RUI: Where does all the molybdenum go? Evaluating the role of iron sulfides in molybdenum accumulation in marine sediments

该项目将研究沉积物铁硫化物在从海洋中去除溶解的钼方面的作用。钼是一种存在于海洋沉积物中的痕量金属。在过去，它被用作低氧和高硫化物条件的指示器。了解海洋氧合的历史对于监测现代沿海水域的低氧“死区”和预测气候变化对海洋的影响很重要。然而，钼在沉积物中积累的过程还不是很清楚。该项目将利用实验室实验来研究溶液钼与固体硫化铁之间的相互作用。这项研究将在一所本科院校进行。这项研究将为STEM中历史上代表性不足的群体的学生提供一个有系统的经历过程。它将包括在学生进入校园之前以及在他们的科学入门课程期间为学生提供支持的队列-导师方法。暑期研究学生(正在崛起的三年级和四年级学生)将参与这个项目的各个方面。这些大三和大四的学生还将担任研究队列学生(上升的大二学生)的同伴导师，组成一个有效的研究团队。学生将通过与宾夕法尼亚州立大学的研究小组会面来获得研究生院的机会。最后，将通过在普通化学入门课程中开发以环境为主题的实验室模块来培养对地球化学的兴趣。这项工作的目的是阐明在海洋沉积物中的相对重要性：(1)钼在硫化铁上的吸附，(2)在黄铁矿形成过程中钼的保留和/或在黄铁矿晶区生长时钼的排斥，以及(3)可能形成不同的FeMoS产物。最近的工作证实，从硫化水中去除钼是在亚稳硫化铁的初始形成过程中进行的，在这些前驱物相转变为热力学稳定的黄铁矿的过程中，钼被保留下来。有了这些基础工作，应该有可能通过以下方式最终确定硫化铁在海洋沉积物中钼的积累中所起的作用：1)合成亚稳态硫化铁，并评估不同条件下这些物种对钼的吸附，以确定钼硫化作用、硫化铁表面化学和氧化还原行为的影响；2)监测这些物质向黄铁矿的转化，以确定各种初始条件如何改变钼的保留或并入；以及3)建立一个模型，通过这些过程追踪钼的命运，并与不同的文献证据相一致。了解硫化铁及其相变在钼的积累中所起的作用，将为使用钼作为古示踪剂的工作提供坚实的基础。该奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

The project will study the role of sediment iron sulfides in removing dissolved molybdenum from the ocean. Molybdenum (Mo) is a trace metal found in ocean sediments. It is used as an indicator of low-oxygen and high-sulfide conditions in the past. Understanding the history of ocean oxygenation is important for monitoring low-oxygen “dead zones” in modern coastal waters and predicting the impact of climate change on the oceans. However, the processes by which Mo accumulates in sediments are not well understood. This project will use laboratory experiments to study the interactions between solution Mo and solid iron sulfides. The research will be carried out at an undergraduate institution. The study will provide a structured progression of experiences for students from historically underrepresented groups in STEM. It will include a cohort-mentor approach to support students before they arrive on campus and throughout their introductory science courses. Summer Research Students (rising juniors and seniors) will be engaged in every aspect of this project. These juniors and seniors will also serve as peer mentors for Research Cohort students (rising sophomores), forming an effective research team. Student exposure to graduate school opportunities will occur through meetings with research groups at the Pennsylvania State University. Finally, interest in geochemistry will be fostered by developing an environmentally-themed laboratory module within an introductory General Chemistry course. The goal of this work is to clarify the relative importance in marine sediments of: (1) sorption of Mo to iron sulfides, (2) retention of Mo during the formation of pyrite and/or exclusion of Mo upon growth of crystalline pyrite domains, and (3) possible formation of a distinct FeMoS product. Recent work established that Mo removal from sulfidic waters occurs with the initial formation of metastable iron sulfides and Mo is retained during the transformation of these precursor phases to the thermodynamically stable form of pyrite. With this groundwork, it should be possible to conclusively determine the role that iron sulfides play in the accumulation of Mo in marine sediments by: 1) synthesizing metastable iron sulfides and evaluating Mo adsorption to these species under various conditions to determine the impact of Mo thiolation, iron sulfide surface chemistry, and redox behavior; 2) monitoring the transformation of these materials to pyrite to determine how various initial conditions alter the retention or incorporation of Mo; and 3) developing a model that traces the fate of Mo through these processes and reconciles the disparate literature evidence. Understanding the role that iron sulfides and their phase transformations play in Mo accumulation will provide a strong foundation for work that employs Mo as a paleotracer.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.