Plutonium Speciation\nand Oxidation State Distributions\nin the Presence of Citrate

We explored the speciation and kinetics of the Pu(VI)-citrate\nand\nPu(III)-citrate systems (pH<sub>m</sub> = 2.5–11.0, <i>I</i> = 0.1 M NaCl, <i>T</i> = 23 °C, O<sub>2</sub>(g) < 2 ppm) using ultraviolet–visible–near-infrared\n(UV–vis–NIR) spectrophotometry, solvent extraction,\nand PHREEQC modeling. Formation constants were determined for PuO<sub>2</sub>(HcitH)(aq) (log <i>K</i>°<sub>1,1</sub> = 1.09 ± 0.05) and PuO<sub>2</sub>(HcitH)(citH)<sup>3–</sup> (log <i>K</i>°<sub>1,2</sub> = −0.20\n± 0.07), and evidence for (PuO<sub>2</sub>)<sub><i>m</i></sub>(citH<sub>–<i>k</i></sub>)<sub><i>n</i></sub>(OH)<sub><i>x</i></sub><sup>2<i>m</i>(3+<i>k</i>)<i>n</i>–<i>x</i></sup> was\nidentified under alkaline conditions. Pu(VI) species were found to\nbe less stable in the presence of citrate than in the absence of citrate\n(<i>t</i> ≤ 168 days); the rate of reduction increased\nwith increasing pH. The direct reduction of Pu(VI) to Pu(IV) was required\nto fit experimental data in the presence of citrate but did not improve\nthe fit for Pu in the absence of citrate. We also observed increased\nPu(III) stability in the presence of citrate (<i>t</i> ≤\n293 days), with higher concentrations of Pu(III) favored at lower\npH. Finally, we provide evidence of a radiolysis-driven mechanism\nfor the citrate-mediated reduction of plutonium that involves electron\ntransfer from the oxidative breakdown of citrate. Our work highlights\nthe need to investigate the redox effect of organic ligands on plutonium\noxidation states under repository-relevant conditions.