Neptunium(V) and Uranium(VI) Reactions at the Magnetite (111) Surface

Abstract

Neptunium and uranium are important radionuclides in many aspects of the nuclear fuel cycle and are often present in radioactive wastes which require long term management. Understanding the environmental behaviour and mobility of these actinides is essential in underpinning remediation strategies and safety assessments for wastes containing these radionuclides. By combining state-of-the-art X-ray techniques (synchrotron-based Grazing Incidence XAS, and XPS) with wet chemistry techniques (ICP-MS, liquid scintillation counting and UV-Vis spectroscopy), we determined that contrary to uranium(VI), neptunium(V) interaction with magnetite is not significantly affected by the presence of bicarbonate. Uranium interactions with a magnetite surface resulted in XAS and XPS signals dominated by surface complexes of U(VI), while neptunium on the surface of magnetite was dominated by Np(IV) species. UV-Vis spectroscopy on the aqueous Np(V) species before and after interaction with magnetite showed different speciation due to the presence of carbonate. Interestingly, in the presence of bicarbonate after equilibration with magnetite, an unknown aqueous NpO2+ species was detected using UV-Vis spectroscopy, which we postulate is a ternary complex of Np(V) with carbonate and (likely) an iron species. Regardless, the Np speciation in the aqueous phase (Np(V)) and on the magnetite (111) surfaces (Np(IV)) indicate that with and without bicarbonate the interaction of Np(V) with magnetite proceeds via a surface mediated reduction mechanism. Overall, the results presented highlight the differences between uranium and neptunium interaction with magnetite, and reaffirm the potential importance of bicarbonate present in the aqueous phase.

A Tetrahedral Neptunium(V) Complex

“…This result opens the opportunity to correlate changes in bonding across the mid-actinides (U-Pu) with changes in the thermochemistry of a fundamental reaction. PCET reactions have been invoked to explain the chemical, electrochemical, and biochemical reduction of hexavalent uranyl and penta-and hexavalent neptunyl, plutonyl, and americyl [35][36][37][38][39][40][41][42][43][44][45][46] , and in the reactions of uranium nitride 47,48 or imido complexes 49 . In these chemical transformations of actinyl moieties, the fundamental PCET reaction step is masked by subsequent reactions including disproportionation, oligomerization, and hydrolysis.…”

In Situ EXAFS Study of Sr Adsorption on TiO2(110) under High Ionic Strength Wastewater Conditions

“…The F-test and dimensional Hamilton test was performed to justify each path included in the fit [33]. Finally, shells were chosen that gave the best fit, e.g., split oxygen shell, and that were chemically reasonable based on the chemistry of each system (see Supplementary Material SS1) [28][29][30][34][35][36][37][38][39].…”

Proton-Coupled Electron Transfer at the Pu5+/4+ Couple

“…The recent renewed interest in plutonium chemistry has greatly expanded the number of well-characterized complexes. ,,− However, comparative reactivity studies across the mid-actinides are rare. ,,− Recently, we reported the redox properties of a valence series of four-coordinate neptunium complexes supported by the NPC ligand (NPC = [NP t Bu(pyrr) 2 ] − ; t Bu = C(CH 3 ) 3 ; pyrr = pyrrolidinyl). , These studies established the first Np 5+/4+ couple in a complex without a metal–ligand multiple bond (MLMB), the isolation of a 4-coordinate Np 5+ complex, and the characterization of the proton-coupled electron transfer (PCET) reactivity of this Np 5+ complex in tetrahydrofuran (THF) . PCET chemistry has been invoked as a critical mechanism in the redox chemistry of the actinyls, ,,− but the thermodynamic and kinetic parameters of this fundamental transformation have not been measured. The lack of mechanistic analysis of these reactions is due in part to chemical issues, as the fundamental step in aqueous transformations of actinyl complexes is often masked by subsequent transformations including disproportionation, hydrolysis, and oligomerization.…”