Adsorption Parameters for Radioactive Liquid-Waste Migration
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RADIOACTIVE LIQUID-WASTE MIGRATION L. C. HULL, M. N. PACE, G. D. REDDEN Idaho National Engineering and Environmental Laboratory, Idaho Falls, ID 83415
ABSTRACT Proton titration experiments have been conducted at the Idaho National Engineering and Environmental Laboratory on synthetic goethite and soil in an effort to develop adsorption parameters that will help predict migration of radioactive liquid waste. This is the initial step in a reactive transport project to understand contaminant migration in a system characterized by strong chemical gradients. For this stage, two levels of pretreatment were applied to the soil to remove carbonate minerals and soluble salts to focus on the remaining mineral fraction. Without some sort of treatment or conditioning, native soil has a large buffer capacity that interferes with proton titration experiments. In this report, results are presented from the initial stages of the project.
INTRODUCTION One of the most significant problems in the application of geochemistry to fate and transport modeling is the lack of an approach to adsorption that can account for changes in the subsurface geochemical environment such as changes in pH, ionic strength, and competing cations. An essential requirement to advance the study of geochemistry is to integrate mechanistic ion exchange and surface complexation into reactive transport models to allow practical application for performance and risk assessment. At the Idaho National Engineering and Environmental Laboratory (INEEL), liquid radioactive waste from reprocessing spent nuclear fuel is 1 Mnitric acid with milligram/liter levels of uranium. Past spills of the liquid waste to the soil were rapidly neutralized by calcite in the soil matrix, with concurrent large pH changes, generation of carbon dioxide gas, strong chemical gradients, and adsorption of uranium onto soil minerals. Adsorption parameters that can accommodate associated changes in the subsurface geochemical environment are needed to understand the migration potential of the waste. A reactive transport project is under way at the INEEL to study the mechanisms controlling contaminant transport in the vadose zone. The working hypothesis of the reactive transport project is that uranium transport in the vadose zone is primarily controlled by surface complexation reactions on mineral oxide surfaces and requires knowledge of uranium partitioning on individual soil components under the influence of the carbonate system. Many laboratory experiments have been conducted on the adsorption of uranium on single, synthetic minerals [1, 2, 3]. However, little work has been done to expand these studies to adsorption on mixtures of minerals or on natural soil materials. Proton titration experiments are being conducted on single minerals and on soil to quantify surface charge and to evaluate buffer capacity. Future experiments will measure adsorption of uranium on these materials.
EXPERIMENT Goethite Preparation Goethite (c-FeOOH) was prepared under C0 2-free conditions following a method simi
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