报告题目：Uranium isotope fractionation: Experiments, environmental applications and a predictive understanding

各位老师和同学：
应徐义刚院士邀请，英国皇家霍洛威学院的Anirban Basu博士访问我所，并将于本周三上午做“同位素室学术报告”2018年第25次报告。欢迎大家参加并积极参与讨论！

报告题目：Uranium isotope fractionation: Experiments, environmental applications and a predictive understanding
报 告 人：Dr. Anirban Basu (Royal Holloway, Univ. of London, UK)
报告时间：7月25日（周三）上午10:00
报告地点：综合楼701会议室

报告人简介：
Dr. Anirban Basu从事低温地球化学研究，2013年获得美国伊利诺伊大学（香槟分校）的博士学位，2013-2016年在美国加州大学（伯克利分校）Lawrence Berkeley National Laboratory (Berkeley Lab)做博士后研究工作，并于2016年入职皇家霍洛威学院。Dr. Basu的研究兴趣广泛，主要集中在用非传统稳定同位素示踪现今与历史时期的生物化学过程。他当前的研究课题包括“用Te同位素示踪Te氧化物在自然界中的活动性”与“从大气中移除甲烷气体的新方法”。近年来，Dr. Basu在Geochimica et Cosmochimica Acta、Chemical Geology和Environmental Science and Technology等国际一流期刊已发表SCI论文15篇。

Abstract：
Uranium (U) isotope ratio (238U/235U) has emerged as a powerful tool to investigate a range ofgeochemical problems from contaminant remediation to the extent of ocean anoxia. Thegeochemical behavior of U is governed by its oxidation state. The oxidized species U(VI) isthermodynamically stable and highly soluble, while the reduced species U(IV) is insoluble atnear-neutral pH conditions. The reduction of soluble U(VI) to insoluble U(IV) immobilizes U inthe environment, and fractionates U isotopes by partitioning 238U in the U(IV) phases. As thereaction proceeds, the remaining dissolved U(VI) becomes progressively enriched in 235U.This “reverse” sense of isotopic fractionation (compared with lower z number elements wherethe product is enriched in lighter isotopes) is attributed to equilibrium isotopic fractionationdue to nuclear volume effect. The isotopic enrichment is quantified by measuring 238U/235U inthe remaining unreacted pool of U(VI) and the magnitude of the isotopic fractionation isexpressed by the isotopic enrichment factor ε (ε=1000*(α-1); α =(238U/235U)Product/(238U/235U)Reactant).
We observe significant U isotope fractionation (ε = 0.8‰) for the first time during abioticreduction of U(VI) by the synthetic iron monosulfide (FeS). The ε increases with decreasingU(VI) reduction rate and with increasing amounts of neutrally charged aqueous Ca-U-CO3species. This suggests that abiotic U isotope fractionation is likely to occur in any reducingenvironment with aqueous Ca ≥ 1mM. We demonstrate large U isotopic fractionation duringU(VI) reduction by a metabolically diverse group of bacteria. The ε for microbial U(VI)reduction ranges from 0.68‰ to 0.99‰. The fractionation tends to be the largest (ε ≈ 1‰)under environmentally relevant electron-donor concentrations. The variation in ε is inverselyrelated to the U(VI) reaction rate and can be modeled as a two step process: diffusion ofU(VI) to the enzymes followed by reduction. The results of this study reveals the mechanismand controlling factors for isotopic fractionation during microbial U(VI) reduction. Ourexperimental results establish U isotope ratio as an excellent proxy for U reduction regardlessof the mechanism (i.e., microbial vs. abiotic).
We demonstrate naturally occurring U reduction at ISR U mines using U isotope ratios ofgroundwater samples collected from wells drilled within, upgradient and downgradient ofmined U ore. Along the hydraulic gradient, the δ238U values measured in groundwatersamples range from 0.6‰ to ？2.5‰ at Rosita, TX, USA, and from ？0.9‰ to ？2.8‰ at SmithRanch, WY, USA. We also report U isotopic characterization ((234U/238U) and δ238U) of a rollfrontU deposit to understand the relative contributions of groundwater hydrology and porescalechemical reactions. Our results show a systematic pattern of decreasing δ238U alongthe hydraulic gradient at the center of the mineralized zone, caused by reactive transport of Uacross the redox gradient. This predictable pattern helps understand the formation andevolution of roll-fronts as well as aids in U exploration and efficient mining. These results helpinterpret existing U isotope data from Black Sea, modern oceans, and have implications forthe study of anoxia in the ancient oceans and other environments such as roll-front U deposits and U contaminated aquifers.