The Kinetics and Surface Chemistry of Mineral Corrosion

对于低温地球化学来说，没有什么过程比导致不同溶质物种之间以及矿物与邻近含水流体之间的传质更重要的了。我们建议继续研究将溶解反应组织成一个预测框架。该方法是为了证明金属从表面去除的速率系数与溶解配合物周围的配体交换机制之间的定量相似性（Casey, 1991; Casey and Westrich, 1992; Ludwig etal ., 1995 5a, b）。这些过程之间的定量相似性允许建立线性-自由能关系（LFER）来预测速率系数。在适当的情况下，我们可以在矿物表面确定可能的速率控制基本步骤反应，例如活化配合物中水的运动速率，我们现在知道这是重要的（例如，Ludwig等人，1995a）。新的研究将扩展预测框架，以检查有机配体对金属转移速率的影响。我们测试了这样一个假设，即小分子量配合物的配体促进速率系数（KL）可以通过相应溶解金属配体配合物的溶剂运动速率或描述溶液中金属配体形态的平衡常数来预测（见Ludwig等人，1995a,b）。成功的关键是获取高质量的氧化矿物速率数据，这些氧化矿物在金属-氧键强度上存在很大差异。同样重要的是：(1)选择在溶解反应中形成单一显性表面复合物的配体，(2)理解吸附质子在反应中的作用，以及(3)表征降解产物。本研究解决了对污染物地球化学平衡模型的普遍批评（例如，Oreskes et al., 1994），即重要的反应是缓慢的，速率系数未知。对一些配体的系统研究可能有助于结合多种化合物的反应活性，因为空间拥挤最终必须限制与表面金属配位的配体的大小、刚性和数量。

Casey 9626553 No processes are more essential to low-temperature geochemistry than those leading to mass transfer among distinct solute species, and between minerals and adjacent aqueous fluids. We propose to continue research that organizes dissolution reactions into a predictive framework. The approach is to demonstrate quantitative similarity between the rate coefficients for metal removal from a surface and the better-characterized mechanisms of ligand exchange around dissolved complexes (Casey, 1991; Casey and Westrich, 1992; Ludwig et al., 1995a, b). The quantitative similarity between these processes allows establishment of linear-free-energy relations (LFER) to predict rate coefficients. In suitable cases we can identify possible rate-controlling elementary step reactions at mineral surfaces, such as the rates of water movement in the activated complexes, which we now know to be important (e.g., Ludwig et al., 1995a). New research will expand the predictive framework to examine the effect of organic ligands on rates of metal transfer. We test the hypothesis that ligand-promoted rate coefficients (KL) for small-molecular-weight complexes are predictable using either the rates of solvent motion in the corresponding dissolved metal-ligand complex or the equilibrium constants that describe metal-ligand speciation in solution (see Ludwig et al., 1995a,b). Key to success is acquisition of high-quality rate data on oxide minerals that differ considerably in metal-oxygen bond strengths. Also critical are: (I) choice of ligands that form a single dominant surface complex stoichiometry in the dissolution reaction, (ii) in understanding the role of adsorbed protons in reaction, and (iii) in characterizing the degradation products. This research addresses the common criticism of equilibrium models of contaminant geochemistry (e.g., Oreskes et al., 1994) that the important reactions are slow and the rate coefficients unknown. Systematic study of a few ligands may serve to bound the re activities of a wide range of compounds because steric crowding must ultimately limit the size, rigidity, and number of ligands that can coordinate to a surface metal.