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Ion Exchange Processes and Mechanisms in Glasses Supported by Environmental Management Science Project. Recent performance assessment calculations(1) of a disposal system at Hanford, Washington, for low-activity-waste glass show that a Na ion-exchange reaction can effectively increase the radionuclide release rate by over a factor of 1000 and so is a major factor that currently limits waste loading. However, low-temperature ion exchange has not been thought to be important in recent analyses of waste glass durability. The objective of this work is to develop an understanding of the processes and mechanisms controlling alkali ion exchange and to correlate the kinetics of the ion-exchange reaction with glass structural properties. The fundamental understanding of the ion-exchange process developed under this study is targeted at developing lower ion-exchange rate glasses that would remain durable at higher alkali waste loading. This multidisciplinary research program involves two primary tasks to develop an understanding of the processes and mechanisms that control ion exchange, specifically Na ion exchange, in waste glass materials: 1) reaction mechanisms, and 2) glass structure correlations. The objective of the reaction mechanisms task is to identify specific ion-exchange mechanism(s) by using surface and near-surface analytical techniques to probe the distribution of selected elements in the hydrated layers on glass surfaces. Differences in the uptake and distribution of selected isotopes will provide a signature characteristic of specific ion-exchange reactions. The objective of the glass structure task is to determine whether differences in key structural properties, such as the number of nonbridging oxygens (NBO), bonding of alkali to other elements in the glass, and alkali coordination, can be correlated with differences in measured rates of alkali exchange. The reaction mechanisms and glass structure tasks are discussed herein in detail. This project was initiated in FY98 and is being performed cooperatively between PNNL and LBNL. Progress has primarily focused on three activities 1) developing and manufacturing the glasses, 2) characterizing these glasses, and 3) conducting flow-through tests for measuring Na ion-exchange kinetics. The primary role of the EMSL staff and capabilities involves measurements of isotope distributions using nuclear reaction analysis (NRA) and elemental depth profiles using Rutherford backscattering spectroscopy (RBS). An accurate measurement of the rate of Na ion exchange from the test glasses is one of the key challenges on this project. The experiments must be performed under controlled conditions of constant pH, temperature, and solution composition. The kinetics of Na-Al-Si glass dissolution and ion exchange were determined with a unique single-pass flow-through (SPFT) system. Glass coupons (up to five) were placed into columns fashioned from polyetheretherketone containing five individual cells. The cells are interconnected by a narrow tunnel that passes from the bottom to the top of the column, allowing solutions to flow from the reservoir, past each glass coupon, and out of the column to the sample collection vial. In this manner, the solution reacts with the samples and individual coupons can be removed from the column for analysis of reaction layers without disrupting the other coupons. Solutions were made up by adding THAM (tris hydroxymethyl aminomethane) to deionized water to bring the concentration up to 0.01 or 0.05 M. Ultra pure nitric acid was then added to the solution to bring the solution pH to the desired value (8 or 9). We found that the maximum buffering capacity of this weak buffer solution is at pH values of 8. We additionally added silicon to the solution (2.3 x 10-3 M) in the form of amorphous silica. The solutions were heated to 90° C to facilitate the dissolution of silica into solution. The high concentrations of Si in solution correspond to oversaturated conditions, which simulates high reaction progress and minimizes glass matrix dissolution. In summary, the solutions are designed to maintain a uniform pH and to fix the reaction affinity at a constant high level, conditions that are lacking in previous investigations. The number of Na atoms in the leached layer were determined by comparing the RBS spectra for the reacted coupons to that of non-reacted (‘blank’) coupons. Spectra were collected by accelerating 2.04 MeV He ions at the target coupons. The accumulated charge was 60 mC and the scattering angle was 170 degrees. Hydrogen-uptake profiles in the leached layer were determined by NRA by comparing spectra between reacted and non-reacted (‘blank’) samples. The analyses were performed using 19F ions with energies between 6.4 to 9.0 MeV. A typical RBS spectra plot is illustrated in Figure 9.4 for a leached and a non-reacted (‘blank’) coupon. This illustration plots channel versus counts, which is roughly equivalent to plotting energy versus concentration. The number of counts in this illustration has been normalized to the number of Si atoms so the two spectra can be compared. Several features of the spectra should be noted, beginning with the sharp rise in the number of counts from the left of the diagram (towards low ‘channels’ or energy). This feature represents the Si edge, and the Na edge is evident in the ‘blank’ coupon spectra further to the right of the diagram. The smaller number of counts in the energy region corresponding to Na in the leached coupon, compared with the ‘blank’ coupon, represents sodium loss from the glass. The approximate depth of Na depletion is estimated to be between 2000 and 3000 Å.
A resonant nuclear reaction using a 19F beam was used to measure the H profiles across leached and unleached glass samples. The depth of Na depletion in Figure 9.4 roughly corresponds to the depth of hydrogen uptake, as illustrated in Figure 9.5. Comparing leached and ‘blank’ glass coupons shows that hydrogen concentrations are relatively enriched in a region approximately 500 to 1000 Å.
A series of dissolution experiments using D2O and D218O have been conducted. Measurements of D and 18O uptake in the glass coupons will provide crucial information on the exact mechanism of alkali H-species exchange. A series of measurements of 18O uptake on glasses are shown in Figure 9.6. The amount of O uptake increases linearly with time. These measurements are of particular interest in comparison with the measurements of d uptake. The ratio of O to deuterium uptake is shown in Figure 9.7. These results show a ratio that decreases with time. The important implications of this measurement are that the glass dissociates and repolymerizes. This reaction removes the deuterium but retains some of the 18O. Thus ion exchange occurs and the surface/ reaction layer is changing with time.
William R. Wiley Environmental Molecular Sciences Laboratory Feedback: webmaster@emsl.pnl.gov Revised: June 12, 2001 Security & Privacy PNNL-13147 |
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