Pentavalent Uranium Incorporated in the Structure of Proterozoic Hematite

Abstract

Characterizing the chemical state and physical disposition of uranium that has persisted over geologic time scales is key for modeling the long-term geologic sequestration of nuclear waste, accurate uranium–lead dating, and the use of uranium isotopes as paleo redox proxies. X-ray absorption spectroscopy coupled with molecular dynamics modeling demonstrated that pentavalent uranium is incorporated in the structure of 1.6 billion year old hematite (α-Fe2O3), attesting to the robustness of Fe oxides as waste forms and revealing the reason for the great success in using hematite for petrogenic dating. The extreme antiquity of this specimen suggests that the pentavalent state of uranium, considered a transient, is stable when incorporated into hematite, a ubiquitous phase that spans the crustal continuum. Thus, it would appear overly simplistic to assume that only the tetravalent and hexavalent states are relevant when interpreting the uranium isotopic record from ancient crust and contained ore systems.

Pentavalent U Reactivity Impacts U Isotopic Fractionation during Reduction by Magnetite

“…Experimental evidence for U(V) persistence in iron oxides has been accumulating for more than 10 years . The most recent work has evidenced the presence of U(V) in a 1.6 billion years old hematite . Previous work has both reported the persistence of U(V) up to 4 days as well as the formation of transient nanowires composed of individual uranium oxide nanoparticles that later collapse into UO 2 nanoclusters .…”

“…Due to its redox sensitivity and long residence time in the ocean (∼500 ky), U is considered as a reliable paleo-redox tracer to reconstruct Earth’s past atmosphere and oceans − as well as a monitoring tool to trace U(VI) reduction in the modern environments. − While the current understanding is grounded in the isotope fractionation that occurs during the transition from U(VI) to U(IV), the existence of U(V) and its role in the fractionation processes have been largely overlooked. In light of the identification of U(V) in 1.6 billion years old hematite, it is increasingly clear that U isotope fractionation during Fe(II)-mediated reduction must consider intermediate valence states and their attendant fractionation behavior.…”

“…Therefore, they are relevant for contaminated aquifers and nuclear waste disposal. As a result, the reduction of U­(VI) by Fe­(II)-containing mineral phases has been extensively studied to pinpoint the underlying reduction mechanism(s), such as the roles of the Fe­(II)/Fe­(III) ratio at the mineral surface, , of U­(VI) loading, and of pH and aqueous chemistry. , While crystalline uraninite U­(IV) has been considered the major abiotic reduction product, ,,, there are several studies documenting the formation and persistence of pentavalent U [U­(V)] during U­(VI) reduction by forming or dissolving/reprecipitating iron-bearing minerals. ,,− In particular, the incorporation of U­(V) in iron oxide mineral phases has been reported during the coprecipitation of U­(VI) with magnetite or green rust, , the reduction of U­(VI) concomitantly with the dissolution and recrystallization of iron oxides, ,,,, or even within the structure of a Proterozoic hematite . At the magnetite surface, the presence of surface U­(V) has been observed under electrochemically controlled U­(VI)-reducing conditions. , Moreover, the nanoscale reductive mineralization mechanism has been uncovered, evidencing the formation of U oxide nanowires as an intermediate morphology, and the presence of U­(V) as a transient valence state followed by reduction to U­(IV) .…”

Mechanism of Reduction of Aqueous U(V)-dpaea and Solid-Phase U(VI)-dpaea Complexes: The Role of Multiheme c-Type Cytochromes

“…Hence, these ligands are promising candidates for the stabilization of U(V), and recent studies have suggested that dpa could stabilize U(V) in aqueous solution. 50,51 Very little is known about the distribution of U(V) in the environment, even though it was identified in the matrix of a 1.6 billion year-old hematite by Ilton et al, 52 opening the door to a quest for U(V) in the environment. Indeed, U(V) chemistry and its resulting stability is dependent on the local chemical environment: available organic carbon, presence of mineral phases, physical state of U (solid/aqueous), pH, electrochemical potential, and O 2 levels.…”

Exploring the Valence Diversity of Uranium by Flux Growth of Uranium Silicate under Inert Atmosphere

“…Similarly, soddyite, uranophane, boltwoodite, and sklodowskite are formed by the oxidation of uraninite (UO 2 ) in nature. , In the natural environment, the most stable oxidation states of uranium are U­(IV) and U­(VI), and the latter dominates in many crystalline minerals . In contrast, the fact that U­(V) species in aqueous solution is easily oxidized to uranium­(VI) or disproportionates into U­(IV) and U­(VI) species rapidly makes it rarely observed in naturally occurring minerals. , The exploration of new uranium silicate compounds under extreme conditions in the laboratory leads to the creation of many new phases of uranium silicate with different kinds of structural units. For example, Lii and co-workers have reported hydrothermal syntheses of a U­(IV) silicate, Cs 2 U IV Si 6 O 15 , two U­(V) silicates, K­(U V O)­Si 2 O 6 and K 3 (U V 3 O 6 )­(Si 2 O 7 ), and several mixed-valence uranium silicates, including [Na 9 F 2 ]­[(U V O 2 )­(U VI O 2 ) 2 (Si 2 O 7 ) 2 ], Na 7 U IV O 2 (U V O) 2 (U V/VI O 2 ) 2 SiO 6 , and Cs 4 (U V O)­(U IV/V O) 2 (Si 2 O 7 ) 2 .…”