Effects of Iron and pH on Glass Dissolution Rate
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EFFECTS OF IRON AND pH ON GLASS DISSOLUTION RATE Seung-Young Jeong and William L. Ebert Chemical Technology Division, Argonne National Laboratory, 9700 S. Cass Ave. Argonne, IL 60439 ABSTRACT Short-term static tests were conducted with a surrogate high-level waste glass to measure the effects of pH and dissolved iron on the glass dissolution rate. The tests were conducted to determine if a term to account for the effects of dissolved iron is needed in the glass degradation model developed for Total System Performance Assessment (TSPA) calculations for the Yucca Mountain disposal system license application. The glass degradation model includes terms for dependencies on temperature, pH, and chemical affinity. A series of tests was conducted at 90 °C in various pH solutions without iron and with added FeCl3, Fe2O3, Fe3O4, and FeOOH. Tests were conducted at glass surface area/solution volume (S/V) ratios about 2 and 10 m-1 for between 2 and 21 days. Solution concentrations of boron were used to measure the extent of reaction and calculate the glass dissolution rates. Similar rates were measured in tests conducted with and without iron at each pH. Both the results of the tests with and without iron showed Vshaped pH dependence curves with minima at near-neutral pH values. The pH dependencies (η) are about 0.44 in basic solutions and –0.49 in acidic solutions, based on the combined results of tests with and without iron. These are within the range of values for the pH dependence in the TSPA model for site recommendation. INTRODUCTION Short-term static tests were conducted with a surrogate high-level waste glass in pH solutions with and without iron at 90 °C. The purpose of this work was to (1) determine the model parameter values for pH dependency used in the Total System Performance Assessment glass degradation model for License Application (TSPA-LA) and (2) measure the effect of dissolved iron and iron contain products on the degradation rates of a representative Defense Waste Processing Facility (DWPF) glass in various pH solutions. The Total System Performance Assessment for Site Recommendation (TSPA-SR) glass degradation model is based on a mechanistic model for which the dissolution rate is expressed as the product of three terms that represent the intrinsic rate constant (k0), pH (η), and temperature (Ea), and a fourth term representing the chemical affinity (1 – Q/K), which represents feedback effects of the solution [1]. The rate expression can be written as rate = k0 • 10 η•pH • exp(-Ea/RT) • (1 – Q/K)
(1)
The possible effects on the glass dissolution rate due to (1) the corrosion of other materials, (2) the formation of alteration phases, and (3) dissolution of the glass itself are taken into account through the chemical affinity term. The values of η and Ea used in the TSPA-SR model were determined from the results of tests in which the value of the chemical affinity term remained near 1 so that the dissolution rate was not affected by the chemical affinity term.
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The short-term static tes
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