Testing Hypotheses of Near-Equilibrium Kinetics For Silicate Minerals with an Innovative Silicon Isotope Tracer Method

对在地质储存库中储存核废料和二氧化碳的安全性的评估需要对遥远的未来进行预测。例如，这些废物与周围的矿物和岩石之间的相互作用是否会导致有害物质在1万年或10万年后释放到环境中？在这样的时间跨度内进行可靠的预测，需要对这些相互作用期间化学反应的速度或速度的基本原理有更好的了解。但这些科目一直是研究的一个挑战。例如，90%的地壳由硅酸盐矿物组成，其中硅(Si)和氧是主要的化学元素，它们的反应速度很慢，很难被检测到。这项拟议的研究使用了一种创新的实验方法，即同位素示踪法，在实验室中测量以前不可能出现的条件下的反应速度。这些新的费率最终将被纳入社会一般环境项目的安全评估模型。除了科技创新，该项目还致力于人力资源开发。基础科学是热力学和动力学，这对学生来说是很难学的科目。尽管热力学和动力学课程意义重大，但它正在从地质学课程中消失，大多数热力学专家要么退休，要么接近退休年龄。为了培训下一代专家，该项目将开发一系列关于热力学和动力学的网络研讨会，并与印第安纳大学创新教学和学习中心合作进行广播和记录。最后，这笔赠款将利用印第安纳大学的少数族裔服务机构STEM倡议和女性技术卓越中心来招募代表人数不足的教职员工和潜在的博士生参加印第安纳大学的夏季研究。拟议的研究将侧重于近平衡条件下的地球化学动力学，并使用从同位素示踪实验方法获得的单向溶解速率数据来检验各种假说。关键假设是，近平衡数据的分散和冲突是由无法解释的第二相沉淀引起的。消除或考虑这种可能性是检验近平衡反应机制和界面过程的其他假设的关键的第一步。在所提出的实验中，在实验溶液中加入稀有的硅同位素(29Si或30Si)，它与主要含有28Si的天然硅酸盐反应。采用高分辨率多收集电感耦合等离子体质谱技术，通过反应溶液的29Si/28Si比值跟踪反应速率。相比之下，传统的实验方法基于硅浓度来测量速率。由于用同位素比而不是浓度来计算溶解速率，含硅第二相的析出不会干扰溶解速率的测定，因为含硅第二相的析出消耗了硅的浓度，而对硅同位素比的影响可以忽略不计。在溶解和沉淀过程中(2-4？)，由于29Si压倒了系统而导致了巨大的同位素不平衡，导致了Si同位素分馏。对于速率确定并不重要。目前，为了将实验室实验数据转化为地质和环境过程的模型，有必要从远离平衡状态到接近平衡状态进行外推。但由于缺乏接近均衡的数据和未经支持的假设，人们对这些推断几乎没有信心。该项目将填补这一重大的知识空白。同位素示踪方法是通用和新颖的，可以扩展到使用钡、钙和镁同位素来研究硫酸盐、碳酸盐和其他矿物，并应用于化学、化学工程和材料科学领域。该奖项反映了NSF的法定使命，并通过使用基金会的智力价值和更广泛的影响审查标准进行评估，被认为值得支持。

The assessment of the safety of storing nuclear wastes and carbon dioxide in geological repositories requires projections into a distant future. For example, would the interactions between these wastes and surrounding minerals and rocks cause the release of harmful materials into the environment in 10,000 years or 100,000 years? Reliable predictions over such a time span require a greater understanding of the fundamentals regarding the rates or speed of chemical reactions during these interactions. But these subjects have been a challenge to study. For example, 90% of Earth's crust is made of silicate minerals, for which silicon (Si) and oxygen are the main chemical elements, and their reactions are slow and difficult to detect. The proposed research uses an innovative experimental method, the isotope tracer method, to measure reaction rates in the laboratory under conditions not possible before. These new rates will be eventually incorporated into safety assessment models for society's general environmental projects. In addition to scientific innovations, this project also aims at human resources development. The underlying fundamental sciences are thermodynamics and kinetics, which are difficult learning subjects for students. Despite its significance, courses in thermodynamics and kinetics are disappearing from the geology curriculum and most experts on thermodynamics are either retired or near retirement age. To train the next generation of experts, this project will develop a webinar series on thermodynamics and kinetics and work with Indiana University's Center for Innovative Teaching and Learning for broadcasting and recording. Finally, the grant will leverage Indiana University's Minority-Serving Institution STEM initiative and the Center of Excellence for Women in Technology to recruit underrepresented faculty and potential doctoral students to summer research at Indiana University. The proposed research will focus on geochemical kinetics at near-equilibrium conditions and test various hypotheses using unidirectional dissolution rate data obtained from the isotope tracer experimental method. The key hypothesis is that the scatter and conflicts of near-equilibrium data are caused by unaccounted-for secondary phase precipitation. Eliminating or accounting for this possibility is a critical first step toward testing other hypotheses of near-equilibrium reaction mechanisms and interfacial processes. In the proposed experiments, a rare Si isotope (29Si or 30Si) is added to the experimental solutions, which reacts with natural silicates having mostly 28Si. The reaction rates are tracked by 29Si/28Si ratios of reacted solutions using High-Resolution Multi-Collector ICP-MS technology. In contrast, the conventional experimental method measures rates based on Si concentrations. Because isotope ratios are used instead of concentrations to calculate rates, the precipitation of Si-containing secondary phases, which consumes Si concentrations while having a negligible effect on Si isotope ratios, does not interfere with dissolution rate determination. The huge isotope disequilibrium introduced by overwhelming the system with 29Si makes Si isotope fractionation during dissolution and precipitation (2-4?) unimportant for rate determination. Currently, extrapolation from far-from-equilibrium to near-equilibrium conditions is necessary to translate laboratory experimental data to models of geological and environmental processes. But little confidence can be placed on these extrapolations because of the lack of near-equilibrium data and unsupported assumptions. The project will fill this significant knowledge gap. The isotope tracer method is general and novel and can be expanded to using Barium, Calcium, and Magnesium isotopes to study sulfate, carbonate, and other minerals, and applied to the fields of chemistry, chemical engineering, and materials science.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.