Plutonium(IV) and (V) Sorption to Goethite at Sub-Femtomolar to Micromolar Concentrations: Redox Transformations and Surface Precipitation

Abstract

Pu(IV) and Pu(V) sorption to goethite was investigated over a concentration range of 10(-15)-10(-5) M at pH 8. Experiments with initial Pu concentrations of 10(-15) - 10(-8) M produced linear Pu sorption isotherms, demonstrating that Pu sorption to goethite is not concentration-dependent across this concentration range. Equivalent Pu(IV) and Pu(V) sorption Kd values obtained at 1 and 2-week sampling time points indicated that Pu(V) is rapidly reduced to Pu(IV) on the goethite surface. Further, it suggested that Pu surface redox transformations are sufficiently rapid to achieve an equilibrium state within 1 week, regardless of the initial Pu oxidation state. At initial concentrations >10(-8) M, both Pu oxidation states exhibited deviations from linear sorption behavior and less Pu was adsorbed than at lower concentrations. NanoSIMS and HRTEM analysis of samples with initial Pu concentrations of 10(-8) - 10(-6) M indicated that Pu surface and/or bulk precipitation was likely responsible for this deviation. In 10(-6) M Pu(IV) and Pu(V) samples, HRTEM analysis showed the formation of a body centered cubic (bcc) Pu4O7 structure on the goethite surface, confirming that reduction of Pu(V) had occurred on the mineral surface and that epitaxial distortion previously observed for Pu(IV) sorption occurs with Pu(V) as well.

Recent advances in the detection of specific natural organic compounds as carriers for radionuclides in soil and water environments, with examples of radioiodine and plutonium

“…Unfortunately, for analytical convenience, most laboratory Pu-NOM interaction experiments are often performed at Pu concentrations that greatly exceed those observed in contaminated environments, and the results are then simply extrapolated to ambient environmental conditions. However, this assumption may be unrealistic due to the availability of limited surface-mediated oxidation-reduction reactions sites and strong Pubinding sites, and Pu (IV) tends to form polymeric or colloidal Pu-oxide particles (i.e., intrinsic colloids) at higher concentrations (Zhao et al, 2016) The strategy to resolve this issue is to use a suites of techniques including solvent extraction, ultrafiltration, radiolabelling and isoelectric focusing (IEF) electrophoresis, followed by alpha-counting or ICP-MS determination to obtain natural organic macromolecules that are complexing and immobilizing/remobilizing Pu (Figs. 4a and 4b, also see Xu et al, 2008Xu et al, , 2015bXu et al, , 2016.…”

Ultra-selective ligand-driven separation of strategic actinides

“…Since the 1940’s, worldwide Pu inventory has evolved from almost 0 to ~2,500,000 kg due to anthropogenic activities 33,34 , and is estimated to increase by 70,000 kg/year 35 based on civilian nuclear power generation forecasts. Pu materials must be properly safeguarded throughout their lifespan, which necessitates advanced nuclear forensic controls and reprocessing activities.…”

Speciation of Uranium and Plutonium From Nuclear Legacy Sites to the Environment: A Mini Review

“…The plutonium sorption is complicated by redox reactions; hence, this topic remains open for discussion and study (Romanchuk et al, 2016b ). Recently, it was found that Pu(IV) stabilization onto mineral surfaces may result in Pu surface precipitation in the form of PuO 2 nanoparticles (Kirsch et al, 2011 ; Romanchuk et al, 2013 , 2016a ; Schmidt et al, 2013 ; Zhao et al, 2016 ) ( Figure 2 ). Therefore, the study of PuO 2 nanoparticles becomes increasingly important, and it is actively being conducted (Powell et al, 2011 ; Romanchuk et al, 2018 ; Kvashnina et al, 2019 ; Gerber et al, 2020 ).…”