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.

该奖项是根据2009年《美国复苏与再投资法》（公法111-5）资助的。六价形式Cr（VI）的铬在环境中是可移动和有毒的。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不动并且毒性较小。还原诱导稳定的同位素分馏，PIS导致努力开发53CR/52CR同位素比率测量作为一种新的且急需的还原指标。这种方法比传统的质量平衡方法更快且复杂，并且似乎可以按预期工作。 CR稳定同位素的应用正在扩展到污染物地球化学超出污染物地球化学的研究，以探索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。但是，尽管已经详细研究了由Cr（VI）减少引起的Cr同位素分馏，但由于CR（III）对CR（VI）的氧化而引起的分馏以及CR（III）-CR（VI） - CR（VI）交换反应的氧化引起的，必须探索，并且必须探索以准确地解释CR同位素数据。使用MNO2诱导的Cr（III）氧化的初步实验表明，在较重的同位素中，反应产物在重量富集（高达1.1O/OO）。这不能是简单的动力学同位素效应，这会产生相反的转移。观察到的同位素分馏必须是多步氧化还原反应的复合作用。预计有很大的变化随pH，溶液化学和氧化物氧化物化学的函数，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，以在CR（III）氧化过程中对CR同位素分馏的系统理解，并确定与自然条件相关的分级因子。先前关于CR（III）和CR（VI）之间的同位素交换的研究表明该过程很慢，但是这两个物种可以在含水层中共存多年。目前尚不清楚速率是否足够慢，可以使用雷利蒸馏模型从CR同位素数据确定Cr（VI）降低的程度，该模型没有交换。该项目包括在可行的反应持续时间（不到几个月）检测和量化缓慢交换所需的高度敏感实验。该项目将衡量汇率，并确定CR（III）-CR（VI）平衡的同位素分级因子。开发CR同位素检测减少的方法将产生学术界的影响；这项新技术已经需要环境顾问对CR（VI）污染物研究。在更广泛的地球科学研究中，使用CR同位素的使用才刚刚开始，但是我们预计在海洋学，地球历史（例如，地球氧化还原变化的历史）和地面表面过程研究（例如，页岩风化在碳循环研究中的作用）的重要应用是对CRSOTOPE SYSTAMEs的理解变得更加完整的。该项目将在这两个机构中协同效果，因为它将从代表性不足的群体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流行正常运作的质谱学设施，并通过一群从事相同技术的学生提供。