visible spectroscopy bring a new and more complete perspective into the plutonium redox transformations. This allows elucidation of the mechanisms of the involved redox reactions facilitating an in-depth understanding of the relative stability of the Pu oxidation states as a function of redox potentials and nitric acid concentrations. It is observed that oxidation of Pu(III) results in generation of Pu(IV) and Pu(VI) (the latter as PuO22+), bypassing the Pu(V) oxidation state. Further, with increasing acid concentrations, the formation of the Pu(VI) species progressively decreases so that the dynamic equilibrium between the Pu(III) and Pu(IV) oxidation states dominates. These findings have significant implications for developing separation processes for used nuclear fuel reprocessing and treatment.

Method for the in situ Measurement of pH and Alteration Extent for Aluminoborosilicate Glasses Using Raman Spectroscopy.

Characterization of long-term processes occurring during alteration of aluminoborosilicate glasses is relevant for natural as well as man-made materials. Static dissolution tests are a common setup for such studies, but the obtained results and related errors are impacted by the frequency and protocol of samplings performed to determine release via solution analysis, e.g., ICP-OES. A noninvasive method was developed to continuously monitor glass alteration based on in situ Raman spectrometry of the solution contained in the alteration vessel. The alteration of a benchmark glass, the environment assessment (EA) glass, for 7 days at 90 °C showed that the pH and boron concentration results obtained from solution monitoring and ICP-OES quantification were similar to the pH and boron results obtained from chemometric modeling of the Raman spectra and within error of previously published results in similar conditions. The errors on altered amounts of glass based on B release were similar for both in situ Raman and ICP-OES. The new Raman method provides a more detailed picture of real time monitoring of an alteration experiment, with intervals between monitoring times as short as dozens of seconds. The in situ Raman method also helps to reduce perturbation to experiments caused by the physical sampling of aliquots (including temperature excursions, re-equilibration with atmosphere, volume variation, and potential chemical contamination) by limiting their number and frequency.

Micro-Raman Technology to Interrogate Two-Phase Extraction on a Microfluidic Device.

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 HNO3 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.

Development and testing of a novel micro-Raman probe and application of calibration method for the quantitative analysis of microfluidic nitric acid streams.

To simplify and improve the safety of reprocessing used nuclear fuel, an initial assessment was made of Raman microscopy applied to microfluidic volumes with a view toward the on-line spectroscopic measurement of highly radioactive solutions. This study compares a microscopic Raman probe (excitation focal point diameter 70 µm) to a larger, well studied probe (excitation focal point diameter 125 µm) used in prior investigations. This was done by chemometrically modeling and predicting concentrations of HNO3 solutions (0 M to 8 M) as they flowed through microfluidic cells based upon spectra from each probe. Spectra recorded for each probe using the same static HNO3 solution set contained in 2 dram glass vials were used as training sets to produce models for the respective probes. Modeling required baseline, normalization and smoothing preprocessing to compensate for a reduced path length between the static glass vial training set (4 cm) and the reduced path length flow cell (1 cm), wide ranging solution concentrations, and the associated non-linear spectral changes, and abrupt and uneven concentration changes of flowing solutions. The micro-Raman probe is able to produce spectra that may be analyzed chemometrically to accurately predict the concentration of flowing HNO3 solutions down to microliter volumes. Based upon RMSECV and RMSEP modeling statistics concentration predictions of the micro-Raman probe are comparable to those obtained for a macro-Raman on identical samples.

Multivariate Analysis To Quantify Species in the Presence of Direct Interferents: Micro-Raman Analysis of HNO3 in Microfluidic Devices.

Microfluidic devices are a growing field with significant potential for applications to small scale processing of solutions. Much like large scale processing, fast, reliable, and cost-effective means of monitoring streams during processing are needed. Here we apply a novel micro-Raman probe to the online monitoring of streams within a microfluidic device. For either macro- or microscale process monitoring via spectroscopic response, interfering or confounded bands can obfuscate results. By utilizing chemometric analysis, a form of multivariate analysis, species can be accurately quantified in solution despite the presence of overlapping or confounding spectroscopic bands. This is demonstrated on solutions of HNO3 and NaNO3 within microflow and microfluidic devices.

Incorporating spectroscopic on-line monitoring as a method of detection for a Lewis cell setup.

A Lewis cell was designed and constructed for investigating solvent extraction systems by spectrophotometrically monitoring both the organic and aqueous phases in real time. This new Lewis cell was tested and shown to perform well compared to other previously reported Lewis cell designs. The advantage of the new design is that the spectroscopic measurement allows determination of not only metal ion concentrations, but also information regarding chemical speciation - information not available with previous Lewis cell designs. For convenience, the new Lewis cell design was dubbed COSMOFLEX (COntinuous Spectroscopic MOnitoring of Forrest's Liquid-liquid EXtraction cell). After construction performance testing was done for establishing the ideal stir speed range, UV-Vis measured concentration and D value determination. Each one of these tests was satisfactorily passed.

Development of Online Spectroscopic pH Monitoring for Nuclear Fuel Reprocessing Plants: Weak Acid Schemes.

In nuclear fuel reprocessing, separating trivalent minor actinides and lanthanide fission products is extremely challenging and often necessitates tight pH control in TALSPEAK (Trivalent Actinide-Lanthanide Separation by Phosphorus reagent Extraction from Aqueous Komplexes) separations. In TALSPEAK and similar advanced processes, aqueous pH is one of the most important factors governing the partitioning of lanthanides and actinides between an aqueous phase containing a polyaminopolycarboxylate complexing agent and a weak carboxylic acid buffer and an organic phase containing an acidic organophosphorus extractant. Real-time pH monitoring would significantly increase confidence in the separation performance. Our research is focused on developing a general method for online determination of the pH of aqueous solutions through chemometric analysis of Raman spectra. Spectroscopic process-monitoring capabilities, incorporated in a counter-current centrifugal contactor bank, provide a pathway for online, real-time measurement of solution pH. The spectroscopic techniques are process-friendly and can be easily configured for online applications, whereas classic potentiometric pH measurements require frequent calibration/maintenance and have poor long-term stability in aggressive chemical and radiation environments. Raman spectroscopy discriminates between the protonated and deprotonated forms of the carboxylic acid buffer, and the chemometric processing of the Raman spectral data with PLS (partial least-squares) regression provides a means to quantify their respective abundances and therefore determine the solution pH. Interpretive quantitative models have been developed and validated under a range of chemical composition and pH conditions using a lactic acid/lactate buffer system. The developed model was applied to new spectra obtained from online spectral measurements during a solvent extraction experiment using a counter-current centrifugal contactor bank. The model predicted the pH of this validation data set within 11% for pH > 2, thus demonstrating that this technique could provide the capability of monitoring pH online in applications such as nuclear fuel reprocessing.

Water O-H stretching Raman signature for strong acid monitoring via multivariate analysis.

A distinct need exists for real time information on an acid concentration of industrial aqueous streams. Acid strength affects efficiency and selectivity of many separation processes, including nuclear fuel reprocessing. Despite the seeming simplicity of the problem, no practical solution has been offered yet, particularly for the large-scale schemes involving toxic streams such as highly radioactive nuclear wastes. The classic potentiometric technique is not amiable for online measurements due to the requirements of frequent calibration/maintenance and poor long-term stability in aggressive chemical and radiation environments. Therefore, an alternative analytical method is needed. In this work, the potential of using Raman spectroscopic measurements for online monitoring of strong acid concentration in solutions relevant to dissolved used nuclear fuel was investigated. The Raman water signature was monitored for solution systems containing nitric and hydrochloric acids and their sodium salts of systematically varied composition, ionic strength, and temperature. The trivalent neodymium ion simulated the presence of multivalent f metals. The gaussian deconvolution analysis was used to interpret observed effects of the solution nature on the Raman water O-H stretching spectrum. The generated Raman spectroscopic database was used to develop predictive multivariate regression models for the quantification of the acid and other solution components, as well as selected physicochemical properties. This method was validated using independent experiments conducted in a flow solvent extraction system.