各位老師和同學(xué)：
應徐義剛院士邀請，英國皇家霍洛威學(xué)院的Anirban Basu博士訪(fǎng)問(wèn)我所，并將于本周三上午做“同位素室學(xué)術(shù)報告”2018年第25次報告。歡迎大家參加并積極參與討論！

報告題目：Uranium isotope fractionation: Experiments, environmental applications and a predictive understanding
報 告 人：Dr. Anirban Basu (Royal Holloway, Univ. of London, UK)
報告時(shí)間：7月25日（周三）上午10:00
報告地點(diǎn)：綜合樓701會(huì )議室

報告人簡(jiǎn)介：
Dr. Anirban Basu從事低溫地球化學(xué)研究，2013年獲得美國伊利諾伊大學(xué)（香檳分校）的博士學(xué)位，2013-2016年在美國加州大學(xué)（伯克利分校）Lawrence Berkeley National Laboratory (Berkeley Lab)做博士后研究工作，并于2016年入職皇家霍洛威學(xué)院。Dr. Basu的研究興趣廣泛，主要集中在用非傳統穩定同位素示蹤現今與歷史時(shí)期的生物化學(xué)過(guò)程。他當前的研究課題包括“用Te同位素示蹤Te氧化物在自然界中的活動(dòng)性”與“從大氣中移除甲烷氣體的新方法”。近年來(lái)，Dr. Basu在Geochimica et Cosmochimica Acta、Chemical Geology和Environmental Science and Technology等國際一流期刊已發(fā)表SCI論文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.