Uranium Isotope Fractionation during Adsorption, (Co)precipitation, and Biotic Reduction

Abstract

Uranium contamination of surface environments is a problem associated with both U-ore extraction/processing and situations in which groundwater comes into contact with geological formations high in uranium. Apart from the environmental concerns about U contamination, its accumulation and isotope composition have been used in marine sediments as a paleoproxy of the Earth's oxygenation history. Understanding U isotope geochemistry is then essential either to develop sustainable remediation procedures as well as for use in paleotracer applications. We report on parameters controlling U immobilization and U isotope fractionation by adsorption onto Mn/Fe oxides, precipitation with phosphate, and biotic reduction. The light U isotope (U) is preferentially adsorbed on Mn/Fe oxides in an oxic system. When adsorbed onto Mn/Fe oxides, dissolved organic carbon and carbonate are the most efficient ligands limiting U binding resulting in slight differences in U isotope composition (δU = 0.22 ± 0.06‰) compared to the DOC/DIC-free configuration (δU = 0.39 ± 0.04‰). Uranium precipitation with phosphate does not induce isotope fractionation. In contrast, during U biotic reduction, the heavy U isotope (U) is accumulated in reduced species (δU up to -1‰). The different trends of U isotope fractionation in oxic and anoxic environments makes its isotope composition a useful tracer for both environmental and paleogeochemical applications.

Biogenic non-crystalline U(IV) revealed as major component in uranium ore deposits

“…Thus, sediment δ 238 U can serve as a signature for past or present biological activity in sedimentary rocks. In three out of four samples, the sandstone of the roll-front deposit investigated here displays higher δ 238 U than those typically observed for the continental crust (δ 238 U∼−0.3‰)41424344 consistent with a predominantly biotic reduction mechanism (Fig. 3 and Supplementary Table 5).…”

Uranium Bio-Transformations: Chemical or Biological Processes?

“…They found that the biotic process produced a very similar negative epsilon (enrichment factor) value for both bacteria, while the abiotic iron process resulted in the same ratio as natural abundance [60]. Dang et al also compared biotic and abiotic pathways and came to the same conclusion, but with the addition that certain minerals such as mixtures of iron and manganese have an isotope preference for the lighter isotope, U-235 [148]. The Bernier-Latmani group has also measured a uranium signature with preference for the heavy isotope and has used this signature to detect microbial activity in rocks from multiple ages of Earth history including Archean and Cretaceous [149].…”

Uranium isotope geochemistry in modern coastal sediments: Insights from Toulon Bay, France

“…All the acid solutions were double-distilled trace metal analysis grade (BDH Aristar plus; DuoPUR, Milestone). The analytical procedures to determine U concentration and U isotope composition were detailed in a previous study (Dang et al, 2016). Briefly, U concentration was analyzed at the Water Quality Centre (WQC) at Trent University (Peterborough, Canada)…”

“…The U-P precipitation system in a seawater matrix at different pH values was modeled with PHREEQC (version 2.18.3,40) similar to an approach previously described (Dang et al, 2016). Briefly, the chemical speciation and the mineral saturation index in mixtures consisting of U and P (in the concentration range of our samples: 1 to 500 nM and 1 μM to 100 M, respectively) in the seawater matrix was calculated ( Figure S9) for a range of pH values (6)(7)(8) that are relevant when considering the observed variations in the porewaters.…”

“…In addition to reductive reactions, other geochemical reactions such as adsorption, (co)precipitation, as well as biogenic U input could contribute to the variation in isotopic U composition (Andersen et al, 2014;Brennecka et al, 2011;Dang et al, 2016;Holmden et al, 2015;Stirling et al, 2015Stirling et al, , 2007. Even though, ab initio computations and experimental experiments on U biotic reduction have shown that U fractionation could reach a full fractionation factor of +1.2 ‰ (Δ 238 U) (Abe et al, 2008;Andersen et al, 2014;Bigeleisen, 1996;Dang et al, 2016;Fujii et al, 2006;Stylo et al, 2015), most of natural sedimentary systems record only a U-half fractionation factor (Δ 238 U up to +0.6‰, Andersen et al, 2016 and references therein). Recent studies suggest that a U-full fractionation factor could be observed by either a partial U removal from porewater (Andersen et al, 2014) or an early U reduction in the water column which is not transport-diffusion restricted (Andersen et al, 2017).…”