Collaborative Research: Chromium isotopes as redox indicators- Oxidation and isotopic equilibration experiments

该奖项是根据2009年《美国复苏和再投资法案》(公法111-5)提供资金的。六价铬在环境中具有流动性和毒性。将六价铬还原为三价铬是一个非常重要的过程，因为它使铬不能移动，毒性较小。还原导致稳定的同位素分馏，而PI已领导了一项努力，将53Cr/52Cr同位素比值测量作为一种新的、迫切需要的还原指标。这种方法比传统的质量平衡方法更快、更简单，而且似乎如预期的那样起作用。铬稳定同位素的应用正在从污染地球化学扩展到探索铬的生物地球化学循环，并试图限制过去和现在的海洋氧化还原反应和条件。然而，由于铬(VI)还原引起的铬同位素分馏已经得到了较详细的研究，而由铬(III)氧化成铬(VI)和由铬(III)-铬(VI)交换反应引起的分馏却知之甚少，必须加以探索才能准确地解释铬同位素数据。对MnO2诱导的铬(III)氧化的初步实验表明，反应产物在较重的同位素中有不同程度的浓缩(最高可达1.1o/oo)。这不可能是简单的动力学同位素效应，它会产生相反的变化。观测到的同位素分馏一定是多步氧化还原反应的综合效应。作为pH、溶液化学和氧化锰化学的函数，预期会有相当大的变化，因为这些变量会影响反应步骤的相对速率。该项目将探索这些变量的范围，以系统地了解铬(III)氧化过程中的铬同位素分馏，并确定与自然条件有关的分馏系数。以往对铬(III)和铬(VI)之间的同位素交换的研究表明，这一过程是缓慢的，但这两种物种可以在含水层中共存多年。目前还不清楚这个速度是否足够慢，以至于可以使用假设没有交换的瑞利蒸馏模型从铬同位素数据中确定铬(VI)的还原程度。该项目包括在可行的反应持续时间(少于几个月)内检测和量化缓慢交换所需的高灵敏度实验。该项目将测量交换率，并确定铬(III)-铬(VI)平衡的同位素分馏系数。开发铬同位素方法来检测还原将产生超出学术界的影响；环境顾问已经需要这项新技术来进行铬(VI)污染物研究。铬同位素在更广泛的地学研究中的使用才刚刚开始，但随着对铬同位素系统学理解的更加完整，我们预计将在海洋学、地球历史(例如地球氧化还原变化史)和地球表面过程研究(例如页岩风化在碳循环研究中的作用)中得到重要的应用。该项目将使两所学校协同受益，因为它将把代表不足的群体的学生带到伊利诺伊州校区，以增加其多样性，同时为这些学生提供使用平稳运行的质谱学设施的机会，以及一批从事相同技术的学生。

This award is funded under the American Recovery and Reinvestment Act of 2009 (Public Law 111-5). Chromium in its hexavalent form, Cr(VI), is mobile and toxic in the environment. Reduction of Cr(VI) to the trivalent form, Cr(III), is a very important process as it renders Cr immobile and less toxic. Reduction induces stable isotope fractionation, and the PIs have led an effort to develop 53Cr/52Cr isotope ratio measurements as a new and much needed reduction indicator. This approach is quicker and less complicated than the traditional mass balance approach, and it appears to work as expected. Application of Cr stable isotopes is expanding beyond contaminant geochemistry to studies exploring biogeochemical cycling of Cr and attempting to constrain past and present marine redox reactions and conditions. However, whereas Cr isotope fractionation caused by Cr(VI) reduction has been studied in some detail, fractionation caused by oxidation of Cr(III) to Cr(VI) and by Cr(III)-Cr(VI) exchange reactions is poorly understood and must be explored to enable accurate interpretation of Cr isotope data. Preliminary experiments with MnO2-induced Cr(III) oxidation have revealed that the reaction product is variably enriched (up to 1.1o/oo) in the heavier isotope. This cannot be a simple kinetic isotope effect, which would produce an opposite shift. The observed isotopic fractionation must be the composite effect of a multi-step redox reaction. Considerable variation as a function of pH, solution chemistry, and Mn oxide chemistry is expected, as these variables can affect the relative rates of the reaction steps. This project will explore a range of these variables to develop a systematic understanding of Cr isotopic fractionation during Cr(III) oxidation, and to determine fractionation factors relevant to natural conditions. Previous studies of isotopic exchange between Cr(III) and Cr(VI) indicate that the process is slow, but the two species can coexist in aquifers for many years. It is not clear if the rate is slow enough that the extent of Cr(VI) reduction can be determined from Cr isotope data using Rayleigh distillation models, which assume no exchange. This project includes highly sensitive experiments needed to detect and quantify slow exchange over workable reaction durations (less than several months). This project will measure exchange rates, and also determine the isotopic fractionation factor for Cr(III)-Cr(VI) equilibrium. Development of the Cr isotope approach to detecting reduction will have impacts beyond academia; this new technique is already in demand for Cr(VI) contaminant studies by environmental consultants. Use of Cr isotopes in a wider array of geoscience studies has just begun, but we expect important applications in oceanography, earth history (e.g., the history of earth redox changes), and earth surface process studies (e.g., the role of shale weathering in carbon cycle studies) as the understanding of Cr isotope systematics becomes more complete. The project will synergistically benefit both institutions, as it will bring students from an underrepresented group to the Illinois campus to increase its diversity, while providing those students access to a smoothly functioning mass spectrometry facility with a cadre of students working on the same techniques.