磁铁矿中五价铀同位素特征的持久性,Environmental Science & Technology

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磁铁矿中五价铀同位素特征的持久性
Environmental Science & Technology ( IF 10.8 ) Pub Date : 2022-01-21 , DOI: 10.1021/acs.est.1c06865
Zezhen Pan _{1,

2} , Yvonne Roebbert ₃ , Aaron Beck ₄ , Barbora Bartova ₂ , Tonya Vitova ₄ , Stefan Weyer ₃ , Rizlan Bernier-Latmani ₂

Affiliation

铀同位素特征可用于监测地下污染的还原修复或重建古氧化还原环境。然而，与 U 还原相关的同位素分馏的机制基础仍然知之甚少。在这里，我们提出了一项共沉淀研究，其中六价 U (U(VI)) 在磁铁矿的合成过程中被还原，五价 U (U(V)) 是主要物种。测量的未还原 U(VI) (∼-1.0‰)、合并 U (96 ± 2% U(V), ∼-0.1‰) 和提取表面 U (主要是 U(IV), ∼0.3) 的 δ ²³⁸ U 值‰) 表明重同位素在还原物种中的优先积累。U-磁铁矿共沉淀物暴露在空气中后，U(V) 部分再氧化为 U(VI)，δ ^{238没有显着变化}U 值。相比之下，重同位素掺杂的 U(VI) 溶液的缺氧修正导致掺入的 U 物种的 δ ²³⁸ U 随着时间的推移而增加，表明掺入的 U 和表面/含水 U 之间存在交换。总体而言，结果支持存在具有轻同位素特征的持久 U（V），并表明氧化铁的矿物动力学可能允许叠印掺入的 U 物种的同位素特征。这项工作进一步加深了对现代和古环境中与氧化铁相关的 U 同位素分馏的理解。

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Persistence of the Isotopic Signature of Pentavalent Uranium in Magnetite

Uranium isotopic signatures can be harnessed to monitor the reductive remediation of subsurface contamination or to reconstruct paleo-redox environments. However, the mechanistic underpinnings of the isotope fractionation associated with U reduction remain poorly understood. Here, we present a coprecipitation study, in which hexavalent U (U(VI)) was reduced during the synthesis of magnetite and pentavalent U (U(V)) was the dominant species. The measured δ²³⁸U values for unreduced U(VI) (∼−1.0‰), incorporated U (96 ± 2% U(V), ∼−0.1‰), and extracted surface U (mostly U(IV), ∼0.3‰) suggested the preferential accumulation of the heavy isotope in reduced species. Upon exposure of the U-magnetite coprecipitate to air, U(V) was partially reoxidized to U(VI) with no significant change in the δ²³⁸U value. In contrast, anoxic amendment of a heavy isotope-doped U(VI) solution resulted in an increase in the δ²³⁸U of the incorporated U species over time, suggesting an exchange between incorporated and surface/aqueous U. Overall, the results support the presence of persistent U(V) with a light isotope signature and suggest that the mineral dynamics of iron oxides may allow overprinting of the isotopic signature of incorporated U species. This work furthers the understanding of the isotope fractionation of U associated with iron oxides in both modern and paleo-environments.

更新日期：2022-02-01

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