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

Abstract

The biological reduction of soluble U(VI) complexes to form immobile U(IV) species has been proposed to remediate contaminated sites. It is well established that multiheme c-type cytochromes (MHCs) are key mediators of electron transfer to aqueous phase U(VI) complexes for bacteria such as Shewanella oneidensis MR-1. Recent studies have confirmed that the reduction proceeds via a first electron transfer forming pentavalent U(V) species that readily disproportionate. However, in the presence of the stabilizing aminocarboxylate ligand, dpaea 2− (dpaeaH 2 �bis(pyridyl-6-methyl-2-carboxylate)-ethylamine), biologically produced U(V) persisted in aqueous solution at pH 7. We aim to pinpoint the role of MHC in the reduction of U(V)-dpaea and to establish the mechanism of solidphase U(VI)-dpaea reduction. To that end, we investigated U-dpaea reduction by two deletion mutants of S. oneidensis MR-1−one lacking outer membrane MHCs and the other lacking all outer membrane MHCs and a transmembrane MHC−and by the purified outer membrane MHC, MtrC. Our results suggest that solid-phase U(VI)-dpaea is reduced primarily by outer membrane MHCs. Additionally, MtrC can directly transfer electrons to U(V)-dpaea to form U(IV) species but is not strictly necessary, underscoring the primary involvement of outer membrane MHCs in the reduction of this pentavalent U species but not excluding that of periplasmic MHCs.

The isotopic signature of UV during bacterial reduction

“…During microbiological U VI reduction, two distinct mechanisms for the complete reduction to U IV can occur: either via disproportionation of two uranyl V atoms (generating U VI and U IV ) (Vettese et al, 2020), or via a second biologically mediated electron transfer to U V (Molinas et al, 2021(Molinas et al, , 2023. However, due to the challenges associated with the chemical stabilisation and separation of U V , there is a lack of experimental evidence for its isotopic fractionation, and thus its role in the fractionation mechanism remains unresolved.…”

Chelating Effect of Siderophore Desferrioxamine-B on Uranyl Biomineralization Mediated by Shewanella putrefaciens

“…The development of nuclear energy is important for the sustainable energy industry. The uranium ore processing, nuclear energy generation, and spent-fuel treatment leads to uranium contamination in underground water and soil environment. , Therefore, the effective disposal of radioactive uranium-containing wastewater and the study of uranium cycling in biogeochemistry are mandatory in environmental science. , Species of two predominant oxidation states, hexavalent U­(VI) in uranyl (UO 2 2+ ) and tetravalent U­(IV) or U 4+ , of environmental uranium species have been studied extensively. − Noncrystalline U­(IV) species have been detected to be markedly more labile than solvable species of uranyl because they tend to be promptly oxidized and mobilized by O 2 , persulfate and bicarbonate. , Uranyl species with high aqueous solubility and mobility have huge effects on the hydrosphere and geosphere. − Under anaerobic conditions, uranyl species are rapidly reduced by organophosphates to crystalline U­(IV)-phosphate minerals, which are more difficult to oxidatively reactivate than products of microbial uranyl reduction . Biological treatments are widely used for the immobilization of uranyl in aqueous solution. , Especially, biomineralization is a process in which organisms coordinated and controlled by living organisms generate inorganic minerals, with virtue of low cost and high stability of the products.…”

Unusual redox stability of pentavalent uranium with hetero-bifunctional phosphonocarboxylate: insight into aqueous speciation

“…(3) to interpret the U(V) biological reduction, [41][42][43] (4) in photocatalysis applications and molecular magnetism due to its 5f 1 electronic configuration, 10,11 and (5) to provide insight into the control of the neptunium redox chemistry. 43,44 In the pentavalent state, U can form solid phases based on the reduction and precipitation mechanisms, which lead to the precipitation of a mixed U(V)/U(VI) phase under acidic conditions, thus affecting the mobility of U in the environment.…”

“…34–37 Its aqueous chemistry is very relevant for the speciation of U in the environment and for the development of subsequent remediation strategies. 8,34–37 The chemistry of pentavalent uranium and its role as an intermediate species in the U redox conversion is an important aspect (1) for the removal of uranium from the aqueous solution when U interacts with minerals and microbes, 38–40 (2) to decipher the isotopic signature of U 36 and (3) to interpret the U( v ) biological reduction, 41–43 (4) in photocatalysis applications and molecular magnetism due to its 5f 1 electronic configuration, 10,11 and (5) to provide insight into the control of the neptunium redox chemistry. 43,44 In the pentavalent state, U can form solid phases based on the reduction and precipitation mechanisms, which lead to the precipitation of a mixed U( v )/U( vi ) phase under acidic conditions, thus affecting the mobility of U in the environment.…”