Uranium redox transition pathways in acetate-amended sediments

Abstract

Redox transitions of uranium [from U(VI) to U(IV)] in low-temperature sediments govern the mobility of uranium in the environment and the accumulation of uranium in ore bodies, and inform our understanding of Earth's geochemical history. The molecular-scale mechanistic pathways of these transitions determine the U(IV) products formed, thus influencing uranium isotope fractionation, reoxidation, and transport in sediments. Studies that improve our understanding of these pathways have the potential to substantially advance process understanding across a number of earth sciences disciplines. Detailed mechanistic information regarding uranium redox transitions in field sediments is largely nonexistent, owing to the difficulty of directly observing molecular-scale processes in the subsurface and the compositional/physical complexity of subsurface systems. Here, we present results from an in situ study of uranium redox transitions occurring in aquifer sediments under sulfate-reducing conditions. Based on molecular-scale spectroscopic, pore-scale geochemical, and macroscale aqueous evidence, we propose a biotic-abiotic transition pathway in which biomass-hosted mackinawite (FeS) is an electron source to reduce U(VI) to U(IV), which subsequently reacts with biomass to produce monomeric U(IV) species. A species resembling nanoscale uraninite is also present, implying the operation of at least two redox transition pathways. The presence of multiple pathways in low-temperature sediments unifies apparently contrasting prior observations and helps to explain sustained uranium reduction under disparate biogeochemical conditions. These findings have direct implications for our understanding of uranium bioremediation, ore formation, and global geochemical processes. metal reduction | roll front | sulfate reduction | sulfide | bioreduction

Uranium isotopes fingerprint biotic reduction

“…This report suggests that microbial U reduction, i.e., the direct enzymatic pathway, played an important role in the generation of the observed U isotope fractionation in the aquifer. A more recent field-based study at the same uranium-contaminated site (35) suggested that U immobilization was due to biomass-associated mackinawite that acts as an electron source to reduce U(VI) to U(IV). However, according to the isotope signature of the process and the present findings, biogenic mackinawite could not have acted exclusively as a reducing agent in field-stimulated bioremediation.…”

Effect of biogeochemical redox processes on the fate and transport of As and U at an abandoned uranium mine site: an X-ray absorption spectroscopy study

“…2). [13,15] The redox conditions of the pond promote microbial reduction of uranyl to uraninite or alternately molecular U IV that is likely to settle to the bottom of the pond and not be further transported. [64][13] The chemical structure of U IV could not be confirmed by the collected XANES spectra.…”

“…Sediment sulfur concentrations (measured by Larson et al [4] ) and sediment Fe concentrations in the pond could allow for the precipitation of amorphous FeS, following microbial reduction of sulfate and Fe III oxy(hydr)oxides, which can abiotically reduce uranyl to uraninite or non-uraninite U IV . [4,13,15,27,65] Because of the low environmental sediment U concentrations, U XANES spectra were only obtained for the top of the tailings, toe of the tailings, pond and pond outlet (Figs S7, S8, Table S2 of the Supplementary material). Of the collected spectra, the U XANES spectrum from the pond outlet was fit with the greatest percentage of U IV (45.1 %).…”

“…[10][11][12][13] Although U is less mobile in its reduced forms than in its oxidised forms, As is generally more mobile in its reduced form as As III . [14][15][16] Arsenic can adsorb to iron oxides under environmental pH as both As III (H 2 AsO 3 0 ) and As V (H 2 AsO 4 -). [17] Although As III has a greater sorption affinity to iron oxides than As V at pH 7, As V binds more strongly and is not as easily desorbed and released into surface waters.…”

“…[22,23] Uranium can also be reduced by precipitated or biogenically produced Fe-sulfide and by aqueous sulfide under anoxic conditions. [15,[24][25][26][27] Enzymatic reduction of U VI has been observed by iron and sulfate reducing bacteria. [28,29] Although both As III and As V can be adsorbed to iron minerals, As can be released, not only by desorption, but by microbial reduction of iron mineral phases, resulting in mineral dissolution.…”