Quantitative Separation of Monomeric U(IV) from UO₂ in Products of U(VI) Reduction

Abstract

The reduction of soluble hexavalent uranium to tetravalent uranium can be catalyzed by bacteria and minerals. The end-product of this reduction is often the mineral uraninite, which was long assumed to be the only product of U(VI) reduction. However, recent studies report the formation of other species including an adsorbed U(IV) species, operationally referred to as monomeric U(IV). The discovery of monomeric U(IV) is important because the species is likely to be more labile and more susceptible to reoxidation than uraninite. Because there is a need to distinguish between these two U(IV) species, we propose here a wet chemical method of differentiating monomeric U(IV) from uraninite in environmental samples. To calibrate the method, U(IV) was extracted from known mixtures of uraninite and monomeric U(IV) and testted using X-ray absorption spectroscopy (XAS). Monomeric U(IV) was efficiently removed from biomass and Fe(II)-bearing phases by bicarbonate extraction, without affecting uraninite stability. After confirming that the method effectively separates monomeric U(IV) and uraninite, it is further evaluated for a system containing those reduced U species and adsorbed U(VI). The method provides a rapid complement, and in some cases alternative, to XAS analyses for quantifying monomeric U(IV), uraninite, and adsorbed U(VI) species in environmental samples.

Biogeochemical Controls on the Product of Microbial U(VI) Reduction

“…Our results confirm that the biouraninite is indeed a mixture of uraninite and nonuraninite U(IV) species, in an approximately 1:1 ratio, formed under simple conditions similar to those examined in past studies 9,12 (sample BP 400). The geochemical conditions can be divided into three groups based on the contribution of the nonuraninite U(IV) fraction in comparison to the baseline case, BP 400 (previously studied 9,12 ). An even higher U concentration (BP 600) or the addition of calcium or magnesium (Ca-1, Ca-2, and Mg) do not alter the U(IV) product.…”

“…9 As was discussed above, it is crucial to remember that bio-UO 2 is a mixture of crystalline uraninite and amorphous nonuraninite U(IV) species. 12 However, depending on the chemical composition of the reduction medium, the ratio of the two U(IV) species can vary. Therefore, to identify conditions that lead predominantly to the formation of nonuraninite U(IV) species, we conducted U(VI) bioreduction experiments in BP medium amended with individual solutes or mixtures thereof.…”

“…9−11 It is essential to understand that, in the literature, what is referred to as "bio-UO 2 " (microbially produced uraninite), is actually a mixture of both species: crystalline uraninite and amorphous nonuraninite U(IV). 12 Depending on the geochemical conditions present during U(VI) reduction, the ratio of the two U(IV) species can vary. Laboratory-scale research showed that it is possible to control this ratio.…”

Impact of Microbial Mn Oxidation on the Remobilization of Bioreduced U(IV)

“…The more dramatic increase in oxidation of the monomeric U(IV) could possibly be due to the fact that, unlike the mineral UO 2 , it is amorphous and likely more easily oxidized. 13 The XAS LCF data and the extraction data are summarized in Table 2 display remarkable agreement. For instance, for UO 2 after 40 days, both methods indicate that U(VI) represents 68% of the total U.…”

Rapid Mobilization of Noncrystalline U(IV) Coupled with FeS Oxidation

“…Only a few studies, however, have examined the reactivity of biogenic noncrystalline U(IV) in laboratory systems. 16,17 Alessi et al 16 showed that the noncrystalline U(IV) fraction can be selectively extracted from a U(IV)−U(VI) mixture by an anoxic 1 M bicarbonate solution. Cerrato et al 17 further demonstrated that the U(IV) species can be readily oxidized by dissolved oxygen (DO) and persulfate.…”