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)构成了络合痕量金属的重要配位体。确定NOM对地下水系统中痕量金属形态和迁移的影响是开发准确的环境和人类健康风险评估模型的关键。反应性迁移模型通常包含等温线(线性或非线性)或系数分布(例如Kd)来表示金属的吸附特性。然而，等温线参数和Kd值取决于感兴趣体系的化学组成，并且对许多体系来说，化学组成在时间和空间上是不同的。因此，等温线或KD方法对体系化学的变化不敏感，包括金属离子络合有机配体的存在，如NOM。表面络合模型(SCMS)通过考虑颗粒表面官能团和溶解化学物种之间的化学反应来考虑吸附，扩展了溶液络合的化学和数学形式。然而，尽管NOM在金属离子形态和迁移中的重要性已经得到了充分的证明，等温线和Kd值的条件性，以及表面络合理论在评估无有机物体系中金属离子的形态形成方面取得了成功，但SCM尚未被应用于评估含有NOM的体系中的金属离子迁移。初始体系将由地球化学非均质性较低的砂岩、铀(VI)和柠檬酸组成。在后续的任务中，将增加复杂性，将苏万尼河黄腐酸(标称分子量=800)作为有机配位体和固定相，其地球化学性质的不均一性比“模型”沙所呈现的更大。在富里酸存在的情况下，在示踪剂水平上使用一套放射性金属离子(例如，镍、锌、钴、铅的同位素)的第二层实验将为测试反应迁移模型提供一个扩展的数据库。反应迁移模拟将遵循多组分模拟方法，明确考虑溶液和表面相过程，包括表面络合。被调用和测试的界面分子假说的范围从非静电构型到分布式电荷模型(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)。本研究的结果将为以下工作提供依据：1)选择合适的模型复杂性水平来预测可变体系化学条件下饱和体系中的金属离子迁移；2)将NOM引入多组分迁移模型。

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.