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

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

The +5 state is an unusual oxidation state of uranium due to its instability in the aqueous phase. As a result, gaining information about its aqueous speciation is extremely difficult. The present work is an attempt in that direction and it provides insight into the existence of a new pentavalent species in the presence of hetero-bifunctional phosphonocarboxylate (PC) chelators, other than the carbonate ion, in the aqueous medium. The aqueous chemistry of pentavalent uranium species with three environmentally relevant PCs was probed using electrochemical and DFT methods to understand the redox energy and kinetics of conversion of the U(VI)/U(V) couple, stability, structure, stoichiometry, binding modes, etc. Interestingly, pentavalent uranium complexes with PCs are quite persistent over a wide range of pH starting from acidic to alkaline conditions. The PC chelators block the cation-cation interaction (CCI) of U(V) through strong hetero-bidentate chelation and intermolecular hydrogen bonding (IMHB) interactions which stabilize the pentavalent metal ion against disproportionation. For uranyl species in the presence of PCs, acting as chelators, CV plots were obtained at varying pH values from 2 to 8. The obtained results indicate an irreversible single redox peak involving U(VI) to U(V) conversion and association of a coupled chemical reaction with the electron transfer step. ESI-MS studies were performed to understand the speciation effect on the U(VI)/U(V) redox couple with varying pH. Speciation modelling of U(V) with the PC ligands was carried out, which indicated that the U(V) is redox stable in nearly 47% of the pH region in the presence of the PCs as compared to the carboxylate-based chelators. The free energy and reduction potential of the U(V) complexes and the reduction free energy and disproportionation free energy for the U(VI)/U(V) couple were determined by DFT computations in the presence of the PCs. In situ spectroelectrochemical spectra were recorded to provide evidence for the existence of U(V) species with PCs in the aqueous medium and to acquire its absorption spectra. The present study is highly significant for understanding the coordination chemistry of pentavalent uranium species, accurate modelling of uranium, and isolation of U(V).

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

In contaminated water and soil, little is known about the role and mechanism of the biometabolic molecule siderophore desferrioxamine-B (DFO) in the biogeochemical cycle of uranium due to complicated coordination and reaction networks. Here, a joint experimental and quantum chemical investigation is carried out to probe the biomineralization of uranyl (UO22+, referred to as U(VI) hereafter) induced by Shewanella putrefaciens (abbreviated as S. putrefaciens) in the presence of DFO and Fe3+ ion. The results show that the production of mineralized solids {hydrogen-uranium mica [H2(UO2)2(PO4)2·8H2O]} via S. putrefaciens binding with UO22+ is inhibited by DFO, which can both chelate preferentially UO22+ to form a U(VI)-DFO complex in solution and seize it from U(VI)-biominerals upon solvation. However, with Fe3+ ion introduced, the strong specificity of DFO binding with Fe3+ causes re-emergence of biomineralization of UO22+ {bassetite [Fe(UO2)2(PO4)2·8(H2O)]} by S. putrefaciens, owing to competitive complexation between Fe3+ and UO22+ for DFO. As DFO possesses three hydroxamic functional groups, it forms hexadentate coordination with Fe3+ and UO22+ ions via these functional groups. The stability of the Fe3+-DFO complex is much higher than that of U(VI)-DFO, resulting in some DFO-released UO22+ to be remobilized by S. putrefaciens. Our finding not only adds to the understanding of the fate of toxic U(VI)-containing substances in the environment and biogeochemical cycles in the future but also suggests the promising potential of utilizing functionalized DFO ligands for uranium processing.

Implementation of cytochrome c proteins and carbon nanotubes hybrids in bioelectrodes towards bioelectrochemical systems applications

Multiheme cytochrome c (Cyt c) can function as a redox protein on electrode to accomplish bioelectrocatalysis. However, the direct electron transfer (DET) between the redox site of Cyt c and electrode is low due to the large coupling distance. A close proximity or a connection pathway from the deeply buried active site to the protein surface can be established by modifying the electrode with carbon nanotubes (CNTs) to improve the DET. Therefore, the isolated Cyt c has been assembled or casted with CNTs by various processes to form Cyt c-CNTs bioelectrodes that can be further applied to biosensing and bioanalysis. These strategies can be transplanted to the fabrication of biofilm-CNTs based electrodes by complexing the out membrane (OM) Cyt c of natural electricigen with CNTs to realize the application of the electrochemical properties of "in vivo" Cyt c to bioelectrochemical systems (BESs). This review intends to highlight the preparation strategies of bioelectrodes that have been well studied in electrochemical biosensors and improving approaches of the DET from the CNTs surface to Cyt c in their hybrids. The efficient fabrication processes of the biofilm-CNTs based electrodes that can be considered as "in vivo" Cyt c-CNTs based electrodes for BES designs are also summarized, aiming to provide an inspiration source and a reference to the related studies of BES downstream.

Strengthening Fe(II)/Fe(III) Dynamic Cycling by Surface Sulfation to Achieve Efficient Electrochemical Uranium Extraction at Ultralow Cell Voltage

Electrochemically mediated Fe(II)/Fe(III) redox-coupled uranium extraction can efficiently reduce the cell voltage of electrochemical uranium extraction (EUE). How to regulate the surface structure to enhance the uranium acyl ion adsorption capacity and strengthen the Fe(II)/Fe(III) redox cycle process is crucial for EUE. In this work, we developed surface sulfated nanoreduced iron (S-NRI) for EUE and exhibited improved properties for EUE at an ultralow cell voltage (-0.1 V). Compared with a nanoreduced iron (NRI) adsorbent, S-NRI displayed faster electrochemical extraction kinetics properties and higher extraction efficiency and capacity for uranium. In a more complex seawater electrolyte containing uranyl ion concentration ranging from 1 to 20 ppm, the removal efficiency could reach almost ∼100% after EUE for 24 h. At a higher 50 ppm uranium acyl ion concentration in a seawater electrolyte, S-NRI exhibited higher extraction capacity (755.03 mg/g), which is better than 528.53 mg/g of NRI at a cell voltage of -0.1 V. Outstanding EUE property could be attributed to the fact that sulfate species (M-SO42-) on the S-NRI surface not only enhanced selective adsorption of uranyl ions but also strengthened the Fe(II)/Fe(III) redox cycle, which accelerated electron transfer between Fe(II) and U(VI), promoted the regeneration of Fe(II) active sites, and finally enhanced the EUE property.