Ion-Specific Effects on PuO₂ Nanoparticle Aggregation and Dissolution in Concentrated Electrolytes

Hydrolytic PuO₂ nanoparticles (NPs) are a dominant aqueous Pu-bearing phase in high ionic strength nuclear wastes, yet their reactivity in nonideal brines remains poorly constrained. We quantify how electrolyte identity and concentration control PuO₂ NP aggregation and ligand-assisted dissolution in acidic, high salinity solutions (NaCl, NaNO₃, NaClO₄, Na₂SO₄, Na₂C₂O₄ up to 5 M). A multitechnique workflow combining liquid scintillation counting (operationally defined aqueous [Pu]), scattering/electrokinetic measurements (aggregate size and zeta potential), and spectroscopy (UV-vis, XPS, Raman) resolves electrolyte-dependent partitioning between colloidal and molecular Pu species. Weakly coordinating anions (ClO₄^-, Cl^-, NO₃^-) largely preserve the (aggregated) nanoparticulate fraction but generate distinct dissolved species at high concentration, i.e., Pu(IV)-nitrato complexes in NaNO₃ and Pu(VI)-chloro complexes in NaCl. In contrast, stronger ligands substantially perturb PuO₂ NP stability: sulfate promotes partial dissolution to Pu(IV)-sulfate complexes at low concentration but reduces aqueous [Pu] at higher sulfate levels via secondary Pu(IV) sulfate formation, whereas oxalate drives strong dissolution to aqueous Pu-oxalate complexes. Aged NPs show similar trends with reduced aqueous fractions and more dominant aggregation mechanisms. These spectroscopically constrained speciation data provide a foundation for incorporating PuO₂ NP reactivity into thermodynamic and reactive transport models for high salinity waste and brine environments.