Available literature related to hexavalent plutonium in the presence of oxalate (C2O42–) is severely limited, primarily due to the reduction of Pu(VI) to lower oxidation states and the low solubility of the resulting Pu-oxalate complexes. The low solubility of tri- and tetravalent plutonium oxalates has been utilized as a means to purify plutonium on an industrial scale for decades. However, the presence of the slightly more soluble hexavalent plutonium oxalate should not be ruled out due to the possibility of incomplete reduction of Pu(VI) in Pu-oxalate systems. A spectrophotometric study was performed to investigate the aqueous behavior of Pu(VI) in the presence of oxalate as a function of pH (1, 3, &gt;6), metal-to-ligand (M:L) ratio (1:10 – 10:1), and time. Solutions containing oxalate were pH adjusted and then were spiked with a Pu(VI) stock solution to reach the desired M:L ratio. The reaction mixture was subsequently analyzed by ultraviolet-visible near-infrared (UV-vis-NIR) spectroscopy and compared to Pu(VI) in the absence of oxalate at the same pH. Solutions were sampled as a function of time to monitor the reduction of plutonium. Preliminary thermodynamic calculations were performed to determine formation constants for possible Pu(VI)-oxalate complexes. Two wavelength ranges (820-870 and 900-1050 nm) in the NIR spectrum were observed to contain absorption bands indicative of Pu(VI)-oxalate complexation. These absorption bands, identified in systems at pH 1, 3, and &gt;6, were sensitive to changes in pH and M:L ratio, resulting in isosbestic points that indicate the presence of at least two Pu(VI)-oxalate complexes in solution. UV-vis-NIR data coupled with thermodynamic calculations have provided more insight into the behavior of Pu(VI) in the presence of oxalate.

Spectrophotometric study of Pu(VI)-oxalate speciation and the impact of pH

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