EAR-PF Understanding the Effects of Incomplete Mixing on Mixing Corrosion: Pore-scale Visualization and Upscaling

Michael A.陈博士已获得美国国家科学基金会博士后奖学金，在明尼苏达大学双城校区与彼得·K·金教授一起研究混合诱导的方解石溶解并开发推广材料。康和戴安娜·达尔博顿博士碳酸盐矿物，如方解石，构成了许多天然岩石和土壤，包括许多岩溶系统，也是碳循环中碳的重要汇。通过溶解作用去除这些矿物质会推动地下景观的演变，并导致碳释放回到碳循环中，即使是简单地混合含有不同来源方解石的饱和沃茨也会刺激剧烈的溶解作用。这种特定的过程，称为混合腐蚀，基本上是一个土壤孔隙尺度的过程，被称为岩溶形成的关键机制，但我们仍然不明白这个小规模的过程如何导致更大规模的溶解模式在时间和空间。用于研究混合腐蚀的含水层尺度模型通常假设溶液一遇到彼此就充分混合，然而，在较小尺度下不一定如此，不完全混合可能使溶液分离，从而降低总方解石溶解速率。因此，该奖学金的主要目标将是在微观尺度上实验研究方解石的混合腐蚀，并开发一个框架，以了解微尺度流动，混合和化学反应如何影响方解石溶解在更大的尺度。这个框架和通过这些研究获得的基本知识可以扩展到了解其他微尺度过程如何影响更大尺度的过程，这可以用来改善景观演变的预测，发展预测模型的碳封存，并加强努力，修复地下水污染物。该项目还将通过从地球科学中代表性不足的群体中招聘和指导实习生，支持少数群体参与地球科学，以及通过与明尼苏达州科学博物馆的外联活动，利用新的示范实验提高公众对岩石和土壤的流动和化学的理解。继续需要更好地了解流动和地球化学的耦合过程如何改变溶解或方解石的沉淀与碳循环和自然地质构造的发展有关，特别是喀斯特。这在混合腐蚀过程中得到了最好的例证，其中两种溶液与矿物（即方解石）平衡，但矿物成分（即CO2和Ca）的量不同，混合，导致溶解方解石的不饱和溶液。虽然这一过程已被广泛研究与达西规模的模型，假设在孔隙尺度的混合条件，有没有报告的观察混合腐蚀在孔隙尺度，其中不完全混合可以强烈影响净溶解混合腐蚀。也有越来越多的认识到，在现场或含水层规模的流动和运输的影响，孔隙规模的流动和反应，因此有必要建立一个框架，可以集成到更大规模的模型孔隙规模的过程。这些过程的关键是溶液的不完全混合如何驱动地球化学反应速率和溶解矿物的运输变化。使用方解石矿物的创新微流体实验和台式反应器用于研究流动和地球化学反应的参数如何影响方解石的混合和随后的溶解。然后将实验结果合成到一个框架中，该框架能够将整体溶解速率与流动、孔隙几何形状和地球化学条件的关键参数相关联。该框架奠定了基础，然后了解如何耦合流和地球化学在孔隙尺度影响地球化学反应在更大的尺度，这将是广泛相关的许多环境processes.This奖项反映了NSF的法定使命，并已被认为是值得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的支持。

Dr. Michael A. Chen has been granted an NSF EAR Postdoctoral fellowship to study mixing induced calcite dissolution and develop outreach materials at the University of Minnesota – Twin Cities campus with Professor Peter K. Kang and Dr. Diana Dalbotten. Carbonate minerals, such as calcite, make up many natural rocks and soils, including many karst systems, and are also an important sink for carbon in the carbon cycle. Removal of these minerals by dissolution drives the evolution of subsurface landscapes and results in carbon release back into the carbon cycle, and vigorous dissolution can be spurred by even the simple mixing of waters saturated with different sources of calcite. This specific process, called mixing corrosion, is fundamentally a soil pore-scale process and known as a key mechanism for karst formation, but we still do not understand how this small-scale process can lead to the larger scale patterns of dissolution in time and space. Aquifer scale models used to study mixing corrosion typically assume that solutions are well mixed as soon as they encounter each other, however, this is not necessarily true at the smaller scale where incomplete mixing may leave solutions segregated, thus reducing the total calcite dissolution rate. Thus, the primary goal of this fellowship will be to experimentally study mixing corrosion of calcite at the microscale, and develop a framework for understanding how microscale flow, mixing, and chemical reaction affect calcite dissolution at larger scales. This framework and the fundamental knowledge gained through these studies can be extended to understand how other microscale processes influence larger scale processes, which can be used to improve predictions of landscape evolution, develop predictive models of carbon sequestration, and enhance efforts to remediate groundwater contaminants. This project will also support minority involvement in Earth Sciences through recruiting and mentoring of interns from underrepresented groups in the Earth Sciences, as well as through outreach with the Science Museum of Minnesota to improve public understanding of flow and chemistry of rocks and soils using new demonstration experiments.There is a continuing need to better understand how the coupled processes of flow and geochemistry alter the dissolution or precipitation of calcite for its relevance in carbon cycling and the development of natural geologic formations, particularly karst. This is best exemplified in the process of mixing corrosion, where two solutions that are equilibrated with a mineral (i.e. calcite), but differing amounts of mineral constituents (i.e. CO2 and Ca), are mixed, resulting in an undersaturated solution that will dissolve calcite. While this process has been studied extensively with Darcy-scale models that assumes well-mixed conditions at pore scale, there are no reported observations of mixing corrosion at the pore scale, where incomplete mixing can strongly influence net dissolution by mixing corrosion. There is also increasing recognition that flow and transport at the field or aquifer scale is impacted by pore scale flow and reaction, thus there is need for a framework that can integrate pore-scale processes into the larger scale models. Key among these processes is how incomplete mixing of solutions drives variation in geochemical reaction rates and transport of dissolved minerals. Innovative microfluidic experiments and benchtop reactors using calcite minerals are used to study how parameters of flow and geochemical reaction influence mixing and subsequent dissolution of calcite. The experimental results will then be synthesized into a framework which is able to relate overall dissolution rate to the key parameters of flow, pore geometry, and geochemical conditions. This framework lays the groundwork to then understand how coupled flow and geochemistry at the pore scale influences geochemical reaction at larger scales, which will be broadly relevant to many environmental processes.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.