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）资助的。六价铬Cr（VI）在环境中具有移动的和毒性。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。还原诱导稳定的同位素分馏，和PI已经导致努力开发53 Cr/52 Cr同位素比测量作为一个新的和急需的还原指标。这种方法比传统的质量平衡方法更快，更简单，而且它看起来像预期的那样工作。铬稳定同位素的应用正在扩展到污染物地球化学之外，探索铬的地球化学循环，并试图限制过去和现在的海洋氧化还原反应和条件。然而，尽管已经详细研究了由Cr（VI）还原引起的Cr同位素分馏，但对由Cr（III）氧化为Cr（VI）和由Cr（III）-Cr（VI）交换反应引起的分馏了解甚少，必须进行探索以准确解释Cr同位素数据。MnO 2诱导的Cr（III）氧化的初步实验表明，反应产物中较重的同位素是富铬的（高达1.1o/oo）。这不可能是一个简单的动力学同位素效应，它会产生相反的位移。所观察到的同位素分馏必须是多步氧化还原反应的复合效应。预计pH值、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。本项目将探讨这些变量的范围，以发展在铬（三价铬）氧化过程中的铬同位素分馏系统的理解，并确定分馏因素相关的自然条件。以前的研究表明，铬（III）和铬（VI）之间的同位素交换的过程是缓慢的，但这两个物种可以共存于含水层多年。目前尚不清楚，如果速度是足够慢的程度，铬（VI）的减少，可以确定从铬同位素数据使用瑞利蒸馏模型，假设没有交换。该项目包括高灵敏度的实验，需要检测和量化在可行的反应持续时间（不到几个月）的缓慢交换。该项目将测量交换率，并确定Cr（III）-Cr（VI）平衡的同位素分馏系数。发展铬同位素的方法来检测减少将有学术界以外的影响，这种新技术已经在铬（VI）污染物研究的环境顾问的需求。铬同位素在更广泛的地球科学研究中的应用才刚刚开始，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，页岩风化作用在碳循环研究中的作用）。该项目将协同受益于两个机构，因为它将使学生从一个代表性不足的群体到伊利诺伊州校园，以增加其多样性，同时为这些学生提供一个顺利运作的质谱设施与学生的干部工作在相同的技术。

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.