Characterizing Transport and Sorption in Ion-Specific Resin Columns Using Nuclear Magnetic Resonance (NMR) Imaging
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ABSTRACT The goal of this work is to assess the physical transport properties of Gd through an ion exchange column while determining the sorption properties of the resin. By coupling the physical transport with the chemical sorption, further insight into the behavior of the ion exchange resin can be gained. NMR imaging provides a powerful, non-destructive, means to extract spatial information from complex systems on a near real-time basis. An important example is liquid flow through granular media. With the use of a chemically reactive NMR contrast agent, the chemical speciation can be traced along the physical flow path of the granular media. In this study, trivalent gadolinium (Gd 3+)was selected based on its chemical similarity to typical high-level waste components, 241"Am and 244Cm, and for its paramagnetic contrasting abilities in NMR experiments. NMR imaging results of flow experiments are provided showing a characteristic flow phenomena and resin column loading profiles. ICP-AES data are provided to show resin ion exchange capacities (IECs) and breakthrough curves. The use of NMR imaging with a Gd3+ tracer will lead to a better understanding of the transport and sorption properties of these ion-specific resins. This technique can be applied to other complex flow systems such as environmental transport. INTRODUCTION Ion-specific resins have been developed to partition similar inorganic chemical species, such as Cs from Na, from waste streams in radioactive waste reprocessing operations. The resins used in these studies are synthetic organic structures with phenol functional groups. The resins are made selective by incorporation of a chelating compound within the resin structure. These types of resins have been shown to form stable complexes with a variety of radionuclides [I]. Due to the recent development of these synthetic resins, there is a general lack of information on the behavior of the resins under operational conditions. Many analytical methods exist to provide information on the bulk characteristics of a packed resin column, however, few methods exist to extract real-time spatial information from the columns under operational conditions. The method chosen in this study is nuclear magnetic resonance (NMR) imaging with the use of Gd tracer to examine new ion-specific resins for the separation of trivalent metal ions. NMR imaging is a non-invasive technique to probe the spatial structure of a variety of complex systems [2-5]. By using a paramagnetic ion in a water-based solution, the behavior of that ion can be traced as it moves through the NMR sampling volume, since the paramagnetic ion reduces the NMR relaxation time of neighboring water spins. The ability to use an ion as an NMR tracer provides a means to assess the physical structure of porous media while identifying the chemical behavior along the flow path. Specifically, the chemical speciation (sorption, complexation, colloid formation, and precipitation) can be traced along a flow path, which leads to a greater understanding of the ultimate f
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