Mineral Dissolution and Growth: A Laboratory Study of Kinetic Responses to Variable Conditions

矿物溶解和生长是岩石与水圈和生物圈（即地球表面的环境）之间化学交流的基本形式。该提案描述了一项研究，该研究使用速率松弛动力学和非稳态实验作为在氧化矿物溶解和生长过程中操作的表面化学过程的强大新探针。地球表面的条件是多变的。这种环境中的化学过程受到频繁的瞬态和周期性扰动和条件波动的影响；很少能达到稳定状态。相比之下，我们对矿物溶解和生长动力学的理解主要局限于稳态。我们对利率放松时间（即从一个稳态过渡到另一个稳态所需的时间）知之甚少，尽管有强有力的证据表明它们存在。这就限制了我们模拟自然界反应速率的能力。本研究将测试和约束非稳态（NSS）溶解和生长动力学理论，以解决这一限制。本文将NSS数据系统化的理论框架结合了表面络合模型（SCMs）、吸收动力学和Burton-Cabrera-Frank （BCF）理论。这种方法直接关系到其他几个长期存在的问题：1)总热力学驱动力与速率之间的关系；2)矿物表面的结构-反应性对比；3)影响速率的吸附物质的作用。重要的是，它还创造了一种将原位扫描探针显微镜的运动学（步进和扭结运动和相互作用）新数据用于约束理论的方法。因此，NSS动力学提供了一个独特的机会，在实验室和现场尺度上建立微观表面过程和宏观动力学行为之间的联系。使用简单的（单组分）Al， Fe和Si氧化物矿物（加上一些结构类似物）以及后来使用钠长石测试的特定假设是：存在速率弛豫时间；即，作为对突然变化（如pH值）的响应，溶解速率或生长速率预计相对于变化的突然性缓慢地接近一个新的稳定状态。这将使用专门设计的流动反应器进行湿化学测试。系统对方波和正弦波变化（如pH）的响应将被记录下来。这种来自化学工程的“频率响应”技术为反应机制的中间步骤提供了有价值的数据。速率弛豫时间预计与相应的水离子的h2o置换速率相关（溶解和吸附速率也相关）。速率弛豫是由于吸附的“营养物”物质（如Al203上的Al）的浓度和/或结构的变化造成的。这种吸附的平衡和动力学数据将通过以下方法获得：a)同位素稀释，b) FTIR光谱的新应用，c)新的高分辨率静电原子力显微镜（AFM）技术，以及d) AFM衍生的阶跃运动学数据与前面讨论的BCF-SCM理论预测的比较。等结构（刚玉和针铁矿结构）氧化物被使用，这样线性自由能量关系可以用来系统化动力学数据。

Eggleston 9527031 Mineral dissolution and growth are basic forms of chemical communication between rocks and the hydro-, and bio-spheres (i.e., environments at the Earth's surface). This proposal describes a study that uses rate-relaxation kinetics and non steady-state experiments as a powerful new probe of surface chemical processes operating during dissolution and growth of oxide minerals. Conditions at the Earth's surface are variable. Chemical processes in this environment are subject to frequent transient and periodic perturbations and fluctuations of conditions; steady-state is rarely achieved. Our understanding of mineral dissolution and growth kinetics, in contrast is largely limited to steady-state. We know very little about rate-relaxation times (i.e., the time needed for transition from one steady-state to another), despite strong evidence that they exist. This imposes a fundamental limit on our ability to model reaction rates in nature. This study will test and constrain a theory of non-steady-state (NSS) dissolution and growth kinetics that addresses this limitation. The theoretical framework developed here to systematize NSS data combines surface complexation models (SCMs), absorption kinetics, and Burton-Cabrera-Frank (BCF) theory. This approach has direct bearing on several other long-standing problems: 1) relationship(s) between overall thermodynamic driving force and rate; 2) structure-reactivity correlation's for mineral surfaces; and 3) the role of rate-affecting adsorbed species. Importantly, it also creates a way to use new data on kinematics (step and kink motion and interaction) from in-situ scanning probe microscopy to constrain theory. NSS kinetics thus provides a unique opportunity to establish connections between microscopic surface processes and macroscopic kinetic behavior at both laboratory and field scales. Specific hypotheses tested, using simple (single component) Al, Fe, and Si oxide minerals (plus a few structural analogs), and later usi ng albite, are: Rate-relaxation times exist; i.e., in response to an abrupt change (e.g., in pH), dissolution or growth rates are expected to approach a new steady-state slowly relative to the abruptness of the change. This will be tested wet-chemically using specially designed flow-through reactors. The system response to square-wave and sine-wave variations of , e.g., pH, will be recorded. This "frequency-response" technique from chemical engineering provides valuable data on intermediate steps in reaction mechanisms. Rate-relaxation times are expected to correlate with H2O-replacement rates for corresponding aqueous ions (as do dissolution and adsorption rates). Rate-relaxation results from changes in the concentration and/or structure of adsorbed "nutrient" species (e.g., of Al on Al203). Equilibrium and kinetic data on such adsorption will be gained using : a) isotopic dilution, b) a novel use of FTIR spectroscopy, c) a new, high-resolution electrostatic atomic force microscopy (AFM) technique, and d) comparison of AFM-derived step kinematics data to the predictions of BCF-SCM theory discussed above. Iso-structural (corundum and goethite-structure oxides are used so that linear-free energy relationships can be employed to systematize kinetic data.