Brian A. Powell

The Hanford Site in Washington State in the United States generated large quantities of radioactive waste during plutonium production and separations activities conducted from the 1940s through the...

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.

Suspended solids can be the primary vector for transporting contaminants in streams. The objective of this study was to determine whether changes in the properties of suspended solids during rain events impacted contaminant transport. Stream water was collected during five episodic events downstream from a U-contaminated wetland located in South Carolina, USA. The suspended particles were initially composed of Fe-flocs (particles formed in situ prior to the rain event) that had significantly greater Fe, Mn, organic-C, and U content than particles collected later during a sampling rain event. XANES and EXAFS revealed that U in the Fe-flocs was U(VI) and that it was not incorporated in a mineral structure but existed as inner- or outer-sphere adsorbed uranyl species associated with organic matter and Fe-oxides. The uranyl had an extraordinarily high affinity for the suspended solids, with solid to liquid U ratios of >72,000 (μg/kg)/(μg/L). After the initial flush of Fe-flocs, a greater fraction of the suspended solids had lower organic-C, Fe, Mn, and amorphous phases and were composed of more quartz, kaolinite, and gibbsite, resulting in lower U concentrations than those in the solids collected earlier in the rain event. This study highlights the importance of understanding suspended solids as transport vectors and their potential dynamic nature during rain events.

Extracting radionuclides from natural and anthropogenic sources addresses two challenges: remediating contaminated environments and enabling the separation of technologically important isotopes. Understanding how actinides interact and form complexes with polymer-based ligands allows the development of efficient extractants for selective remediation, supporting sustainable environmental protection and resource recovery strategies. Here, we report on the structure and actinide sorption properties of eco-friendly hybrid nanocomposite systems based on the natural mineral halloysite and multifunctional conjugated polymers. Three polyaniline (PANI)-based polymers and their nanocomposites with halloysite nanotubes (HNT) were synthesized and evaluated as sorbents for removing uranium and plutonium from aqueous solutions. The study included two PANI variants synthesized at high (60 °C) and low (1-2 °C) temperatures using different acid-dopants, H₂SO₄ and p-toluenesulfonic acid, and a PANI copolymer incorporating aminoterephthalic acid (PANIATA), which introduces carboxylic functional groups. X-ray photoelectron spectroscopy and electron microscopy revealed the formation of core-shell nanocomposites with HNT as the core and a polymer shell several nanometers thick. The HNT/PANI nanocomposites exhibited a synergistic enhancement in actinide retention capacity, exceeding the additive performance of the individual components. Sorption experiments with artificial groundwater and seawater confirmed the nanocomposites' selectivity for uranium. Analysis of the FTIR spectra of the polymer composites before and after uranium sorption provided detailed insight into uranyl cation-polymer complexation mechanisms responsible for high selectivity and sorption performance.