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

合作研究：铬同位素作为氧化还原指示剂-氧化和同位素平衡实验

• 批准号: 0843631
• 负责人: Thomas Johnson
• 金额: $ 16.3万
• 依托单位: University of Illinois at Urbana-Champaign
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2009
• 资助国家: 美国
• 起止时间: 2009-09-01 至 2012-08-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0843631&HistoricalAwards=false
• 关键词: Collaborative; Research; Chromium; isotopes; redox

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.

该奖项根据 2009 年美国复苏和再投资法案（公法 111-5）提供资金。六价铬 Cr(VI) 在环境中具有流动性和毒性。将 Cr(VI) 还原为三价形式 Cr(III) 是一个非常重要的过程，因为它使 Cr 固定且毒性较小。还原会导致稳定同位素分馏，PI 领导了开发 53Cr/52Cr 同位素比测量的工作，作为一种新的、急需的还原指标。这种方法比传统的质量平衡方法更快、更简单，而且看起来效果符合预期。 Cr 稳定同位素的应用正在从污染物地球化学扩展到探索 Cr 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的研究。然而，虽然对 Cr(VI) 还原引起的 Cr 同位素分馏已经进行了一些详细的研究，但对 Cr(III) 氧化成 Cr(VI) 以及 Cr(III)-Cr(VI) 交换反应引起的分馏还知之甚少。必须进行探索才能准确解释 Cr 同位素数据。 MnO2 诱导 Cr(III) 氧化的初步实验表明，反应产物中的重同位素有所富集（高达 1.1o/oo）。这不可能是一个简单的动力学同位素效应，它会产生相反的变化。观察到的同位素分馏必定是多步氧化还原反应的复合效应。预计 pH、溶液化学和氧化锰化学会产生相当大的变化，因为这些变量会影响反应步骤的相对速率。该项目将探索一系列这些变量，以系统地了解 Cr(III) 氧化过程中的 Cr 同位素分馏，并确定与自然条件相关的分馏因子。先前对 Cr(III) 和 Cr(VI) 之间同位素交换的研究表明，该过程很缓慢，但两种物质可以在含水层中共存多年。目前尚不清楚该速率是否足够慢，以至于可以使用瑞利蒸馏模型从 Cr 同位素数据确定 Cr(VI) 的还原程度，该模型假设没有交换。该项目包括检测和量化可行反应持续时间（少于几个月）内缓慢交换所需的高度敏感实验。该项目将测量汇率，并确定 Cr(III)-Cr(VI) 平衡的同位素分馏因子。开发用于检测还原的 Cr 同位素方法将产生学术界以外的影响；环境顾问已经在 Cr(VI) 污染物研究中需要这种新技术。铬同位素在更广泛的地球科学研究中的使用刚刚开始，但我们预计在海洋学、地球历史（例如，地球氧化还原变化的历史）和地球表面过程研究（例如，页岩风化的作用）中会有重要应用。随着对 Cr 同位素系统学的理解变得更加完整，碳循环研究）。该项目将为两个机构带来协同效益，因为它将把来自代表性不足群体的学生带到伊利诺伊州校园，以增加其多样性，同时为这些学生提供一个平稳运行的质谱设施，并由一批从事相同技术的学生组成。