Role of Organic Acids in Transport of U(VI) and PB(II) Through Saturated Porous Media: Application of Surface Chemical Models to Transport Simulations of Bench-Scale Experiments

在大多数水生系统中，溶解的天然有机物（NOM）是络合微量金属的重要配体。确定NOM对微量金属形态和通过地下水系统迁移的影响，对于开发准确的环境和人类健康风险评估模型至关重要。反应输运模型通常采用等温线（线性或非线性）或系数分布（例如Kd）来表示金属吸附特性。然而，等温线参数和Kd值取决于感兴趣系统的化学组成，对于许多系统，化学组成随时间和空间而变化。因此，等温线或Kd方法将对系统化学的变化不敏感，包括金属离子络合有机配体（如no）的存在。表面络合模型（SCMs）通过考虑颗粒表面官能团和溶解化学物质之间化学反应的吸附，扩展了溶液相络合的化学和数学形式。然而，尽管有充分的文献证明了NOM在金属离子形态形成和运输中的重要性，恒温和Kd值的条件性质，以及表面络合理论成功地应用于评估有机无物质体系中金属离子形态，但SCMs尚未应用于评估含NOM体系中的金属离子运输。本项目的实验任务将从相对简单的到更复杂的。初始体系将由地球化学非均质性较低的砂、U（VI）和柠檬酸（NOM替代物）组成。随着Suwannee河富里酸（标称MW =800）作为有机配体和比“模型”砂具有更大地球化学非均质性的不流动相，后续任务将增加复杂性。在黄腐酸存在的情况下，使用一套示踪剂水平的放射性金属离子（例如，Ni、Zn、Co、Pb的同位素）进行二级实验，将为测试反应传输模型提供扩展的数据库。反应输运建模将遵循多组分模拟方法，明确考虑溶液相和表面相过程，包括表面络合。调用和测试的界面分子假设将从非静电配置到分布式电荷模型（例如，van Riemsdijk和同事）。如有必要，将明确考虑反应动力学（例如，在Szecsody和同事之后）。本研究的结果将为：1)在可变系统化学条件下，选择合适的模型复杂性水平来预测饱和系统中的金属离子输运提供依据；2)在多组分输运模型中引入NOM。

9909477HoneymanIn most aquatic systems species of dissolved natural organic matter (NOM) constitute an important pool of ligands for complexing trace metals. The determination of the effects of NOM on trace metal speciation and migration through groundwater systems is critical to the development of accurate models for environmental and human health risk assessment.Reactive transport models have generally incorporated isotherms (linear or non-linear) or distribution of coefficients (e.g., Kd ) to represent metal sorption characteristics. However, isotherm parameters and Kd values are conditional on the chemical composition of the system of interest and, for many systems, chemical composition varies in time and space. As consequence, the isotherm or Kd approach will be insensitive to variations in system chemistry, including the presence of metal-ion-complexing organic ligands such as NOM.Surface complexation model (SCMs) extend the chemical and mathematical formatisms of solution-phase complexation by considering adsorption in terms of chemical reactions between particle surface functional groups and dissolved chemical species. However, in spite of the well-documented importance of NOM in metal-ion speciation and transport, the conditional nature of iotherms and Kd values, and the success of surface complexation theory as applied to evaluation of metal-ion speciation in organic matter-free systems, SCMs have not yet been applied to the evaluation of metal-ion transport in systems containing NOM.The experimental tasks in this project will proceed from the relatively simple to the more complex. The initial systems will be composed of a sand with low geochemical heterogeneity, U(VI) and citric acid as an NOM surrogate. Increased complexity will be introduced in subsequent tasks with Suwannee River Fulvic Acid (nominal MW =800) as the organic ligand and an immobile phase of greater geochemical heterogeneity than presented by the 'model' sand. Second-tier experiments using a suite of radioactive metal-ions (e.g., isotopes of Ni, Zn, Co, Pb)at tracer levels in the presence of fulvic acid will provide an extended data base for testing reactive transport models.Reactive transport modeling will follow the multicomponent simulation approach by explicitly considering both solution- and surface-phase processes, including surface complexation. Interfacial molecular hypotheses invoked and tested will range from non-electrostatic configurations to distributed charge models (e.g., van Riemsdijk and co-workers). Reaction kinetics will be explicitly considered if necessary (e.g., after Szecsody and co-workers). Results of this study will provide a basis for: 1) selecting the appropriate level of model complexity necessary to predict metal-ion transport in saturated systems under variable system chemical conditions; 2) the incorporation of NOM in multicomponent transport models.