Coupled Silicate Reaction Kinetics in an Aquifer

该提案寻求更新由美国国家科学基金会赞助的综合研究和教育计划，对亚利桑那州布莱克台地的纳瓦霍砂岩含水层地下水中的硅酸盐反应动力学进行现场、微观和模拟研究。现代水文地质学和地球化学中的一个基本问题是流域和土壤剖面的硅酸盐溶解速率与实验室测量的硅酸盐溶解速率之间存在着数量级的差异。这种巨大的差异表明我们对控制自然界中硅酸盐溶解动力学的基本物理和化学过程缺乏基本的了解。拟议的研究将检验两个假设来解释这种差异：(1)现场和实验室速率之间的差异很大程度上是由于长石在自然体系和大多数实验室实验中的不同饱和状态造成的。虽然大多数实验室实验试图测量远离平衡的单一一致溶解反应的速率，但现场研究在反应网络的复杂网络中测量现场速率，并且相对于长石在非常接近平衡的条件下测量。这些反应中包括粘土的沉淀，它从地下水中去除溶质，从而促进长石的持续溶解。然而，与粘土与地下水处于平衡状态的普遍假设相反，我们认为粘土的沉淀比长石的溶解慢得多，因此地下水化学被约束为与长石接近平衡。随着时间的推移，地下水化学达到稳定状态，长石溶解和粘土沉淀的速率接近恒定；(2)风化长石表面形成淋溶层，这是自然界中硅酸盐溶解发生的重要部分，必须在描述反应动力学的速率定律中考虑。拟议中的研究将通过分析含水层中溶解的Al3+浓度，评估饱和指数及其沿流动路径的变化来验证第一个假设，并通过数值地球化学模拟来阐明与含水层中长稀疏溶解相关的复杂反应网络。我们的目标是建立一个解释实验室实验和现场数据的理论框架。为了验证第二个假设，我们将使用原子尺度的场发射枪电子显微镜来表征长石-粘土界面的微观结构和详细的化学成分。总体目标是促进我们对实验室和现场速率差异的两种主要可能性的理解：热力学驱动减弱和风化长石表面的特征。广泛的影响PIS将把研究活动整合到本科生和研究生水平的地质学课程中。暑期指导计划将资助本科生，特别是来自代表性不足群体的本科生，在PI的实验室进行研究。将与国家能源技术实验室和洛斯阿拉莫斯国家实验室建立伙伴关系，以促进研究成果在碳汇和环境恢复方案中的传播和应用。地下水是饮用水的主要来源，对地球上的生命至关重要。地下水也代表了98%的人类随时可用的淡水。因此，含水层中的反应速度对水资源和水质管理、废物处理和全球变暖缓解战略至关重要。因此，该项目的发现将有助于我们对环境的了解，并有助于为环境政策和战略建立科学基础。

0509755ZhuThis proposal seeks renewal of a NSF-sponsored integrated research and education program infield, microscopic, and modeling studies of silicate reaction kinetics in a groundwater aquifer theNavajo sandstone aquifer at Black Mesa, Arizona. One of the fundamental problems in modernhydrogeology and geochemistry is the orders of magnitude discrepancy between silicate dissolutionrates derived from watersheds and soil profiles versus those derived from laboratory measurements.This large discrepancy indicates our lack of basic understanding of the fundamental physical andchemical processes controlling silicate dissolution kinetics in nature.The proposed study will test two hypotheses to explain this discrepancy: (1) A large part of thediscrepancy between in situ and laboratory rates results from different saturation states under whichfeldspars dissolve in natural systems and in most laboratory experiments. While most laboratoryexperiments attempt to measure the rate of a single congruent dissolution reaction far from equilibrium,field studies measure in situ rates amid a complex web of reaction networks and at a condition veryclose to equilibrium with respect to feldspars. Among these reactions is the precipitation of clay, whichremoves solutes from groundwater and hence promotes continued feldspar dissolution. However,contrary to the prevailing assumption that clays are at equilibrium with groundwater, we believe thatclay precipitation is much slower than feldspar dissolution, and hence groundwater chemistry isconstrained to be close to equilibrium with feldspars. Over time, a steady state of groundwaterchemistry is reached with near constant rates of feldspar dissolution and clay precipitation; and (2) Aleached layer forms on weathered feldspar surfaces, which is an important part of how silicatedissolution occurs in nature and must be considered in rate laws that properly describe reactionkinetics.The proposed study will test the first hypothesis by analyzing dissolved Al3+ concentrations inthe aquifer, evaluating saturation indices and their variations along a flow path, and numericalgeochemical modeling to elucidate the complex network of reactions associated with feldspardissolution in aquifers. We aim to establish a theoretical framework for interpreting laboratoryexperiments and field data. To test the second hypothesis, we will characterize microstructures anddetailed chemistry at the feldspar-clay interfaces using an atomic scale Field Emission GunTransmission Electron Microscope. The overall objectives are to advance our understanding of the twokey possibilities for discrepancies between laboratory and field rates: the diminishing thermodynamicdrive and the characteristics of weathered feldspar surfaces.Broader ImpactsThe PIs will integrate the research activities into undergraduate and graduate-level geologycourses. A summer mentorship program will sponsor undergraduate students, particularly fromunderrepresented groups, to conduct research in the PI's laboratories. Partnerships with the NationalEnergy Technology Laboratory and Los Alamos National Laboratory will be developed to facilitatedissemination and applications of the research results to the carbon sequestration and environmentalrestoration programs.Groundwater is a key source of drinking water and is essential to life on Earth. Groundwateralso represents 98% of fresh water readily available to humans. Therefore, reaction rates in aquifers arecritical to water resource and water quality management, waste disposal, and global warmingmitigation strategies. The findings from this project will therefore contribute to our understanding ofthe environment, and help to build a scientific basis for environmental policies and strategies.