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

Colorado School Of Mines, Golden CO

Investigators

Abstract

9909477 Honeyman In 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.