Silicate Dissolution in Organic-Rich Aqueous Systems: Reaction Mechanisms and Rate Controls

本研究的目的是研究有机硅和有机铝配合物在长石溶解中的作用。有机酸在许多风化环境中被认为是重要的反应物质，并被认为可以加速长石的溶解，增加长石的溶解度，调动溶液中的铝和二氧化硅，改变溶液与沉淀次生相之间的平衡。然而，有机酸影响长石溶蚀的确切机制及其在不同地表风化过程中的意义尚未得到很好的确定。在环中性到酸性pH的天然水中，似乎有四个主要过程：质子水解，铝的溶液络合，二氧化硅的溶液络合，以及导致加速溶解的表面配位。根据环境条件和存在的有机酸种类，主要机制可能是四种可能性中的一种或任何一种的组合。提出的研究方法是结合三种独立的方法来研究一个简单的长石-水电解质系统。溶解实验将量化各种有机酸阴离子对溶解动力学的影响，作为pH、温度和溶液组成的函数。这些实验旨在分离酸性水解，溶液络合和表面相互作用的平行机制。光谱实验将研究铝和二氧化硅溶液配合物的稳定性和形成动力学。拉曼、FTIR、UV-VIS、C和Si方法将用于获得键合环境、化学计量学和稳定性作为pH和温度的函数的信息，使用与溶解实验相同的溶液组成。这些研究将结合有机酸-金属组合的分子轨道建模关系作为有机酸官能团化学的功能。溶液络合研究和建模的结果将用于校准硅酸盐表面的大规模分子力学模型，该模型将用于测试来自溶解实验的假设相互作用。这是一项为期两年的研究，将与正在进行的实地研究相结合，在实地研究中，硅-有机络合已被确定为一种重要的风化机制，这些实地地点将用于测试可能的相互作用模型。此外，该研究将涉及与洛斯阿拉莫斯国家实验室合作，开发水系统中有机金属配合物的光谱分析方法。

The goal of the proposed research is to examine the role of organic-silica and organic-aluminum complexes in feldspar dissolution. Organic acids have been implicated as important reactants in a number of weathering environments, and are suggested to accelerate feldspar dissolution, increase feldspar solubility, mobilize aluminum and silica in solution, and alter the equilibrium between the solution and the precipitated secondary phases. The precise mechanism by which organic acids influence feldspar dissolution, however, and their significance in varying surficial weathering processes, is not well established. In natural waters at circum-neutral to acidic pH, there appears to be four primary processes: proton hydrolysis, solution complexation of aluminum, solution complexation of silica, and surface coordination resulting in accelerated dissolution. Depending on the environmental conditions and the organic acid species present, the dominant mechanism may be one or a combination of any of the four possibilities. The approach for the proposed study is to combine three independent methodologies to investigate a simple feldspar-aqueous- -electrolyte system. Dissolution experiments will be done to quantify the effects of various organic-acid anions on dissolution kinetics as a function of pH, temperature, and solution composition. These experiments are designed to separate the parallel mechanisms of acidic hydrolysis, solution complexation, and surface interactions. Spectroscopic experiments will investigate the stability and formation kinetics of the solution complexes of aluminum and silica. Raman, FTIR, UV-VIS, C and Si methods will be used to gain information on bonding environment, stoichiometry, and stability as a function of pH and temperature, using the same solution compositions as in the dissolution experiments. These studies will be combined with molecular orbital modeling relationships of the organic-acid--metal combinations as a function of organic-acid functional-group chemistry. The results of the solution complexation studies and modeling will be used to calibrate large-scale molecular-mechanics models of silicate surfaces that will be used to test the hypothetical interactions derived from the dissolution experiments. This is a two-year study which will interface with ongoing studies of field locations at which silica-organic complexation has been identified as a significant weathering mechanism, and these field sites will be used to test possible models of interaction. In addition, the study will involve collaboration with Los Alamos National Laboratory in developing methods of spectroscopic analysis of organic-metal complexes in aqueous systems.