Interfacial redox processes of plutonium with iron phases and organic ligands in repository-relevant brine systems

The redox behavior and solubility of plutonium (Pu) were investigated in complex, high-ionic-strength brines containing organic ligands EDTA (ethylenediaminetetraacetic acid), oxalate, citrate, and acetate, oxalate, citrate, acetate) and iron phases (magnetite and zero-valent iron, Fe 0 ) under an inert atmosphere. Long-term batch solubility experiments were conducted using an undersaturation approach to evaluate Pu oxidation-state distribution and the solid phases controlling solubility under reducing, alkaline conditions relevant to some deep geologic repositories. A novel dual-sampling strategy was implemented to simultaneously determine Pu oxidation states in (i) Pu-bearing precipitates and (ii) Pu associated with iron phases after long-term reaction periods of up to 1300 days, providing a realistic, interface-resolved view of redox processes in saline environments. Pu L III -edge X-ray absorption near-edge structure (XANES) revealed that Pu(III) dominates in Fe 0 systems, whereas Pu(IV) predominates in magnetite systems, independent of organic ligands. The ligands caused only minor shifts in oxidation state and did not substantially alter overall solubility trends. Pu L III -edge EXAFS indicated predominantly inner-sphere Pu–Fe coordination, with only minor PuO 2 -like contributions. Bulk powder X-ray diffraction (XRD) showed no detectable bulk structural alteration or measurable bulk phase transformation of either metallic iron or magnetite over the three-year equilibration period. Plutonium solubility varied by up to two orders of magnitude (10 −5 –10 −8 M) across pC H + 7–10. Small organic-induced changes in Pu oxidation state were not accompanied by significant changes in solubility. X-ray photoelectron spectroscopy (XPS) was consistent with XANES, showing the same oxidation-state trends and confirming Pu(III) and Pu(IV) dominance in Fe 0 -and magnetite-bearing systems, respectively, under the investigated conditions. Scanning electron microscopy-energy dispersive X-ray (SEM–EDX) mapping demonstrated a clear spatial association between Pu and Fe on particle surfaces, supporting close Pu–Fe coupling within the interfacial microstructure. Overall, this long-term dataset provides direct experimental evidence for stable, metallic Fe 0 and magnetite-dependent Pu redox partitioning within the specific brine compositions, iron materials, and experimental conditions investigated and highlights the critical role of surface-mediated redox interactions in controlling Pu speciation and solubility. These findings inform long-term safety assessments of salt-based repositories and may also be relevant to other geological disposal concepts in high-salinity groundwater environments.