1. ELECTROCHEMICAL DETERMINATION OF URANIUM IN SALINE SOLUTIONS Matthew Kirby, 1 Pascal Salaun, 2 Jonathan Watson, 1 and Dominik Weiss. 1 1 Department of Earth Science and Engineering, Imperial College London. (mek15@ic.ac.uk) 2 School of Environmental Sciences, University of Liverpool Abstract In the United Kingdom, uranium (U) is a major component of Intermediate Level Waste. A key problem is the leaching of the cement backfill in a geological disposal facility, leading to the transport of uranium in a high pH and ionic strength plume. A rapid, cost effective, analytical method that can measure uranium concentrations at the ug /ml in solutions with high ionic strengths is needed to analyse samples collected from uranium mobility studies. To this end we optimised an adsorptive stripping voltammetric method. The lower limit of detection is 0.464ng/ml (3σ), and has a linear range between 0.464-30ng/ml. The typical precision is 7%, an accuracy within 10%, reproducibility is 7%, and sensitivity is 0.92nA/ppb. It has been successfully used to analyse the breakthrough of 10ug U/ml in columns packed with quartz. Motivation In the United Kingdom, Intermediate Level Waste (ILW) will be grouted in steel canisters, placed in a repository at approximately 600m depth, and backfilled with cement Over time, the canisters will fail and cement will degrade, leading to uranium transport in a high pH and ionic strength plume A key challenge is to analyse low concentrations of uranium (ng/ml) in high ionic strength solutions Traditionally methods such as ICPMS are used, however high ionic strengths drop the sensitivity and stability of the system, therefore many dilutions are needed. Therefore we need an analytical method that allows us to rapidly analyse low uranium concentrations (ug/ml) in high ionic strength solutions which can be applied to our uranium mobility studies. FIGURE 1 Application to Column Experiment A breakthrough curve for a 10ug U/ml, pH 4.32 and 0.1M potassium nitrate solution passing through a column packed with quartz has been developed with voltammetry and ICPMS. The results are shown in figure 2. As the pH in the cell needs to be pH 2.45, initial pH of experiment is irrelevant The results from the voltammetric method and ICPMS are in close agreement This means that we have accurate results with the voltammetric method The breakthrough occurs rapidly, with 80% breakthrough after 1.75 pore volumes. This agrees with the literature [3] and is due to there being little sorption of uranium on the quartz surface (UO 2 2+ species, and very few negative surface sites on quartz) Figure 1a shows a typical voltammogram generated using the optimised method. Key input parameters are pH 2.45, -0.04V deposition potential, 15s deposition time, and 25Hz frequency. A 0.1M KNO 3 solution was spiked with 5ng U/ml. Two standard additions of 5ng U/ml were then measured. Figure 1b shows the peak height versus added uranium concentration, extrapolated to determine the uranium concentration in the sample. Uranium concentration is 4.73 +/- 0.24ng/ml. FIGURE 2 TABLE 1 shows the quality measures for the optimised method ACKNOWLEDGEMENTS Thank you to the NERC RATE project for funding this research REFERENCES [1] Sander, S. and Henze, G.1994. Fresenius’ Journal of Analytical Chemistry 349 654-658pp [2] Metrohm. 2007. Application: Voltammetric Trace Determination 9-12pp [3] Cheng, T., Barnett, M.O., Roden, E.E., and Zhunag, J.L. 2007. Chemosphere. 68. 1218-1223 Aims and Objectives The aim of this research is to test whether an electrochemical method can be used to analyse uranium concentrations leaving a column packed with quartz sand We will do this by optimising an analytical method to directly and rapidly determine uranium concentrations in saline solutions We will then apply this method to samples collected from the quartz columns and compare them to results obtained from ICPMS Electrochemical Method An adsorptive stripping voltammetric method [1][2] has been optimised. A HMDE is used as the working electrode, a Ag/AgCl as the reference electrode, and a glassy carbon electrode as the auxiliary electrode 0.1mM chloranilic acid is used as a complexing agent, and 0.1M potassium nitrate as a background electrolyte The method was optimised in terms of time per measurement, peak shape, and sensitivity The key parameters are pH, deposition potential, deposition time, and frequency in square wave pulse mode Quality MeasuresResult Linear Detection Range0.464-30ng/ml Precisionca. 7% Sensitivity0.92nA/ppb Accuracywithin 10% Reproducibility7% b Conclusion The aim of this research was to test whether an electrochemical method could be used to analyse low uranium concentrations in high ionic strength solutions, in uranium mobility studies The method has been optimised in terms of time taken, peak shape and sensitivity This method has then been used to successfully analyse the breakthrough of 10ug U/ml, pH 4.32, 0.1M potassium nitrate solution passing through a column packed with quartz sand. It is cost effective compared to ICPMS, and takes a similar amount of time to prepare and analyse the samples (three days). Each measurement takes 20 minutes, but no prior sample preparation is required. It uses the standard addition technique, automatically taking into account the matrix of the solution (ionic strength), and has an internal calibration Cell solution needs a high ionic strength (0.1M KNO 3 ), so samples can be analysed directly, unlike ICPMS where dilutions are needed. One major issue is that at high sample concentrations, small volumes of sample are added (0.02ml) to a big cell (22.25ml). therefore it is very sensitive to errors in sample addition. Figure 2 shows a comparison of the breakthrough of a 10ug U/ml, pH 4.32, 0.1M potassium nitrate, solution in a column packed with quartz. The x axis is normalised in terms of pore volumes of solution passed through the column, the concentration leaving the column is normalised against initial uranium concentration of the spiked solution, and plotted on the y axis Optimisation Results pH 2.45 0.1M KNO 3 solution spiked with 5ppb U a