Uranium Isotope Fractionation during Adsorption to Mn-Oxyhydroxides
Uranium Isotope Fractionation during Adsorption to Mn-Oxyhydroxides
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DOI：
10.1021/es103061v
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发表时间：
2011-02-15
影响因子：
11.4
通讯作者：
Anbar, Ariel D.
中科院分区：
文献类型：
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作者：
Brennecka, Gregory A.;Wasylenki, Laura E.;Anbar, Ariel D.
以前的工作已经显示了铀(U)同位素在天然锰铁结壳和海水之间的分馏。了解氧化锰铁吸附过程中U-238/U-235分异的机理是利用U-238/U-235作为地下水中U吸附反应的示踪剂和潜在的海洋古氧化还原代理的关键步骤。我们用合成的k -偏镁石和含铀溶液进行了U吸附实验。这些实验揭示了海水和天然锰铁沉积物之间的分异匹配：吸附的铀同位素比溶解的铀轻0.2%o （δ (238)/U-235）。在吸附过程中，铀的氧化还原状态没有改变，溶解的和吸附的铀之间的配位环境的差异可能是造成这种影响的原因。为了验证这一假设，我们在实验研究中使用扩展x射线吸收精细结构（EXAFS）光谱分析了吸附在k -伯氏石上的U，以获得吸附配合物中U的配位信息。我们的EXAFS光谱与水态U的比较揭示了溶解态和吸附态U- o配位壳之间微妙但重要的差异。我们假设这些差异是我们实验中观察到的分异以及自然界中一些U同位素变化的原因。
Previous work has shown uranium (U) isotope fractionation between natural ferromanganese crusts and seawater. Understanding the mechanism that causes U-238/U-235 fractionation during adsorption to ferromanganese oxides is a critical step in the utilization of U-238/U-235 as a tracer of U adsorption reactions in groundwater as well as a potential marine paleoredox proxy. We conducted U adsorption experiments using synthetic K-bimessite and U-bearing solutions. These experiments revealed a fractionation matching that observed between seawater and natural ferromanganese sediments: adsorbed U is isotopically lighter by similar to 0.2%o (delta(238)/U-235) than cissolved U. As the redox state of U does not change during adsorption, a difference in the coordination environment between dissolved and adsorbed U is likely responsible for this effect. To test this hypothesis, we analyzed U adsorbed to K-birnessite in our experimental study using extended X-ray absorption fine structure (EXAFS) spectroscopy, to obtain information about U coordination in the adsorbed complex. Comparison of our EXAFS spectra with those for aqueous U species reveals subtle, but important, differences in the U-O coordination shell between dissolved and adsorbed U. We hypothesize that these differences are responsible for the fractionation observed in our experiments as well as for some U isotope variations in nature.