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

评估在地质储存库中储存核废料和二氧化碳的安全性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？在这样一个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率或速度的基本原理有更深入的了解。但这些课题的研究一直是一个挑战。例如，90%的地壳是由硅酸盐矿物构成的，其中硅（Si）和氧是主要的化学元素，它们的反应缓慢且难以检测。提出的研究使用了一种创新的实验方法，同位素示踪法，在以前不可能的条件下测量实验室的反应速率。这些新费率最终将纳入社会一般环境项目的安全评估模型。除科学创新外，该项目还着眼于人力资源开发。基础科学是热力学和动力学，这是学生很难学习的科目。尽管热力学和动力学具有重要意义，但它正在从地质学课程中消失，而且大多数热力学专家不是已经退休，就是接近退休年龄。为了培养下一代专家，该项目将开发一个关于热力学和动力学的网络研讨会系列，并与印第安纳大学创新教学和学习中心合作进行广播和录音。最后，这笔拨款将利用印第安纳大学的少数族裔服务机构STEM倡议和技术领域女性卓越中心，招募代表性不足的教师和潜在的博士生到印第安纳大学进行暑期研究。拟开展的研究将侧重于近平衡条件下的地球化学动力学，并利用同位素示踪剂实验方法获得的单向溶解速率数据验证各种假设。关键的假设是，近平衡数据的分散和冲突是由未解释的二次相沉淀引起的。排除或解释这种可能性是测试其他近平衡反应机制和界面过程假设的关键的第一步。在提出的实验中，在实验溶液中加入稀有的Si同位素（29Si或30Si），与主要含有28Si的天然硅酸盐反应。采用高分辨率多收集器ICP-MS技术，通过反应溶液的29Si/28Si比率来跟踪反应速率。相比之下，传统的实验方法是基于Si浓度来测量速率的。由于使用同位素比率而不是浓度来计算溶解速率，因此含硅次级相的析出不会干扰溶解速率的测定，虽然它消耗了Si浓度，但对Si同位素比率的影响可以忽略不计。在溶解和沉淀过程中（2-4?）Si同位素分馏对速率的测定不重要。目前，从远离平衡状态到接近平衡状态的外推是将实验室实验数据转化为地质和环境过程模型的必要条件。但由于缺乏接近平衡的数据和不受支持的假设，这些推断几乎没有可信度。该项目将填补这一重要的知识空白。同位素示踪方法具有通用性和新见性，可扩展到利用钡、钙、镁同位素研究硫酸盐、碳酸盐等矿物，并可应用于化学、化学工程、材料科学等领域。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识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.